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Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..fdce707 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #50671 (https://www.gutenberg.org/ebooks/50671) diff --git a/old/50671-0.txt b/old/50671-0.txt deleted file mode 100644 index 4ba56b4..0000000 --- a/old/50671-0.txt +++ /dev/null @@ -1,22869 +0,0 @@ -The Project Gutenberg eBook, Earth Features and Their Meaning, by William -Herbert Hobbs - - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - - - - -Title: Earth Features and Their Meaning - An Introduction to Geology for the Student and the General Reader - - -Author: William Herbert Hobbs - - - -Release Date: December 12, 2015 [eBook #50671] - -Language: English - -Character set encoding: UTF-8 - - -***START OF THE PROJECT GUTENBERG EBOOK EARTH FEATURES AND THEIR MEANING*** - - -E-text prepared by Giovanni Fini and the Online Distributed Proofreading -Team (http://www.pgdp.net) from page images generously made available by -Internet Archive (https://archive.org) - - - -Note: Project Gutenberg also has an HTML version of this file which - includes the more than 500 original illustrations. - See 50671-h.htm or 50671-h.zip: - (http://www.gutenberg.org/files/50671/50671-h/50671-h.htm) - or - (http://www.gutenberg.org/files/50671/50671-h.zip) - - - Images of the original pages are available through - Internet Archive. See - https://archive.org/details/cu31924004975763 - - -Transcriber’s note: - - Text enclosed by underscores is in italics (_italics_). - - Text enclosed by equal signs is in bold face (=bold=). - - A carat character is used to denote superscription. A - single character following the carat is superscripted - (example: b^1). Multiple superscripted characters are - enclosed by curly brackets (example: ^{1-2}). - - Subscript numbers in chemical formulas have been - rendered as _n or _{n}. - - - - - -EARTH FEATURES AND THEIR MEANING - - - * * * * * * - -[Illustration: LOGO] - -The Macmillan Company -New York · Boston · Chicago -Dallas · San Francisco - -Macmillan & Co., Limited -London · Bombay · Calcutta -Melbourne - -The Macmillan Co. Of Canada, Ltd. -Toronto - - * * * * * * - - -+-------------------------------------------------------------+ -¦ PLATE 1. ¦ -¦ ¦ -¦ [Illustration: Mount Balfour and the Balfour Glacier in the ¦ -¦ Selkirks.] ¦ -+-------------------------------------------------------------+ - - -EARTH FEATURES AND THEIR MEANING - -An Introduction to Geology for the Student and the General Reader - -by - -WILLIAM HERBERT HOBBS - -Professor of Geology in the University of Michigan -Author of “Earthquakes. an Introduction to Seismic Geology”; -“Characteristics of Existing Glaciers”; etc. - - - - - - - -New York -The Macmillan Company -1921 -All rights reserved - - -Copyright, 1912, -By The Macmillan Company. - -Norwood Press -J. S. Cushing Co.—Berwick & Smith Co. -Norwood, Mass., U.S.A. - - - - - TO THE MEMORY - - OF - - GEORGE HUNTINGTON WILLIAMS - - - - -PREFACE - - -THE series of readings contained in the present volume give in somewhat -expanded form the substance of a course of illustrated lectures -which has now for several years been delivered each semester at the -University of Michigan. The keynote of the course may be found in -the dominant characteristics of the different earth features and the -geological processes which have been betrayed in the shaping of them. -Such a geological examination of landscape is replete with fascinating -revelations, and it lends to the study of Nature a deep meaning which -cannot but enhance the enjoyment of her varied aspects. - -That there is a real place for such a cultural study of geology within -the University is believed to be shown by the increasing number of -students who have elected the work. Even more than in former years the -American travels afar by car or steamship, and the earth’s surface -features in all their manifold diversity are thus one after the other -unrolled before him. The thousands who each year cross the Atlantic -to roam over European countries may by historical, literary, or -artistic studies prepare themselves to derive an exquisite pleasure -as they visit places identified with past achievement of one form or -another. Yet the Channel coast, the gorge of the Rhine, the glaciers of -Switzerland, and the wild scenery of Norway or Scotland have each their -fascinating story to tell of a history far more remote and varied. To -read this history, the runic characters in which it is written must -first of all be mastered; for in every landscape there are strong -individual lines of character such as the pen artist would skillfully -extract for an outline sketch. Such _character profiles_ are often many -times repeated in each landscape, and in them we have a key to the -historical record. - -An object of the present readings has thus been to enable the -student to himself pick out in each landscape these more significant -lines and so read directly from Nature. In the landscapes which -have been represented, the aim has been to draw as far as possible -upon localities well known to travelers and likely to be visited, -either because of their historical interest or their purely scenic -attractions. It should thus be possible for a tourist in America or -Europe to pursue his landscape studies whenever he sets out upon his -travels. The better to aid him in this endeavor, some suggestions -concerning the itinerary of journeys have been supplied in an appendix. - -Regarded as a textbook of geology, the present work offers some -departures from existing examples. Though it has been customary to -combine in a single text historical with dynamical and structural -geology, a tendency has already become apparent to treat the historical -division apart from the others. Again, a desire to treat the science -of geology comprehensively has led some authors into including so -many subjects as to render their texts unnecessarily encyclopedic -and correspondingly uninteresting to the general reader. It is the -author’s belief that there is a real need for a book which may be read -intelligently by the general public, and it must be recognized that -the beginner in the subject cannot cover the entire field by a single -course of readings. The present work has, therefore, been prepared with -a view to selecting for study those dominant geological processes which -are best illustrated by features in northern North America and Europe. -It is this desire to illustrate the readings by travels afield, which -accounts for the prominence given to the subject of glaciation; for the -larger number of colleges and universities in both America and Europe -are surrounded by the heavy accumulations that have resulted from -former glaciations. - -Emphasis has also been placed upon the dependence of the dominant -geological processes of any region upon existing climatic conditions, -a fact to which too little attention has generally been given. This -explains the rather full treatment of desert regions, of which, -in our own country particularly, much may be illustrated upon the -transcontinental railway journeys. - -More than in most texts the attempt has here been made to teach -directly through the eye with the efficient aid of apt illustrations -intimately interwoven with the text. For such success as has been -reached in this endeavor, the author is greatly indebted to two -students of the University of Michigan,—Mr. James H. Meier, who has -prepared the line drawings of landscapes, and Mr. Hugh M. Pierce, who -has draughted the diagrams. Though credit has in most cases been given -where illustrations have been made from another’s photographs, yet -especial mention should here be made of the debt to Dr. H. W. Fairbanks -of Berkeley, California, whose beautiful and instructive photographs -are reproduced upon many a page. - -As given at the University of Michigan, the lectures reflected in the -present volume are supplemented by excursions and by so much laboratory -practice as is necessary to become familiar with the more common -minerals and rocks, and to read intelligently the usual topographical -and geological maps. In the appendices the means for carrying out such -studies, in part with newly devised apparatus, have been indicated. - -The scope of the book precludes the possibility of furnishing the -reader with the sources for the body of fact and theory which is -presented, although much may be inferred from the names which appear -beneath the illustrations, and more definite knowledge will be found in -the references to literature supplied at the ends of chapters. A large -amount of original and unpublished material is for a similar reason -unlabeled, and it has been left for the professional geologist to -detect these new strands which have been drawn into the web. - - WILLIAM HERBERT HOBBS. - - ANN ARBOR, MICHIGAN, - October 25, 1911. - - - - -CONTENTS - - - CHAPTER I - - THE COMPILATION OF EARTH HISTORY - - PAGE - - 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 _versus_ - speculative philosophy—The scientific attitude and temper—The - value of the hypothesis—Heading references 1 - - - CHAPTER II - - THE FIGURE OF THE EARTH - - The lithosphere and its envelopes—The evolution of ideas - concerning the earth’s figure—The oblateness of the earth—The - arrangement of oceans and continents—The figure toward - which the earth is tending—Astronomical _versus_ geodetic - observations—Changes of figure during 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 hydrosphere 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 surface shell—The essential nature of crystals—The - lithosphere a complex of interlocking crystals—Some properties of - natural crystals, minerals—The alterations of minerals—Reading - references 20 - - - CHAPTER IV - - THE ROCKS OF THE EARTH’S SURFACE SHELL - - The processes by which rocks are formed—The marks of origin—The - metamorphic rocks—Characteristic textures of the igneous - rocks—The classification of rocks—Subdivisions of the sedimentary - rocks—The different deposits of ocean, lake, and river—Special - marks of littoral deposits—The order of deposition during a - transgression of the sea—The basins of deposition of earlier - ages—The deposits of the deep sea—Reading references 30 - - - CHAPTER V - - CONTORTIONS OF THE STRATA WITHIN THE ZONE OF FLOW - - The zones of fracture and flow—Experiments which illustrate - the fracture and flow of solid bodies—The arches and troughs - of the folded strata—The elements of folds—The shapes of rock - folds—The overthrust 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 references 40 - - - CHAPTER VI - - THE ARCHITECTURE OF THE FRACTURED SUPERSTRUCTURE - - The system of the fractures—The space intervals of joints—The - displacements upon joints: faults—Methods of detecting faults—The - base of the geological map—The field 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: EARTHQUAKES 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: EARTHQUAKES AND - 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 - - - CHAPTER IX - - THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE; VOLCANIC - MOUNTAINS OF EXUDATION - - Prevalent misconceptions about volcanoes—Early views concerning - volcanic mountains—The birth of volcanoes—Active and extinct - volcanoes—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 OF 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 cinder 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 - Pelé—The aftermath of mud flows—The dissection of volcanoes—The - formation of lava reservoirs—Character profiles—Reading - references 115 - - - CHAPTER XI - - THE ATTACK OF THE WEATHER - - The two contrasted processes of weathering—The rôle of the - 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 mantle and its shield in the mat of - vegetation—Reading references 149 - - - CHAPTER XII - - THE LIFE HISTORIES OF RIVERS - - The intricate pattern of river etchings—The motive power of - rivers—Old land and new land—The earlier aspects of rivers—The - meshes of the river network—The upper and lower reaches - of a river contrasted—The balance between degradation and - aggradation—The accordance of tributary valleys—The grading of - the flood plain—The cycles of stream meanders—The cut-off of the - meander—Meander scars—River terraces—The delta of the river—The - levee—The sections of delta deposits 158 - - - CHAPTER XIII - - EARTH FEATURES SHAPED BY RUNNING WATER - - 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 - rocks—The capture of one river by another—Water and wind - gaps—Character profiles—Reading references 169 - - - CHAPTER XIV - - THE TRAVELS OF THE UNDERGROUND WATER - - The descent within the unsaturated zone—The trunk channels - of descending 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 - - - CHAPTER XVII - - REPEATING PATTERNS IN THE EARTH RELIEF - - The weathering processes under control of the fracture system—The - fracture control of the drainage lines—The repeating pattern in - drainage networks—The dividing lines of the relief patterns: - lineaments—The composite repeating patterns of the higher - orders—Reading references 223 - - - CHAPTER XVIII - - THE FORMS CARVED 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 barrier—The land-tied island—A barrier series—Character - profiles—Reading references 231 - - - CHAPTER XIX - - COAST RECORDS OF 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 - cuesta—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 movements 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 mountain barriers in initiating glaciers—Sensitiveness of - glaciers to temperature 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 rock islands—Rock fragments which travel - with the 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 GLACIERS OF THE “ICE AGE” - - Earlier cycles of glaciation—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 OF THE LAST ICE AGE - - Interference of glaciers with drainage—Temporary lakes due to ice - blocking—The “parallel roads” of the Scottish glens—The glacial - Lake Agassiz—Episodes of the glacial lake history within the St. - Lawrence Valley—The crescentic 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 Hudson’s Bay - drainage basin—Reading references 320 - - - CHAPTER XXIV - - THE UPTILT OF THE LAND AT THE CLOSE OF THE ICE AGE - - The response of the earth’s shell to its ice mantle—The - abandoned 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 references 340 - - - CHAPTER XXV - - NIAGARA FALLS A CLOCK OF RECENT GEOLOGICAL TIME - - 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 correlation 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 - distribution 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 amphitheater 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 OF 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 séracs—Bodies given up by the - _Glacier des Bossons_—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 - - - CHAPTER XXIX - - A STUDY OF LAKE BASINS - - Fresh water and saline lakes—Newland lakes—Basin-range - lakes—Rift-valley lakes—Earthquake lakes—Crater lakes—Coulée - lakes—Morainal lakes—Pit lakes—Glint or colk lakes—Ice-dam - lakes—Glacier-lobe lakes—Rock-basin lakes—Valley moraine - lakes—Landslide lakes—Border lakes—Ox-bow lakes—Saucer - lakes—Crescentic levee lakes—Raft lakes—Side-delta lakes—Delta - lakes—Barrier lakes—Dune lakes—Sink lakes—Karst lakes: - _poljen_—Playa lakes—Salines—Alluvial-dam lakes—Résumé—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 off of - bays—Lake extinction by peat growth—Extinction of lakes in desert - regions—The rôle of lakes in the economy of nature—Ice ramparts - on lake shores—Reading 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 effect 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_ - - FACING PAGE - - 2. A. Layers compressed in experiments and showing the effect of - a competent layer in the process of folding 44 - B. Experimental production of a series of parallel thrusts - within closely folded strata 44 - C. Apparatus to illustrate shearing action within the - overturned limb of a fold 44 - - 3. A. An earthquake fault opened in Formosa in 1906 with vertical - 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 - intersected by master joints 82 - - 5. A. Once wooded region in China now reduced to desert through - deforestation 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 Cañon, New Mexico 216 - - 9. Map of the High Plains at the eastern front of the Rocky - Mountains 220 - - 10. A. View in Spitzbergen to illustrate the disintegration of - rock 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 shore 232 - - 12. A. V-shaped cañon 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 272 - B. Lateral stream beside the Benedict Glacier outlet, - Greenland 272 - - 14. View of the margin of the Antarctic continental glacier in - Kaiser Wilhelm Land 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 - striæ 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, Philipsburg Quadrangle, - Montana 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 for comparison of coasts upon an upright and upon an - inverted tetrahedron 17 - - 7. The continents, including submerged portions 18 - - 8. Diagram to indicate the altitude of different parts of the - lithosphere surface 18 - - 9. Diagram to show how the terrestrial rocks grade into the - meteorites 22 - - 10. Comparison of a crystalline with an amorphous substance 24 - - 11. “Light figure” seen upon etched surface of calcite 25 - - 12. Battered sand grains which have developed crystal faces 26 - - 13. Unassimilated grains of quartz within a garnet crystal 28 - - 14. New minerals developed about the core of an augite crystal 28 - - 15. A common rim of new mineral developed by reaction where - earlier minerals come into contact 28 - - 16. Laminated structure of a sedimentary rock 30 - - 17. Characteristic textures of igneous rocks 33 - - 18. Diagram to show the order of sediments laid down during a - transgression of the sea 37 - - 19. Fractures produced by compression of a block of molder’s wax 41 - - 20. Apparatus to illustrate the folding of strata 41 - - 21. Diagrams of fold types 42 - - 22. Diagrams to illustrate crustal shortening 42 - - 23. Anticlinal and synclinal folds 43 - - 24. Diagrams to illustrate the shapes of rock folds 44 - - 25. Secondary and tertiary flexures superimposed upon the - primary ones 44 - - 26. A bent stratum to illustrate tension and compression upon - opposite sides 45 - - 27. A geological section with truncated arches restored 47 - - 28. Diagram to illustrate the nature of strike and dip 47 - - 29. Diagram to show the use of T symbols for strike and dip - observation 48 - - 30. Diagram to show how the thickness of a formation is - determined 49 - - 31. A plunging anticline 50 - - 32. A plunging syncline 50 - - 33. An unconformity upon the coast of California 51 - - 34. Series of diagrams to illustrate the episodes involved in - the production of an angular unconformity 52 - - 35. Types of deceptive or erosional unconformities 53 - - 36. A set of master joints in shale 55 - - 37. Diagram to show the manner of replacement of one set of - joints by another 56 - - 38. Diagram to show the different combinations of joint series 56 - - 39. View of the shore in West Greenland 57 - - 40. View in Iceland which shows joint intervals of more than one - order 57 - - 41. Faulted blocks of basalt near Woodbury, Connecticut 58 - - 42. A fault in previously disturbed strata 59 - - 43. Diagram to show the effect of erosion upon a fault 60 - - 44. A fault plane exhibiting drag 60 - - 45. Map to show how a fault may be indicated by abrupt changes - in strike and dip 61 - - 46. A series of parallel faults revealed by offsets 61 - - 47. Field map prepared from the laboratory table 64 - - 48. Areal geological map based upon the field map 64 - - 49. A portion of the ruins of Messina 67 - - 50. Ruins of the Carnegie Palace of Peace at Cartaga, Costa Rica 68 - - 51. Overturned bowlders from Assam earthquake of 1897 69 - - 52. Post sunk into ground during Charleston earthquake 69 - - 53. Map showing localities where shocks have been reported at - sea off Cape Mendocino, California 70 - - 54. Effect of seismic water wave in Japan 70 - - 55. A fault of vertical displacement 71 - - 56. Escarpment produced by an earthquake fault in India 72 - - 57. A fault of lateral displacement 72 - - 58. Fence parted and displaced by lateral displacement on fault - during California earthquake 72 - - 59. Fault with vertical and lateral displacements combined 72 - - 60. Diagram to show how small faults may be masked at the - earth’s surface 73 - - 61. “Mole hill” effect above buried earthquake fault 73 - - 62. Post-glacial earthquake faults 74 - - 63. Earthquake cracks in Colorado desert 74 - - 64. Railway tracks broken or buckled at time of earthquake 75 - - 65. Railroad bridge in Japan damaged by earthquake 75 - - 66. Diagrams to show contraction of earth’s crust during an - earthquake 76 - - 67. Map of the Chedrang fault of India 76 - - 68. Displacements along earthquake fault in Alaska 77 - - 69. Abrupt change in direction of throw upon an earthquake fault 77 - - 70. Map of faults in the Owens Valley, California, formed during - earthquake of 1872 78 - - 71. Marquetry of the rock floor in the Tonopah district, Nevada 79 - - 72. Map of Alaskan coast to show adjustments of level during an - earthquake 79 - - 73. An Alaskan shore elevated seventeen feet during the - earthquake of 1899 80 - - 74. Partially submerged forest from depression of shore in - Alaska during earthquake 80 - - 75. Effect of settlement of the shore at Port Royal during - earthquake of 1907 80 - - 76. Diagrams to illustrate the draining of lakes during - earthquakes 83 - - 77. Diagram to illustrate the derangements of water flow during - an earthquake 84 - - 78. Mud cones aligned upon an earthquake fissure in Servia 84 - - 79. Craterlet formed near Charleston, South Carolina, during the - earthquake of 1886 85 - - 80. Cross section of a craterlet 85 - - 81. Map of the island of Ischia to show the concentration of - earthquake shocks 87 - - 82. A line of earth fracture revealed in the plan of the relief 87 - - 83. Seismotectonic lines of the West Indies 88 - - 84. Device to illustrate the different effects of earthquakes in - firm rock and in loose materials 88 - - 85. House wrecked in San Francisco earthquake 90 - - 86. Building wrecked in California earthquake by roof and upper - floor battering down the upper walls 91 - - 87. Breached volcanic cone in New Zealand showing the bending - down of the strata near the vent 96 - - 88. View of the new Camiguin volcano formed in 1871 in the - Philippines 97 - - 89. Map to show the belts of active volcanoes 98 - - 90. A portion of the “fire girdle” of the Pacific 98 - - 91. Volcanic cones formed in 1783 above the Skaptár fissure in - Iceland 99 - - 92. Diagrams to illustrate the location of volcanic vents upon - fissure lines 100 - - 93. Outline map showing the arrangement of volcanic vents upon - the island of Java 100 - - 94. Map showing the migration of volcanoes along a fissure 101 - - 95. Basaltic plateau of the northwestern United States due to - fissure eruptions of lava 102 - - 96. Lava plains about the Snake River in Idaho 102 - - 97. Characteristic profiles of lava volcanoes 103 - - 98. A driblet cone 104 - - 99. Leffingwell Crater, a cinder cone in the Owens Valley, - California 104 - - 100. Map of Hawaii and its lava volcanoes 106 - - 101. Section through Mauna Loa and Kilauea 106 - - 102. Schematic diagram to illustrate the moving platform in the - crater of Kilauea 107 - - 103. View of the open lava lake of Halemaumau 108 - - 104. Map to show the manner of outflow of the lava from Kilauea - in the eruption of 1840 109 - - 105. Lava of Matavanu flowing down to the sea during the eruption - of 1906 110 - - 106. Lava stream discharging into the sea from a lava tunnel 111 - - 107. Diagrammatic representation of the structure of lava - volcanoes as a result of the draining of frozen lava streams 112 - - 108. Diagram to show the formation of mesas by outflow of lava in - valleys and subsequent erosion 112 - - 109. Surface of lava of the Pahoehoe type 113 - - 110. Three successive views to show the growth of the island of - Savaii, from lava outflow in 1906 113 - - 111. View of the volcano of Stromboli showing the excentric - position of the crater 116 - - 112. Diagrams to illustrate the eruptions within the crater of - Stromboli 117 - - 113. Map of Volcano in the Æolian Islands 118 - - 114. “Bread-crust” lava projectile from the eruption of Volcano - in 1888 119 - - 115. “Cauliflower cloud” of steam and ash rising above the cinder - cone of Volcano 120 - - 116. Eruption of Taal volcano in 1911 seen from a distance of six - miles 120 - - 117. The thick mud veneer upon the island of Taal (after a - photograph by Deniston) 121 - - 118. A pear-shaped lava projectile 121 - - 119. Artificial production of a cinder cone 122 - - 120. Diagram to show the contrast between a lava dome and a - cinder cone 123 - - 121. Mayon volcano on the island of Luzon, Philippine Islands 123 - - 122. A series of breached cinder cones due to migration of the - eruption along a fissure 124 - - 123. The mouth upon the inner cone of Mount Vesuvius from which - flowed the lava of 1872 124 - - 124. A row of parasitic cones raised above a fissure opened on - the flanks of Etna in 1892 125 - - 125. View of Etna, showing the parasitic cones upon its flanks 125 - - 126. Sketch map of Etna to show the areas covered by lava and - tuff respectively 126 - - 127. Panum crater showing the caldera 126 - - 128. View of Mount Vesuvius before the eruption of 1906 127 - - 129. Sketches of the summit of the Vesuvian cone to bring out the - changes in its outline 128 - - 130. Night view of Vesuvius from Naples before the outbreak of - 1906, showing a small lava stream descending the central cone 129 - - 131. Scoriaceous lava encroaching upon the tracks of the Vesuvian - railway 130 - - 132. Map of Vesuvius, showing the position of the lava mouths - opened upon its flanks during the eruption of 1906 131 - - 133. The ash curtain over Vesuvius lifting and disclosing the - outlines of the mountain 132 - - 134. The central cone of Vesuvius as it appeared after the - eruption of 1906 132 - - 135. A sunken road upon Vesuvius filled with indrifted ash 133 - - 136. View of Vesuvius from the southwest during the waning stages - of the eruption 133 - - 137. The main lava stream advancing upon Boscotrecase 133 - - 138. A pine snapped off by the lava and carried forward upon its - surface 133 - - 139. Lava front pushing over and running around a wall in its - path 134 - - 140. One of the ruined villas in Boscotrecase 134 - - 141. Three diagrams to illustrate the sequence of events during - the cone-building and crater-producing periods 135 - - 142. The spine of Pelé rising above the chimney of the volcano - after the eruption of 1902 136 - - 143. Successive outlines of the Pelé spine 137 - - 144. Corrugated surface of the Vesuvian cone due to the mud flows - which followed the eruption of 1906 138 - - 145. View of the Kammerbühl near Eger in Bohemia 139 - - 146. Volcanic plug exposed by natural dissection of a volcanic - cone in Colorado 140 - - 147. A dike cutting beds of tuff in a partly dissected volcano of - southwestern Colorado 140 - - 148. Map and general view of St. Paul’s rocks, a volcanic cone - dissected by waves 141 - - 149. Dissection by explosion of Little Bandai-san in 1888 141 - - 150. The half-submerged volcano of Krakatoa before and after the - eruption of 1883 142 - - 151. The cicatrice of the Banat 142 - - 152. Diagram to illustrate a probable cause of formation of lava - reservoirs and the connection with volcanoes upon the surface 143 - - 153. Effect of relief of load upon rocks by arching of a - competent formation 144 - - 154. Character profiles connected with volcanoes 146 - - 155. Diagrams to show the effect of decomposition in producing - spheroidal bowlders 150 - - 156. Spheroidal weathering of an igneous rock 151 - - 157. Dome structure in granite mass 152 - - 158. Talus slope beneath a cliff 153 - - 159. Striped ground from soil flow 154 - - 160. Pavement of horizontal surface due to soil flow 154 - - 161. Tree roots prying rock apart on fissure 154 - - 162. Bowlder split by a growing tree 155 - - 163. Rock mantle beneath soil and vegetable mat 155 - - 164. Diagram to show the varying thickness of mantle rock upon - the different portions of a hill surface 156 - - 165. Gullies from earliest stage of a river’s life 160 - - 166. Partially dissected upland 160 - - 167. Longitudinal sections of upper portion of a river valley 161 - - 168. Map and sections of a stream meander 163 - - 169. Tree undermined on the outer bank of a meander 164 - - 170. Diagrams to show the successive positions of stream meanders 164 - - 171. An ox-bow lake in the flood plain of a river 165 - - 172. Schematic representation of a series of river terraces 165 - - 173. “Bird-foot” delta of the Mississippi River 167 - - 174. Diagrams to show the nature of delta deposits as exhibited - in sections 168 - - 175. Gorge of the River Rhine near St. Goars 169 - - 176. Valley with rounded shoulders characteristic of the stage of - adolescence 170 - - 177. View of a maturely dissected upland 170 - - 178. Hogarth’s line of beauty 171 - - 179. View of the oldland of New England, with Mount Monadnock - rising in the distance 171 - - 180. Comparison of the cross sections of river valleys of - different stages 172 - - 181. The Beavertail Bend of the Yakima River 173 - - 182. A rejuvenated river valley 174 - - 183. Plan of a river narrows 174 - - 184. Successive diagrams to illustrate the origin of “trellis - drainage” 175 - - 185. Sketch maps to show the earlier and present drainage near - Harper’s Ferry 176 - - 186. Section to illustrate the history of Snickers Gap 177 - - 187. Character profiles of landscapes shaped by stream erosion in - humid climates 177 - - 188. Diagram to show the seasonal range in the position of the - water table 180 - - 189. Diagram to show the effect of an impervious layer upon the - descending water 181 - - 190. Sketch map to illustrate corrosion of limestone along two - series of vertical joints 181 - - 191. Diagram to show the relation of limestone caverns to the - river system of the district 182 - - 192. Plan of a portion of Mammoth Cave, Kentucky 183 - - 193. Trees and shrubs growing upon the bottoms of limestone sinks 183 - - 194. Diagrams to show the manner of formation of stalactites and - stalagmites 185 - - 195. Sinter formations in the Luray caverns 186 - - 196. Map of the dolines of the Karst region 187 - - 197. Cross section of a doline formed by inbreak 187 - - 198. Sharp Karren of the Ifenplatte 188 - - 199. The Zirknitz seasonal lake 189 - - 200. Fissure springs arranged at intersections of rock fractures 190 - - 201. Schematic diagrams to illustrate the different types of - artesian wells 191 - - 202. Cross section of Geysir, Iceland 192 - - 203. Apparatus for simulating geyser action 193 - - 204. Cone of siliceous sinter about the Lone Star Geyser 194 - - 205. Former shore lines in the Great Basin 198 - - 206. Map of the former Lake Bonneville 199 - - 207. Borax deposits in Death Valley, California 201 - - 208. Hollowed forms of weathered granite in a desert of Central - Asia 201 - - 209. Hollow hewn blocks in a wall in the Wadi Guerraui 202 - - 210. Smooth granite domes shaped by exfoliation 203 - - 211. Granite blocks rent by diffission 204 - - 212. “Mushroom Rock” from a desert in Wyoming 205 - - 213. Windkanten shaped by sand blast in the desert 205 - - 214. The “stone lattice” of the desert 206 - - 215. Shadow erosion in the desert 206 - - 216. Cliffs in loess with characteristic vertical jointing 207 - - 217. A cañon in loess worn by traffic and wind 207 - - 218. Diagrams to illustrate the effects of obstructions in - arresting wind-driven sand 209 - - 219. Sand accumulating on either side of a firm and impenetrable - obstruction 210 - - 220. Successive diagrams to illustrate the history of the town of - Kunzen upon the Kurische Nehrung 210 - - 221. View of desert barchans 211 - - 222. Diagrams to show the relationships of dunes to sand supply - and wind direction 211 - - 223. Ideal section showing the rising mountain wall about a - desert and the neighboring slope 212 - - 224. Dry delta at the foot of a range upon the borders of a - desert 213 - - 225. Map of distributaries of streams which issue at the western - base of the Sierra Nevadas 213 - - 226. A group of “demoiselles” in the “bad lands” 214 - - 227. Amphitheater at the head of the Wadi Beni Sur 215 - - 228. Mesa and outlier in the Leucite Hills of Wyoming 216 - - 229. Flat-bottomed basin separating dunes 216 - - 230. Billowy surface of the salt crust on the central sink of the - desert of Lop 217 - - 231. Schematic diagram to show the zones of deposition in their - order from the margin to the center of a desert 217 - - 232. Mounds upon the site of the buried city of Nippur 218 - - 233. Exhumed structures in the buried city of Nippur 218 - - 234. Section across the High Plains 219 - - 235. Section across the lenticular threads of alluvial deposits - of the High Plains 220 - - 236. Distributaries of the foot hills superimposed upon an - earlier series 220 - - 237. Character profiles in the landscapes of arid lands 220 - - 238. Rain sculpturing under control by joints 224 - - 239. Sagging of limestone above joints 224 - - 240. Map of the joint-controlled Abisko Cañon in Northern Lapland 225 - - 241. Map of the gorge of the Zambesi River below Victoria Falls 225 - - 242. Controlled drainage network of the Shepaug River in - Connecticut 226 - - 243. A river network of repeating rectangular pattern 226 - - 244. Squared mountain masses which reveal a distribution of - joints in block patterns of different orders 228 - - 245. Island groups of the Lofoten Archipelago 229 - - 246. Diagrams to illustrate the composite profiles of the islands - on the Norwegian coast 229 - - 247. Diagram to show the nature of the motions within a free - water wave 231 - - 248. Diagram to illustrate the transformation of a free wave into - a breaker 232 - - 249. Notched rock cliff and fallen blocks 233 - - 250. A wave-cut chasm under control by joints 233 - - 251. Grand Arch upon one of the Apostle Islands in Lake Superior 234 - - 252. Stack near the shore of Lake Superior 234 - - 253. The Marble Islands, stacks in a lake of the southern Andes 235 - - 254. Squared stacks revealing the position of the joint planes on - which they were carved 235 - - 255. Ideal section cut by waves upon a steep rocky shore 236 - - 256. Map showing the outlines of the island of Heligoland at - different stages in its history 236 - - 257. Ideal section carved by waves upon a steep shore of loose - materials 237 - - 258. Sloping cliff and boulder pavement at Scituate, - Massachusetts 237 - - 259. Map to show the nature of the shore current and the forms - which are molded by it 238 - - 260. Crescent-shaped beach in the lee of a headland 239 - - 261. Cross section of a beach pebble 239 - - 262. A storm beach on the northeast shore of Green Bay 240 - - 263. Spit of shingle on Au Train Island, Lake Superior 240 - - 264. Barrier beach in front of a lagoon 241 - - 265. Cross section of a barrier beach with lagoon in its rear 242 - - 266. Cross section of a series of barriers and an outer bar 242 - - 267. A barrier series and an outer bar on Lake Mendota at - Madison, Wisconsin 242 - - 268. Series of barriers at the western end of Lake Superior 243 - - 269. Character profiles resulting from wave action upon shores 243 - - 270. The even shore line of a raised coast 246 - - 271. The ragged coast line produced by subsidence 246 - - 272. Portion of the Atlantic coastal plain at the base of the - oldland 246 - - 273. Ideal form of cuestas and intermediate lowlands carved from - a coastal plain 247 - - 274. Uplifted sea cave on the coast of California 248 - - 275. Double-notched cliff near Cape Tiro, Celebes 248 - - 276. Uplifted stacks on the coast of California 249 - - 277. Uplifted shingle beach across the entrance to a former bay - upon the coast of California 250 - - 278. Raised beach terraces near Elie, Fife, Scotland 250 - - 279. Uplifted sea cliffs and terraces on the Alaskan coast 250 - - 280. Diagrams to show how excessive sinking upon the sea floor - will cause the shore to migrate landward 251 - - 281. A drowned river mouth or estuary upon a coastal plain 251 - - 282. Archipelago of steep rocky islets due to submergence 252 - - 283. The submerged Hudsonian channel which continues the Hudson - River across the continental shelf 252 - - 284. Marine clay deposits near the mouths of the Maine rivers - which preserve a record of earlier subsidence and later - elevation 253 - - 285. View of the three standing columns of the Temple of Jupiter - Serapis, at Pozzuoli 254 - - 286. Three successive views to set forth the recent oscillations - of level on the northern shore of the Bay of Naples 255 - - 287. Relief map of San Clemente Island, California 256 - - 288. Relief map of Santa Catalina Island, California 257 - - 289. Cross section of the Blue Grotto, on the island of Capri 258 - - 290. Character profiles of coast elevation and subsidence 259 - - 291. Map showing the distribution of existing glaciers and the - two important wind poles of the earth 263 - - 292. An Alaskan glacier spreading out at the foot of the range - which nourishes it 264 - - 293. Surface of a glacier whose upper layers spread with but - slight restraint from retaining walls 265 - - 294. Section through a mountain glacier 267 - - 295. Profile across the largest of the Icelandic ice caps 267 - - 296. Ideal section across a continental glacier 267 - - 297. View of the Eyriks Jökull, an ice cap of Iceland 268 - - 298. The zones of the lower atmosphere as revealed by recent kite - and balloon exploration 269 - - 299. Map of Greenland, showing the area of inland ice and the - routes of explorers 271 - - 300. Profile in natural proportions across the southern end of - the continental glacier of Greenland 272 - - 301. Map of a glacier tongue with dimple above 273 - - 302. Edge of the Greenland inland ice, showing the nunataks - diminishing in size toward the interior 274 - - 303. Moat surrounding a nunatak in Victoria Land 274 - - 304. A glacier pavement of Permo-Carboniferous age in South - Africa 276 - - 305. Diagrams to illustrate the manner of formation of scape - colks 277 - - 306. Marginal moraine now forming at the edge of the continental - glacier of Greenland 279 - - 307. Small lake between the ice front and a moraine which it has - recently built 279 - - 308. View of a drained lake bottom between the ice front and an - abandoned moraine 280 - - 309. Diagrams to show the manner of formation and the structure - of an outwash plain and fosse 280 - - 310. Map of the ice masses of Victoria Land, Antarctica 282 - - 311. Sections across the inland ice and the shelf ice of - Antarctica 283 - - 312. Diagram to show the nature of the fixed glacial anticyclone - above continental glaciers 284 - - 313. Snow deltas about the margins of a glacier tongue in - Greenland 285 - - 314. View of the sea ice of the Arctic region 286 - - 315. Map of the north polar regions, showing the area of drift - ice and the tracks of the _Jeannette_ and the _Fram_ 288 - - 316. The shelf ice of Coats Land with surrounding pack ice 290 - - 317. Tidewater cliff on a glacier tongue from which icebergs are - born 290 - - 318. A Greenlandic iceberg after a long journey in warm latitudes 291 - - 319. Diagram showing one way in which northern icebergs are born - from the glacier tongue 291 - - 320. A northern iceberg surrounded by sea ice 292 - - 321. Tabular Antarctic iceberg separating from the shelf ice 293 - - 322. Map of the globe, showing the areas covered by continental - glaciers during the “ice age” 297 - - 323. Glaciated granite bowlder weathered out of a moraine of - Permo-Carboniferous age, South Australia 298 - - 324. Map to show the glaciated and nonglaciated regions of North - America 298 - - 325. Map of the glaciated and nonglaciated areas of northern - Europe 299 - - 326. An unstable erosion remnant characteristic of the “driftless - area” 300 - - 327. Diagram showing the manner in which a continental glacier - obliterates existing valleys 301 - - 328. Lake and marsh district in northern Wisconsin 302 - - 329. Cross section in natural proportion of the latest North - American continental glacier 303 - - 330. Diagram showing the earlier and the later glacier records - together upon the same limestone surface 304 - - 331. Map to show the outcroppings of peculiar rock types in the - region of the Great Lakes, and some localities where “drift - copper” has been collected 305 - - 332. Map of the “bowlder train” from Iron Hill, Rhode Island 306 - - 333. Shapes and approximate natural sizes of some of the diamonds - from the Great Lakes region 307 - - 334. Glacial map of a portion of the Great Lakes region 308 - - 335. Section in coarse till 310 - - 336. Sketch map of portions of Michigan, Ohio, and Indiana, - showing the distribution of moraines 312 - - 337. Map of the vicinity of Devil’s Lake, Wisconsin, partly - covered by the continental glacier 313 - - 338. Moraine with outwash apron in front 313 - - 339. Fosse between an outwash plain and a moraine 314 - - 340. View along an esker in southern Maine 315 - - 341. Outline map of moraines and eskers in Finland 315 - - 342. Sketch maps showing the relationships of drumlins and eskers 316 - - 343. View of a drumlin, showing an opening in the till 317 - - 344. Outline map of the front of the Green Bay lobe to show the - relationships of drumlins, moraines, outwash plains, and - ground moraine 317 - - 345. Character profiles referable to continental glacier 318 - - 346. View of the flood plain of the ancient Illinois River near - Peoria 320 - - 347. Broadly terraced valleys which mark the floods that once - issued from the continental glacier of North America 321 - - 348. Border drainage about the retreating ice front south of Lake - Erie 321 - - 349. The “parallel roads” of Glen Roy in the Scottish Highlands 322 - - 350. Map of Glen Roy and neighboring valleys of the Scottish - Highlands 322 - - 351. Three successive diagrams to set forth the late glacial lake - history of the Scottish glens 324 - - 352. Harvesting time on the fertile floor of the glacial Lake - Agassiz 325 - - 353. Map of Lake Agassiz 325 - - 354. Map showing some of the beaches of Lake Agassiz and its - outlet 326 - - 355. Narrows of the Warren River where it passed between jaws of - granite and gneiss 327 - - 356. Map of the valley of the Warren River near Minneapolis 327 - - 357. Portion of the Herman beach on the shore of the former Lake - Agassiz 328 - - 358. Map of the continental glacier of North America when it - covered the entire St. Lawrence basin 329 - - 359. Outline map of the early Lake Maumee 330 - - 360. Map to show the first stages of the ice-dammed lakes within - the St. Lawrence basin 330 - - 361. Outline map of the later Lake Maumee and its outlet 332 - - 362. Outline map of lakes Whittlesey and Saginaw 333 - - 363. Map of the glacial Lake Warren 333 - - 364. Map of the glacial Lake Algonquin 334 - - 365. Outline map of the Nipissing Great Lakes 335 - - 366. Probable preglacial drainage of the upper Ohio region 337 - - 367. Diagrams to illustrate the episodes in the recent history of - a Connecticut river 338 - - 368. The notched rock headland of Boyer Bluff on Lake Michigan 341 - - 369. View of Mackinac Island from the direction of St. Ignace 342 - - 370. The “Sugar Loaf”, a stack of Lake Algonquin upon Mackinac - Island 342 - - 371. Beach ridges in series on Mackinac Island 343 - - 372. Notched stack of the Nipissing Great Lakes at St. Ignace 343 - - 373. Series of diagrams to illustrate the evolution of ideas - concerning the uplift of the lake region since the Ice Age 344 - - 374. Map of the Great Lakes region to show the isobases and hinge - lines of uptilt 345 - - 375. Series of diagrams to indicate the nature of the recovery of - the crust by uplift when unloaded of an ice mantle 346 - - 376. Portion of the Inner Sandusky Bay, for comparison of the - shore line of 1820 with that of to-day 350 - - 377. Ideal cross section of the Niagara Gorge to show the - marginal terrace 353 - - 378. View of the bed of the Niagara River above the cataract - where water has been drained off 353 - - 379. View of the Falls of St. Anthony in 1851 354 - - 380. Ideal section to show the nature of the drilling process - beneath the cataract 355 - - 381. Plan and section of the gorge, showing how the depth is - proportional to the width 355 - - 382. Comparative views of the Canadian Falls in 1827 and 1895 356 - - 383. Map to show the recession of the Canadian Fall 357 - - 384. Comparison of the present with the future falls 358 - - 385. Bird’s-eye view of the captured Canadian Fall at Wintergreen - Flats 358 - - 386. Map of the Whirlpool Basin 360 - - 387. Map of the cuestas which have played so important a part in - fixing the boundaries of the lake basins 361 - - 388. Bird’s-eye view of the cuestas south of Lakes Ontario and - Erie 362 - - 389. Sketch map of the greater portion of the Niagara Gorge to - illustrate Niagara history 363 - - 390. Snowdrift hollowing its bed by nivation 368 - - 391. Amphitheater formed upon a drift site in northern Lapland 369 - - 392. The marginal crevasse on the highest margin of a glacier 370 - - 393. Niches and cirques in the Bighorn Mountains of Wyoming 371 - - 394. Subordinate cirques in the amphitheater on the west face of - the Wannehorn 371 - - 395. “Biscuit cutting” effect of glacial sculpture in the Uinta - Mountains of Wyoming 372 - - 396. Diagram to show the cause of the hyperbolic curve of cols 372 - - 397. A col in the Selkirks 373 - - 398. Diagrams to illustrate the formation of comb ridges, cols, - and horns 374 - - 399. The U-shaped Kern Valley in the Sierra Nevadas of California 375 - - 400. Glaciated valley wall, showing the sharp line which - separates the abraded from the undermined rock surface 375 - - 401. View of the Vale of Chamonix from the séracs of the _Glacier - des Bossons_ 376 - - 402. Map of an area near the continental divide in Colorado 377 - - 403. Gorge of the Albula River in the Engadine cut through a rock - bar 378 - - 404. Idealistic sketch, showing glaciated and nonglaciated side - valleys 378 - - 405. Character profiles sculptured by mountain glaciers 379 - - 406. Flat dome shaped under the margin of a Norwegian ice cap 379 - - 407. Two views which illustrate successive stages in the shaping - of tinds 380 - - 408. Schematic diagram to bring out the relationships of the - various types of mountain glaciers 383 - - 409. Map of the Malaspina Glacier of Alaska 384 - - 410. Map of the Baltoro Glacier of the Himalayas 385 - - 411. View of the Triest Glacier, a hanging glacieret 385 - - 412. Map of the Harriman Fjord Glacier of Alaska 386 - - 413. Map of the Rotmoos Glacier, a radiating glacier of - Switzerland 386 - - 414. Outline map of the Asulkan Glacier in the Selkirks, a - horseshoe glacier 387 - - 415. Outline map of the Illecillewaet Glacier of the Selkirks, an - inherited-basin glacier 388 - - 416. Diagram to illustrate the surface flow of glaciers 390 - - 417. Diagram to show the transformation of crevasses into séracs 391 - - 418. View of the _Glacier des Bossons_, showing the position of - accidents to Alpinists 392 - - 419. Lines of flow upon the surface of the _Hintereisferner_ - Glacier in the Alps 393 - - 420. Lateral and medial moraines of the _Mer de Glace_ and its - tributaries 393 - - 421. Ideal cross section of a mountain glacier 394 - - 422. Diagrams to illustrate the melting effects upon glacier ice - of rock fragments of different sizes 394 - - 423. Small glacier table upon the Great Aletsch Glacier 395 - - 424. Effects of differential melting and subsequent refreezing - upon a glacier surface 396 - - 425. Dirt cone with its casing in part removed 396 - - 426. Schematic diagram to show the manner of formation of glacier - cornices 397 - - 427. Superglacial stream upon the Great Aletsch Glacier 398 - - 428. Ideal form of the surface left on the site of a piedmont - glacier apron 399 - - 429. Map of the site of the earlier piedmont glacier of the Upper - Rhine 399 - - 430. Diagram and map to bring out the characteristics of newland - lakes 402 - - 431. View of the Warner Lakes, Oregon 402 - - 432. Schematic diagram to illustrate the characteristics of - basin-range lakes 403 - - 433. Schematic diagram of rift-valley lakes and the valley of the - Jordan 403 - - 434. Map of the rift-valley lakes of East Central Africa 404 - - 435. Earthquake lakes formed in 1811 in the flood plain of the - Lower Mississippi 404 - - 436. View of a crater lake in Costa Rica 405 - - 437. Diagrams to illustrate the characteristics of crater lakes 406 - - 438. View of Snag Lake, a coulée lake in California 406 - - 439. Diagrams to illustrate the characteristics of morainal lakes 407 - - 440. Diagram to show the manner of formation of pit lakes 408 - - 441. Diagrams to illustrate the characteristics of pit lakes 408 - - 442. Diagram to show the manner of formation of glint lakes 409 - - 443. Map of a series of glint lakes on the boundary of Sweden and - Norway 409 - - 444. Map of ice-dam lakes near the Norwegian boundary of Sweden 410 - - 445. Wave-cut terrace of a former ice-dam lake in Sweden 410 - - 446. View of the Márjelen Lake from the summit of the Eggishorn 411 - - 447. Diagrams to illustrate the arrangement and the characters of - rock-basin lakes 412 - - 448. Convict Lake, a valley-moraine lake of California 413 - - 449. Lake basins produced by successive slides from the steep - walls of a glaciated mountain valley 414 - - 450. Lake Garda, a border lake upon the site of a piedmont apron 414 - - 451. Diagrams to bring out the characteristics of ox-bow lakes 415 - - 452. Diagrammatic section to illustrate the formation of - saucer-like basins between the levees of streams on a flood - plain 415 - - 453. Saucer lakes upon the bed of the former river Warren 416 - - 454. Levee lakes developed in series within meanders in a delta - plain 417 - - 455. Raft lakes along the banks of the Red River in Arkansas and - Louisiana 418 - - 456. Map of the Swiss lakes Thun and Brienz 419 - - 457. Delta lakes formed at the mouth of the Mississippi 419 - - 458. Delta lakes at the margin of the Nile delta 420 - - 459. Diagrams to illustrate the characteristics of barrier lakes 420 - - 460. Dune lakes on the coast of France 421 - - 461. Sink lakes in Florida, with a schematic diagram to - illustrate the manner of their formation 421 - - 462. Map of the Arve and the Upper Rhone 426 - - 463. View of the Arve and the Rhone at their junction 427 - - 464. A village in Switzerland built upon a strath at the head of - Lake Poschiavo 428 - - 465. View of the floating bog and surrounding zones of vegetation - in a small glacial lake 429 - - 466. Diagram to show how small lakes are transformed into peat - bogs 430 - - 467. Map to show the anomalous position of the delta in Lake St. - Clair 431 - - 468. A bowlder wall upon the shore of a small lake 432 - - 469. Diagrams to show the effect of ice shove in producing ice - ramparts upon the shores of lakes 433 - - 470. Various forms of ice ramparts 433 - - 471. Map of Lake Mendota, showing the position of the ridge which - forms from ice expansion and the ice ramparts upon the shores 434 - - 472. The great multiple mountain arc of Sewestan, British India 436 - - 473. Diagrams to illustrate the theories of origin of mountain - arcs 437 - - 474. Festoons of mountain arcs about the borders of the Pacific - Ocean 438 - - 475. The interrupted Armorican Mountains common to western Europe - and eastern North America 438 - - 476. A zone of diverse displacement in the western United States 439 - - 477. Section of an East African block mountain 439 - - 478. Tilted crust blocks in the Queantoweap valley 440 - - 479. View of the laccolite of the Carriso Mountain 441 - - 480. Map of laccolitic mountains 441 - - 481. Ideal sections of laccolite and bysmalite 442 - - 482. The gabled façade largely developed in desert landscapes 443 - - 483. Balloon view of the Mythen in Switzerland 444 - - 484. The battlement type of erosion mountain 445 - - 485. Symmetrically formed low islands repeated in ranks upon - Temagami Lake, Ontario 445 - - 486. Forms of crystals of a number of minerals 454 - - 487. Forms of crystals of a number of minerals 457 - - 488. A student’s contour map 469 - - 489. Models to represent outcrops of rock 472 - - 490. Special laboratory table set with a problem in geological - mapping which is solved in Figs. 47 and 48 472 - - 491. Three field maps to be used as suggestions in arranging - laboratory table for problems in the preparation of areal - geological maps 473 - - 492. Sketch map of Western Scotland and the Inner Hebrides to - show location of some points of special geological interest 481 - - 493. Outline map of a geological pilgrimage across the continent - of Europe 483 - - - - -EXPLANATORY LIST OF ABBREVIATIONS FOR JOURNAL NAMES IN READING -REFERENCES - - - Am. Geol.: American Geologist. - - Am. Jour. Sci.: American Journal of Science, New Haven. - - Ann. de Géogr.: Annales de Géographie, Paris. - - Ann. Rept. Geol. and Geogr. Surv. Ter.: Annual Report of the - Geological and Geographical Survey of the Territories (Hayden), - Washington. - - Ann. Rept. Geol. and Nat. Hist. Surv. Minn.: Annual Report of the - Geological and Natural History Survey of Minnesota, Minneapolis. - - Ann. Rept. Mich. Geol. Surv.: Annual Report of the Michigan Geological - Survey, Lansing. - - Ann. Rept. U. S. Geol. Surv.: Annual Report of the United States - Geological Survey, Washington. - - Bull. Am. Geogr. Soc.: Bulletin of the American Geographical Society, - New York. - - Bull. Earthq. Inv. Com. Japan: Bulletin of the Earthquake - Investigation Committee of Japan, Tokyo. - - Bull. Geogr. Soc. Philadelphia: Bulletin of the Geographical Society - of Philadelphia. - - Bull. Geol. Soc. Am.: Bulletin of the Geological Society of America. - - Bull. Mus. Comp. Zoöl.: Bulletin of the Museum of Comparative Zoölogy, - Harvard College, Cambridge. - - Bull. N. Y. State Mus.: Bulletin of the New York State Museum, Albany. - - Bull. Soc. Belge d’Astronomie: Bulletin de la Société Belge - d’Astronomie, Brussels. - - Bull. Soc. Belge Géol.: Bulletin de la Société Belge de Géologie, - Brussels. - - Bull. Soc. Sc. Nat. Neuchâtel: Bulletin de la Société des Sciences - Naturelles de Neuchâtel. - - Bull. Univ. Calif. Dept. Geol.: Bulletin of the University of - California, Department of Geology, Berkeley. - - Bull. U. S. Geol. Surv.: Bulletin of the United States Geological - Survey, Washington. - - Bull. Wis. Geol. and Nat. Hist. Surv.: Bulletin of the Wisconsin - Geological and Natural History Survey, Madison. - - C. R. Cong. Géol. Intern.: Comptes Rendus de la Congrès Géologique - Internationale. - - Dept. of Mines, Geol. Surv. Branch, Canada: Department of Mines, - Geological Survey Branch, Canada. - - Geogr. Abh.: Geographische Abhandlungen. - - Geogr. Jour.: Geographical Journal, London. - - Geol. Folio U. S. Geol. Surv.: Geological Folio of the United States - Geological Survey. - - Geol. Mag.: Geological Magazine, London (sections designated by - decades). - - Jour. Am. Geogr. Soc.: Journal of the American Geographical Society, - New York. - - Jour. Coll. Sci. Imp. Univ. Tokyo: Journal of the College of Science - of the Imperial University, Tokyo, Japan. - - Jour. Geol.: Journal of Geology, Chicago. - - Jour. Sch. Geogr.: Journal of School Geography. - - Livret Guide Cong. Géol. Intern.: Livret Guide Congrès Géologique - Internationale. - - Mem. Geol. Surv. India: Memoirs of the Geological Survey of India, - Calcutta. - - Mitt. Geogr. Ges. Hamb.: Mitteilungen der Geographische Gesellschaft, - Hamburg. - - Mon. U. S. Geol. Surv.: Monograph of the United States Geological - Survey, Washington. - - Nat. Geogr. Mag.: National Geographic Magazine, Washington. - - Nat. Geogr. Mon.: National Geographic Monographs, American Book - Company, New York. - - Naturw. Wochenschr.: Naturwissenschaftliche Wochenschrift. - - Pet. Mitt.: Petermanns Mittheilungen aus Justus Perthes’ - Geographischer Anstalt, Gotha. - - Pet. Mitt., Ergänzungsh. or Erg.: Petermanns Mittheilungen, Gotha - (Ergänzungsheft or Supplementary Paper). - - Phil. Jour. Sci.: Philippine Journal of Science, Manila. - - Phil. Trans.: Philosophical Transactions of the Royal Society, London. - - Proc. Am. Acad. Arts and Sci.: Proceedings of the American Academy of - Arts and Sciences. - - Proc. Am. Assoc. Adv. Sci.: Proceedings of the American Association - for the Advancement of Science. - - Proc. Am. Phil. Soc.: Proceedings of the American Philosophical - Society, Philadelphia. - - Proc. Bost. Soc. Nat. Hist.: Proceedings of the Boston Society of - Natural History, Boston. - - Proc. Ind. Acad. Sci.: Proceedings of the Indiana Academy of Science. - - Proc. Linn. Soc. New South Wales: Proceedings of the Linnean Society - of New South Wales. - - Proc. Ohio State Acad. Sci.: Proceedings of the Ohio State Academy of - Science. - - Prof. Pap. U. S. Geol. Surv.: Professional Paper of the United States - Geological Survey, Washington. - - Pub. Carneg. Inst.: Publication of the Carnegie Institution of - Washington. - - Pub. Mich. Geol. and Biol. Surv.: Publication of the Michigan - Geological and Biological Survey, Lansing. - - Quart. Jour. Geol. Soc. Lond.: Quarterly Journal of the Geological - Society, London. - - Rept. Brit. Assoc. Adv. Sci.: Report of the British Association for - the Advancement of Science. - - Rept. Geol. Surv. Mich.: Report of the Geological Survey of Michigan, - Lansing. - - Rept. Mich. Acad. Sci.: Report of the Michigan Academy of Science, - Lansing. - - Rept. Nat. Conserv. Com.: Report of the National Conservation - Commission, Washington. - - Rept. Smithson. Inst.: Report of the Smithsonian Institution, - Washington. - - Sci. Bull. Brooklyn Inst. Arts and Sci.: Science Bulletin of the - Brooklyn Institute of Arts and Sciences. - - Scot. Geogr. Mag.: Scottish Geographic Magazine, Edinburgh. - - Smith. Cont. to Knowl.: Smithsonian Contributions to Knowledge, - Washington. - - Tech. Quart.: Technology Quarterly of the Massachusetts Institute of - Technology, Boston. - - Trans. Am. Inst. Min. Eng.: Transactions of the American Institute of - Mining Engineers, New York. - - Trans. Roy. Dublin Soc.: Transactions of the Royal Dublin Society. - - Trans. Seis. Soc. Japan: Transactions of the Seismological Society of - Japan, Tokyo. - - Trans. Wis. Acad. Sci.: Transactions of the Wisconsin Academy of - Sciences, Arts, and Letters, Madison. - - U. S. Geogr. and Geol. Surv. Rocky Mt. Region: United States - Geographical and Geological Survey of the Rocky Mountain Region - (Powell), Washington. - - Zeit. d. Gesell. f. Erdk. z. Berlin: Zeitschrift der Gesellschaft für - Erdkunde zu Berlin. - - Zeit. f. Gletscherk: Zeitschrift für Gletscherkunde, Berlin. - - - - -EARTH FEATURES AND THEIR MEANING - - - - -CHAPTER I - -THE COMPILATION OF EARTH HISTORY - - -=The sources of the history.=—The science which deals with the -chapters of earth history that antedate the earliest human writings -is geology. The pages of the record are the layers of rock which make -up the outer shell of our world. Here as in old manuscripts pages are -sometimes found to be missing, and on others the writing is largely -effaced so as to be indistinct or even illegible. An intelligent -interpretation of this record requires a knowledge of the materials -and the structure of the earth, as well as a proper conception of the -agencies which have caused change and so developed the history. These -agencies in operation are physical and chemical processes, and so the -sciences of physics and chemistry are fundamental in any extended study -of geology. Not only is geology, so to speak, founded upon chemistry -and physics, but its field overlaps that of many other important -sciences. The earliest earth history has to do with the form, size, and -physical condition of a minor planet in the solar system. The earliest -portion of the story belongs therefore to astronomy, and no sharp line -can be drawn to separate this chapter from those later ones which are -more clearly within the domain of geology. - - -=Subdivisions of geology.=—The terms “cosmic geology” and “astronomic -geology” have sometimes been used to cover the astronomy of the earth -planet. The later earth history develops, among other things, the -varied forms of animal and vegetable life which have had a definite -order of appearance. Their study is to a large extent zoölogy and -botany, though here considered from an essentially different viewpoint. -This subdivision of our science is called paleontological geology or -paleontology, which in common usage includes the plant as well as the -animal world, or what is sometimes called paleobotany. In order to fix -the order of events in geological history, these biological studies -are necessary, for the pages of the record have many of them been -misplaced as a result of the vicissitudes of earth history, and the -remains of life in the rock layers supply a pagination from which it is -possible to correctly rearrange the misplaced pages. As compiled into a -consecutive history of the earth since life appeared upon it, we have -the division of historical geology; though this differs but little from -stratigraphical geology, the emphasis in the case of the former being -placed on the history itself and in the latter upon the arrangement of -events—the pagination of the record. - -So far as they are known to us, the materials of which the earth is -composed are minerals grouped into various characteristic aggregates -known as rocks. Here the science is founded upon mineralogy as well as -chemistry, and a study of the rock materials of the earth is designated -petrographical geology or petrography. The various rocks which enter -into the composition of the earth’s outer shell—the only portion known -to us from direct observation—are built into it in an architecture -which, when carefully studied, discloses important events in the -earth’s history. The division of the science which is concerned with -earth architecture is geotectonic or structural geology. - - -=The study of earth features and their significance.=—The features -upon the surface of the earth have all their deep significance, and -if properly understood, a flood of light is thrown, not only upon -present conditions, but upon many chapters of the earth’s earlier -history. Here the relation of our study to topography and geography is -very close, so that the lines of separation are but ill defined. The -terms “physiographical geology”, “physiography”, and “geomorphology” -are concerned with the configuration of the earth’s surface—its -physiognomy—and with the genesis of its individual surface features. -It is this genetical side of physiography which separates it from -topography and lends it an absorbing interest, though it causes it -to largely overlap the division of dynamical geology or the study of -geological processes. In fact, the difference between dynamical geology -and physiography is largely one of emphasis, the stress being laid -upon the processes in the former and upon the resultant features in the -latter. - -Under dynamical geology are included important subdivisions, such -as seismic geology, or the study of earthquakes, and vulcanology, -or the study of volcanoes. Another large subject, glacial geology, -belongs within the broad frontier common to both dynamical geology -and physiography. A relatively new subdivision of geological science -is orientational geology, which is concerned with the trend of earth -features, and is closely related both to physiography and to dynamical -and structural geology. - - -=Tabular recapitulation.=—In a slightly different arrangement from the -above order of mention, the subdivisions of geology are as follows:— - -_Subdivisions of Geology_ - - _Petrographical Geology._ Materials of the earth. - _Geotectonic Geology._ Architecture of the earth’s - outer shell. - _Dynamical Geology._ Earth processes. - Seismic Geology—earthquakes. - Vulcanology—volcanoes. 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. - _Statigraphic Geology._ The pagination of earth records. - _Historical Geology._ The compiled record and its - interpretation. - _Paleontological Geology._ The evolution of life upon the earth. - -In every attempt at systematic arrangement difficulties are -encountered, usually because no one consideration can be used -throughout as the basis of classification. Such terms as “economic -geology” and “mining geology” have either a pedagogical or a commercial -significance, and so would hardly fit into the system which we have -outlined. - - -=Geological processes not universal.=—It is inevitable that the -geology of regions which are easily accessible for study should -have absorbed the larger measure of attention; but it should not be -forgotten that geology is concerned with the history of the entire -world, and that perspective will be lost and erroneous conclusions -drawn if local conditions are kept too often before the eyes. To -illustrate by a single instance, the best studied regions of the globe -are those in which fairly abundant precipitation in the form of rain -has fitted the land for easy conditions of life, and has thus permitted -the development of a high civilization. In degree, and to some extent -also in kind, geologic processes are markedly different within those -widely extended regions which, because either arid or cold, have been -but ill fitted for human habitation. Yet in the historical development -of the earth, those geologic processes which obtain in desert or -polar regions are none the less important because less often and less -carefully observed. - - -=Change, and not stability, the order of nature.=—Man is ever prone to -emphasize the importance of apparent facts to the disadvantage of those -less clearly revealed though equally potent. The ancient notion of the -_terra firma_, the safe and solid ground, arose because of its contrast -with the far more mobile bodies of water; but this illusion is quickly -dispelled with the sudden quaking of the ground. Experience has clearly -shown that, both upon and beneath the earth’s surface, chemical and -physical changes are going on, subject to but little interruption. “The -hills rock-ribbed and ancient as the sun” is a poetical metaphor; for -the Himalayas, the loftiest mountains upon the globe, were, to speak -in geological terms, raised from the sea but yesterday. Even to-day -they are pushing up their heads, only to be relentlessly planed down -through the action of the atmosphere, of ice, and of running water. -Even more than has generally been supposed, the earth suffers change. -Often within the space of a few seconds, to the accompaniment of a -heavy earthquake, many square miles of territory are bodily uplifted, -while neighboring areas may be relatively depressed. Thus change, and -not stability, is the order of nature. - - -=Observational geology _versus_ speculative philosophy.=—There appears -to be a more or less prevalent notion that the views which are held -by scientists in one generation are abandoned by those of the next; -and this is apt to lead to the belief that little is really known and -that much is largely guessed. Some ground there undoubtedly is for -such skepticism, though much of it may be accounted for by a general -failure among scientists, as well as others, to clearly differentiate -that which is essentially speculative from what is based broadly upon -observed facts. Even with extended observation, the possibility of -explaining the facts in more than one way is not excluded; but the -line is nevertheless a broad one which separates this entire field -of observation from what is essentially speculative philosophy. To -illustrate: the mechanics of the action which goes on within volcanic -craters is now fairly well understood as a result of many and extended -observations, and it is little likely that future generations of -geologists will discredit the main conclusions which have been reached. -The cause of the rise of the lava to the earth’s surface is, on the -other hand, much less clearly demonstrated, and the views which are -held express rather the differing opinions than any clear deductions -from observation. Again, and similarly, the physical history of the -great continental glaciers of the so-called “ice age” is far more -thoroughly known than that of any existing glacier of the same type; -but the cause of the climatic changes which brought on the glaciation -is still largely a matter for speculation. - -In the present work, the attempt will be, so far as possible, to give -an exposition of geologic processes and the earth features which result -from them, with hints only at those ultimate causes which lie hidden in -the background. - - -=The scientific attitude and temper.=—The student of science should -make it his aim, not only clearly to separate in his studies the -proximate from the ultimate causes of observed phenomena, but he should -keep his mind always open for reaching individual conclusions. No -doctrines should be accepted finally upon faith merely, but subject -rather to his own reasoning processes. This should not be interpreted -to mean that concerning matters of which he knows little or nothing -he should not pay respect to the recognized authorities; but his -acceptance of any theory should be subject to review so soon as his -own horizon has been sufficiently enlarged. False theories could hardly -have endured so long in the past, had not too great respect been given -to authorities, and individual reasoning processes been held too long -in subjection. - - -=The value of the hypothesis.=—Because all the facts necessary for a -full interpretation of observed phenomena are not at one’s hand, this -should not be made to stand in the way of provisional explanations. If -science is to advance, the use of hypothesis is absolutely essential; -but the particular hypothesis adopted should be regarded as temporary -and as indicating a line of observation or of experimentation which -is to be followed in testing it. Thus regarded with an open mind, -inadequate hypotheses are eventually found to be untenable, whereas -correct explanations of the facts by the same process are confirmed. -Most hypotheses of science are but partially correct, for we now “see -through a glass darkly”; but even so, if properly tested, the false -elements in the hypothesis are one after the other eliminated as the -embodied truth is confirmed and enlarged. Thus “working hypothesis” -passes into theory and becomes an integral part of science. - - - READING REFERENCES FOR CHAPTER I - - The most comprehensive of general geological texts written in English - is Chamberlin and Salisbury’s “Geology” in three volumes (Henry Holt, - 1904-1906), the first volume of which is devoted exclusively to - geological processes and their results. An abridged one-volume edition - of the work intended for use as a college text was issued in 1906 - (College Geology, Henry Holt). Other standard texts are:— - - SIR ARCHIBALD GEIKIE. Text-book of Geology, 4th ed. 2 vols. London, - 1902, pp. 1472. - - W. B. SCOTT. An Introduction to Geology. 2d ed. Macmillan, 1907, pp. - 816. - - J. D. DANA. Manual of Geology. New edition. American Book Company, - 1895, pp. 1087. - - JOSEPH LECONTE. Elements of Geology. (Revised by Fairchild.) Appleton, - 1905, pp. 667. - -A very valuable guide to the recent literature of dynamical and -structural geology is Branner’s “Syllabus of a Course of Lectures on -Elementary Geology” (Stanford University, 1908). - -On the relation of geology to landscape, a number of interesting books -have been written:— - - JAMES GEIKIE. Earth Sculpture or the Origin of Land-Forms. New York - and London, 1896, pp. 397. - - JOHN E. MARR. The Scientific Study of Scenery. Methuen, London, 1900, - pp. 368. - - SIR A. GEIKIE. The Scenery of Scotland. 3d ed. Macmillan, London, - 1901, pp. 540. - - SIR JOHN LUBBOCK. The Scenery of Switzerland and the Causes to which - it is Due. Macmillan, London, 1896, pp. 480. - - LORD AVEBURY. The Scenery of England. Macmillan, London, 1902, pp. 534. - - SIR A. GEIKIE. Landscape in History, and Other Essays. Macmillan, - London, 1905, pp. 352. - - N. S. SHALER. Aspects of the Earth. Scribners, New York, 1889, pp. 344. - - G. DE LA NOE ET EMM. DE MARGERIE. Les Formes du Terrain, Service - Géographique de l’Armée. Paris, 1888, pp. 205, pls. 48. - - W. M. DAVIS. Practical Exercises in Physical Geography, with - Accompanying Atlas. Ginn and Co., Boston, 1908, pp. 148, pls. 45. - - JOHN MUIR. The Mountains of California. Unwin, London, 1894, pp. 381. - -Upon the use and interpretation of topographic maps in illustration of -characteristic earth features, the following are recommended:— - - R. D. SALISBURY and W. W. ATWOOD. The Interpretation of Topographic - Maps, Prof. Pap., 60 U.S. Geol. Surv., pp. 84, pls. 170. - - D. W. JOHNSON and F. E. MATTHES. The Relation of Geology to - Topography, in Breed and Hosmer’s Principles and Practice of - Surveying, vol. 2. Wiley, New York, 1908. - - GÉNÉRAL BERTHAUT. Topologie, Étude du Terrain, Service Géographique de - l’Armée. Paris, 1909, 2 vols., pp. 330 and 674, pls. 265. - -The United States Geological Survey issues free of charge a list of -100 topographic atlas sheets which illustrate the more important -physiographic types. In his “Traité de Géographie Physique”, Professor -E. de Martonne has given at the end of each chapter the important -foreign maps which illustrate the physiographic types there described. - -“The Principles of Geology”, by Sir Charles Lyell, published first in -three volumes, appeared in the years 1830-1833, and may be said to -mark the beginning of modern geology. Later reduced to two volumes, an -eleventh edition of the work was issued in 1872 (Appleton) and may be -profitably read and studied to-day by all students of geology. Those -familiar with the German language will derive both pleasure and profit -from a perusal of Neumayr’s “Erdgeschichte” (2d ed. revised by Uhlig. -Leipzig and Vienna, 2 vols., 1895-1897), and especially the first -volume, “Allgemeine Geologie.” A recent French work to be recommended -is Haug’s “Traité de Géologie” (Paris, 1907). - -Some texts of physical geography may well be consulted, especially Emm. -de Martonne’s “Traité de Géographie Physique.” Colin, Paris, 1909, pp. -910, pls. 48, and figs. 396. - - * * * * * - -NOTE. An explanatory list of abbreviations used in the reading -references follows the List of Illustrations. - - - - -CHAPTER II - -THE FIGURE OF THE EARTH - - -=The lithosphere and its envelopes.=—The stony part of the earth is -known as the _lithosphere_, of which only a thin surface shell is known -to us from direct observation. The relatively unknown central portion, -or “core”, is sometimes referred to as the centrosphere. Inclosing the -lithosphere is a water envelope, the _hydrosphere_, which comprises -the oceans and inland bodies of water, and has a mass 1/4540 that of -the lithosphere. If uniformly distributed, the hydrosphere would cover -the lithosphere to the depth of about two miles, instead of being -collected in basins as it now is. Though apparently not continuous, if -we take into account the zone of underground water upon the continents, -the hydrosphere may properly be considered as a continuous film about -the lithosphere. It is a fact of much significance that all the ocean -basins are connected, so that the levels are adjusted to furnish a -common record of deposits over the entire surface that is sea-covered. - -Enveloping the hydrosphere is the gaseous envelope, the _atmosphere_, -with a mass 1/1200000 that of the lithosphere. The atmosphere is a -mixture of the gases oxygen and nitrogen in parts by volume of one of -the former to four of the latter, with a relatively small percentage -of carbon dioxide. Locally, and at special seasons, the atmosphere -may be charged with relatively large percentages of water vapor; and -we shall see that both the carbon dioxide and the vapor contents are -of the utmost importance in geological processes and in the influence -upon climate. Unlike the water which composes the hydrosphere, the -gases of the atmosphere are compressible. Forced down by the weight of -superincumbent gas, the layers of the atmosphere at the level of the -sea sustain a pressure of about fifteen pounds to the square inch; but -this pressure steadily decreases in ascending to higher levels. From -direct instrumental observation, the air has now been investigated to -a height of more than twelve miles from the earth’s surface. - - -=The evolution of ideas concerning the earth’s figure.=—The ideas -which in all ages have been promulgated concerning the figure of the -earth have been many and varied. Though among them are not wanting the -purely speculative and fantastic, it will be interesting to pass in -review such theories as have grown directly out of observation. - -The ancient Hebrews and the Babylonians were dwellers of the desert, -and in the mountains which bounded their horizon they saw the confines -of the earth. Pushing at last westward beyond the mountains, they -found the Mediterranean, and thus arrived at the view that the earth -was a disk with a rim of mountains which was floated upon water. The -rare but violent rainfalls to which they were accustomed—the desert -cloudburst—further led them to the belief that the mountain rim was -continued upward in a dome or firmament of transparent crystal upon -which the heavenly bodies were hung and from which out of “windows -of heaven” the water “which is above the earth” was poured out upon -the earth’s surface. Fantastic as this theory may seem to-day, it -was founded upon observation, and it well illustrates the dangers of -reasoning from observation within too limited a field. - -As soon as men began to sail the sea, it was noticed that the water -surface is convex, for the masts of ships were found to remain -visible long after their hulls had disappeared below the horizon. It -is difficult to say how soon the idea of the earth’s rotundity was -acquired, but it is certainly of great antiquity. The Dominican monk -Vincentius of Beauvais, in a work completed in 1244, declared that the -surfaces of the earth and the sea were both spherical. The poet Dante -made it clear that these surfaces were one, and in his famous address -upon “The Water and the Land”, which was delivered in Verona on the -20th of January, 1320, he added a statement that the continents rise -higher than the ocean. His explanation of this was that the continents -are pulled up by the attraction of the fixed stars after the manner -of attraction of magnets, thus giving an early hint of the force of -gravitation. - -The earth’s rotundity may be said to have been first proven when -Magellan’s ships in 1521 had accomplished the circumnavigation of -the globe. Circumnavigation, soon after again carried out by Sir -Francis Drake, proved that the earth is a closed body bounded by -curving surfaces in part enveloped by the oceans and everywhere by the -atmosphere. The great discovery of Copernicus in 1530 that the earth, -like Venus, Mars, and the other planets, revolves about the sun as a -part of a system, left little room for doubt that the figure of the -earth was essentially that of a sphere. - - -=The oblateness of the earth.=—Every schoolboy is to-day familiar with -the fact that the earth departs from a perfect spherical figure by -being flattened at the ends of its axis of rotation. The polar diameter -is usually given as 1/299 shorter than the equatorial one. This -oblateness of the spheroid was proven by geodesists when they came to -compare the lengths of measured degrees of arc upon meridians in high -and in low latitudes. - -[Illustration: - -FIG. 1.—Diagrams to afford a correct impression of the measure of the -inequalities upon the earth’s surface compared to the earth’s radius. -The shell represented in _b_ is 1/100 of the earth’s radius, and in _a_ -this zone is magnified for comparison with surface inequalities.] - -The oblateness of the geoid is well understood from accepted hypotheses -to be the result of the once more rapid rotation of the planet when its -materials were more plastic, and hence more responsive to deformation. -An elastic hoop rotating rapidly about an axis in its plane appears to -the eye as a solid, and becomes flattened at the ends of its axis in -proportion as the velocity of rotation is increased. Like the earth, -the other planets in the solar system are similarly oblate and by -amounts dependent on the relative velocities of rotation. - -The departure of the geoid from the spherical surface, owing to its -oblateness, is so small that in the figures which we shall use for -illustration it would be less than the thickness of a line. Since it -is well recognized and not important in our present consideration, we -shall for the time being speak of the figure of the earth in terms of -departures from a standard spherical surface. - - -=The arrangement of oceans and continents.=—There are other departures -from a spherical surface than the oblateness just referred to, -and these departures, while not large, are believed to be full of -significance. Lest the reader should gain a wrong impression of their -magnitude, it may be well to introduce a diagram drawn to scale and -representing prominent elevations and depressions of the earth (Fig. 1). - -Wrong impressions concerning the figure of the lithosphere are -sometimes gained because its depressions are obliterated by the oceans. -The oceans are, indeed, useful to us in showing where the depressions -are located, but the figure of the earth which we are considering -is the naked surface of the rock. In a broad way, the earth’s shape -will be given by the arrangement of the oceans and the continents. As -soon as we take up the study of this arrangement, we find that quite -significant facts of distribution are disclosed. - -[Illustration: - -FIG. 2.—Map on Mercator’s projection to show the reciprocal relation -of the land and sea areas (after Gregory and Arldt).] - -One of the most significant facts involved in the distribution -of land and sea, is a concentration of the land areas within the -northern and the seas within the southern hemisphere. The noteworthy -exception is the occurrence of the great and high Antarctic continent -centered near the earth’s south pole; and there are extensions of the -northern continent as narrowing land masses to the southward of the -equator. Hardly less significant than the existence of land and water -hemispheres is the reciprocal or antipodal distribution of land and sea -(Fig. 2). A third fact of significance is a dovetailing together of sea -and land along an east-and-west direction. While the seas are generally -A-shaped and narrow northward, the land masses are V-shaped and narrow -southward, _but this occurs mainly in the southern hemisphere_. Lastly, -there is some indication of a belt of sea dividing the land masses -into northern and southern portions along the course of a great circle -which makes a small angle with the earth’s equator. Thus the western -continent is nearly divided by a mediterranean sea,—the Caribbean,—and -the eastern is in part so divided by the separation of Europe from -Africa. - -[Illustration: - -FIG. 3.—The form toward which the figure of the earth is tending, a -tetrahedron with symmetrically truncated angles.] - - -=The figure toward which the earth is tending.=—Thus far in our -discussion of the earth’s figure we have been guided entirely by the -present distribution of land and water. There are, however, depressions -upon the surface of the land, in some cases extending below the level -of the sea, which are not to-day occupied by water. By far the most -notable of these is the great Caspian Depression, which with its -extension divides central and eastern Asia upon the east from Africa -and Europe upon the west. This depression was quite recently occupied -by the sea, and when added to the present ocean basins to indicate -depressions of the lithosphere, it shows that the earth’s figure -departs from the standard spheroid _in the direction_ of the form -represented in Fig. 3. This form approximates to a tetrahedron, a -figure bounded by four equal triangular faces, here with symmetrically -truncated angles. Of all regular figures with plane surfaces the -tetrahedron has the smallest volume for a given surface, and it -presents moreover a reciprocal relation of projection to depression. -Every line passing through its center thus finds the surface nearer -than the average distance upon one side and correspondingly farther -upon the other (Fig. 4). - - -=Astronomical _versus_ geodetic observations.=—Confirmation of the -conclusions arrived at from the arrangement of oceans and continents -has been secured in other fields. It was pointed out that the earth’s -oblateness was proven by comparison of the measured degrees of latitude -upon the earth’s surface in lower and higher latitudes, the degree -being found longer as the pole is approached. Any variation from the -spherical surface must obviously increase the size of the measured -degree of latitude in proportion to the departure from the standard -form, and so the tetrahedral figure with one of its angles at the -south pole will require that the degrees of latitude be longer in the -southern than they are in the northern hemisphere. This has been found -by measurement to be the case, and the result is further confirmed -by pendulum studies upon the distribution of the earth’s attraction -or gravity. If less of the mass of the earth is concentrated in the -southern hemisphere, its attraction as measured in vibrations of the -pendulum should be correspondingly smaller. - -[Illustration: - -FIG. 4.—A truncated tetrahedron, showing how the depression upon one -side of the center is balanced by the opposite projection.] - -Other confirmations of the tetrahedral figure of the earth have been -derived from a comparison of astronomical data, which assume the earth -to be a perfect spheroid, with geodetic measurements, which are based -upon direct measurements. Thus the arc measured in an east-and-west -direction across Europe revealed a different curvature near the angle -of the tetrahedral figure from what was found farther to the eastward. - - -=Changes of figure during contraction of a spherical body.=—If -we inquire why the earth in cooling should tend to approach the -tetrahedral figure, an answer is easily found. When formed, the earth -appears to have been a but slightly oblate spheroid, or practically -a sphere—the shape which of all incloses the most space for a given -surface. Cooled and solidified at the surface to the temperature of -the surrounding air, and the core still hot and continuing to lose -heat, the core must continue to contract though the outer shell is -no longer able to do so. The superficial area being thus maintained -constant while the volume continues to diminish, the figure must change -from the initial one of greatest bulk to others of smaller volume, -and ultimately, if the process should continue indefinitely, to the -tetrahedron, which of all regular figures has the minimum volume for a -given surface. - -That a contracting sphere does indeed pass through such a series of -changes has been shown by the behavior of contracting soap bubbles and -of rubber balloons, as well as by experiments upon the exhaustion of -air contained in hollow metal spheres of only moderate strength. In -all these instances, the ultimate form produced indicates an indenting -of four sides of the sphere which have the positions of the faces -of a tetrahedron. The late Professor Prinz of Brussels secured some -extremely interesting results in which he obtained intermediate forms -with six angles, but unfortunately these studies were not prepared for -publication at the time of his death. - -The earth’s departure from the spheroid in the direction of the -modified tetrahedron is, as we have seen, no hypothesis, but observed -fact revealed in (1) the concentration of the land about a central -ocean in the northern hemisphere; in (2) the antipodal relation of the -land to the water areas, and in (3) the threefold subdivision of the -surface into north and south belts by the two greater oceans and the -Caspian Depression. - - -=The earlier figures of the earth.=—The manner in which continent and -ocean are dovetailed into each other in an east-and-west direction -has been generally adduced as additional evidence for the tetrahedral -figure as above described. Closer examination shows that instead of -being in harmony with this figure, it indicates a departure from it, -and, as we shall see, a significant departure which undoubtedly has its -origin in the earlier history of the planet. The mediterranean seas of -both the eastern and the western hemispheres likewise interfere with -the perfection of the tetrahedral figure and require an explanation. - -Let us then examine in outline the past history of the world with -reference especially to the evolution of the continents and to the -times and the manners of surface change. It is now well known that -there have been three major periods of great deformation of the earth’s -shell. The first of these of which we have record came at the end of -the first great era of geologic history, the so-called Eozoic era; -a second great transformation came at the close of the second or -Paleozoic era; and a third began at the end of the next or Mesozoic -era, an adjustment which is apparently continuing to-day. Each of -these great surface deformations was accompanied by great volcanic -eruptions of which we have the evidence in the lavas remaining for our -inspection, and each was followed by the formation of great glaciers -which spread over large areas of the existing continents. - -Before the earliest of these great changes, the earth appears to have -approximated in its figure somewhat closely to the ideal spheroid, for -it was everywhere enveloped in the hydrosphere as a universal ocean. -Toward the close of this period came the adjustments which brought -the lithosphere to protrude through the hydrosphere in shield-like -continents whose distribution, as shown by the rocks of this period, is -of great significance. Within the northern hemisphere rose three land -shields spaced at nearly equal intervals and at nearly equal distances -from the northern pole. One of these was centered where now is Hudson -Bay, another about the present Baltic Sea, and the relics of the third -are found in northeastern Siberia. These earliest continents have been -referred to as the Laurentian, Baltic, and Angara shields. Within -the southern hemisphere shields appear to have developed in somewhat -similar grouping, namely, in South America, in South Africa, and in -Australia (Figs. 3 and 5). - -[Illustration: - -FIG. 5.—Approximations to earlier and present figures of the earth.] - -These coigns or angles of a form into which the earlier spheroid of the -earth was being transformed have persisted through the greater part -of subsequent geologic time, and have been enlarged by the growth of -sediments about them as well as by the later elevation and wrinkling -of these deposits into marginal mountain ranges. - - -=The continents and oceans which arose at the close of the Paleozoic -era.=—At the close of the second great era in the recorded history -of the earth, the now somewhat enlarged continents were profoundly -altered during a series of convulsive movements within the surface -shell of the lithosphere. When these convulsions were over, there was -a new disposition of land and sea, but one quite different from the -present arrangement. Instead of being extended in north-south belts, as -they are at present, the continents stretched out in broad east-west -zones, one in the northern and the other in the southern hemisphere. -To the broad southern continent of which so little now remains, the -name “Gondwana Land” has been given, and to the sea which divided the -northern from the southern continent the name “Ocean of Tethys.” The -northern continent stretched across the site of the present Atlantic -Ocean as the “North Atlantis”, its northern shore to the westward -being somewhat farther south than the present northern coast of North -America, since life forms migrated in the northern ocean from the site -of Behring Sea to that of the present North Atlantic. - -This arrangement of land and water during the middle period of the -earth’s recorded history, when considered with reference both to its -earlier and to its later evolution, may perhaps be best accounted -for by the assumption that the lithosphere was then shaped like Fig. -5 (middle). In this figure two truncated tetrahedrons are joined in -a common plane of contact which may be described as the twin plane. -This medial depression upon the lithosphere was occupied by the -intercontinental sea, the Ocean of Tethys. - -Near the close of this second great era of the earth’s continental -history, crustal convulsions, which were perhaps the most remarkable in -the entire record, resulted in the almost complete disappearance of the -southern continent and a concentration of the land within the northern -hemisphere as a somewhat interrupted belt surrounding a central polar -ocean (Figs. 3 and 5). - -Upon the assumption of twin tetrahedrons in the intermediate era of -continental evolution, both the Ocean of Tethys of that time and its -present remnants, the Caribbean and Mediterranean seas, are accounted -for. The V-shaped continent extensions and the A-shaped oceans of the -southern hemisphere (Fig. 2) may likewise be considered as relics of -the now largely submerged tetrahedron of the southern hemisphere, since -this had its apex to the northward (Fig. 6). - -[Illustration: - -FIG. 6.—Diagrams for comparison of shore lines upon tetrahedrons which -have an angle, the first at the south and the second at the north.] - -Thus we see that the lithosphere can scarcely be regarded as a perfect -spheroid, since in the course of geologic ages it has undergone -successive departures from this original form. In its present state it -has been described as tetrahedral, though we must keep in mind that -the sharp angles of that figure are deeply truncated. The soundings -first by Nansen and more recently by Peary in the Arctic basin, far to -the north of the continental border, showed that this depression is -characterized by profound depths, and so have afforded confirmation -of the tetrahedral figure. To match this depression at the northern -extremity of the earth’s axis, a high continent reaching to elevations -in excess of 10,000 feet has been penetrated by Sir Ernest Shackleton -at the opposite extremity of this polar diameter. Considering its size -and its elevation, the Antarctic continent with its glacier mantle is -the largest protuberance upon the surface of the lithosphere. - -In our study of the departures of the earth from the standard -spheroidal surface, we might even go a step farther and show how -the tetrahedron, which best represents the symmetry of the present -figure, is somewhat deformed by a flattening perpendicular to the -Pacific Ocean. To draw attention to this flattening of the earth, it -has sometimes been described as “potato-shaped”, since the outline -perpendicular to this face is imperfectly heart-shaped or like a -flattened “peg top.” - -[Illustration: - -FIG. 7.—The continents with submerged portions added (after Gilbert).] - - -=The flooded portions of the present continents.=—We are accustomed -to think of the continents as ending at the shores of the oceans. -If, however, we are to regard them as platforms which rise from the -ocean depressions, their margins should be considerably extended, for -a submerged shelf now practically surrounds all the continents to a -nearly uniform depth of 100 fathoms or 600 feet. The oceans thus more -than fill their basins and may be thought of as spilling over upon the -continents. In Fig. 7, the submerged portions of the continents have -been joined to those usually represented, thus adding about 10,000,000 -square miles to their area, and giving them one third, instead of one -fourth, of the lithosphere surface. - -[Illustration: - -FIG. 8.—Diagram to indicate the altitude of different parts of the -lithosphere surface.] - - -=The floors of the hydrosphere and atmosphere.=—The highest altitudes -upon the continents and the profoundest deeps of the ocean are each -removed about 30,000 feet, or nearly 6 miles, from the level of the -sea. In comparison with the entire surface of the lithosphere, these -extremes of elevation represent such small areas as to be almost -inappreciable. Only about 1/80 of the lithosphere surface rises more -than 6000 feet above sea level, and about the same proportion lies -deeper than 18,000 feet below the same datum plane (Fig. 8). Almost -the entire area of the lithosphere is included either in the so-called -continental plateau or platform, in the oceanic platform, or in the -slope which separates the two. The continental platform includes the -continental shelf above referred to, and represents about one third of -the entire area of the planet. This platform has a range of elevation -from 6000 feet above to 600 feet below sea level and has an average -altitude of about 2300 feet. The oceanic platform slopes more steeply, -ranges in depth from 12,000 to 18,000 feet below sea level, and -comprises about one half the lithosphere surface. The remaining portion -of the surface, something less than one eighth of all, is included in -the steep slopes between the two platforms, between 600 and 12,000 -feet below sea. The two platforms and the slope between them must -not, however, be thought of as continuous features upon the surface, -but merely as representing the average elevations of portions of the -lithosphere. - - - READING REFERENCES FOR CHAPTER II - - On the evolution of ideas concerning the earth’s figure:— - - SUESS. The Face of the Earth (Clarendon Press, 1906), vol. 2, Chapter - 1. - - V. ZITTEL. History of Geology and Paleontology (Walter Scott, London, - 1901), Chapters 1-2. - -The departure of the spheroid toward the tetrahedron:— - - W. LOWTHIAN GREEN. Vestiges of the Molten Globe, Part 1. London, 1875. - (Now a rare work, but it contains the original statement of the idea.) - - J. W. GREGORY. The Plan of the Earth and Its Causes, Geogr. Jour., - vol. 13, 1899, pp. 225-251 (the best general statement of the - arguments for a tetrahedral form). - - W. PRINZ. L’échelle reduite des expériences géologiques, Bull. Soc. - Belge d’Astronomie, 1899. - - B. K. EMERSON. The Tetrahedral Earth and Zone of the Intercontinental - Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pls. 9-14. - - M. P. RUDSKI. Physik der Erde (Tauchnitz, Leipzig, 1911), Chapters - 1-3 (the best discussion of the geoid from the purely mathematical - standpoint, so far as the spheroid is concerned). - -The earlier figures of the earth:— - - TH. ARLDT. Die Entwicklung der Kontinente und ihrer Lebewelt. - Engelmann, Leipzig, 1907. (Contains a valuable series of map plates, - showing the probable boundaries of the continents in the different - geological periods). - - - - -CHAPTER III - -THE NATURE OF THE MATERIALS IN THE LITHOSPHERE - - -=The rigid quality of our planet.=—For a long time it was supposed -that the solid earth constituted a crust only which was floated -upon a liquid interior. This notion was clearly an outgrowth of the -then generally accepted Laplacian hypothesis of the origin of the -universe, which assumed fluid interiors for the planets, the crust -being suggested by the winter crust of frozen water upon the surface of -our inland lakes. To-day the nebular hypothesis in the Laplacian form -is fast giving place to quite different conceptions, in which solid -particles, and not gaseous ones, are conceived to have built up the -lithosphere. The analogy with frozen water has likewise been abandoned -with the discovery that frozen rock, instead of floating, sinks in its -molten equivalent. - -Yet even more cogent arguments have been brought forward to show that -whatever may be the state of aggregation within the earth’s core—and -it may be different from any now known to us—it nevertheless has many -of the properties recognized as belonging to solid and rigid bodies. -Provisionally, therefore, we may regard the earth’s core as rigid and -essentially solid. It was long ago pointed out by the late Lord Kelvin -that if our lithosphere were not more rigid than a ball of glass of the -same size, it would be constantly passing through periodic six-hourly -distortions of great amplitude in response to the varying attractions -of the moon. An equally striking argument emanating from the same high -authority is furnished by the well-known egg-spinning demonstration. -For illustration, Kelvin was accustomed to take two eggs, one boiled -and the other raw, and attempt to spin them upon their ends. For the -boiled, and essentially solid, egg this is easily accomplished, but -internal friction of the liquid contents of the raw egg quickly stops -any rotary motion which is imparted to it. Upon the same grounds it -is argued that had the earth’s interior possessed the properties of a -liquid, rotation must long since have ceased. - -A stronger proof of earth rigidity than either of these has been lately -furnished by the instrumental study of earthquakes. With the delicate -apparatus which is now installed for the purpose, heavy earthquakes may -be sensed which have occurred anywhere upon the earth’s surface, the -earth movement sending its own message by the shortest route through -the core of the earth to the observing station. A heavy shock which -occurs in New Zealand is recorded in England, almost diametrically -opposite, in about twenty-one minutes after its occurrence. The laws -of wave propagation and their relation to the properties of the -transmitting medium are well known, and in order to explain such -extraordinary velocity it is necessary to assume that for such impulses -the earth’s interior is much more rigid than the finest tool steel. - - -=Probable composition of the earth’s core.=—In deriving views -concerning the nature of the earth’s interior we are greatly aided -by astronomical studies. The common origin long ago indicated for -the planets of the solar system and the sun has been confirmed by -the analysis of light with the aid of the spectroscope. It has thus -been found that the same chemical elements which we find in the earth -are present also in the sun and in the other stellar bodies. Again, -the group of planets of the solar system which are nearest to the -sun—Mercury, Venus, the Earth, and Mars—have each a high density, all -except Mars, the most distant, having specific gravities very closely -5½, that of Mars being about 4. This average specific gravity is also -that of the solid bodies, the so-called meteorites, which reach the -surface of our planet from the surrounding space. Yet though the earth -as a whole is thus found to have a specific gravity five and a half -times that of water, its surface shell has an average density of less -than half this value, or 2.7. - -The study of meteorites has given us a possible clew to the nature of -the earth’s interior; for when both terrestrial and celestial rock -types are classified and placed in orderly arrangement, it is found -that the chemical elements which compose the two groups are identical, -and that these are united according to the same physical and chemical -laws. No new element has been discovered in the one group that has not -been found in the other, and though some compounds of these elements, -the minerals, occur in the earth’s crust that have not been found in -meteorites, and though some occur in meteorites which are not known -from the earth, yet of those which are common to both bodies there is -agreement, even to the minor details (Fig. 9). It is found, however, -that the commonest of the minerals in the earth’s shell are absent from -meteorites, as the commoner constituents of meteorites are wanting in -the earth’s crust. This observation would go far to show that we may in -the two cases be examining different portions of quite similar bodies; -and this view is strikingly confirmed when the rocks of the two groups -are arranged in the order of their densities (Fig. 9). - -[Illustration: - -FIG. 9.—Diagram to show how terrestrial rocks grade into those of the -meteorites. 1, oxygen; 2, silicon; 3, aluminium; 4, alkali metals; 5, -alkaline earth metals; 6, iron, nickel, cobalt, etc.; _a_, granites and -rhyolites; _b_, syenites and trachytes; _c_, diorites and andesites; -_d_, gabbros and basalts; _e_, ultra-basic rocks; _f_, basic inclosures -in basalt, etc.; _g_, iron basalts of west Greenland; _h_, iron masses -of Ovifak, west Greenland; _a’-d’_, meteorites in order of density -(after Judd).] - -In a broad way, density, structure, and chemical composition are all -similarly involved in the gradations illustrated by the diagram; and it -is significant that while there are terrestrial rocks not represented -by meteorites, the densest and the most unusual of the terrestrial -rocks are chemically almost identical with the less dense of the -celestial bodies. - - -=The earth a magnet.=—The denser, and likewise the more common, of -the meteorite rocks—the so-called meteoric irons—are composed almost -entirely of the elements iron, nickel, and cobalt. Such aggregates -are not known as yet from terrestrial sources, although transitional -types appear to exist upon the island of Disco off the west coast of -Greenland. If it were possible to explore the earth’s interior, would -such combinations of the iron minerals be encountered? Apart from the -surprising velocity of transmission of earthquake waves, the strongest -argument for an iron core to the lithosphere is found in the magnetic -property of the earth as a whole. The only magnetic elements known to -us are those of the heavy meteorites—iron, nickel, and cobalt,—and -the earth is, as we know, a great magnet whose northern pole in British -America and whose southern pole in Antarctica have at last been visited -by Amundsen and David, respectively. The specific gravity of iron is -7.15, and those of nickel and cobalt, which in the meteorites are -present in relatively small amounts, are 7.8 and 7.5, respectively. -Considering that the surface shell of the earth has a specific gravity -of 2.7, these values must be regarded as agreeing well with the -determined density of the earth (5.6) and the other planets of its -group (Mercury 5.7, Venus 5.4, Mars 4). - - -=The chemical constitution of the earth’s surface shell.=—The number -of the so-called chemical elements which enter into the earth’s -composition is more than eighty, but few of these figure as important -constituents of the portion known to us. Nearly one half of the mass -of this shell is oxygen, and more than a quarter is silicon. The -remaining quarter is largely made up of aluminium, iron, calcium, -magnesium, and the alkalies sodium and potassium, in the order named. -These eight constituent elements are thus the only ones which play any -important rôle in the composition of the earth’s surface shell. They -are not found there in the free condition, but combined in the definite -proportions characteristic of chemical compounds, and as such they are -known as _minerals_. - - -=The essential nature of crystals.=—A crystal we are accustomed to -think of as something transparent bounded by sharp edges and angles, -our ideas having been obtained largely from the gem minerals. This -outward symmetry of form is, however, but an expression of a power -which resides, so to speak, in the heart or soul of the crystal -individual—it has its own structural make-up, its individuality. No -more correct estimates of the comparison of crystal individualities -would be obtained by the study of outward forms alone of two minerals -than would be gained by a judgment of persons from the cut of their -clothing. Too often this outward dress tells only of the conditions by -which both men and crystals have been surrounded, and but little of the -power inherent in the individual. Many a battered mineral fragment with -little beauty to recommend it, when placed under suitable conditions -for its development, has grown into a marvel of beauty. Few minerals -are so mean that they have not within them this inherent power of -individuality which lifts them above the world of the amorphous and -shapeless. - -[Illustration: - -FIG. 10.—Comparison of a crystalline with an amorphous substance when -expanded by heat and when attacked by acid.] - -Just as the real nature of a person is first disclosed by his behavior -under trying circumstances, so of a crystal it is its conduct under -stress of one sort or another which brings out its real character. By -way of illustration let us prepare a sphere from the mineral quartz—it -matters not whether we destroy the beautiful outlines of the crystal or -employ a battered fragment—and then prepare a sphere of similar size -and shape from a noncrystalline or amorphous substance like glass. If -now these two spheres be introduced into a bath of oil and raised to a -higher temperature, the glass globe undergoes an enlargement without -change of its form; but the crystal ball reveals its individuality -by expanding into a spheroid in which each new dimension is nicely -adjusted to this more complex figure (Fig. 10). - -We may, instead of submitting the two balls to the “trial by fire”, -allow each to be attacked by the powerful reagent, hydrofluoric acid. -The common glass under the attack of the acid remains as it was before, -a sphere, but with shrunken dimensions. The crystal, on the other hand, -is able to control the action of the solvent, and in so doing its -individuality is again revealed in a beautifully etched figure having -many curving outlines—it is as though the crystal had possessed a soul -which under this trial has been revealed. This glimpse into the nature -of the crystal, so as to reveal its structural beauty, is still more -surprising when the crystal is taken from the acid in the early stages -of the action and held close beneath the eye. Now the little etchings -upon the surface display each the individuality of the substance, and -joining with their neighbors they send out a beautifully symmetrical -and entirely characteristic picture (Fig. 11). - -[Illustration: - -FIG. 11.—“Light figure” seen upon an etched surface of a crystal of -calcite (after Goldschmidt and Wright).] - - -=The lithosphere a complex of interlocking crystals.=—To the layman -the crystal is something rare and expensive, to be obtained from -a jeweler or to be seen displayed in the show cases of the great -museums. Yet the one most striking quality of the lithosphere which -separates it from the hydrosphere and the atmosphere is its crystalline -structure,—a structure belonging also to the meteorite, and with -little doubt to all the planets of the earth group. A snowflake caught -during its fall from the sky reveals all the delicate tracery of -crystal boundary; collected from a thick layer lying upon the ground, -it appears as an intricate aggregate of broken fragments more or less -firmly cemented together. And so it is of the lithosphere, for the -myriads of individuals are either the ruins of former crystals, or -they are grown together in such a manner that crystal facets had no -opportunity to develop. - -Such mineral individuals as once possessed the crystal form may have -been broken and their surfaces ground away by mutual attrition under -the rhythmic beating of the waves upon a shore or in the continuous -rolling of pebbles on a stream bed, until as battered relics they -are piled away together in a bed of sand. Yet no amount of such rough -handling is sufficient to destroy the crystal individuality, and if -they are now surrounded with conditions which are suitable for their -growth, their individual nature again becomes revealed in new crystal -outlines. Many of our sandstones when turned in the bright sunlight -send out flashes of light to rival a bank of snow in early spring. -These bright flashes proceed from the facets of minute crystals formed -about each rounded grain of the sand, and if we examine them under a -lens, we may note the beauty of line formed with such exactness that -the most delicate instruments can detect no difference between the -similar angles of neighboring crystals (Fig. 12). - -[Illustration: - -FIG. 12.—Battered sand grains which have taken on a new lease of life -and have developed a crystal form. _a_, a single grain grown into an -individual crystal; _b_, a parallel growth about a single grain; _c_, -growth of neighboring grains until they have mutually interfered and so -destroyed the crystal facets—the common condition within the mass of a -rock (after Irving and Van Hise).] - -This individual nature of the crystal is believed to reside in a -symmetrical grouping of the chemical molecules of the substance into -larger and so-called “crystal molecules.” The crystal quality belongs -to the chemical elements and to their compounds in the solid condition, -but not to ordinary mixtures of them. - - -=Some properties of natural crystals, minerals.=—No two mineral -species appear in crystals of the same appearance, any more than two -animal species have been given the same form; and so minerals may be -recognized by the individual peculiarities of their crystals. Yet -for the reason that crystals have so generally been prevented from -developing or retaining their characteristic faces, in the vast number -of instances it is the behavior, and not the appearance, of the mineral -substance which is made use of for identification. - -When a mineral is broken under the blow of a hammer, instead of -yielding an irregular fracture, like that of glass, it generally -tends to part along one or more directions so as to leave plane -surfaces. This property of _cleavage_ is strikingly illustrated -for a single direction in the mineral mica, for two directions in -feldspar, and for three directions in calcite or Iceland spar. Other -properties of minerals, such as hardness, specific gravity, luster, -color, fusibility, etc., are all made use of in rough determinations -of the minerals. Far more delicate methods depend upon the behavior -of minerals when observed in polarized light, and such behavior is -the basis of those branches of geological science known as optical -mineralogy and as microscopical petrography. An outline description of -some of the common minerals and the means for identifying them will be -found in appendix A. - - -=The alterations of minerals.=—By far the larger number of minerals -have been formed in the cooling and consequent consolidation of molten -rock material such as during a volcanic eruption reaches the earth’s -surface as lava. Beginning their growth at many points within the -viscous mass, the individual crystals eventually may grow together and -so prevent a development of their crystal faces. - -Another class of minerals are deposited from solution in water within -the cavities and fissures of the rocks; and if this process ceases -before the cavities have been completely closed, the minerals are -found projecting from the walls in a beautiful lining of crystal—the -_Krystallkeller_ or “crystal cellar.” It is from such pockets or veins -within the rocks that the valuable ores are obtained, as are the -crystals which are displayed in our mineral cabinets. - -[Illustration: - -FIG. 13.—Crystal of garnet developed in a schist with grains of quartz -included because not assimilated.] - -There is, however, a third process by which minerals are formed, -and minerals of this class are produced within the solid rock as a -product of the alteration of preëxisting minerals. Under the enormous -pressures of the rocks deep below the earth’s surface, they are as -permeable to the percolating waters as is a sponge at the surface. -Under these conditions certain minerals are dissolved and their -material redeposited after traveling in the solution, or solution -and redeposition of mineral matter may go on together within the mass -of the same rock. One new mineral may have been produced from the -dissolved materials of a number of earlier species, or several new -minerals may be the result of the alteration of a preëxisting mineral -with a more complex chemical structure. Where the new mineral has been -formed “in place”, it has sometimes been able to utilize the materials -of all the minerals which before existed there, or it may have been -obliged to inclose within itself those earlier constituents which it -could not assimilate in its own structure (Fig. 13). - -[Illustration: - -FIG. 14.—A crystal of augite within the mass of a rock altered in -part to form a rim of the minerals hornblende and magnetite. Note the -original outline of the augite crystal.] - -At other times a crystal which is imbedded in rock has been attacked -upon its surface by the percolating solutions, and the dissolved -materials have been deposited in place as a crown of new minerals -which steadily widens its zone until the center is reached and the -original crystal has been entirely transformed (Fig. 14). It is -sometimes possible to say that the action by which these changes have -been brought about has involved a nice adjustment of supply of the -chemical constituents necessary to the formation of the new mineral or -minerals. In rocks which are aggregates of several mineral species, a -newly formed mineral may appear only at the common margin of certain of -these species, thus showing that they supply those chemical elements -which were necessary to the formation of the new substance (Fig. 15). -Thus it is seen that below the earth’s surface chemical reactions are -constantly going on, and the earlier rocks are thus locally being -transformed into others of a different mineral constitution. - -[Illustration: - -FIG. 15.—A new mineral (hornblende) forming as an intermediate -“reaction rim” between the mineral having irregular fractures (olivine) -and the dusty white mineral (lime-soda feldspar).] - -Near the earth’s surface the carbon dioxide and the moisture which are -present in the atmosphere are constantly changing the exposed portions -of the lithosphere into carbonates, hydrates, and oxides. These -compounds are more soluble than are the minerals out of which they were -formed, and they are also more bulky and so tend to crack off from -the parent mass on which they were formed. As we are to see, for both -of these reasons the surface rocks of the lithosphere succumb to this -attack from the atmosphere. - -In connection with those wrinklings of the surface shell of the -lithosphere from which mountains result, the underlying rocks are -subjected to great strains, and even where no visible partings are -produced, the rocks are deformed so that individual minerals may be -bent into crescent-shaped or S-shaped forms, or they are parted into -one or more fragments which remain imbedded within the rock. - - -READING REFERENCES FOR CHAPTER III - - Theories of origin of the earth:— - - THOMSON and TAIT. Natural Philosophy. 2d ed. Cambridge, 1883, pp. 422. - - T. C. CHAMBERLIN. Chamberlin and Salisbury’s Geology, vol. 2, pp. 1-81. - -Rigidity of the earth:— - - LORD KELVIN. The Internal Condition of the Earth as to Temperature, - Fluidity, and Rigidity, Popular Lectures and Addresses, vol. 2, pp. - 299-318; Review of evidence regarding the physical condition of the - earth, _ibid._, pp. 238-272. - - HOBBS. Earthquakes (Appleton, New York, 1907), Chapters xvi and xvii. - -Composition of the earth’s core and shell:— - - O. C. FARRINGTON. The Preterrestrial History of Meteorites, Jour. - Geol., vol. 9, 1901, pp. 623-236. - - E. S. DANA. Minerals and How to Study Them (a book for beginners in - mineralogy). Wiley, New York, 1895. - -On the nature of crystals:— - - VICTOR GOLDSCHMIDT. Ueber das Wesen der Krystalle, Ostwalds Annalen - der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419. - - - - -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 ways. 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 disintegrated 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 sediment. 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 _subaërial_ or _eolian_ -sediments; but if water, they are known as _subaqueous_ deposits. Still -another class are ice-deposited and are known as _glacial_ deposits. - -[Illustration: - -FIG. 16.—Laminated structure of sedimentary rock, Western Kansas -(after a photograph by E. S. Tucker).] - -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, accompanied by thorough -disintegration, due to the direct attack of the atmosphere, the -resulting rocks are called _residual_ rocks. - - -=The marks of origin.=—Each of the three great classes of rocks, -the igneous, sedimentary, and metamorphic, is characterized by both -coarser and finer structures, in the examination of which they may be -identified. The igneous rocks having been produced from magmas, which -are essentially homogeneous, are usually without definite directional -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 fragments 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 transportation and deposition of -sediments, for it is characteristic of the sedimentary rocks that their -constituent grains are well rounded. Eolian sediments have usually more -perfectly rounded grains than subaqueous deposits. - -Glacial deposits, if laid down directly by the ice, are unstratified, -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-glacial_ 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 constituent grains become rounded off by a -sort of peripheral crushing or granulation. Metamorphic rocks are -therefore characterized by an imperfectly banded structure described -as _schistosity_ or _gneiss banding_, 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 determine, -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 boundaries of -their constituents, there are many additional textures which are -characteristic of the igneous rocks. While those that have consolidated -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 surface. Magmas which thus reach the surface -of the earth are described 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 -_pumiceous_ 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 consolidated, these earlier 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 -consolidate rapidly as a rock glass, its texture is described as -_glassy_ (Fig. 17 _c_). - -When the crystals of the earlier generation are numerous and -needle-like in form, as is very often the case, they arrange themselves -“end on” during the rock flow, so that when consolidation 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 sedimentary rocks, so that wrong -conclusions have been reached 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 -National Park. - -[Illustration: - -FIG. 17.—Characteristic textures of igneous rocks. _a_, granitic -texture characteristic of the deep-seated intrusive rocks; _b_, -porphyritic texture characteristic of the extrusive and of the -near-surface intrusive rocks; _c_, glassy texture of an extrusive rock.] - -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_ texture (Fig. 17 _a_). - - -=Classification of rocks.=—In tabular form rocks may thus be -classified as follows:— - - {_Intrusive._ Granitic or porphyritic - { texture. - _Igneous._ Massive and {_Extrusive._ Glassy or porphyritic - with sharply angular { texture; often also with vesicular, - grains. { scoriaceous, pumiceous, fluxion, - { or spherulitic textures. - - {_Subaërial._ Sands and loess. - {_Subaqueous._ (See below.) - _Sedimentary._ Laminate { _Glacial._ Coarse, unstratified - and with rounded { deposits with faceted pebbles. Till - grains. { and tillite. - {_Fluvio-glacial._ Stratified sands - { and gravels with “worked over” - { glacial characters. - - _Metamorphic._ Schistose {_Metamorphic proper._ Due to below - and with grains either { surface 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 vegetable 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._ - - { _Subaqueous_ Conglomerate, sandstone - { Deposited by water. and shale. - { _Subaërial_ or _Eolian_ Sandstone and loess. - _Mechanical_ { Deposited by wind. - { _Glacial_ Till and tillite. - { Deposited by ice. - { _Fluvio-glacial_ Sands and gravels. - { Glacier-water deposits. - - { Calcareous tufa Deposited in springs - { and rivers. - _Chemical_ { Oölitic limestone Deposited at the - { mouths of rivers - { between high and - { low tide. - - { Formed of plant remains. Peats and coals. - _Organic_ { Formed of animal remains. Limestones and - { marls. - -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 deserts 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 -subaqueous 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 -sedimentation 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 subject 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 considerable -uncertainty. Some, like the shell limestone or coquina of the Florida -coast, are an aggregation of remains of mollusks which live near -the border of the sea. Other limestones are deposited 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 continents, 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 continental 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 sandstone -which results from the cementation of such deposits (pl. 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 distinguish the upper from the lower -surface. - -In the finer sands and muds of sheltered tidal flats may be preserved -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 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 deposits from the subaërial 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 peculiar “plunge -and flow” or “cross-bedded” structure, a structure which is extremely -common in littoral deposits, though simulated in rocks of eolian origin. - - -=The order of deposition during a transgression of the sea.=—Many -shore lines of the continents are almost constantly migrating either -landward or seaward. When the shore line advances 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 evident that each must advance in turn in -the direction of the shore and so be 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. - -[Illustration: FIG. 18.—Diagram to show the order of the sediments -laid down during a transgression of the sea.] - -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. Later, when cemented -together, these become in order, coarser and finer sandstones, shales, -and limestones. The order of superposition, reading from the bottom to -the top, thus gives the order of decreasing age of the formations. - -A subsequent uplift 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 doctrine -of evolution. - - -=The basins of earlier ages.=—It was the great Viennese geologist, -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 formations 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 mountain 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 uplift. - - -=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_ expedition 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, these deposits are the so-called “ooze”, composed of the -calcareous or chitinous parts of algæ 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 countless -myriads. The hard parts of both plant and animal organisms descend to -the bottom and there form the ooze in which are sometimes 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. DILLER. The Educational Series of Rock Specimens collected and - distributed by the United States Geological Survey, Bull. 150 U. S. - Geol. Surv., 1898, pp. 1-400. - - L. V. PIRSSON. Rocks and Rock Minerals. Wiley, New York, 1908. - - SIR JOHN MURRAY. Deep-sea Deposits, Reports of the _Challenger_ - expedition, Chapter iii. - - L. W. COLLET. Les dépôts marins. Doin, Paris, 1907 (Encyclopédie - Scientifique). - - - - -CHAPTER V - -CONTORTIONS OF THE STRATA WITHIN THE ZONE OF FLOW - - -=The zones of fracture and flow.=—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 represented by the -length of such a column, rocks cannot yield to pressure by fracture, -for the opening of a crack implies that the rock upon either side is -strong enough to prevent the walls from closing. 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_. - -[Illustration: - -FIG. 19.—Two intersecting parallel series of fractures produced upon -each free surface of a prismatic block of stiff molders’ wax when -broken by compression from the ends (after Daubrée and Tresca).] - -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 of -intersecting fractures which are perpendicular to the free surfaces -of the block and take two directions each inclined by half 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 experiment. 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 admixture of varying proportions of plaster of Paris, or -weakened by the use of Venice turpentine. Such a series of layers may -represent rocks of as widely different characters as limestone and -shale. The load which is to represent superincumbent rock is supplied -in the experiment by a deep layer of shot. - -[Illustration: FIG. 20.—Apparatus to illustrate the folding of strata -within the zone of flow (after Willis).] - -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 latter have -been protected under an arch of the more competent layer (pl. 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_. - -[Illustration: - -FIG. 21.—Diagrams representing _a_, an anticline; _b_, a syncline; and -_c_, 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 shortening 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_). - -[Illustration: - -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.] - - -=The elements of folds.=—Folds must always be thought of as having -extension 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 intersects 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 horizontal, 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 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. - -[Illustration: FIG. 23.—Anticlinal and synclinal folds in strata -(after Willis).] - - -[Illustration: - -FIG. 24.—Diagrams to illustrate the different shapes of rock folds.] - -=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 _overturned_. 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, though generally less easily, in passing through the -Appalachian Mountains. - -[Illustration: FIG. 25.—Secondary and tertiary flexures superimposed -upon the primary ones.] - -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 possess 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 _rock 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. - -┌────────────────────────────────────────────────────────────────────────┐ -│ PLATE 2. │ -│ │ -│ [Illustration: _A._ Layers compressed in experiments and showing the │ -│ effect of a competent layer in the process of folding (after Willis).] │ -│ │ -│ [Illustration: _B._ Experimental production of a series of parallel │ -│ thrusts within closely folded strata (after Willis).] │ -│ │ -│ [Illustration: _C._ Apparatus to illustrate shearing action within the │ -│ overturned limb of a fold.] │ -└────────────────────────────────────────────────────────────────────────┘ - -=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 concave side are crowded closer -together—there is tension in the former case and compression in the -latter (Fig. 26). Within an unsymmetrical or an overturned fold, the -peculiar distortions in the different sections of the stratum are less -simple and are best illustrated by pl. 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 that those rock particles which before the -bending were included in the circle have been moved past each other in -the manner of the blades of a pair of shears. Such extreme “shearing” -action is thus localized in the underturned 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 probably 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_. - -[Illustration: - -FIG. 26.—A bent stratum to illustrate tension upon the convex and -compression upon the concave side (after Van Hise).] - -In the long series of experiments conducted by Mr. Bailey Willis of the -United States Geological Survey, all the stages between the overturned -fold and the overthrust fold were reproduced. Where a series of folds -was closely compressed, a parallel series of thrusts developed (pl. 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 Scotland, where the overriding is measured in -miles. Near the thrust planes the rocks show a crushing of the grains, -and the planes themselves 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 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 superincumbent -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 incoherent 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 exposure 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 district 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 -locations, either with or without assumptions concerning the covered -areas, is known as a _geological map_. - -[Illustration: FIG. 27.—A geological section based upon observations -at outcrops, but with the truncated arches restored.] - -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 surface, will -give the directions of continuation of the individual strata. This -strike direction is usually determined at each exposure 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 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 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_). - -[Illustration: - -FIG. 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; _b_, a compass with the east -and west initials reversed upon the dial; _c_, home-made clinometer in -position to determine the dip.] - -By the dip is meant the inclination of the stratum at any exposure, and -this must obviously be measured in a vertical plane 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 indicator 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 geologist’s compass is always provided with a clinometer attachment -to the dial. - -[Illustration: - -FIG. 29.—Diagram to show the use of T symbols to indicate the dip and -strike of outcroppings.] - -Since the strike is the intersection of the bedding plane with -a horizontal surface, and the dip is the intersection with that -particular 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 exposures. To illustrate, in Fig. 29, the symbol marked _a_ -represents an outcrop of limestone, 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 horizontal beds, which -have in consequence 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. - - -[Illustration: - -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 exposures.] - -=Measurement of the thickness of formations.=—When formations still -lie in horizontal beds, we may sometimes learn their thickness -directly either from the depth of borings to the underlying rock, or -by measurements upon steep cañon 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 vertical, the thickness must be obtained indirectly from -the width of the exposures and the angle of the dip. The factor by -which the exposure width must be multiplied 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. - -_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 - -[Illustration: FIG. 31.—Combined surface and sectional views of a -plunging anticline (after Willis).] - -[Illustration: FIG. 32.—Combined surface and sectional views of a -plunging syncline (after Willis).] - -=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, the first representing a pitching -anticline and the second a pitching syncline. In both these reciprocal -cases the strikes of the 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 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 Appalachian 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. - - -[Illustration: - -FIG. 33.—Unconformity between a lower and an upper series of beds upon -the coast of California. Note how the hard layer stands in relief upon -the connecting surface (after Fairbanks).] - -=The meaning of an unconformity.=—The rock beds, which are deposited -one above the other during a transgression of the sea, are usually -parallel and thus represent a continuous process of deposition. 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 _unconformable_ series, and the break -or surface of junction is an _unconformity_ (Fig. 33). - -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 -delimits the lower series from the upper is somewhat 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. - -[Illustration: - -FIG. 34.—Series of diagrams to illustrate in succession the episodes -involved in the historical development of an angular unconformity. The -vertical arrows indicate direction of movement of the land, and the -horizontal arrows the direction of shore migration.] - -In reality, an unconformity between formations must be interpreted to -mean that the lower series is not only 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 transgression of the sea. - -(_b_) Continued subsidence and burial of the lower series beneath -overlying sediments, and flexuring in the zone of flow. - -(_c_) Elevation of the combined deposits to and far above sea level and -removal by erosion of vast thicknesses of the upper sediments. - -(_d_) A new subsidence 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. - -[Illustration: - -FIG. 35.—Types of deceptive or erosional unconformities.] - -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 repeated operations -is so vast as at first to stagger the mind in contemplating 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 unconformity_. -It may be, however, that the lower series was not so far depressed -as to enter the zone of flow, and that its beds meet those of the -upper series with apparent conformity. Such an unconformity is often -extremely difficult to recognize, and it is described as a _deceptive_ -or _erosional unconformity_. - -With a deceptive unconformity the clew to its real nature is usually -some fact which indicates that the lower series of sediments had been -raised above the 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 furnished by a basal conglomerate in the upper -series, or some indication 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 member of the lower -series, hence the frequent occurrence of the discordant cross bedding -in sandstone should not deceive even the novice into the assumption of -an unconformity. - - -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, Pt. I, pp. 581-603. - - BAILEY WILLIS. Mechanics of Appalachian Structure, 13th Ann. Rept. - U.S. Geol. Surv., 1893, Pt. II, pp. 217-253. - - A. DAUBRÉE. Études Synthétiques de Géologie Expérimentale. Paris, - 1879, pp. 306-328, pl. II. - - W. PRINZ. Quelques remarques générales à propos de l’essai de carte - tectonique de la belgique, etc., Bull. Soc. Belge Geol., vol. 18, - 1904, p. 143, pl. V. - -Analysis of folds:— - - VAN HISE and WILLIS as above; DE MARGERIE et HEIM; Les dislocations de - l’écorce 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 SUPERSTRUCTURE - - -[Illustration: - -FIG. 36.—A set of master joints developed in shale upon the shores of -Cayuga Lake near Ithaca, New York (after U. S. G. S.).] - -[Illustration: - -FIG. 37.—Diagram to show how sets of master joints differing in -direction by half a right angle may abruptly replace each other.] - -[Illustration: - -FIG. 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.] - -=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 of a right angle to the direction of compression, and -thus perpendicular 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 compression. From mechanical -considerations involving the resolution of forces with reference 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 attitudes, -they are, in fact, seen to be intersected by two parallel series of -fractures which are perpendicular to the earth’s surface 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 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 master joints may remain practically -constant, or this set may quite suddenly give place to a similar 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 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 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 regular joint -series observed which do not stand perpendicular to the original -attitude of the rock beds. In a few localities, however, rectangular -joint sets have been discovered 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 joints may be such as to defy all attempts -at orderly arrangement. - -[Illustration: FIG. 39.—View on the shore at Holstensborg, West -Greenland, to show the subequal spacing of the joints (after Kornerup).] - -[Illustration: - -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).] - - -=The space intervals of joints.=—The same kind of subequal spacing -which characterizes the fractures near the surface of the block in -Daubrée’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 fractures 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 without apparent -displacement. Joints are therefore planes of disjunction only, and not -planes of displacement. - -[Illustration: - -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 preparing the sketch from a photograph.] - -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 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 rôles in the movement. A simple case of such displacements -in rocks intersected by a single set of master joints is represented -in the model of plate 4 C. The most prominent 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 the fault is called its _throw_. 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 -system 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 additional technical terms (Fig. 42). The inclination of a sloping -fault plane measured against the vertical is called the _hade_ 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. - -[Illustration: - -FIG. 42.—A fault in previously disturbed strata. _AB_, displacement; -_AC_, throw; _BD_, stratigraphic throw; _BC_, heave; angle _CAB_, hade.] - - -=Methods of detecting faults.=—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 escarpment 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 necessarily planes -of faulting. In many cases the actual displacements lie buried under -loose rock débris near to and paralleling the escarpment, and in some -cases as a result of the erosional processes working upon alternately -hard and soft layers of rock, the escarpment may later appear upon the -downthrown side or limb of the fault (Fig. 43). As an illustration of a -fault escarpment, the façade of El Capitan and many other rock faces of -the Yosemite valley may be instanced. - -[Illustration: - -FIG. 43.—Diagrams to show how an escarpment originally on the upthrown -side of the fault may, through erosion, appear upon the downthrown -side.] - -[Illustration: - -FIG. 44.—A fault plane exhibiting “drag.” The opening is artificial -(after Scott).] - -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, whereas fold structures will long remain in -evidence. Many faults will thus be overlooked, and too great weight -is likely to be ascribed to the folds in accounting for the existing -attitudes and positions of the rock masses. Faults must therefore be -sought out if mistakes of interpretation 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 breccia_. 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 produces little fracture of the walls, but they slide over -each other in such a manner as to yield either a smoothly corrugated or -an evenly polished surface which is described as “slickensides.” It may -be, however, that during the movement 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 displacement -of a fault is not open to inspection, the movement may be proven by -the observation of abrupt, as contrasted with gradual, changes in the -strikes and dips of neighboring exposures (Fig. 45); or by noting -that some easily recognized formation has been sharply offset in its -outcrops (Fig. 46). - -[Illustration: - -FIG. 45.—Map to show how a fault may be indicated in abrupt changes of -the strike and dip of neighboring exposures.] - -[Illustration: - -FIG. 46.—A series of parallel faults indicated by successive offsets -in the course of an easily recognizable rock formation.] - -There are in addition many indications 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 -neighboring formations may terminate abruptly upon a straight line -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 preëxisting -fractures. - - -=The base of the geological map.=—Modern topographic maps form an -important 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 publishing the results -in the form of an atlas of quadrangles. Thus 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 peculiarity 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 relief 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. - - -=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 individual hue joined to each other like the parts -of a child’s picture 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 judgment 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. Experience 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 geological map, and -every student should be advised to follow this by practical study of -some small area where rocks are found in outcrop. - -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 numbered 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 sedimentary rock types are so -constructed as to show the strike and dip of the beds. (See Appendix D.) - - -[Illustration: FIG. 47.—Field map prepared from a laboratory table.] - -=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 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 there have been uncertainties in the -correlation of exposures or in determining the position of the bedding -plane. - -[Illustration: FIG. 48.—Areal geological map constructed from the -field map of Fig. 47, with two selected geological sections.] - -To prepare the areal geological map from the field map, it is first -necessary to fix the _boundaries_ which separate formations at 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 of them—where -the facts are more nearly adequate—may correspond. In Fig. 48 is -represented the areal geological map constructed from the field map, -with the doubtful area at one side left blank. - -Some conclusions from this map may now be profitably considered. 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 formations 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_ thicknesses of -the different formations may be calculated. In one instance 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 generally long and straight -ridges. More complex flexures, since they generally 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 fracture structures are for this reason much more likely to be -overlooked, 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 - District. London, 1836, pp. 90-98. - - SAMUEL HAUGHTON. On the Physical Structure of the Old Red Sandstone of - the County of Waterford, considered with reference to cleavage, joint - surfaces, and faults, Trans. Roy. Soc. London, vol. 148, 1858, pp. - 333-348. - - W. C. BRÖGGER. 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, Connecticut, - 21st Ann. Rept. U. S. Geol. Surv., Pt. III, 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 MOVEMENTS: 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 disillusionment -from the most fundamental of his beliefs. Were he better advised, the -long periods of quiet which separate earthquakes, and not the lighter -shocks which follow all grander disturbances, would occasion him -concern. - -[Illustration: FIG. 49.—View of a portion of the ruins of Messina -after the earthquake of December 28, 1908.] - -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 -life 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 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. - -[Illustration: - -FIG. 50.—Ruins of the Carnegie Palace of Peace at Cartago, Costa Rica, -destroyed when almost completed by the great earthquake of May 4, 1910 -(after a photograph by Rear-Admiral Singer, U.S.N.).] - -Earthquakes are usually preceded for a brief instant by subterranean -rumblings whose intensity appears to bear no relation to the shocks -which follow. The ground then rocks in wavelike 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 sometimes thrown up from the ground (Fig. 51), -and at other times 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 movement of the ground had been suddenly arrested, while the -burial of heavy bodies in the earth is probably due to a movement which -begins suddenly and is less abruptly terminated. - -[Illustration: FIG. 51.—Bowlders thrown into the air and overturned -during the Assam earthquake of 1897 (after R. D. Oldham).] - -[Illustration: - -FIG. 52.—Heavy post sunk deeper into the ground during the Charleston -earthquake of August 31, 1886 (after Dutton).] - - -=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 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 definite straight lines has been often illustrated -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. - -[Illustration: - -FIG. 53.—Map showing the localities at which shocks have been reported -at sea off Cape Mendocino, California.] - -Far grander disturbances upon the floor of the ocean have been revealed -by the great sea waves—the 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 approaching 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 country 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 depressions owe -their exceptional depth. The withdrawal of the water from neighboring -shores may be presumed to be connected with a descent of the floor -of the depression and the consequent drawing-in of the ocean surface -above. The later high wave would thus represent the dispersion of the -mountain of water which is raised by the meeting of the waters from the -different sides of the depression. - - -[Illustration: FIG. 54.—Effect of a seismic water wave at Kamaishi, -Japan, in 1896 (after E. R. Scidmore).] - -[Illustration: FIG. 55.—A fault of vertical displacement.] - -=The grander and the lesser earth movements.=—Upon the land the -grander and so-called catastrophic earthquakes are usually the -accompaniment of important changes in the surface of the ground that -will be discussed in later sections. Those shocks which do little -damage to structures produce no visible changes in the earth’s surface, -except, it may be, to shake down some water-soaked masses of earth upon -the steeper slopes. Still other movements, and these too slight to be -felt even in the night when the animal world is at rest, may yet be -distinguished by their sounds, the unmistakable rumblings which are -characteristic alike of the heaviest and the lightest of earthquake -shocks. - - -[Illustration: - -FIG. 56.—Escarpment produced by an earthquake fault of vertical -displacement which cut across the Chedrang River and thus produced a -waterfall, Assam earthquake of 1897 (after R. D. Oldham).] - -=Changes in the earth’s surface during earthquakes—faults and -fissures.=—Each of the grander among historic earthquakes has been -accompanied by noteworthy changes in the configuration of the earth’s -surface within the district where the shocks were most intense. A -section of the ground is usually found to have moved with reference -to another upon the other side of a vertical plane which is usually -to be seen; we have here to do with the actual making of a fault -or displacement such as we find the fossil examples of within the -rocks. The displacement, or throw, upon the fault plane may be either -upward or downward or laterally in one direction or the other, or -these movements may be combined. A movement of adjacent sections of -the ground upward or downward with reference to each other (Fig. 55) -has been often observed, notably at Midori after the great Japanese -earthquake of 1891, and in the Chedrang valley of Assam after the -earthquake of 1897 (Fig. 56). - -[Illustration: FIG. 57.—A fault of lateral displacement.] - -[Illustration: - -FIG. 58.—Fence parted and displaced fifteen feet by a transverse fault -formed during the California earthquake of 1906 (after W. B. Scott).] - -[Illustration: - -FIG. 59.—Fault with vertical and lateral displacements combined.] - -A lateral throw, unaccompanied by appreciable vertical displacement -(Fig. 57), is especially well illustrated by the fault in California -which was formed during the earthquake of 1906 (Fig. 58). A combination -of the two types of displacement in one (Fig. 59) is exemplified by the -Baishiko fault of Formosa at the place shown in plate 3 A. - - -=The measure of displacement.=—To afford some measure of the -displacements which have been observed upon earthquake faults, it may -be stated that the maximum vertical throw measured upon the fault in -the Neo valley of Japan (1891) was 18 feet, in the Chedrang valley of -Assam (1897) 35 feet, and of the Alaskan coast (1899) 47 feet. Large -sections of land were bodily uplifted in these cases within the space -of a few seconds, or at most a few minutes, by the amounts given. The -largest recorded lateral displacement measured upon an earthquake fault -is about 21 feet upon the California rift after the earthquake of 1906; -though an amount only slightly less than this is indicated in the -shifting of roads and arroyas dating from the earthquake of 1872 in the -Owens valley, California. Fault lines once established are planes of -special weakness and become later the seat of repeated movements of the -same kind. - -┌──────────────────────────────────────────────────────────────────────┐ -│ PLATE 3. │ -│ │ -│ [Illustration: _A._ An earthquake fault opened in Formosa in 1906, │ -│ with vertical and lateral displacements combined (after Omori).] │ -│ │ -│ [Illustration: _B._ Earthquake faults opened in Alaska in 1889, on │ -│ which vertical slices of the earth’s shell have undergone individual │ -│ adjustments (after Tarr and Martin).] │ -└──────────────────────────────────────────────────────────────────────┘ - -[Illustration: - -FIG. 60.—Diagram to show how small faults in the rock basement may be -masked at the surface through adjustments within the loose rock mantle.] - -The greater number of earthquake faults are found in the loose rock -cover which so generally mantles the firmer rock basement, and it is -almost certain that the throws within the solid rock are considerably -larger than those which are here measured at the surface, owing to -the adjustments which so readily take place in the looser materials. -Those lighter shocks of earthquake which are accompanied by no visible -displacements at the surface do, however, in some instances affect -in a measure the flow of water upon the surface, and thus indicate -that small changes of surface level have occurred without breaks -sufficiently sharp to be perceived (Fig. 60). Intermediate between the -steep escarpment and the masked displacement just described is the -so-called “mole-hill” effect,—a rounded and variously cracked slope or -ridge above the position of a buried fault (Fig. 61). - -[Illustration: - -FIG. 61.—Diagram to show the appearance of a “mole hill” above a -buried earthquake fault (after Kotô).] - -The escarpments due to earthquake faults in loose materials at the -earth’s surface can obviously retain their steepness for a few years -or decades at the most; for because of their verticality they must -gradually disappear in rounded slopes under the action of the elements. -Smaller displacements within a rock which rapidly disintegrates under -the action of frost and sun will likewise before long be effaced. In -those exceptional instances where a resistant rock type has had all -altered upper layers planed away until a fresh and hard surface is -exposed, and has further been protected from the frost and sun beneath -a thin layer of soil, its original surface may be retained unaltered -for many centuries. Upon such a surface the lightest of sensible -shocks, or even the smaller earth movements which are not perceived -at the time, may leave an almost indelible record. Such records -particularly show that the movements which they register occur upon the -planes of jointing within the rock, and that these ready formed cracks -have probably been the seats of repeated and cumulative adjustments -(Fig. 62). - -[Illustration: - -FIG. 62.—Post-glacial earthquake faults of small but cumulative -displacement, eastern New York (after Woodworth).] - -[Illustration: FIG. 63.—Earthquake cracks in Colorado desert (after a -photograph by Sauerven).] - - -=Contraction of the earth’s surface during earthquakes.=—The wide -variations in the amount of the lateral displacement upon earthquake -faults, like those opened in California in 1906, show that at the -time of a heavy earthquake there must be large local changes in the -density of the surface materials. Literally, thousands of fissures may -appear in the lowlands, many of them no doubt a secondary effect of -the shaking, but others, like the _quebradas_ of the southern Andes or -the “earthquake cracks” in the Colorado desert (Fig. 63), may have a -deeper-seated origin. Many facts go to show, however, that though local -expansion does occur in some localities, a surface contraction is a -far more general consequence of earth movement. In civilized countries -of high industrial development, where lines of metal of one kind or -another run for long distances beneath or upon the surface of the -ground, such general contraction of the surface may be easily proven. -Comparatively seldom are lines of metal pulled apart in such a way as -to show an expansion of the surface; whereas bucklings and kinkings of -the lines appear in many places to prove that the area within which -they are found has, as a whole, been reduced. - -[Illustration: FIG. 64.—Diagrams to show how railway tracks are either -broken or buckled locally within the district visited by an earthquake.] - -[Illustration: FIG. 65.—The Biwajima railroad bridge in Japan after -the earthquake of 1891 (after Milne and Burton).] - -[Illustration: - -FIG. 66.—Diagrams to show how the compression of a district and its -consequent contraction during an earthquake may close up the joint -spaces within the rock basement and concentrate the contraction of the -overlying mantle where this is partially cut through and so weakened in -the valley sections.] - -Water pipes laid in the ground at a depth of some feet may be bowed up -into an arch which appears above the surface; lines of curbing are -raised into broken arches, and the tracks of railways are thrown into -local loops and kinks which imply a very considerable local contraction -of the surface (Fig. 64). With unvarying regularity railway or other -bridges which cross rivers or ravines, if the structures are seriously -damaged, indicate that the river banks have drawn nearer together at -the time of the disturbance. In such cases, whenever the bridge girder -has remained in place upon its abutments, these have either been broken -or back-tilted as a whole in such a manner as to indicate an approach -of the foundations which was prevented at the top by the stiffness of -the girder (Fig. 65). - -[Illustration: - -FIG. 67.—Map of the Chedrang fault which made its appearance during -the Assam earthquake of 1897. The figures give the amounts of the local -vertical displacement measured in feet (after R. D. Oldham).] - -The simplest explanation of such an approach of the banks at the -sides of the valleys cut in loose surface material is to be found in -a general closing up of the joint spaces within the underlying rock, -and an adjustment of the mantle upon the floor mainly in the valley -sections (Fig. 66). - - -[Illustration: - -FIG. 68.—Map giving the displacements in feet measured along an -earthquake fault formed in Alaska in 1899 (after Tarr and Martin).] - -=The plan of an earthquake fault.=—In our consideration of earthquake -faults we have thus far given our attention to the displacement as -viewed at a single locality only. Such displacements are, however, -continued for many miles, and sometimes for hundreds of miles; and when -now we examine a map or plan of such a line of faulting, new facts of -large significance make their appearance. This may be well illustrated -by a study of the plan of the Chedrang fault which appeared at the -time of the Assam earthquake of 1897 (Fig. 67). From this map it -will be noticed that the upward or downward displacement upon the -perpendicular plane of the fault is not uniform, but is subject to -large and _sudden_ changes. Thus in order the measurements in feet are -32, 0, 18, 35, 0, 8, 25, 12, 8, 2, 0. The fault formed in 1899 upon -the shores of Russell Fjord in Alaska (Fig. 68) reveals similar sudden -changes of throw, only that here the direction of the movement is often -reversed; or, otherwise expressed, the upthrow is suddenly transferred -from one side of the fault to the other. Such abrupt changes in the -direction of the displacement have been observed upon many earthquake -faults, and a particularly striking one is represented in Fig. 69. - -[Illustration: - -FIG. 69.—Abrupt change in the direction of throw upon an earthquake -fault which was formed in the Owens valley, California, in 1872. The -observer looks directly along the course of the fault from the left -foreground to the cliff beyond and to the left of the impounded water -(after a photograph by W. D. Johnson).] - - -=The block movements of the disturbed district.=—The displacements -upon earthquake faults are thus seen to be subdivided into sections, -each of which differs from its neighbors upon either side and is -sharply separated from them, at least in many instances. These points -of abrupt change of displacement are, in many cases at least, the -intersection points with transverse faults (Fig. 69). Such points of -abrupt change in the degree or in the direction of the displacement may -be, when looked at from above, abrupt turning points in the direction -of extension of the fault, whose course upon the map appears as a -zigzag line made up of straight sections connected by sharp elbows -(Fig. 70). - -[Illustration: - -FIG. 70.—Map of the faults within an area of the Owens valley, -California, formed in part during the earthquake of 1872, and in part -due to early disturbances, In the western portions the displacements -cut across firm rock and alluvial deposits alike without deviation of -direction (after a map by W. D. Johnson).] - -Such a grouping of surface faults as are represented upon the map is -evidence that the area of the earth’s shell, which is included, has at -the time of the earthquake been subject to adjustments as a series of -separate units or blocks, certain of the boundaries of which are the -fault lines represented. The changes in displacement measured upon the -larger faults make it clear that the observed faults can represent but -a fraction of the total number of lines of displacement, the others -being masked by variations in the compactness of the loose mantling -deposits. Could we but have this mantle removed, we should doubtless -find a rock floor separated into parts like an ancient Pompeiian -pavement, the individual blocks in which have been thrown, some upward -and some downward, by varying amounts. Less than a hundred miles away -to the eastward from the Owens Valley, a portion of this pavement has -been uncovered in the extensive operations of the Tonapah Mining -District, so that there we may study in all its detail the elaborate -pattern of earth marquetry (Fig. 71) which for the floor of the Owens -valley is as yet denied us. - -[Illustration: - -FIG. 71.—Marquetry of the rock floor of the Tonapah Mining District, -Nevada (after Spurr).] - -[Illustration: - -FIG. 72.—Map of a portion of the Alaskan coast to show the adjustments -in level during the earthquake of 1899 (after Tarr and Martin).] - - -=The earth blocks adjusted during the Alaskan earthquake of 1899.=—For -a study of the adjustments which take place between neighboring earth -blocks during a great earthquake, the recent Alaskan disturbance -has offered the advantage that the most affected district was upon -the seacoast, where changes of level could be referred to the datum -of the sea’s surface. Here a great island and large sections of the -neighboring shore underwent movements both as a whole in large blocks -and in adjustments of their subordinate parts among themselves (Fig. -72). Some sections of the coast were here elevated by as much as 47 -feet, while neighboring sections were uplifted by smaller amounts (Fig. -73), and certain smaller sections were even dropped below the level -of the sea. - -[Illustration: - -FIG. 73.—View on Haencke Island, Disenchantment Bay, Alaska, revealing -the shore that rose seventeen feet above the sea during the earthquake -of 1899, and was found with barnacles still clinging to the rock (after -Tarr and Martin).] - -The amount of such subsidence is, however, difficult -to ascertain, for the reason that the former shore features are now -covered with water and thus removed from observation. In favorable -localities the minimum amount of submergence may sometimes be measured -upon forest trees which are now flooded with sea water. In Fig. 74 -a portion of the coast is represented where the beach sand is now -extended back into the spruce forest, a distance of a hundred feet or -more, and where sedgy beach grass is growing among trees whose roots -are now laved in salt water. At the front of this forest the great -storm waves overturn the trees and pile the wreckage in front of those -that still remain standing. - -[Illustration: - -FIG. 74.—Partially submerged forest upon the shore of Knight Island, -Alaska, due to the sinking of a section of the coast during the -earthquake of 1899 (after Tarr and Martin).] - -[Illustration: - -FIG. 75.—Settlement of a section of the shore at Port Royal, Jamaica, -during the earthquake of January 14, 1907, adjacent to a similar but -larger settlement of the near shore during the earthquake of 1692 -(after a photograph by Brown).] - -Upon the glaciated rock surfaces of the Alaskan coast, exceptionally -favorable opportunities are found for study of the intricate pattern -of the earth mosaic which is under adjustment at the time of an -earthquake. Upon Gannett Nunatak the surface was found divided by -parallel faults into distinct slices which individually underwent small -changes of level (plate 3 B). - - - - -CHAPTER VIII - -THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTHQUAKES AND SEAQUAKES -(Concluded) - - -=Experimental demonstration of earth movements.=—The study of the -Alaskan earthquake of 1899 showed that during this adjustment within -the earth’s shell some of the local blocks moved upward and by larger -amounts than their neighbors, and that still others were actually -depressed so that the sea flowed over them. It must be evident that -such differential vertical movements of neighboring blocks at the -earth’s surface can only take place if lateral transfers of material -are made beneath it. From under those strips of coast land which were -depressed, material must have been moved so as to fill the void which -would otherwise have formed beneath the sections that were uplifted. -If we take into consideration much larger fractions upon the surface -of our planet, we are taught by the great seaquakes which are now -registered upon earthquake instruments at distant stations that large -_downward_ movements are to-day in progress beneath the sea much more -than sufficient to compensate all extensions of the earth’s surface -within those districts where the land is rising in mountains. From -under the offshore deeps of the ocean to beneath the growing mountains -upon the shore, a transfer of earth material must be assumed to take -place when disturbances are registered. - -Within the time interval that separates the sudden adjustments of -the surface which are manifested in earthquakes, the condition of -strain which brings them about is steadily accumulating, due, as we -generally assume, to earth contraction through loss of its heat. It -seems probable that the resistance to an immediate adjustment is found -in the rigidity of the shell because of the compression to which it is -subjected. To illustrate: a row of blocks well fitted to each other -may be held firmly as a bridge between the jaws of a vice, because so -soon as each block starts to fall a large resistance from friction upon -its surface is called into existence, a force which increases with the -degree of compression. - -It is thus possible upon this assumption crudely to demonstrate the -adjustment of earth blocks by the simple device represented in plate -4 A. The construction of this experimental tank is so simple that -little explanation is necessary. Wooden blocks of different heights -are supported in water within a tank having a glass front, and are -kept in a strained condition at other than their natural positions of -flotation by the compression of a simple vice at the top. Held firmly -in this position, they may thus represent the neighboring blocks within -the earth’s outer shell which are supported upon relatively yielding -materials beneath, and prevented from at once adjusting themselves -to their natural positions through the compression to which they are -subjected. Held as they now are, the water near the ends of the tank -is forced up beneath the blocks to higher than its natural level, and -thus tends to flow from both ends toward the center. Such a movement -would permit the end blocks to drop and force the middle ones to rise. -The end blocks are, let us say, the sections of Alaskan coast line -which sunk during the earthquake, as the center blocks are the sections -which rose the full measure of 47 feet. Upon a larger scale the end -blocks may equally well be considered as the floor of the great deeps -off the Alaskan coast, whose sinking at the time of the earthquake was -the cause of the great sea wave. Upon this assumption the center blocks -would represent the Alaskan coast regarded as a whole, which underwent -a general uplift. - -Though we may not, in our experiment, vary the tendency to adjustment -by any contractional changes in either the water or the blocks, we may -reduce the compression of the vice, which leads to the same general -result. As the compression of the vice is slowly relaxed, a point is -at last reached at which friction upon the block surfaces is no longer -sufficient to prevent an adjustment taking place, and this now suddenly -occurs with the result shown in plate 4 B. In the case of the earth -blocks, this sudden adjustment is accompanied by mass movements of the -ground separated by faults, and these movements produce successional -vibrations that are particularly large near the block margins, and -other frictional vibrations of such small measure as to be generally -appreciated by sounds only. The jolt of the adjustments has thrown some -blocks beyond their natural position of rest, and these sink and rise -subsequently in order to readjust themselves with lighter vibrations, -which may be repeated and continued for some time. In the case of the -earth these later adjustments are the so-called _aftershocks_ which -usually continue throughout a considerable period following every great -earthquake. Gradually they fall off in intensity and frequency until -they can no longer be felt, and are thereafter continued for a time as -rumblings only. - - -┌───────────────────────────────────────────────────────────────────────┐ -│ PLATE 4. │ -│ │ -│ [Illustration: │ -│ │ -│ _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.] │ -│ │ -│ [Illustration: _B._ The same apparatus after a sudden adjustment.] │ -│ │ -│ [Illustration: _C._ Model to illustrate a block displacement in rocks │ -│ which are intersected by master joints.] │ -└───────────────────────────────────────────────────────────────────────┘ - -=Derangement of water flow by earth movement.=—The water which -supported the blocks in our experiment has represented the more mobile -portion of the earth’s substance beneath its outer zone of fracture. -The surface water layers in the tank may, however, be considered -in a different way, since their behavior is remarkably like that -of the water within and upon the earth’s surface during an earth -adjustment. At the instant when adjustment takes place in the tank, -water frequently spurts upward from the cracks between the sinking end -blocks; and if in place of one of the higher center blocks we insert -one whose top is below the level of the water in the tank, a “lake” -will be formed above it. When the adjustment occurs, this lake is -immediately drained by outflow of the water at its bottom along one of -the cracks between the blocks (Fig. 76). - -[Illustration: - -FIG. 76.—Diagrams to illustrate the draining of lakes during -earthquakes.] - -Such derangements of water flow as have been illustrated by the -experiment are among the commonest of the phenomena which accompany -earthquakes. Lakes and swamp lands have during earthquakes been -suddenly drained, fountains of water have been seen to shoot up -from the surface and have played for some minutes or hours before -their sudden disappearance in a sucking down of the water with later -readjustment. During the great earthquake of the lower Mississippi -valley in 1811, known as the New Madrid earthquake, the earlier Lake -Eulalie was completely drained, and upon the now exposed bed there -appeared parallel fissures on which were ranged funnel-like openings -down which the water had been sucked. In other sections of the affected -region the water shot up in sheets along fissures to the tops of high -trees. Areas where such spurting up of the water has been observed have -in most cases been shown to correspond to areas of depression, and such -areas have sometimes been left flooded with water. During the Indian -earthquake of 1819 an area of some 200 square miles suddenly sank and -was transformed into a lake. - -[Illustration: - -FIG. 77.—Diagram to illustrate-the derangements of flow of water at -the time of an earthquake; water issuing at the surface over downthrown -rocks, and being sucked down in upthrown blocks.] - -[Illustration: - -FIG. 78.—Mud cones aligned upon a fissure opened at Moraza, Servia, -during the earthquake of April 4, 1904 (after Michailovitch).] - - -=Sand or mud cones and craterlets.=—From a very moderate depth below -the surface to that of several miles, all pore spaces and all larger -openings within the rock are completely filled with water, the “trunk -lines” of whose circulation is by way of the joints or along the -bedding planes of the rocks. The principal reservoirs, so to speak, of -this water inclosed within the rock are the porous sand formations. -When, now, during an earthquake a block of the earth’s shell is -suddenly sunk and as suddenly arrested in its downward movement, the -effect is to compress the porous layers and so force the contained -water upward along the joints to the surface, carrying with it large -quantities of the sand (Fig. 77). - -[Illustration: - -FIG. 79.—One of the many craterlets formed near Charleston, South -Carolina, during the earthquake of August 31, 1886. The opening is -twenty feet across, and the leaves about it are encased in sand as were -those upon the branches of the overhanging trees to a height of some -twenty feet (after Dutton).] - -[Illustration: - -FIG. 80.—Cross section of a craterlet to show the trumpet-like form of -the sand column.] - -Ejected at the surface this water appears in fountains usually arranged -in line over joints, or even in continuous sheets, and the sand -collecting about the jets builds up lines of _sand_ or _mud cones_ -sometimes described as “mud volcanoes” (Fig. 78). The amount of sand -thus poured out is sometimes so great that blankets of quicksand are -spread over large sections of the country. Most frequently, however, -the sand is not built above the general level of the surface, but forms -a series of _craterlets_ which are largely shaped as the water is -sucked down at the time of the readjustment with which the play of such -earthquake fountains is terminated (Fig. 79). Subsequent excavations -made about such craterlets have shown them to have the form of a -trumpet, and that in the sand which so largely fills them there are -generally found scales of mica and such light bodies as would be picked -out from the heterogeneous materials of the sand layers and carried -upward in the rush of water to the surface (Fig. 80). - - -=The earth’s zones of heavy earthquake.=—Since earthquakes give notice -of a change of level of the ground, the special danger zones from this -source are the growing mountain systems which are usually found near -the borders of the sea. Such lines of mountains are to-day rising where -for long periods in the past were the basins of deposition of former -seas. They thus represent the zones upon the earth’s surface which -are the most unstable—which in the recent period have undergone the -greatest changes of level. - -[Illustration: - -FIG. 81.—Map of the island of Ischia to show how the shocks of recent -earthquakes have been concentrated at the crossing point of two -fractures (after Mercalli and Johnston-Lavis).] - -By far the most unstable belt upon the earth’s surface is the rim -surrounding the Pacific Ocean, within which margin it has been -estimated that about 54 per cent of the recorded shocks of earthquake -have occurred. Next in importance for seismic instability is the -zone which borders both the Mediterranean Sea and the Caribbean—the -American Mediterranean—and is extended across central Asia through -the Himalayas into Malaysia. Both zones approximate to great circles -upon the earth’s surface and intersect each other at an angle of about -67°. It has been estimated that about 95 per cent of the recorded -continental earthquakes have emanated from these belts. - - -[Illustration: - -FIG. 82.—A line of earth fracture indicated in the plan of the relief, -which may at any time become the seat of movement and resultant shock.] - -=The special lines of heavy shock.=—Within any earthquake district -the shocks are not felt with equal severity at all places, but there -are, on the contrary, definite lines which the disturbance seems to -search out for special damage. From their relations to the relief of -the land these lines would appear to be lines of fracture upon the -boundaries of those sections of the crust that play individual rôles in -the block adjustment which takes place. More or less masked as these -lines are beneath the rounded curves of the landscape, they are given -an altogether unenviable prominence with each succeeding earthquake. At -such times we may think of the earth’s surface as specially sensitized -for laying bare its hidden structure, as is the sensitized plate under -the magical influence of the X rays. - -When, at the time of an earthquake, blocks are adjusted with reference -to their neighbors, the movements of oscillation are greatest in -those marginal portions of direct contact. Corners of blocks—the -intersecting points of the important faults—should for the same -reason be shaken with a double violence, and this assumption appears -to be confirmed by observation. Upon the island of Ischia, off the -Bay of Naples, the shocks from recent earthquakes have been strangely -concentrated near the town of Casamicciola, which was last destroyed -in 1883. This unfortunate city lies at the crossing point of important -fractures whose course upon the island is marked by numerous springs -and _suffioni_ (Fig. 81). - - -=Seismotectonic lines.=—The lines of important earth fractures, as -will be more clearly shown in the sequel (p. 227), are often indicated -with some clearness by straight lines in the plan of the surface -relief (Fig. 82). Lines of this nature are easily made out upon -the map of the West Indies, and if we represent upon it by circles -of different diameters the combined intensities of the recorded -earthquakes in the various cities, it appears that the heavily shaken -localities are ranged upon lines stamped out in the relief, with -the most severely damaged places at their intersections (Fig. 83). -These lines of exceptional instability are known as _seismotectonic -lines_—earthquake structure lines. - -[Illustration: FIG. 83.—Seismotectonic lines of the West Indies.] - - -=The heavy shocks above loose foundations.=—It is characteristic of -faults that they soon bury themselves from sight under loose materials, -and are thus made difficult of inspection. The escarpment which is the -direct consequence of a vertical displacement upon a fault tends to -migrate from the place of its formation, rounding the surface as it -does so and burying the fault line beneath its deposits (Fig. 43, p. -60). - -This is not, however, the sole reason why loose foundations should be -places of special danger at the time of earth shocks, for the reason -that earthquake waves are sent out in all directions from the surfaces -of displacement through the medium of the underlying rock. These -waves travel within the firm rock for considerable distances with -only a gradual dissipation of their energy, but with their entry into -the loose surface deposits their energy is quickly used up in local -vibrations of large amplitude, and hence destructive to buildings. - -[Illustration: - -FIG. 84.—Device to illustrate the different effects upon the -transmission and the character of shocks which are produced by firm -rock and by loose materials.] - -The essential difference between firm rock and such loose materials as -are found upon a river bottom or in the “made land” about our cities -may be illustrated by the simple device which is represented in Fig. -84. Two similar metal pans are suspended from a firm support by bands -of steel and “elastic” braid of similar size and shape, and carry each -a small block of wood standing upon its end. Similar light blows are -now administered directly to the pans with the effect of upsetting that -block which is supported by the loose braid because of the large range -or amplitude of movement that is imparted to the pan. The “elastic” -braid, because of these large vibrations of which it is susceptible, -may represent the loose materials when an earthquake wave passes into -them. In the case of the steel support, the energy of the blow, instead -of being dissipated in local swingings of the pan, is to a large extent -transmitted through the elastic metal to materials beyond. The steel -thus resembles in its high elasticity the firmer rock basement, which -receives and transmits the earthquake shocks, but except when ruptured -in a fault is subject to vibrations of small amplitude only. - - -=Construction in earthquake regions.=—Wherever earthquakes have -been felt, they are certain to occur again; and wherever mountains -are growing or changes of level are in progress, there no record of -past earthquakes is required in order to forecast the future seismic -history. Although the future earthquakes may be predicted, the time of -their coming is, fortunately or unfortunately, still hidden from us. If -one’s lot is to be cast in an earthquake country, the only sane course -to pursue is to build with due regard to future contingencies. - -The danger, from destructive fires may to-day be largely met by methods -of construction which levy an additional burden of cost. Though the -danger from seismic disturbances can hardly be met as fully as that -from fire, yet it is true that buildings may be so constructed as to -withstand all save those heaviest shocks in the immediate vicinity of -the lines of large displacement. Here, also, a considerable additional -expense is involved in the method of construction, in the case of -residences particularly. - -From what has been said, it is obvious that much of the danger from -earthquakes can be met by a choice of site away from lines of important -fracture and from areas of relatively loose foundation. The choice of -building materials is next of importance. Those buildings which succumb -to earthquakes are in most cases racked or shaken apart, and thus they -become a prey to their own inherent properties of inertia. Each part of -a structure may be regarded as a weight which is balanced upon a stiff -rod and pivoted upon the ground. When shocks arrive, each part tends -to be thrown into vibration after the manner of an inverted pendulum. -In proportion, therefore, as the weights are large and rest upon long -supports, the danger of overthrow and of tearing apart is increased. -In general, structures are best constructed of light materials whose -weight is concentrated near the ground. Masonry structures, and -especially high ones, are, therefore, the least suited for resisting -earthquakes, of which the late complete destruction of the city of -Messina is a grewsome reminder. Despite repeated warnings in the past, -the buildings of that stricken city were generally constructed of heavy -rubble, which in addition had been poorly cemented (Fig. 49, p. 67). -Such structures are usually first ruptured at the edges and corners, -since here the vibrations which tend to tear the building asunder are -resisted by no supports and are reënforced from neighboring walls. - -[Illustration: - -FIG. 85.—House wrecked in San Francisco earthquake of 1906 because the -floors and partitions were not securely fastened to the walls (after R. -L. Humphrey).] - -An advantage of the first importance is evidently secured if the rods -of the pendulum, of which the building is conceived to be composed, -have sufficient elasticity to be considerably distorted without -rupture and to again recover their original position. This is the -supreme advantage of structural steel for all large buildings, which -is coupled, however, with the disadvantage that the riveted fastenings -are apt to be quickly sheered off under the vibrations. Large and high -buildings, when sufficiently elastic, have fortunately the property of -destroying the earth waves by interference before they have traveled -above the lower stories. - -For large structures in which wood cannot be used, strongly reënforced -concrete is well adapted, for it has in general the same advantages -as steel with somewhat reduced elasticity, but with a more effective -binding together of the parts. This requirement of thorough bracing -and tying together of the several parts of a building causes it to -vibrate, not as many pendulums, but as one body. If met, it removes -largely the danger from racking strains, and for small structures -particularly it is the requirement which is most easily complied with. -For such buildings it is therefore necessary that the framework should -be built in a close network with every joint firmly braced and with -all parts securely tied together. Especial attention should be given -to the fastenings of floor and partition ends. The house shown in Fig. -85 could not have been subjected to heavy shocks, for though the walls -are thrown down, the floors and partitions have been left near their -original positions. - -[Illustration: - -FIG. 86.—Building wrecked at San Mateo, California, during the late -earthquake. The heavy roof and upper floor, acting as a unit, have -battered down the upper walls (after J. C. Branner).] - -This tendency of the walls, floors, partitions, and roof to act as -individual units in the vibration, is one that must be reckoned with -and be met by specially effective bracing and tying at the junctions. -Otherwise these larger parts of the structure may act like battering -rams to throw over the walls or portions of them (Fig. 86). - - -READING REFERENCES FOR CHAPTERS VII AND VIII - - General works:— - - JOHN MILNE. Seismology. London, 1898, pp. 320. - - C. E. DUTTON. Earthquakes in the Light of the New Seismology. Putnam, - New York, 1904, pp. 314. - - A. SIEBERG. Handbuch der Erdbebenkunde. Braunschweig, 1904, pp. 362. - - COUNT F. DE MONTESSUS DE BALLORE. Les Tremblements de Terre, - Géographie Séismologique. Paris, 1906, pp. 475; La Science - Séismologique. Paris, 1907, pp. 579. - - WILLIAM HERBERT HOBBS. Earthquakes, an Introduction to Seismic - Geology. Appleton, New York, 1907, pp. 336. - - C. G. KNOTT. The Physics of Earthquake Phenomena. Clarendon Press, - Oxford, 1908, pp. 283. - - E. RUDOLPH. Ueber Submarine Erdbeben und Eruptionen, Beiträge zur - Geophysik, vol. 1, 1887, pp. 133-365; vol. 2, 1895, pp. 537-666; vol. - 3, 1898, pp. 273-536. - -Descriptive reports of some important earthquakes:— - - C. E. DUTTON. The Charleston Earthquake of August 31, 1886, 9th Ann. - Rept. U. S. Geol. Surv., 1889, pp. 203-528. - - B. KOTÔ. On the Cause of the Great Earthquake in Central Japan, 1891, - Jour. Coll. Sci. Imp. Univ., Tokyo, Japan, vol. 5, 1893, pp. 295-353, - pls. 28-35. - - JOHN MILNE and W. K. BURTON. The Great Earthquake of Central Japan. - 1891, pp. 10, pls. 30. - - R. D. OLDHAM. Report on the Great Earthquake of 12th June, 1897, Mem. - Geol. Surv. India. Vol. 29, 1899, pp. 379, pls. 42. - - A. C. LAWSON, and others. The California Earthquake of April 18, 1906, - Report of the State Earthquake Investigation Commission, three quarto - vols. (Carnegie Institution of Washington); many plates and figures. - - _Italian Photographic Society_, Messina and Reggio before and after - the Earthquake of December 28, 1908 (an interesting collection of - pictures). Florence, 1909. - - R. S. TARR and L. MARTIN. Recent Changes of Level in the Yakutat Bay - Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, pls. - 12-23. - - WILLIAM HERBERT HOBBS. The Earthquake of 1872 in the Owens Valley, - California, Beiträge zur Geophysik, vol. 10, 1910, pp. 352-385, pls, - 10-23. - -Faults in connection with earthquakes:— - - WILLIAM H. HOBBS. On Some Principles of Seismic Geology, Beiträge zur - Geophysik, vol. 8, 1907, Chapters iv-v. - -Expansion or contraction of the earth’s surface during earthquakes:— - - WILLIAM H. HOBBS. A Study of the Damage to Bridges during Earthquakes, - Jour. Geol., vol. 16, 1908, pp. 636-653; The Evolution and the Outlook - of Seismic Geology, Proc. Am. Phil. Soc., vol. 48, 1909, pp. 27-29. - -Earthquake construction:— - - JOHN MILNE. Construction in Earthquake Countries, Trans. Seis. Soc., - Japan, vol. 14, 1889-1890, pp. 1-246. - - F. DE MONTESSUS DE BALLORE. L’art de bâtir dans les pays à - tremblements de terre (34th Congress of French Architects), - L’Architecture, 193 Année, 1906, pp. 1-31. - - GILBERT, HUMPHREY, SEWELL, and SOULÉ. The San Francisco Earthquake and - Fire of April 18, 1906, and their Effects on Structures and Structural - Materials, Bull. 324, U. S. Geol. Surv., 1907, pp. 1-170, pls. 1-57. - - WILLIAM H. HOBBS. Construction in Earthquake Countries, The - Engineering Magazine, vol. 37, 1909, pp. 1-19. - - LEWIS ALDEN ESTES. Earthquake-proof Construction, a discussion of the - effects of earthquakes on building construction with special reference - to structures of reënforced concrete, published by Trussed Concrete - Steel Company. Detroit, 1911, pp. 46. - - - - -CHAPTER IX - -THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE - -VOLCANIC MOUNTAINS OF EXUDATION - - -=Prevalent misconceptions about volcanoes.=—The more or less common -impression that a volcano is a “burning mountain” or a “smoking -mountain” has been much fostered by the school texts in physical -geography in use during an earlier period. The best introduction -to a discussion of volcanoes is, therefore, a disillusionment from -this notion. Far from being burning or smoking, there is normally -no combustion whatever in connection with a volcanic eruption. The -unsophisticated tourist who, looking out from Naples, sees the steam -cap which overhangs the Vesuvian crater tinged with brown, easily -receives the impression that the material of the cloud is smoke. Even -more at night, when a bright glow is reflected to his eye and soon -fades away, only to again glow brightly after a few moments have -passed, is it difficult to remove the impression that one is watching -an intermittent combustion within the crater. The cloud which floats -away from the crest of the mountain is in reality composed of steam -with which is admixed a larger or smaller proportion of fine rock -powder which gives to the cloud its brownish tone. The glow observed at -night is only a reflection from molten lava within the crater, and the -variation of its brightness is explained by the alternating rise and -fall of the lava surface by a process presently to be explained. - -Not only is there no combustion in connection with volcanic eruptions, -but so far as the volcano is a mountain it is a product of its own -action. The grandest of volcanic eruptions have produced no mountains -whatever, but only vast plains or plateaus of consolidated molten rock, -and every volcanic mountain at some time in its history has risen out -of a relatively level surface. - -When the traditional notions about volcanoes grew up, it was supposed -that the solid earth was merely a “crust” enveloping still molten -material. As has already been pointed out in an earlier chapter, this -view is no longer tenable, for we now know that the condition of matter -within the earth’s interior, while perhaps not directly comparable to -any that is known, yet has properties most resembling known matter in -a solid state; it is much more rigid than the best tool steel. While -there must be reservoirs of molten rock beneath active volcanoes, it -is none the less clear that they are small, local, and temporary. -This is shown by the comparative study of volcanic outlets within any -circumscribed district. - -It is perhaps not easy to frame a definition of a volcano, but its -essential part, instead of being a mountain, is rather a vent or -channel which opens up connection between a subsurface reservoir of -molten rock and the surface of the earth. An eruption occurs whenever -there is a rise of this material, together with more or less steam and -admixed gases, to the surface. Such molten rock arriving at the surface -is designated _lava_. The changes in pressure upon this material during -its elevation induce secondary phenomena as the surface is approached, -and these manifestations are often most awe inspiring. While often -locally destructive, the geological importance of such phenomena is by -reason of their terrifying aspect likely to be greatly exaggerated. - - -=Early views concerning volcanic mountains.=—As already pointed out, -a volcano at its birth is not a mountain at all, but only, so to -speak, a shaft or channel of communication between the surface and a -subterranean reservoir of molten rock. By bringing this melted rock to -the surface there is built up a local elevation which may be designated -a mountain, except where the volume of the material is so large and is -spread to such distances as to produce a plain (see fissure eruptions -below). - -In the early history of geology it was the view of the great German -geologist von Buch and his friend and colleague von Humboldt, that a -volcanic mountain was produced in much the same manner as is a blister -upon the body. The fluids which push up the cuticle in the blister were -here replaced by fluid rock which elevated the sedimentary rock layers -at the surface into a dome or mound which was open at the top—the -so-called _crater_. This “elevation-crater” theory of volcanoes long -held the stage in geological science, although it ignored the very -patent fact that the layers on the flanks of volcanic cones are not -of sedimentary rock at all, but, on the contrary, of the volcanic -materials which are brought up to the surface during the eruption. -The observational phase of science was, however, dawning, and the -English geologists Scrope and Lyell were able to show by study of -volcanic mountains that the mound about the volcanic vent was due to -the accumulation of once molten rock which had been either exuded or -ejected. Making use of data derived from New Zealand, Scrope showed -that, instead of being elevated during the formation of a volcanic -mountain, the sedimentary strata of the vicinity may be depressed near -the volcanic vent (Fig. 87). - -[Illustration: - -FIG. 87.—Breached volcanic cone near Auckland, New Zealand, showing -the bending down of the sedimentary strata in the neighborhood of the -vent (after Heaphy and Scrope).] - - -=The birth of volcanoes.=—To confirm the impression that the formation -of the volcanic mountain is in reality a secondary phenomenon -connected with eruptions, we may cite the observed birth of a number -of volcanoes. On the 20th of September, 1538, a new volcano, since -known as Monte Nuovo (new mountain), rose on the border of the ancient -Lake Lucrinus to the westward of Naples. This small mountain attained -a height of 440 feet, and is still to be seen on the shore of the bay -of Naples. From Mexico have been recorded the births of several new -volcanoes: Jorullo in 1759, Pochutla in 1870, and in 1881 a new volcano -in the Ajusco Mountains about midway between the Gulf of Mexico and the -Pacific Ocean. The latest of new volcanoes is that raised in Japan on -November 9, 1910, in connection with the eruption of Usu-san. This “New -Mountain” reached an elevation of 690 feet. - -[Illustration: - -FIG. 88.—View of the new Camiguin volcano from the sea. It was formed -in 1871 over a nearly level plain. The town of Catarman appears at the -right near the shore (after an unpublished photograph by Professor Dean -C. Worcester).] - -As described by von Humboldt, Jorullo rose in the night of the 28th -of September, 1759, from a fissure which opened in a broad plain at -a point 35 miles distant from any then existing volcano. The most -remarkable of new volcanoes rose in 1871 on the island of Camiguin -northward from Mindanao in the Philippine archipelago. This mountain -was visited by the _Challenger_ expedition in 1875, and was first -ascended and studied thirty years later by a party under the leadership -of Professor Dean C. Worcester, the Secretary of the Interior of the -Philippine Islands, to whom the writer is indebted for this description -and the accompanying illustration of this largest and most interesting -of new-born volcanoes. As in the case of Jorullo, the eruption began -with the formation of a fissure in a level plain, some 400 yards -distant from the town of Catarman (Fig. 88). The eruption continued -for four years, at the end of which time the height of the summit was -estimated by the _Challenger_ expedition to be 1900 feet. At the time -of the first ascent in 1905, the height was determined by aneroid as -1750 feet, with sharp rock pinnacles projecting some 50 or 75 feet -higher. - - -=Active and extinct volcanoes.=—The terms “active” and “extinct” -have come into more or less common use to describe respectively those -volcanoes which show signs of eruptive activity, and those which are -not at the time active. The term “dormant” is applied to volcanoes -recently active and supposed to be in a doubtfully extinct condition. -From a well-known volcano in the vicinity of Naples, volcanoes which no -longer erupt lava or cinder, but show gaseous emanations (_fumeroles_) -are said to be in the _solfatara_ condition, or to show _solfataric_ -activity. - -Experience shows that the term “extinct”, while useful, must always be -interpreted to mean apparently extinct. This may be illustrated by the -history of Mount Vesuvius, which before the Christian era was forested -in the crater and showed no signs of activity; and in fact it is -known that for several centuries no eruption of the volcano had taken -place. Following a premonitory earthquake felt in the year 63, the -mountain burst out in grand explosive eruption in 79 A.D. This eruption -profoundly altered the aspect of the mountain and buried the cities -of Pompeii, Stabeii, and Herculaneum from sight. Once more, this time -during the middle ages, for nearly five centuries (1139 to 1631) there -was complete inactivity, if we except a light ash eruption in the year -1500. During this period of rest the crater was again forested, but the -repose was suddenly terminated by one of the grandest eruptions in the -mountain’s history. - - -[Illustration: FIG. 89.—Map showing the location of the belts of -active volcanoes.] - -=The earth’s volcano belts.=—The distribution of volcanoes is not -uniform, but, on the contrary, volcanic vents appear in definite zones -or belts, either upon the margins of the continents or included within -the oceanic areas (Fig. 89). The most important of these belts girdles -the Pacific Ocean, and is represented either by chains or by more -widely spaced volcanic mountains throughout the Cordilleran Mountain -system of South and Central America and Mexico, by the volcanoes of the -Coast and Cascade ranges of North America, the festooned volcanic chain -of the Aleutian Islands, and the similar island arcs off the eastern -coast of the Eurasian continent. The belt is further continued through -the islands of Malaysia to New Zealand, and on the Pacific’s southern -margin are found the volcanoes of Victoria Land, King Edward Land, and -West Antarctica. - -[Illustration: FIG. 90.—A portion of the “fire girdle” of the Pacific, -showing the relation of the chains of volcanic mountains to the deeps -of the neighboring ocean floor.] - -This volcano girdle is by no means a perfect one, for in addition -to the principal festoons of the western border there are many -secondary ones, and still other arcs are found well toward the center -of the oceanic area. Another broad belt of volcanoes borders the -Mediterranean Sea, and is extended westward into the Atlantic Ocean. -Narrower belts are found in both the northern and southern portions -of the Atlantic Ocean, on the margins of the Caribbean Sea, etc. The -fact of greatest significance in the distribution seems to be that -bands of active volcanoes are to be found wherever mountain ranges are -paralleled by deeps on the neighboring ocean floor (Fig. 90). As has -been already pointed out in the chapter upon earthquakes, it is just -such places as these which are the seat of earthquakes; these are zones -of the earth’s crust which are undergoing the most rapid changes of -level at the present time. Thus the rise of the land in mountains is -proceeding simultaneously with the sinking of the sea floor to form the -neighboring deeps. - -[Illustration: FIG. 91.—Volcanic cones formed in 1783 above the -Skaptár fissure in Iceland (after Helland).] - - -[Illustration: - -FIG. 92.—Diagrams to illustrate the location of volcanic vents upon -fissure lines, _a_, openings caused by lateral movement of fissure -walls; _b_, openings formed at fissure intersections.] - -=Arrangement of volcanic vents along fissures and especially at their -intersections.=—Within those districts in which volcanoes are widely -separated from their neighbors, the law of their arrangement is -difficult to decipher, but the view that volcanic vents are aligned -over fissures is now supported by so much evidence that illustrations -may be supplied from many regions. An exceptionally perfect line of -small cones is found along the Skaptár cleft in Iceland, upon which -stands the large volcano of Laki. This fissure reopened in 1783, and -great volumes of lava were exuded. Over the cleft there was left a long -line of volcanic cones (Fig. 91). There are in Iceland two dominating -series of parallel fissures of the same character which take their -directions respectively northeast-southwest and north-south. Many such -fissures are traceable at the surface as deep and nearly straight -clefts or _gjás_, usually a few yards in width, but extending for many -miles. The Eldgjá has a length of more than 18 English miles and a -depth varying from 400 to 600 feet. On some of these fissures no lava -has risen to the surface, whereas others have at numerous points exuded -molten rock. Sometimes one end only of a fissure, the more widely -gaping portion, has supplied the conduits for the molten lava. This -is well illustrated by the cratered monticules raised by the common -ant over the cracks which separate the blocks of cement sidewalk, the -hillocks being located where the most favorable channel was found for -the elevation of the materials. - -[Illustration: - -FIG. 93.—Outline map of the eastern portion of the island of Java, -displaying the arrangement of volcanic vents in alignment upon fissures -with the larger mountains at fissure intersections (after Verbeek).] - -Those places upon fissures which become lava conduits appear to be the -ones where the cleft gapes widest so as to furnish the widest channel. -Wherever a differential lateral movement of the walls has occurred, -openings will be found in the neighborhood of each minor variation from -a straight line (Fig. 92_a_). Wherever there are two or more series -of fissures, and this would appear to be the normal condition, places -favorable for lava conduits occur at fissure intersections. Within such -veritable volcano gardens as are to be found in Malaysia, the law of -volcano distribution became apparent so soon as accurate maps had been -prepared. Thus the outline map of a portion of the island of Java (Fig. -93) shows us that while the volcanoes of the island present at first -sight a more or less irregular band or zone, there are a number of -fissures intersecting in a network, and that the volcanoes are aligned -upon the fissures with the larger cones located at the intersections. -So also in Iceland, the great eruption of Askja in 1875 occurred at -the intersection of two lines of fissure. - -Outside these closely packed volcanic regions, similar though less -marked networks are indicated; as, for example, in and near the Gulf of -Guinea. If now, instead of reducing the scale of our volcano maps, we -increase it, the same law of distribution is no less clearly brought -out. The monticules or small volcanic cones which form upon the flanks -of larger volcanic mountains are likewise built up over fissures which -on numerous occasions have been observed to open and the cones to form -upon them. - -[Illustration: - -FIG. 94.—Map of the Puy Pariou in the Auvergne of central France. The -seat of eruption has migrated along the fissure upon which the earlier -cone had been built up (after Scrope).] - -Still further reducing now the area of our studies and considering for -the moment the “frozen” surface of the boiling lava within the caldron -of Kilauea, this when observed at night reveals in great perfection -the sudden formation of fissures in the crust with the appearance -of miniature volcanoes rising successively at more or less regular -intervals along them. - -It not infrequently happens that after a volcanic vent has become -established above some conduit in a fissure, the conduit migrates along -the fissure, thus establishing a new cone with more or less complete -destruction of the old one (Fig. 94). - - -=The so-called fissure eruptions.=—The grandest of all volcanic -eruptions have been those in which the entire length and breadth of -the fissures have been the passageway for the upwelling lava. Such -grander eruptions have been for the most part prehistoric, and in later -geologic history have occurred chiefly in India, in Abyssinia, in -northwestern Europe, and in the northwestern United States. In western -India the singularly horizontal plateaus of basaltic lava, the Dekkan -traps, cover some 200,000 square miles and are more than a mile in -depth. The underlying basement where it appears about the margins of -the basalt is in many places intersected by dikes or fissure fillings -of the same material. No cones or definite vents have been found. - -[Illustration: - -FIG. 95.—Basaltic plateau of the northwestern United States due to -fissure eruptions of lava.] - -The larger portion of the northwestern British Isles would appear to -have been at one time similarly blanketed by nearly horizontal beds of -basaltic lava, which beds extended northwestward across the sea through -the Orkney and Faroe islands to Iceland. Remnants of this vast plateau -are to-day found in all the island groups as well as in large areas of -northeastern Ireland, and fissure fillings of the same material occur -throughout large areas of the British Isles. In many cases these dikes -represent once molten rock which may never have communicated with the -surface at the time of the lava outpouring, yet they well illustrate -what we might expect to find if the basalt sheets of Iceland or Ireland -were to be removed. - -The floods of basaltic lava which in the northwestern United States -have yielded the barren plateau of the Cascade Mountains (Fig. 95) -would appear to offer another example of fissure eruption, though cones -appear upon the surface and perhaps indicate the position of lava -outlets during the later phases of the eruptive period. The barrenness -and desolation of these lava plains is suggested by Fig. 96. - -[Illustration: FIG. 96.—Lava plains about the Snake River in Idaho.] - -Though the greater effusions of lava have occurred in prehistoric -times, and the manner of extrusion has necessarily been largely -inferred from the immense volume of the exuded materials and the -existence of basaltic dikes in neighboring regions, yet in Iceland -we are able to observe the connection between the dikes and the lava -outflows. Professor Thoroddsen has stated that in the great basaltic -plateau of Iceland, lava has welled out quietly from the whole length -of fissures and often on both sides without giving rise to the -formation of cones. At three wider portions of the great Eld cleft, -lava welled out quietly without the formation of cones, though here in -the southern prolongation of the fissure, where it was narrower, a row -of low slag cones appeared. Where the lava outwellings occurred, an -area of 270 square miles was flooded. - -[Illustration: FIG. 97.—Characteristic profiles of lava volcanoes. 1, -basaltic lava mountain; 2, mountain of siliceous lava (after Judd).] - - -=The composition and the properties of lava.=—In our study of igneous -rocks (Chapter IV) it was learned that they are composed for the most -part of silicate minerals, and that in their chemical composition they -represent various proportions of silica, alumina, iron, magnesia, -lime, potash, and soda. The more abundant of these constituents is -silica, which varies from 35 to 70 per cent of the whole. Whenever the -content of silica is relatively low,—basic or basaltic lava,—the -cooled rock is dark in color and relatively heavy. It melts at a -relatively low temperature, and is in consequence relatively fluid -at the temperatures which lavas usually have on reaching the earth’s -surface. Furthermore, from being more fluid, the water which is nearly -always present in large quantity within the lava more readily makes its -escape upon reaching the surface. Eruptions of such lava are for this -reason without the violent aspects which belong to extrusions of more -siliceous (more “acidic”) lavas. For the same reason, also, basaltic -lava flows more freely and can spread much farther before it has -cooled sufficiently to consolidate. This is equivalent to saying that -its surface will assume a flatter angle of slope, which in the case -of basaltic lava seldom exceeds ten degrees and may be less than one -degree (Fig. 97). - -[Illustration: FIG. 98.—A driblet cone (after J. D. Dana).] - -Siliceous lavas, on the other hand, are, when consolidated, -relatively light both in color and weight and melt at relatively high -temperatures. They are, therefore, usually but partly fused and of a -viscous consistency when they arrive at the earth’s surface. Because -of this viscosity they offer much resistance to the liberation of the -contained water, which therefore is released only to the accompaniment -of more or less violent explosions. The lava is blown into the air -and usually falls as consolidated fragments of various degrees of -coarseness. - -[Illustration: - -FIG. 99.—View of Leffingwell crater, a cinder cone in the Owens -valley, California (after an unpublished photograph by W. D. Johnson).] - -It must not, however, be assumed that the temperature of lava is always -the same when it arrives at the surface, and hence it may happen that -a siliceous lava is exuded at so high a temperature that it behaves -like a normal basaltic lava. On the other hand, basaltic lavas may be -extruded at unusually low temperatures, in which case their behavior -may resemble that of the normal siliceous lavas. If, however, as is -generally the case, the energy of explosion of a basaltic lava is -relatively small, any ejected portions of the liquid lava travel to -a moderate height only in the air, so that on falling they are still -sufficiently pasty to adhere to rock surfaces and thus build up the -remarkably steep cones and spines known as “spatter cones” or “driblet -cones” (Fig. 98). When, on the other hand, the energy of explosion is -great, as is normally the case with siliceous lavas, the portions of -ejected lava have been fully consolidated before their fall to the -surface, so that they build up the same type of accumulation as would -sand falling in the same manner. The structures which they form are -known as tuff, cinder, or ash cones (Fig. 99). - -Whenever the contained water passes off from siliceous lavas without -violent explosions, the lava may flow from the vent, but in contrast to -basaltic lavas it travels a short distance only before consolidating. -The resulting mountain is in consequence proportionately high and steep -(Fig. 97). Eruptions characterized by violent explosions accompanied by -a fall of cinder are described as _explosive_ eruptions. Those which -are relatively quiet, and in which the chief product is in the form of -streams of flowing lava, are spoken of as _convulsive_ eruptions. - - -=The three main types of volcanic mountain.=—If the eruptions at a -volcanic vent are exclusively of the explosive type, the material -of the mountain which results is throughout tuff or cinder, and the -volcano is described as a _cinder cone_. If, on the other hand, the -vent at every eruption exudes lava, a mountain of solid rock results -which is a _lava dome_. It is, however, the exception for a volcano -which has a long history to manifest but a single kind of eruption. At -one time exuding lava comparatively quietly, at another the violence -with which the steam is liberated yields only cinder, and the mountain -is a composite of the two materials and is known as a _composite -volcanic cone_. - - -=The lava dome.=—When successive lava flows come from a crater, the -structure which results has the form of a more or less perfect dome. If -the lava be of the basaltic or fluid type, the slopes are flat, seldom -making an angle of as much as ten degrees with the horizon and flatter -toward the summit (Fig. 101, p. 106). If of siliceous or viscous lava, -on the other hand, the slopes are correspondingly steep and in some -cases precipitous. To this latter class belong some of the _Kuppen_ of -Germany, the _puys_ of central France, and the _mamelons_ of the Island -of Bourbon. - - -[Illustration: - -FIG. 100.—Map of Hawaii and the lava volcanoes of Mokuaweoweo (Mauna -Loa) and Kilauea (after the government map by Alexander).] - -=The basaltic lava domes of Hawaii.=—At the “crossroads of the -Pacific” rises a double line of lava volcanoes which reach from 20,000 -to 30,000 feet above the floor of the ocean, some of them among -the grandest volcanic mountains that are known. More than half the -height and a much larger proportion of the bulk of the largest of -these are hidden beneath the ocean’s surface. The two great active -vents are Mokuaweoweo (on Mauna Loa) and Kilauea, distinct volcanoes -notwithstanding the fact that their lava extravasations have been -merged in a single mass. The rim of the crater of Mauna Loa is at an -elevation of 13,675 feet above the sea, whereas that of Kilauea is -less than 4000 feet and appears to rest upon the flank of the larger -mountain (Figs. 100 and 101). Although one crater is but 20 miles -distant from the other and nearly 10,000 feet lower, their eruptions -have apparently been unsympathetic. Nowhere have still active lava -mountains been subjected to such frequent observations extending -throughout a long period, and the dynamics of their eruptions are -fairly well understood. To put this before the reader, it will be -best to consider both mountains, for though they have much in common, -the observations from one are strangely complementary to those of the -other. The lower crater being easily accessible, Kilauea has been often -visited, and there exists a long series of more or less consecutive -observations upon it, which have been assembled and studied by Dana and -Hitchcock. The place of outflow of the Kilauea lavas has not generally -been visible, whereas Mokuaweoweo has slopes rising nearly 14,000 feet -above the sea and displays the records of outflow of many eruptions, -some of which were accompanied by the grandest of volcanic phenomena. - -[Illustration: FIG. 101.—Section through Mauna Loa and Kilauea.] - - -=Lava movements within the caldron of Kilauea.=—The craters of these -mountains are the largest of active ones, each being in excess of -seven miles in circumference. In shape they are irregularly elliptical -and consist of a series of steps or terraces descending to a pit at the -bottom, in which are open lakes of boiling lava. Enough is known of the -history of Kilauea to state that the steep cliffs bounding the terraces -are fault walls produced by inbreak of a frozen lava surface. The cliff -below the so-called “black ledge” was produced by the falling in of -the frozen lava surface at the time of the outflow of 1840, the lava -issuing upon the eastern flank of the mountain and pouring into the sea -near Nanawale. Since that date the floor of the pit below the level of -this ledge has been essentially a movable platform of frozen lava of -unknown and doubtless variable thickness which has risen and descended -like the floor of an elevator car between its guiding ways (Fig. 102). -The floor has, however, never been complete, for one or more open lakes -are always to be seen, that of Halemaumau located near the southwestern -margin having been much the most persistent. Within the open lakes the -boiling lava is apparently white hot at the depth of but a few inches -below the surface, and in the overturnings of the mass these hotter -portions are brought to the surface and appear as white streaks marking -the redder surface portions. From time to time the surface freezes -over, then cracks open and erupt at favored points along the fissures, -sending up jets and fountains of lava, the material of which falls in -pasty fragments that build up driblet cones. Small fluid clots are -shot out, carrying a threadlike line of lava glass behind them, the -well-known “Pelé’s hair.” Sometimes the open lakes build up congealed -walls, rising above the general level of the pit, and from their rim -the lava spills over in cascades to spread out upon the frozen floor, -thus increasing its thickness from above (Fig. 103). At other times -a great dome of lava has been pushed up from the pit of Halemaumau -under a frozen shell, the molten lava shining red through cracks in -its surface and exuding so as to heal each widely opened fissure as it -forms. - -[Illustration: - -FIG. 102.—Schematic diagram to illustrate the moving platform of -frozen lava which rises and falls in the crater of Kilauea.] - -At intervals of from a few years to nine or ten years the crater has -been periodically drained, at which times the moving platform of -frozen lava has sunk more or less rapidly to levels far below the black -ledge and from 900 to 1700 feet below the crater rim. Following this -descent a slow progressive rise is inaugurated, which has sometimes -gone on at a rate of more than a hundred feet per year, though it is -usually much slower than this. When the platform has reached a height -varying from 700 to 350 feet below the crater rim, another sudden -settlement occurs which again carries the pit floor downward a distance -of from 300 to 700 feet. - -[Illustration: - -FIG. 103.—View of the open lava lake of Halemaumau within the crater -of Kilauea, the molten lava shown cascading over the raised lava walls -on to the floor of the pit (after Pavlow).] - - -=The draining of the lava caldrons.=—The changes which go on within -the crater of Mokuaweoweo, though less studied than those of Kilauea, -appear to be in some respects different. Here every eruption seems -to be preceded by a more or less rapid influx of melted lava to the -pit of the crater, this phenomenon being observed from a distance as -a brilliant light above the crater—the reflection of the glow from -overhanging vapor clouds. The uprising of the lava has often been -accompanied by the formation of high lava fountains upon the surface, -and the molten lava sometimes appears in fissures near the crater rim -at levels well above the lava surface within the pit. - -Although in many cases the lava which has thus flooded the crater has -suddenly drained away without again becoming visible, it is probable -that in such cases an outlet has been found to some submarine exit, -since under-ocean discharge effects have been observed in connection -with eruptions of each of the volcanoes. - -[Illustration: - -FIG. 104.—Map showing the manner of outflow of lava from Kilauea -during the eruption of 1840. The outflowing lava made its appearance -successively at the points _A_, _B_, _C_, _m_, _n_, and finally at a -point below _n_, from whence it issued in volume and flowed down to the -sea at Nanawale (after J. D. Dana).] - -Inasmuch as no earthquakes are felt in connection with such outflows -as have been described, it is probable that the hot lava fuses a -passageway for itself into some open channel underneath the flanks -of the mountain. Such a course is well illustrated by the outflow -of Kilauea in 1840, when, it will be remembered, occurred the great -down-plunge of the crater that yielded the pit below the black ledge. -At this time the lava first made its appearance upon the flanks of the -mountain at the bottom of a small pit or inbreak crater which opened -five miles southeast of the main crater of Kilauea (Fig. 104). Within -this new crater the lava rose, and small ejections soon followed from -fissures formed in its neighborhood. Some time after, the lava sank -in the first new crater, only to reappear successively at other small -openings (Fig. 104, _B_, _C_, _m_, _n_) and finally to issue in volume -at a point eleven miles from the shore and flow thereafter _upon -the surface_ of the mountain until it had reached the sea. Only the -slightest earth tremors were felt, and as no rumblings were heard, it -is evident that the lava fused its way along a buried channel largely -open at the time (see below, p. 112). - -In a majority of the eruptions of Mokuaweoweo, when the outflowing -lavas have become visible, the molten rock has apparently fused its way -out to the surface of the mountain at points from 1000 to 3000 feet -below the bottom of the crater, and this discharge has corresponded -in time to the lowering of the lava surface within the crater. There -are, however, three instances upon record in which the lava issued -from definite rents which were formed upon the mountain flanks at -comparatively low levels. In contrast to the formation of fused -outlets, these ruptures of a portion of the mountain’s flank were -always accompanied by vigorous local earthquakes of short duration. In -one instance (the eruption of 1851) such a rent appeared under the same -conditions but at an elevation of 12,500 feet, or near the level of the -lava in the crater. - - -=The outflow of the lava floods.=—In order to properly comprehend -these and many otherwise puzzling phenomena connected with -volcanoes, it is necessary to keep ever in mind the quite remarkable -heat-insulating property of congealed lava. So soon as a thin crust -has formed upon the surface of molten rock, the heat of the underlying -fluid mass is given off with extreme slowness, so that lava streams no -longer connected with their internal lava reservoirs may remain molten -for decades. - -[Illustration: - -FIG. 105.—Lava of Matavanu upon the Island of Savaii flowing down to -the sea during the eruption of 1906. The course may be followed by the -jets of steam escaping from the surface down to the great steam cloud -which rises where the fluid lava discharges into the sea (after H. I. -Jensen).] - -We have seen that for Mokuaweoweo and Kilauea, lava either quietly -melts its way to the surface at the time of outflow, or else produces a -rent for its egress to the accompaniment of vigorous local earthquakes. -In either case if the lava issues at a point far below the crater, -gigantic lava fountains arise at the point of outflow, the fluid rock -shooting up to heights which range from 250 to 600 or more feet above -the surface. A certain proportion of this fluid lava is sufficiently -cooled to consolidate while traveling in the air, and falling, it -builds up a cinder cone which is left as a location monument for -the place of discharge. From this outlet the molten lava begins its -journey down the slope of the mountain, and quickly freezes over to -produce a tunnel, beneath the roof of which the fluid lava flows with -comparatively slow further loss of heat. Save for occasional steam jets -issuing from its surface, it may give little indication of its presence -until it has reached the sea (Fig. 105). - -[Illustration: FIG. 106.—Lava stream discharging into the sea from -beneath the frozen roof of a lava tunnel. Eruption of Matavanu on -Savaii in 1906 (after Sapper).] - -If sufficient in volume and the shore be not too distant, the stream -of lava arrives at the sea, where, discharging from the mouth of its -tunnel, it throws up vast volumes of steam and induces ebullition of -the water over a wide area (Fig. 106). Professor Dana, who visited -Hawaii a few months only after the great outflow of 1840, states that -the lava, upon reaching the ocean, was shivered like melted glass and -thrown up in millions of particles which darkened the sky and fell like -hail over the surrounding country. The light was so bright that at a -distance of forty miles fine print could be read at midnight. - -[Illustration: - -FIG. 107.—Diagrammatic representation of the structure of the flanks -of lava volcanoes as a result of the draining of frozen lava streams.] - -Protected from any extensive consolidation by its congealed cover, the -lava within a stream may all drain away, leaving behind an empty lava -tunnel, which in the case of the Hawaiian volcanoes sometimes has its -roof hung with beautiful lava stalactites and its floor studded with -thin lava spines. Later lava outflows over the same or neighboring -courses bury such tunnels beneath others of similar nature, giving -to the mountain flanks an elongated cellular structure illustrated -schematically in Fig. 107. These buried channels may in the future be -again utilized for outflows similar in character to that of Kilauea in -1840. - -[Illustration: - -FIG. 108.—Diagram to show the manner of formation of mesas or table -mountains by the outflow of lava in valleys and the subsequent more -rapid erosion of the intervening ridges. _R_, earlier river valley; -_R’R’_, later valleys.] - -While the formation of lava stalactites of such perfection and beauty -is peculiar to the Hawaiian lava tunnels, the formation of the tunnel -in connection with lava outflow is the rule wherever a dissipation at -the end has permitted of drainage. A few hours only after the flow has -begun, the frozen surface has usually a thickness of a few inches, and -this cover may be walked over with the lava still molten below. At -first in part supported by the molten lava, the tunnel roof sometimes -caves in so soon as drainage has occurred. - -[Illustration: FIG. 109.—Surface of lava of the Pahoehoe type.] - -Wherever basaltic lava has spread out in valleys on the surface of -more easily eroded material, either cinder or sedimentary formations, -the softer intervening ridges are first carried away by the eroding -agencies, leaving the lava as cappings upon residual elevations. -Thus are derived a type of table mountain or _mesa_ of the sort well -illustrated upon the western slopes of the Sierra Nevadas in California -(Fig. 108). - -[Illustration: - -FIG. 110.—Three successive views to illustrate the growth of the -Island of Savaii from the outflow of lava at Matavanu in the year 1906. -_a_, near the beginning of the outflow; _b_, some weeks later than _a_; -_c_, some weeks later than _b_ (after H. I. Jensen).] - -The surface which flowing lava assumes, while subject to considerable -variation, may yet be classified into two rather distinct types. On -the one hand there is the billowy surface in which ellipsoidal or -kidney-shaped masses, each with dimensions of from one to several feet, -lie merged in one another, not unlike an irregular collection of sofa -pillows. This type of lava has become known as the _Pahoehoe_, from -the Hawaiian occurrence (Fig. 109). A variation from this type is the -“corded” or “ropy” lava, the surface of which much resembles rope as it -is coiled along the deck of a vessel, the coils being here the lines of -scum or scoriæ arranged in this manner by the currents at the surface -of the stream (Fig. 123, p. 124). A quite different type is the block -lava (_Aa_ type) which usually has a ragged scoriaceous surface and -consists of more or less separate fragments of cooled lava (Fig. 131, -p. 130). - -Wherever lava flows into the sea in quantity, it extends the margin of -the shore, often by considerable areas. The outflow of Kilauea in 1840 -extended the shore of Hawaii outward for the distance of a quarter of a -mile, and a more recent illustration of such extension of land masses -is furnished by Fig. 110. - - - - -CHAPTER X - -THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE - -VOLCANIC MOUNTAINS OF EJECTED MATERIALS - - -=The mechanics of crater explosions.=—If we now turn from the lava -volcano to the active cinder cone, we encounter an entire change of -scene. In place of the quiet flow and convulsive movements of the -molten lava, we here meet with repeated explosions of greater or less -violence. If we are to profitably study the manner of the explosions, -considering the volcanic vent as a great experimental apparatus, it -would be well to select for our purpose a volcano which is in a not too -violent mood. The well-known cinder cone of Stromboli in the Eolian -group of islands north of Sicily has, with short and unimportant -interruptions, remained in a state of light explosive activity since -the beginning of the Christian era. Rising as it does some three -thousand feet directly out of the Mediterranean, and displaying by day -a white steam cap and an intermittent glow by night, its summit can be -seen for a distance of a hundred miles at sea and it has justly been -called the “Lighthouse of the Mediterranean.” The “flash” interval -of this beacon may vary from one to twenty minutes, and it may show, -furthermore, considerable variation of intensity. - -For the reason that the crater of the mountain is located at one side -and at a considerable distance below the actual summit, the opportunity -here afforded of looking into the crater is most favorable whenever -the direction of the wind is such as to push aside the overhanging -steam cloud (Fig. 111). Long ago the Italian vulcanologist Spallanzani -undertook to make observations from above the crater, and many others -since his day have profited by his example. - -Within the crater of the volcano there is seen a lava surface lightly -frozen over and traversed by many cracks from which vapor jets -are issuing. Here, as in the Kilauea crater, there are open pools -of boiling lava. From some of these, lava is seen welling out to -overflow the frozen surface; from others, steam is ejected in puffs -as though from the stack of a locomotive. Within others lava is seen -heaving up and down in violent ebullition, and at intervals a great -bubble of steam is ejected with explosive violence, carrying up with -it a considerable quantity of the still molten lava, together with -its scumlike surface, to fall outside the crater and rattle down -the mountain’s slope into the sea. Following this explosion the -lava surface in the pool is lowered and the agitation is renewed, -to culminate after the further lapse of a few minutes in a second -explosion of the same nature. The rise of the lava which precedes the -ejection appears at night as a brighter reflection or glow from the -overhanging steam cloud—the flash seen by the mariner from his vessel. - -[Illustration: - -FIG. 111.—The volcano of Stromboli, showing the excentric position of -the crater (after a sketch by Judd).] - -What is going on within the crater of Stromboli we may perhaps best -illustrate by the boiling of a stiff porridge over a hot fire. Any one -who has made corn mush over a hot camp fire is fully aware that in -proportion as the mush becomes thicker by the addition of the meal, it -is necessary to stir the mass with redoubled vigor if anything is to be -retained within the kettle. The thickening of the mush increases its -viscosity to such an extent that the steam which is generated within it -is unable to make its escape unless aided by openings continually made -for it by the stirring spoon. If the stirring motion be stopped for a -moment, the steam expands to form great bubbles which soon eject the -pasty mass from the kettle. - -For the crater of Stromboli this process is illustrated by the series -of diagrams in Fig. 112. As the lava rises toward the surface, -presumably as a result of convectional currents within the chimney of -the volcano, the contained steam is relieved from pressure, so that -at some depth below the surface it begins to separate out in minute -vesicles or bubbles, which, expanding as they rise, acquire a rapidly -accelerating velocity. Soon they flow together with a quite sudden -increase of their expansive energy, and now shooting upward with -further accelerated velocity, a layer of liquid lava with its cover of -scum is raised on the surface of a gigantic bubble and thrown high into -the air. Cooled during their flight, the quickly congealed lava masses -become the tuff or volcanic ash which is the material of the cinder -cone. - -[Illustration: FIG. 112.—Diagrams to illustrate the nature of -eruptions within the crater of Stromboli.] - - -=Grander volcanic eruptions of cinder cones.=—Most cinder and -composite cones, in the intervals between their grander eruptions, if -not entirely quiescent, lapse into a period, of light activity during -which their crater eruptions appear to be in all essential respects -like the habitual explosions within the Strombolian crater. This phase -of activity is, therefore, described as _Strombolian_. By contrast, -the occasional grander eruptions which have punctuated the history -of all larger volcanoes are described in the language of Mercalli as -_Vulcanian_ eruptions, from the best studied example. - -Just what it is that at intervals brings on the grander Vulcanian -outburst within a volcano is not known with certainty; but it is -important to note that there is an approach to periodicity in the -grander eruptions. It is generally possible to distinguish eruptions of -at least two orders of intensity greater than the Strombolian phase; -a grander one, the examples of which may be separated by centuries, -and one or more orders of relatively moderate intensity which recur -at intervals perhaps of decades, their time intervals subdividing the -larger periods marked off by the eruptions of the first order. - -[Illustration: - -FIG. 113.—Map of Volcano in the Eolian group of islands. The smaller -craters partially dissected by the waves belong to Vulcanello (after -Judd).] - - -=The eruption of Volcano in 1888.=—In the Eolian Islands to the north -of Sicily was located the mythical forge of Vulcan. From this locality -has come our word “volcano”, and both the island and the mountain bear -no other name to-day (Fig. 113). There is in the structure of the -island the record of a somewhat complex volcanic history, but the form -of the large central cinder cone was, according to Scrope, acquired -during the eruption of 1786, at which time the crater is reported to -have vomited ash for a period of fifteen days. Passing after this -eruption into the solfatara condition, with the exception of a light -eruption in 1873, the volcano remained quiet until 1886. So active -had been the fumeroles within the crater during the latter part of -this period that an extensive plant had been established there for -the collection especially of boracic acid. In 1886 occurred a slight -eruption, sufficient to clear out the bottom of the crater, though -not seriously to disturb the English planter whose vineyards and fig -orchards were in the valley or _atrio_ near the point _d_ upon the map -(Fig. 113), nearly a mile from the crater rim. On the 3d of August, -1888, came the opening discharge of an eruption, which, while not of -the first order of magnitude, was yet the greatest in more than a -century of the mountain’s history, and may serve us to illustrate the -Vulcanian phase of activity within a cinder cone. During the day, to -the accompaniment of explosions of considerable violence, projectiles -fell outside the crater rim and rolled down the steep slopes toward the -_atrio_. These explosions were repeated at intervals of from twenty to -thirty minutes, each beginning in a great upward rush of steam and -ash, accompanied by a low rumbling sound. During the following night -the eruptions increased in violence, and the anxious planter remained -on watch in his villa a mile from the crater. Falling asleep toward -morning, he was rudely awakened by a rain of projectiles falling upon -his roof. Hastily snatching up his two children he ran toward the door -just as a red hot projectile, some two feet in diameter, descended -through the roof, ceiling, and floor of the drawing room, setting fire -to the building. A second projectile similar to the first was smashed -into fragments at his feet as he was emerging from the house, burning -one of the children. Making his escape to Vulcanello at the extremity -of the island, the remainder of the night and the following day, until -rescue came from Lipari, were spent just beyond the range of the -falling masses. - -[Illustration: - -FIG. 114.—“Bread-crust” lava projectile from the eruption of Volcano -in 1888 (after Mercalli).] - -When the writer visited the island some months later, the eruption was -still so vigorous that the crater could not be reached. The ruined -villa, smashed and charred, stood with its walls half buried in ash and -lapilli, among which were partly smashed pumiceous lava projectiles. -The entire _atrio_ about the mountain lay buried in cinder to the depth -of several feet and was strewn with projectiles which varied in size -from a man’s fist to several feet in diameter (Fig. 114). The larger of -these exhibited the peculiar “bread-crust” surface and had generally -been smashed by the force of their fall after the manner of a pumpkin -which has been thrown hard against the ground. One of these projectiles -fully three feet in diameter was found at the distance of a mile and -a half from the crater. Though diminished considerably in intensity, -the rhythmic explosions within the crater still recurred at intervals -varying from four minutes to half an hour, and were accompanied -by a dull roar easily heard at Lipari on a neighboring island six -miles away. Simultaneously, a dark cloud of “smoke”, the peculiar -“cauliflower cloud” or _pino_ mounted for a couple of miles above the -crater (Fig. 115), and the rise was succeeded by a rain of small lava -fragments or _lapilli_ outside the crater rim. - -[Illustration: - -FIG. 115.—Peculiar “cauliflower cloud” or _pino_ composed of steam and -ash, rising above the cinder cone of Volcano during the waning phases -of the explosive eruption of 1888 (after a photograph by B. Hobson).] - -There seems to be no good reason to doubt that Vulcanian cinder -eruptions of this type differ chiefly in magnitude from the rhythmic -explosion within the crater of Stromboli, if we except the elevation of -a considerable quantity of accessory and older tuff which is derived -from the inner walls of the crater and carried upward into the air -together with the pasty cakes of fresh lava derived from the chimney. -It is this accessory material which gives to the _pino_ its dark or -even black appearance. - -[Illustration: - -FIG. 116.—Double explosive eruption of Taal volcano on the morning of -January 30, 1911.] - - -=The eruption of Taal volcano on January 30, 1911.=—The recent -eruption of the cinder cone known as Taal volcano is of interest, -not only because so fresh in mind, but because two neighboring vents -erupted simultaneously with explosions of nearly equal violence (Fig. -116). This Philippine volcano lies near the center of a lake some -fifteen miles in diameter and about fifty miles south of the city of -Manila. After a period of rest extending over one hundred and fifty -years, the symptoms of the coming eruption developed rapidly, and on -the morning of January 30 grand explosions of steam and ash occurred -simultaneously in the neighboring craters, and the condensed moisture -brought down the ash in an avalanche of scalding mud which buried the -entire island. Almost the entire population of the island, numbering -several hundreds, was literally buried in the blistering mud (Fig. -117); and the gases from the explosions carried to the distant shores -of the lake added to this number many hundred victims. - -[Illustration: - -FIG. 117.—The thick mud veneer upon the island of Taal (after a -photograph by Deniston).] - -[Illustration: FIG. 118.—A pear-shaped lava projectile.] - -The shocks which accompanied the explosions raised a great wave upon -the surface of the lake, which, advancing upon the shores, washed away -structures for a distance of nearly a half mile. - - -=The materials and the structure of cinder cones.=—Obviously the -materials which compose cinder cones are the cooled lava fragments of -various degrees of coarseness which have been ejected from the crater. -If larger than a finger joint, such fragments are referred to as -_volcanic projectiles_, or, incorrectly, as “volcanic bombs.” Of the -larger masses it is often true that the force of expulsion has not been -applied opposite the center of mass of the body. Thus it follows that -they undergo complex whirling motions during their flight, and being -still semiliquid, they develop curious pear-shaped or less regular -forms (Fig. 118). When crystals have already separated out in the lava -before its rise in the chimney of the volcano, the surrounding fluid -lava may be blown to finely divided volcanic dust which floats away -upon the wind, thus leaving the crystals intact to descend as a crystal -rain about the crater. Such a shower occurred in connection with the -eruption of Etna in 1669, and the black augite crystals may to-day be -gathered by the handful from the slopes of the Monti Rossi (Fig. 125, -p. 125). - -[Illustration: - -FIG. 119.—Artificial production of the structure of a cinder cone with -use of colored sands carried up in alternation by a current of air -(after G. Linck).] - -The term _lapilli_, or sometimes _rapilli_, is applied to the ejected -lava fragments when of the average size of a finger joint. This is the -material which still partially covers the unexhumed portions of the -city of Pompeii. Volcanic _sand_, _ash_, and _dust_ are terms applied -in order to increasingly fine particles of the ejected lava. The -finest material, the volcanic dust, is often carried for hundreds and -sometimes even for thousands of miles from the crater in the high-level -currents of the atmosphere. Inasmuch as this material is deposited far -from the crater and in layers more or less horizontal, such material -plays a small rôle in the formation of the cinder cone. The coarser -sands and ash, on the other hand, are the materials from which the -cinder cone is largely constructed. - -The manner of formation and the structure of cinder cones may be -illustrated by use of a simple laboratory apparatus (Fig. 119). Through -an opening in a board, first white and then colored sand is sent up in -a light current of air or gas supplied from suitable apparatus. The -alternating layers of the sand form in the attitudes shown; that is -to say, dipping inward or toward the chimney of the volcano at all -points within the crater rim, and outward or away from it at all points -outside (Fig. 119). If the experiment is carried so far that at its -termination sand slides down the crater walls into the chimney below, -the inward dipping layers will be truncated, or even removed entirely, -as shown in Fig. 119 _b_. - - -[Illustration: - -FIG. 120.—Diagram to show the contrast between a lava dome and a -cinder cone. _AAA_, cinder cone; _BabC_, lava dome; _DE_, line of low -cinder cones above a fissure (after Thoroddsen).] - -=The profile lines of cinder cones.=—The shapes of cinder cones are -notably different from those of lava mountains. While the latter are -domes, the mountains constructed of cinder are conical and have curves -of profile that are concave upward instead of convex (Fig. 120). In -the earlier stages of its growth the cinder cone has a crater which in -proportion to the height of the mountain is relatively broad (Fig. 99, -p. 104). - -[Illustration: - -FIG. 121.—Mayon volcano on the island of Luzon, P.I. A remarkably -perfect high cinder cone.] - -Speaking broadly, the diameter of the crater is a measure of the -violence of the explosions within the chimney. A single series of short -and violent explosive eruptions builds a low and broad cinder cone. -A long-continued succession of moderately violent explosions, on the -other hand, builds a high cone with crater diameter small if compared -with the mountain’s altitude, and the profile afforded is a remarkably -beautiful sweeping curve (Fig. 121). Toward the summit of such a cone -the loose materials of which it is composed are at as steep an angle as -they can lie, the so-called angle of repose of the material; whereas -lower down the flatter slopes have been determined by the distribution -of the cinder during its fall from the air. When one makes the ascent -of such a mountain, he encounters continually steeper grades, with the -most difficult slope just below the crest. - - -[Illustration: - -FIG. 122.—A series of breached cinder cones where the place of -eruption has migrated along the underlying fissure. The Puys Noir, -Solas, and La Vache in the Mont Dore Province of central France (after -Scrope).] - -=The composite cone.=—The life histories of volcanoes are generally -so varied that lava domes and the pure types of cinder cones are less -common than volcanoes in which paroxysmal eruptions have alternated -with explosions, and where, therefore, the structure of the mountain -represents a composite of lava and cinder. Such composite cones possess -a skeleton of solid rock upon which have been built up alternate -sloping layers of cinder and lava. In most respects such cones stand in -an intermediate position between lava domes and cinder cones. - -[Illustration: - -FIG. 123.—The _bocca_ or mouth upon the inner cone of Mount Vesuvius -from which flowed the lava stream of 1872. This lava stream appears in -the foreground with its characteristic “ropy” surface.] - -Regarded as a retaining wall for the lava which mounts in the chimney, -the cinder cone is obviously the weakest of all. Should lava rise in a -cinder cone without an explosion occurring, the cone is at once broken -through upon one side by the outwelling of the lava near the base. Thus -arises the characteristic _breached_ cone of horseshoe form (Fig. 122). - -[Illustration: - -FIG. 124.—A row of parasitic cones raised above a fissure which was -opened upon the flanks of Mount Etna during the eruption of 1892 (after -De Lorenzo).] - -Quite in contrast with the weak cinder cone is the lava dome with -its rock walls and relatively flat slopes. Considered as a retaining -wall for lava it is much the strongest type of volcanic mountain, and -it is likely that the hydrostatic pressure of the lava within the -crater would seldom suffice to rupture the walls, were it not that -the molten rock first fuses its way into old stream tunnels buried -under the mountain slopes (see _ante_, p. 112). Composite cones have -a strength as retaining walls for lava which is intermediate between -that of the other types. Their Vulcanian eruptions of the convulsive -type are initiated by the formation of a rent or fissure upon the -mountain flanks at elevations well above the base, the opening of the -fissure being generally accompanied by a local earthquake of greater or -less violence. - -From one or more such fissures the lava issues usually with sufficient -violence at the place of outflow to build up over it either an enlarged -type of driblet cone, referred to as a “mouth”, or _bocca_[1] (Fig. -123), or one or more cinder cones which from their position upon the -flanks of the larger volcano are referred to as _parasitic cones_ (Fig. -124). The lava of Vesuvius more frequently yields _bocchi_ at the place -of outflow, whereas the flanks of Etna are pimpled with great numbers -of parasitic cinder cones, each the monument to some earlier eruption -(Fig. 125). - -[Illustration: - -FIG. 125.—View looking toward the summit of Etna from a position upon -the southern flank near the village of Nicolosi. The two breached -parasitic cones seen behind this village are the Monti Rossi which were -thrown up in 1669 and from which flowed the lava which overran Catania -(after a photograph by Sommer).] - -It is generally the case that a single eruption makes but a relatively -small contribution to the bulk of the mountain. From each new cone or -_bocca_ there proceeds a stream of lava spread in a relatively narrow -stream extending down the slopes (Fig. 126). - -[Illustration: - -FIG. 126.—Sketch map of Etna, showing the individual surface lava -streams (in black) and the tuff covered surface (stippled).] - - -=The caldera of composite cones.=—Because of the varied episodes -in the history of composite cones, they lack the regular lines -characteristic of the two simpler types. The larger number of the more -important composite cones have been built up within an outer crater of -relatively large diameter, the Somma cone or _caldera_, which surrounds -them like a gigantic ruff or collar. This caldera is clearly in most -cases at least the relic of an earlier explosive crater, after which -successive eruptions of lesser violence have built a more sharply -conical structure. This can only be interpreted to mean that most -larger and long-active volcanoes have been born in the grandest throes -of their life history, and that a larger or smaller lateral migration -of the vent has been responsible for the partial destruction of the -explosion crater. Upon Vesuvius we find the crescent-like rim of Monte -Somma; on Etna it is the Val del Bove, etc. It is this caldera of -composite cones which gave rise to the theory of the “elevation crater” -of von Buch (see _ante_, p. 95, and Fig. 127). - - -[Illustration: - -FIG. 127.—Panum crater, showing the caldera and the later interior -cones (after Russell).] - -=The eruption of Vesuvius in 1906.=—The volcano Vesuvius rises on the -shores of the beautiful bay of Naples only about ten miles distant -from the city of Naples. The mountain consists of the remnant of an -earlier broad-mouthed explosion crater, the Monte Somma, and an inner, -more conical elevation, the Monte Vesuvio. Before the eruption of 1906 -this central cone was sharply conical and rose to a height of about -4300 feet above the surface of the bay, or above the highest point of -the ancient caldera. The base of this inner cone is at an elevation of -something less than half that of the entire mass, and is separated from -the encircling ring wall of the old crater by the _atrio_, to which -corresponds in height a perceptible shelf or _piano_ upon the slope -toward the bay of Naples (Fig. 128). - -[Illustration: - -FIG. 128.—View of Mount Vesuvius as it appeared from the Bay of Naples -shortly before the eruption of 1906. The horn to the left is Monte -Somma.] - -An active composite cone like that of Vesuvius is for the greater part -of the time in the Strombolian condition; that is to say, light crater -explosions continue with varying intensity and interval, except when -the mountain has been excited to the periodic Vulcanian outbreaks with -which its history has been punctuated. The Strombolian explosions -have sufficient violence to eject small fragments of hot lava, which, -falling about the crater, slowly built up a rather sharp cone. The -period of Strombolian activity has, therefore, been called the -_cone-producing period_. Just before each new outbreak of the Vulcanian -type, the altitude of the mountain has, therefore, reached a maximum, -and since the larger explosive eruptions remove portions of this cone -at the same time that they increase the dimensions of the crater, -the Vulcanian stage in contrast to the other has been called the -_crater-producing period_. In this period, then, the material ejected -during the explosions does not consist solely of fresh lava cakes, -but in part of the older débris derived from the crater walls, whence -it is avalanched upon the chimney after each larger explosion. The -overhanging cloud, which during the Strombolian period has consisted -largely of steam and is noticeably white, now assumes a darker tone, -the “smoke” which characterizes the Vulcanian eruption. - -[Illustration: - -FIG. 129.—A series of consecutive sketches of the summit of the -Vesuvian cone, showing the modifications in its outline (after Sir -William Hamilton).] - -On several historical occasions the cone of Vesuvius has been lowered -by several hundred feet, the greatest of relatively recent truncations -having occurred in 1822 and in 1906. Between Vulcanian eruptions the -Strombolian activity is by no means uniform, and so the upward growth -of the cone is subject to lesser interruptions and truncations (Fig. -129). - -The Vesuvian eruption of 1906 has been selected as a type of the -larger Vulcanian eruption of composite cones, because it combined the -explosive and paroxysmal elements, and because it has been observed and -studied with greater thoroughness than any other. The latest previous -eruption of the Vulcanian order had occurred in 1872. Some two years -later the period of active cone building began and proceeded with -such rapidity that by 1880 the new cone began to appear above the rim -of the crater of 1872. From this time on occasional light eruptions -interrupted the upbuilding process, and as the repairs were not in -all cases completed before a new interruption, a nest of cones, each -smaller than the last, arose in series like the outdrawn sections of -an old-time spyglass. At one time no less than five concentric craters -were to be seen. - -For a brief period in the fall of 1904 Vesuvius had been in almost -absolute repose, but soon thereafter the Strombolian crater explosions -were resumed. On May 25, 1905, a small stream of lava began to issue -from a fissure high up upon the central cone, and from this time on -the lava continued to flow down to the valley or _atrio_, separating -the inner cone from the caldera remnant of Monte Somma. Seen in the -night, this stream of lava appeared from Naples like a red hot wire -laid against the mountain’s side (Fig. 130). With gradual augmentation -of Strombolian explosions and increase in volume of the flowing lava -stream, the same condition continued until the first days of April in -1906. The flowing lava had then overrun the tracks of the mountain -railway and accumulated in considerable quantity within the _atrio_ -(Fig. 131). - -[Illustration: - -FIG. 130.—Night view of Vesuvius from Naples before the outbreak of -1906. A small lava stream is seen descending from a high point upon the -central cone (after Mercalli).] - -On the morning of April 4, a preliminary stage of the eruption was -inaugurated by the opening of a new radial fissure about 500 feet -below the summit of the cone (Fig. 132 _a_), and by early afternoon -the cone-destroying stage began with the rise of a dark “cauliflower -cloud” or _pino_ to replace the lighter colored steam cloud. The -cone was beginning to fall into the crater, and old lava débris was -mingled in the ejections with the lava clots blown from the still fluid -material within the chimney. From now on short and snappy lightning -flashes played about the black cloud, giving out a sharp staccato -“tack-a-tack.” The volume and density of the cloud and the intensity -of the crater explosions continued to increase until the culmination -on April 7. On April 5 at midnight a new lava mouth appeared upon -the same fissure which had opened near the summit, but now some 300 -feet lower (Fig. 132 _b_). The lava now welled out in larger volume -corresponding to its greater head, and the stream which for ten months -had been flowing from the highest outlet upon the cone now ceased to -flow. The next morning, April 6, at about 8 o’clock, lava broke out at -several points some distance east of the opening _b_, and evidently -upon another fissure transverse to the first (Fig. 132 _c_). The lava -surface within the chimney must still have remained near its old -level,—effective draining had not yet begun,—since early upon the -following morning a small outflow began nearly at the top of the cone -upon the opposite side and at least a thousand feet higher. - -[Illustration: FIG. 131.—Scoriaceous lava encroaching upon the tracks -of the Vesuvian railway (after a photograph by Sommer).] - -[Illustration: - -FIG. 132.—Map of Vesuvius, showing the position and order of formation -of the lava mouths upon its flanks during the eruption of 1906 (after -Johnston-Lavis).] - -The culmination of the eruption came in the evening of April 7, when, -to the accompaniment of light earthquakes felt as far as Naples, lava -issued for the first time in great volume from a mouth more than -halfway down the mountain side (Fig. 132 _f_), and thus began the -drainage of the chimney. At about the same time with loud detonations -a huge black cloud rose above the crater in connection with heavy -explosions, and a rain of cinder was general in the region about the -mountain but especially within the northeast quadrant. Those who were -so fortunate as to be in Pompeii had a clear view of the mountain’s -summit where red hot masses of lava were thrown far into the air. The -direction of these projections was reported to have been not directly -upward, but inclined toward the northeast quadrant of the mountain; but -since with a northeast surface wind the heaviest deposit of ash and -dust should have been upon the southwestern quadrant of the mountain, -it is evident that the material was carried upward until it reached the -contrary upper currents of the atmosphere, to be by them distributed. - -[Illustration: FIG. 133.—The ash curtain which had overhung Vesuvius -lifting and disclosing the outlines of the mountain on April 10, 1911 -(after De Lorenzo).] - -[Illustration: - -FIG. 134.—The central cone of Vesuvius as it appeared after the -eruption of 1906, but with the earlier profile indicated. The -truncation represents a lowering of the summit by some five hundred -feet, with corresponding increase in the diameter of the crater (after -Johnston-Lavis).] - -When the heavy curtain of ash, which now for a number of succeeding -days overhung all the circum-Vesuvian country, began to lift (Fig. -133), it was seen that the summit of the cone had been truncated -an average of some 500 feet (Fig. 134). All the slopes and much of -the surrounding country had the aspect of being buried beneath a -cocoa-colored snow of a depth to the northeastward of several feet, -where it had drifted into all the hollow ways so as almost to efface -them (Fig. 135). More than thrice as heavy as water, the weak roof -timbers of the houses at the base of the mountain gave way beneath the -added load upon them, thus making many victims. Inasmuch, however, as -the ash-fall partakes of the same general characters as in eruptions -from cinder cones, we may here give our attention especially to the -streams of lava which issued upon the opposite flank of the mountain -(Fig. 136). - -[Illustration: - -FIG. 135.—A sunken road filled with indrifted cocoa-colored ash from -the Vesuvian eruption of 1906.] - -[Illustration: - -FIG. 136.—View of Vesuvius taken from the southwest during the waning -stages of the eruption of 1906. In the middle distance may be discerned -the several lava mouths aligned upon a fissure, and the courses of the -streams which descend from them. In the foreground is the main lava -stream with scoriaceous surface (after W. Prinz).] - -The main lava stream descended the first steep slopes with the velocity -of a mile in twenty-five minutes, about the strolling speed of a -pedestrian, but this rate was gradually reduced as the stream advanced -farther from the mouth. Taking advantage of each depression of the -surface, the black stream advanced slowly but relentlessly toward -the cities at the southwest base of the mountain. With a motion not -unlike that of a heap of coal falling over itself down a slope, the -block lava advances without burning the objects in its path (Fig. -137). - -[Illustration: - -FIG. 137.—The main lava stream of 1906 advancing upon the village of -Boscotrecase.] - -[Illustration: - -FIG. 138.—An Italian pine snapped off by the lava and carried forward -upon its surface as a passenger (after Haug).] - -The beautiful pines are merely charred where snapped off and are -carried forward upon the surface of the stream (Fig. 138). When a real -obstruction, such as a bridge or a villa, is encountered, the stream is -at first halted, but the rear crowding upon the van, unless a passage -is found at the side, the lava front rises higher and higher until by -its weight the obstruction is forced to give way (Figs. 139 and 140). - -[Illustration: - -FIG. 139.—Lava front both pushing over and running around a wall which -lies athwart its course (after Johnston-Lavis).] - -[Illustration: - -FIG. 140.—One of the villas in Boscotrecase which was ruined by the -Vesuvian lava flow of 1906. The fragments of masonry from the ruined -walls traveled upon the lava current, where they sometimes became -incased in lava.] - - -=The sequence of events within the chimney.=—The thorough study of -this Vesuvian eruption has placed us in a position to infer with -some confidence in our conclusions the sequence of events within the -chimney and crater of the volcano, both before and during the eruption. -Anticipating some conclusions derived from the observed dissection of -volcanoes, which will be discussed below, it may be stated that what -might be termed the core of the composite cone—the chimney—is a more -or less cylindrical plug of cooled lava which during the active period -of the vent has an interior bore of probably variable caliber. This -plug in its lower section appears in solid black in all the diagrams -of Fig. 141. During the cone-building period (Fig. 141 _a_ and _b_) -the plug is obviously built upward along with the cone, for lava often -flows out at a level a few hundred feet only below the crater rim. By -what process this chimney building goes on is not well understood, -though some light is thrown upon it by the post-eruption stage of Mont -Pelé in 1902-1903 (see below). - -[Illustration: - -FIG. 141.—Three diagrams to illustrate the sequence of events -within the crater of a composite cone during the cone-building and -crater-producing periods. _a_ and _b_, two successive stages of the -cone building or Strombolian period; _c_, enlargement of the crater, -truncation of the cone, and destruction of the upper chimney during the -relatively brief crater-producing or Vulcanian period.] - -Both the older and newer sections of this plug or chimney are furnished -some support against the outward pressure of the contained lava by -the surrounding wall of tuff; and they are, therefore, in a condition -not unlike that of the inner barrel of a great gun over which sleeves -of metal have been shrunk so as to give support against bursting -pressures. On the other hand, when not sustaining the hydrostatic -pressure of the liquid lava within, the chimney would tend to be -crushed in by the pressure of the surrounding tuff. Its strength to -withstand bursting pressures is dependent not alone upon the thickness -of its rock walls, but also upon its internal diameter or caliber. A -steam cylinder of given thickness of wall, as is well known, can resist -bursting pressures in proportion as its internal diameter is small. So -in the volcanic chimney, any tendency to remelt from within the chimney -walls must weaken them in a twofold ratio. - -We are yet without accurate temperature observations upon the lava in -volcanic chimneys, but it seems almost certain that these temperatures -rise as the Vulcanian stage is approaching, and such elevation of -temperature must be followed by a greater or less re-fusion of the -chimney walls. The sequence of events during the late Vesuvian eruption -is, then, naturally explained by progressive re-fusion and consequent -weakening of the chimney walls, thus permitting a radial fissure to -open near the top and gradually extend downwards. Thus at first small -and high outlets were opened insufficient to drain the chimney, but -later, on April 7, after this fissure had been much extended and a new -and larger one had opened at a lower level, the draining began and the -surface of lava commenced rapidly to sink. - -[Illustration: FIG. 142.—The spine of Pelé rising above the chimney of -the volcano after the eruption of 1902 (after Hovey).] - -When the rapid sinking of the lava surface occurred, the lower lava -layers were almost immediately relieved of pressure, thus causing a -sudden expansion of the contained steam and resulting in grand crater -explosions. The partially refused and fissured upper chimney, now -unable to withstand the inward pressure of the surrounding tuff walls, -since outward pressures no longer existed, crushed in and contributed -its materials and those of the surrounding tuff to the fragments of -fresh lava rising in volume in the grand explosions (Fig. 141 _c_). In -outline, then, these seem to be the conditions which are indicated by -the sequence of observed events in connection with the late Vesuvian -outbreak. - -[Illustration: - -FIG. 143.—Outlines of the Pelé spine upon successive dates. The full -line represents its outline on December 26, 1902; the dotted-dashed -line is a profile of January 3, 1902; while the dotted line is that of -January 9, 1903. The dark line is a fissure (after E. O. Hovey).] - - -=The spine of Pelé.=—The disastrous eruption of Mont Pelé upon -Martinique in the year 1902 is of importance in connection with the -interesting problem of the upward growth of volcanic chimneys during -the cone-building period of a volcano. After the conclusion of this -great Vulcanian eruption, a spine of lava grew upward from the -chimney of the main crater until it had reached an elevation of more -then a thousand feet above its base, a figure of the same order of -magnitude as the probable height of the upper section of the Vesuvian -chimney previous to the eruption of 1906. The Pelé spine (Fig. 142) -did not grow at a uniform rate, but was subject to smaller or larger -truncations, but for a period of 18 days the upward growth was at the -rate of about 41 feet per day. Later, the mass split upon a vertical -plane revealing a concave inner surface, and was somewhat rapidly -reduced in altitude to 600 feet (Fig. 143), only to rise again to its -full height of about 1000 feet some three months later. - -While apparently unique as an observed phenomenon, and not free from -uncertainty as to its interpretation, the growth of this obelisk has -at least shown us that a mass of rock can push its way up above the -chimney of an active volcano even when there are no walls of tuff about -it to sustain its outward pressures. - - -[Illustration: - -FIG. 144.—Corrugated surface of the Vesuvian cone after the mud flows -which followed the eruption in 1906 (after Johnston-Lavis).] - -=The aftermath of mud flows.=—When the late Vulcanian explosions of -Vesuvius had come to an end, all slopes of the mountain, but especially -the higher ones, were buried in thick deposits of the cocoa-colored -ash, included in which were larger and smaller projectiles. As this -material is extremely porous, it greedily sucks up the water which -falls during the first succeeding rains. When nearly saturated, it -begins to descend the slopes of the mountain and soon develops a -velocity quite in contrast with that of the slow-moving lava. The upper -slopes are thus denuded, while the fields and even the houses about the -base are invaded by these torrents of mud (_lava d’acqua_). Inasmuch as -these mud flows are the inevitable aftermath of all grander explosive -eruptions, the Italian government has of late spent large sums of -money in the construction of dikes intended to arrest their progress -in the future. It was streams of this sort that buried the city of -Herculaneum after the explosive eruption of 79 A.D. - -After the mud flows have occurred, the Vesuvian cone, like all -similar volcanic cones under the same conditions, is found with deep -radial corrugations (Fig. 144), such as were long ago described as -“barrancoes” and supposed to support the “elevation crater” theory of -volcano formation. - - -=The dissection of volcanoes.=—To the uninitiated it might appear a -hopeless undertaking to attempt to learn by observation the internal -structure of a volcano, and especially of a complex volcano of the -composite type. The earliest successful attempt appears to have been -made by Count Caspar von Sternberg in order to prove the correctness -of the theory of his friend, the poet Goethe. Goethe had claimed that -a little hill in the vicinity of Eger, on the borders of Bohemia, was -an extinct volcano, though the foremost geologist of the time the -famous Werner, had promulgated the doctrine that this hill, in common -with others of similar aspect, originated in the combustion of a bed -of coal. The elevation in question, which is known as the Kammerbühl, -consists mainly of cinder, and Goethe had maintained that if a tunnel -were to be driven horizontally into the mountain from one of its -slopes, a core or plug of lava would be encountered beneath the summit. -The excavations, which were completed in 1837, fully verified the -poet’s view, for a lava plug was found to occupy the center of the mass -and to connect with a small lava stream upon the side of the hill (Fig. -145). - -[Illustration: - -FIG. 145.—The Kammerbühl near Eger, showing the tunnel completed in -1837 which proved the volcanic nature of the mountain (after Judd).] - -It is not, however, to such expensive projects that reference is here -made, but rather to processes which are continually going on in nature, -and on a far grander scale. The most important dissecting agent for our -purpose is running water, which is continually paring down the earth’s -surface and disclosing its buried structures. How much more convincing -than any results of artificial excavation, as evidence of the internal -structure of a volcano, is the monument represented in Fig. 146, -since here the lava plug stands in relief like a gigantic thumb still -surrounded by a remnant of cinder deposits. Such exposed chimneys of -former volcanoes are found in many regions, and have become known as -volcanic _necks_, _pipes_, or _plugs_. - -[Illustration: FIG. 146.—Volcanic plug exposed by natural dissection -of a volcanic cone in Colorado (U. S. G. S.).] - -[Illustration: - -FIG. 147.—A dike cutting beds of tuff in a partly dissected volcano of -southwestern Colorado (after Howe, U. S. G. S.).] - -Not infrequently the beds of tuff composing the flanks of the volcano, -upon dissection by the same process, bring to light walls of cooled -lava standing in relief (Fig. 147)—the filling of the fissure which -gave outlet to the flanks of the mountain at the time of the eruption. -Study of exposed dikes formed in connection with recent eruptions of -Vesuvius has shown that in many instances they are still hollow, the -lava having drained from them before complete consolidation. - -Another agent which is effective in uncovering the buried structures -of volcanoes is the action of waves on shores. Always a relatively -vigorous erosive agency, the softer structures of volcanic cones are -removed with especial facility by this agent. On the shores of the -island of Volcano, the little cone of Vulcanello has been nearly half -carried away by the waves, so as to reveal with especial perfection the -structure of the cinder beds as well as the internal rock skeleton of -the mass. Here the characteristic dips of lava streams, intercalated as -they now are between tuff deposits and the lava which consolidated in -fissures, are both revealed. - -[Illustration: - -FIG. 148.—Map and general view of St. Paul’s Rocks, a volcanic cone -dissected by waves.] - -In mid-Atlantic a quite perfect crater, the St. Paul’s Rocks, has been -cut nearly in half so as to produce a natural harbor (Fig. 148). - -In still other instances we may thank the volcano itself for opening -up the interior of the mountain for our inspection. The eruption in -1888 of the Japanese volcano of Bandai-san, by removing a considerable -part of the ancient cone, has afforded us a section completely through -the mountain. The summit and one side of the small Bandai was carried -completely away, and there was substituted a yawning crater eccentric -to the former mountain and having its highest wall no less than 1500 -feet in height (Fig. 149). In two hours from the first warning of the -explosion the catastrophe was complete and the eruption over. - -[Illustration: - -FIG. 149.—Dissection by explosion of Little Bandai-san in 1888 (after -Sekiya).] - -The eruption of Krakatoa in 1883, probably the grandest observed -volcanic explosion in historic times, left a volcanic cone divided -almost in half and open to inspection (Fig. 150). Rakata, Danan, -and Perbuatan had before constituted a line of cones built up round -individual craters subsequent to the partial destruction of an -earlier caldera, portions of which were still existent in the islands -Verlaten and Lang. By the eruption of 1883 all the exposed parts and -considerable submerged portions of the two smaller cones were entirely -destroyed, and the larger one, known as Rakata, was divided just -outside the plug so as to leave a precipitous wall rising directly from -the sea and showing lava streams in alternation with somewhat thicker -tuff layers, the whole knit together by numerous lava dikes. - -[Illustration: - -FIG. 150.—The half-submerged volcano of Krakatoa in the Sunda Straits -before and after the eruption of 1883 (after Verbeek).] - -In order to carry our dissecting process down to levels below the -base of the volcanic mountain, it is usually necessary to inspect the -results of erosion by running water. Here the plug or chimney, instead -of being surrounded by tuff, is inclosed by the country rock of the -region, which is commonly a sedimentary formation. Such exposed lower -sections of volcanic chimneys are numerous along the northwestern -shores of the British Isles. Where aligned upon a dislocation or -noteworthy fissure in the rocks, the group of plugs has been referred -to as a scar or _cicatrice_ (Fig. 151). Associated with the plugs of -the cicatrice are not infrequently dikes, or, it may be, sheets of lava -extended between layers of sediment and known as _sills_. - -[Illustration: FIG. 151.—The cicatrice of the Banat (after Suess).] - -If we are able to continue the dissection process to still greater -depths, we encounter at last igneous rock having a texture known as -granitic and indicating that the process of consolidation was not -only exceedingly slow but also uninterrupted. This rock is found in -masses of larger dimensions, and though generally of more or less -irregular form, no one dimension is of a different order of magnitude -from the others. Such masses are commonly described as _bosses_, or, -if especially large, as _batholites_ (Fig. 152). Wherever the rock -beds appear as though they had been forced up by the upward pressure -of the igneous mass, the latter takes the form of a mushroom and has -been described as a _laccolite_ (Figs. 479-481, pp. 441-442). Evidence -seems, however, to accumulate that in the greater number of cases the -molten rock has fused its way upward, in part assimilating and in part -inclosing the rock which it encountered. This process of upward fusion -has been likened to the progress of a red hot iron burning its way -through a board. - - -=The formation of lava reservoirs.=—The discarding of the earlier -notion that the earth has a liquid interior makes it proper in -discussing the subject of volcanoes to at least touch upon the origin -of the molten rock material. As already pointed out, such reservoirs as -exist must be local and temporary, or it would be difficult to see how -the existing condition of earth rigidity could be maintained. From the -rate at which rock temperatures rise, at increasing depths below the -surface, it is clear that all rocks would be melted at very moderate -depths only, if they were not kept in a solid state by the prodigious -loads which they sustain. Any relief from this load should at once -result in fusion of the rock. - -[Illustration: - -FIG. 152.—Diagram to illustrate a probable cause of formation of lava -reservoirs, and to show the connection between such reservoirs and the -volcanoes at the surface.] - -Now the restriction of active volcanoes to those zones of the earth’s -surface within which mountains are rising, and where in consequence -earthquakes are felt, has furnished us at least a clew to the origin -of the lava. Regarded as a structure capable of sustaining a load, -the competency of an arch is something quite remarkable, so that the -arching up of strong rock formations into anticlines within the upper -layers of the zone of flow, or of combined fracture and flow, would -be sufficient to remove the load from relatively weak underlying beds, -which in consequence would be fused and form local reservoirs of lava -(Figs. 152 and 153). - -It has been further quite generally observed that lines of volcanoes, -in so far as they betray any relation in position to neighboring -mountain ranges, tend to appear upon the rear or flatter limb of -unsymmetrical arches, or where local tension would favor the opening of -channels toward the surface. Moreover, wherever recent block movements -of surface portions of the earth’s shell have been disclosed in the -neighborhood of volcanoes, the latter appear to be connected with -downthrown blocks, as though the lava had, so to speak, been squeezed -out from beneath the depressed block or blocks. - -[Illustration: - -FIG. 153.—Result of experiment with layers of composition to -illustrate the effect of relief of load upon rocks by arching of -competent formation (after Willis).] - -We must not, however, forget that the igneous rocks are greatly -restricted in the range of their chemical composition. No igneous -rock type is known which could be formed by the fusion of any of -the carbonate rocks such as limestone or dolomite, or of the more -siliceous rocks, such as sandstone or quartzite. There remains only -the argillaceous class of sediments, the shales and slates, and so -soon as we examine the composition of these rocks we are struck by -the remarkable resemblance to that of the class of igneous rocks. For -purposes of comparison there is given below the composite or average -constitution of igneous rocks in parallel column, with the average -attained by combining the analyses of 56 slates and shales, the latter -recalculated with water excluded: - - - ══════════════╤══════════════════════════════════╤════════════════ - │ AVERAGE IGNEOUS ROCK │ - ├——————————————————┬———————————————┤ AVERAGE SHALE - │ (Clark) │ (Washington) │ - ——————————————┼——————————————————┼———————————————┼———————————————— - │ │ │ - SiO_2 │ 61.25 │ 61.69 │ 63.34 - Al_{2}O_3 │ 15.81 │ 15.94 │ 16.56 - Fe_{2}O_3 │ 2.70} 6.31 │ 1.88} 4.53 │ 4.41} 7.89 - FeO │ 3.61} │ 2.65} │ 3.48} - MgO │ 4.47 │ 4.90 │ 3.54 - CaO │ 5.03 │ 5.02 │ 3.33 - Na_{2}O │ 3.64 │ 4.09 │ 1.29 - K_{2}O │ 2.87 │ 3.35 │ 3.52 - TiO_2 │ .62 │ .48 │ .53 - │ —————— │ —————— │ —————— - │ 100.00 │ 100.00 │ 100.00 - ══════════════╧══════════════════╧═══════════════╧════════════════ - -This close resemblance is probably of deep significance, for the reason -that shales and slates are structurally the weakest of all rocks and -for the further reason that they rather generally directly underlie -the carbonate rocks, which are by contrast the strongest (see _ante_, -p. 37). For these reasons shales and slates are the only rocks which -are likely to be fused by relief from load through the formation of -anticlinal arches within the earth’s zone of flow. If this view is -well founded, lavas and other igneous rocks are in large part fused -argillaceous sediments formed in connection with the process of -folding, or are refused rocks of igneous origin and similar composition. - - -=Character profiles.=—The character profiles of features connected in -their origin with volcanoes are particularly easy to recognize, and in -a few cases in which they might be confused with others of a different -origin, an examination of the materials of the features should lead to -a definitive judgment. - -The lava plains which result from massive outflows of basalt might -perhaps strictly be regarded as lack of feature, so great may be their -continuous extent. Wherever definite vents exist, a broad flat dome is -the usual result of the extravasation of a basaltic lava. The puys of -France and many of the Kuppen of Germany, being formed from less fluid -lava, have afforded profiles with relatively small radius of curvature. - -In its youthful stage, the cinder cone usually presents a broad summit -sag and relatively short side slopes, whereas the cone of later stages -is apt to present long sweeping and upwardly concave curves with both -the gradient and the radius of curvature increasing rapidly toward -the summit. In contrast, too, with the earlier stage, the crest is -relatively small. A marked reduction in the high symmetry of such -profiles is noted wherever a breaching by lava outflow has occurred -(Fig. 154). - -With the composite cone, complexity and corresponding lack of symmetry -is introduced, especially in the partially ruined caldera, and by -the more or less accidental distribution of parasitic cones, as well -as by migrations of the central cone. Peculiarly similar acuminated -profiles result from spatter-cone formation, from the formation of -a superchimney spine, and by the uncovering of the chimney through -denudational processes—the volcanic neck. - -[Illustration: FIG. 154.—Character profiles connected with volcanoes.] - -Another important feature resulting from denudation is the Mesa or -table mountain with its protecting basalt cap above softer rocks. Its -profile most resembles that of table mountains due to differential -erosion of alternately strong and weak horizontally bedded rocks, such -as compose the upper portion of the section in the Grand Cañon of the -Colorado. Here, however, in place of a single unusually strong top -layer there are found several strong layers in alternation with weaker -ones so as to produce additional steps in the profile. - - -READING REFERENCES TO CHAPTERS IX AND X - - General works:— - - PAULETT SCROPE. The Geology of the Extinct Volcanoes of Central - France. John Murray, London, 1858, pp. 258. (An epoch-making work of - early date which, like the following reference, may be studied to - advantage to-day.) - - SIR CHARLES LYELL. Principles of Geology, vol. 1, Chapters xxiii-xxv. - - MELCHIOR NEUMAYR. Erdgeschichte, vol. 1, Allgemeine Geologie, revised - edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable - information clearly presented). - - J. D. DANA. Characteristics of Volcanoes, with Contributions of Facts - and Principles from the Hawaiian Islands. Dodd, Mead, and Company, New - York, 1890, pp. 397. - - TEMPEST ANDERSON. Volcanic Studies in Many Lands, being reproductions - of photographs by the author with explanatory notes. John Murray, - London, 1903, pp. 200, pls. 105. - - T. G. BONNEY. Volcanoes, their Structure and Significance. John - Murray, London, 1899, pp. 331. - - I. C. RUSSELL. Volcanoes of North America. Macmillan, New York, 1897, - pp. 346. - - ELISÉE RÉCLUS. Les volcans de la terre, Belgian Society of Astronomy, - Meteorology, and Physics of the Globe, 1906-1910 (a valuable - descriptive geographical and bibliographical work of reference). - - G. MERCALLI. I vulcani attivi della terre. Hoepli, Milan, 1907, pp. - 421. (A most valuable work, beautifully illustrated, but in the - Italian language.) - -Arrangement of volcanic vents:— - - TH. THORODDSEN. Die Bruchlinien und ihre Beziehungen zu den Vulkanen, - Pet. Mitt., vol. 51, 1905, pp. 1-5, pl. 5. - - R. D. M. VERBEEK. Various volumes and atlases of maps covering the - Dutch East Indies and fully cited in the following reference (p. 21). - - WILLIAM H. HOBBS. The Evolution and the Outlook of Seismic Geology, - Proc. Am. Phil. Soc., vol. 48, 1909, pp. 17-27. - -Birth of volcanoes:— - - F. OMORI. The Usu-san Eruption and Earthquake and Elevation Phenomena, - Bull. Earthq. Inv. Com., Japan, vol. 5, No. 1, 1911, pp. 1-37, pls. - 1-13. - -Fissure eruptions:— - - TH. THORODDSEN. Island, IV, Vulkane, Pet. Mitt., Ergänzungsh. 153, - 1906, pp. 108-111. - - A. GEIKIE. Text-book of Geology, 4th ed., pp. 342-346. - -Lava domes of Hawaii:— - - J. D. DANA. Characteristics of Volcanoes (as above). - - C. H. HITCHCOCK. Hawaii and Its Volcanoes. Honolulu, 1909, pp. 314. - -Eruption of Matavanu volcano in 1906:— - - KARL SAPPER. Der Matavanu-Ausbruch auf Savaii, 1905-1906, Zeit. d. - Gesell. f. Erdk. z. Berlin, vol. 19, 1906, pp. 686-709, 4 pls. - - H. J. JENSEN. The Geology of Samoa, and the Eruptions in Savaii, Proc. - Linn. Soc., New South Wales, vol. 31, 1906, pp. 641-672, pls. 54-64. - - TEMPEST ANDERSON. The Volcano of Matavanu in Savaii, Quart. Jour. - Geol. Soc., London, vol. 66, 1910, pp. 621-639, pls. 45-52. - -Eruption of Volcano in 1888:— - - H. J. JOHNSTON-LAVIS. The South Italian Volcanoes. Naples, 1891, pp. - 342, pls. 16. - -Eruption of Taal volcano in 1911:— - - W. E. PRATT. The Eruption of Taal Volcano, January 30, 1911, Phil. - Jour. Sci., vol. 6, No. 2, Sec. A, 1911, pp. 63-86, pls. 1-14. - - F. H. NOBLE. Taal Volcano, album of views of 1911 eruption, Manila, - 1911, pp. 1-48. - -The volcano of Etna:— - - G. VOM RATH. Der Aetna. Bonn, 1872, pp. 1-33. (A beautiful piece of - descriptive writing from both the geological and scenic standpoints.) - - SARTORIUS VON WALTERSHAUSEN. Der Aetna. Leipzig, 1880, 2 quarto vols., - pp. 371 and 548. - -The eruption of Vesuvius in 1906:— - - H. J. JOHNSTON-LAVIS. Geological Map of Monte Somma and Vesuvius, with - a short and concise account, etc. Geo. Philip & Son, London, 1891. - - H. J. JOHNSTON-LAVIS. The Eruption of Vesuvius in April, 1906, Trans. - Roy. Dublin Soc., vol. 9, 1909, Pt. VIII, pp. 139-200, pls. 3-23 (the - most authoritative work upon the subject). - - T. A. JAGGAR, JR. The Volcano Vesuvius in 1906, Tech. Quart., vol. 19, - 1906, pp. 105-115. - - W. PRINZ. L’éruption du Vesuv d’avril, 1906, Ciel et Terre, 27e Année, - 1906, pp. 1-49. - - FRANK A. PERRET. Notes on the Electrical Phenomena of the Vesuvian - Eruption, April, 1906, Sci. Bull., Brooklyn Inst. Arts and Sci., vol. - 1, No. 11, pp. 307-312; Vesuvius, Characteristics and Phenomena of the - Present Repose Period, Am. Jour. Sci., vol. 28, 1909, pp. 413-430. - - WILLIAM H. HOBBS. The Grand Eruption of Vesuvius in 1906, Jour. Geol., - vol. 14, 1906, pp. 636-655. - -The spine of Pelée:— - - E. O. HOVEY. The New Cone of Mont Pelée and the Gorge of the Rivière - Blanche, Martinique, Am. Jour. Sci., vol. 16, 1903, pp. 269-281, pls. - 11-14. - - A. HEILPRIN. The Tower of Pelée. Philadelphia, 1904, pp. 62, pls. 22. - - A. LACROIX. La montagne Pelée et ses éruptions, Acad. des Sciences, - Paris, 1904, Chapter iii. - - KARL SAPPER. In den Vulkangebieten Mittelamerikas und Westindiens, - Stuttgart, 1905, pp. 172-178. - - A. C. LANE. Absorbed Gases of Vulcanism, Science, N.S., vol. 18, 1903, - p. 760. - - G. K. GILBERT. The Mechanism of the Mont Pelée Spine, _ibid._, vol. - 19, 1904, pp. 927-928. - - I. C. RUSSELL. Pelée Obelisk once More, _ibid._, vol. 21, 1905, pp. - 924-931. - -The dissection of volcanoes:— - - J. W. JUDD. Volcanoes, Chapter v. - - S. SEKYA and Y. KIKUCHI. The Eruption of Bandai-San, Trans. Seis. - Soc., Japan, vol. 13, Pt. 2, 1890, pp. 140-222, pls. 1-9. - - R. D. M. VERBEEK. Krakatau. Batavia, 1885, pp. 557, pls. 25. - - ROYAL SOCIETY. The Eruption of Krakatoa and Subsequent Phenomena. - London, 1888, pp. 494. - - G. K. GILBERT. Report on the Geology of the Henry Mountains, U.S. - Geogr. and Geol. Surv., Rocky Mt. Region, Washington, 1877, pp. 22-60. - - SIR A. GEIKIE. Ancient Volcanoes of Great Britain, vol. 2 especially. - - D. W. JOHNSON. Volcanic Necks of the Mount Taylor Region, New Mexico, - Bull. Geol. Soc. Am., vol. 18, 1907, pp. 303-324, pls. 25-30. - - - - -CHAPTER XI - -THE ATTACK OF THE WEATHER - - -=The two contrasted processes of weathering.=—It has already been -pointed out that change and not stability is the order of nature. -Within the earth’s outer shell and upon it rock alteration goes on -continually, and from some portions of its surface the changed material -is as constantly migrating to neighboring or even far distant regions. -Before such transportation can begin the hard rock must first be broken -down and reduced to fragments which the transporting agencies are -competent to move. - -To accomplish this breaking down, or _degeneration_, of the rock -masses, either a wide range in temperature or chemical reaction is -essential. In the atmosphere are found such active chemical agents as -oxygen and carbon dioxide, the so-called carbonic acid gas; and these -agents in the presence of water react chemically with the minerals of -the rocks and form other minerals such as the hydrates and carbonates, -which are lighter in weight and more soluble. This _chemical_ attack -upon the outer shell of the lithosphere is described as _decomposition_. - -On the other hand the rock may succumb to changes which are purely -mechanical and are due either to the stresses set up by differences -between surface and interior temperatures, or to the prying action -of the frost in the crevices. Such purely mechanical degeneration -of the rocks is in contrast with decomposition and is described as -_disintegration_. The two processes of decomposition and disintegration -may, however, go on together; and the changes of volume that are caused -by decomposition may result directly in considerable disintegration, as -we are to see. - - -=The rôle of the percolating water.=—In order to effect chemical -change or reaction, it is essential that the substances which are -to react must be brought into such intimate contact with each other -as it is seldom possible to attain except by solution. The chemical -reactions which go on between the gaseous atmosphere and the solid -lithosphere are accomplished through solution of the gases in water. -This water, derived from rain or snow, percolates into the ground or -descends along the crevices in the rocks, carrying with it a certain -measure of dissolved air. This air differs from that of the surrounding -atmospheric envelope by containing relatively large amounts of oxygen -and of the other active element carbon dioxide. It follows from the -important rôle thus performed by the percolating water that the process -of decomposition will be relatively important in humid regions where -the atmospheric precipitation is sufficient for the purpose. - -[Illustration: - -FIG. 155.—Successive diagrams to show the effect of decomposition and -resulting disintegration upon joint blocks so as to produce spheroidal -bowlders by weathering.] - -Within hot and dry regions there is a larger measure of rock -disintegration, and distinct chemical changes unlike those of humid -regions take place in the higher temperatures and with the more -concentrated saline solutions. The discussion of such changes will be -deferred until desert conditions are treated in another chapter. - - -=Mechanical results of decomposition—spheroidal weathering.=—From -an earlier chapter it has been learned that the rocks of the earth’s -outermost shell are generally intersected by a system of vertical -fissures which at each locality tend to divide the rock into parallel -and upright rectangular prisms. It is these joints which offer -relatively easy paths for the descent of the water into the rocks. -In rocks of sedimentary origin there are found, in addition to the -vertical joints, planes of bedding originally horizontal, and in the -intrusive and volcanic rocks a somewhat similar parting, likewise -parallel to the surface of the ground. The combined effect of the -joints and the additional parting planes is thus to separate the rock -mass into more or less perfect squared blocks (Fig. 155, upper figure) -which stand in vertical columns. - -The water which percolates downward upon the joints, finds its way -laterally along the parting planes, and so subjects the entire surface -of each block to simultaneous attack by its reagents. Though all -parts of the surface of each block are alike subject to attack, it -is the angles and the edges which are most vigorously acted upon. In -the narrow crevices the solutions move but sluggishly, and as they -are soon impoverished of their reagents in the attack upon the rock, -fresh solution can reach the middle of the faces from relatively few -directions. The edges are at the same time being reached from many more -directions, and the corners from a still larger number. - -The minerals newly formed by these chemical processes of hydration -and carbonization are notably lighter, and hence more bulky than the -minerals from whose constituents they have been largely formed. Strains -are thus set up which tend to separate the bulkier new material from -the core of unaltered rock below. As the process continues, distinct -channels for the moving waters are developed favorable to action at -the edges and corners of the blocks. Eventually, the squared block is -by this process transformed into a spheroidal core of still unaltered -rock wrapped in layers of decomposed material, like the outer wrappings -of an onion. These in turn are usually imbedded in more thoroughly -disintegrated material from which the shell structure has disappeared -(Fig. 156). - -[Illustration: - -FIG. 156.—Spheroidal weathering of an igneous rock.] - - -=Exfoliation or scaling.=—A fact of much importance to geologists, but -one far too often overlooked, is that rocks are but poor conductors -for heat. It results from this that in the bright sun of a summer’s -day a thin skin, as it were, upon the rock surface may be heated to -a relatively high temperature, although the layer immediately below -it is practically unaffected. The consequent expansion of the surface -layer causes stresses that tend to scale it off from the layer below, -which, uncovered in its turn, develops new strains of the same sort. -This process of exfoliation acquires exceptional importance in desert -regions where the rock surfaces are daily elevated to excessively high -temperatures (see Chapter XV). - - -[Illustration: - -FIG. 157.—Dome structure in granite mass, Yosemite valley, California -(after a photograph by Sinclair).] - -=Dome structure in granite masses.=—In large granite masses, such -as are to be found in the ranges of the Sierra Nevada of California, -a peculiar dome structure is sometimes found developed upon a large -scale, and has had an important influence upon the breaking down of the -rock and upon the shaping of the mountain (Fig. 157). Such a structure, -made up as it is of prodigious layers, can have little in common with -the veneers of weathered minerals which are the result of exfoliation, -and it is quite likely that the dome structure is in some way connected -with the relief of these massive rocks from their load—the rock which -once rested upon them, but has been carried away by erosion since the -uplift of the range. - - -=The prying work of frost.=—In all countries where winter temperatures -range below the freezing point of water, a most potent agent of rock -disintegration is the frost which pries at every crevice and cranny -of the surface rock. Important in the temperate zones, in the polar -regions it becomes almost the sole effective agent of rock weathering. -There, as elsewhere, its efficiency as a disintegrating agent is -directly dependent upon the nature of the crevices within the rock, so -that the omnipresent joints are able to exercise a degree of control -over the sculpturing of the surface features which is hardly to be -looked for elsewhere (see plate 10 A). - - -[Illustration: FIG. 158.—Talus slope beneath a cliff.] - -=Talus.=—Wherever the earth’s surface rises in steep cliffs, the -rock fragments derived from frost action, or by other processes of -disintegration, as they become detached either fall or slide rapidly -downward until arrested upon a flatter slope. Upon the earlier -accumulations of this kind, the later ones are deposited, until their -surface slopes up to the cliff face as steeply as the material will -lie—the angle of repose. Such débris accumulations at the base of a -cliff (Fig. 158) are known as _talus_, and the slope is described as a -talus slope, or in Scotland as a “scree.” - - -[Illustration: - -FIG. 159.—Striped ground from soil flow of chipped rock fragments upon -a slope, Snow Hill Island, West Antarctica (after Otto Nordenskiöld).] - -=Soil flow in the continued presence of thaw water.=—So soon as the -rocks are broken down by the weathering processes, they are easily -moved, usually to lower levels. In part this transportation may be -accomplished by gravity slowly acting upon the disintegrated rock and -causing it to creep down the slope. Yet even in such cases water is -usually present in quantity sufficient to fill the spaces between the -grains, and so act as a lubricant to facilitate the migration. - -Upon a large scale rocks which were either originally incoherent or -have been made so by weathering, after they have become saturated with -water, may start into sudden motion as great landslides or avalanches, -which in the space of a few moments materially change the face of the -country, and by burying the bottom lands leave disaster and misery in -their wake. - -[Illustration: - -FIG. 160.—Pavement of horizontal surface due to soil flow, Spitzbergen -(after Otto Nordenskiöld).] - -Within the subpolar regions, where a large part of the surface is for -much of the year covered with snow, the underlying rocks are for long -periods saturated with thaw water, and in alternation are repeatedly -frozen and thawed. Essentially similar conditions are met with in the -high, snow-capped mountains of temperate or torrid regions. For the -subpolar regions particularly it is now generally recognized that -somewhat special processes of soil flow, described under the name -_solifluction_, are characteristic. The exact nature of these processes -is as yet imperfectly understood, but there can be little doubt -concerning the large rôle which they have played in the transportation -of surface materials. Such soil flow is clearly manifested under -different aspects, and it is likely that by this comprehensive term -distinct processes have been brought together. - -[Illustration: - -FIG. 161.—Tree roots entering fissured rock and prying its sections -apart.] - -Possibly the most striking aspect of the soil flow in subpolar regions -is furnished by the remarkable “stone rivers” and “rock glaciers”; -though the more generally characteristic are peculiar stripings or -other markings which appear upon the surface of the ground and thus -betray the movements of the underlying materials. Upon slopes it is not -uncommon for the surface to be composed of angular rock fragments riven -by the frost and crossed by broad parallel furrows as though a gigantic -plow had gone over it (Fig. 159). The direction of the furrows is -always up and down the slope, and the striping is marked in proportion -as the slope is steep. Where the bottom is reached, the furrows are -replaced by a sort of mosaic pavement of hexagonal repeating figures, -each of which may be an area of the surface six feet or more across -(Fig. 160, and Fig. 390, p. 368). The depressions which separate the -“blocks” of the pavement are often filled with clay, while the inclosed -surfaces are made up of coarsely chipped stone. - - -=The splitting wedges of roots and trees.=—In the mechanical breakdown -of the rocks within humid regions a not unimportant part is sometimes -taken by the trees, which insinuate the tenuous extremities of their -rootlets into the smallest cracks and by continued growth slowly wedge -even the firmer rocks apart (Fig. 161). In a similar manner the small -tree trunk growing within a crevice of the rock may in time split its -parts asunder (Fig. 162). - -[Illustration: - -FIG. 162.—A large glacial bowlder split by a growing tree near East -Lansing, Michigan (after a photograph by Bertha Thompson).] - - -=The rock mantle and its shield in the mat of vegetation.=—Through -the action of weathering, the rocks, as we have seen, lose their -integrity within a surface layer, which, though it may be as much as a -hundred feet or more in thickness, must still be accounted a mere film -above the underlying bed rock. The mechanical agents of the breakdown -operate only within a few feet of the surface, and the agents of rock -decomposition, derived as they are from the atmosphere, become inert -before they have descended to any considerable depth. The surface layer -of incoherent rock is usually referred to as the _rock mantle_ (Fig. -163). Where the rock mantle is relatively deep, as it is in the states -south of the Ohio in the eastern United States, there is found, deep -below the outer layer of soil, a partially decomposed and disintegrated -rock, of which the unaltered minerals lie unchanged in position but -separated by the new minerals which have resulted from the breakdown -of their more susceptible associates. While thus in a certain sense -possessing the original structure, this altered material is essentially -incoherent and easily succumbs to attack by the pick and spade, so that -it is only at considerably greater depths that the unaltered rock is -encountered. - -[Illustration: - -FIG. 163.—Rock mantle consisting of broken rock, above which is -soil and a vegetable mat. Coast of California (after a photograph by -Fairbanks).] - -Because of the tendency of mantle rock to creep down upon slopes it is -generally found thicker upon the crests and at the bases of hills and -thinnest upon their slopes (Fig. 164). - -In the transformation of the upper portion of the mantle rock into -soil, additional chemical processes to those of weathering are carried -through by the agency of earthworms, bacteria, and other organisms, and -by the action of humus and other acids derived from the decomposition -of vegetation. The bacteria particularly play a part in the formation -of carbonates, as they do also in changing the nitrogen of the air -into nitrates which become available as plant food. Within the humid -tropical regions ants and other insects enter as a large factor in rock -decomposition, as they do also in producing not unimportant surface -irregularities. - -[Illustration: - -FIG. 164.—Diagram to show the varying thickness of mantle rock -upon the different portions of a hill surface (after Chamberlin and -Salisbury).] - -How important is the cover of vegetation in retaining the rock -mantle and the upper soil layer in their respective positions, as -required for agricultural purposes, may be best illustrated by the -disastrous consequences of allowing it to be destroyed. Wherever, by -the destruction of forests, by the excessive grazing of animals, or -by other causes, the mat of turf has been destroyed, the surface is -opened in gullies by the first hard rain, and the fertile layer of soil -is carried from the slopes and distributed with the coarser mantle -upon the bottom lands. Thus the face of the country is completely -transformed from fertile hills into the most desolate of deserts where -no spear of grass is to be seen and no animal food to be obtained -(plate 5 A). The soil once washed away is not again renewed, for the -continuation of the gullying process now effectively prevents its -accumulation. - -┌────────────────────────────────────────────────────────────────┐ -│ PLATE 5. │ -│ │ -│ [Illustration: _A._ Once wooded region in China now reduced to │ -│ desert │ -│ through deforestation (after Willis).] │ -│ │ -│ [Illustration: _B._ “Bad Lands” in the Colorado Desert (after │ -│ Mendenhall).] │ -└────────────────────────────────────────────────────────────────┘ - - - READING REFERENCES TO CHAPTER XI - - Decomposition and disintegration:— - - GEORGE P. MERRILL. The Principles of Rock Weathering, Jour. Geol., - vol. 4, 1896, pp. 704-724, 850-871. Rocks, Rock Weathering, and Soils. - Macmillan, New York, 1897, Pt. iii, pp. 172-411. - - ALEXIS A. JULIEN. On the Geological Action of the Humus Acids, Proc. - Am. Assoc. Adv. Sci., vol. 28, 1879, pp. 311-410. - -Corrosion of rocks:— - - C. W. HAYES. Solution of Silica under Atmospheric Conditions, Bull. - Geol. Soc. Am., vol. 8, 1897, pp. 213-220, pls. 17-19. - - M. L. FULLER. Etching of Quartz in the Interior of Conglomerates, - Jour. Geol., vol. 10, 1902, pp. 815-821. - - C. H. SMYTH, JR. Replacement of Quartz by Pyrites and Corrosion of - Quartz Pebbles, Am. Jour. Sci. (4), vol. 19, 1905, pp. 282-285. - -Dome structure of granite masses:— - - G. K. GILBERT. Domes and Dome Structure of the High Sierra, Bull. - Geol. Soc. Am., vol. 15, 1904, pp. 29-36, pls. 1-4. - - RALPH ARNOLD. Dome Structure in Conglomerate, _ibid._, vol. 18, 1907, - pp. 615-616. - -Soil flow:— - - J. GUNNAR ANDERSSON. Solifluction, a Component of Subaërial - Denudation, Jour. Geol., vol. 14, 1906, pp. 91-112. - - OTTO NORDENSKIÖLD. Die Polarwelt und ihre Nachbarländer, Leipzig, - 1909, pp. 60-65. - - ERNEST HOWE. Landslides in the San Juan Mountains, Colorado, etc., - Prof. Pap., 67 U. S. Geol. Surv., 1909, pp. 1-58, pls. 1-20. - - G. E. MITCHELL. Landslides and Rock Avalanches, Nat. Geogr. Mag., vol. - 21, 1910, pp. 277-287. - - WILLIAM H. HOBBS. Soil Stripes in Cold Humid Regions and a Kindred - Phenomenon, 12th Rept. Mich. Acad. Sci., 1910, pp. 51-53, pls. 1-2. - -Relation of deforestation to erosion:— - - N. S. SHALER. Origin and Nature of Soils, 12th Ann. Rept. U. S. Geol. - Surv., 1891, Pt. 1, pp. 268-287. - - W. J. MCGEE. The Lafayette Formation, _ibid._, pp. 430-448. - - F. H. KING. Soils. Macmillan, New York, 1908, pp. 50-54. - - BAILEY WILLIS. Water Circulation and Its Control, Rept. Nat. Conserv. - Com., 1909, vol. 2, pp. 687-710. - - W. J. MCGEE. Soil erosion, Bull. 71, U. S. Bureau of Soils, 1911, pp. - 60, pls. 33. - - - - -CHAPTER XII - -THE LIFE HISTORIES OF RIVERS - - -=The intricate pattern of river etchings.=—The attack of the weather -upon the solid lithosphere destroys the integrity of its surface -layer, and through reducing it to rock débris makes it the natural -prey of any agent competent to carry it along the surface. We have -seen how, for short distances, gravity unaided may pile up the débris -in accumulations of talus, and how, when assisted by thaw water which -has soaked into the material, it may accomplish a slow migration by a -peculiar type of soil flow. Yet far more potent transporting agencies -are at work, and of these the one of first importance is running -water. Only in the hearts of great deserts or in the equally remote -white deserts of the polar regions is the sound of its murmurings -never heard. Every other part of the earth’s surface has at some time -its running water coursing in valleys which it has itself etched into -the surface. It is this etching out of the continents in an intricate -pattern of anastomosing valleys which constitutes the chief difference -between the land surface and the relatively even floor of the oceans. - - -=The motive power of rivers.=—Every river is born in throes of Mother -Earth by which the land is uplifted and left at a higher level than -it was before. It is the difference of elevation thus brought about -between separated portions of the land areas that makes it possible for -the water which falls upon the higher portions to descend by gravity to -the lower. This natural “head” due to differences of elevation is the -motive power of the local streams, and for each increase in elevation -there is an immediate response in renewed vigor of the streams. The -elevated area off which the rivers flow is here termed an upland. - -The velocity of a stream will be dependent not only upon the difference -in altitude between its source and its mouth, but upon the distance -which separates them, since this will determine the grade. The level -of the mouth being the lowest which the stream can reach is termed -the _base level_, and the current is fixed by the slope or declivity. -The capacity to lift and transport rock débris is augmented at a quite -surprising rate with every increase in current velocity, the law being -that the weight of the heaviest transportable fragment varies with the -sixth power of the velocity of the current. Thus if one stream flows -twice as rapidly as another, it can transport fragments which are -sixty-four times as heavy. - - -=Old land and new land.=—The uplifts of the continents may proceed -without changes in the position of the shore lines, in which case -areas, already carved by streams but no longer actively modified by -them, are worked upon by tools freshly sharpened and driven by greater -power. The land thus subjected to active stream cutting is described -as old land, and has already had engraved upon it the characteristic -pattern of river etchings, albeit the design has been in part effaced. - -If, upon the other hand, the shore line migrates seaward with the -uplift, a portion of the relatively even sea floor, or new land, is -elevated and laid under the action of the running water. As we are to -see, stream cutting is to some extent modified when a river pattern is -inherited from the uplift. The uplift, whether of old land only or of -both old land and new land, marks the starting point of a new river -history, usually described as an _erosion cycle_. - - -[Illustration: FIG. 165.—Two successive forms of gullies from the -earliest stage of a river’s life (after Salisbury and Atwood).] - -=The earlier aspects of rivers.=—Though geologists have sometimes -regarded the uplift of the continents as a sort of upwarping in a -continuous curved surface, the discussions of river histories and -the pictorial illustrations of them have alike clearly assumed that -the uplift has been essentially in blocks and that the elevated area -meets the lower lying country or the sea in a more or less definite -escarpment. The first rivers to develop after the uplift may be -described as gullies shaped by the sudden downrush of storm waters and -spaced more or less regularly along the margin of the escarpment (Fig. -165). These gullies are relatively short, straight, and steep; they -have precipitous walls and few, if any, tributaries. - -[Illustration: FIG. 166.—Partially dissected upland (after Salisbury -and Atwood).] - -With time the gully heads advance into the upland as they take on -tributaries; and so at length they in part invest it and dissect it -into numerous irregularly bounded and flat-topped tables which are -separated by cañons (Fig. 166). At the same time the grade of the -channel is becoming flatter, and its precipitous walls are being -replaced by curving slopes, as will be more fully described in the -sequel. It is because of this progressive reduction of grades with -increasing age that the early stages of a river’s life are much the -most turbulent of its history. The water then rushes down the steep -grades in rapids, and is often at times opened out in some basin to -form a lake where differences of uplift have been characteristic of -neighboring sections. For several reasons such basins in the course of -a stream are relatively short lived (Chapter XXX), and they disappear -with the earlier stages of the river history. - - -=The meshes of the river network.=—From the continued throwing out -of new tributaries by the streams, the meshes in the river network -draw more closely together as the stages of its history advance. The -closeness of texture which is at last developed upon the upland is in -part determined by the quantity of rainfall, so that in New Jersey with -heavy annual precipitation the meshes in the network are much smaller -than they are, for example, upon the semiarid or arid plains of the -western United States. Its design will, however, in either case more -or less clearly express the plan of rock architecture which is hidden -beneath the surface (Chapter XVII). - - -=The upper and lower reaches of a river contrasted.=—From the fact -that the river progressively invades new portions of the upland and -lays the acquired sections under more and more thorough investment, it -has near its headwaters for a long time a frontier district which may -be regarded as youthful even though the sections near its mouth have -reached a somewhat advanced stage. The newly acquired sections of river -valley may thus possess the steep grade and precipitous walls which are -characteristic of early gullies and cañons and are in contrast with the -more rounded and flat-bottomed sections below. Lateral streams, from -the fact that they are newer than the main or trunk stream to which -they are tributary, likewise descend upon somewhat steeper grades (Fig. -167). - -[Illustration: FIG. 167.—Characteristic longitudinal sections of the -upper portion of a river valley and its tributaries (after scaled -sections by Nussbaum).] - - -=The balance between degradation and aggradation.=—We have seen -that the power to transport rock fragments is augmented at a most -surprising rate with every increase in the current velocity. While the -lighter particles of rock may be carried as high up as the surface of -the water, the heavier ones are moved forward upon the bottom with -a combined rolling and hopping motion aided by local eddies. Those -particles which come in contact with the bottom or sides of the -channel abrade its surface so as ever to deepen and widen the valley. -This cutting accomplished by partially suspended débris in rapidly -moving currents of water is known as _corrasion_ and the stream is said -to be _incising_ its valley. - -As the current is checked upon the lower and flatter grades, some of -its load of sediment, and especially the coarser portion, will be -deposited and so partially fill in the channel. A nice balance is thus -established between _degradation_ and the contrasted process known -as _aggradation_. The older the river valley the flatter become the -grades at any section of its course, and thus the point which separates -the lower zone of aggradation from the upper one of degradation moves -steadily upstream with the lapse of time. - - -=The accordance of tributary valleys.=—It is a consequence of the -great sensitiveness of stream corrasion to current velocity that -no side stream may enter the trunk valley at a level above that -of the main stream—the tributary streams enter the trunk stream -_accordantly_. Each has carved its own valley, and any abrupt increase -in gradient of the side streams near where they enter the main stream -would have increased the local corrasion at an accelerated rate and so -have cut down the channel to the level of the trunk stream. - - -=The grading of the flood plain.=—All rivers are subject to seasonal -variations in the volume of their waters. Where there are wet and -dry seasons these differences are greatest, and for a large part of -the year the valleys in such regions may be empty of water, and are -in fact often utilized for thoroughfares. In the temperate climates -of middle latitudes rivers are generally flooded in the spring when -the winter snows are melted, though they may dwindle to comparatively -small streams during the late summer. In the upper reaches of the -river the current velocities are such that the usual river channel may -carry all the water of flood time; but lower down and in the zone of -aggradation, where the current has been checked, the level of the water -rises in flood above the banks of its usual channel and spreads over -the surrounding lowlands. As a deposit of sediment is spread upon the -surface, the succession of the annual deposits from this source raises -the general level as a broad floor described as the _flood plain_ of -the river. - - -=The cycles of stream meanders.=—The annual flooding with water -and simultaneous deposition of silt is not, however, the only -grading process which is in operation upon the flood plain. It is -characteristic of swift currents that their course is maintained -in relatively straight lines because of the inertia of the rapidly -moving water. In proportion as their currents become sluggish, rivers -are turned aside by the smallest of obstructions; and once diverted -from their straight course, a law of nature becomes operative which -increases the curvature of the stream at an accelerated rate up to -a critical point, when by a change, sudden and catastrophic, a new -and direct course is taken, to be in its turn carried through a -similar cycle of changes. This so-called _meandering_ of a stream is -accompanied by a transfer of sediment from one bend or meander of the -river to those below and from one bank to the other. Inasmuch as the -later meanders cross the earlier ones and in time occupy all portions -of the plain to the same average extent, a process of rough grading is -accomplished to which the annual overflow deposit is supplementary. - -[Illustration: - -FIG. 168.—Map and sections of a stream meander. The course of the main -current is indicated by the dashed line.] - -The course of the current in consecutive meanders and the cross -sections of the channel which result directly from the meandering -process will be made clear from examination of Fig. 168. So soon -as diverted from its direct course, the current, by its inertia of -motion, is thrown against the outer or convex side so as to scour or -corrade that bank. Upon the concave or inner side of the curve there -is in consequence an area of slack water, and here the silt scoured -from higher meanders is deposited. The scouring of the current upon -the outer bank and the filling upon the inner thus gives to the cross -section of the stream a generally unsymmetrical character (Fig. 168 -_ab_). Between meanders near the point of inflection of the curve, and -there only, the current is centered in the middle of the channel and -the cross section is symmetrical (Fig. 168 _cd_). - -[Illustration: FIG. 169.—Tree in part undermined upon the outer bank -of a meander.] - -The scour upon the convex side of a meander causes the river to swing -ever farther in that direction, and through invasion of the silted -flood plain to migrate across it. Trees which lie in its path are -undermined and fall outward in the stream with tops directed with the -current (Fig. 169). Whenever the flood plain is forested, the fallen -trees may be so numerous as to lie in ranks along the shore, and at the -time of the next flood they are carried downstream to jam in narrow -places along the channel and give the erroneous impression that the -flood has itself uprooted a section of forest (see p. 418). - -[Illustration: - -FIG. 170.—Diagrams to show the successive positions of stream meanders -and the relatively stationary point near the sharpest curvature.] - - -=The cut-off of the meander.=—As the meander swings toward its -extreme position it becomes more and more closely looped. Adjacent -loops thus approach nearer and nearer to each other, but in the -successive positions a nearly stationary point is established near -where the river makes its sharpest turn (Fig. 170, _G_, and Fig. 454, -p. 417). At length the neck of land which separates meanders is so -narrow that in the next freshet a temporary jamming of logs within the -channel may direct the waters across the neck, and once started in -the new direction a channel is scoured out in the soft silt. Thus by -a breaking through of the bank of the stream, a so-called “crevasse”, -the river suddenly straightens its course, though up to this time it -has steadily become more and more sharply serpentine. After the cut-off -has occurred, the old channel may for a time continue to be used by the -stream in common with the new one, but the advantage in velocity of -current being with the cut-off, the old channel contains slacker water -and so begins to fill with silt both at the beginning and the end of -the loop. Eventually closed up at both ends, this loop or “ox-bow” -is entirely separated from the new channel, and once abandoned of the -stream is transformed into an ox-bow lake (Fig. 171 and p. 415). - -[Illustration: FIG. 171.—An ox-bow lake in the flood plain of a river.] - - -=Meander scars.=—Swinging as it occasionally does in its meanderings -quite across the flood plain and against the bank of the earlier -degrading river in this section, the meander at times scours the high -bank which bounds the flood plain, and undermining it in the same -manner, it excavates a recess of amphitheatral form which is known as -a meander _scar_ (Fig. 172). At length the entire bank is scarred in -this manner so as to present to the stream a series of concave scallops -separated by sharp intermediate salients of cuspate form. - - -[Illustration: - -FIG. 172.—Schematic representation of a series of river terraces. _a_, -_b_, _c_, _e_, successive terraces in order of age. _d_, _d_, _d_, _d_, -terrace slopes formed of meander scars.] - -=River terraces.=—Whenever the river’s history is interrupted by a -small uplift, or the base level is for any reason lowered, the stream -at once begins to sink its channel into the flood plain. Once more -flowing upon a low grade, it again meanders, and so produces new walls -at a lower level, but formed, like the first, of intersecting meander -scars. Thus there is produced a new flood plain with cliff and terrace -above, which is known as a _river terrace_. A succession of uplifts or -of depressions of the base level yields terraces in series, as they -appear schematically represented in Fig. 172. Such terraces are to be -found well developed upon most of our larger rivers to the northward of -the Ohio and Missouri. The highest terrace is obviously the remnant of -the earliest flood plain, as the lowest represents the latest. - - -=The delta of the river.=—As it approaches its mouth the river moves -more and more sluggishly over the flat grades, and swings in broader -meanders as it flows. Yet it still carries a quantity of silt which is -only laid down after its current has been stopped on meeting the body -of standing water into which it discharges. If this be the ocean, the -salinity of the sea water greatly aids in a quick precipitation of the -finest material. This clarifying effect upon the water of the dissolved -salt may be strikingly illustrated by taking two similar jars, the -one filled with fresh and the other with salt water, and stirring the -same quantity of fine clay into each. The clay in the salt water is -deposited and the water cleared long before the murkiness of the other -has disappeared. - -By the laying down of the residue of its burden of sediment where it -meets the sea, the river builds up vast plains of silt and clay which -are known as deltas and which often form large local extensions of the -continents into the sea. Whereas in its upper reaches the river with -its tributary streams appears in the plan like a tree and its branches, -in the delta region the stream, by dividing into diverging channels -called distributaries (Fig. 458, p. 420), completes the resemblance to -the tree by adding the roots. From the divergence of the distributaries -upon the delta plain the Greek capital letter Δ is suggested and has -supplied the name for these deposits. Of great fertility, the delta -plains of rivers have become the densely populated regions of the -globe, among which it is necessary to mention only the delta of the -Nile in Egypt, those of the Ganges and Brahmaputra in India, and those -of the Hoang and Yangtse rivers in China. - - -=The levee.=—When the snows thaw upon the mountains at the headwaters -of large rivers, freshets result and the delta regions are flooded. At -such times heavily charged with sediment, a thin deposit of fertile -soil is left upon the surface of the delta plain, and in Egypt -particularly this is depended upon for the annual enrichment of the -cultivated fields. Though at this time the waters spread broadly over -the plain, the current still continues to flow largely within the -normal channel, so that the slack water upon either side becomes the -locus for the main deposit of the sediment. There is thus built up on -either side of the channel a ridge of silt which is known as a _levee_, -and this bank is steadily increased in height from year to year (Fig. -452). - -To prevent the danger of floods upon the inhabited plains, artificial -levees are usually raised upon the natural ones, and in a country like -Holland, such levees (dikes) involve a large expenditure of money and -no small degree of engineering skill and experience to construct. So -important to the life of the nation is the proper management of its -dikes, that in the past history of China each weak administration has -been marked by the development of graft in this important department -and by floods which have destroyed the lives of hundreds of thousands -of people. - -[Illustration: - -FIG. 173.—“Bird-foot” delta of the Mississippi River.] - -Wherever there has been a markedly rapid sinking upon a delta region, -and depressions are common in delta territory, no doubt as a result of -the loading down of the crust, the river may present the paradoxical -condition of flowing at a higher level than the surrounding country. -Between the levees of neighboring distributaries there are peculiar -saucer-shaped depressions of the country which easily become filled -with water. At the extremity of the delta the levee may be the only -land which shows above the ocean surface, and so present the peculiar -“bird-foot” outline which is characteristic of the extremity of the -Mississippi delta, though other processes than the mere sinking of the -deposits may contribute to this result (Fig. 173). - - -=The sections of delta deposits.=—If now we leave the plan of the -delta to consider the section of its deposits, we find them so -characteristic as to be easily recognized. Considered broadly, the -delta advances seaward after the manner of a railroad embankment -which is being carried across a lake. Though the greater portion of -the deposit is unloaded upon a steep slope at the front, a smaller -amount of material is dropped along the way, and a layer of extremely -fine material settles in advance as the water clears of its finely -suspended particles (Fig. 174). Simultaneous deposits within a delta -thus comprise a nearly horizontal layer of coarser materials, the -so-called top-set bed; the bulk of the deposit in a forward sloping -layer, the so-called fore-set bed; and a thin film of clay which is -extended far in advance, the bottom-set bed (Fig. 174, 2). If at any -point a vertical section is made through the deposits, beds deposited -in different periods are encountered; the oldest at the bottom in a -horizontal position, the next younger above them and with forward -dip, and the youngest and coarsest upon the top in nearly horizontal -position (Fig. 174, 3). - -[Illustration: - -FIG. 174.—Diagrams to show the nature of delta deposits as exhibited -in section.] - -It has been estimated that the surface of the United States is now -being pared down by erosion at the average rate of an inch in 760 -years. The derived material is being deposited in the flood plain -and delta regions of its principal rivers. Some 513 million tons of -suspended matter is in the United States carried to tidewater each -year, and about half as much more goes out to sea as dissolved matter. -If this material were removed from the Panama Canal cutting, an 85-foot -sea-level canal would be excavated in about 73 days. The Mississippi -River alone carries annually to the sea 340 million tons of suspended -matter, or two thirds of the entire amount removed from the area of the -United States as a whole. It is thus little wonder that great deltas -have extended their boundaries so rapidly and that the crust is so -generally sinking beneath the load. - - - - -CHAPTER XIII - -EARTH FEATURES SHAPED BY RUNNING WATER - - -=The newly incised upland and its sharp salients.=—The successive -stages of incising, sculpturing, and finally of reducing an uplifted -land area, are each of them possessed of distinctive characters -which are all to be read either from the map or in the lines of the -landscape. Upon the newly uplifted plain the incising by the young -rivers is to be found chiefly in the neighborhood of the margins. In -this stage the valleys are described as V-shaped cañons, for the valley -wall meets the upland surface in sharp salients (plate 12 A), and the -lines of the landscape are throughout made up from straight elements. -Though the landscapes of this stage present the grandest scenery that -is known and may be cut out in massive proportions, often with rushing -river or placid lake to enhance the effect of crag and gorge, they lack -the softness and grace of outline which belong only to the maturer -erosion stages. The grand cañon of the Colorado presents the features -characteristic of this stage in the grandest and most sublime of all -examples, and the castled Rhine is a gorge of rugged beauty, carved out -from the newly elevated plateau of western Prussia, through which the -water swirls in eddying rapids (Fig. 175). - -[Illustration: - -FIG. 175.—Gorge of the River Rhine near St. Goars, incised within an -uplifted plain which forms the hill tops.] - - -=The stage of adolescence.=—As the upland becomes more largely -invaded as a consequence of the headward advance of the cañons and -their sending out of tributary side cañons, the sharp angles in which -the cañon walls intersect the plain become gradually replaced by -well-rounded shoulders. Thus the lines in the landscape of this stage -are a combination of the straight line with a simple curve convex -toward the sky (Fig. 176). In this stage large sections of the original -plateau remain, though cut into small areas by the extensions of the -tributary valleys. - - -[Illustration: - -FIG. 176.—V-shaped valley with well-rounded shoulders characteristic of -the stage of adolescence. Allegheny plateau in West Virginia.] - -=The maturely dissected upland.=—Continued ramifications by the -rivers eventually divide the entire upland area into separated parts, -and the rounding of the shoulders of valleys proceeds simultaneously -until of the original upland no easily recognizable compartments -are to be found. Where before were flat hilltops are now ridges or -watersheds, the well-known _divides_. The upland is now said to be -completely dissected or to have arrived at _maturity_. The streams are -still vigorous, for they make the full descent from the upland level -to base level, and yet a critical turning point of their history has -been reached, and from now on they are to show a steady falling off in -efficiency as sculpturing agents. - -[Illustration: - -FIG. 177.—View of a maturely dissected upland from one of its -hilltops, Klamath Mountains, California (after a photograph by -Fairbanks).] - -Viewed from one of the hilltops, the landscape of this stage bears a -marked resemblance to a sea in which the numberless divides are the -crests of billows, and these, as distance reduces their importance in -the landscape, fade away into the even line of the horizon (Fig. 177). - - -=The Hogarthian line of beauty.=—Since the youthful stage of the -upland, when the lines of its landscape were straight, its character -rugged, and its rivers wild and turbulent, there has been effected -a complete transformation. The only straight line to be seen is the -distant horizon, for the landscape is now molded in softened outlines, -among which there is a repeated recurrence of the line of beauty made -famous by Hogarth in his “Analysis of Beauty.” As well known to all art -students, this is a sinuous line of reversed or double curvature—a -curve which passes insensibly at a point of inflection from convex -to concave (Fig. 178). The curve of beauty is now found in every -section of the hills, and it imparts to the landscape a gracefulness -and a measure of restfulness as well, which are not to be found in the -landscapes of earlier stages in the erosion cycle. In the bottoms of -the valleys also the initial windings of the rivers within their narrow -flood plains add silver beauty lines which stand out prominently from -the more somber background of the hills. - -[Illustration: FIG. 178.—Hogarth’s line of beauty.] - -Considered from the commercial viewpoint, the mature upland is one -of the least adaptable as a habitation for highly civilized man. -Direct lines of communication run up hill and down dale in monotonous -alternation, and almost the only way of carrying a railroad through the -region, without an expenditure for trestles which would be prohibitive, -is to follow the tortuous crest of a main divide or the equally winding -bed of one of the larger valleys. - -[Illustration: - -FIG. 179.—View of the old land of New England, with Mount Monadnock -rising in the distance.] - - -=The final product of river sculpture—the peneplain.=—When maturity -has been reached in the history of a river, its energies are devoted -to a paring down of the valley slopes and crests so as to reduce the -general level. From this time on hill summits no longer fall into a -common level—that of the original upland—for some mount notably -higher than others, and with increasing age such differences become -accentuated. There is now also a larger aggradation of the valleys to -form the level floors of flood plains, out of which at length the now -slight elevations rise upon such gentle slopes that the process of -land sculpture approaches its end. Gradually the vigor of the stream -has faded away, and can now only be renewed through a fresh uplift of -the land, or, what would amount to the same thing, a depression of -the base level. Upland and river have reached old age together, and -the approximation to a new plain but little elevated above base level -is so marked that the name _peneplain_ is applied to it. Scattered -elevations, which because of some favoring circumstance rise to -greater heights above the general level of the peneplain, are known as -_monadnocks_ after the type example of Mount Monadnock in New Hampshire -(Fig. 179). - - -[Illustration: FIG. 180.—Comparison of the cross sections of river -valleys for the different stages of the erosion cycle.] - -=The river cross sections of successive stages.=—To the successive -stages of a river’s life it has been common to carry over the names -from the well-marked periods of a human life. If neglecting for the -moment the general aspect of the upland, we fix our attention upon the -characteristic cross sections of the river valley, we find that here -also there are clearly marked characters to distinguish each stage -of the river’s life (Fig. 180). In infancy the steep, narrow, and -sharp-angled cañon is a characteristic; with youth the wider V-form -has already developed; in adolescence the angles of the cañon are -transformed into well-rounded shoulders, and the valley broadens so -as in the lower reaches to lay down a flood plain; in maturity the -divides and the double curves of the line of beauty appear; while in -the decline of old age the valleys are extremely broad and flat and are -floored by an extended flood plain. - - -=The entrenchment of meanders with renewed uplift.=—Upon the reduced -grades which are characteristic of the declining stage of a river’s -life, the current has little power to modify the surface configuration. -On the old land of this stage a renewed uplift starts the streams again -into action. This infusion of driving power into moving water, regarded -as a machine capable of accomplishing certain work, is like winding up -a clock that has run down. Once more the streams acquire a velocity -sufficient to enable them to cut their valleys into the land surface, -and so a new erosional cycle may be inaugurated upon the old land -surface—the peneplain. After such an uplift has been accomplished -and the rivers have sunk their early valleys within the new upland, we -may look out from this now elevated surface and the eye take in but a -single horizontal line, since we view the plain along its edge. - -[Illustration: FIG. 181.—The Beavertail Bend of the Yakima Cañon in -central Washington (after George Otis Smith).] - -By the uplift the meanders of the earlier rivers may become entrenched -in the new upland, the wide lobes of the individual meanders being now -separated by mountains where before had been plains of silt only. The -New River of the Cumberland plateau and the Yakima River of central -Washington (Fig. 181) furnish excellent American examples of intrenched -meanders, as the Moselle River does in Europe. Upon the course of the -latter river near the town of Zell a tunnel of the railroad a quarter -of a mile in length pierces a mountain in the neck of a meander lobe -in which the river itself travels a distance of more than six miles in -order to make the same advance. The Kaiser Wilhelm tunnel in the same -district penetrates a larger mountain included in a double meander of -the river. Although intrenched, river meanders are still competent to -scour and so undermine the outer bank, and with favoring conditions -they may by this process erode extended “bottoms” out of the plateau. -(See Lockport quadrangle, U. S. G. S.) - - -=The valley of the rejuvenated river.=—Whenever a new uplift occurs -before an erosional cycle has been completed, the rivers become -intrenched, not in a peneplain, but in the bottoms of broad valleys. -The sweeping curves which characterize mature landscapes may thus be -brought into striking contrast with the straight lines of youthful -cañons which with V-sections descend from their lowest levels (Fig. -182). The full cross section of such a valley shows a central V whose -sharp shoulders are extended outward and upward in the softened curves -of later erosion stages. - -[Illustration: FIG. 182.—A rejuvenated river valley (after a -photograph by Fairbanks).] - - -=The arrest of stream erosion by the more resistant rocks.=—The -capacity of a river to erode and carry away the rock material that -lies along its course is dependent not only upon the velocity of the -current, but also upon the hardness, the firmness of texture, and the -solubility of the material. Particularly in arid and semiarid regions, -where no mantle of vegetation is at hand to mask the surfaces of the -firmer rock masses, differences of this kind are stamped deeply upon -the landscape. The rock terraces in the Grand Cañon of the Colorado -together represent the stronger rock formations of the region, while -sloping talus accumulations bury the weaker beds from sight. - -[Illustration: FIG. 183.—Plan of a river narrows.] - -Each area of harder rock which rises athwart the course of a stream -causes a temporary arrest in the process of valley erosion and is -responsible for a noteworthy local contraction of the river valley. -The valley is carved less widely as well as less deeply, and since a -river can never corrade below its base, a “temporary base level” is -for a time established above the area of harder rock. Owing to the -contraction of the valley under these conditions, the locality is -described as a river _narrows_ (Fig. 183). The narrows upon the Hudson -River occur in the Highlands where the river leaves a broad expanse -occupied by softer sediments to traverse an island-like area of hard -crystalline rocks. Within the narrows of a river the steep walls, -characteristic of youth and the turbulent current as well, are often -retained long after other portions of the river have acquired the more -restful lines of river maturity. The picturesque crag and the generally -rugged character of river narrows render them points of special -interest upon every navigable river. - -[Illustration: - -FIG. 184.—Successive diagrams to illustrate repeated river piracy and -the development of “trellis drainage”, (after Russell).] - - -=The capture of one river’s territory by another.=—The effect of a -hard layer of rock interposed in the course of a stream is thus always -to delay the advance of the erosional process at all levels above the -obstruction. When a stream in incising its valley degrades its channel -through a veneer of softer rocks into harder materials below, it is -technically described as having _discovered_ the harder layer. Where -several neighboring streams flow by similar routes to their common base -level, those which discover a harder rock will advance their headwaters -less rapidly into the upland and so will be at a disadvantage in -extending their drainage territory. A stream which is not thus hindered -will in the course of time rob the others of a portion of their -territory, for it is able to erode its lower reaches nearer to base -level and thus acquire for its upper reaches, where erosion is chiefly -accomplished, an advantage in declivity. The divide which separates its -headwaters from those of its less favored neighbor will in consequence -migrate steadily into the neighbor’s territory. The divide is thus a -sort of boundary wall separating the drainage basins of neighboring -streams, and any migration must extend the territory of the one at -the expense of the other. As more and more territory is brought under -the dominion of the more favored stream, there will come a time when -the divide in its migration will arrive at the channel of the stream -that is being robbed, and so by a sudden act of annexation draw off -all the upper waters into its own basin. By this _capture_ the stream -whose territory has been invaded is said to have been _beheaded_. -By this act of _piracy_ the stronger stream now develops exceptional -activity because of the local steep grades near the point of capture, -and with this newly acquired cutting power the invader is competent to -advance still further and enter the territory of the stream that lies -next beyond. The type of drainage network which results from repeated -captures of this kind is known as “trellis drainage” (Fig. 184), a type -well illustrated by the rivers of the southern Appalachians. - -In general it may be said that, other conditions being the same, of -two neighboring streams which have a common base level, that one which -takes the longest route will lose territory to the other, since it must -have the flatter average slope. Stream capture may thus come about -without the discovery of hard rock layers which are more unfavorable to -one stream than another. - -[Illustration: - -FIG. 185.—Sketch maps to show the earlier and the present drainage -condition about the Blue Ridge near Harper’s Ferry.] - - -=Water and wind gaps.=—In the Allegheny plateau rivers cross, the -range of harder rocks in deep mountain narrows which upon the horizon -appear as gateways through the barrier of the mountain wall. Such -gateways are sometimes referred to as “water gaps”, of which the -Delaware Water Gap is perhaps the best known example, though the -Potomac crosses the Blue Ridge at the historic Harper’s Ferry through -a similar portal. The valley of the tributary Shenandoah has been the -scene of an interesting episode in the struggle of rival streams which -is typical of others in the same upland region. The records which may -be made out from the landscapes show clearly that in an earlier but -recent period, when the general surface stood at a higher level which -has been called the Kittatinny Plain, the younger Potomac of that time -and a younger but larger ancestor of Beaverdam Creek each crossed -the Blue Ridge of the time through similar water gaps (Fig. 185, map, -and Fig. 186). The Potomac of that time was, however, the more deeply -intrenched, and possessing an advantage in slope it was able to advance -the divide at the head of its tributary, the Shenandoah, into the -territory of Beaverdam Creek. Thus the beheading of the Beaverdam by -the Shenandoah was accomplished (Fig. 185, second map) and its upper -waters annexed to the Potomac system. With the subsequent lowering of -the general level of the country which yielded the present Shenandoah -Plain, the former water gap of Beaverdam Creek was abandoned of its -stream at a high level in the range. Known as Snickers Gap, it may -serve as a type of the “wind gaps” of similar origin which are not -altogether uncommon in the Appalachian Mountain system (Fig. 186). - - -[Illustration: FIG. 186.—Section to illustrate the history of Snickers -Gap.] - -=Character profiles.=—For humid regions the landscapes possess -characters which, speaking broadly, depend upon the stage of the -erosion cycle. For the earliest stages the straight line enters as -almost the only element in the design; as the cycle advances to -adolescence the rounded forms begin to replace the angles of the -immature stages, and with full maturity the lines of beauty alone -are characteristic. As this critical stage is passed irregularity of -feature and ever more flattened curves are found to correspond to the -decline of the river’s vital energies. There are thus marks of senility -in the work of rivers (Fig. 187). - -[Illustration: FIG. 187.—Character profiles of landscapes shaped by -stream erosion in humid climates.] - - -READING REFERENCES FOR CHAPTERS XII AND XIII - - General:— - - SIR JOHN PLAYFAIR. Illustrations of the Huttonian Theory of the Earth. - Edinburgh, 1802, pp. 350-371. - - J. W. POWELL. Exploration of the Colorado River of the West and its - Tributaries. Washington, 1875, pp. 149-214. - - G. K. GILBERT. Report on the Geology of the Henry Mountains. - Washington, 1877, pp. 99-150. (A classic upon the work of rivers.) - - C. E. DUTTON. Tertiary History of the Grand Cañon District (with - atlas), Mon. 2, U. S. Geol. Surv., 1882, pp. 264. - - W. M. DAVIS. The Rivers and Valleys of Pennsylvania, Nat. Geogr. Mag. - vol. 1, 1889, pp. 203-219; The Triassic Formation of Connecticut, 18th - Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 144-153. - - SIR A. GEIKIE. The Scenery of Scotland. London, 1901, pp. 1-12. - - I. C. RUSSELL. Rivers of North America. Putnam. New York, 1898, pp. - 327. - - M. R. CAMPBELL. Drainage Modifications and their Interpretation, Jour. - Geol., vol. 4, 1896, pp. 567-581, 657-678. - - HENRY GANNETT. Physiographic Types, U. S. Geol. Surv., Topographic - Atlas, Folios 1-2, 1896, 1900. - - W. M. DAVIS. The Geographical Cycle, Geogr. Jour., vol. 14, 1899, pp. - 481-504. - -The flood plain:— - - HENRY GANNETT. The Flood of April, 1897, in the Lower Mississippi, - Scot. Geogr. Mag., vol. 13, 1897, pp. 419-421. - - W. M. DAVIS. The Development of River Meanders, Geol. Mag., Decade iv, - vol. 10, 1903, pp. 145-148. - - W. S. TOWER. The Development of Cut-off Meanders, Bull. Am. Geogr. - Soc., vol. 36, 1904, pp. 589-599. - -River terraces:— - - W. M. DAVIS. The Terraces of the Westfield River, Massachusetts, Am. - Jour. Sci., vol. 14, 1902, pp. 77-94, pl. 4; River Terraces in New - England, Bull. Mus. Comp. Zoöl., vol. 38, 1902, pp. 281-346. - -River deltas:— - - G. K. GILBERT. The Topographic Features of Lake Shores, 5th Ann. - Rept. U. S. Geol. Surv., 1885, pp. 104-108; Lake Bonneville, Mon. I, - U. S. Geol. Surv., 1890, pp. 153-167. - - Charts of Mississippi River Commission. - - G. R. CREDNER. Die Deltas, ihre Morphologie, geographische Verbreitung - und Entstehungsbedingungen, Pet. Mitt. Ergh. 56, 1878, pp. 1-74, pls. - 1-3. - -The peneplain:— - - W. M. DAVIS. Plains of Marine and Subaërial Denudation, Bull. Geol. - Soc. Am., vol. 7, 1896, pp. 377-398; The Peneplain, Am. Geol., vol. - 23, 1899, pp. 207-239. - -Intrenchment of meanders:— - - W. M. DAVIS. The Seine, the Meuse, and the Moselle, Nat. Geogr. Mag., - vol. 7, 1896, pp. 189-202. - -Stream capture:— - - N. H. DARTON. Examples of Stream Robbing in the Catskill Mountains, - Bull. Geol. Soc. Am., vol. 7, 1896, pp. 505-507, pl. 23. - - COLLIER COBB. A Recapture from a River Pirate, Science, vol. 22, 1893, - p. 195. - - WILLIAM H. HOBBS. The Still Rivers of Western Connecticut, Bull. Geol. - Soc. Am., vol. 13, 1902, pp. 17-22, pl. 1. - - ISAIAH BOWMAN. A Typical Case of Stream Capture in Michigan, Jour. - Geol., vol. 12, 1904, pp. 326-334. - - - - -CHAPTER XIV - -THE TRAVELS OF THE UNDERGROUND WATER - - -=The descent within the unsaturated zone.=—Of the moisture -precipitated from the atmosphere, that portion which neither evaporates -into the air nor runs off upon the surface, sinks into the ground and -is described as the _ground water_. Here it descends by gravity through -the pores and open spaces, and at a quite moderate depth arrives at a -zone which is completely saturated with water. The depth of the upper -surface of this saturated zone varies with the humidity of the climate, -with the altitude of the earth’s surface, and with many other similarly -varying factors. Within humid regions its depth may vary from a few -feet to a few hundred feet, while in desert areas the surface may lie -as low as a thousand feet or more. - -The surface of the zone of the lithosphere that is saturated with water -is called the _water table_, and though less accentuated it conforms in -general to the relief of the country (Fig. 188). Its depth at any point -is found from the levels of all perennial streams and from the levels -at which water stands in wells. - -[Illustration: FIG. 188.—Diagram to show the seasonal range in the -position of the water table and the cause of intermittent streams.] - -During the season of small precipitation the water table is lowered, -and if at such times it falls below the bed of a valley, the surface -stream within the valley dries up, to be revived when, after heavier -precipitation, the water table has in turn been raised. Such streams -are said to be _intermittent_, and are especially characteristic of -semiarid regions (Fig. 188). - -Wherever in descending from the surface an impervious layer, such as -clay, is encountered, the further downward progress of the water is -arrested. Now conducted in a lateral direction it issues at the surface -as a spring at the line of emergence of the upper surface of the -impervious layer (Fig. 189). - -[Illustration: FIG. 189.—Diagram to show how an impervious layer -conducts the descending water in a lateral direction to issue in -surface springs.] - - -=The trunk channels of descending water.=—While within the -unconsolidated rock materials near the surface of the earth, it is -clear that water can circulate in proportion as the materials are -porous and so relatively pervious. As the pore spaces become minute and -capillary, the difficulty of permeation through the materials becomes -very great. Thus in the noncoherent rocks it is the coarse gravel and -the layers of sand which serve as the underground channels, while the -fine clays have the effect of an impervious wall upon the circulating -waters. In coarse sand as much as a third of the volume of the material -is pore space for the absorption and transmission of water. Even under -these favorable conditions the movement of the water is exceedingly -slow and usually less than a fifth of a mile a year. - -[Illustration: - -FIG. 190.—Sketch map of the Oucane de Chabrières near Chorges in the -High Alps, to illustrate the corrosion of limestone along two series of -vertical joints (after Martel).] - -Within the hard rocks it is the sandstones which have the largest pore -spaces, but in nearly all consolidated rocks there are additional -spaces along certain of the bedding planes, the joint openings (Fig. -190), and the crushed zones of displacement, so that these parting -planes become the trunk channels, so to speak, of the circulating -water. It is along such crevices that in the course of time the -mineral matter carried in solution by the water is deposited to produce -the ore veins and the associated crystallized minerals. - - -=The caverns of limestones.=—Where limestone formations have a nearly -flat upper surface, a large part of the surface water enters the rock -by way of the joint spaces, which it soon widens by solution into -broad crevices with well-rounded shoulders. At joint intersections -solution of the limestone is so favored that the water may here descend -in a sort of vertical shaft until it meets a bedding plane extending -laterally and offering more favorable conditions for corrosion. Its -journey now begins in a lateral direction, and solution of the rock -continuing, a tunnel may be etched out and extended until another -joint is encountered which is favorable to its further descent into -the formation. By this process on alternating shafts and galleries -the water descends to near the surface of the water table by a series -of steps, and is eventually discharged into the river system of the -district (Fig. 191). Within the larger caverns the water at the lowest -level usually flows as a subterranean river to emerge later into the -light from beneath a rock arch. - -[Illustration: - -FIG. 191.—Diagram to show the relation of caverns in limestone to -the river system of the district and to the “swallow holes” upon the -surface.] - -From the plan of a system of connecting caverns it may often be -observed that the galleries of the several levels are alike directed -along two rectangular directions which indicate the master joint -directions within the limestone formation. This is especially clear -from the map of the galleries in the explored portions of the Mammoth -Cave (Fig. 192). - - -=Swallow holes and limestone sinks.=—Above the caverns of limestone -formations there are selected points where the water has descended -in the largest volume, and here funnel-shaped depressions have -been dissolved out from the surface of the rock. In different -districts such depressions have become known as “sinks”, “swallow -holes”, _entonnoirs_, and _Orgeln_. Wherever the depressions have -a characteristic circular outline, there can be little doubt that -they are the product of solution by the descending water, and have -relatively small connections only with the subterranean caverns. They -have thus naturally collected upon their bottoms the insoluble clay -which was contained in the impure limestone as well as a certain amount -of slope wash from the surface. Inasmuch as the clays are impervious -to water, the bottoms of these swallow holes are better supplied with -moisture than the surrounding rock surfaces, and by nourishing a more -vigorous plant growth are strongly impressed upon the landscape (Fig. -193). - -[Illustration: FIG. 192.—Plan of a portion of Mammoth Cave, Kentucky -(after H. C. Hovey).] - -[Illustration: - -FIG. 193.—Trees and shrubs growing luxuriantly upon the bottoms of -sinks within a limestone country (after a photograph by H. T. A. de L. -Hus).] - -Certain of the depressions above caverns are, however, less regular in -outline, and their bottoms are occupied by a mass of limestone rubble. -In some instances, at least, these depressions appear to be the result -of local incaving of the cavern roofs. An incaving of this nature may -close up an earlier gallery in the cavern and divert the cave waters -to a new course. The destruction of the roofs of caverns through this -process of incaving may continue until only relatively small remnants -are left. From long subterranean tunnels the caves are thus transformed -into subaërial rock bridges that have become known as “natural -bridges.” The best-known American example is the Natural Bridge near -Lexington, Virginia. Much grander natural bridges have been formed in -sandstone by a totally different process, and must not be confused with -these limestone remnants of caverns. - - -=The sinter deposits.=—Just as water can dissolve the calcareous -rocks with the formation of caverns, it can under other conditions -deposit the material which has thus been taken into solution. Its -power to hold carbonate of lime in solution is dependent upon the -presence of carbonic acid gas within the water. Water charged with gas -and dissolved lime carbonate is said to be “hard”, and if the gas be -driven off by boiling or otherwise, the dissolved lime is thrown out of -solution and deposited in a form well known to all housekeepers. - -Hard water flowing in a surface stream, if dashed into spray at a -cascade, may deposit its lime carbonate in an ever thickening veneer -wherever the spray is dashed about the falls. This material, when cut -in section, has waving parallel layers and is known as _travertine_ -or _calcareous sinter_. Some of the most remarkable deposits of this -nature may be seen at the cascade of Tivoli near Rome, and most of the -Roman buildings have been constructed from travertine that has been -quarried in the vicinity. - - -=The growth of stalactites.=—Water, after percolating slowly through -the crevices of limestone, where it becomes charged with the carbonic -acid gas and with dissolved carbonate of lime, may trickle from the -roof of a cavern. Emerging from the narrow crevice, it may give off -some of its contained gas and is usually subject to evaporation, -with the result that the lime carbonate is left adhering to the rock -surface from which evaporation took place. If the water collects upon -the cavern roof so slowly that it can entirely evaporate before a drop -can form, the entire content of carbonate will be left adhering to the -roof. Evaporation is most rapid near the margins and over the center of -each drop as it develops, and the deposit which is left thus takes the -form of tiny white rings at those points upon the crevice where there -is the easiest passage for the trickling water. To the outer surface -of these rings water will first adhere and then evaporate, as it will -also slowly ooze through the passage in the ring, but here without -evaporation until it reaches the lower surface. A pendant structure -will, therefore, develop, growing outward in all directions by the -deposition of concentric layers which are thickest near the roof, and -downward into the form of a rock “icicle” through evaporation of the -water which collects near the tip. These pendant sinter formations are -known as stalactites and are thus formed of concentric layers arranged -like a series of nested cornucopias with a perforation of nearly -uniform caliber along the axis of the structure (Fig. 194). - -[Illustration: - -FIG. 194.—Diagrams to show the manner of formation of stalactites, -stalagmites, and sinter columns beneath parallel crevices upon the -roofs of caverns (in part after von Knebel).] - - -=Formation of stalagmites.=—Wherever the water percolates through -the roof of the cavern so rapidly that it cannot entirely evaporate -upon the roof, a portion falls to the floor, and, spattering as it -strikes, builds up a relatively thick cone of sinter known as a -stalagmite, and this is accurately centered beneath a stalactite -upon the roof. In proportion as the cavern is high, the dropping -water is widely dispersed as it strikes the floor, with the formation -of a correspondingly thick and blunt stalagmite. As this rises by -growth toward the roof, it often develops upon its summit a distinct -crater-like depression (Fig. 194, lower figure). When the process is -long continued, stalactites and stalagmites may grow together to form -columns which may be ranged with their neighbors like the pipes of an -organ, and like them they give out clear tones when struck lightly with -a mallet. At other times the columns are joined to their neighbors to -form hangings and draperies of the most fantastic and beautiful design -(Fig. 195). - -[Illustration: FIG. 195.—Sinter formations in the Luray caverns, -Virginia.] - -In remote antiquity limestone caverns afforded a refuge to many species -of predatory birds and animals as well as to our earliest ancestors. -The bones of all these denizens of the caves lie entombed within the -clays and the sinter formations upon the cavern floors, and they tell -the story of a fierce and long-continued warfare for the possession of -these natural strongholds. The evidence is clear that these cave men -with their primitive weapons were able at times to drive away the cave -bears, lions, and hyenas, and to set up in the cavern their simple -hearths, only in their turn to be conquered by the ferocity of their -enemies. Some of the European caves have yielded many wagonloads of the -skeletons of these fierce predatory animals, together with the simple -weapons of the primitive man. - - -=The Karst and its features.=—Most so-called limestones have a large -admixture of argillaceous materials (clays) and of siliceous or sandy -particles. Such impurities make up the bulk of the clays and muds which -are left behind when the soluble portions of the limestone have been -dissolved. - -[Illustration: FIG. 196.—Map of the dolines of the Karst region near -Divača.] - -Swallow holes we have found to be characteristic features within such -districts. When limestones are more nearly pure, as in the Karst -region east of the Adriatic Sea, similar features are developed, but -upon a grander scale, and certain additional forms are encountered. In -place of the sink or swallow hole, there appears the “karst funnel” or -_doline_, a deep, bowl-shaped depression having a flat bottom. Such -funnels may be 30 to 3000 feet across and from 6 to 300 feet in depth -(Fig. 196). Though in one or two instances known to be the result of -the break down of cavern roofs (Fig. 197), yet like the swallow holes -of other regions these larger funnels appear generally to be the work -of solution by the descending waters. Where they have been opened in -artificial cuttings along railroads or in mines, the original rock is -found intact at the bottom, with small crevices only going down to -lower levels. Over the bottoms of the dolines there is spread a layer -of fertile red clay, the _terra rossa_, like that which is obtained -as a residue when a fragment of the limestone has been dissolved in -laboratory experiments. - -[Illustration: - -FIG. 197.—Cross section of the doline formed by inbreak of a cavern -roof. The Stara Apnenka doline in Carinthia (after Martel).] - - -=A desert from the destruction of forests.=—Between the dolines is -found a veritable desert with jutting limestone angles and little if -any vegetation. The water which falls upon the surface either runs off -quickly or goes down to the subterranean caverns by which so much of -the country is undermined. Hence it is that the gardens which furnish -the sustenance for the scattered population are all included within -the narrow limits of the doline bottoms. Although to-day so largely a -barren waste, we know that the Karst upon the Adriatic was in remote -antiquity a heavily forested region and that it supplied the myriads of -wooden piles upon which the city of Venice is supported. The vessels -which brought to this port upon the Adriatic its ancient prosperity -were built from wood brought from this tract of modern desert. In the -days of Venetian grandeur the fertile terra rossa formed a veneer upon -the rock surface of the Karst and so retained the surface waters for -the support of the luxuriant forest cover. After deforestation this -veneer of rich soil was washed by the rains into the dolines or into -the few stream courses of the region, thus leaving a barren tract which -it will be all but impossible to reclaim (plate 6 A). - -[Illustration: FIG. 198.—Sharp _Karren_ of the Ifenplatte Allgäu -(after Eckert).] - -Upon the steeper slopes over the purer limestones, the rain water runs -away, guided by the joints within the rock. There is thus etched out a -more or less complete network of narrow channels (Fig. 190, p. 181), -between which the remnants rise in sharp blades to produce a structure -often simulated upon the fissured surface of a glacier that has been -melted in the sun’s rays (Fig. 401). These almost impassable areas of -karst country are described as _Schratten_ or _Karrenfelder_ (Fig. 198). - - -=The ponore and the polje.=—To-day large areas of the Karst are -devoid of surface streams, nearly all the surface water finding its -way down the crevices of the limestone into caverns, and there flowing -in subterranean courses. The foot traveler in the Karst country is -sometimes suddenly arrested to find a precipice yawning at his feet, -and looking down a well-like opening to the depth of a hundred feet or -more, he may see at the bottom a large river which emerges from beneath -the one wall to disappear beneath the other. These well-like shafts are -in the Austrian Karst known as _Ponores_, while to the southward in -Greece they are called _Katavothren_. - -┌──────────────────────────────────────────────────────────────────────┐ -│ PLATE 6. │ -│ │ -│ [Illustration: _A._ Barren Karst landscape near the famous Adelsberg │ -│ grottoes. │ -│ (_Photograph by I. D. Scott._)] │ -│ │ -│ [Illustration: _B._ Surface of a limestone ledge where joints have │ -│ been widened through solution. │ -│ Syracuse, N.Y. │ -│ (_Photograph by I. D. Scott._)] │ -└──────────────────────────────────────────────────────────────────────┘ - -Elsewhere the karst river may emerge from its subterranean course in -a broader depressed area bounded by vertical cliffs, from which it -later disappears beneath the limestone wall. Such depressions of the -karst are known as _poljen_, and appear in most cases to be above the -downthrown blocks in the intricate fault mosaic of the region. Some of -these steeply walled inclosures have an area of several hundred square -miles, and especially at the time of the spring snow melting they are -flooded with water and so transformed into seasonal lakes (Fig. 199 -and p. 422). It appears that at such times the cave galleries of the -region with their local narrows are not able to carry off all the water -which is conducted to them; and in consequence there is a temporary -impounding of the flood waters in those portions of the river’s course -which are open to the sky and more extended. The rush of water at -such times may bring the red clay into the subterranean channels in -sufficient quantity to clog the passages. The Zirknitz Lake usually has -high water two or three times a year, and exceptionally the flooding -has continued for a number of years. It has thus in some districts been -necessary to afford relief to the population through the construction -of expensive drainage tunnels. - -[Illustration: - -FIG. 199.—The Zirknitz seasonal lake within a polje of the Karst -(after Berghaus).] - -The conditions which are typified in the Karst area to the east of the -Adriatic Sea are encountered also in many other lands; as, for example, -in the Vorarlberg and Swiss Alps, in Lebanon, and in Sicily. - - -=The return of the water to the surface.=—Water which has descended -from the surface and been there held between impervious layers, may be -under the pressure of its own weight or “head”; and will later find its -way upward, it may be to the surface or higher, where a perforation is -discovered in its otherwise impervious cover. Such local perforations -are produced naturally by lines of fracture or faulting (widened at -their intersections), and artificially through the sinking of deep -wells. The water, which at ordinary times reaches the surface upon -fissures, is usually concentrated locally at the intersections of the -fracture network, where it issues in lines of fissure springs (Fig. -200); but at the time of earthquakes the water may rise above the -surface in lines of fountains (p. 83), or occasionally as sheets of -water which may mount some tens of feet into the air. - -[Illustration: - -FIG. 200.—Fissure springs arranged upon lines of rock fracture at -intersections, Pomperaug valley, Connecticut.] - -In contrast to the flow of surface springs, which varies with the -season through wide ranges both in its volume and in temperature of -the water, the volume of fissure springs is but slightly affected by -the seasonal precipitation, and the water temperature is maintained -relatively constant. Rock is but a poor heat conductor, and the -seasonal temperature changes descend a few feet only into the ground. -Thus water which rises from depths of a few hundred feet only is -apt to be icy cold, while from greater depths the effect of the -earth’s internal heat is apparent in a uniform but relatively higher -temperature of the water. Such “warm” or _thermal_ springs are apt to -contain considerable mineral matter in solution, both because the water -is far traveled and because its higher temperature has considerably -increased its solvent properties. - -It has long been recognized that lines of junction of different rock -formations at the base of mountain ranges are localities favorable for -the occurrence of thermal springs. These junction lines are usually -within zones where by movement upon fractures the widest openings -in the rock have formed, and the catchment area of the neighboring -mountain highland has supplied head for the ground water. A map of the -hot springs within the Great Basin of the western United States would -present in the main a map of its principal faults. - - -=Artesian wells.=—From the natural fissure spring an artesian -well differs in the artificial character of the perforation of the -impervious cover to the water layer. The water of artesian wells may -flow out at the surface under pressure, or it may require pumping to -raise it from some lower level. Ideal conditions are furnished where -the geological structure of the district is that of a broad basin -or syncline. The water which falls in a neighboring upland is here -impounded between two parallel, saucer-like walls and will flow under -its head if the upper wall be perforated at some low level (Fig. 201, -3). - -[Illustration: - -FIG. 201.—Schematic diagrams to illustrate the different types of -artesian wells, (1) A non-flowing well; (2) flowing wells without basin -structure caused by clogging of the pervious formation; (3) flowing -wells in an artesian basin. The dotted lines are the water levels -within the pervious layers (after Chamberlin).] - -A monoclinal structure may furnish artesian conditions when the -generally pervious layer has become clogged at a low level so as -to hold back the water (Fig. 248, 2). Pumping wells may be used -successfully even when such clogging does not exist, for the -slow-moving underground water flows readily in the direction of all -free outlets (Fig. 201, 1). - - -=Hot springs and geysers.=—Thermal springs whose temperature -approaches the boiling point of water are known as _hot springs_. -A _geyser_ is a hot spring which intermittently ejects a column of -water and steam. Both hot springs and geysers are to be found only in -volcanic regions, and appear to be connected with uncooled masses of -siliceous lava. In two of the three known geyser regions, Iceland and -New Zealand, the volcanoes of the neighborhood are still active, and -the lavas of the Yellowstone National Park date from the quite recent -geological period which immediately preceded the so-called “Ice Age.” - -Wherever found, geysers are in the low levels along lines of drainage -where the underground water would most naturally reappear at the -surface. Their water has penetrated to considerable depths below the -surface, but has been chiefly heated by ascending steam or other -vapors. The water journey has been chiefly made along fissures, as is -shown by the cool springs which often issue near them. Though some hot -springs and geysers may disappear from a district, others are found -to be forming, and there is no good reason to think that geysers are -rapidly dying out, as was at one time supposed. - -The action of a geyser was first satisfactorily explained by the great -German chemist Bunsen after he had made studies of the Icelandic -geysers, and the mechanics of the eruption was later strikingly -illustrated in the laboratory by an artificial geyser constructed by -the Irish physicist Tyndall. In many respects this action is like that -of the Strombolian eruption within a cinder cone, since it is connected -with the viscosity of the fluid and the resistance which this opposes -to the liberation of the developing vapor. In the case of the geyser, a -column of heated water stands within a vertical tube and is heated near -the bottom of the column. - -[Illustration: - -FIG. 202.—Cross section of Geysir, Iceland, with simultaneously -observed temperatures recorded at the left, and the boiling -temperatures for the same levels at the right (after Campbell).] - -Though the water may at its surface have the normal boiling temperature -and be there in quiet ebullition, the boiling point for all lower -levels is raised by the weight of the column of superincumbent liquid, -and so for a time the formation of steam within the mass is prevented. -In Fig. 202 is shown a cross section of the Icelandic _Geysir_ from -which our name for such phenomena has been derived, and to this section -have been added the actual observed temperatures of the water at the -different levels as well as the temperatures at which boiling can take -place at these levels. From this it will be seen that at a depth of 45 -feet the water is but 2° Centigrade below its boiling point. A slight -increase of temperature at this level, due to the constantly ascending -steam, will not only carry this layer above the boiling point, but the -expansion of the steam within the mass will elevate the upper layers of -the water into zones where the boiling points are lower, and thus bring -about a sudden and violent ebullition of all these upper portions. Thus -is explained the almost universal observation that just before geysers -erupt the hot water rises in the bowls and generally overflows them. - -[Illustration: - -FIG. 203.—Apparatus for simulating geyser action in the lecture room -(by courtesy of Professor B. W. Snow).] - -The water ejected from the geyser is considerably cooled in the air; -and after its return to the tube must be again heated by the ascending -vapors before another eruption can occur. The measure of the cooling, -the time necessary to fill the tube, and the supply of rising steam, -all play a part in fixing the period which separates consecutive -eruptions. If the top of the tube be narrowed from its average -caliber, as is commonly observed to be true of the geysers within the -Yellowstone National Park, the escape of the steam is further hindered, -and frequent geyser eruption promoted. - -An artificial geyser for demonstration of the phenomenon in the lecture -room is represented in Fig. 203. The cut has been prepared from a -photograph of an apparatus designed by Professor B. W. Snow of the -University of Wisconsin. In this design the tube is contracted so as -to have a top diameter one fourth only of what it is at the bottom, -where heat is directly applied by multiple Bunsen lamps. The water once -sufficiently heated, this artificial geyser erupts at regular intervals -of time which are dependent upon the dimensions of the apparatus and -the quantity of heat applied. - -In case of natural geysers a considerable quantity of heat escapes -between eruptions in steam which issues quietly from the bowl of the -geyser. If this heat be retained by plugging the mouth of the tube with -a barrowful of turf, as is sometimes done with the geyser _Strokr_ -in Iceland, eruption is promoted and so takes place earlier. Another -method of securing the same result is to increase the viscosity of the -water through the addition of soap, as was accidentally discovered by -a Chinaman who was utilizing the geyser water in the Yellowstone Park -for laundry operations. After this discovery it became a common custom -to “soap” the Yellowstone geysers in order to make them play; but this -method was prohibited under heavy penalty after the disastrous eruption -of the Excelsior Geyser. - - -=The deposition of siliceous sinter by plant growth.=—Geysers are -known only from areas of siliceous volcanic lava, and this may perhaps -have its cause in the easier solution of the geyser tube from such -materials. The silica dissolved in the heated waters is _again_ -deposited at the surface to form _siliceous sinter_ or _geyserite_. -This material forms terraces surrounding the geysers or is built up -into mounds which are often quite symmetrical, such as those of the Bee -Hive and Lone Star geysers of the Yellowstone Park (Fig. 204). - -[Illustration: - -FIG. 204.—Cone of siliceous sinter built up about the mouth of the -Lone Star Geyser in the Yellowstone National Park.] - -The greater part of this separation of silica from the heated geyser -waters is due to the action of plants or algæ that are able to -grow in the boiling waters and which produce the beautiful colors -in the linings to the hot springs. The wonderful variety of the -tints displayed is accounted for by the fact that the algæ take on -different colors at different temperatures. The silica is deposited -from the water in the gelatinous hydrated form, which, however, dries -in the sun to a white sand. The growth within the pools goes on in -a manner similar to that of a coral reef, the algæ dying below and -there becoming encased in the rock lining while still continuing to -grow upon the surface. Whereas sinter of this nature, when deposited -by evaporation alone, can produce a maximum thickness of layer of a -twentieth of an inch each year, the growth from alga deposition within -limited areas may be as much as eight inches during the same period. - - -READING REFERENCES FOR CHAPTER XIV - - General:— - - F. H. KING. Principles and Conditions of the Movements of Ground - Water, 19th Ann. Rept. U. S. Geol. Surv., 1899, Pt. ii, pp. 59-294, - pls. 6-16. - - C. S. SLICHTER. The Motions of the Underground Waters, Water Supply - Paper No. 67, U. S. Geol. Surv., 1902, pp. 1-106, pls. 1-8; Field - Measurements of the Rate of Movement of Underground Waters, _ibid._, - No. 140, 1905, pp. 1-122, pls. 1-15. - - M. L. FULLER. Occurrence of Underground Water, _ibid._. No. 114, 1905, - pp. 18-40, pls. 4; Bibliographic review and index of papers relating - to underground waters published by the United States Geological - Survey, 1879-1904, _ibid._, No. 120, 1905, pp. 1-128. - -Caverns:— - - E. A. MARTEL. Les abimes, les eaux souterraines, les cavernes, - les sources, la spélæologie. Delagrave, Paris, pp. 578. (Lavishly - illustrated.) - - H. C. HOVEY. Celebrated American Caverns. Cincinnati, 1896, pp. 228; - The Mammoth Cave of Kentucky. Louisville, 1897, pp. 111. - - J. W. BEEDE. Cycle of Subterranean Drainage in the Bloomington - Quadrangle, Proc. Ind. Acad. Sci., 1910, pp. 1-31. - -Karst conditions:— - - J. CVIJIC. Das Karstphänomen, Geogr. Abh., vol. 5, 1893. - - ÉMILE CHAIX. La topographie du desert de platé (Hautes Savoie), Le - Globe, vol. 34, 1895, pp. 1-44, pls. 1-16, pp. 217-330. - - W. V. KNEBEL. Höhlenkunde mit Berücksichtigung der Karstphänomene. - Vieweg, Braunschweig, 1906, pp. 222. - - A. GRUND. Die Karsthydrographie, Studien aus Westbosnien, Geogr. Abh., - vol. 7, No. 3, 1903, pp. 200. - - ÉMILE CHAIX-DU BOIS et ANDRÉ CHAIX. Contribution a l’étude des lapies - en Carniole et au Steinernes Meer, Le Globe, vol. 46, 1907, pp. 17-56, - pls. 26. - - P. ARBENZ. Die Karrenbildungen geschildert am Beispiele der - Karrenfelder bei der Frutt in Kanton Obwalden (Schweiz). Deutsch. - Alpenzeitung, Munich, 1909, pp. 1-9. - - F. KATZER. Karst und Karsthydrographie. Sarejevo, 1909, pp. 95. - - M. NEUMAYR. Erdgeschichte, vol. 1, pp. 500-510. - - E. DE MARTONNE. Traité de Géographie Physique, pp. 462-472 (excellent - summaries in this and the last reference). - - E. A. MARTEL. The Land of the Causses, Appalachia, vol. 7, 1893, pp. - 18-149, pls. 4-13. - -Fissure springs:— - - A. C. PEALE. Natural Mineral Waters of the United States, 14th Ann. - Rept. U. S. Geol. Surv., Pt. ii, 1894, pp. 49-88. - - WILLIAM H. HOBBS. The Newark System of the Pomperaug Valley. - Connecticut, 21st Ann. Rept. U. S. Geol. Surv., Pt. iii, 1901, pp. - 91-93. - -Artesian wells:— - - T. C. CHAMBERLIN. Requisite and Qualifying Conditions of Artesian - Wells, 5th Ann. Rept. U. S. Geol. Surv., 1885, pp. 131-173. - -Hot springs and geysers:— - - A. C. PEALE. Yellowstone Park, Thermal Springs, 12th Ann. Rept. Geol. - and Geogr. Surv. Ter. (Hayden), Pt. ii, Sec. ii, pp. 63-454 (many - plates and maps). - - W. H. WEED. Geysers, Rept. Smithson. Inst., 1891, pp. 163-178. - - ARNOLD HAGUE and W. H. WEED (on hot springs and geysers of Yellowstone - National Park), C. R. Cong. Géol. Intern., Washington, 1891, pp. - 346-363. - - W. H. WEED. Formation of Travertine and Siliceous Sinter by the - Vegetation of Hot Springs, 9th Ann. Rept. U. S. Geol. Surv., 1889, pp. - 613-676, pls. 78-87. - - M. NEUMAYR. Erdgeschichte, vol. 1, pp. 500-510. - - ARNOLD HAGUE. Soaping Geysers, Trans. Am. Inst. Min. Eng., vol. 17, - 1889, pp. 546-553. - - JOHN TYNDALL. Heat as a Mode of Motion, New York, 1873, pp. 115-121 - (artificial geyser). - - - - -CHAPTER XV - -SUN AND WIND IN THE LANDS OF INFREQUENT RAINS - - -=The law of the desert.=—It is well to keep ever in mind that there is -no universal law which dominates Nature’s processes in all the sections -of her realm. Those changes which, because often observed, are most -familiar, may not be of general application, for the reason that the -areas habitually occupied by highly civilized races together comprise -but a small portion of the earth’s surface. In the dank tropical -jungle, upon the vast arid sand plains, and in the cold white spaces -near the poles, Nature has instituted peculiar and widely different -processes. - -The fundamental condition of the desert is aridity, and this -necessitates an exclusion from it of all save the exceptional rain -cloud. Thus deserts are walled in by mountain ranges which serve -as barriers to intercept the moisture-bringing clouds. They are in -consequence saucer-shaped depressions, often with short mountain ranges -rising out of the bottoms, and such rain as falls within the inclosure -is largely upon the borders. Of this rainfall none flows out from the -desert, for the water is largely returned to the atmosphere through -evaporation. - -The desert history is thus begun in isolation from the sea from which -the cloud moisture is derived, a balance being struck between inflow -and evaporation. Yet if deserts have no outlets, it is not true that -they have no rivers. These are occasionally permanent, often periodic, -but generally ephemeral and violent. The characteristic drainage of -deserts comes as the immediate result of sudden cloudburst. As a -consequence, the desert stream flows from the mountain wall choked -with sediment, and entering the depressed basin, is for the most part -either sucked down into the floor or evaporated and returned to the -atmosphere. The dissolved material which was carried in the water is -eventually left in saline deposits, and the great burden of sediment -accumulates in thick stratified masses which in magnitude outstrip the -largest deltas in the ocean. - - -=The self-registering gauge of past climates.=—From the initiation -of the desert in its isolation from the lands tributary to the sea, -its history becomes an individual and independent one. An increasing -quantity of rainfall will be marked by larger inflow to the basin, and -the lakes which form in its lowest depression will, as a consequence, -rise and expand over larger areas. A contrary climatic change will -bring about a lowering of the lakes and leave behind the marks of -former shorelines above the water level (Fig. 205). Deserts are thus -in a sense self-registering climatic gauges whose records go back far -beyond the historic past. From them it is learned that there have been -alternating periods of larger and smaller precipitation, which are -referred to as _pluvial_ and _interpluvial_ periods. - -[Illustration: FIG. 205.—Former shore lines on the mountain wall -surrounding the desert of the Great Basin. View from the temple in Salt -Lake City (after Gilbert).] - -From such records it is learned that the Great Basin of the western -United States was at one time occupied by two great desert lakes, the -one in the eastern portion being known as Lake Bonneville (Fig. 206). -With the desiccation which followed upon the series of pluvial periods, -which in other latitudes resulted in great continental glaciers and -has become known as the Glacial Period, this former desert lake dried -up to the limits of Great Salt Lake and a few smaller isolated basins. -Between 1850 and 1869 the waters of Great Salt Lake were rising, while -from 1876 to 1890 their level was falling, though subject to periodic -fluctuations, and in recent years the waters of the lake have risen so -high as to pass all records since the occupation of the country. As -a consequence the so-called Salt Lake “cut-off” of the Union Pacific -Railway, constructed at great expense across a shallow portion of the -lake, has been overflowed by its waters. The Sawa Lake in the Persian -Desert, which disappeared some five hundred years ago, again came into -existence in 1888 so as to cover the caravan route to Teheran. - -[Illustration: - -FIG. 206.—Map of the former Lake Bonneville (dotted shores), and the -boundaries of the Great Salt Lake of 1869 (smaller area) and that of -the present (after Berghaus).] - -The record in the rocks of the distant past reveals the fact that in -some former deserts barriers were, in the course of time, broken down, -with the result that an invading sea entered through the breached -wall. The result was the sudden destruction of land life, the remains -of which are preserved in “bone beds”, now covered by true marine -deposits. A still later episode of the history was begun when the sea -had disappeared and land animals again roamed above the earlier desert. -Such an alternation of marine deposits with the remains of land plants -and animals in the deposits of the Paris Basin, led the great Cuvier -to his belief that geologic history was comprised of a succession of -cataclysms in which life was alternately destroyed and re-created in -new forms—a view which later, under the powerful influence of Lyell -and Darwin, gave way to that of more gradual changes and the evolution -of life forms. - - -=Some characteristics of the desert wastes.=—The great stretches of -the arid lands have been often compared to the ocean, and the Bedouin’s -camel is known as “the ship of the desert.” Though a deceptive -resemblance for the most part, the comparison is not without its value. -Both are closed basins, and it is in this respect that the desert and -the ocean may be said to most resemble each other, for none of the -water and none of the sediment is lost to either except as boundaries -are, with the progress of time, transposed or destroyed. Flatness of -surface and monotony of scenery both have in common, and the waters and -the sand are in each case salt; yet the ocean, from the tropics to the -poles, has the same salts in essentially the same proportions, while in -the desert the widest variations are found both in the salts which are -present and in their relative quantities. - -Upon the borders of the ocean are found ridges of yellow sand heaped up -by the wind, but these ramparts are small in comparison to those which -in deserts are found upon the borders (plate 7 A). - -The desert is a land of geographic paradoxes. As Walther has pointed -out, we have rain in the desert which does not wet, springs which yield -no brooks, rivers without mouths, forests preserved in stone, lakes -without outlets, valleys without streams, lake basins without lakes, -depressions below the level of the sea yet barren of water, intense -weathering with no mantle of disintegrated rock, a decomposition of the -rocks from within instead of from without, and valleys which branch -sometimes upstream and sometimes down. - -Within the deserts curious mushroom-like remnants of erosion afford a -local relief from the searching rays of the desert sun. Pocket-like -openings large enough for a hermit’s habitation are hollowed out by the -wind from the disintegrated rock masses. Amphitheaters open out from -little erosion valleys or wadi, and isolated outliers of the mountains -stand like sentinels before their massive fronts. - -Because of the general absence of clouds above a desert, no shield -such as is common in humid regions is provided against the blinding -intensity of the sun’s rays. Sun temperatures as high as 180° -Fahrenheit have been registered over the deserts of western Africa. -Every one is familiar with the fact that a blanket of thick clouds is a -prevention of frosts at night, for, with the setting of the sun and the -consequent radiation of heat from the earth, these rays are intercepted -by the clouds, returned and re-returned in many successive exchanges. -Over desert regions the absence of any such blanket of moisture is -responsible for the remarkable falls of temperature at sunset. Though -shortly before temperatures of 100° Fahrenheit or greater may have been -measured, it is not uncommon for water to freeze during the following -night. Much the same conditions of sudden temperature change with -nightfall are experienced in high mountains when one has ascended above -the blanketing clouds. - - -[Illustration: - -FIG. 207.—Borax deposits upon the floor of Death valley, California -(after a photograph by Fairbanks).] - -=Dry weathering—the red and brown desert varnish.=—In desert lands -the fierce rays of the sun suck up all the available moisture, and -the water table may be hundreds of feet below the surface. Roots of -trees a hundred feet or more in length have been found to testify to -the fierce struggle of the desert plant with the arid conditions. In -humid regions the meteoric water dissolves the more soluble sodium -salts near the surface of the rock and carries them out to the ocean, -where they add to the saltness of the sea. In the desert the rare -precipitations prevent an outflow, but the sun’s strong rays suck out -with the moisture the salts from within the rock, and evaporating upon -the surface, the salts are left as a coat of “alkali”, which is in part -carried away on the wind and in part washed off in one of the rare -cloudbursts. In either case these constituents find their way to the -lowest depressions of the basin, where they contribute to the saline -deposits of the desert lakes (Fig. 207). - -[Illustration: FIG. 208.—Hollowed forms of weathered granite in a -desert of central Asia (after Walther).] - -Certain of the saline constituents of the rocks, as they are thus -drawn out by the sun’s rays, fuse with the rock at the surface to form -a dense brown substance with smooth surface coat, known as _desert -varnish_. Within the interior a portion of the salts crystallize within -the capillary fissures, and like water freezing within a pipe, they -rend the walls apart. As a direct consequence of this disintegrating -process the interior of rock masses may crumble into sand; and if the -hard shell of varnish be broken at any point, the wind makes its -entrance and removes the interior portion so as to leave a hollow -shell—the characteristic “pocket rock” (Fig. 208) of the desert. The -nummulitic limestone of Mokkatan and many of the great hewn blocks of -Egyptian limestone sound hollow under the tap of the hammer, and when -broken, they reveal a shell a few inches only in thickness (Fig. 209). - -[Illustration: FIG. 209.—Hollow hewn blocks in a wall in the Wadi -Guerraui (after Walther).] - -The brown desert varnish is one of the most characteristic marks -of an arid country. It is found in all deserts under much the same -conditions, and is especially apt to be present in sandstone. When -scratched, the surface of the rock becomes either cherry-red, -indicating anhydrous ferric oxide, or it is yellowish, due to the -hydrated iron oxide which we know as iron rust. Thus it is seen that -the sands of deserts, in contrast to those yielded by other processes -within humid regions, have a characteristic red color, and this may -vary from brownish red upon the one hand to a rich carmine upon the -other. - - -=The mechanical breakdown of the desert rocks.=—The chemical changes -of decomposition within desert rocks are, as we have seen, largely due -to the action of concentrated solutions of salts at high temperatures. -That there is a certain mechanical rending of these rocks, due to the -“freezing” of salts within the capillary fissures, has been already -mentioned. A further strain effect arises in rocks like granite, which -are a mixture of different minerals. Heated to a high temperature -during the day and cooled through a considerable range at night, the -different minerals alternately expand and contract at different rates -and by different relative amounts, so that strains are set up, tending -to tear them apart. The effect of these strains is thus a surface -crumbling of rocks. - -But rock is, as already pointed out, a relatively poor conductor of -heat, and hence it is a relatively thin skin only which passes through -the daily round of wide temperature range. This outer shell when heated -is expanded, and so tends to peel off, or exfoliate, like the outer -skin of an onion. The process is therefore described as _exfoliation_. -In all rocks of homogeneous texture the continued action of this -process results in convexly spherical surfaces, the material scaled -off in the process remaining as a slope or talus which surrounds the -projecting knob (Fig. 210). Naked, these projecting domes rise above -the rim of débris at their bases. Not a particle of dust adheres to the -fresh rock surface—no dirt interferes with its glaring whiteness. Yet -close at hand lie masses of débris into which wells may be carried to -depths of more than six hundred feet without encountering either solid -rock or ground water. The bare walls of granite sometimes mount upwards -for thousands of feet into the air, as steep and as inaccessible as the -squared towers of the Tyrolean Dolomites. - -[Illustration: FIG. 210.—Smooth granite domes shaped by exfoliation -and surrounded by a rim of talus. Gebel Karsala, Nubian Desert (after -Walther).] - -Rock is such a poor conductor of heat that special strains are set -up at the margin of sunlight and shade. This localization of the -disintegration on the margin of the shaded portions of rock masses is -known as _shadow weathering_ (see Fig. 215, p. 206). - -There is, however, still another mechanical disintegrating process -characteristic of the desert regions, which is likewise dependent -upon the sudden changes of temperature. Rains, though they may not -occur for a year or more, come as sudden downpours of great volume and -violence. Rock masses, which are highly heated beneath the desert sun, -if suddenly dashed with water, may be rent apart by the differential -strains set up near the surface. That rocks may be easily rent as a -result of sudden chilling is well known to our Northern farmers, who -are accustomed to rid themselves of objectionable bowlders by first -building a fire about them and then dashing water upon their surface. -Thus split into fragments, even the larger bowlders may be handled and -so removed from the farming land. The natural process of rock rending -by the occasional cloudburst may be described as _diffission_. Blocks -as much as twenty-five feet in diameter have been observed in the -desert of western Texas, soon after being broken into several fragments -at the time of a downpour of rain (Fig. 211). - -[Illustration: - -FIG. 211.—Granite blocks in the Sierra de los Dolores of Texas, rent -into several fragments by the dash of rain (after Walther).] - - -=The natural sand blast.=—Because of the saucer-like shape, the vast -expanse, and the absence of wind breaks, the potency of wind as a -geological agent is in desert areas not easily overestimated. While -most of its work is accomplished with the aid of tools, it has been -proven that even without this help, considerable work is done through -the friction of the wind alone, particularly when moving as powerful -eddies in cracks and crannies. This wear of the wind, unaided by -cutting tools, is known as _deflation_. - -The greater work of the wind is, however, accomplished with the aid -of larger or smaller rock particles, the sand and dust, with which it -is so generally charged above the deserts. Unprotected by any mat of -vegetation the materials of the desert surface are easily lifted and -are constantly migrating with the wind. The finest dust is raised high -into the air, and is carried beyond the marginal barriers, but none of -the sand or coarser materials ever passes beyond the borders. - -[Illustration: - -FIG. 212.—“Mushroom rock” from a desert in Wyoming (after Fairbanks).] - -The efficiency of this sand as a cutting tool when carried by the wind -is directly proportioned to the size of the grain, since with larger -fragments a heavier blow is struck when carried at any given velocity. -These more effective grains are, however, not lifted far above the -ground, but advance with a squirming or hopping motion, much as do -the larger pebbles upon the bottom of a river at the time of a spring -freshet. To quote Professor Walther: “Whoever has had the opportunity -to travel over a surface of dune sand when a strong wind is blowing has -found it easy to convince himself of the grinding action of the wind. -At such times the ground becomes alive, everywhere the sand is creeping -over the surface with snake-like squirmings, and the eye quickly tires -of these writhing movements of the currents of sand and cannot long -endure the scene.” - -[Illustration: - -FIG. 213.—Windkanten shaped by the desert sand blast (after Chamberlin -and Salisbury).] - -A direct consequence of this restriction of the more effective cutting -tools to the layer of air just above the ground, is the strong tendency -to cut away all projecting masses near their bases. The “mushroom -rocks”, which are so characteristic of desert landscapes, have been -shaped in this manner (Fig. 212). Another product of the desert -sand blast is the so-called _Windkante_ (wind-edge) or _Dreikante_ -(three-edge), a pebble which is usually shaped in the form of a pyramid -(Fig. 213). - -Whenever a rock face, open to direct attack by the drifting sand, is -constituted of parts which have different hardness, the blast of sand -pecks away at the softer places and leaves the harder ones in relief. -Thus is produced the well-known “stone lattice” of the desert (Fig. -214). Particularly upon the neck of the great Sphinx have the flying -sand grains, by removing the softer layers, brought the sedimentary -structures of the sandstone into strong relief. - -[Illustration: - -FIG. 214.—The “stone lattice” of the desert, the work of the natural -sand blast (after Walther).] - -When guided both by planes of sedimentation and planes of jointing, -forms of a very high degree of ornamentation are developed. Some of -the most remarkable forms are due to the protection afforded to the -sun-exposed surfaces by the shell of desert varnish. In the shaded -portions of projecting masses there is no such protection, and here the -sand blast insinuates itself into every crack and cranny. In this it is -aided by shadow weathering due to the differential strains set up at -the border of the expanded sun-heated surface. As a result, projecting -rock masses are sometimes etched away beneath and give the effect of a -squatting animal. These forms, due to shadow erosion, have also been -likened to projecting faucets. (Fig. 215). - -[Illustration: - -FIG. 215.—Projecting rock carved by the drifting sand into the form of -a couchant animal as a result of shadow weathering and erosion. Cut in -granite on the north Indian Desert (after Walther).] - -Worn by its impact upon neighboring sand grains while in transport, but -much more as it is thrown against the ground or hard rock surfaces, -the wind-driven or _eolian_ sand is at last worn into smoothly rounded -granules which approach the form of a sphere. Compared to the surface -which sea sand acquires by attrition, this shaping process is much -the more efficient, since in the water the beach sand is buoyed up -and is more effectively cushioned against its neighboring grains. The -grains of beach sand when examined under a microscope are found to be -much more irregular in form and usually display the original fracture -surfaces only in part abraded. - - -[Illustration: - -FIG. 216.—Cliffs in loess 200 feet in height which exhibit the -characteristic vertical jointing (after von Richtofen).] - -=The dust carried out of the desert.=—When, standing upon the mountain -wall that surrounds a desert, the traveler gazes out to windward over -the great depression, his field of view is generally obscured by the -yellow haze of the dust clouds moving across the margins. Upon the -mountain flanks and extending far outside the borders, this cloud of -dust settles as a shrouding mantle of impalpable yellow powder, which -is known as _loess_. These deposits are continually deepening, and -have sometimes accumulated until they are hundreds or even thousands -of feet in thickness. Before reaching its final resting place the dust -of this deposit may have settled many times, and has certainly been in -part redistributed by the streams near the desert margin. In it are -the ingredients which are necessary for the nourishment of plants, and -it constitutes the most important of natural soils. Continually fed by -new deposits from the desert, and refertilized from below by a natural -process so soon as the upper layers become impoverished, it requires no -artificial fertilization. Without artificial aids the loess of northern -China has been tilled for thousands of years without any signs of -exhaustion. - -[Illustration: - -FIG. 217.—A cañon in loess worn by traffic and wind. A highway in -northern China (after von Richtofen).] - -Though easily pulverized between the fingers, loess is none the less -characterized by a perfect vertical jointing and stands on vertical -faces as does the solid rock (Fig. 216), but it is absolutely devoid -of layers or bedding. Its capacity of standing in vertical cliffs the -loess owes to a never failing content of lime carbonate which acts as a -cement, and to a peculiar porous structure caused by capillary canals -that run vertically through the mass, branching like rootlets and lined -with carbonate of lime. This texture once destroyed, loess resolves -itself into a common sticky clay. - -By the feet of passing animals or by wheels of vehicles, the loess is -crushed, and a portion is lifted and carried away by the wind. Thus in -the course of time roadways sink deep into the mass as steep-walled -cañons (Fig. 217). A portion of the now structureless clay remaining -upon the roadway is at the time of the rains transformed into a thick -mud which makes traveling all but impossible, though before its -structure has been destroyed the loess is perfectly drained to the -bottom of its deposits. - -The particles which compose the loess are sharply angular quartz -fragments, so fine that all but a few grains can be rubbed into the -pores of the skin. Fine scales of mica, such as are easily lifted by -the wind, are disseminated uniformly throughout the mass. The only -inclosures which are arranged in layers consist of irregularly shaped -concretions of clay. These show a striking resemblance to ginger roots -and are called by the Chinese “stone ginger”, though they are elsewhere -more generally known by their German name of _Loessmännchen_, or loess -dolls. These concretions are so disposed in the loess that their longer -axes are vertical, and they were evidently separated from the mass and -not deposited with it. - - - - -CHAPTER XVI - -THE FEATURES IN DESERT LANDSCAPES - - -[Illustration: - -FIG. 218.—Diagrams to illustrate the effects of obstructions of -different types in arresting wind-driven sand. _a_, An unyielding -obstruction which permits the wind to pass through it; _b_, a flexible -and perforated obstruction; _c_, an unyielding closed barrier (after -Schulze).] - -=The wandering dunes.=—Over the broad expanse of the desert, sand -and dust, and occasionally gypsum from the saline deposits, are ever -migrating with the wind; on quiet days in the eddying “sand devils”, -but especially during the terrifying sand storms such as in the windy -season darken the air of northern China and southern Manchuria. This -drift of the sand is halted only when an obstruction is encountered—a -projecting rock, a bush, or a bunch of grass, or again the buildings of -a city or a town. The manner in which the sand is arrested by obstacles -of different kinds is of great interest and importance, and is utilized -in raising defenses against its encroachments. If the obstacle is -unyielding but allows some of the wind to pass through it, no eddies -are produced and the sand is deposited both to windward and to leeward -of the obstruction to form a fairly symmetrical mound (Fig. 218 _a_). -An obstruction which yields to the wind causes the sand to deposit -in a mound which is largely to leeward of the obstruction (Fig. 218 -_b_). A solid wall, on the other hand, by inducing eddies, is at first -protected from the sand and mounds deposit both to windward and to -leeward (Fig. 218 _c_ and Fig. 219). - -Except when held up by an obstruction, the drifting sand travels -to leeward in slowly migrating mounds or ridges which are known as -_dunes_. Their motion is due to the wind lifting the sand from the -windward side and carrying it over the crest, from where it slides down -the leeward slope and assumes a surface which is the angle of repose -of the material. In contrast with this the windward slope is notably -gradual, being shaped in conformity to the wind currents. - -[Illustration: - -FIG. 219.—Sand accumulating both to windward and to leeward of a firm -and impenetrable obstruction. The wind comes from the left (after a -photograph by Bastin).] - -The dunes which are raised upon seashores, like those of the desert, -are constantly migrating, those upon the shores of the North Sea at the -average rate of about twenty feet per year. Relentlessly they advance, -and despite all attempts to halt them, have many times overwhelmed the -villages along the coast. Upon the great barrier beach known as the -_Kurische Nehrung_, on the southeastern shore of the Baltic Sea, such -a burial of villages has more than once occurred, but as in the course -of time further migration of the dune has proceeded, the ruins of the -buried villages have been exhumed by this natural excavating process -(Fig. 220). - -[Illustration: - -FIG. 220.—Successive diagrams to show how the town of Kunzen was -buried, and subsequently exhumed in the continued migration of a great -dune upon the Kurische Nehrung (after Behrendt).] - -┌─────────────────────────────────────────────────────────────────────┐ -│ PLATE 7. │ -│ │ -│ [Illustration: _A._ Ranges of dunes upon the margin of the Colorado │ -│ Desert (after Mendenhall).] │ -│ │ -│ [Illustration: _B._ Sand dunes encroaching upon the oasis of Wed │ -│ Souf. Algeria (after T. H. Kearney).] │ -└─────────────────────────────────────────────────────────────────────┘ - - -=The forms of dunes.=—The forms assumed by dunes are dependent to -a very large extent upon the strength of the wind and the available -supply of sand. With small quantities of sand and with moderate winds, -sickle-shaped dunes known as _barchans_ (Fig. 221) are formed, whose -convex and flatter slopes are toward the wind and whose steep concave -leeward slopes are maintained at the angle of repose. The barchan is -shaped by the wind going both over and around the dune, constantly -removing sand from the windward side and depositing it to leeward. -With larger supplies of sand and winds which are not too violent a -series of barchans is built up, and these are arranged transversely to -the wind direction (Fig. 222 _b_). If the winds are more violent, the -minor depressions in the crests of the dunes become wind channels, and -the sand is then trailed out along them until the arrangement of the -ridges is parallel to the wind (Fig. 222 _c_). The surfaces of dunes -are generally marked by beautiful ripples in the sand, which, seen from -a little distance, may give the appearance of watered silk (plate 7 A). - -[Illustration: FIG. 221.—View of desert barchans (after Haug).] - -[Illustration: - -FIG. 222.—Diagrams to show the relationships in form and in -orientation of dunes to the supply of sand and to the strength of the -wind. _a_, barchans formed by small supplies of sand and moderate -winds; _b_, transverse dune ridges, formed when supply of sand is large -and winds are moderate; _c_, dune ridges formed with large sand supply -and violent winds (after Walther and Cornish).] - -Under normal conditions dunes are not stationary but continue to wander -with the prevailing winds until they have reached the outer edge of -the zone of vegetation near the base of the foothills at the margin of -the desert. Here the grasses and other desert plants arrest the first -sand grains that reach them, and they continue to grow higher as the -sands accumulate. Some of the desert plants, like the yuccas, have so -adapted themselves to desert conditions that they may grow upward with -the sand for many feet and so keep their crowns above its surface. - - -=The cloudburst in the desert.=—Such clouds as enter the desert -through its mountain ramparts, and those derived from evaporation -from the hot desert soil, usually precipitate their moisture before -passing out of the basin. Above the highly heated floor the heavy -rain clouds are unable to drop their burden. The rain can sometimes -be seen descending, but long before it has reached the ground it has -again passed into vapor, and through repetition of this process the -clouds become so charged with moisture that when they encounter a -mountain wall and are thus forced to rise, there is a sudden downpour -not equaled in the humid regions. Desert rains are rare, but violent -beyond comparison. Often for a year or more there is no rainfall upon -the loose sand or porous clay, and the few plants which survive must -push their roots deep down until they have reached the zone of ground -water. When the clouds burst, each small cañon or _wed_ (pl. _wadi_) -within the mountain wall is quickly occupied by a swollen current -which carries a thick paste of sediment and drowns everything before -it. Ere it has flowed a mile, it may be that the water has disappeared -entirely, leaving a layer of mud and sand which rapidly dries out with -the reappearance of the sun. - -[Illustration: FIG. 223.—Ideal section across the rising mountain wall -surrounding a desert and a part of the neighboring slope (after R. W. -Pumpelly).] - -As the mountains upon seacoasts are generally rising with reference to -the neighboring sea bottom, so the mountains which hem in the deserts -are generally growing upward with reference to the inclosed desert -floor. The marginal dislocations which separate the two are often in -evidence at the foot of the steep slope (Fig. 223), and these may even -appear as visible earthquake faults to indicate that the uplift is -more accelerated than the deposition along the mountain front. - - -[Illustration: - -FIG. 224.—Dry delta or alluvial fan at the foot of a mountain range -upon the borders of a desert.] - -=The zone of the dwindling river.=—The rapid uplift so generally -characteristic of desert margins gives to the torrential streams which -develop after each cloudburst such an unusual velocity that when they -emerge from the mountain valleys on to the desert floor, the current -is suddenly checked and the burden of sediment in large part deposited -at the mouth of the valley so as to form a coarse delta deposit which -is described as a _dry delta_ (Fig. 224). Dependent upon its steepness -of slope, this delta is variously referred to as an _alluvial fan_ -or _apron_, or as an _alluvial cone_. Over the conical slopes of the -delta surface the stream is broken up into numerous distributaries -which divide and subdivide as do the roots of a tree. In the Mohammedan -countries described as _wadi_, these distributaries upon dry deltas are -on the Pacific coast of the United States referred to as “washes” (Fig. -225). - -[Illustration: FIG. 225.—Map of the distributaries of neighboring -streams which emerge at the western base of the Sierra Nevadas in -California (after W. D. Johnson).] - -Fast losing their velocity after emerging from the mountains, the -various distributaries drop first of all the heavy bowlders, then -the large pebbles and the sand, so that only the finer sand and the -silt are carried to the margin of the delta. As they enlarge their -boundaries, the neighboring deltas eventually coalesce and so form an -_alluvial bench_ or “gravel piedmont” at the foot of the range. Only -the larger streams are able to entirely cross this bench of parched -deposits with its coarsely porous structure, for the water is soon -sucked up by the thirsty materials. Encountering in its descent more -clayey layers, this water is conducted to the surface near the margin -of the bench and may there appear as a line of springs. At this level -there develops, therefore, a zone of vegetation, though there is no -local rain. - -The alluvial bench grows upward by accretion of layers which are -thickest at the mountain end, so that the steepness of the bench -increases with time. - - -=Erosion in and about the desert.=—The violent cloudburst that is -characteristic of the arid lands is a most potent agent in modeling -the surface of the ground wherever the rock materials are not too -firmly coherent. Under the dash of the rain a peculiar type of “bad -land” topography is developed (plate 5 B and Fig. 226). Such a -rain-cut surface is a veritable maze of alternating gully and ridge, a -country worthless for agricultural purposes and offering the greatest -difficulty in the way of penetrating it. When composed of stiff clay -with scattered pebbles and bowlders, the effect of the “rain erosion” -is to fashion steep clay pillars each capped by a pebble and described -as “demoiselles” (Fig. 226). - -[Illustration: - -FIG. 226.—A group of “demoiselles” in the “bad lands” (after a -photograph by Fairbanks).] - -Behind the mountain front the valleys out of which the torrents are -discharged are usually short with steep side walls and a relatively -flat bottom, ending headward in an amphitheater with precipitous walls -(Fig. 227). In the western United States such valleys are referred to -as “box cañons”, but in Mohammedan countries the name “wed” applies to -the river valley within the mountains and to the distributaries as well. - - -=Characteristic features of the arid lands.=—It is characteristic of -erosion and deposition within humid regions that all outlines become -softened into flowing curves, due to the protective mat of vegetation. -In arid lands those massive rocks which are without structural planes -of separation, partly as a consequence of exfoliation, develop broad -domes which are projected upon the horizon as great semicircles, -broken in half it may be by displacement. The same massive rocks where -intersected by vertical joint planes yield, on the contrary, sharp -granite needles like those of Harney Peak (plate 8 A). Similarly, -schistose or bedded rocks, when tilted at a high angle, may yield forms -which are almost identical. Examples of such needles are found in the -Garden of the Gods in Colorado. - -At lower levels, where the flying sand becomes effective as an eroding -agent, flat bedded rocks become etched into shelves and cornices, and -if intersected by joints, the shelves and cornices are transformed -into groups of castellated towers and pinnacles of a high degree of -ornamentation. These fantastic erosion remnants are usually referred to -as “chimneys” and may be seen in numbers in the bad lands of Dakota, as -they may in Colorado either in Monument Park or in the new Monolithic -National Park (plate 8 B). - -[Illustration: - -FIG. 227.—Amphitheater at the head of the Wed Beni Sur (after -Walther).] - -Where wind erosion plays a smaller rôle in the sculpture, but where -after an uplift a river has made its way, horizontally bedded rocks -are apt to be carved into broad _rock terraces_, nowhere shown upon so -grand a scale as about the Grand Cañon of the Colorado. Each harder -layer has here produced a floor or terrace which ends in a vertical -escarpment, and this is separated from the next lower layer of more -resistant rock by a slope of talus which largely hides the softer -intermediate beds. The great Desert of Sahara is shaped in a series of -rock terraces or steppes which descend toward the interior of the basin. - -A single harder layer of resistant rock comes often to form the -flat capping of a plateau, and is then known as a _mesa_, or table -mountain. Along its front, detached outliers usually stand like -sentinels before the larger mass, and according to their relative -proportions, these are referred to either as small mesas or as the -smaller _buttes_ (Fig. 228). - -[Illustration: FIG. 228.—Mesa and outlying butte in the Leucite Hills -of Wyoming (after Whitman Cross, U. S. G. S.).] - - -=The war of dune and oasis.=—In every desert the deposits are arranged -in consecutive belts or zones which are alternately the work of wind -and water. Surrounding the desert and upon the flanks of the mountain -wall there is found (1) the deposit of loess derived from the dust that -is carried out of the desert by the wind. Immediately within the desert -border at the base of the mountains is (2) the zone of the dwindling -river with its sloping bench of coarse rubble and gravel. - -[Illustration: FIG. 229.—Flat-bottomed basin separating dunes—_bajir_ -or _takyr_ (after Ellsworth Huntington).] - -Next in order is (3) the belt of the flying sand, a zone of dune -ridges often separated by narrow, flat-bottomed basins (Fig. 229) into -which the strongest streams bring the finer sands and silt from the -mountains. Lastly, there is (4) the central sink or sinks, into which -all water not at once absorbed within the zone of alluviation or in -the zone of dunes is finally collected. Here are the true lacustrine -deposits of clay and separated salts (Fig. 230 and Fig. 207, p. 201). -The lake deposits fill in all the original irregularities of the desert -floor, out of which the tops of isolated ranges of mountains now -project like islands out of the surface of the sea. The several zones -of deposits in their order from the margin to the center of the desert -are given schematically in Fig. 231. - -┌─────────────────────────────────────────────────────────────────────┐ -│ PLATE 8. │ -│ │ -│ [Illustration: _A._ The granite needles of Harney Peak in the Black │ -│ Hills of South Dakota (after Darton).] │ -│ │ -│ [Illustration: _B._ Castellated erosion chimneys in El Cobra Cañon, │ -│ New Mexico. │ -│ (_Photograph by E. C. Case._)] │ -└─────────────────────────────────────────────────────────────────────┘ - -[Illustration: - -FIG. 230.—Billowy surface of the salt crust on the central sink in the -Lop Desert of central Asia (after Ellsworth Huntington).] - -The zone of vegetation, as already stated, lies near the foot of -the alluvial bench, so that here are found the oases about which -have clustered the cities of the desert from the earliest records of -antiquity until now. Just without the line of oases is the wall of -dunes held back from further advance only by the vegetation which in -turn is dependent upon the rains in the neighboring mountains. With -every diminution in the water supply, the dunes advance and encroach -upon the oases (plate 7 B); while with every considerable increase in -this supply of moisture the alluvial bench advances over the dunes and -acquires a strip of their territory. Thus with varying fortunes a war -is continually waged between the withering river and the flying sand, -and the alternations of climate are later recorded in the dovetailing -together of the eolian and alluvial deposits at their common junction -(Fig. 231). - -[Illustration: FIG. 231.—Schematic diagram to show the zones of -deposition in their order from the margin to the center of a desert.] - -[Illustration: - -FIG. 232.—Mounds upon the site of the buried city of Nippur (after the -cast by Muret).] - -In addition to the smaller periodic alternations of pluvial and -interpluvial climate—the “pulse of Asia”—the record of the Asiatic -deserts indicates a progressive desiccation of the entire region, which -has now given the victory to the dune. The ancient history of the -cities of the plains supplies the records of many that have been buried -in the dunes. To-day, where once were prosperous cities, nothing is to -be seen at the surface but a group of mounds (Fig. 232). Exhumed after -much painstaking labor, the walls and palaces of these ancient cities -have once more been brought to the light of day, and much has thus been -learned of the civilization of these early times (Fig. 233). Quite -recently the mounds which cover between one and two hundred buried -villages have been found upon the borders of the Tarim basin of central -Asia, where they were lost to history when they were overwhelmed in the -early centuries of the Christian Era. - -[Illustration: FIG. 233.—Exhumed structures in the buried city of -Nippur (after Hilprecht).] - - -=The origin of the high plains which front the Rocky Mountains.=—To -the eastward of the great backbone of the North American continent -stretches a vast plain gently inclined away from the range and -generally known as the High Plains region (plate 9). The tourist who -travels westward by train ascends this slope so gradually that when he -has reached the mountain front it is difficult to realize that he has -climbed to an altitude of five thousand feet above the level of the -sea. That he has also passed through several climatic zones—a humid, a -semiarid, and an arid—and has now entered a semiarid district, is more -easily appreciated from study of the vegetation (Fig. 234). The surface -of the High Plains, where not cut into by rivers, is remarkably even, -so that it might be compared to the quiet surface of a great lake. - -[Illustration: FIG. 234.—Section across the High Plains, showing the -position of the terrace and the climatic zones above it (after W. D. -Johnson).] - -The materials which compose the surface veneer of these plains are -coarse conglomerates, gravels, and sands, and the so-called “mortar -beds”, which are nothing but sands cemented into sandstone by carbonate -of lime. The pebbles in all these deposits are far-traveled and appear -to have been derived from erosion of those crystalline rocks which -compose the eastern front of the Rocky Mountains. These different -materials are not arranged in strictly parallel beds, as are the -deposits of a lake or sea; but the beds are made up of long threads of -lenticular cross section which are interlaced in the most intricate -fashion and which extend down the slope, or outward from the mountain -front (Fig. 235). It is thus shown that the High Plains are a bench or -plain of alluviation formed at the front of the Rocky Mountains during -an earlier series of pluvial periods, and that subsequent uplift has -produced the modern river valleys which are cut out of the plain. The -plexus of long threads of the coarser materials are the courses of -dwindling rivers which interlaced over the former plain, and which in -time were buried under other channel deposits of the same nature but in -different positions (Fig. 236). The pluvial periods in which this bench -was formed produced in other latitudes the great continental glaciers -which wrought such marvelous changes in northern North America and in -northern Europe. - -[Illustration: FIG. 235.—Section across the great lenticular threads -of alluvial deposits which compose the veneer of the High Plains (after -W. D. Johnson).] - -[Illustration: FIG. 236.—Distributaries of the foothills superimposed -upon an earlier series (after W. D. Johnson).] - - -=Character profiles.=—In contrast with the profiles in the landscapes -of humid regions (see Fig. 187, p. 177), those of arid lands are -marked by straighter elements (Fig. 237). Almost the only exception -of importance is furnished by the domes of massive granite monoliths, -which are sometimes broken in half by great displacements. Below -the horizon the secondary lines in the landscape betray the same -straightness of the component elements by the gabled slopes of talus -which are many times repeated so as almost to reproduce the lines in -a house of cards, since the sloping lines are maintained at the angle -of repose of the materials (Fig. 482, p. 443). Wherever the waves of -desert lakes have made an attack upon the rocks and have retired the -projecting spurs, other gables characterized by slightly different -slopes are introduced into the landscape. - -[Illustration: FIG. 237.—Character profiles in the landscapes of arid -lands.] - -┌─────────────────────────────────┐ -│ PLATE 9. │ -│ │ -│ [Illustration: THE HIGH PLAINS] │ -└─────────────────────────────────┘ - - -READING REFERENCES FOR CHAPTERS XV AND XVI - - General:— - - JOHANNES WALTHER. Das Gesetz der Wüstenbildung in Gegenwart und - Vorzeit. Berlin, 1900, pp. 175, many plates. (This extremely valuable - work is now out of print, but both a revised edition and an English - translation are promised for 1912.) - - SIEGFRIED PASSARGE. Die Kalihari. Berlin, 1904, pp. 662. - - W. M. DAVIS. The Geographic Cycle in an Arid Climate, Jour. Geol., - vol. 13, 1905, pp. 381-407. - - ELLSWORTH HUNTINGTON. The Pulse of Asia. New York and Boston, 1907, - pp. 415. - - SVEN HEDIN. Scientific Results of a Journey through Central Asia, - 1899-1900. Stockholm, 1904-1905, vols. 1 and 2, pp. 523 and 717, pls. - 56 and 76. - - JOSEPH BARRELL. Relative Geological Importance of Continental, - Littoral and Marine Sedimentation, Jour. Geol., vol. 14, 1906, pp. - 316-356, 429-457, 524-568. - - E. F. GAUTIER. Études sahariennes, Ann. de Géogr., vol. 16, 1907, pp. - 46-69, 117-138. - -The self-registering gauge of past climates:— - - G. K. GILBERT. Lake Bonneville, Mon. I, U. S. Geol. Surv., Chapter vi, - pp. 214-318. - - T. F. JAMIESON. The Inland Seas and Salt Lakes of the Glacial Period, - Geol. Mag. decade III, vol. 2, 1885, pp. 193-200. - - J. E. TALMAGE. The Great Salt Lake, Present and Past. Salt Lake City, - 1900, pp. 116, plates. - - E. HUNTINGTON. Some Characteristics of the Glacial Period in - Non-glaciated Regions, Bull. Geol. Soc. Am., vol. 18, 1907, pp. - 351-388, pls. 31-39. - - T. C. CHAMBERLIN. The Future Habitability of the Earth, Rept. - Smithson. Inst., 1910, pp. 371-389. - -The red and brown desert varnish:— - - I. C. RUSSELL. Subaërial Decay of Rocks and Origin of the Red Color of - Certain Formations. Bull. 52, U. S. Geol. Surv., 1889, pp. 65, pls. 5. - -Erosion in the desert:— - - J. A. UDDEN. Erosion, Transportation, and Sedimentation performed by - the Atmosphere, Jour. Geol., vol. 2, 1894, pp. 318-331. - - S. PASSARGE. Die pfannenförmigen Hohlformen der südafrikanischen - Steppen, Pet. Mitt., vol. 57, 1911, pp. 57-61, 130-135. - -The dust carried out of the desert:— - - F. VON RICHTOFEN. China, Ergebnisse eigene Reisen und darauf - gegründeten Studien, Berlin, 1877, vol. 1, pp. 56-125. - - E. HILGARD. The Loess of the Mississippi Valley, Am. Jour. Sci., (3), - vol. 18, 1879, pp. 106-112. - - T. C. CHAMBERLIN and R. D. SALISBURY. Preliminary Paper on the - Driftless Area of the Upper Mississippi Valley, 6th Ann. Rept. U. S. - Geol. Surv., 1885, pp. 278-307. - - E. E. FREE. The movement of soil material by the wind, with a - bibliography of eolian geology by S. C. Stuntz and E. E. Free, Bull. - 68, U. S. Bureau of Soils, 1911, pp. 272, pls. 5. - - M. NEUMAYR. Erdgeschichte, vol. 1, pp. 510-514. - - E. DE MARTONNE. Géographie physique, pp. 663-668. - -Dunes:— - - VAUGHAN CORNISH. On the Formation of Sand-dunes, Geogr. Jour., vol. 9, - 1897, pp. 278-309 (a most important paper). - - F. SOLGER and Others. Dünenbuch. Enke, Stuttgart, 1910, pp. 373. - -The zone of the dwindling river:— - - E. HUNTINGTON. The Border Belts of the Tarim Basin, Bull. Am. Geogr. - Soc., vol. 38, 1906, pp. 91-96; The Pulse of Asia, pp. 210-222, - 262-279. - -The war of dune and oasis:— - - R. PUMPELLY. Explorations in Turkestan, Expedition of 1904, etc., Pub. - 73, Carneg. Inst., Washington, vol. 1, pp. 1-13. - - E. HUNTINGTON. The Oasis of Kharga, Bull. Am. Geogr. Soc., vol. 42. - 1910, pp. 641-661. - - TH. H. KEARNEY. The Country of the Ant Men, Nat. Geogr. Mag., vol. 22, - 1911, pp. 367-382. - -Features of the arid lands:— - - C. E. DUTTON. Tertiary History of the Grand Cañon District, with - Atlas, Mon. II, U. S. Geol. Surv., 1882, pp. 264, pls. 42, maps 23. - - G. SWEINFURTH. Map Sheets of the Eastern Egyptian Desert. Berlin, - 1901-1902, 8 sheets. - -The origin of the high plains:— - - W. D. JOHNSON. The High Plains and their Utilization, 21st Ann. Rept. - U. S. Geol. Surv., Pt. iv, 1901, pp. 601-741. - - - - -CHAPTER XVII - -REPEATING PATTERNS IN THE EARTH RELIEF - - -=The weathering processes under control of the fracture system.=—In -an earlier chapter it was learned that the rocks which compose the -earth’s surface shell are intersected by a system of joint fractures -which in little-disturbed areas divide the surface beds into nearly -square perpendicular prisms (Fig. 36, p. 55), more or less modified -by additional diagonal joints, and often also by more disorderly -fractures. Throughout large areas these fractures may maintain nearly -constant directions, though either one or more of the master series -may be locally absent. This distinctive architecture of the surface -shell of the lithosphere has exercised its influence upon the various -weathering processes, as it has also upon the activities of running -water and of other less common transporting agencies at the surface. - -Within high latitudes, where frost action is the dominant weathering -process, the water, by insinuating itself along the joints and -through repeated freezings, has broken down the rock in the immediate -neighborhood of these fractures, and so has impressed upon the surface -an image of the underlying pattern of structure lines (plate 10 A). - -In much lower latitudes and in regions of insufficient rainfall, the -same structures are impressed upon the relief, but by other weathering -processes. In the case of the less coherent deposits in these -provinces, the initial forms of their erosional surface have sometimes -been determined by the dash of rain from the sudden cloudburst. Thus -the “bad lands” may have their initial gullies directed and spaced in -conformity with the underlying joint structures (Fig. 238). - -[Illustration: - -FIG. 238.—Rain sculpturing under control by joints. Coast of southern -California (after a photograph by Fairbanks).] - -In such portions of the temperate regions as are favored by a humid -climate, the mat of vegetation holds down a layer of soil, and mat -and soil in coöperation are effective in preventing any such large -measure of frostwork as is characteristic of the subpolar regions or -of high levels in the arid lands. In humid regions the rocks become a -prey especially to the processes of solution and accompanying chemical -decomposition, and these processes, although guided by the course of -the percolating ground water along the fracture planes, do not afford -such striking examples of the control of surface relief. - -Those limestones which slowly pass into solution in the percolating -water do, however, quite generally indicate a localization of the -solution along the joint channels (Fig. 239 and plate 6 B). Though in -other rocks not so apparent, yet solutions generally take their courses -along the same channels, and upon them is localized the development -of the newly formed hydrated and carbonate minerals, as is well -illustrated by the phenomenon of spheroidal weathering (Fig. 155, p. -150). - - -[Illustration: - -FIG. 239.—Outcrop of flaggy limestone which shows the effects of -solution along neighboring joints in a sagging of the upper beds (after -Gilbert, U. S. G. S.).] - -=The fracture control of the drainage lines.=—The etching out of -the earth’s architectural plan in the surface relief, which we have -seen begun in the processes of weathering, is continued after the -transporting agents have become effective. It is often easy to see -that a river has taken its course in rectangular zigzags like the -elbows of a jointed stove pipe, and that its walls are formed of joint -planes from which an occasional squared buttress projects into the -channel. This structure is rendered in the plan of the Abisko Cañon of -northern Lapland (Fig. 240). We are later to learn that another great -transporting agent, the water wave, makes a selective attack upon the -lithosphere along the fractures of the joint system (Fig. 250, p. 233 -and Fig. 254, p. 235). - -[Illustration: - -FIG. 240.—Map of the joint-controlled Abisko Cañon in northern Lapland -(after Otto Sjögren).] - -Where the scale of the example is large, as in the cases which have -been above cited, the actual position and directions of the joint wall -are easily compared with the near-by elements of the river’s course, -so that the connection of the drainage lines with the underlying -structure is at once apparent. In many examples where the scale -is small, the evidence for the controlling influence of the rock -structure in determining the courses of streams may be found in the -peculiar character of the drainage plan. To illustrate: the course of -the Zambesi River, within the gorge below the famous Victoria Falls, -not only makes repeated turnings at a right angle, but its tributary -streams, instead of making the usual sharp angle where they join the -main stream, also affect the right angle in their junctions (Fig. 241). - -[Illustration: - -FIG. 241.—Map of the gorge of the Zambesi River below the Victoria -Falls (after Lamplugh).] - - -=The repeating pattern in drainage networks.=—It is a characteristic -of the joint system that the fractures within each series are spaced -with approximation to uniformity. If the plan of a drainage system has -been regulated in conformity with the architecture of the underlying -rock basement, the same repeating rectangles of the master joints may -be expected to appear in the lines of drainage—the so-called drainage -network. - -Such rectangular patterns do very generally appear in the drainage -network, though they are often masked upon modern maps by what, to -the geologist, seems impertinent intrusion of the black lines of -overprinting which indicate railways, lines of highway, and other -culture elements. On river maps, which are printed without culture, the -pattern is much more easily recognized (Figs. 242 and 243). Wherever -the relief is strong, as is the case in the Adirondack Mountain -province of the State of New York, individual hills may stand in relief -between the bounding streams which compose the rectangular network, -like the squared pedestals of monuments. Such a type of relief carved -in repeating patterns has been described as “checkerboard topography.” - -[Illustration: - -FIG. 242.—Controlled drainage network of the Shepaug River in -Connecticut.] - -[Illustration: - -FIG. 243.—A river network of repeating rectangular pattern. Near Lake -Temiskaming, Ontario (from the map by the Dominion Government).] - - -=The dividing lines of the relief patterns—lineaments.=—The repeating -design outlined in the river network of the Temiskaming district -(Fig. 243) would appear in greater perfection if we could reproduce -the relief without at the same time obscuring the lines of drainage; -for where the pattern is not completely closed by the course of the -stream, there is generally found either a dry valley or a ravine to -complete the design. If these are not present, a bit of straight -coast line, a visible line of fracture, a zone of fault breccia, or -the boundary line separating different formations may one or more of -them fill in the gaps of the parallel straight drainage lines which by -their intersection bring out the pattern. These significant lines of -landscapes which reveal the hidden architecture of the rock basement -are described as _lineaments_ (Fig. 82, p. 87). They are the character -lines of the earth’s physiognomy. - -It is important to emphasize the essentially composite expression of -the lineament. At one locality it appears as a drainage line, a little -farther on it may be a line of coast; then, again, it is a series of -aligned waterfalls, a visible fault trace, or a rectilinear boundary -between formations; but in every case it is some surface expression -of a buried fracture. Hidden as they so generally are, the fracture -lines must be searched out by every means at our disposal, if we are -not to be misled in accounting for the positions and the attitudes of -disturbed rock masses. - -As we have learned, during earthquake shocks, as at no other time, -the surface of the earth is so sensitized as to betray the position -of its buried fractures. As the boundaries of orographic blocks, -certain of the fractures are at such times the seats of especially -heavy vibrations; they are the seismotectonic lines of the earthquake -province. Many lineaments are identical with seismotectonic lines, and -they therefore afford a means of to some extent determining in advance -the lines of greatest danger from earthquake shock. - - -=The composite repeating patterns of the higher orders.=—Not only -do the larger joint blocks become impressed upon the earth relief as -repeating diaper patterns, but larger and still larger composite units -of the same type may, in favorable districts, be found to present the -same characters. Attention has already been more than once directed -to the fact that the more perfect and prominent fracture planes recur -among the joints of any series at more or less regular intervals (Fig. -40, p. 57, and Fig. 41, p. 58). Nowhere, perhaps, is this larger order -of the repeating pattern more perfectly exemplified than in some recent -deposits in the Syrian desert (plate 10 B). It is usually, however, -in the older sediments that such structures may be recognized; as, for -example, in the squared towers and buttresses of the Tyrolean Dolomites -(Fig. 244). Here the larger blocks appear in the thick bedded lower -formation, the dolomite, divided into subordinate sections of large -dimensions; but in the overlying formations in blocks of relatively -small size, yet with similarly perfect subequal spacing. - -[Illustration: - -FIG. 244.—Squared mountain masses which reveal a distribution of the -joints in block patterns of different orders of magnitude. The Pordoi -range of the Sella group of the Dolomites, seen from the Cima di Rossi -(after Mojsisovics).] - -The observing traveler who is privileged to make the journey by -steamer, threading its course in and out among the many islands and -skerries of the Norwegian coast, will hardly fail to be struck by the -remarkable profiles of most of the lower islands (Fig. 245). These -profiles are generally convexly scalloped with a noteworthy regularity, -and not in one unit only, but in at least two with one a multiple of -the other (Fig. 246). As the steamer passes near to the islands, it is -discovered that the smaller recognizable units in the island profiles -are separated by widely gaping joints which do not, however, belong -to the unit series, but to a larger composite group (Fig. 246 _b_). -Frostwork, which depends for its action upon open spaces within the -rocks, has here been the cause of the excessive weathering above the -more widely gaping joints. - -┌──────────────────────────────────────────────────────────────────────┐ -│ PLATE 10. │ -│ │ -│ [Illustration: _A._ View in Spitzbergen to illustrate the │ -│ disintegration of rock under the control of joints. │ -│ (_Photograph by O. Haldin._)] │ -│ │ -│ [Illustration: _B._ Composite pattern of the joint structures within │ -│ recent alluvial deposits. │ -│ (_Photograph by Ellsworth Huntington._)] │ -└──────────────────────────────────────────────────────────────────────┘ - -[Illustration: - -FIG. 245.—Island groups of the Lofoten archipelago off the northwest -coast of Norway, which reveal repeating patterns of the relief in two -orders of magnitude (after a photograph by Knudsen).] - -[Illustration: - -FIG. 246.—Diagrams to illustrate the composite profiles of the islands -on the Norwegian coast. _a_, distant view; _b_, near view, showing the -individual joints and the more widely gaping fractures beneath each sag -in the profile.] - -High northern latitudes are thus especially favorable for revealing all -the details in the architectural pattern of the lithosphere shell, and -we need not be surprised that when the modern maps of the Norwegian -coast are examined, still larger repeating patterns than any that may -be seen in the field are to be made out. The Norwegian coast was long -ago shown to be a complexly faulted region, and these larger divisions -of the relief pattern, instead of being explained as a consequence -solely of selective weathering, must be regarded as due largely to -fault displacements of the type represented in our model (plate 4 C). -Yet whether due to displacements or to the more numerous joints, all -belong to the same composite system of fractures expressed in the -relief. - - -READING REFERENCES FOR CHAPTER XVII - - WILLIAM H. HOBBS. The River System of Connecticut, Jour. Geol., - vol. 9, 1901, pp. 469-485, pl. 1; Lineaments of the Atlantic Border - Region, Bull. Geol. Soc. Am., vol. 15, 1904, pp. 483-506, pls. 45-47; - The Correlation of Fracture Systems and the Evidences for Planetary - Dislocations within the Earth’s Crust, Trans. Wis. Acad. Sci., etc., - vol. 15, 1905, pp. 15-29; Repeating Patterns in the Relief and in - the Structure of the Land, Bull. Geol. Soc. Am., vol. 22, 1911, pp. - 123-176, pls. 7-13. - - - - -CHAPTER XVIII - -THE FORMS CARVED AND MOLDED BY WAVES - - -=The motion of a water wave.=—The motions within a wave upon the -surface of a body of water may be thought of in two different ways. -First of all, there is the motion of each particle of water within -an orbit of its own; and there is, further, the forward motion of -propagation of the wave considered as a whole. - -[Illustration: FIG. 247.—Diagram to show the nature of the motions -within a free water wave.] - -The water particle in a wave has a continued motion round and round its -orbit like that of a horse circling a race course, only that here the -track is in a vertical plane, directed along the line of propagation of -the wave (Fig. 247). Each particle of water, through its friction upon -neighboring particles, is able to transmit its motion both along the -surface and downward into the water below. The force which starts the -water in motion and develops the wave, is the friction of wind blowing -over the water surface, and the size of the orbit of the water particle -at any point is proportional to the wind’s force and to the stretch -of water over which it has blown. The wind’s effect is, therefore, -cumulative—the wave is proportional to the wind’s effect upon all -water particles in its rear, added to the local wind friction. - -The size or _height_ of the wave is measured by the diameter of the -orbit of motion of the surface particle, and this is the difference in -height between trough and crest. The distance from crest to crest, or -from trough to trough, is called the _wave length_. Though the wave -motion is transmitted downward into the water there is a continued -loss of energy which is here not compensated by added wind friction, -and so the orbital motion grows smaller and smaller, until at the -depth of about a wave length it has completely died out. This level -of no motion is called the _wave base_. In quiet weather the level of -no motion is practically at the water’s surface, and inasmuch as the -geological work of waves is in large part accomplished during the great -storms, the term “wave base” refers to the lowest level of wave motion -at the time of the heaviest storms. Upon the ocean the highest waves -that have been measured have an amplitude of about fifty feet and a -wave length of about six hundred feet. - - -=Free waves and breakers.=—So long as the depth of the water is below -wave base, there is obviously no possibility of interference with the -wave through friction upon the bottom. Under these conditions waves are -described as _free waves_, and their forms are symmetrical except in so -far as their crests are pulled over and more or less dissipated in the -spray of the “white caps” at the time of high winds. - -[Illustration: FIG. 248.—Diagram to illustrate the transformation of a -free wave into a breaker as it approaches the shore.] - -As a wave approaches a shore, which generally has a gentle outward -sloping surface, there is interposed in the way of a free forward -movement the friction upon the bottom. This friction begins when the -depth of water is less than wave base, and its effect is to hold back -the wave at the bottom. Carried slowly upward in the water by the -friction of particle upon particle, the effect of this holding back -is a piling up of the water, which increases the wave height as it -diminishes the wave length, and also interferes with wave symmetry -(Fig. 248). Moving forward at the top under its inertia of motion and -held back at the bottom by constantly increasing friction, a strong -turning motion or couple is started about a horizontal axis, the -immediate effect of which is to steepen the forward slope of the wave, -and this continues until it overhangs, and, falling, “breaks” into -surf. Such a breaking wave is called a “comber” or “breaker” (plate 11 -B). - -┌─────────────────────────────────────────────────────────────────┐ -│ PLATE 11. │ -│ │ -│ [Illustration: _A._ Ripple markings within an ancient sandstone │ -│ (courtesy of U. S. Grant).] │ -│ │ -│ [Illustration: _B._ A wave breaking as it approaches the shore. │ -│ (_Photograph by Fairbanks._)] │ -└─────────────────────────────────────────────────────────────────┘ - -[Illustration: - -FIG. 249.—Notched rock cliff cut by waves and the fallen blocks -derived from the cliff through undermining. Profile Rock at Farwell’s -Point near Madison, Wisconsin.] - - -=Effect of the breaking wave upon a steep rocky shore—the notched -cliff.=—If the shore rises abruptly from deeper water, the top of -the breaking wave is hurled against the cliff with the force of a -battering ram. During storms the water of shore waves is charged with -sand, and each sand particle is driven like a stone cutter’s tool -under the stroke of his hammer. The effect is thus both to chip and -to batter away the rock of the shore to the height reached by the -wave, undermining it and notching the rock at its base (Fig. 249). -When the notch has been cut in this manner to a sufficient depth, the -overhanging rock falls by its own weight in blocks which are bounded -by the ever present joints, leaving the upper cliff face essentially -vertical. - -[Illustration: - -FIG. 250.—A wave-cut chasm under control by joints, coast of Maine -(after Tarr).] - - -=Coves, sea arches, and stacks.=—It is the headland which is most -exposed to the work of the waves, since with change of wind direction -it is exposed upon more than a single face. The study of headlands -which have been cut by waves shows that the joints within the rock -play a large rôle in the shaping of shore features. The attack of the -waves under the direction of these planes of ready separation opens -out indentations of the shore (Fig. 250) or forms _sea caves_ which, as -they extend to the top of the cliff by the process of sapping, yield -the _coves_ which are so common a feature upon our rock-bound shores -(Fig. 259, p. 238). With continuation of this process, the caves formed -on opposite sides of the headland may be united to form a _sea arch_ -(Fig. 251). - -[Illustration: - -FIG. 251.—The sea arch known as the Grand Arch upon one of the -Apostle Islands in Lake Superior (after a photograph by the Detroit -Photographic Company).] - -A later stage in this selective wave carving under the control of -joints is reached when the bridge above the arch has fallen in, leaving -a detached rock island with precipitous walls. Such an offshore island -of rock with precipitous sides is known as a _stack_ (Fig. 252), or -sometimes as a “chimney”, though this latter term is best restricted to -other and similar forms which are the product of selective weathering -(p. 300). - -[Illustration: FIG. 252.—Stack near the shore of Lake Superior.] - -Whenever the rock is less firmly consolidated, and so does not stand -upon such steep planes, the stack is apt to have a more conical form, -and may not be preceded in its formation by the development of the -sea arch (Fig. 260, p. 239). In the reverse case, or where the rock -possesses an unusual tenacity, the stack may be largely undermined -and stand supported like a table upon thick legs or pillars of rock -(Fig. 253). In Fig. 254 is seen a group of stacks upon the coast of -California, which show with clearness the control of the joints in -their formation, but unlike the marble of the South American example -the forms are not rounded, but retain their sharp angles. - -[Illustration: - -FIG. 253.—The Marble Islands, stacks in Lake Buenos Aires, southern -Andes (after F. P. Moreno).] - - -=The cut rock terrace.=—When waves first begin their attack upon a -steep, rocky shore, the lower limit of the action is near the wave -base. The action at this depth is, however, less efficient, and as the -recession of the cliff is one of the most rapid of erosional processes, -the rock floor outside the receding cliff comes to slope gradually -downward from the cliff to a maximum depth at the edge of the terrace, -approximately equal to wave base (Fig. 255). This cut terrace is -extended seaward or lakeward, as the case may be, in a _built terrace_ -constructed from a portion of the rock débris acquired from the cliff. - -[Illustration: - -FIG. 254.—Squared stacks which reveal the position of the joint planes -which have controlled in the process of carving by the waves. Pt. -Buchon, California (after a photograph by Fairbanks).] - -[Illustration: - -FIG. 255.—Ideal section of a steep rocky shore carved by waves into a -notched cliff and cut terrace, and extended by a built terrace.] - -The broken wave, after rising upon the terrace under the inertia of -its motion until all its energy has been dissipated, slides outward -by gravity, and though checked and overridden by succeeding breakers, -it continues its outward slide as the “undertow” until it reaches the -end of the terrace. Here it suddenly enters deep water, and losing its -velocity, drops its burden of rock, and builds the terrace seaward -after the manner of construction of an embankment. As we are to see, -the larger portion of the wave-quarried material is diverted to a -different quarter. - -[Illustration: - -FIG. 256.—Map showing the outlines of the Island of Heligoland at -different stages in its recent history. The peripheries given are in -miles.] - -To gain some conception of the importance of wave cutting as an eroding -process, we may consider the late history of Heligoland, a sandstone -island off the mouth of the Elbe in the North Sea (Fig. 256). From a -periphery of 120 miles, which it possessed in the ninth century of the -Christian era, the island has reduced its outline to 45 miles in the -fourteenth century, 8 miles in the seventeenth, and to about 3 miles at -the beginning of the twentieth century. The German government, which -recently acquired this little remnant from England, has expended large -sums of money in an effort to save this last relic. - - -[Illustration: - -FIG. 257.—Cut and built terrace with bowlder pavement shaped by waves -on a steep shore formed of loose materials.] - -=The cut and built terrace on a steep shore of loose materials.=—In -materials which lack the coherence of firm rock, no vertical cliff can -form; for as fast as undermined by the waves the loose materials slide -down and assume a surface of practically constant slope—the “angle of -repose” of the materials (Fig. 257). The terrace below this sloping -cliff will not differ in shape from that cut upon a rocky shore; but -whenever the materials of the shore include disseminated blocks too -large for the waves to handle, they collect upon the terrace near where -they have been exhumed, thus forming what has been called a “bowlder -pavement” (Fig. 258). - -[Illustration: - -FIG. 258.—Sloping cliff and terrace with bowlder pavement exposed at -low tide upon the shore at Scituate, Massachusetts.] - -The edge of the cut and built terrace is, as already mentioned, -maintained at the depth of wave base. If one will study the submerged -contours of any of our inland lakes, it will be found that these -basins are surrounded by a gently sloping marginal shelf,—the cut and -built terrace,—and that the depth of this shelf at its outer edge is -proportioned to the size of the lake. Upon Lake Mendota at Madison, -Wisconsin, the large storm waves have a length of about twenty feet, -which is the depth of the outer edge of the shore terraces (Fig. 267, -p. 242). The shelf surrounding the continents has, with few local -exceptions, a uniform depth of 100 fathoms, or about the wave base of -the heaviest storm waves. - - -=The work of the shore current.=—In describing the formation of -the built terrace, it was stated that the greater part of the rock -material quarried upon headlands by the waves is diverted from the -offshore terrace. This diversion is the work of the shore current -produced by the wave. - -[Illustration: FIG. 259.—Map to show the nature of the shore current -and the forms which are molded by it.] - -At but few places upon a shore will the storm waves beat -perpendicularly, and then for but short periods only. The broken wave, -as it crawls ever more slowly up the beach, carries the sand with it in -a sweeping curve, and by the time gravity has put a stop to its forward -movement, it is directed for a brief instant parallel to the shore. -Soon, however, the pull of gravity upon it has started the backward -journey in a more direct course down the slope of the terrace; and -here encountering the next succeeding breaker, a portion of the water -and the coarser sand particles with it are again carried forward for a -repetition of the zigzag journey. This many times interrupted movement -of the sand particles may be observed during a high wind upon any sandy -lee shore. The “set” of the water along the shore as a result of its -zigzag journeyings is described as the _shore current_ (Fig. 259), -and the effect upon sand distribution is the same as though a steady -current moved parallel to the shore in the direction of the average -trend of the moving particles. - - -=The sand beach.=—The first effect of the shore current is to deposit -some portion of the sand within the first slight recess upon the shore -in the lee of the cliff. The earlier deposits near the cliff gradually -force the shore current farther from the shore and so lay down a sand -selvage to the shore, which is shaped in the form of an arc or crescent -and known as a _beach_ (Fig. 259 and Fig. 260). - -[Illustration: FIG. 260.—Crescent-shaped beach formed in the lee of -a headland. Santa Catalina Island, California (after a photograph by -Fairbanks).] - -[Illustration: FIG. 261.—Cross section of a beach pebble.] - - -=The shingle beach.=—With heavy storms and an exceptional reach of the -waves, the shore currents are competent to move, not the sand alone, -but pebbles, the area of whose broader surface may be as great as the -palm of one’s hand. Such rock fragments are shaped by the continued -wear against their neighbors under the restless breakers, until they -have a lenticular or watch-shaped form (Fig. 261). Such beach pebbles -are described as _shingle_, and they are usually built up into distinct -ridges upon the shore, which, under the fury of the high breakers, may -be piled several feet above the level of quiet water (Fig. 262). Such -storm beaches have a gentle forward slope graded by the shore current, -but a steep backward slope on the angle of repose. Most storm beaches -have been largely shaped by the last great storm, such as comes only at -intervals of a number of years. - - -[Illustration: - -FIG. 262.—Storm beach of coarse shingle about four feet in height -at the base of Burnt Bluff on the northeast shore of Green Bay, Lake -Michigan.] - -=Bar, spit, and barrier.=—Wherever the shore upon which a beach is -building makes a sudden landward turn at the entrance to a bay, the -shore currents, by virtue of their inertia of motion, are unable longer -to follow the shore. The débris which they carry is thus transported -into deeper water in a direction corresponding to a continuation of -the shore just before the point of turning (see Fig. 259, p. 238). -The result is the formation of a _bar_, which rises to near the -water surface and is extended across the entrance to the bay through -continued growth at its end, after the manner of constructing a railway -embankment across a valley. - -[Illustration: FIG. 263.—Spit of shingle on Au Train Island, Lake -Superior (after Gilbert).] - -Over the deeper water near the bar the waves are at first not generally -halted and broken, as they are upon the shore, and so the bar does not -at once build itself to the surface, but remains an invisible bar to -navigation. From its shoreward end, however, the waves of even moderate -storms are broken, and the bar is there built above the water surface, -where it appears as a narrow cape of sand or shingle which gradually -thins in approaching its apex. This feature is the well-known _spit_ -(Fig. 263) which, as it grows across the entrance to the bay, becomes a -_barrier_ or _barrier beach_ (Fig. 264). - -The continuation of the visible in the usually invisible bar, is at the -time of high winds made strikingly apparent, for the wave base is below -the crest of the bar, and at such times its crescentic course beyond -the spit can be followed by the eye in a white arc of broken water. - -[Illustration: - -FIG. 264.—Barrier beach in front of a lagoon on Lake Mendota at -Madison, Wisconsin. The shallow lagoon behind the barrier is filling up -and is largely hidden in vegetation.] - -The construction of a barrier across the entrance to a bay transforms -the latter into a separate body of water, a lagoon, within which -silting up and peat formation usually lead to an early extinction -(p. 429). The formation of barriers thus tends to straighten out -the irregularities of coast lines, and opens the way to a natural -enlargement of the land areas. While the coasts of the United Kingdom -of Great Britain have been losing some four thousand acres through wave -erosion, there has been a gain by growth in quiet lagoons which amounts -to nearly seven times that amount. As evidence of the straightening -of the shore line which results from this process, the coast of the -Carolinas or of Nantucket (Fig. 459) may serve for illustration. - - -=The land-tied island.=—We have seen that wave erosion operates to -separate small islands from the headlands, but the shore currents -counteract this loss to the continents by throwing out barriers which -join many separated islands to the mainland. Such land-tied islands are -a common feature on many rocky coasts, and upon the New England coast -they usually have been given the name of “neck.” The long arc of Lynn -Beach joins the former island of Nahant, through its smaller neighbor -Little Nahant, to the coast of Massachusetts. A similar land-tied -island is Marblehead Neck. The Rock of Gibraltar, formerly an island, -is now joined to Spain by the low beach known as the “neutral ground.” -The Spanish name, _tombola_, has sometimes been employed to describe an -island thus connected to the shore. - - -[Illustration: FIG. 265.—Cross section of a barrier beach with lagoon -in its rear.] - -=A barrier series.=—The cross section of a barrier beach, like that of -a storm beach upon the shore, slopes gently upon the forward side, and -more steeply at the angle, of repose upon the rear or landward margin -(Fig. 265). The thinning wedge of shore deposits which the barrier -throws out to seaward raises the level of the lake bottom (Fig. 266), -and when coast irregularities are favorable to it, new spits will -develop upon the shore outside the earlier one, and a new bar, and in -its turn a barrier, will be found outside the initial one, taking a -course in a direction more or less parallel to it (Fig. 267). - -[Illustration: FIG. 266.—Cross section of a series of barriers and an -outer bar.] - -[Illustration: - -FIG. 267.—Formation of barrier series and an outer bar in University -Bay of Lake Mendota, at Madison, Wisconsin. The water contour interval -is five feet, and the land contour interval ten feet (based on a map by -the Wisconsin Geological Survey).] - -[Illustration: FIG. 268.—Series of barriers at the western end of Lake -Superior (after Gilbert).] - -So soon as the first barrier is formed, processes are set in operation -which tend to transform the newly formed lagoon into land, and so with -a series of barriers, a zone of water lilies between the outer barrier -and the bar, a bog, and a land platform may represent the successive -stages in this acquisition of territory by the lands. A noteworthy -example of barrier series and extension of the land behind them, is -afforded by the bay at the western end of Lake Superior (Fig. 268). - - -[Illustration: FIG. 269.—Character profiles resulting from wave action -upon shores.] - -=Character profiles.=—The character profiles yielded by the work of -waves are easy of recognition (Fig. 269). The vertical cliff with notch -at its base is varied by the stack of sugar-loaf form carved in softer -rocks, or the steeper notched variety cut from harder masses. Sea caves -and sea arches yield variations of a curve common to the undercut -forms. Wherever the materials of the shore are loosely consolidated -only, the sloping cliff is formed at the angle of repose of the -materials. The barrier beach, though projecting but a short distance -above the waves, shows an unsymmetrical curve of cross section with the -steeper slope toward the land. - - -READING REFERENCES FOR CHAPTER XVIII - - G. K. GILBERT. The Topographic Features of Lake Shores, 5th Ann. Rept. - U. S. Geol. Surv., 1885, pp. 69-123, pls. 3-20; Lake Bonneville, Mon. - I, U. S. Geol. Surv., 1890, Chapters ii-iv, pp. 23-187. - - VAUGHAN CORNISH. On Sea Beaches and Sand Banks, Geogr. Jour., vol. 11, - 1898, pp. 528-543, 628-658. - - F. P. GULLIVER. Shore Line Topography, Proc. Am. Acad. Arts and Sci., - vol. 34, 1899, pp. 149-258. - - N. S. SHALER. The Geological History of Harbors, 13th Ann. Rept. U. S. - Geol. Surv., 1893, pp. 93-209. - - SIR A. GEIKIE. The Scenery of Scotland, 1901, pp. 46-89. - - W. H. WHEELER. The Sea Coast. Longmans, London, 1902, pp. 1-78. - - G. W. VON ZAHN. Die zerstörende Arbeit des Meeres an Steilküsten nach - Beobachtungen in der Bretagne und Normandie in den Jahren 1907 und - 1908, Mitt. d. Geogr. Ges. Hamb., vol. 24, 1910, pp. 193-284, pls. - 12-27. - - - - -CHAPTER XIX - -COAST RECORDS OF THE RISE OR FALL OF THE LAND - - -=The characters in which the record has been preserved.=—The peculiar -forms into which the sea has etched and molded its shores have been -considered in the last chapter. Of these the more significant are -the notched rock cliff, the cut rock terrace, the sea cave, the sea -arch, the stack, and the sloping cliff and terrace, among the carved -features; and the barrier beach and built terrace, among the molded -forms. It is important to remember that the molded forms, by the very -manner of their formation, stand in a definite relationship to the -carved features; so that when either one has been in part effaced and -made difficult of determination, the discovery of the other in its -correct natural position may remove all doubt as to the origin of the -relic. - -[Illustration: - -FIG. 270.—The east coast of Florida, with shore line characteristic of -a raised coast.] - -In studies of the change of level of the land, it is customary to refer -all variations to the sea level as a zero plane of reference. It is not -on this account necessary to assume that the changes measured from this -arbitrary datum plane are the absolute upward or downward oscillations -which would be measured from the earth’s center; for the sea, like the -land, has been subject to its changes of level. There need, however, be -no apology for the use of the sea surface as a plane of reference; for -it is all that we have available for the purpose, and the changes in -level, even if relative only, are of the greatest significance. It is -probable that in most cases where the coast line is rising from uplift, -some portion of the sea basin not far distant is becoming deepened, -so that the visible change of level is the algebraic sum of the two -effects. - - -=Even coast line the mark of uplift.=—It was early pointed out in -this volume (p. 158) that the floor of the sea in the neighborhood of -the land presents a relatively even surface. The carving by waves, -combined with the process of deposition of sediments, tends to fill up -the minor irregularities of surface and preserve only the features of -larger scale, and these in much softened outlines. Upon the continents, -on the contrary, the running water, taking advantage of every slight -difference in elevation and searching out the hidden structure planes -within the rock, soon etches out a surface of the most intricate -detail. The effect of elevation of the sea floor into the light of day -will therefore be to produce an even shore line devoid of harbors (Fig. -270). If the coast has risen along visible planes of faulting near the -sea margin, the coast line, in addition to being even, will usually be -made up of notably straight elements joined to one another. - - -[Illustration: - -FIG. 271.—Ragged coast line of Alaska, the effect of subsidence.] - -=A ragged coast line the mark of subsidence.=—When in place of uplift -a subsidence occurs upon the coast, the intricately etched surface, -resulting from erosion beneath the sky, comes to be invaded by the sea -along each trench and furrow, so that a most ragged outline is the -result (Fig. 271). Such a coast has many harbors, while the uplifted -coast is as remarkable for its lack of them. - - -=Slow uplift of the coast—the coastal plain and cuesta.=—A gradual -uplift of the coast is made apparent in a progressive retirement of the -sea across a portion of its floor, thus exposing this even surface of -recent sediments. The former shore land will be easily recognized by -it’s etched surface, which will now come into sharp contrast with the -new plain. It is therefore referred to as the _oldland_ and the newly -exposed _coastal plain_ as the _newland_ (Fig. 272). - -[Illustration: - -FIG. 272.—Portion of Atlantic coastal plain and neighboring oldland of -the Appalachian Mountains.] - -But the near-shore deposits upon the sea floor had an initial dip -or slope to seaward, and this inclination has been increased in the -process of uplift. The streams from the oldland have trenched their way -across these deposits while the shore was rising. But the process being -a slow one, deposits have formed upon the seaward side of the plain -after the landward portion was above tide, and the coastal plain may -come to have a “belted” or zoned character. The streams tributary to -those larger ones which have trenched the plain may encounter in turn -harder and softer layers of the plain deposits, and at each hard layer -will be deflected along its margin so as to enter the main streams -more nearly at right angles. They will also, as time goes on, migrate -laterally seaward through undermining of the harder layers, and thus -will be shaped alternating belts of lowland separated by escarpments -in the harder rock from the residual higher slopes. Belts of upland of -this character upon a coastal plain are called _cuestas_ (Fig. 273). - -[Illustration: - -FIG. 273.—Ideal form of cuestas and intermediate lowlands carved from -a coastal plain (after Davis).] - - -=The sudden uplifts of the coasts.=—Elevations of the coast which -yield the coastal plain must be accounted among the slower earth -movements that result in changes of level. Such movements, instead of -being accompanied by disastrous earthquakes, were probably marked by -frequent slight shocks only, by subterranean rumblings, or, it may be, -the land rose gradually without manifestations of a sensible character. - -Upon those coasts which are often in the throes of seismic disturbance, -a quite different effect is to be observed. Here within the rocks -we will probably find the marks of recent faulting with large -displacements, and the movements have been upon such a scale that shore -features, little modified by subsequent weathering, stand well above -the present level of the seas. Above such coasts, then, we recognize -the characteristic marks of wave action, and the evidence that they -have been suddenly uplifted is beyond question. - -[Illustration: FIG. 274.—Uplifted sea cave, ten feet above the water -upon the coast of California; the monument to a former earthquake -(after a photograph by Fairbanks).] - -[Illustration: FIG. 275.—Double-notched cliff near Cape Tiro, Celebes -(after a photograph by Sarasin).] - - -=The upraised cliff.=—Upon the coast of southern California may be -found all the features of wave-cut shores now in perfect preservation, -and in some cases as much as fifteen hundred feet above the level of -the sea. These features are monuments to the grandest of earthquake -disturbances which in recent time have visited the region (Fig. -274). Quite as striking an example of similar movements is afforded -by notched cliffs in hard limestone upon the shore of the Island of -Celebes (Fig. 275). But the coast of California furnishes the other -characteristic coast features in the high sea arch and the stack -as additional monuments to the recent uplift. Let one but imagine -the stacks which now form the Seal Rocks off the Cliff House at San -Francisco to be suddenly raised high above the sea, and the forms which -they would then present would differ but little from those which are -shown in Fig. 276. - -[Illustration: FIG. 276.—Jasper rock stacks uplifted on the coast of -California (after a photograph by Fairbanks).] - - -=The uplifted barrier beach.=—Within the reëntrants of the shore, the -wave-cut cliff is, as we know, replaced by the barrier beach, which -takes its course across the entrance to a bay. After an uplift, such -a barrier composed of sand or shingle should be connected with the -headlands, often with a partially filled lagoon behind it. Its cross -section should be steep in the direction of the lagoon, but quite -gradual in front (Fig. 277). - -[Illustration: FIG. 277.—Uplifted shingle beach across the entrance to -a former bay upon the coast of southern California (after a photograph -by Fairbanks).] - -[Illustration: FIG. 278.—Raised beach terraces near Elie, Fife, -Scotland.] - - -=Coast terraces.=—Upon those shores where to-day high mountains -front the sea, the coast may generally be seen to rise in a series of -terraces (Fig. 278). This is notably true of those coasts which are -to-day racked by earthquakes, such as is the eastern margin of the -Pacific from Alaska to Patagonia. The traveler by steamer along the -coast from San Francisco to Chili has for weeks almost constantly in -sight these giant steps on which the mountains have been uplifted from -the sea. In Alaska we are fortunate in having the history of the later -stages in this uplift (Fig. 279). As described in a former chapter, -portions of this shore rose in the month of September of the year 1899 -in some places as high as forty-seven feet, to the accompaniment of -a terrific earthquake and sea wave. Above the terrace which marks -the beach line of 1899 there is a higher terrace of similar form now -overgrown with trees, but none the less clearly to be recognized as a -shore line of the past century which preceded in the long sequence the -uplift of 1899. - -[Illustration: FIG. 279.—Uplifted sea cliffs and terraces on the coast -of Russell Fjord, Alaska (after Tarr and Martin).] - -[Illustration: FIG. 280.—Diagrams to show how excessive sinking -upon the sea floor will cause the shore to migrate landward as it is -uplifted.] - -[Illustration: - -FIG. 281.—A drowned river mouth, or estuary upon a coastal plain.] - -As was noted in our study of earthquakes, the recent instrumental -records of distant earthquakes tell us that the movements upon the sea -floor are many times larger than those upon the continents, and that -while the mountainous coasts are generally rising, the deeps of the sea -are sinking. The effect of this over-balance of sinking, or resultant -shrinking of the earth’s shell, may be to compress the mountain -district and so cause the shore line to move landward at the same time -that it moves upward (Fig. 280). - - -=The sunk or embayed coast.=—When now, upon the other hand, a section -of the coast line sinks with reference to the sea, the water invades -all the near-shore valleys, thus “drowning” them and yielding the -drowned river mouth or _estuary_. If the relief of the shore was -slight, as it generally is upon a coastal plain, slight depression only -will produce broad estuaries, such as Chesapeake Bay at the drowned -mouth of the Susquehanna (Fig. 281). - -If, on the other hand, the relief of the shore is strong and the -subsidence is large, the entire coast line will be transformed into -an archipelago of steep-walled rocky islets which rise abruptly from -the sea (Figs. 282 and 284). A plateau which is intersected by deep -and steep-walled valleys of U-section (p. 341) under large submergence -yields the _fjords_ so characteristic of Scandinavia or Alaska. A -ragged coast line, fringed with islands as a result of submergence, is -described as an _embayed coast_. - -[Illustration: - -FIG. 282.—Archipelago of steep rocky islets due to large submergence -of a coast having strong relief. Entrance to Esquimalt Harbor, -Vancouver Island (after a photograph by Fairbanks).] - -[Illustration: - -FIG. 283.—The submerged Hudsonian channel which continues the Hudson -River across the continental shelf.] - - -=Submerged river channels.=—The sinking of a coast of small relief be -sufficient to completely submerge river valleys, whose channels then -begin to fill with sediment and whose courses can only be followed in -soundings. One of the most interesting of such channels is that which -continues the Hudson River across the continental shelf into the deeper -sea (Fig. 283). - - -=Records of an oscillation of movement.=—Because a coast is deeply -embayed is no ground for assuming that a subsidence is now in progress, -or is, in fact, the latest movement recorded upon the coast. In many -cases it is easy to see that such is not the case. The coast of Maine -is perhaps as typical of an embayed shore line as any that might be -cited, but a study of the river valleys in the neighborhood shows -clearly that the present submergence of their mouths is a fraction only -of an earlier one which has left a record of its existence in beds of -marine clay which outline the earlier and far deeper indentations (Fig. -284). - -[Illustration: - -FIG. 284.—Marine clay deposits near the mouths of the rivers of Maine -which preserve a record of earlier subsidence (after Stone).] - -If now we give a closer examination to the coast, it is found that -there are marks of recent uplift in an abandoned shore line now far -above the reach of the waves. There is here, then, the record, first of -subsidence and consequent embayment, and, later, of an uplift which has -reduced the raggedness of the coast outline exposed the clay deposits, -and raised the strands of the period of deep subsidence to their -present position. - -In countries which possess a more ancient civilization than our -own, the record of such oscillations in the level of the ground has -sometimes been entered upon human monuments, so that it is possible to -date more or less definitely the periods of subsidence or elevation. At -the little town of Pozzuoli, upon the shore of the Bay of Naples, is -found one of the mos instructive of these records. - -[Illustration: - -FIG. 285.—View of the three standing columns of the temple of Jupiter -Serapis at Pozzuoli, showing the dark and rough band nine feet in width -affected by the rock-boring mollusks which now live in the Bay of -Naples.] - -In the ruins of the ancient temple of Jupiter Serapis are three marble -monoliths 40 feet in height, curiously marked by a roughened surface -between the heights of 12 and 21 feet above their pedestals (Fig. 285). -Closer inspection shows that this roughened surface has been produced -by a marine, rock-boring mollusk, the _lithodomus_, which lives in -the waters of the Bay of Naples, and the shells of this animal are -still to be found within the cavities upon the surface of the columns. -Without recounting details which have been many times recited since -these interesting monuments were first geologically explored by Babbage -and Lyell, it may be stated that a record is here preserved, first of -subsidence amounting to some 40 feet, and of subsequent elevation, of -the low coast land on which stood the temple in the old Roman city of -Puteoli (Fig. 286). - -At the time of deepest submergence the top of the lithodomus zone -upon the column stood at the level of the water in the Bay of Naples, -the smoother lower zone being buried at the time in the sand at the -bottom, and thus made inaccessible for the lithodomi. It is to be added -that studies made in the environs of Pozzuoli have fully confirmed -the changes of level revealed by the columns, through the discovery -of now elevated shore lines which are referable to the period of deep -submergence. - - -=Simultaneous contrary movements upon a coast.=—In our study of -the changes in the level of the ground that take place during -earthquakes, it was learned that neighboring sections of the earth’s -crust may be moved at different rates or even in opposite directions, -notwithstanding the fact that the general movement of the province is -one of uplift. Thus during the Alaskan earthquake of 1899, although -portions of the coast line were elevated by as much as forty-seven -feet, neighboring sections were raised by smaller amounts, and some -small sections were sunk and so far submerged that the salt water and -the beach sand were washed about the roots of forest trees. - -[Illustration: FIG. 286.—Pozzuoli in the 3rd, 9th, and 20th -Centuries.] - -A region racked by heavy earthquakes, where the present configuration -of the ground speaks strongly for a movement of somewhat similar -nature, but with average movement of elevation much greater to the -northward than in the opposite direction, is the extended coast line -of Chili. This country is characterized by a great central north and -south valley which separates the coast range from the high chain of the -Cordilleras to the eastward. To the southward the floor of this valley -descends, and has its continuance in the Gulf of Corcovado behind the -island of Chiloe and the Chonos archipelago. The known recent uplift of -the coast of Chili, particularly in the northern sections and during -the earthquakes of the eighteenth, nineteenth, and twentieth centuries, -lends great interest to this topographic peculiarity. Indications are -not lacking that, during the earthquake of Concepcion in 1835, and of -Valparaiso in 1907, the measure of uplift was greater to the north than -it was to the south. - -[Illustration: FIG. 287.—Map of San Clemente Island, California, -showing the characteristic topography of recent uplift (after U. S. -Coast and Geodetic Survey).] - - -=The contrasted islands of San Clemente and Santa Catalina.=—Perhaps -the most striking example of simultaneous opposite movements observable -in neighboring portions of the earth’s crust is furnished by the coast -of southern California. The coast itself at San Pedro and the island -of San Clemente, some fifty miles off this point, in common with most -portions of the neighboring coast land, have been rising in interrupted -movements from the sea, and offer in rare perfection the characteristic -coast terraces (Fig. 287 and Fig. 278, p. 250). Midway between these -two rising sections of the crust, and less than twenty-five miles -distant from either, is the island of Santa Catalina, which has been -sinking beneath the waves, and apparently at a similarly rapid rate -(Fig. 288). The topography of the island shows the intricate detail of -a maturely eroded surface, while that of the neighboring San Clemente -shows only the widely spaced, deep cañons of the infantile stage of -erosion (Fig. 165 and pl. 12 A). While Santa Catalina has been sinking, -San Pedro Hill has risen 1240 feet, and San Clemente, 1500 feet. It -is characteristic of a sinking coast line that the cliff recession is -abnormally rapid, and evidence for this is furnished by the shores of -Santa Catalina, upon which the waves are cutting the cliffs back into -the beds of cañons, and so causing small falls to develop at the cañon -mouths. - -┌─────────────────────────────────────────────────────────────────────┐ -│ PLATE 12. │ -│ │ -│ [Illustration: _A._ V-shaped cañon cut in an upland recently │ -│ elevated from the sea, San Clemente Island, California (after W. S. │ -│ Tangier-Smith).] │ -│ │ -│ [Illustration: _B._ A “hogback” at the base of the Bighorn │ -│ Mountains, Wyoming (after Darton).] │ -└─────────────────────────────────────────────────────────────────────┘ - -[Illustration: - -FIG. 288.—Map of Santa Catalina Island, California, showing the -characteristic surface of an area which has long been above the waves, -and the entire absence of coast terraces (after U. S. C. and G. S.).] - - -=The Blue Grotto of Capri.=—We may now return to the Bay of Naples for -additional evidence that oscillations of level in neighboring portions -of the same coast are not necessarily synchronous, and that near-lying -sections may even move in opposite directions at the same time, as has -already been shown for the islands off the California coast. For the -Pozzuoli shore of the bay it was learned that within the Christian -Era a complete cycle of downward, followed by later upward, movement -has been largely accomplished. Across the bay, and less than 20 miles -distant, is the Blue Grotto of Capri, a sea cave cut in limestone above -an earlier cave of the same nature which is now deep below the water -surface. It is the refracted sunlight which enters the cave through -this lower submerged opening and has been robbed on the way of all but -its blue rays which gives to the famous grotto its special charm (Fig. -289). - -[Illustration: - -FIG. 289.—Cross section of the Blue Grotto on the Island of Capri, -showing the submerged sea cave through which most of the light enters -the grotto, and the higher artificial window now widened by wave action -(after von Knebel).] - -It is known that the former, and now submerged, sea cave was in use -by Roman patricians as a cool retreat from the oppressive hot wind -known as the sirocco, and that an artificial entrance or window was -cut where is now the only accessible entrance to the grotto. In the -ancient writings, no mention is made, however, of the remarkable blue -illumination for which it is now famous, and the conditions at the -time, as we may see, were not such as to make this possible. Later -subsidence of the coast has brought the ancient window to the sea -level, where it has been considerably enlarged by the waves. The -earlier grotto, abandoned as its entrance was closed, was rediscovered -in 1826 by the painter and poet, August Kopisch. - -A grotto with green illumination (the Grotto Verde) is situated upon -the opposite side of the island, and a blue grotto, having its origin -in similar conditions to those of the famous Blue Grotto, is found upon -the island of Busi off the Dalmatian coast. - - -=Character profiles.=—In the landscape of a coast which has been -slowly uplifted the characteristic line is the profile of the cuesta, -with short perpendicular element joined to a gently sloping and longer -section and continued in the horizontal portion corresponding to the -lowland (Fig. 290). Rapidly uplifted coasts offer in contrast the -lines characteristic of wave erosion and deposition, but at higher -levels and in repeated sections. Most prominent of all is the staircase -constructed of coast terraces, with either vertical or sloping risers -and with outwardly inclining and gently graded treads. Near the steep -riser in the staircase may sometimes be seen the sugar-loaf outline of -the stack cut in softer material, or the obelisk-like pillar undercut -at its base, which is carved in firmer rock masses. With excessively -rapid uplift, the double-notched cliff or the double sea arch may -appear in the landscape. Upon a submerged coast the most significant -lines in the view are those of the rock islet and the steep-walled -fjord. - -[Illustration: FIG. 290.—Character profiles in coast landscapes where -there has been either elevation or depression.] - - -READING REFERENCES FOR CHAPTER XIX - - General:— - - SIR CH. LYELL. Principles of Geology, vol. 2, pp. 180-197. - - ED. SUESS. The Face of the Earth, Clarendon Press, Oxford, 1906, vol. - 2, Chapters i and xiv, pp. 1-29, 535-556. - - ROBERT SIEGER. Seenschwankungen und Strandverschiebungen in - Scandinavien, Zeit. d. Gesell. f. Erdk., Berlin, vol. 28, 1893, pp. - 1-106, 393-688, pl. 7. - -Elevated shore lines:— - - F. B. TAYLOR. The Highest Old Shore Line of Mackinac Island, Am. Jour. - Sci., vol. 43, 1892, pp. 210-218. - - THOMAS L. WATSON. Evidences of Recent Elevation of the Southern Coast - of Baffins Land, Jour. Geol., vol. 5, 1897, pp. 17-33. - - J. W. GOLDTHWAIT. The Abandoned Shore Lines of Eastern Wisconsin. - Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 1-37. - -Evidences of depression:— - - W. B. SCOTT. Introduction to Geology, New York, 1907, pp. 33-36. - - W. J. MCGEE. The Gulf of Mexico as a Measure of Isostacy, Am. Jour. - Sci. (3), vol. 44, 1892, pp. 177-192. - - A. LINDENKOHL. Notes on the Submarine Channel of the Hudson River, - etc., Am. Jour. Sci. (3), vol. 41, 1891, pp. 489-499, pl. 18. - - J. W. SPENCER. The Submarine Great Cañon of the Hudson River, _ibid._ - (4), vol. 19, 1905, pp. 1-15; Submarine Valleys off the American Coast - and in the North Atlantic, Bull. Geol. Soc. Am., vol. 14, 1903, pp. - 207-226, pls. 19-20. - - F. NANSEN. The Bathymetrical Features of the North Polar Sea, with - a Discussion of the Continental Shelves and Previous Oscillations - of Shore Line, Norwegian North Polar Expedition, vol. 4, 1904, pp. - 99-231, pl. 1. - - W. V. KNEBEL. Höhlenkunde, etc., Braunschweig, 1906, pp. 175-177 (the - blue grotto of Capri). - -Oscillation of movement:— - - C. LYELL. Principles of Geology, vol. 2, pp. 164-176 (Temple of - Jupiter Serapis). - - E. RAY LANKESTER. Extinct Animals, New York, 1905, pp. 31-42. - - H. W. FAIRBANKS. Oscillations of the Coast of California during the - Pliocene and Pleistocene, Am. Geol., vol. 20, 1897, pp. 213-245. - - G. H. STONE. Mon. 34, U. S. Geol. Surv., 1899, pp. 56-58, pl. 2. - - BAILEY WILLIS. Ames Knob, North Haven, Maine. Bull. Geol. Soc. Am., - vol. 14, 1903, pp. 201-206, pls. 17-18. - -Simultaneous contrary movements on a coast:— - - A. C. LAWSON. The Post-Pliocene Diastrophism of the Coast of Southern - California, Bull. Univ. Calif. Dept. Geol., vol. 1, 1893, pp. 115-160, - pls. 8-9. - - W. S. TANGIER-SMITH. A Geological Sketch of San Clemente Island, 18th - Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 459-496, pls. 84-96. - - R. S. TARR and L. MARTIN. Recent Changes of Level in the Yakutat Bay - Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, pls. - 12-23. - - - - -CHAPTER XX - -THE GLACIERS OF MOUNTAIN AND CONTINENT - - -=Conditions essential to glaciation.=—Wherever for a sufficiently -protracted period the annual snowfall of a district is in excess of -the snow that is melted, a residue must remain from each season to be -added to that of succeeding ones. Eventually so much snow will have -accumulated that under its own weight and in obedience to its peculiar -properties, a movement will begin within the mass tending to spread -it and so to reduce the slope of its upper surface (Frontispiece -plate). The conditions favorable to glaciation are, therefore, heavy -precipitation and low annual temperature. If the precipitation is -scanty, the small snowfall is soon melted; and if the temperature be -too high, the moisture is precipitated not in the form of snow but as -rain. It is important here to keep in mind that snow is a poor heat -conductor and itself protects its deeper layers from melting. - - -=The snow-line.=—Because of the low temperatures glaciers should -be most abundant or most extensive in high latitudes and in high -altitudes. The largest are found in polar and subpolar regions, and -they are elsewhere encountered only at considerable elevations. -The largest glaciers are the vast sheets of ice which inwrap the -continents of Greenland and Antarctica, but glaciers of large size -are to be found upon other large land masses of the Arctic, as well -as in Alaska, in the southern Andes, and in New Zealand. Much smaller -glaciers are characteristic of certain highlands within temperate and -tropical regions, but because of specially favorable conditions both of -altitude and precipitation the Himalayas, although in relatively low -latitudes, nourish glaciers of large proportions. In general, it may -be said that the nourishing grounds of glaciers are largely restricted -to those areas where snow covers the ground throughout the year. The -lower margin of such areas is designated the _snow line_, and varies -but little from the line on which the average summer temperature is at -the freezing point of water—the so-called _summer isotherm of 32° -Fahrenheit_. Within the tropics this line may rise as high as 18,000 -feet above the sea, whereas in polar latitudes it descends to sea level. - - -=Importance of mountain barriers in initiating glaciers.=—The -precipitation within any district depends, however, not alone upon the -amount of moisture which is brought to it in the clouds, but upon the -amount which is abstracted before the clouds have passed over it. The -capacity of space to hold moisture increases with its temperature, and -hence any lowering of this temperature will reduce the capacity. If -lowered sufficiently, the point of complete saturation will be reached -and further cooling must result in precipitation. Hence, anything which -forces an air current to rise into more rarefied zones above, will -lower the pressure upon it and so bring about a cooling effect in which -no heat is abstracted. This so-called _adiabatic refrigeration_ of a -gas may be illustrated by the cool current which issues in a jet from -a warm expanded rubber tire after the cock has been opened; or even -better, by the instant solidification at extreme low temperatures of -such normal gases as carbonic acid when they are allowed to issue under -heavy pressure from a small orifice. - -As applied to moisture-laden and near-surface winds, the effective -agents of adiabatic cooling are the upland areas upon the continents, -and especially the ranges of mountains. These barriers force the moving -clouds to rise, cool, and deposit their moisture. It is, therefore, the -highland barriers which face the oncoming, moisture-laden winds that -receive the heaviest precipitation. Within temperate regions, because -of the prevalence of westerly winds, those barriers which face the -western shores receive the heaviest fall. Within the tropics, on the -other hand, it is the barriers facing the eastern shores which, because -of the easterly “trades”, are most favorable to precipitation. - -Thus it is in the Sierra Nevadas of California, and not in the Rockies -or the Appalachians, that the glaciers of the United States are found. -The highland of the Swiss Alps lying likewise athwart the “westerlies” -of the temperate zone acquires the moisture for nourishment of its -glaciers from the western ocean—here the Atlantic (Fig. 291). Within -the tropics the conditions are reversed, and it is in general the -ranges which lie nearer the eastern coasts that are the more favored. -If no barrier is found upon this coast, the clouds may travel over -vast stretches of country before being arrested by mountains and robbed -of their moisture. Thus in tropical Brazil the glaciers are found in -the Andes upon the Pacific coast though nourished by clouds from the -Atlantic. - -[Illustration: FIG. 291.—Map showing the distribution of existing -glaciers, and the two important wind poles of the earth.] - - -=Sensitiveness of glaciers to temperature changes.=—How sensitive -is the adjustment between snow precipitation and temperature may be -strikingly illustrated by the statement on excellent authority that if -the average annual temperature of the air within the Scottish Highlands -should be lowered by only three degrees Fahrenheit, small glaciers -would be the result; and a moderate temperature fall within the region -surrounding the Laurentian lakes of North America would bring on -glaciation, otherwise expressed as a depression of the snow line of the -region. - - -=The cycle of glaciation.=—Though to-day buried beneath its ice -mantle, it is known that Greenland had more than once in earlier -geological ages a notably mild climate, and in some future age it may -revert to this condition. In other regions, also, we have evidence -that such a rotation of climatic changes has been successively -accomplished, the climate having steadily increased in severity towards -a culminating point, and been followed by a reverse series of changes. -Such a complete period may be called a _cycle of glaciation_. While -the climate is steadily becoming more rigorous, we have to do with -an _advancing hemicycle_ of glaciation, but after the culminating -point has been reached, the period of amelioration of climate is the -_receding hemicycle_. - - -=The advancing hemicycle.=—There is little reason to doubt that -whatever be the cause of the climatic changes which bring on glacial -conditions, these changes come on by insensible gradations. The -first visible evidence of the increased severity of the climate is -the longer persistence of the winter snows, at first within the more -elevated districts. In such positions drifts must eventually continue -throughout the warm season and so contribute to the snow accumulations -of the succeeding winter. This point once reached, small glaciers are -inevitable, even should the average temperature fall no further, for -the snow left over in each season must steadily increase the depth of -the deposits until the weight brings about an internal motion of the -mass from higher to lower levels. - -[Illustration: FIG. 292.—An Alaskan glacier spreading out at the foot -of the range which nourishes it.] - -The inherited depressions of the upland—the gentle hollows at the -heads of rivers—will first be filled, and so the valleys below become -the natural channels for the outflow of the early glaciers. With a -continued lowering of the annual temperature and consequent increased -snowfall, the early glaciers become more and more amply nourished. Snow -and ice will, therefore, cover larger areas of the upland, and the -glaciers will push their fronts farther down the valleys before they -are wasted in the warm air of the lower levels. As the valleys become -thus more completely invested by the glacier they are likewise filled -to greater and greater depths, and they may thus submerge portions of -the walls that separate adjacent valleys. Reaching at last the front -of the upland area, the glaciers may now be so well nourished at -their heads that they push out upon the flatter foreland and without -restraint from retaining walls spread broadly upon it (Fig. 292). - -[Illustration: FIG. 293.—Surface of a glacier whose upper layers -spread with slight restraint from retaining walls. Surface of the -Folgefond, an ice cap of southern Norway.] - -The culmination of the progressive climatic change may ere this have -been reached and milder conditions have ensued. If, however, the -severity of the climate should be still further increased, the expanded -fronts of neighboring glaciers will coalesce to form a common ice fan -or apron along the foot of the upland (Plate 18 B). This could hardly -take place without a still further deepening of the ice within the -valleys above, and, probably, a progressive submergence of the lower -crests in the valley walls. This may even continue until all parts of -the upland area have been buried. The snow and ice now take the form -of a covering cap or carapace, and the upper portions being no longer -restrained at the sides, now spread into a broad dome, as would a -viscous liquid like thick molasses when poured out upon the floor (Fig. -293). The lower zones of the mass and the thinner marginal portions -still have their motion to a greater or less extent controlled by the -irregularity of the rock floor against which they rest. - -The reverse series of changes in the glacier is inaugurated by an -amelioration of the climate, and here, therefore, the advancing -hemicycle becomes merged in the receding hemicycle of glaciation. - - -=Continental and mountain glaciers contrasted.=—The time when the -rock surface becomes submerged beneath the glacier is, as regards -both the surface forms and the erosive work, a critical point of much -significance; for the ice cap and larger continental glacier obviously -protect the rock surface from the action of those chemical and -mechanical processes in which the atmosphere enters as chief agent, and -which are collectively known as weathering processes. Until submergence -is accomplished, larger or smaller portions of the rock surface project -either through or between the ice masses and are, therefore, exposed to -direct attack by the weather (see below, p. 370). - -[Illustration: FIG. 294.—Section through a mountain glacier (in solid -black), showing how its surface is determined by the irregularities in -the rock basement (after Hess).] - -Snow which falls in the mountains is not allowed to remain long where -it falls. By the first high wind it is swept off the more elevated and -exposed surfaces and collected under eddies in any existing hollows, -but especially those upon the lee slopes of the range. We are to learn -that glaciers carve the mountains by enlarging the hollows which they -find and producing great basins for the collection of their snows; but -with the initiation of glaciation the inherited hollows are in most -cases the unimportant depressions at the heads of streams. Whatever -they may be and however formed, the snow first fills those hollows -which are sheltered from the wind, and as it accumulates and becomes -distributed as ice, assumes a surface of its own that is dependent upon -the form and the position of the basin which it occupies (see Fig. 294). - -[Illustration: FIG. 295.—Profile across the largest of the Icelandic -ice caps, with the vertical scale greatly exaggerated (after Thoroddsen -and Spethmann).] - -When the quantity of accumulated snow is so great that all hollows of -the rock surface are filled, its own surface is no longer controlled -by retaining rock walls, and it now assumes a form largely independent -of the irregularities in the upland. Experience shows that this surface -is approximately that of a flat dome or shield, and as it covers all -the upland, save where the ice thins upon its margins, this type of -glacier is called an _ice cap_ (Fig. 295). All types of glacier in -which rock projects above the highest levels of the ice and snow are -known as _mountain glaciers_. - -[Illustration: FIG. 296.—Ideal section across a continental glacier, -with the vertical scale and the projecting rock masses of the marginal -zone greatly magnified.] - -The flat domes of ice which mantle the continents of Greenland and -Antarctica, though resembling in form the smaller ice cap, are yet -because of their vast size so distinct from them, particularly in the -manner of their nourishment, that they belong in a separate class -described as _inland ice_ or _continental glaciers_. Though they have -some affinities with ice caps, they are most sharply differentiated -from all types of mountain glaciers. Of them it is true that the -lithosphere projects through them only in the neighborhood of their -margins (Fig. 296), whereas in the case of mountain glaciers rock may -project at any level but _always above the highest snow surface_. Ice -caps may be regarded as intermediate between the two main classes of -mountain and continental glaciers (Fig. 297). Because of the large rôle -which continental glaciers have played in geological history, it is -thought best to consider them first, leaving for later discussion the -no less interesting but less important mountain glaciers. - -[Illustration: FIG. 297.—View of the Eyriks-Jökull, an ice-cap of -Iceland (after Grossman).] - - -=The nourishment of glaciers.=—The life of a glacier is dependent upon -the continued deposition of snow in aggregate amount in excess of that -which is lost by melting or by other depleting processes. Whenever, on -the other hand, the waste exceeds the precipitation, the glacier is in -a receding condition and must eventually disappear, if such conditions -are sufficiently long continued. The source of the snow is the water -of the ocean evaporated into the atmosphere and transported over the -land in the form of clouds. We are to learn that the changes which -this moisture undergoes before its delivery to the glacier are notably -different for the classes of continental and mountain glacier. - - -=The upper and lower cloud zones of the atmosphere.=—Before we can -comprehend the nature of the processes by which glaciers are nourished, -it will be necessary to review the results of recent studies made upon -the earth’s atmospheric envelope. It must be kept in mind that the -sun’s rays are chiefly effective in warming the atmosphere through -being first absorbed by some solid body such as rock or water and -their heat then communicated by contact to the immediately adjacent -air layers. The layers thus warmed being now lighter than before, -they rise and are replaced by colder air, which in its turn is warmed -and likewise set in upward motion. Such currents developed in the air -by contact with warmer solid bodies constitute the process known as -convection. - -[Illustration: FIG. 298.—The zones of the lower atmosphere as revealed -by recent kite and balloon explorations.] - -To a relatively small degree the atmosphere is heated by the direct -absorption of the sun’s rays which pass through it. Since air has -weight, it compresses the lower layers near the earth, and hence as we -ascend from the earth’s surface the air becomes continually lighter. -Convection currents must, therefore, adjust themselves by the air -expanding as it rises. But expansion of a gas always results in its -cooling, as every one must have observed who has placed his finger in -the air current which escapes from the open valve of a warm rubber -tire. Dry air is cooled a degree Fahrenheit for every six hundred feet -of ascent in the atmosphere. At a height of about seven miles above -the earth’s surface all rising air currents have cooled to about 68° -below the zero of the Fahrenheit scale, and exploration with balloons -has shown that the currents rise no farther. At this level they move -horizontally, just as rising vapor spreads out in a room beneath the -ceiling. Above this level, as far as exploration has gone, or to a -height of more than twelve miles, the temperature remains nearly -constant, and this upper zone is, therefore, called the _isothermal_ -or the _advective zone_—the uniform temperature zone of the lower -atmosphere. Beneath the convective ceiling the process of convection is -characteristic, and this zone is therefore described as the _convective -zone_ (Fig. 298). - -A large part of the moisture which rises from the ocean’s surface is -condensed to vapor before it has ascended three miles, and in this form -it makes its transit over land as fleecy or stratiform clouds—the -so-called cumulus and stratus clouds and their many intermediate -varieties (see Frontispiece). This lower layer within the convective -zone is, therefore, a moist one overlaid by a relatively drier middle -layer of the convective zone. That moisture which rises above the lower -cloud layer is congealed by adiabatic cooling to fine ice needles -visible as the so-called cirrus clouds which float as feathery fronds -beneath the convective ceiling (see frontispiece at right upper corner -of picture). Thus we have within the convective zone an upper layer -more or less charged with water in the form of ice needles. It is the -clouds of the lower zone whose moisture in the form of vapor supplies -the nourishment of mountain glaciers, and the high cirrus clouds whose -congealed moisture, after interesting transformations, is responsible -for the continued existence of continental glaciers. - -As we are to see, there are other noteworthy differences between -continental and mountain glaciers, in the manner of their sculpture -of the lithosphere, so that long after they have disappeared the -characters of each are easily identified in their handiwork. How the -lower clouds are forced upward and so compelled to give up their -moisture to feed the mountain glaciers, and how the upper clouds are -pulled downward to nourish the glaciers of continents, can be best -understood after the characteristics of each glacier class have been -studied. - - - - -CHAPTER XXI - -THE CONTINENTAL GLACIERS OF POLAR REGIONS - - -[Illustration: - -FIG. 299.—Map of Greenland showing the area of inland-ice and the -routes of different explorers.] - -=The inland ice of Greenland.=—In Greenland and in Antarctica the land -is almost or quite buried under a cover of snow and ice—the so-called -“inland ice”—which always assumes the surface of a very flat dome or -shield. In Greenland there is found a marginal ribbon of land generally -from five to twenty-five miles in width (Fig. 299), but in Antarctica -all the land, with the exception of a few mountain peaks, is inwrapped -in a mantle of ice which is also extended upon the sea in a broad shelf -of snow and ice. Neither of these vast glaciers has been explored -except in its marginal portion, yet such is the symmetry of the -profiles along the routes traversed, and such the flatness and monotony -of the snow surface within the margins, that there is little reason to -doubt that the profile made along Nansen’s route in southern Greenland -would, save only for magnitude, fairly represent a section across the -middle of the continent (Fig. 300). - - -=The mountain rampart and its portals.=—As soon as we examine the -coastal belt we observe that the “Great Ice” of Greenland is held -in by a wall of mountains and so prevented from spreading out to its -natural surface in the marginal portions. Through portals of the -inclosing mountain ranges—the _outlets_—it sends out _tongues_ of ice -which in many respects resemble certain types of mountain glaciers. - -[Illustration: - -FIG. 300.—Profile in natural proportions across the southern end of -the continental glacier of Greenland, constructed upon an arc of the -earth’s surface and based upon Nansen’s profile corrected by Hess. The -marginal portions of the profile are represented below upon a magnified -scale in order to bring out the characters of the marginal slopes.] - -Such measurements as have been made upon the inland ice of Greenland -at points back from, but yet comparatively near to, the outlets, show -that it has here a surface rate of motion amounting to less than an -inch per day, and it is highly probable that at moderate distances from -the margin this amount diminishes to zero. Upon the outlets, on the -contrary, surface rates as high as 59 feet per day have been measured, -and even 100 feet per day has been reported. We are thus justified in -saying that glacier flow within the outlets is from 700 to 1000 times -as great as it is upon the near-by inland ice, and that the glacier -is in a measure drained through the portals of the inclosing ranges. -Back from these outlet streams of ice, or tongues, the surface of the -inland ice is depressed to form a dimple or “basin of exudation” as is -the surface of a reservoir above the raceway when the water is being -rapidly drawn away (Fig. 301). - -┌────────────────────────────────────────────────────────────────────┐ -│ PLATE 13. │ -│ │ -│ [Illustration: _A._ Precipitous front of the Bryant glacier outlet │ -│ of the Greenland inland-ice (after Chamberlin).] │ -│ │ -│ [Illustration: _B._ Lateral stream beside the Benedict glacier │ -│ outlet, Greenland (after R. E. Peary).] │ -└────────────────────────────────────────────────────────────────────┘ - -[Illustration: - -FIG. 301.—Map of a glacier tongue, with dimple showing above and due -to indraught of the ice. Umanakfjord, Greenland (after von Drygalski).] - -Fissures in the ice, the so-called crevasses, are the recognized -marks of ice movement, and these are always concentrated at the steep -slopes of the ice surface in the neighborhood of its margins. Upon the -Greenland ice, crevasses are restricted in their distribution to a zone -which extends from seven to twenty-five miles within the ice border. - - -=The marginal rock islands.=—From its margin the ice surface rises -so steeply as to be climbed only with difficulty, but this gradient -steadily diminishes until at a distance of between seventy-five and -a hundred miles its slope is less than two degrees. Where crossed by -Nansen near latitude 64° N. the broad central area of ice was so nearly -level as to appear to be a plain. - -As we pass across the irregular ice margin in the direction of the -interior, larger and larger proportions of the land’s surface are -submerged, until only projecting peaks rise above the ice as islands -which are described as _nunataks_ (Fig. 302). - -Though not a universal observation, it has been often noted that the -absorption of the sun’s rays by rock masses projecting through the snow -results in a radiation of the heat and a lowering by melting of the -surrounding snow and ice. For this reason nunataks are often surrounded -by a deep trench due to a melting of the snow. Such a depression in -the ice surface about the margin of a nunatak, from its resemblance to -a trench about an ancient castle, has been designated a _moat_ (Fig. -303). For the same reason, the outlet tongues of ice which descend in -deep fjords between walls of rock are melted away from the walls and a -lateral stream of water is sometimes found to flow between ice and rock -(pl. 13 B). - - -[Illustration: - -FIG. 302.—Edge of the Greenland inland ice, showing the nunataks -diminishing in size toward the interior. The lines upon the ice are -medial moraines starting from nunataks (after Libbey).] - -=Rock fragments which travel with the ice.=—Rock surfaces which are -exposed to the atmosphere are in high latitudes broken down through -the freezing of water within their crevices. The fragments resulting -from this rending process fall upon the glacier surface and are carried -forward as passengers in the direction of the ice margin. They are -either visible as long and narrow ridges or trains following the -directions of the steepest slope (Fig. 302), or they become buried -under fresh falls of snow and only again become visible where summer -melting has lowered the glacier surface in the vicinity of its margin. -These longitudinal trains of rock fragments upon the glacier surface -always have their starting point at the lower margin of one of the -nunataks, and are known as _medial moraines_ (Fig. 301, p. 273, and -Fig. 302). Inside the zone of nunataks the glacier surface is, however, -clear of rock débris except where dust has been blown on by the wind, -and this extends for a few miles only. The material of the medial -moraines is a collection of angular blocks whose surfaces are the -result of frost rending, for in their travel above the ice they are -subjected to no abrading processes. - -[Illustration: FIG. 303.—Moat surrounding a nunatak in Victoria Land -(after Scott).] - -A contrasted type of surface moraines upon the Greenland glacier, -instead of being parallel to the direction of ice movement, is directed -transversely or parallel to the margins. The materials of these -moraines are more rounded fragments of rock which have come up from -the bottom layers, and we shall again refer to the origin of such -moraines after the subglacial conditions have been considered. - - -=The grinding mill beneath the ice.=—If, now, we examine the front of -a glacier tongue which goes out from the inland ice, we find that while -the upper portion is white and mainly free from rock débris (plate -13 A), the lower zone is of a dark color and crowded with layers of -pebbles and bowlders which have been planed, polished, and scratched in -a quite remarkable manner. The ice front is itself subject to forward -and retrograde migrations of short period, but it is easily seen that -in the main its larger movement has been a retrograde one. The ground -from which it has lately withdrawn is generally a hard rock floor -unweathered, but smooth, polished, and scratched in the same manner -as the bowlders which are imbedded within the ice. It is perfectly -apparent that the latter have been derived from some portion of the -rock basement upon which the glacier still rests, and that floor and -bowlders have alike been ground smooth by mutual contact under pressure. - -This erosion beneath the ice is accomplished by two processes; namely, -_plucking_ and _abrasion_. Wherever the rock over which the glacier -moves has stood up in projecting masses and is riven by fissure planes -of any kind, the ice has found it easy to remove it in larger or -smaller fragments by a quarrying process described as plucking. The -rock may be said to be torn away in blocks which are largely bounded by -the preëxisting fissure planes. Over relatively even surfaces plucking -has little importance, but where there are noteworthy inequalities -of surface upon the glacier bed, those sides which are away from the -oncoming ice (_lee_ side) are degraded by plucking in such a manner as -sometimes to leave steep and ragged fracture surfaces. The tools of -the ice thus acquired in the process of plucking are quickly frozen -into the lowest ice layers, and being now dragged along the floor -they abrade in the same manner as does a rasp or file. These tools of -the ice are themselves worn away in the process and are thus given -their characteristic shapes. Just as the lapidary grinds the surface -of a jewel into facets by imbedding the gem in a matrix, first in one -and then in another position, each time wearing down the projecting -irregularities through contact with the abrading surface; so in like -manner the rock fragment is held fast at the bottom of the glacier -until “soled” or “shod”, first upon one side and then upon another. -Accidental contact with some obstruction upon the floor may suffice -to turn the fragment and so expose a new surface to wear upon the -abrading floor. Minor obstructions coming in contact with one side of -the fragment only, may turn it in its own plane without overturning. -Evidence of such interruptions can be later read in the different -directions of striæ upon the same facet (plate 17 A). - -[Illustration: - - FIG. 304.—A glacier pavement of Permo-Carboniferous age in South - Africa. The striæ running in the direction of the observer are - prominent and a noteworthy gouging of the surface is to be noted to - the right in the middle distance (after Davis).] - -The floor beneath the glacier is reduced by the abrading process to a -more or less smooth and generally flattened or rounded surface—the -so-called _glacier pavement_ (Fig. 304). To accomplish this all former -mantle rock due to weathering processes must first be cleared away, and -the firm unaltered rock beneath is wherever susceptible of it given a -smooth polish although locally scored and scratched by the grinding -bowlders. The earlier projections of the surface of the floor, if not -entirely planed away, are at least transformed into rounded shoulders -of rock, which from their resemblance to closely crowded backs in a -flock of sheep have been called “sheep backs” or “_roches moutonnées_.” -Thus the effect of the combined action of the processes of plucking and -abrasion is to reduce the accent of the relief and to mold the contours -of the rock in smoothly flowing curves, generally of large radius. - - -=The lifting of the grinding tools and their incorporation within the -ice.=—Wherever the ice is locally held in check by the projecting -nunataks, relief is found between such obstructions, and there the flow -of the ice has a correspondingly increased velocity (Fig. 305 _b_). -If the obstructions are not of large dimensions, the ice which flows -around the outer edges is soon joined to that which passes between the -obstructions and so normal conditions of flow are restored below the -nunataks. The locally rapid flow of the ice is, therefore, restricted -to a relatively short distance, the passageway between the nunataks, -and the conditions are thus to be likened to the fall of water at a -raceway due to the sudden descent of its surface from the level of -the reservoir to the level of the stream in the outlet. As is well -known, there is under these conditions a prodigious scour upon the -bottom which tends to dig a pit just above and below the dam—a _scape -colk_—and carry the materials up to the surface below the pit. Such -a tendency was well illustrated by the behavior of the water at the -opening of the Neu Haufen dam below the city of Vienna (Fig. 305 _a_). -In the case of ice, material from the bottom may by the upward current -be brought up to the surface of the glacier at the lower edge of the -colk and thus produce a type of local surface moraine of horseshoe -form with its direction generally transverse to the direction of ice -movement (Fig. 305 _b_). - -[Illustration: - -FIG. 305.—_a_, Map showing pit excavated by the current below the -opening in a dam. _b_, Nunataks and surface moraines on the Greenland -ice. Dalager’s Nunataks (after Suess).] - -Any obstruction upon the pavement of the glacier apparently exerts a -larger or smaller tendency to elevate the bowlders and pebbles and -incorporate them within the ice. Rock débris thus incorporated is -described as _englacial_ drift. In the case of Greenland glaciers this -material seems at the ice front to be largely restricted to the lower -100 feet (plate 13 A). - -Near the front of the inland ice the increased slope of the upper -surface greatly increases the flow of the upper ice layers in -comparison with those nearer the bottom, so that the upper layers -override the lower as they would an obstruction. The englacial drift -is either for this reason or because of rock obstructions brought to -the surface, where it yields parallel ridges corresponding in direction -to the glacier margin. Such transverse surface moraines are thus in -many respects analogous to those which appear about the lower margins -of scape colks. In contrast to the longitudinal or medial surface -moraines the materials of the transverse moraines are more faceted and -rounded—they have been abraded upon the glacier pavement. - - -=Melting upon the glacier margins in Greenland.=—During the short but -warm summer season, the margins of the Greenland ice are subject to -considerable losses through surface melting. When the uppermost ice -layer has attained a temperature of 32° Fahrenheit, melting begins -and moves rapidly inward from the glacier margin. In late spring the -surface of the outer marginal zone is saturated with water, and this -zone of slush advances inward with the season, but apparently never -transgresses the inner border of what we have generally referred to as -the marginal zone of the ice characterized by relatively steep slopes, -crevasses, and nunataks. Upon the ice within this zone are found -streams large enough to be designated as rivers and these are connected -with pools, lakes, and morasses. The dirt and rock fragments imbedded -in the ice are melted out in the lowering of the surface, so that late -in the season the ice presents a most dirty aspect. At the front of the -great mountain glaciers of Alaska, a more vigorous operation of the -same process has yielded a surface soil in which grow such rank forests -as entirely to mask the presence of the ice beneath. - -In addition to the visible streams upon the surface of the Greenland -ice, there are others which flow beneath and can be heard by putting -the ear to the surface. All surface streams eventually encounter the -marginal crevasses and plunge down in foaming cascades, producing the -well known “glacier wells” or “glacier mills.” The progress of the -water is now throughout in tunnels within the ice until it again makes -its appearance at the glacier margin. - - -[Illustration: - -FIG. 306.—Marginal moraine now forming at the edge of Greenland inland -ice, showing a smooth rock pavement outside it. A small lake with a -partial covering of lake ice occupies a hollow of this pavement (after -von Drygalski).] - -=The marginal moraines.=—Study of both the Greenland and Antarctic -glaciers has shown that if we disregard the smaller and short-period -migrations of the ice front, the general later movement has been a -retrograde one—we live in a receding hemicycle of glaciation. The -earlier Greenland glacier has now receded so as to expose large areas -of the former glacier pavement. In places this pavement is largely -bare, indicating a relatively rapid retirement of the ice front, but -at all points at which the ice margin was halted there is now found a -ridge of unassorted rock materials which were dropped by the ice as it -melted (Fig. 306). Such ridges, composed of the unassorted materials -described as _till_, come to have a festooned arrangement largely -concentric to the ice margin, and are the _marginal_ or _terminal -moraines_ (see Fig. 336, p. 312). Marginal moraines, if of large -dimensions, usually have a hummocky surface, and are apt to be composed -of rock fragments of a wide range of size from rock flour (clay) to -large bowlders (plate 17 A), which may represent many types since they -have been plucked by the glacier or gathered in at its surface from -many widely separated localities. - -[Illustration: - -FIG. 307.—Small lake impounded between the ice front and a moraine -which it has recently built. Greenland (after von Drygalski).] - -As the glacier front retires from the moraine which it has built up, -the water which emerges from beneath the ice is impounded behind the -new dam so as to form a lake of crescentic outline (Fig. 307). Such -lakes are particularly short-lived, for the reason that the water finds -an outlet over the lowest point in the crest of the moraine and easily -cuts a gorge through the loose materials, thus draining the lake (Fig. -308). Thereafter, the escaping water flows in a braided stream across -the late lake bottom and thence at the bottom of the gorge through the -moraine. - -[Illustration: - -FIG. 308.—View of a drained lake bottom between the moraine-covered -ice front in the foreground and an abandoned marginal moraine in the -middle distance. The water flows from the ice front in a braided stream -and passes out through the moraine in a narrow gorge. Variegated -glacier, Alaska (after Lawrence Martin).] - - -=The outwash plain or apron.=—The water which descends from the -glacier surface in the glacier wells or mills, eventually arrives at -the bottom, where it follows a sinuous course within a tunnel melted -out in the ice. Much of this water may issue at the ice front beneath -the coarse rock materials which are found there, and so be discovered -with the ear rather than by the eye. The water within the tunnels not -flowing with a free surface but being confined as though it were in -a pipe, may, however, reach the glacier margin under a hydrostatic -pressure sufficient to carry it up rising grades. Inasmuch as it is -heavily charged with rock débris and is suddenly checked upon arriving -at the front it deposits its burden about the ice margin so as to build -up plains of assorted sands and gravels, and over this surface it flows -in ever shifting serpentine channels of braided type (Fig. 308). Such -plains of glacier outwash are described as _outwash plains_ or _outwash -aprons_. - -Rising as it does under hydrostatic pressure the water issuing at the -glacier front may find its way upward in some of the crevasses and so -emerge at a level considerably above the glacial floor. It may thus -come about that the outwash plain is built up about the nose of the -glacier so as partially to bury it from sight. When now the ice front -begins a rapid retirement, a depression or _fosse_ (Fig. 309 and Fig. -339, p. 314) is left behind the outwash plain and in front of the -moraine which is built up at the next halting place. - -[Illustration: FIG. 309.—Diagrams to show the manner of formation -and the structure of an outwash plain, and the position of the fosse -between this and the moraine.] - - -=The continental glacier of Antarctica.=—In Victoria Land, upon the -continent of Antarctica, so far as exploration has yet gone, the -continental glacier is held back by a rampart of mountains, as has been -shown to be true of the inland ice of Greenland. The same flat dome or -shield has likewise been found to characterize its upper surface (Fig. -310). - -The most noteworthy differences between the inland ice masses of -Greenland and Antarctica are to be ascribed to the greater severity of -the Antarctic climate and to the more ample nourishment of the southern -glacier measured by the land area which it has submerged. There is here -no marginal land ribbon as in Greenland, but the glacier covers all -the land and is, moreover, extended upon the sea as a broad floating -terrace—the shelf ice (Fig. 311). This barrier at its margin puts a -bar to all further navigation, rising as it does in some cases 280 feet -above the sea and descending to even greater depths below (plate 15 B). - -[Illustration: - -FIG. 310.—Map showing the inland ice of Victoria Land bordered by the -shelf ice of the Great Ross Barrier. The arrows show the direction of -the prevailing winds (based on maps by Scott and Shackleton).] - -In that portion of Antarctica which was explored by the German -expedition, the inland ice is not as in Victoria Land restrained -within walls of rock, but is spread out upon the continent so as -to assume its natural ice slopes, which are therefore much flatter -than those examined in Greenland and Victoria Land. Here in Kaiser -Wilhelm Land the ice rises at its sea margin in a cliff which is -from 130 to 165 feet in height, then upon a fairly steeply curving -slope to an elevation of perhaps a thousand feet. Here the grades -have become relatively level, and on ever flatter slopes the surface -appears to continue into the distant interior (plate 14). Near the -ice margin numerous fissures betray a motion within the mass which -exact measurements indicate to be but one foot per day, and at a -distance of a mile and a quarter from the margin even this slight -value has diminished by fully one eighth. It can hardly be doubted -that at moderate distances only within the ice margin, the glacier is -practically without motion. - -┌────────────────────────────────────────────────────────────────┐ -│ PLATE 14. │ -│ │ -│ [Illustration: View of the margin of the Antarctic continental │ -│ glacier in Kaiser Wilhelm Land (after E. v. Drygalski).] │ -└────────────────────────────────────────────────────────────────┘ - -Rain or general melting conditions being unknown in Antarctica, a -striking contrast is offered to the marginal zone of the Greenland -continent. This is to a large extent explained by the existence upon -the northern land mass of a coastland ribbon which becomes quickly -heated in the sun’s rays, and both by warming the air and by radiating -heat to the ice it causes melting and produces local air temperatures -which in summer may even be described as hot. About Independence Bay in -latitude 82° N. and near the northernmost extremity of Greenland, Peary -descended from the inland ice into a little valley within which musk -oxen were lazily grazing and where bees buzzed from blossom to blossom -over a gorgeous carpet of flowers. - -[Illustration: FIG. 311.—Sections across the inland ice of Victoria -Land, Antarctica, with the shelf ice in front (after Shackleton).] - - -=Nourishment of continental glaciers.=—Explorations upon and about the -glaciers of Greenland and Antarctica have shown that the circulation -of air above these vast ice shields conforms to a quite simple and -symmetrical model subject to spasmodic pulsations of a very pronounced -type. Each great ice mass with its atmospheric cover constitutes a sort -of refrigerating air engine and plays an important part in the wind -system of the globe. (See Fig. 291, p. 263). Both the domed surface and -the low temperature of the glacier are essential to the continuation of -this pulsating movement within the atmosphere (Fig. 312). The air layer -in contact with the ice is during a period of calm cooled, contracted, -and rendered heavier, so that it begins to slide downward and outward -upon the domed surface in all directions. The extreme flatness of -the greater portion of the glacier surface—a fraction only of one -degree—makes the engine extremely slow in starting, but like all -bodies which slide upon inclined planes, the velocity of its movement -is rapidly accelerated, until a blizzard is developed whose vigor is -unsurpassed by any elsewhere experienced. - -[Illustration: FIG. 312.—Diagram to show the nature of the fixed -glacial anticyclone above continental glaciers and the process by which -their surface is shaped.] - -The effect of such centrifugal air currents above the glacier is to -suck down the air of the upper currents in order to supply the void -which soon tends to develop over the central portion of the glacier -dome. This downward vortex, fed as it is by inward-blowing, high-level -currents, and drained by outwardly directed surface currents, is what -is known as an _anticyclone_, here fixed in position by the central -embossment of the dome. - -The air which descends in the central column is warmed by compression, -or adiabatically, just as air is warmed which is forced into a rubber -tire by the use of a pump. The moisture congealed in the cirrus clouds -floating in the uppermost layer of the convective zone, is carried -down in this vortex and first melted and in turn evaporated, due to -the adiabatic effect. This fusion and evaporation of the ice by its -transformation of latent, to sensible, heat, in a measure counteracts, -and so retards, the adiabatic elevation of temperature within the -column. Eventually the warm air now charged with water vapor reaches -the ice surface, is at once chilled, and its burden of moisture -precipitated in the form of fine snow needles, the so-called “frost -snow”, which in accompaniment to the sudden elevation of temperature is -precipitated at the termination of a blizzard. - -The warming of the air has, however, had the effect of damping as it -were, the engine stroke, and, as the process is continued, to start a -reverse or upward current within the chimney of the anticyclone. The -blizzard is thus suddenly ended in a precipitation of the snow, which -by changing the latent heat of condensation to sensible heat tends to -increase this counter current. - - -[Illustration: FIG. 313.—Snow deltas about the margins of the Fan -glacier outlet of Greenland (after Chamberlin).] - -=The glacier broom.=—During the calm which succeeds to the blizzard, -heat is once more abstracted from the surface air layer, and a new -outwardly directed engine stroke is begun. The tempest which later -develops acts as a gigantic centrifugal broom which sweeps out to -the margins of the glacier all portions of the latest snowfall which -have not become firmly attached to the ice surface. The sweepings -piled up about the margin of continental glaciers have been described -as fringing glaciers, or the glacial fringe. The northern coast of -Greenland and Grant Land are bordered by a fringe of this nature (plate -14 A, and Fig. 315, p. 288). It is by the operation of the glacier -broom that the inland ice is given its characteristic shield-like shape -(Fig. 312). The granular nature of the snow carried by the wind is well -brought out by the little snow deltas about the margins of Greenland -ice tongues (Fig. 313). Obviously because of the presence of the -vigorous anticyclone, no snows such as nourish mountain glaciers can be -precipitated upon continental glaciers except within a narrow marginal -zone, and, as shown by Nansen rock dust from the coastland ribbon and -from the nunataks of Greenland, is carried by a few miles inside the -western margin, and not at all within the eastern. - - -[Illustration: FIG. 314.—Sea ice of the Arctic region in lat. 80° 5´ -N. and long. 2° 52´ E. (after Duc d’Orleans).] - -=Field and pack ice.=—Within polar regions the surface of the sea -freezes during the long winter season, the product being known as -_sea-ice_ or _field-ice_ (Fig. 314). This ice cover may reach a -thickness by direct freezing of eight or more feet, and by breaking up -and being crowded above and below neighboring fragments may increase to -a considerably greater thickness. Ice thus crowded together and more or -less crushed is described as _pack ice_ or _the pack_. - -The pack does not remain stationary but is continually drifting with -the wind and tide, first in one direction and then in another, but -with a general drift in the direction of the prevailing winds. Because -of the vast dimensions of the pack, the winds over widely separated -parts may be contrary in direction, and hence when currents blow -toward each other or when the ice is forced against a land area, it -is locally crushed under mighty pressures and forced up into lines of -_hummocks_—the so-called _pressure ridges_. At other times, when the -winds of widely separated areas blow away from each other, the pack is -parted, with the formation of lanes or _leads_ of open water. - -If seen in bird’s-eye view the lines of hummocks would according to -Nansen be arranged like the meshes of a net having roughly squared -angles and reaching to heights of 15 to 25, rarely 30, feet above the -general surface of the pack. The ice within each mesh of the network -is a _floe_, which at the times of pressure is ground against its -neighbors and variously shifted in position. At the margin of the pack -these floes become separated and float toward lower latitudes until -they are melted. - - -=The drift of the pack.=—The discovery of the drift in the Arctic -pack is a romantic chapter in the history of polar exploration, and -has furnished an example of faith in scientific reasoning and judgment -which may well be compared with that of Columbus. The great figure in -this later discovery is the Norwegian explorer Fridtjof Nansen, and to -the final achievement the ill-fated _Jeannette_ expedition contributed -an important part. - -The _Jeannette_ carrying the American exploring expedition was in 1879 -caught in the pack to the northward of Wrangel Island (Fig. 315), and -two years later was crushed by the ice and sunk to the northward of -the New Siberian Islands. In 1884 various articles, including a list -of stores in the handwriting of the commander of the _Jeannette_, were -picked up at Julianehaab near the southern extremity of Greenland -but upon the western side of Cape Farewell. Nansen, having carefully -verified the facts, concluded that the recovered articles could have -found their way to Julianehaab only by drifting in the pack across the -polar sea, and that at the longest only five years had been consumed -in the transit. After being separated from the pack the articles must -have floated in the current which makes southward along the east coast -of Greenland and after doubling Cape Farewell flows northward upon the -west coast. It was clear that if they had come through Smith Sound they -would inevitably have been found upon the other shore of Baffin Bay. In -confirmation of this view there was found at Godthaab, a short distance -to the northward of Julianehaab (Fig. 315), an ornamented Alaskan -“throwing stick” which probably came by the same route. Moreover, -large quantities of driftwood reach the shores of Greenland which have -clearly come from the Siberian coast, since the Siberian larch has -furnished the larger quantity. - -[Illustration: FIG. 315.—Map of the north polar regions, showing the -area of drift ice and the tracks of the _Jeannette_ and the _Fram_ -(compiled from various maps).] - -Pinning his faith to these indubitable facts, Nansen built the _Fram_ -in such a manner as to resist and elude the enormous pressures of -the ice pack, stocked her with provisions sufficient for five years, -and by allowing the vessel to be frozen into the pack north of the -New Siberian Islands, he consigned himself and his companions to the -mercy of the elements. The world knows the result as one of the most -remarkable achievements in the long history of polar exploration. The -track of the _Fram_, charted in Fig. 315, considered in connection with -that of the _Jeannette_, shows that the Arctic pack drifts from Bering -Sea westward until near the northeastern coast of Greenland. - -Special casks were for experimental purposes fastened in the ice to -the north of Behring Strait by Melville and Bryant, and two of these -were afterwards recovered, the one near the North Cape in northern -Norway, and the other in northeastern Iceland (see map, Fig. 315). -Peary’s trips northward in 1906 and 1909 from the vicinity of Smith -Sound have indicated that between the Pole and the shores of Greenland -and Grant Land the drift is throughout to the eastward, corresponding -to the westerly wind. Upon this border the great area of Arctic drift -ice is in contact with great continental glaciers bordered by a glacier -fringe. Admiral Peary has shown that instead of consisting of frozen -sea ice, the pack is here made up of great floes from 20 to 100 feet in -thickness and that these have been derived from the glacier fringe. - -Whenever the blizzards blow off the inland ice from the south, leads -are opened at the margin of the fringe and may carry strips from the -latter northward across the lead. With favorable conditions these -leads may be closed by thick sea ice so that with the occurrence of -counter winds from the north they do not entirely return to their -original position. A continuance of this process may have resulted in -the heavy floe ice to the northward of Greenland, which, acting as -an obstruction, may have forced the thinner drift ice to keep on the -European side of the Arctic pack. - -[Illustration: FIG. 316.—The shelf ice of Coats Land with the -surrounding pack ice showing in the foreground (after Bruce).] - -About the Antarctic continent there is a broad girdle of pack ice -which, while more indolent in its movements than the Arctic pack, has -been shown by the expeditions of the _Belgica_ and the _Pourquoi-Pas_ -to possess the same kind of shifting movements. In the southern spring -this pack floats northward and is to a large extent broken up and -melted on reaching lower latitudes. - - -[Illustration: FIG. 317.—Tidewater cliff at the front of a glacier -tongue from which icebergs are born.] - -=The Antarctic shelf ice.=—It has been already pointed out that the -inland ice of Antarctica is in part at least surrounded by a thick -snow and ice terrace floating upon the sea and rising to heights of -more than 150 feet above it (plate 15 B and Fig. 316). The visible -portions of this shelf-ice are of stratified compact snow, and the -areas which have thus far been studied are found in bays from which -dislodgment is less easily effected. The origin of the shelf ice is -believed to be a sea-ice which because not easily detached at the time -of the spring “break-up” is thickened in succeeding seasons chiefly by -the deposition of precipitated and drifted snow upon its surface, so -that it is bowed down under the weight and sunk to greater and greater -depths in the water. To some extent, also, it is fed upon its inner -margin by overflow of glacier ice from the inland ice masses. - - -=Icebergs and snowbergs and the manner of their birth.=—Greenland -reveals in the character of its valleys the marks of a large subsidence -of the continent—the serpentine inlets or fjords by which its coast is -so deeply indented. Into the heads of these fjords the tongues from the -inland ice descend generally to the sea level and below. The glacier -ice is thus directly attacked by the waves as well as melted in the -water, so that it terminates in the fjords in great cliffs of ice (Fig. -317). It is also believed to extend beneath the water surface as a long -toe resting upon the bottom (Fig. 319). - -┌───────────────────────────────────────────────────────────────────┐ -│ PLATE 15. │ -│ │ -│ [Illustration: _A._ An Antarctic ice foot with boat party landing │ -│ (after R. F. Scott).] │ -│ │ -│ [Illustration: _B._ A near view of the front of the Great Ross │ -│ Barrier, Antarctica (after R. F. Scott).] │ -└───────────────────────────────────────────────────────────────────┘ - -[Illustration: FIG. 318.—A Greenlandic iceberg after a long journey in -warm latitudes.] - -The exposed cliff is notched and undercut by the waves in the same -manner as a rock cliff, and the upper portions override the lower so -that at frequent intervals small masses of ice from this front separate -on crevasses, and toppling over, fall into the water with picturesque -splashes. Such small bergs, whose birth may be often seen at the cliff -front of both the Greenland and Alaskan glaciers, have little in common -with those great floating islands of ice that are drifted by the winds -until, wasted to a fraction only of their former proportions, they -reach the lanes of transatlantic travel and become a serious menace to -navigation (Fig. 318). - -[Illustration: FIG. 319.—Diagram showing one way in which northern -icebergs may be born from the glacier tongue (after Russell).] - -Northern icebergs of large dimensions are born either by the lifting of -a separated portion of the extended glacier toe lying upon the bottom -of the fjord, or else they separate bodily from the cliff itself, -apparently where it reaches water sufficiently deep to float it. In -either case the buoyancy of the sea water plays a large rôle in its -separation. - -If derived from the submerged glacier toe (Fig. 319), a loud noise is -heard before any change is visible, and an instant later the great -mass of ice rises out of the water some distance away from the cliff, -lifting as it does so a great volume of water which pours off on all -sides in thundering cascades and exposes at last a berg of the deepest -sapphire blue. The commotion produced in the fjord is prodigious, and a -vessel in close proximity is placed in jeopardy. - -Even larger bergs are sometimes seen to separate from the ice cliff, in -this case an instant before or simultaneously, with a loud report, but -such bergs float away with comparatively little commotion in the water. - -[Illustration: FIG. 320.—A northern iceberg surrounded by sea ice.] - -The icebergs of the south polar region are usually built upon a far -grander scale than those of the Arctic regions, and are, further, both -distinctly tabular in form and bounded by rectangular outlines (Fig. -321). Whereas the large bergs of Greenlandic origin are of ice and blue -in color, the tabular bergs of Antarctica might better be described -as _snowbergs_, since they are of a blinding whiteness and their -visible portions are either compacted snow or alternating thick layers -of compact snow and thin ribbons of blue ice, the latter thicker and -more abundant toward the base. All such bergs have been derived from -the shelf ice and not from the inland ice itself. Blue icebergs which -have been derived from the inland ice have been described from the -one Antarctic land that has been explored in which that ice descends -directly to the sea. - -[Illustration: FIG. 321.—Tabular Antarctic iceberg separating from the -shelf ice (after Shackleton).] - -In both the northern and southern hemispheres those bergs which have -floated into lower latitudes have suffered profound transformations. -Their exposed surfaces have been melted in the sun, washed by the -rain, and battered by the waves, so that they lose their relatively -simple forms but acquire rounded surfaces in place of the early angular -ones (Fig. 318, p. 291). Sir John Murray, who had such extended -opportunities of studying the southern icebergs from the deck of the -_Challenger_, has thus described their beauties: - - “Waves dash, against the vertical faces of the floating ice island as - against a rocky shore, so that at the sea level they are first cut - into ledges and gullies, and then into caves and caverns of the most - heavenly blue, from out of which there comes the resounding roar of - the ocean, and into which the snow-white and other petrels may be seen - to wing their way through guards of soldier-like penguins stationed - at the entrances. As these ice islands are slowly drifted by wind and - current to the north, they tilt, turn and sometimes capsize, and then - submerged prongs and spits are thrown high into the air, producing - irregular pinnacled bergs higher, possibly, than the original - table-shaped mass.” - - -READING REFERENCES FOR CHAPTERS XX AND XXI - - General:— - - I. C. RUSSELL. Glaciers of North America. Ginn, Boston, 1897, pp. 210, - pls. 22. - - CHAMBERLIN and SALISBURY. Geology, vol. 1, pp. 232-308. - - H. HESS. Die Gletscher, Braunschweig, 1904, pp. 426 (illustrated). - - WILLIAM H. HOBBS. Characteristics of Existing Glaciers. Macmillan, - 1911, pp. 301, pls. 34. - -Special districts of mountain glaciers:— - - JAMES D. FORBES. Travels Through the Alps of Savoy and other Parts - of the Pennine Chain with Observations on the Phenomena of Glaciers. - Edinburgh, 1845, pp. 456, pls. 9, maps 2. - - A. PENCK, E. BRÜCKNER, et L. DU PASQUIER. Le système glaciare des - alpes, etc., Bull. Soc. Sc. Nat. Neuchâtel, vol. 22, 1894, pp. 86. - - E. RICHTER. Die Gletscher der Ostalpen. Stuttgart, 1888, pp. 306, 7 - maps. - - JAMES D. FORBES. Norway and Its Glaciers, etc. Edinburgh, 1853, pp. - 349, pls. 10, map. - - I. C. RUSSELL. Existing Glaciers of the United States, 5th Ann. Rept. - U. S. Geol. Surv., 1885, pp. 307-355, pls. 32-55; Glaciers of Mt. - Ranier, 18th Ann. Rept. U. S. Geol. Surv., 1898, pp. 349-423, pls. - 65-82. - - W. H. SHERZER. Glaciers of the Canadian Rockies and Selkirks, Smith. - Cont. to Knowl. No. 1692, Washington, 1907, pp. 135, pls. 42. - - H. F. REID. Studies of Muir Glacier, Alaska, Nat. Geogr. Mag., vol. 4, - 1892, pp. 19-84, pls. 1-16. - - I. C. RUSSELL. Malaspina Glacier, Jour. Geol., vol. 1, 1893, pp. - 219-245. - - G. K. GILBERT. Harriman Alaska Expedition, vol. 3, Glaciers, 1904, pp. - 231, pls. 37. - - W. M. CONWAY. Climbing and Exploration in the Karakoram Himalayas, - Maps and Scientific Reports, 1894, map sheets I-II. - - FANNY BULLOCK WORKMAN and WILLIAM HUNTER WORKMAN. The Hispar Glacier, - Geogr. Jour., vol. 35, 1910, pp. 105-132, 7 pls. and map. - -The cycle of glaciation:— - - WILLIAM H. HOBBS. The Cycle of Mountain Glaciation, Geogr. Jour., vol. - 36, 1910, pp. 146-163, 268-284. - -Upper and lower cloud zones of the atmosphere:— - - R. ASSMANN, A. BERSON, and H. GROSS. Wissenschaftliche Luftfahrten - ausgeführt vom deutschen Verein zur Förderung der Luftschiffahrt in - Berlin, 1899-1900, 3 vols. - - E. GOLD and W. A. HARWOOD. The Present State of our Knowledge of - the Upper Atmosphere as Obtained by the Use of Kites, Balloons, and - Pilot-balloons, Rept. Brit. Assoc. Adv. Sci., 1909, pp. 1-55. - - W. H. MOORE. Descriptive Meteorology, Appleton, New York, 1910, pp. - 95-136. - - WILLIAM H. HOBBS. The Pleistocene Glaciation of North America Viewed - in the Light of our Knowledge of Existing Continental Glaciers, Bull. - Am. Geogr. Soc., vol. 42, 1911, pp. 647-650. - -The continental glacier of Greenland:— - - F. NANSEN. The First Crossing of Greenland, 2 vols, Longmans, London, - 1890 (the scientific results are contained in an appendix to volume 2, - pp. 443-497). - - R. E. PEARY. A Reconnaissance of the Greenland Inland Ice, Jour. Am. - Geogr. Soc., vol. 19, 1887, pp. 261-289; Journeys in North Greenland, - Geogr. Jour., vol. 11, 1898, pp. 213-240. - - T. C. CHAMBERLIN. Glacier Studies in Greenland, Jour. Geol., vol. - 2, 1894, pp. 649-668, 768-788, vol. 3, pp. 61-69, 198-218, 469-480, - 565-582, 668-681, 833-843, vol. 4, pp. 582-592, 769-810, vol. 5, pp. - 229-245; Recent glacial studies in Greenland (Presidential address), - Bull. Geol. Soc. Am., vol. 6, 1895, pp. 199-220, pls. 3-10. - - R. S. TARR. The Margin of the Cornell Glacier, Am. Geol., vol. 20, - 1897, pp. 139-156, pls. 6-12. - - R. D. SALISBURY. The Greenland Expedition of 1895, Jour. Geol., vol. - 3, 1895, pp. 875-902. - - E. V. DRYGALSKI. Grönland Expedition der Gesellschaft für Erdkunde zu - Berlin 1891-1893, Berlin, 1897, 2 vols., pp. 551 and 571, pls. 53, - maps 10. - - WILLIAM H. HOBBS. Characteristics of the Inland Ice of the Arctic - Regions, Proc. Am. Phil. Soc., vol. 49, 1910, pp. 57-129, pls. 26-30. - -The Antarctic continental glacier:— - - R. F. SCOTT. The Voyage of the _Discovery_. London, 2 vols., 1905. - - E. H. SHACKLETON. The Heart of the Antarctic. London, 2 vols., 1910. - - E. VON DRYGALSKI. Zum Kontinent des eisigen Südens, Deutsche - Südpolar-Expedition, Fahrten und Forschungen des “Gauss”, 1901-1903, - Berlin, 1904, pp. 668, pls. 21. - - OTTO NORDENSKIÖLD and J. S. ANDERSSON. Antarctica or Two Years Amongst - the Ice of the South Pole. London, 1905, pp. 608, illustrated. - - E. PHILIPPI. Ueber die fünf Landeis-Expeditionen, etc., Zeit. f. - Gletscherk., vol. 2, 1907, pp. 1-21. - -Nourishment of continental glaciers:— - - WILLIAM H. HOBBS. Characteristics of the Inland Ice of the Arctic - Regions, Proc. Am. Phil. Soc., vol. 49, 1910, pp. 96-110; The Ice - Masses on and about the Antarctic Continent, Zeit. f. Gletscherk., - vol. 5, 1910, pp. 107-120; Characteristics of Existing Glaciers. New - York, 1911, pp. 143-161, 261-289. Pleistocene Glaciation of North - America Viewed in the Light of our Knowledge of Existing Continental - Glaciers, Bull. Am. Geogr. Soc., vol. 43, 1911, pp. 641-659. - -Field and pack ice:— - - EMMA DE LONG. The Voyage of the _Jeannette_, the ship and ice journals - of George W. de Long, etc. Berlin, 1884, 2 vols., chart in back of - vol. 1. - - ROBERT E. PEARY. The Discovery of the North Pole (for further - references on both sea and pack ice and Antarctic shelf ice, consult - Hobbs’s Characteristics of Existing Glaciers, pp. 210-213, 242-244). - -Icebergs:— - - WYVILLE THOMSON. Challenger Report, Narrative, vol. 1, 1865, Pt. i, - pp. 431-432, pls. B-D. - - I. C. RUSSELL. An Expedition to Mt. St. Elias, Nat. Geogr. Mag., vol. - 3, 1891, pp. 101-102, fig. 1. - - H. F. REID. Studies of Muir Glacier, Alaska, _ibid._, vol. 4, 1892, - pp. 47-48. - - E. VON DRYGALSKI. Grönland-Expedition, etc., vol. 1, pp. 367-404. - - M. C. ENGELL. Ueber die Entstehung der Eisberge, Zeit. f. Gletscherk., - vol. 5, 1910, pp. 112-132. - - - - -CHAPTER XXII - -THE CONTINENTAL GLACIERS OF THE “ICE AGE” - - -=Earlier cycles of glaciation.=—Our study of the rocks composing the -outermost shell of the lithosphere tells us that in at least three -widely separated periods of its history the earth has passed through -cycles of glaciation during which considerable portions of its surface -have been submerged beneath continental glaciers. The latest of these -occurred in the yesterday of geology and has often been referred to as -the “ice age”, because until quite recently it was supposed to be the -only one of which a record was preserved. - -[Illustration: FIG. 322.—Map of the globe showing the areas which were -covered by the continental glaciers of the so-called “ice-age” of the -Pleistocene period. The arrows show the directions of the centrifugal -air currents in the fixed anticyclones above the glaciers.] - -[Illustration: - -FIG. 323.—Glaciated granite bowlder which has weathered out of a -moraine of Permo-Carboniferous age upon which it rests. South Australia -(after Howchin).] - -This latest ice age represents four complete cycles of glaciation, for -it is believed that the continental ice developed and then completely -disappeared during a period of mild climate before the next glacier -had formed in its place, and that this alternation of climates was -no less than three times repeated, making four cycles in all. At -nearly or quite the same time ice masses developed in northern North -America and in northern Europe, the embossments of the ice domes being -located in Canada and in Scandinavia respectively (Fig. 322). There -appears to have been at this time no extensive glaciation of the -southern hemisphere, though in the next earlier of the known great -periods of glaciation—the so-called Permo-Carboniferous—it was the -southern hemisphere, and not the northern, that was affected (Fig. -323 and Fig. 304, p. 276). From the still earlier glacial period our -data are naturally much more meager, but it seems probable that it -was characterized by glaciated areas within both the northern and the -southern hemispheres. - -[Illustration: FIG. 324.—Map to show the glaciated and nonglaciated -regions of North America (after Salisbury and Atwood).] - - -=Contrast of the glaciated and nonglaciated regions.=—Since we have -now studied in brief outline the characteristics of the existing -continental glaciers, we are in a position to review the evidences of -former glaciers, the records of which exist in their carvings, their -gravings, and their deposits. - -[Illustration: - -FIG. 325.—Map of the glaciated and nonglaciated areas of northern -Europe. The strongly marked morainal belts respectively south and north -of the Baltic depression represent halting places in the retreat of the -latest continental glacier (compiled from maps by Penck and Leverett).] - -An observant person familiar with the aspects of Nature in both the -northern and southern portions of the central and eastern United States -must have noticed that the general courses of the Ohio and Missouri -rivers define a somewhat marked common border of areas which in most -respects are sharply contrasted (Fig. 324). Hardly less striking is the -contrast between the glaciated and the nonglaciated regions upon the -continent of Europe (Fig. 325). - -It is the northern of the two areas which in each case reveals -the characteristic evidences of glaciation, while there is entire -absence of such marks to the southward of the common border. Within -the American glaciated region there is, however, an area surrounded -like an island, and within this district (Fig. 324) none of the marks -characteristic of glaciation are to be found. This area is usually -referred to as the “driftless area”, and occupies portions of the -states of Wisconsin, Illinois, Minnesota, and Iowa. Even better than -the area to the southward of the Ohio and Missouri rivers, it permits -of a comparison of the nonglaciated with the drift-covered region. - - -[Illustration: - -FIG. 326.—“Stand Rock” near the “Dells” of the Wisconsin river, an -unstable erosion remnant characteristic of the driftless area of North -America (after Salisbury and Atwood).] - -=The “driftless area.”=—Within this district, then, we have preserved -for our study a landscape which remains largely as it was before -the several ice invasions had so profoundly transformed the general -surface of the surrounding country. Speaking broadly, we may say that -it represents an uplifted and in part dissected plain, which to the -south and east particularly reveals the character of nearly mature -river erosion (Fig. 177, p. 170). The rock surface is here everywhere -mantled by decomposed and disintegrated rock residues of local origin. -The soluble constituents of the rock, such as the carbonates, have -been removed by the process of leaching, so that the clays no longer -effervesce when treated with dilute mineral acid. - -Wherever favored by joints and by an alternation of harder and softer -rock layers, picturesque unstable erosion remnants or “chimneys” may -stand out in relief (Fig. 326). Furthermore, the driftless area is -throughout perfectly drained—it is without lakes or swamps—since -all valleys are characterized throughout by forward grades. The side -valleys enter the main valleys as do the branches a tree trunk; in -other words, the drainage is described as arborescent. In so far as any -portions of a plane surface now remain in the landscape, they are found -at the highest levels (plate 16 A). The topography is thus the result -of a partial removal by erosion of an upland and may be described as -_incised topography_. Nowhere within the area are there found rock -masses foreign to the region, but all mantle rock is the weathered -product of the underlying ledges. - -┌────────────────────────────────────────────────────────────────────┐ -│ PLATE 16. │ -│ │ -│ [Illustration: _A._ Incised topography within the “driftless area” │ -│ (U. S. Geol. Survey).] │ -│ │ -│ [Illustration: _B._ Built-up topography within glaciated region │ -│ (U. S. Geol. Survey).] │ -└────────────────────────────────────────────────────────────────────┘ - - -=Characteristics of the glaciated regions.=—The topography of the -driftless area has been described as _incised_, because due to the -partial destruction of an uplifted plain; and this surface is, -moreover, perfectly drained. The characteristic topography of the -“drift” areas is by contrast _built up_; that is to say, the features -of the region instead of being _carved_ out of a plain are the result -of _molding_ by the process of deposition (plate 16 B). In so far as a -plane is recognizable, it is to be found not at the highest, but at the -lowest level—a surface represented largely by swamps and lakes—and -above this plain rise the characteristic rounded hills of various types -which have been _built up_ through deposition. The process by which -this has been accomplished is one easy to comprehend. As it invaded the -region, the glacier planed away beneath its marginal zone all weathered -mantle rock and deposited the planings within the hollows of the -surface (Fig. 327). The effect has been to flatten out the preëxisting -irregularities of the surface, and to yield at first a gently -undulating plain upon which are many undrained areas and a haphazard -system of drainage (Fig. 328). All unstable erosion remnants, such as -now are to be found within the driftless area, were the first to be -toppled over by the invading glacier, and in their place there is left -at best only rounded and polished “shoulders” of hard and unweathered -rock—the well-known _roches moutonnées_. - -[Illustration: - -FIG. 327.—Diagram showing the manner in which a continental glacier -obliterates existing valleys (after Tarr).] - - -=The glacier gravings.=—The tools with which the glacier works are -never quite evenly edged, and instead of an in all respects perfect -polish upon the rock pavement, there are left furrowings, gougings, and -scratches. Of whatever sort, these scorings indicate the lines of ice -movement and are thus indubitable records graven upon the rock floor. -When mapped over wide areas, a most interesting picture is presented to -our view, and one which supplements in an important way the studies of -existing continental glaciers (Fig. 334, p. 308, and Fig. 336, p. 312). - -[Illustration: FIG. 328.—Lake and marsh district in northern -Wisconsin, the effect of glacial deposition in former valleys (after -Fairbanks).] - -It has been customary to think of the glacier as everywhere eroding -its bed, although the only warrant for assuming degradation by flow of -the ice is restricted to the marginal zone, since here only is there -an appreciable surface grade likely to induce flow. Both upon the -advance and again during the retreat of a glacier, all parts of the -area overridden must be subjected to this action. Heretofore pictured -in the imagination as enlarged models of Alpine glaciers, the vast -ice mantles were conceived to have spread out over the country as the -result of a kind of viscous flow like that of molasses poured upon -a flat surface in cold weather. The maximum thickness of the latest -American glacier of the ice age has been assumed to have been perhaps -10,000 feet near the summit of its dome in central Labrador. From this -point it was assumed that the ice traveled southward up the northern -slope of the Laurentian divide in Canada, and thence to the Ohio river, -a distance of over 1300 miles. If such a mantle of ice be represented -in its natural proportions in vertical section, to cover the distance -from center to margin we may use a line six inches in length, and -only 1/100 of an inch thick. Upon a reduced scale these proportions -are given in Fig. 329. Obviously the force of gravity acting within -a viscous mass of such proportions would be incompetent to effect a -transfer of material from the center to the periphery, even though the -thickness should be doubled or trebled. Yet until the fixed glacial -anticyclone above the glacier had been proven and its efficiency as a -broom recognized, no other hypothesis than that of viscous flow had -been offered in explanation. The inherited conception of a universal -plucking and abrasion on the bed of the glacier is thus made untenable -and can be accepted for the marginal portion only. - -[Illustration: FIG. 329.—Cross section in approximate natural -proportions of the latest North American continental glacier of -Pleistocene age from its center to its margin.] - -Not only do the rock scorings show the lines of ice movement, but -the directions as well may often be read upon the rock. Wherever -there are pronounced irregularities of surface still existing on the -pavement, these are generally found to have gradual slopes upon the -side from which the ice came, and relatively steep falls upon the -lee or “pluck” side. If, however, we consider the irregularities of -smaller size, the unsymmetrical slopes of these protruding portions -of the floor are found to be reversed—it is the steep slope which -faces the oncoming ice and the flatter slope which is upon the lee -side. Such minor projections upon the floor usually have their origin -in some harder nodule which deflects the abrading tools and causes -them to pass, some on the one side and some upon the other. By this -process a staple-shaped groove comes to surround the nodule, leaving an -unsymmetrical elevated ridge within, which is steep upon the stoss side -and slopes gently away to leeward. - - -[Illustration: FIG. 330.—Limestone surface at Sibley, Michigan.] - -=Younger records over older—the glacier palimpsest.=—Many important -historical facts have been recovered from the largely effaced writing -upon ancient palimpsests, or parchments upon which an earlier record -has been intentionally erased to make room for another. In the -gravings upon the glacier pavement, earlier records have been likewise -in large part effaced by later, though in favorable localities the -two may be read together. Thus, as an example, at the great limestone -quarries of Sibley, in southeastern Michigan, the glaciated rock -surface wherever stripped of its drift cover is a smoothly polished and -relatively level floor with striæ which are directed west-northwest. -Beneath this general surface there are, however, a number of elliptical -depressions which have their longer axes directed south-southwest, one -being from twenty-five to thirty feet long and some ten feet in depth -(Fig. 330). These boat-shaped depressions are clearly the remnants of -an earlier more undulating surface which the latest glacier has in -large part planed away, since the bottoms of the depressions are no -less perfectly glaciated but have their striæ directed in general near -the longer axis of the troughs. Palimpsest-like there are here also the -records of more than one graving. - - -[Illustration: - -FIG. 331.—Map to show the outcroppings of peculiar rock types in the -region of the Great Lakes, and some of the localities where “float -copper” has been collected (float copper localities after Salisbury).] - -=The dispersion of the drift.=—Long before the “ice age” had been -conceived in the minds of Agassiz and his contemporaries, it had been -remarked that scattered over the North German plain were rounded -fragments of rock which could not possibly have been derived from their -own neighborhood but which could be matched with the great masses of -red granite in Sweden well known as the “Swedish granite.” Buckland, an -English geologist, had in 1815 accounted for such “erratic” blocks of -his own country, here of Scotch granite, by calling in the deluge of -Noah; but in the late thirties of the nineteenth century, Sir Charles -Lyell, with the results of English Arctic explorers in mind, claimed -that such traveled blocks had been transported by icebergs emanating -from the polar regions. A relic of Buckland’s earlier view we have -in the word “diluvium” still occasionally used in Germany for glacier -transported materials; while the term “drift” still remains in common -use to recall Lyell’s iceberg hypothesis, even though the original -meaning of the term has been abandoned. Drift is now a generic term -and refers to all deposits directly or indirectly referable to the -continental glaciers. - -In general the place of derivation of the glacial drift may be said to -be some point more distant from and within the former ice margin at the -time when it was deposited; in other words, the dispersion of the drift -was centrifugal with reference to the glacier. - -[Illustration: - -FIG. 332.—Map of the “bowlder train” from Iron Hill, R. I. (based upon -Shaler’s map, but with the directions of glacial striæ added).] - -Wherever rocks of unusual and therefore easily recognizable character -can be shown to occur in place and with but limited areas, the -dispersion of such material is easy to trace. The areas of red Swedish -and Scotch granite have been used to follow out in a broad way the -dispersion of drift over northern Europe. Within the region of the -Great Lakes of North America are areas of limited size which are -occupied by well marked rock types, so that the journeyings of their -fragments with the continental glacier can be mapped with some care. -Upon the northern shore of Georgian Bay occurs the beautiful jasper -conglomerate, whose bright red pebbles in their white quartz field -attract such general notice. At Ishpeming in the northern peninsula of -Michigan is found the equally beautiful jaspilite composed of puckered -alternating layers of black hematite and red jasper. On Keweenaw -Peninsula, which protrudes into Lake Superior from its southern shore, -is found that remarkable occurrence of native copper within a series -of igneous rocks of varied types and colors. Fragments of this copper, -some weighing several hundreds of pounds each and masked in a coat -of green malachite, have under the name of “drift” or “float” copper -been collected at many localities within a broad “fan” of dispersal -extending almost to the very limits of glaciation (Fig. 331). - -Some miles to the north of Providence in Rhode Island there is a hill -known as Iron Hill composed in large part of black magnetite rock, -the so-called Cumberlandite. From this hill as an apex there has -been dispersed a great quantity of the rock distributed as a well -marked “bowlder train” within which the size and the frequency of the -dispersed bowlders is in inverse ratio to the distance from the parent -ledge (Fig. 332). Similar though less perfect trains of bowlders are -found on the lee side of most projecting masses of resistant rocks -within the area of the drift. - -Large bowlders when left upon a ledge of notably different appearance -easily attract attention, and have been described as “perched -bowlders.” Resting as they sometimes do upon a relatively small area, -they may be nicely balanced and thus easily given a pendular or rocking -motion. Such “rocking stones” are common enough, especially among the -New England hills (plate 17 B). Many such bowlders have made somewhat -remarkable peregrinations with many interruptions, having been carried -first in one direction by an earlier glacier to be later transported in -wholly different directions at the time of new ice invasions. - -┌───────────────────────────────────────────────────────────────────┐ -│ PLATE 17. │ -│ │ -│ [Illustration: _A._ Soled glacial bowlders which show differently │ -│ directed striæ upon the same facet.] │ -│ │ -│ [Illustration: _B._ Perched bowlder upon a striated ledge of │ -│ different rock type, Bronx Park, New York (after Lungstedt).] │ -│ │ -│ [Illustration: _C._ Characteristic knob and basin surface of a │ -│ moraine.] │ -└───────────────────────────────────────────────────────────────────┘ - - -=The diamonds of the drift.=—Of considerable popular, even if not -economic, interest are the diamonds which have been sown in the drift -after long and interrupted journeyings with the ice from some unknown -home far to the northward in the wilderness of Canada. The first stone -to be discovered was taken by workmen from a well opening near the -little town of Eagle in Wisconsin in the year 1876. Its nature not -being known, it remained where it was found as a curiosity only, and it -was not until 1883 that it was taken to Milwaukee and sold to a jeweler -equally ignorant of its value, and for the merely nominal sum of one -dollar. Later recognized as a diamond of the unusual weight of sixteen -carats, it was sold to the Tiffanys and became the cause of a long -litigation which did not end until the Supreme Court of Wisconsin had -decided that the Milwaukee jeweler, and not the finder, was entitled to -the price of the stone, since he had been ignorant of its value at the -time of purchase. - -[Illustration: - -FIG. 333.—Shapes and approximate natural sizes of some of the more -important diamonds from the Great Lakes region of the United States. In -order from left to right these figures represent the Eagle diamond of -sixteen carats, the Saukville diamond of six and one half carats, the -Milford diamond of six carats, the Oregon diamond of four carats, and -the Burlington diamond of a little over two carats.] - -An even larger diamond, of twenty-one carats weight, was found at -Kohlsville, and smaller ones at Oregon, Saukville, Burlington, and Plum -Creek in the state of Wisconsin; at Dowagiac in Michigan; at Milford in -Ohio, and in Morgan and Brown counties in Indiana. The appearance of -some of the larger stones in their natural size and shape may be seen -in Fig. 333. - -While the number of the diamonds sown in the drift is undoubtedly -large, their dispersion is such that it is little likely they can be -profitably recovered. The distribution of the localities at which -stones have thus far been found is set forth upon Fig. 334. Obviously -those that have been found are the ones of larger size, since these -only attract attention. In 1893, when the finding of the Oregon stone -drew attention to these denizens of the drift, the writer prophesied -that other stones would occasionally be discovered under essentially -the same conditions, and such discoveries are certain to continue in -the future. - -[Illustration: - -FIG. 334.—Glacial map of a portion of the Great Lakes region, -showing the unglaciated area and the areas of older and newer drift. -The driftless area, the moraines of the later ice invasion, and the -distribution of diamond localities upon the latter are also shown. -With the aid of the directions of striæ some attempt has been made to -indicate the probable tracks of more important diamonds, which tracks -converge in the direction of the Labrador peninsula.] - - -=Tabulated comparison of the glaciated and nonglaciated regions.=—It -will now be profitable to sum up in parallel columns the contrasted -peculiarities of the glaciated and the unglaciated regions. - - UNGLACIATED REGION GLACIATED REGION - - TOPOGRAPHY - - The topography is _destructional_; The topography is _constructional_; - the remnants of a plain are found the remnants of a plain are found - at the highest levels or upon the at the lowest levels in lakes and - hill tops; hills are _carved_ out swamps; hills are _molded_ above a - of a high plain; unstable erosion plain in characteristic forms; no - remnants are characteristic. unstable erosion remnants, but only - rounded shoulders of rock. - - DRAINAGE - - The area is completely drained, The area includes undrained - and the drainage network is areas,—lakes and swamps,—and - _arborescent_. the drainage system is _haphazard_. - - ROCK MANTLE - - The exposed rock is decomposed No decomposed or disintegrated - and disintegrated to a rock is “in place”, but only - considerable depth; it is all of hard, fresh surface; loose rock - local derivation and hence of few material is all foreign and of many - types—_homogeneous_; the fragments sizes and types—_heterogeneous_; - are angular; soils are leached and rock bowlders and pebbles are - hence do not contain carbonates. faceted and polished as well as - striated, usually in several - directions upon each facet; soils - are rock flour—the grist of the - glacial mill. - - ROCK SURFACE - - Rock surface is rough and Rock surface is planed or grooved, - irregular. and polished. Shows glacial striæ. - - -=Unassorted and assorted drift.=—The drift is of two distinct types; -namely, that deposited directly by the glacier, which is without -stratification, or unassorted; and that deposited by water flowing -either beneath or from the ice, and this like most fluid deposited -material is assorted or stratified. The unassorted material is -described as _till_, or sometimes as “bowlder clay”; the assorted -is sand or gravel, sometimes with small included bowlders, and is -described as _kame gravel_. To recall the parts which both the glacier -and the streams have played in its deposition, all water-deposited -materials in connection with glaciers are called _fluvio-glacial_. - -[Illustration: FIG. 335.—Section in coarse till. Note the range in -size of the materials, the lack of stratification, and the “soled” form -of the bowlders.] - -Till is, then, characterized by a noteworthy lack of homogeneity, both -as regards the size and the composition of its constituent parts. As -many as twenty different rock types of varied textures and colors may -sometimes be found in a single exposure of this material, and the -entire gamut is run from the finest rock flour upon the one hand to -bowlders whose diameter may be measured in feet (Fig. 335). - -In contrast with those derived by ordinary stream action, the pebbles -and bowlders of the till are faceted or “soled”, and usually show -striations upon their faces. If a number of pebbles are examined, -some at least are sure to be found with striations in more than one -direction upon a single facet. As a criterion for the discrimination -of the material this may be an important mark to be made use of to -distinguish in special cases from rock fragments derived by brecciation -and slickensiding and distributed by the torrents of arid and semiarid -regions. - -Inasmuch as the capacity of ice for handling large masses is greater -than that of water, assorted drift is in general less coarse, and, as -its name implies, it is also stratified. From ordinary stream gravels, -the kame gravels are distinguished by the form of their pebbles, which -are generally faceted and in some cases striated. In proportion, -however, as the materials are much worked over by the water, the -angles between pebble faces become rounded and the original shapes -considerably masked. - - -=Features into which the drift is molded.=—Though the preëxisting -valleys were first filled in by drift materials, thus reducing the -accent of the relief, a continuation of the same process resulted in -the superimposition of features of characteristic shapes upon the -imperfectly evened surface of the earlier stages. These features belong -to several different types, according as they were built up outside -of, at and upon, or within the glacier margin. The extra-marginal -deposits are described as _outwash plains_ or _aprons_, or sometimes as -_valley trains_; the marginal are either _moraines_ or _kames_; while -within the border were formed the _till plain_ or _ground moraine_, -and, locally also, the _drumlin_ and the _esker_ or _os_. These -characteristic features are with few exceptions to be found only within -the area covered by the latest of the ice invasions. For the earlier -ones, so much time has now elapsed that the effect of weathering, -wash, and stream erosion has been such that few of the features are -recognizable. - -Marginal and extra-marginal features are extended in the direction -of the margin or, in other words, perpendicular to the local ice -movement; while the intra-marginal deposits are as noteworthy for being -perpendicular to the margin, or in correspondence with the direction -of local ice movement. Each of these features possesses characteristic -marks in its form, its size, proportions, surface molding and -orientation, as well as in its constituent materials. It should perhaps -be pointed out that the existing continental glaciers, being in high -latitudes, work upon rock materials which have been subjected to -different weathering processes from those characteristic of temperate -latitudes. Moreover, the melting of the Pleistocene glaciers having -taken place in relatively low latitudes, larger quantities of rock -débris were probably released from the ice during the time of definite -climatic changes, and hence heavier drift accumulations have for both -of these reasons resulted. - -=Marginal or “kettle” moraines.=—Wherever for a protracted period the -margin of the glacier was halted, considerable deposits of drift were -built up at the ice margin. These accumulations form, however, not only -about the margin, but upon the ice surface as well; in part due to -materials collected from melting down of the surface, and in part by -the upturning of ice layers near the margin (see _ante_, p. 277). - -[Illustration: - -FIG. 336.—Sketch map of portions of Michigan, Ohio, and Indiana, -showing the festooned outlines of the moraines about the former ice -lobes, and the directions of ice movement as determined by the striæ -upon the rock pavement (after Leverett).] - -An important rôle is played by the thaw water which emerges at the -ice margin, especially within the reëntrants or recesses of the -outline. The materials of moraines are, therefore, till with large -local deposits of kame gravel, and these form in a series of ridges -corresponding to the temporary positions of the ice front. Their width -may range from a few rods to a few miles, their height may reach a -hundred feet or more, and they stretch across the country for distances -of hundreds or even thousands of miles, looped in arcs or scallops -which are always convex outward and which meet in sharp cusps that in -a general way point toward the embossment of the former glacier (Fig. -334, p. 308, and Fig. 336). These festoons of the moraines outline -the ice lobes of the latest ice invasion, which in North America were -centered over the depressions now occupied by the Laurentian lakes. -There was, thus, a Lake Superior lobe, a Lake Michigan lobe, etc. With -the aid of these moraine maps we may thus in imagination picture in -broad lines the frontal contours of the earlier glaciers. At specially -favorable localities where the ice front has crossed a deep valley at -the edge of the Driftless Area, we may, even in a rough way, measure -the slope of the ice face. Thus near Devils Lake in southern Wisconsin -the terminal moraine crosses the former valley of the Wisconsin River, -and in so doing has dropped a distance of about four hundred feet -within the distance of a half mile or thereabouts (Fig. 337). - -The characteristic surface of the marginal moraine is responsible for -the name “kettle” moraine so generally applied to it. The “kettles” are -roughly circular, undrained basins which lie among hummocks or knobs, -so that the surface has often been referred to as “knob and basin” -topography (plate 17 C). - -[Illustration: - -FIG. 337.—Map of the vicinity of Devils Lake, Wisconsin, located -within a reëntrant of the “kettle” moraine upon the margin of the -Driftless Area. The lake lies within an earlier channel of the -Wisconsin River which has been blocked at both ends, first by the -glacier and later by its moraine. The stippled area upon the heights -and next the moraine represents the clay deposits of a former lake -(based on map by Salisbury and Atwood).] - -[Illustration: FIG. 338.—Moraine with outwash apron in front, the -latter in part eroded by a river. Westergötland, Sweden (after H. -Munthe).] - - -[Illustration: - -FIG. 339.—Fosse between an outwash plain (in the foreground) and the -moraine, which rises to the left in the middle distance. Ann Arbor, -Michigan.] - -=Kames.=—Within reëntrants or recesses of the ice margin the drift -deposits were especially heavy, so that high hills of hummocky surface -have been built up, which are described as _kames_. Most of the higher -drift hills have this origin. They rarely have any principal extension -along a single direction, but are composed in large part of assorted -materials. In contrast with other portions of the morainal ridges they -lack the prominent basins known as kettles. Other _kames_ are high -hills of assorted materials not in direct association with moraines and -believed to have been built up beneath glacier wells or mills (p. 278). - - -=Outwash plains.=—Upon the outer margin of the moraine is generally -to be found a plain of glacial “outwash” composed of sand or gravel -deposited by the braided streams (Fig. 308, p. 280) flowing from the -glacier margin. Such plains, while notably flat (Fig. 338), slope -gently away from the moraine. Between the outwash plain and the moraine -there is sometimes found a pit, or _fosse_ (Fig. 309, p. 281), where a -part of the ice front was in part buried in its own outwash (Fig. 339). - - -[Illustration: FIG. 340.—View looking along an esker in southern Maine -(after Stone).] - -=Pitted plains and interlobate moraines.=—Where glacial outwash is -concentrated within a long and narrow reëntrant, separating glacial -lobes, strips of high plain are sometimes built up which overtop the -other glacial deposits of the district. The sand and gravel which -compose such plains have a surface which is pitted by numerous deep and -more or less circular lakes, so that the term “pitted plain” has been -applied to them. The surface of such a plain steadily rises toward its -highest point in the angle between the ice lobes. Though consisting -almost entirely of assorted materials, and built up largely without -the ice margins, such gently sloping pitted platforms are described as -_interlobate moraines_. Upon a topographic map the course of such an -interlobate moraine may often be followed by the belts of small pit -lakes (see Fig. 336). - -[Illustration: - -FIG. 341.—Outline map showing the eskers of Finland trending -southeasterly toward the festooned moraines at the margin of the ice. -The characteristic lakes of a glaciated region appear behind the -moraines (after J. J. Sederholm).] - - -[Illustration: - -FIG. 342.—Small sketch maps showing the relationships in size, -proportions, and orientation of drumlins and eskers in southern -Wisconsin. The eskers are in solid black (after Alden).] - -=Eskers.=—Intra-morainal features, or those developed beneath the -glacier but relatively near its margin, include the “serpentine kame”, -_esker_, or, as it is called in Scandinavia, the _os_ (plural _osar_) -(Fig. 340). These diminutive ridges have a width seldom exceeding a -few rods, and a height a few tens of feet at most, but with slightly -sinuous undulations they may be followed for tens or even hundreds of -miles in the general direction of the local ice movement (Fig. 341). -They are composed of poorly stratified, thick-bedded sands, gravels, -and “worked over” materials, and are believed to have been formed by -subglacial rivers which flowed in tunnels beneath the ice. Inasmuch as -the deposits were piled against the ice walls, the beds were disturbed -at the sides when these walls disappeared, and the stratification, -which was somewhat arched in the beginning, has been altered by -sliding at both margins. As already stated, eskers have not a general -distribution within the glaciated area, but are often found in great -numbers at specially favored localities. Formed as they are beneath -the ice, it is believed that many have their materials redistributed -so soon as uncovered at the glacier margin, because of the vigorous -drainage there. They are thus to be found only at those favored -localities where for some reason border drainage is less active, or -where the ice ended in a body of water. - - -=Drumlins.=—A peculiar type of small hill likewise found behind the -marginal moraine in certain favored districts has the form of an -inverted boat or canoe, the long axis of which is parallel to the -direction of ice movement, as is that of the esker (Fig. 342). Unlike -the esker, this type of hill is composed of till, and from being -found in Ireland it is called a _drumlin_, the Irish word meaning a -little hill (Fig. 343). Drumlins are usually found in groups more or -less radial and not far behind the outermost moraine, to which their -radiating axes are perpendicular. The manner of their formation is -involved in some uncertainty, but it is clear that they have been -formed beneath the margin of the glacier, and have been given their -shape by the last glacier which occupied the district. - -The mutual relationships of nearly all the molded features resulting -from continental glaciation may be read from Fig. 344. - -[Illustration: FIG. 343.—View of a drumlin, showing an opening in the -till. Near Boston, Massachusetts (after Shaler and Davis).] - - -=The shelf ice of the ice age.=—Shelf ice, such as we have become -familiar with in Antarctica as a marginal snow-ice terrace floating -upon the sea, no doubt existed during the ice age above the Gulf of -Maine (see Fig. 324, p. 298), and perhaps also over the deep sea to the -westward of Scotland. Though the inland ice probably covered the North -Sea, and upon the American side of the Atlantic the Long Island Sound, -both these basins are so shallow that the ice must have rested upon the -bottom, for neither is of sufficient depth to entirely submerge one of -the higher European cathedrals. - -[Illustration: - -FIG. 344.—Outline map of the front of the Green Bay lobe of the latest -continental glacier of the United States. Drumlins in solid black, -moraines with diagonal hachure, outwash plains and the till plain or -ground moraine in white (after Alden).] - - -=Character profiles.=—All surface features referable to continental -glaciers, whether carved in rock or molded from loose materials, -present gently flowing outlines which are convex upward (Fig. 345). The -only definite features carved from rock are the _roches moutonnées_, -with their flattened shoulders, while the hillocks upon moraines and -kames, and the drumlins as well, approximate to the same profile. The -esker in its cross sections is much the same, though its serpentine -extension may offer some variety of curvature when viewed from higher -levels. - -[Illustration: FIG. 345.—Character profiles referable to continental -glacier.] - - -READING REFERENCES FOR CHAPTER XXII - - General:— - - JAMES GEIKIE. The Great Ice Age. 3d ed. London, 1894, pp. 850, maps 18. - - CHAMBERLIN and SALISBURY. Geology, vol. 3, 1906, pp. 327-516. - - FRANK LEVERETT. The Illinois Glacial Lobe, Mon. 38, U. S. Geol. Surv., - 1899, pp. 817, pls. 34; Glacial formations and Drainage Features of - the Erie and Ohio Basins, Mon. 41, _ibid._, 1902, pp. 802, pls. 25; - Comparison of North American and European Glacial Deposits, Zeit. f. - Gletscherk., vol. 4, 1910, pp. 241-315, pls. 1-5. - -Former glaciations previous to Ice Age:— - - A. STRAHAN. The Glacial Phenomena of Paleozoic Age in the Varanger - Fjord, Quart. Jour. Geol. Soc., London, vol. 53, 1897, pp. 137-146, - pls. 8-10. - - BAILEY WILLIS and ELIOT BLACKWELDER. Research in China, Pub. 54, - Carnegie Inst. Washington, vol. 1, 1907, pp. 267-269, pls. 37-38. - - A. P. COLEMAN. A Lower Huronian Ice Age, Am. Jour. Sci. (4), vol. 23, - 1907, pp. 187-192. - - W. M. DAVIS. Observations in South Africa, Bull. Geol. Soc. Am., vol. - 17, 1906, pp. 377-450, pls. 47-54. - - DAVID WHITE. Permo-Carboniferous Climatic Changes in South America, - Jour. Geol., vol. 15, 1907, pp. 615-633. - -Driftless and drift areas:— - - T. C. CHAMBERLIN and R. D. SALISBURY. Preliminary Paper on the - Driftless Areas of the Upper Mississippi Valley, 6th Ann. Rept. U. S. - Geol. Surv., 1885, pp. 199-322, pls. 23-29. - - R. D. SALISBURY. The Drift, its Characteristics and Relationships, - Jour. Geol., vol. 2, 1894, pp. 708-724, 837-851. - - R. H. WHITBECK. Contrasts between the Glaciated and the Driftless - Portions of Wisconsin, Bull. Geogr. Soc., Philadelphia, vol. 9, 1911, - pp. 114-123. - -Glacier gravings:— - - T. C. CHAMBERLIN. The Rock Scorings of the Great Ice Invasions, 7th - Ann. Rept. U. S. Geol. Surv., 1888, pp. 147-248, pl. 8. - -The dispersion of the drift:— - - R. D. SALISBURY. Notes on the Dispersion of Drift Copper, Trans. Wis. - Acad. Sci., etc., vol. 6, 1886, pp. 42-50, pl. - - N. S. SHALER. The Conditions of Erosion beneath Deep Glaciers, based - upon a Study of the Bowlder Train from Iron Hill, Cumberland, Rhode - Island, Bull. Mus. Comp. Zoöl. Harv. Coll., vol. 16, No. 11, 1893, pp. - 185-225, pls. 1-4 and map. - - WILLIAM H. HOBBS. The Diamond Field of the Great Lakes, Jour. Geol., - vol. 7, 1899, pp. 375-388, pls. 2 (also Rept. Smithson. Inst., 1901, - pp. 359-366, pls. 1-3). - -Glacial features:— - - T. C. CHAMBERLIN. Preliminary Paper on the Terminal Moraine of the - Second Glacial Epoch, 3d Ann. Rept. U. S. Geol. Surv., 1883, pp. - 291-402, pls. 26-35. - - G. H. STONE. Glacial Gravels of Maine and their Associated Deposits, - Mon. 34, U. S. Geol. Surv., 1899, pp. 489, pls. 52. - - W. C. ALDEN. The Delaven Lobe of the Lake Michigan Glacier of the - Wisconsin Stage of Glaciation and Associated Phenomena. Prof. Pap. - No. 34, U. S. Geol. Surv., 1904, pp. 106, pls. 15; The Drumlins of - Southeastern Wisconsin, Bull. 273, U. S. Geol. Surv., 1905, pp. 46, - pls. 9. - - W. M. DAVIS. Structure and Origin of Glacial Sand Plains, Bull. Geol. - Soc. Am., vol. 1, 1890, pp. 196-202, pl. 3; The Subglacial Origin - of Certain Eskers, Proc. Bost. Soc. Nat. Hist., vol. 35, 1892, pp. - 477-499. - - F. P. GULLIVER. The Newtonville Sand Plain, Jour. Geol., vol. 1, 1893, - pp. 803-812. - - - - -CHAPTER XXIII - -GLACIAL LAKES WHICH MARKED THE DECLINE OF THE LAST ICE AGE - - -[Illustration: - -FIG. 346.—The Illinois River where it passes through the outer moraine -at Peoria, Illinois, showing the flood plain of the ancient stream as -an elevated terrace into which the modern stream has cut its gorge -(after Goldthwait).] - -=Interference of glaciers with drainage.=—Every advance and every -retreat of a continental glacier has been marked by a complex series -of episodes in the history of every river whose territory it has -invaded. Whenever the valley was entered from the direction of its -divide, the effect of the advancing ice front has generally been to -swell the waters of the river into floods to which the present streams -bear little resemblance (Fig. 346). Because of the excessive melting, -this has been even more true of the ice retreat, but here _when the ice -front retired up the valley_ toward the divide. A sufficiently striking -example is furnished by the Wabash, Kaskaskia, Illinois, and other -streams to the southward of the divide which surrounds the basin of the -Great Lakes (Fig. 347). - -[Illustration: - -FIG. 347.—Broadly terraced valleys outside the divide of the St. -Lawrence basin, which remain to mark the floods that issued from the -latest continental glacier during its retreat (after Leverett).] - -Wherever the relief was small there occurred in the immediate vicinity -of the ice front a temporary diversion of the streams by the parallel -moraines, so that the currents tended to parallel the ice front. This -temporary diversion known as “border drainage” was brought to a close -when the partially impounded waters had, by cutting their way through -the moraines, established more permanent valleys (Fig. 348). - - -=Temporary lakes due to ice blocking.=—Whenever, on the contrary, the -advancing ice front entered a valley from the direction of its mouth, -or a _retreating ice front retired down the valley_, quite different -results followed, since the waters were now impounded by the ice front -serving as a dam. Though the histories of such blocking of rivers -are often quite complex, the principles which underlie them are in -reality simple enough. Of the lakes formed during advancing hemicycles -of glaciation, and of all save the latest receding hemicycle, no -satisfactory records are preserved, for the reason that the lake -beaches and the lake deposits were later disturbed and buried by the -overriding ice sheets. We have, however, every reason to suppose that -the histories of each of these hemicycles were in every way as complex -and interesting as that of the one which we are permitted to study. - -[Illustration: - -FIG. 348.—Border drainage about the retreating ice front south of Lake -Erie. The stippled areas are the morainal ridges and the hachured bands -the valleys of border drainage (after Leverett).] - -As an introduction to the study of the ice-blocked lakes of North -America, and to set forth as clearly as may be the fundamental -principles upon which such lakes are dependent, we shall consider -in some detail the late glacial history of certain of the Scottish -glens, since their area is so small and the relief so strong that -relationships are more easily seen; it is, so to speak, a pocket -edition of the history of the more extended glacial lakes. - -[Illustration: - -FIG. 349.—The “parallel roads” of Glen Roy in the southern highlands -of Scotland (after Jamieson).] - -=The “parallel roads” of the Scottish glens.=—In a number of -neighboring glens within the southern highlands of Scotland there are -found faint terraces upon the glen walls which under the name of the -“parallel roads” (Fig. 349) have offered a vexed problem to scientists. -Of the many scientists who long attempted to explain them, though in -vain, was Charles Darwin, the father of modern evolution. He offered it -as his view that the “roads” were beaches formed at a time when the sea -entered the glens and stood at these levels. When, however, Jamieson’s -studies had discovered their true history, Darwin, with a frankness -characteristic of some of the greatest scientists, admitted how far -astray he had been in his reasoning. Let us, then, first examine the -facts, and later their interpretation. The map of Fig. 350 will suffice -to set forth with sufficient clearness the course of the several -“roads.” These “roads” are found in a number of glens tributary to -Loch Lochy, and of the three neighboring valleys, Glen Roy has three, -Glen Glaster two, and Glen Spean one “road.” The facts of greatest -significance in arriving at their interpretation relate to their -elevations with reference to the passes at the valley heads, their -abrupt terminations down-valleyward, and the morainic accumulations -which are found where they terminate. The single “road” of Glen Spean -is found at an elevation of 898 feet, a height which corresponds to -that of the pass or col at the head of its valley and to the lowest of -the “roads” in both Glens Glaster and Roy. Similarly the upper of the -two “roads” in Glen Glaster is at the height of the pass at its head -(1075 feet) and corresponds in elevation to the middle one of the three -“roads” in Glen Roy. Lastly, the highest of the “roads” in Glen Roy is -found at an elevation of 1151 feet, the height of the col at the head -of the Glen. In the neighboring Glen Gloy is a still higher “road” -corresponding likewise in elevation to that of the pass through which -it connects with Glen Roy. - -[Illustration: - -FIG. 350.—Map of Glen Roy and neighboring valleys of the Scottish -highlands with the so-called “roads” entered in heavy lines. Glens Roy, -Glaster, and Spean have three “roads”, two “roads”, and one “road”, -respectively (after Jamieson).] - -To come now to the explanation of the “roads”, it may be said at -the outset that they are, as Darwin supposed, beach terraces cut by -waves, not as he believed of the ocean, but of lakes which once filled -portions of the glens when glaciers proceeding from Ben Nevis to the -southwestward were blocking their lower portions. The several episodes -of this lake history will be clear from a study of the three successive -idealistic diagrams in Fig. 351. - -[Illustration: FIG. 351.—Three successive diagrams to set forth in -order the late glacial lake history of the Scottish glens.] - -To derive the principles underlying this history, it is at once seen -that _all changes are initiated by the retirement of the ice front to -such a point that it unblocks for the waters of a lake an outlet that -is lower than the one in service at the time_. This is the principle -which explains nearly all episodes of glacial lake history. Thus, when -the ice front had retired so as to open direct connections between Glen -Roy and Glen Glaster, the col at the head of Glen Roy was abandoned as -an outlet, and the waters fell to the level fixed for Glen Glaster. -A still further retirement at last opened direct connection between -Glen Glaster and Glen Spean, so that the lake common to Glens Glaster -and Roy fell to the level of the col which was the outlet of the Spean -valley at the time. This stage continued until the ice front had -retired so far that the waters drained naturally down the river Spean -to Loch Lochy and thence to the ocean. - -[Illustration: FIG. 352.—Harvesting time on the fertile floor of the -glacial Lake Agassiz (after Howell).] - -Only in their far grander scale and in the lesser relief of the -land over which they formed, do the complex histories of the great -ice-blocked lakes of North America differ from these little valley -lakes whose beaches may be visited and the relationships worked out, -thanks to Jamieson, in a single day’s strolling. - -[Illustration: FIG. 353.—Map of Lake Agassiz (after Upham).] - - -=The glacial Lake Agassiz.=—The grandest of the temporary lakes -referable to blocking by the continental glaciers of the ice age must -be looked for in the largest valleys that lay within the territory -invaded and _which normally drain toward the retiring ice front_. -In North America these rivers are the Red River of the North in -Minnesota, the Dakotas, and Manitoba; and the St. Lawrence River -system. To the ice dam which lay across the Red River valley we owe the -fertility of that vast plain of lake deposits where is to-day the most -intensive wheat farming of the northwest (Fig. 352). Lakes Winnipeg, -Winnipegoosis, and Manitoba, and the Lake of the Woods, are all that -now remain of this greatest of the glacial lakes, which in honor of -the distinguished founder of the glacial theory has been called Lake -Agassiz (Fig. 353). With their natural outlet blocked by the ice in -northern Manitoba and Keewatin, the waters of the Red were swollen by -melting from the retiring glacier and spread over a vast area before -finding a southern outlet along the course of the present Lake Traverse -and the valley of the Minnesota River. Along this route there flowed a -mighty flood which carved out a broad valley many times too large for -the Minnesota, its present occupant, and this giant prehistoric river -has been called the Warren River (Fig. 354). - -[Illustration: - -FIG. 354.—Map of the southern end of the Lake Agassiz basin, showing -the position of some of the beaches and the outlet through the former -Warren River (after Upham).] - -[Illustration: FIG. 355.—Narrows of the Warren River below Big Stone -Lake, where it passed between jaws of hard granite and gneiss (after -Upham).] - -[Illustration: - -FIG. 356.—Map of the valley of the Warren River in the vicinity of -Minneapolis, with the young valley of the Mississippi entering it at -Fort Snelling (after Sardeson).] - -It is interesting to follow this ancient waterway and to discover -that, like our normal, present-day streams, it was held up in narrows -wherever outcroppings of harder rock had constricted its channel -(Fig. 355). The upper end of the Warren River valley is now occupied -by the long and relatively narrow Lakes Traverse and Big Stone, each -the result of blocking by delta deposits where a tributary stream -has emerged into the valley, but this gigantic channel continues -down to and beyond Minneapolis, occupied as far as Fort Snelling -by the Minnesota River—a mere pygmy compared to its predecessor. -To the earnest student of glacial geology there can be few sights -more impressive than are obtained by standing at Fort Snelling, just -above the confluence of the Minnesota and the Mississippi rivers, and -surveying first the steep and narrow valley of the Mississippi above -the junction,—a stream fitted to its valley for the simple reason that -it has carved it,—and then gazing up and down that broad valley in -which the great Warren River once flowed majestically to the sea, now -the bed of the Minnesota above the Fort and of the Mississippi below it -(Fig. 356). - -[Illustration: - -FIG. 357.—Portion of the Herman quadrangle of Minnesota, showing the -position of the Herman beach on the shore of the former Lake Agassiz. -The lake basin is to the left, and the pitted morainal deposits appear -to the right (U. S. G. S.).] - -Just as the “parallel roads” of Glen Roy, roads in name only, are the -beaches of earlier glacial lake stages, so in Lake Agassiz we have -parallel beaches of the barrier type which are often roads in fact as -well as in name, and which mark the stages of successive lakes within -this vast basin. The Herman beach, corresponding to the highest level -of the lake, is thus a sharp topographic boundary between lake deposits -and morainal accumulations, and is further itself a well-marked -topographic feature composed of wave-washed and hence well-drained -materials (Fig. 357). Farmers of the district have been quick to -realize that these level and slightly elevated ridges lack the clay -which would render them muddy in the wet seasons, and are thus ideally -adapted for roads. They have in many sections been thus used over long -stretches and are known as the “ridge roads.” - - -=Episodes of the glacial lake history within the St. Lawrence -valley.=—Within this great drainage basin it has apparently been -possible to read the records of each stage in the latest lake -history—complex as this has been. We have only to recall the lake -stages cited from the Scottish glens and remember that each new stage -was begun in a retirement of the glacier front which unblocked an -outlet of lower level than the last. This sequence might, however, -have been varied by a temporary readvance of the ice, as indeed once -occurred in the Huron-Erie lobe of the great North American glacier. - -[Illustration: - -FIG. 358.—The continental glacier of North America in an early stage -of its recession, when it covered the entire St. Lawrence drainage -basin. The dashed line is the approximate position of the divide (based -on a map by Goldthwait).] - -[Illustration: - -FIG. 359.—Outline map of the early Lake Maumee, with the bordering -moraine and the water-laid moraine remaining on the site of the former -ice cliff.] - - -=The crescentic lakes of the earlier stages.=—So long as the glacier -covered the entire drainage basin of the St. Lawrence River system, all -water was freely drained away by streams which flowed _away from_ the -ice front (Fig. 358). So soon, however, as at any point the front had -retired behind the divide, impounding of the waters must locally have -occurred. Lakes of this type are to-day to be seen in Greenland and -in the southern Andes; and though upon a diminutive scale, some idea -of their aspect may be obtained from the appearance of the Märjelen -Lake of Switzerland, here blocked by a mountain glacier (Fig. 446, p. -411). Within all areas of small relief, such as the prairie country -surrounding the present Laurentian lakes, the earlier and smaller -stages of such ice-blocked lakes are generally crescentic in outline. -This is because a moraine in most cases forms the land margin of the -lake, and because the ice cliff upon the opposite border, although -somewhat straightened, as a consequence of wave-cutting and iceberg -formation, still retains the convex outlines characteristic of ice -lobes (Fig. 359). - -[Illustration: FIG. 360.—Map to show the first stages of the -ice-dammed lakes within the St. Lawrence basin (after Leverett and -Taylor).] - -Within each of the Great Lake basins a crescentic lake early appeared -at that end of the depression which was first uncovered by the -glacier: Lake Duluth in the Superior basin, Lake Chicago in the -Michigan basin, and Lake Maumee in the Huron-Erie basin (Fig. 360). - -We may now, with profit, trace the successive episodes of the glacial -lake history, considering for the earlier stages those changes which -occurred within the Huron-Erie basin, since, these are in essential -respects like those of the Michigan and Superior basins, although -worked out in greater detail. Lake Chicago must, however, be brought -into consideration, since in all save the earliest and the later -stages, the waters from the Huron-Erie depression were discharged -through the Grand River into this lake and thence by the so-called -“Chicago outlet” into the Mississippi (plate 20 A). - - -=The early Lake Maumee.=—The area, outline, and outlet of this lake -are indicated upon Fig. 360. Its ancient beaches have been traced, as -well as the water-laid moraine beneath its former ice cliff; and no -observant traveler who should take his way down the ancient outlet -from Fort Wayne, Indiana, past the town of Huntington, could fail to -be impressed by its size, suggesting as it does the great volume of -water which must once have flowed along it. Now a channel a mile or -more in width, its bed for the twenty-five miles between Fort Wayne -and Huntington may be seen from the tracks of the Wabash Railway as a -series of swamps merely, while at Huntington the Wabash river enters by -a young V-shaped valley at the side, much as the Mississippi emerges -into the old channel of the Warren River at Fort Snelling, Minnesota -(see p. 327). - -The Huron River of southern Michigan, which now discharges into Lake -Erie, then found its lower course blocked by the glacier and was thus -compelled to find a southerly directed channel now easily followed to -the northern horn of the crescent of Lake Maumee. - - -=The later Lake Maumee.=—When the ice lobe had retired its front -sufficiently, an outlet lower than that at Fort Wayne was uncovered -past the city of Imlay, Michigan, into the Grand River, and thence -through Lake Chicago and its outlet into the Mississippi. This old -outlet south of Chicago follows the course of the present Drainage -Canal and the line of the Chicago & Alton Railway. The traveler -journeying southward by train from Chicago has thus the opportunity of -observing first the beaches of the former lake, and then the several -channels which were joined in the main outlet at the station of Sag -(plate 20 A). - -[Illustration: FIG. 361.—Outline map of the later Lake Maumee and of -its “Imlay outlet” to Lake Chicago (after Leverett).] - -In this stage of our history Lake Maumee pushed a shrunk arm up past -the site of Ypsilanti in Michigan (Fig. 361), the well-marked beach -being found on Summit Street opposite the State Normal College. The -Huron River, which in the first lake stage had followed the valley now -occupied by the Raisin River southward into Indiana, now discharged -directly into a bay upon this arm of Lake Maumee, and so formed a delta -at Ann Arbor. - -[Illustration: FIG. 362.—Outline map of Lakes Whittlesey and Saginaw -(after Leverett).] - -[Illustration: - -FIG. 363.—Map of the glacial Lake Warren, the last of the lakes in the -Huron-Erie basin, which discharged through the “Grand River outlet” -into the Mississippi (after Leverett).] - - -=Lakes Arkona and Whittlesey.=—The ice front in the Huron-Erie basin -now retired so far that the impounded waters, instead of following -the more direct “Imlay outlet” to the Grand, passed at a lower level -completely around “the thumb” of Michigan into the Saginaw basin. -Meanwhile a crescent-shaped lake had developed in that basin, so that -now the waters of the Maumee basin were joined to those in the Saginaw -basin as a common lake, just as the lowering of the waters in Glen Roy -caused a union with those of Glen Glaster in the example cited for -illustration. Our records of this third North American lake stage, -referred to as Lake Arkona, are however most imperfect, for the reason -that it was followed by a readvance of the ice front which closed the -passage around “the thumb” and raised the level of the waters until -an outlet was found past the town of Ubly at a lower level than the -“Imlay outlet.” When the waters of a lake are thus rising, strong beach -formations result, and those of this stage, which is known as the Lake -Whittlesey stage, are much the strongest that are found within the -Huron-Erie basin. Traced for some three hundred miles entirely around -the southern and western margins of Lake Erie, this beach is for much -of the distance the famous “ridge road” (Fig. 362). - - -=Lake Warren.=—As the ice advance which had produced Lake Whittlesey -came to an end, the normal recession was resumed and a lake once more -formed as a body common to the Saginaw and Erie basins. This lake, -known as Lake Warren, extended a shrunk arm far eastward along the ice -front into western New York, though it was still blocked from entering -the great Mohawk valley (Fig. 363). - -[Illustration: FIG. 364.—Map of the Glacial Lake Algonquin (after -Leverett).] - - -=Lakes Iroquois and Algonquin.=—It must be evident that toward the -close of the Lake Warren stage a profound change was imminent—a -transfer of the glacial waters from their course to the Mississippi -and the Gulf to the trench which crosses New York State and enters the -Atlantic. So soon as the ice front had retired sufficiently to lay -bare the bed of the Mohawk, an outlet was found by this route and its -continuation down the Hudson valley to the sea. The Lake Ontario basin -now became occupied by a considerably larger water body known as Lake -Iroquois, and the three upper lakes, then joined as Lake Algonquin, -discharged their combined waters into Lake Iroquois at first through a -great channel now strongly marked across Ontario in the course of the -Trent River and Lake Simcoe, the so-called “Trent outlet.” At this time -a smaller Lake Erie probably occupied the basin of that lake, and later -the Trent outlet was abandoned for the Port Huron outlet (Fig. 364). - -[Illustration: FIG. 365.—Outline map of the Nipissing Great Lakes with -their outlet past North Bay into the Champlain Sea.] - - -=The Nipissing Great Lakes.=—We have now followed the ice front -step by step in its retreat across the valley of the St. Lawrence -system. The successive unblocking of outlets offers but one further -possibility—the opening of the French River-Nipissing Lake-Ottawa -River, or “North Bay outlet.” Though not so to-day, the bed of this -ancient channel was then much lower than that of the “Mohawk outlet”, -and so soon as the glacier had in its retreat uncovered this northern -channel, the waters of the upper lakes discharged through it past the -site of Ottawa and into an arm of the sea which then occupied the lower -St. Lawrence valley and has been called the Champlain Gulf or Sea -(Fig. 365). The level of the waters was lowered and the area of the -lakes correspondingly reduced. - -The reader who has had no opportunity to observe these ancient channels -which carried the swollen waters of the former glacier lakes, will find -it interesting to consider that every one of them has been fixed upon -by engineers for improvement as artificial waterways. Thus we have the -Illinois Drainage Canal and projected ship canal along the “Chicago -outlet”, the projected Mississippi-Lake Erie Canal along the “Fort -Wayne outlet”, the Grand River canal project to connect Lake Michigan -and Saginaw Bay along the course of the “Grand River outlet”, the -Trent Canal along the “Trent outlet”, the Erie Canal along the “Mohawk -outlet”, and, lastly, the proposed Georgian Bay ship canal to the ocean -along the “North Bay” or “Nipissing outlet.” - - -=Summary of lake stages.=—We have omitted in this summary of late lake -history in the Laurentian basin all the less important lake stages, -including some of a transitional nature which were represented by -beaches and outlets easily traced to-day. This is because it is an -outline only which it seems best to present, and the episodes of this -abridged history may be tabulated as follows: - - -EPISODES OF GLACIAL LAKE HISTORY - - MISSISSIPPI DRAINAGE - - Lake Maumee (early), Fort Wayne outlet. - Lake Maumee (late), Imlay City outlet. - Lake Arkona, “thumb” outlet. - Lake Whittlesey (with readvance of glacier), Ubly outlet. - Lake Warren, “thumb” outlet. - - ATLANTIC DRAINAGE - - Lakes Iroquois and Algonquin (early), Trent and Mohawk outlets. - Lakes Iroquois and Algonquin (late), Port Huron and Mohawk outlets. - Nipissing Great Lakes, North Bay outlet. - - -=Permanent changes of drainage affected by the glacier.=—While the -lake history which we have sketched is made up of episodes which -endured only while the ice front lay between certain stations upon its -retreat, there were none the less brought about the profoundest of -permanent modifications in the drainage of the region. It is possible -to restore upon maps in part only the preglacial drainage of the north -central states, but we know at least that it was as different as may be -from that which we find to-day. The Missouri and the Ohio take their -courses to-day along the margin of the glaciated area as an inheritance -from the border drainage of the ice age. Within the glaciated regions -rivers have in many cases been compelled by morainal obstructions to -enter upon new courses, or even to travel in the opposite direction -along their former channels. In districts of considerable relief these -diversions have sometimes caused the streams to plunge over the walls -of deep valleys, and it may truthfully be said that we owe much of our -most beautiful scenery in part to the carving and molding of glaciers, -but especially to the cascades and waterfalls directly due to their -interference with drainage. - -[Illustration: - -FIG. 366.—Probable preglacial drainage of the upper Ohio region (after -Chamberlin and Leverett).] - -Many diversions or reversals of former drainage lines, through the -influence of the continental glacier, are at once suggested by -the abnormal stream courses, which appear upon our maps, and the -correctness of these suggestions may often be confirmed by very simple -observations made upon the ground. The map of Fig. 366 shows how -different was the preglacial drainage of the upper Ohio region from -that of to-day. - -An interesting additional example is furnished by the Still River -which in Connecticut is tributary to the Farmington, and is no less -remarkable for its abnormal northerly course and sluggish current -perpetuated in its name, than for the way in which it is joined to the -Farmington system (Fig. 367 _A_). A careful study of the district has -shown that the Still River was once a part of the Naugatuck and flowed -southward toward Long Island Sound like other rivers of the district -(Fig. 367 _B_). It possessed, however, an advantage in a narrow belt -of softer rock along its course, and because of this advantage it -captured a portion of one of the tributaries to the Farmington (Fig. -367 _C_). The continental glacier later covered the region, and on its -retreat laid down morainal obstructions directly across this river and -also at the head of the severed arm of the Farmington tributary (Fig. -367 _D_). The now impounded waters found their lowest outlet near Sandy -Brook, and in waterfalls and cascades the now reversed river falls one -hundred feet to the bed of that stream. With the aid of the excellent -topographic maps which are now supplied by a generous government at a -merely nominal price, such bits of recent history may be read at many -places within the glaciated region. - -[Illustration: - -FIG. 367.—Diagrams to illustrate the episodes in the recent history -of the Still River tributary to the Farmington in Connecticut. _A_, -present drainage; _B_, early stage; _C_, after capture of a tributary -to the Farmington; _D_, after blocking by morainal obstructions of the -ice age.] - - -=Glacial Lake Ojibway in the Hudson Bay drainage basin.=—When by -passing over the “height of land” in northern Ontario the greatly -reduced continental glacier had vacated the basin of St. Lawrence -drainage, it was in a position to impound those waters which normally -drained to Hudson Bay. The lake which then came into existence has been -called Lake Ojibway and was the latest of the entire series. Though of -but recent discovery in a country till lately a trackless wilderness, -its extension seems to have been that of the clay beds suited for -farming. The beaches and outlets remain to be mapped when the country -has been made more easily accessible. - - -READING REFERENCES FOR CHAPTER XXIII - - Parallel roads of Glen Roy:— - - CHARLES DARWIN. Observations on the Parallel Roads of Glen Roy and of - Other Parts of Lochaber in Scotland, with an attempt to prove that - they are of Marine Origin, Phil. Trans., vol. 8, 1839, pp. 39-82. - - LOUIS AGASSIZ. Geological Sketches, Boston, 1876, vol. 2, pp. 32-76. - - T. T. JAMIESON. On the Parallel Roads of Glen Roy and their Place in - the History of the Glacial Period, Quart. Jour. Geol. Soc. Lond., vol. - 19, 1863, pp. 235-259. - -Glacial Lake Agassiz:— - - WARREN UPHAM. The Glacial Lake Agassiz. Mon. 25, U. S. Geol. Surv., - pp. 658, pls. 38. - - F. W. SARDESON. Beginning and Recession of St. Anthony’s Falls, Bull. - Geol. Soc. Am., vol. 19, 1908, pp. 29-36. - -Glacial lakes in the St. Lawrence valley:— - - CHAMBERLIN AND SALISBURY. Geology, vol. 3, pp. 394-405. - - FRANK LEVERETT. Outline of the History of the Great Lakes - (Presidential Address), 12th Rept. Mich. Acad. Sci., 1910, pp. 19-42. - The Pleistocene Features and Deposits of the Chicago Area. Chicago, - 1897, pp. 86, pls. 8 (Chicago Outlet). - - H. L. FAIRCHILD. Glacial Lakes in Western New York, Bull. Geol. Soc. - Am., vol. 6, 1895, pp. 353-374, pls. 18-23; Glacial Waters in Central - New York. Bull. 127, N. Y. State Mus., 1909, pp. 66, pls. 42, and maps - in cover. - -Early lakes in the Erie basin:— - - FRANK LEVERETT. On the Correlation of Moraines with Raised Beaches of - Lake Erie, Am. Jour. Sci. (3), vol. 43, 1892, pp. 281-301. - - F. B. TAYLOR. The Great Ice Dams of Lakes Maumee, Whittlesey, and - Warren, Am. Geol., vol. 24, 1899, pp. 6-38, pls. 2-3; Relation of Lake - Whittlesey to the Arkona Beaches, 7th Rept. Mich. Acad. Sci., 1905, - pp. 30-36. - - FRANK LEVERETT. The Ann Arbor Folio, Folio No. 155, U. S. Geol. Surv., - 1908, pp. 10-12. - - - - -CHAPTER XXIV - -THE UPTILT OF THE LAND AT THE CLOSE OF THE ICE AGE - - -=The response of the earth’s shell to its ice mantle.=—There is now -good reason to believe that the earth’s outer shell makes a response by -oscillations of level due to the loading by ice, on the one hand, and -to the removal of this burden upon the other. We know, at least, that -both in northern Europe and in North America areas which have undergone -depression during and elevation after the ice age, correspond closely -to the regions which were ice covered. Wherever in these regions there -was high relief before the advent of the ice, river valleys were -drowned at the land margins and were also gouged out into troughs -through erosion by the outlet tongues upon the margin of the ice sheet. -Such furrowed and half-submerged valleys have a characteristic U-shaped -section, so that their walls rise precipitously from the sea. From -their typical occurrence in Scandinavian countries the name _fjord_ has -been applied to them. - -It is now no less clear that the removal of the ice blanket brought -from the earth a relatively quick response in uplift, which began -before the ice front had retired across the present international -boundary of the United States, and that this uplift continued until the -final disappearance of the ice. A far slower elevation of a somewhat -different nature has continued, even to the present day. - -It is obvious that at the time of their formation all shore lines -referable to the work of waves must have been horizontal, and hence -any variations from a perfect level which they reveal to-day must -indicate that a tilting movement of the ground has occurred since the -waters departed from their basins. We have thus provided for us in the -positions of these ancient water planes, particularly because of their -wide extent, a complete record the refinement of which is not easily -overstated. Interpreting this record, we find that it was the uptilt -of the land to the northward which brought the glacial lake history to -an end and inaugurated the present system of St. Lawrence drainage. The -outlet of the Nipissing Great Lakes is to-day more than a hundred feet -above the level of the outlet at Port Huron, where the upper lakes are -now discharging their waters, and this difference in level can only -be ascribed to an upward tilting of the land since the latest of the -glacial lake stages. - - -=The abandoned strands as they appear to-day.=—The traveler by steamer -upon the upper lakes, as he comes within view of each rocky headland, -may note how the profile against the horizon is notched by a series of -steps or terraces (Fig. 368), and if he has followed the discussion in -previous chapters, he will suspect that these terraces mark the now -abandoned shore lines which have come to their present position through -a series of uplifts of the ground accompanied by earthquake shocks. As -his steamer skirts the shore he may chance to note a cave within the -rock cliff which represents the now elevated sea-arch of an ancient -shore. - -[Illustration: - -FIG. 368.—The notched rock headland of Boyer Bluff between Green Bay -and Lake Michigan (after Goldthwait).] - -Disembarking from the steamer and traveling inland at any point where -the shores are high, the traveler is certain to come upon still more -convincing proofs of the ancient strands; perhaps in a storm beach of -the unmistakable “shingle”, half buried though it may be under dunes -of newly drifted sand, or possibly at higher levels the highway has -been cut through a shingle barrier as fresh and unmistakable as though -formed upon the present shore. Sometimes it is the rock cliff and -terrace, at other times barrier ridges of shingle, or, again, it is the -sloping cliff and terrace cut in the drift deposits; but of whatever -sort, if studied with proper regard to the topography of the district, -the evidence is clear and unmistakable. - - -=The records of uplift about Mackinac Island.=—Nowhere are the records -of the recent uplift of the lake region more easily read than about -Mackinac Island in the straits connecting Lake Michigan with Lake -Huron. Approaching the island by steamer from St. Ignace, its profile -upon the horizon is worthy of remark (Fig. 369). From a central crest -broken by minor irregularities and bounded on all sides by a cliff, the -island profile slopes gently away to a still lower cliff, below which -is another terrace. - -[Illustration: - -FIG. 369.—View of Mackinac Island from the direction of St. Ignace. -The irregular central portion is the only part of the island that was -not submerged in Lake Algonquin. The terrace at its base is the old -shore line of Lake Algonquin, and the lower terrace the strand of Lake -Nipissing (after a photograph by Taylor).] - -[Illustration: - -FIG. 370.—The “Sugar Loaf”, a stack near the shore of Lake Algonquin, -as it is seen from the observatory upon Mackinac Island (after a -photograph by Taylor).] - -When we have reached the island and have climbed to the summit, we -there find the surface which is characteristic of erosion by running -water, whereas at lower levels are found the forms carved or molded -by the action of waves. This central “island”, superimposed upon the -larger island, is all that rose above Lake Algonquin, the earliest of -the glacial lakes in this northern district; and as we look out from -the observatory upon the summit, it is easy to call up a picture of the -country when the lake stood at the base of this highest cliff. To the -northward one sees the “Sugar Loaf” rise out of a sea of foliage, as it -formerly did from the waters of Lake Algonquin (Fig. 370). It is a huge -stack near the former island shore. If we turn now to the southward and -direct our gaze toward the Fort, we encounter a veritable succession of -beach ridges formed of shingle and ranged like a series of waves within -the cleared space of the “Short Target Range” (Fig. 371). These ridges -mark each a stage within a series of successive uplifts which have -brought the island to its present height. - -[Illustration: - -FIG. 371.—View from the observatory upon Mackinac Island across the -“Short Target Range” toward the Fort. Beach ridges appear in succession -within the cleared space (after a photograph by Rossiter).] - -[Illustration: FIG. 372.—Notched stack of the Nipissing Great Lakes at -St. Ignace (after a photograph by Taylor).] - -[Illustration: - -FIG. 373.—Series of diagrams to illustrate the evolution of ideas -concerning the uplift of the lake region since the ice age. _A_, -simple northerly up-canting (Gilbert): _B_, northerly acceleration of -the up-canting (Spencer and Upham); _C_, northerly “feathering out” -of beaches (Spencer and Upham); _D_, hinge, line of up-canting found -within the lake region (Leverett); _E_, multiple and northwardly -migrating hinge lines of up-canting (Hobbs).] - -If now we descend from our position and visit the “battlefield”, we -find there a great ridge of level crest, behind which the British -force was stationed in its defense of the island in 1812. Near by in -the woods is Pulpit Rock, a strikingly perfect stack of the Nipissing -Lake. Across the straits at St. Ignace is an even finer example of the -notched stack (Fig. 372). Other less prominent beaches, but all later -than the Nipissing Lakes, intervene between this level and the present -shore to mark the stages in the continued uplift of the land. - - -=The present inclinations of the uplifted strands.=—It is not enough -that we should have recognized the marks of former shores now at -considerable elevations above the existing lakes; if we are to know -the nature of the uplift, we must prepare accurate maps based upon -measurements by precise leveling at many localities. Such methods are, -however, of comparatively recent application in this field; and, as in -the investigation of so many other problems, the earlier observations -were largely of the nature of reconnaissances with the elevation of -beaches estimated by comparatively crude methods only. The evolution of -ideas concerning the uptilt has, therefore, been a gradual one. - -[Illustration: - -FIG. 374.—Map of the Great Lakes region to show isobases and hinge -lines of uptilt. _a_, isobase of the Chicago outlet; _b_, main hinge -line of the Lake Whittlesey beach (Leverett); _b^1_, hinge line of the -Lake Warren beach (Taylor); _c_, isobase of the Port Huron outlet; _d_, -main hinge line of highest Algonquin beach (Goldthwait); _e_, _f_, -_g_, _h_, additional hinge lines of Algonquin beaches in Door County -peninsula (Hobbs); _l_, isobase of the Lake Superior outlet for the -Algonquin beaches (Leverett): _m_, isobase of the same outlet for the -Nipissing beaches (Leverett).] - -It was early observed that the beaches corresponding to a given -lake stage were higher to the northward and northeastward, and the -natural conclusion from this was that the earth’s crust had here been -canted like a trap door (Fig. 373, _A_). As we are to see, this but -half-correct assumption has led to a striking prophecy relating to -future changes within the lake region which we now know to be without -warrant in the facts. Later it was learned that the uptilt of the lake -beaches is much accelerated to the northward (Fig. 373, _B_), and that -new beaches make their appearance from beneath others as we proceed in -this direction—there is a “feathering out” of beaches to the northward -(Fig. 373, _C_). - - -=The hinge lines of uptilt.=—Still later in the study of the region, -it was learned that the axis or fulcrum about which the region has been -uptilted, instead of lying to the southward of the lake district, as -had been assumed by Gilbert, lay within the region and about halfway up -the basin of Lake Michigan (Fig. 373, _D_, and Fig. 374). Similarly, in -the uptilt which followed the ice retreat in northern Europe a definite -hinge line of movement has been discovered. - -Lastly, it has been shown, as a result of the use of precise leveling -methods, that not one but several hinge lines of movement lie within -the region, and that the separate sections into which they divide the -area are each in turn characterized by increased up-cant as we proceed -to the northward (Fig. 373, _E_ and Fig. 374). - -[Illustration: - -FIG. 375.—Series of idealistic diagrams to indicate the nature of the -quick recovery of the crust by uplift in blocks unloaded of the ice in -succession. A further and slower uptilt, added after the completion of -the first movement, is brought out in the last diagram (_b_´).] - -The beaches of Lake Maumee, the earliest of the series of lakes within -the Huron-Erie lobe and within the extreme southern portion of the -Great Lakes area, show only the slightest possible northerly uptilt, -and the well-marked hinge line disclosed in the Whittlesey beach is -evidence that the elastic recoil, as it were, from the weight of -the mantling glacier did not begin until after the draining of Lake -Whittlesey. The determination by Taylor that there is a similar initial -hinge line in the Warren beach—that this strand begins its uptilt some -fifteen miles farther northeast than does the Whittlesey beach—is -one of the greatest importance in obtaining a correct idea of the -recent uplift; for it shows that the draining of Lake Whittlesey was -followed by a period of quick uplift and seismic activity, that the -stage of Lake Warren was one of comparative stability of the land, -and, lastly, that the draining of Lake Warren was followed by a second -period of rapid uplift and earthquake disturbance. The strongly marked -hinge lines, additional to the initial one indicated for the Algonquin -beaches in the profiles by Goldthwait from the west shore of Lake -Michigan, when considered in the light of this northeasterly migration -of the still earlier hinge line in the southern district, are best -explained through the assumption of a succession of quick recoveries of -the crust by uplift, separated by periods of relative stability, and -brought on by the removal in turn of the ice burden from successive -blocks of the shell which are separated by the several hinge lines -(Fig. 375). - -The elaborate study of erosion in the outlet of Lake Agassiz had -indicated identical interruptions in the up-canting process for that -basin. - - -=Future consequences of the continued uptilt within the lake -region.=—One of the most distinguished of American geologists, Dr. -G. K. Gilbert, in order to determine whether the uptilt revealed by -canted beach lines is still in progress, carried out an elaborate -study upon the gauge records preserved at the various gauging stations -about the Great Lakes. Upon the basis of these studies, he concluded -that the uplift continues, that the axes of equal uplift (isobases) -take their course about fifteen degrees north of west, so that the -lines of greatest uptilt should be perpendicular to this direction, -or fifteen degrees east of north. He further believed that the basin -was undergoing an up-cant in the simple manner of a trap door, the -hinge of which lay to the southward of Chicago, and the study of the -gauge records led him to believe that “the rate of change is such -that the two ends of a line one hundred miles long and lying in a -south-southwest direction are relatively displaced four tenths of a -foot in one hundred years.” - - -=Gilbert’s prophecy of a future outlet of the Great Lakes to the -Mississippi.=—The _natural_ rock sill, over which the waters of Lake -Chicago once flowed to the Mississippi, is to-day but eight feet above -the common mean level of Lakes Michigan and Huron, and if the tilting -of the lake region were to continue upon Gilbert’s assumption of a -canting plane with the hinge of the movement to the south of Chicago, a -time must come when the “Chicago outlet” will again come into use and -the lakes once more drain to the Mississippi and the Gulf. Upon the -basis of his measurements, Gilbert ventured the prophecy that the first -high-water discharge into the Mississippi should occur in from five -hundred to six hundred years, and for continuous discharge in fifteen -hundred years. In twenty-five hundred years Niagara Falls should at low -water stages be dry from this cause, and in thirty-five hundred years -it should have become extinct. - -This prophecy, emanating from a high scientific authority and relating -to changes of such profound economic and commercial importance, -has been often quoted and has taken a firm hold upon the popular -imagination. Obviously, it depends upon the now exploded theory that -the lake basin has been canted _as a plane_ and that the axis of uptilt -lies somewhere to the southward of the lake region, or, in any event, -to the southward of the present Port Huron outlet. We know to-day that -instead of being uniformly distributed over the entire lake region, -the uptilting goes on at a much higher rate within the northern areas, -and that since the early stage of Lake Whittlesey the hinge line of -uplift has been steadily migrating northward with the retreat of the -ice and is now well to the northward of the present outlet. There is, -therefore, no known uptilt of the district which separates the present -from the former Chicago outlet, and there is no apparent natural -cause which should result in the reoccupation of the old outlet to -the Mississippi. The prophecy must be regarded as one that has been -outgrown with the progress of science. - - -=Geological evidences of continued uplift.=—It has recently been -claimed, on the basis of a reëxamination of Gilbert’s study of the -lake gauge records, that his methods are open to serious criticism and -that in reality the figures afford no evidence of continued uplift -of the region. However this may be, there are not lacking geological -evidences which do not admit of doubt, and these are in a striking way -confirmatory of the latest conclusions upon the manner of the recent -uplift. - -If our conclusions have been correct, the several lake basins should -now be behaving in different ways as regards the changes upon -their shores. If it is true that the lines of greatest uptilt run -north-northeasterly, there should be, speaking broadly, a “spilling -over” of waters upon the south-southwesterly shores and a laying bare -of the north-northeasterly shore terraces of the basins. This should, -however, be true only of basins whose outlets are to the northeastward -of the existing main hinge line of uptilt. Lake Huron, having its -outlet at the southern margin of its basin, should not have its waters -encroaching upon the southern shore, for the simple reason that any -continued uptilt of the basin can only have the effect of pouring more -water through the outlet. Lake Michigan and Saginaw Bay, which are -arms of the Huron basin, ought, however, to become flooded upon their -southern shores, _were it not that the hinge line of uptilt to-day lies -to the northward of the outlet at Port Huron, and, further, that the -two connecting channels still have their beds lower than the sill of -the outlet channel_. Now the evidence goes to show that no encroachment -of waters is occurring upon the Chicago shore of Lake Michigan, and -although the shores of Saginaw Bay are so excessively flat as to reveal -slight changes of level by large migrations of the strand, yet the -ancient meander posts fixed by the early surveys are still found near -the water’s edge. - - -=Drowning of southwestern shores of Lakes Superior and Erie.=—Within -the basins occupied by Lakes Superior and Erie, a wholly different -condition is found. In each case the outlet is found to the -northeastward (Fig. 374, p. 345), and the northwesterly trend of the -isobases from these outlets is responsible for a continued elevation -from uptilt of the outlets with reference to the western and southern -shores. In consequence, the waters are encroaching upon these shores, -and rivers which there enter the lake are drowned at their mouths, with -the formation of estuaries. Upon Lake Superior these changes are very -marked near Duluth and particularly in the St. Louis River, within -which, since the early treaty with the Indians, certain rapids have -disappeared and submerged trunks of trees are now found in the channel -of the river. As far east as Ontonagon essentially the same conditions -are found. - -Upon the shores within the Porcupine Mountain district, the waters are -clearly rising. Here old cedar trees may be seen, in some cases dead -but still upright and standing in from six to eight inches of water a -number of feet out from the present shore, while others near the shore, -but upon the land and still living, are washed by the waves, and losing -their lower bark in consequence. An old road along the shore has had to -be abandoned because of the encroaching water. - -Upon the opposite or northeastern shore of the lake, on the other hand, -the land is everywhere rising out of the water, and the waves are now -building storm beaches well out upon the wave-cut terrace. Here the -streams, instead of forming estuaries by drowning, drop down in rapids -to the level of the lake. - -[Illustration: - -FIG. 376.—Portion of the Inner Sandusky Bay, to afford a comparison of -the shore line of 1820 with that of to-day (after Moseley).] - -At the southwestern margin of Lake Erie there is everywhere evidence of -a rapid encroachment by the water. In the caves of South Bass Island -stalactites, which must obviously have formed above the lake level, are -now permanently submerged. It is, however, about Sandusky Bay upon the -southwest shore that the most striking observations have been made. -Moseley has collected historical records of the killing of forest -trees through a submergence which was the result of an advance of the -water upon the shores. It seems to be proven from his studies that the -water is now rising in Sandusky Bay at a rate of about 2.14 feet per -century. In Fig. 376 there is a comparison of the shores of the inner -bay separated by an interval of about ninety years. - - -READING REFERENCES FOR CHAPTER XXIV - - Uptilt in basin of Lake Agassiz:— - - WARREN UPHAM. The Glacial Lake Agassiz, Mon. 25, U. S. Geol. Surv., - pp. 474-522. - -Uptilt in Laurentian Basin:— - - G. K. GILBERT. Recent Earth Movement in the Great Lakes Region, 18th - Ann. Rept. U. S. Geol. Surv., 1898, Pt. ii, pp. 595-647. - - J. W. SPENCER. Deformation of the Algonquin Beach, etc., Am. Jour. - Sci. (3), vol. 41, 1891, pp. 14-16. - - F. B. TAYLOR. The Highest Old Shore Line of Mackinac Island, Am. Jour. - Sci. (3), vol. 43, 1892, pp. 210-218. - - A. C. LAWSON. Sketch of the Coastal Topography of the North Side of - Lake Superior, with reference to the abandoned strands, etc., 20th - Ann. Rept. Geol. and Nat. Hist. Surv. Minn., 1893, pp. 181-289, pls. - 7-12. - - J. B. WOODWORTH. Ancient Water Levels of the Champlain and Hudson - Valleys, Bull. 84, N.Y. State Mus., 1905, pp. 265, pls. 28. - - E. L. MOSELEY. Formation of Sandusky Bay and Cedar Point, Proc. Ohio - State Acad. Sci., vol. 4, 1905, Pt. v, pp. 179-238. - - F. E. WRIGHT. Rept. Geol. Surv. Mich. for 1903, 1905, p. 37. - - J. W. GOLDTHWAIT. The Abandoned Shore Lines of Eastern Wisconsin, - Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 37; A - Reconstruction of Water Planes of the Extinct Glacial Lakes in the - Lake Michigan Basin, Jour. Geol., vol. 16, 1908, pp. 459-476; Isobases - of the Algonquin and Iroquois Beaches and their Significance, Bull. - Geol. Soc. Am., vol. 21, 1910, pp. 227-248, pl. 5; An Instrumental - Survey of the Shore Lines of the Extinct Lakes Algonquin and Nipissing - in Southwestern Ontario, Mem. 10, Dept. of Mines, Canada, 1910, pp. - 57, pls. 4. - - WILLIAM H. HOBBS. The Late Glacial and Post-glacial Uplift of the - Michigan Basin, Pub. 5, Mich. Geol. and Biol. Surv., 1911, pp. 68, - pls. 2. - - LAWRENCE MARTIN. [Post-glacial Modifications in and Around the Great - Lakes], Mon. 52, U. S. Geol. Surv., 1911, pp. 455-459. - -Uptilt in northern Europe:— - - G. DE GEER. Quaternary Changes of Level in Scandinavia, Bull. Geol. - Soc. Am., vol. 3, 1892, pp. 65-68, pl. 2. - - H. MUNTHE. Studies in the Late Quaternary History of Southern Sweden, - paper No. 25, Livret Guide, Cong. Géol. Intern., 1910, pp. 96, many - plates and maps. - - - - -CHAPTER XXV - -NIAGARA FALLS A CLOCK OF RECENT GEOLOGICAL TIME - - -=Features in and about the Niagara gorge.=—A striking example of those -permanent alterations of drainage which have resulted from the presence -of the late continental glacier in North America is to be found in the -Niagara gorge between Lakes Erie and Ontario. With the aid of borings -many of the now buried channels of the region have been followed out, -and in a later paragraph we shall refer to some of the stronger lines -of the earlier drainage system. Before undertaking the study of Niagara -history, it is essential that one become somewhat familiar with the -present topography in and about the Niagara gorge. - -Below the present cataract the river flows through a deep gorge for -about seven miles before issuing at the Lewiston Escarpment (Fig. 381, -p. 355). This gorge has been cut in beds of rock sediments which dip -at a gentle angle southward toward Lake Erie. The capping of the rock -series is a compact and relatively resistant limestone which is known -as the Niagara limestone, beneath which there are alternating beds -of shale with thinner limestone and sandstone. The plain formed by -the upper surface of the limestone capping terminates in the Lewiston -Escarpment, which is transverse to the direction of the gorge and seven -miles distant below the Falls. The depth of the gorge varies markedly, -the above-water portion being represented at the upper end by the -height of the cataract, one hundred and sixty-five feet, while at its -lower end near Lewiston it is twice that amount. Halfway down the gorge -a sharp turn is made at an angle of more than ninety degrees, and the -upstream arm is extended to form a basin which contains the famous -whirlpool. This visible extension of the upper gorge is continued in a -buried channel, the St. Davids Gorge, which extends to the escarpment, -broadening as it does so in the form of a trumpet. The materials which -fill this earlier channel are notably coarse glacial deposits (Fig. -389). - -[Illustration: - -FIG. 377.—Ideal cross section of the Niagara gorge to show the -marginal terrace.] - -Directly above the whirlpool the Niagara gorge is first contracted, but -almost immediately swells out into the form of a sausage, which under -the name of the Eddy Basin extends to the constricted channel occupied -by the Whirlpool Rapids. This Gorge of the Whirlpool Rapids extends to -and a little above the railroad bridges, where it again suddenly widens -and deepens and with surprisingly uniform cross section now continues -as far as the cataract. This uppermost section is known as the Upper -Great Gorge. About a mile below the whirlpool is that remarkable -projection into the gorge from the Canadian wall which is known as -Wintergreen Flats, below which and nearer the river are Fosters Flats. -Almost throughout its entire length the Niagara gorge is bordered on -either side by a narrow and gently incurving terrace eroded below the -general level of the plain and meeting the gorge in a sharp angle (Fig. -377). - -The features immediately about the cataract show that the Falls are -to-day in a condition which, so far as we know, has occurred but once -before in their entire history—the waters of the river are divided -unequally by an island, and for this reason, as we shall see, the -cataract enters over the _side wall_ of the gorge instead of at its -_end_ (Fig. 381), although the turning of the channel from this cause -is combined with a bend of the river. - - -[Illustration: - -FIG. 378.—View of the bed of the Niagara River above the cataract, -where water has been drained off in installing a power plant. Some -separated blocks of limestone are still in place (after J. W. Spencer).] - -=The drilling of the gorge.=—There appear to be two important -processes which are responsible for the recession of the Falls, the -rate of which is determined largely by the resistance of the limestone -capping and the tenacity of the looser shale beneath it. One of the -eroding processes operates from below and undermines the cap until -the unsupported cornice falls in blocks to the bottom of the gorge; -the other makes its attack directly from above, selecting for the -purpose the lines of jointing of the rock which it widens by solution -and corrasion until the included blocks are in so far separated that -they are torn out and go over the brink of the Falls (Fig. 378). This -process of overhead attack in the powerful currents just above a -cataract is even better illustrated by the Falls of St. Anthony near -Minneapolis, which have had a similar history of recession to that of -the Niagara Falls (Fig. 379). - -[Illustration: FIG. 379.—Falls of St. Anthony, looking westward from -Hennepin Island in 1851 (after N. H. Winchell, daguerreotype by Hessler -of Chicago).] - -The blocks of the capping limestone at Niagara Falls are to some -extent fixed in size by the joint planes present in them, and as they -fall to the bottom of the gorge, they promote or retard the further -recession of the Falls according as they can or cannot be moved about -by the churning currents beneath the cataract. Of the retarding effect -there is an illustration in the accumulation of the blocks below the -American and the intermediate Luna Falls (plate 23 A), which the weaker -currents upon the American side find too heavy to handle. - -[Illustration: - -FIG. 380.—Ideal section to show the nature of the drilling process -beneath the cataract.] - -[Illustration: - -FIG. 381.—Plan and section of the Niagara gorge, showing how in each -section the depth is proportional to the width, except in the lowest -section where subsequent river action of the normal type has modified -the bed of the channel (plan after Taylor and section after Gilbert).] - -The Canadian Fall, with its much greater power, is an example of the -promotion of recession through the churning about of the blocks at the -base of the cataract. We have here to do with a churn drill which bores -its way into the bottom of the gorge with increasing radius of rotary -motion with each increase in volume of the falling water. Under this -rotary churning the soft shales are torn out near the bottom and in -succession the harder layers above until the capping is reached (Fig. -380). The conditions appear now to be such that the effective work is -largely concentrated, as it usually has been, near the middle of the -channel; and so the gorge recedes with a margin of the earlier river -bed remaining as a terrace on either side and extending to the former -river bank (Fig. 377). - -As must have been noted, one peculiarity of the operation of the churn -drill beneath the cataract is that the depth of the gorge will bear -a direct proportion to its width, and if the volume of water has -varied during the process of recession, these changes in volume will be -registered in the width and also in the depth of that section of the -gorge which was drilled at the time—the cross section of the gorge at -any place is proportional to the volume of the water falling in the -cataract which produced it, modified, however, by the competency to -handle the joint blocks of definite size (Fig. 381). - - -[Illustration: - -FIG. 382.—Comparison of a sketch of the Canadian Fall made with the -aid of a camera lucida in 1827 with a photograph taken from the same -view point in 1895 (after Gilbert).] - -=The present rate of recession.=—There are various sketches, more or -less accurate, made in the early part of the nineteenth century, and -from the later period there are daguerreotypes, photographs, and maps, -which refer especially to the Canadian Fall; and which, taken together, -render possible a comparison of the earlier with the later brinks. By -comparing the earliest with the recent, views it is seen at a glance -that the Falls are receding, and at a quite appreciable rate (Fig. -382). A careful comparison of the maps made in 1842, 1875, 1886, 1890, -and 1905 of the brink of the Canadian Fall (Fig. 383) indicates that -for the period covered the rate of recession has been about five feet -per year, and similar studies made of the American Fall show that it -has been receding at the rate of only three inches per year, or one -twentieth the rate of the recession of the Canadian Fall. - - -[Illustration: - -FIG. 383.—Map to show the recession of the brink of the Canadian Fall, -based upon maps of different dates (after Gilbert).] - -=Future extinction of the American Fall.=—It is because of this many -times more rapid recession of the Canadian Fall that the Niagara -cataract, instead of lying athwart the gorge, enters it from its side. -The Canadian Fall is thus in reality swinging about the American, and -the time can already be roughly estimated when this more effective -drilling tool will have brought about a capture, so to speak, of the -American Fall through the cutting off of its water supply. It will then -be drained and left literally “high and dry”, an enduring witness to -the geological effect of an island in making an unequal division of the -waters for the work of two cataracts. - -As already pointed out, the inefficiency of the American Fall as an -eroding agent is amply attested by the wall of blocks already appearing -above the water below it. The tourist who a thousand years hence pays -a visit to the Niagara cataract, provided the water flow is allowed to -remain as it has been, will find above this rampart of blocks a bare -cliff in part undermined, and surmounted by a nearly flat table surface -which is cut off from the existing cataract by a higher section of the -gorge (Fig. 384). It is quite likely that this table will furnish the -most satisfactory viewpoint of the future cataract of that date. - -[Illustration: - -FIG. 384.—Comparison of the present with the future falls.] - - -=The captured Canadian Fall at Wintergreen Flats.=—What we have -predicted for the future of the present American Fall will be the -better understood from the study of a monument to earlier capture -made long before the upper section of the gorge had been cut or the -whirlpool had come into existence. The tables were then turned, for it -was a fall upon the Canadian side of the gorge that was captured by -one upon the American. The locality is known as Wintergreen Flats, or -sometimes as Fosters Flats; though the first name properly applies to -a higher surface near the brink of the gorge, and Fosters Flats to a -lower plain near the level of the river (see Fig. 381, p. 355). The -peculiar topographic features at this locality are well brought out in -Gilbert’s bird’s-eye view of the locality (Fig. 385); in fact, in some -respects better than they appear to the tourist upon the ground, for -the reason that the abandoned channel and the Flats on the site of the -since undermined island are both heavily forested and so not easy to -include in a single view. For one who has studied the existing cataract -this early monument is full of meaning. Standing, as one may, upon the -very brink of the former cataract, it is easy to call up in imagination -the grandeur of the earlier surroundings and to hear the thunder of the -falling water. A particularly vivid touch is added when, in digging -over the sand about the great blocks of fallen limestone underneath -the brink, one comes upon the shells of an animal still living in the -Niagara River, though only in the continual spray beneath the cataract. - - -[Illustration: - -FIG. 385.—Bird’s-eye view of the captured Canadian Fall at Wintergreen -Flats, showing the section of the river bed above the cliff and the -blocks of fallen Niagara limestone strewn over the abandoned channel -below (after Gilbert).] - -=The Whirlpool Basin excavated from the St. Davids Gorge.=—It has -already been pointed out that a rock channel now filled with glacial -deposits extends from the Whirlpool Basin to the edge of the escarpment -at St. Davids (Fig. 389, p. 363). In plan this buried gorge has a -trumpet form, being more than two miles wide at its mouth and narrowing -to the width of the upper gorge before it has reached the Whirlpool. -Near the Whirlpool it has been in part excavated by Bowman Creek, -thus revealing walls that are well glaciated. Different opinions have -been expressed concerning the origin of this channel, one being that -it is the course either of a preglacial river or one incised between -consecutive glacial invasions; and another that it is a cataract gorge -drilled out between glacial invasions after the manner of the later -Niagara gorge. In either case its contours have been much modified by -the later glacier or glaciers, whose work of planing, polishing, and -widening is revealed in the exposed surfaces; and it is not improbable -that a cataract has receded along the course of an earlier river valley. - -As we shall see, there are facts which point rather clearly to an -earlier cataract which ended its life immediately above the present -Whirlpool. When the later Niagara cataract had receded to near the -upper end of the Cove section, or near the present Whirlpool, the -falling water must have been separated from this older channel and its -filling of till deposits by only a thin wall of rock, and this must -have been constantly weakened as its thickness was further reduced. - -When this weakened dam at last gave way, it must have produced a -debacle grand in the extreme. It is hardly to be conceived that the -“washout” of the ancient channel to form the Whirlpool Basin could -have occupied more than a small fraction of a day, though it is highly -probable that the broken rock partition below the Whirlpool was not -immediately removed entire. The mandible-like termination of the Eddy -Basin immediately above the Whirlpool has led Taylor to believe that -the cataract quickly reëstablished itself at this point upon the last -site of the extinct St. Davids cataract. If reduced in power for a -short interval, as a result of the obstructions still remaining in the -lately broken dam below the Whirlpool, the remarkable narrowing of the -gorge at this point would be sufficiently accounted for. - -Being compelled to turn through more than a right angle after it -enters the Whirlpool Basin, the swift current of the Niagara River is -forced to double upon itself against the opposite bank and dive below -the incoming current before emerging into the Cove section below the -Whirlpool (Fig. 386). - -[Illustration: - -FIG. 386.—Map of the Whirlpool Basin, showing the rock side walls -like those of the Niagara Gorge, and the drift bank which forms the -northwest wall (after Gilbert).] - -In tearing out the loose deposits which had filled this part of -the buried St. Davids Gorge, many bowlders of great size were left -which slid down the slope and in time produced an armor about the -looser deposits beneath, so as to protect them and prevent continued -excavation. Thus it is found that the submerged northwestern wall -of the basin is sheathed with bowlders large enough to retain their -positions and so stop a natural process of placer outwashing upon a -gigantic scale (Fig. 386). - - -=The shaping of the Lewiston Escarpment.=—To understand the formation -of the Lewiston Escarpment cut in the hard Niagara limestone, it is -necessary to consider the geology of a much larger area—that of the -Great Lakes region as a whole. To the north of the Lakes in Canada -is found a most ancient continent which was in existence when all -the area to the southward lay below the waters of the ocean. In a -period still very many times as long ago as the events we have under -discussion, there were laid down off the shore of this oldland a series -of unconsolidated deposits which, hardened in the course of time, and -elevated, are now represented by the shales, sandstone, and limestone -which we find, one above the other, in the Niagara gorge in the order -in which they were laid down upon the ocean floor. The formations -represented in the gorge are but a part of the entire series, for -other higher members are represented by rocks about Lake Erie and even -farther to the southward. These strata, having been formed upon an -outward sloping sea floor, had a small initial dip to the southward, -and this has been probably increased by subsequent uptilt, including -the latest which we have so recently had under discussion. At the -present time the beds dip southward by an angle of less than four -degrees, or about thirty-five feet in each mile. - -[Illustration: - -FIG. 387.—Map to show the cuestas which have played so important -a part in fixing the boundaries of the Lake basins, and also the -principal preglacial rivers by which they have been trenched (based -upon a map by Grabau).] - -When the elevation of the land in the vicinity of this shore had -caused a recession of the waters, there was formed a coastal plain -on the borders of the oldland like that which is now found upon our -Atlantic border between the Appalachians and the sea (Fig. 272, p. -246). The rivers from the oldland cut their way in narrow trenches -across the newland, and because of the harder limestone formations, -their tributaries gradually became diverted from their earlier courses -until they entered the trunk stream nearly at right angles and produced -the type of drainage network which is called “trellis drainage.” It -is characteristic of this drainage that few tributaries of the second -order will flow up the natural slope of the beds, but on the contrary -these natural slopes are followed in the softer rock nearly at right -angles again to the tributaries of the first order of magnitude (Fig. -387). Thus are produced a series of more or less parallel escarpments -formed in the harder rock and having at their base a lowland which -rises gradually in the direction of the oldland until a new escarpment -is reached in the next lower of the hard formations. Such flat-topped -uplands in series with intermediate lowlands and separated by sharp -escarpments are known as _cuestas_ (see p. 246), and the Lewiston -Escarpment limits that formed in Niagara limestone (Figs. 387 and 388). - -[Illustration: - -FIG. 388.—Bird’s-eye view of the cuestas south of Lakes Ontario and -Erie (after Gilbert).] - - -=Episodes of Niagara’s history and their correlation with those of -the Glacial Lakes.=—Of the early episodes of Niagara’s history, our -knowledge is not as perfect as we could desire, but the later events -are fully and trustworthily recorded. The birth of the Falls is to -be dated at the time when the ice front had here first retired into -what is now Canadian territory, thus for the first time allowing the -waters from the Erie basin to discharge over the Lewiston Escarpment -into the basin of the newly formed Lake Iroquois (Fig. 364, p. 334). -Since the level of Lake Iroquois was far above that of the present Lake -Ontario, the new-born cataract was not the equivalent in height of -the escarpment to-day. The Iroquois waters then bathed all the lower -portion of the escarpment, so that the foot of the Fall was upon the -borders of the Lake. - -In order to interpret the history of the Niagara gorge, we must -remember that the effective drilling of this gorge was in each stage -dependent mainly upon the volume of water discharged from Lake Erie, -a large discharge being recorded by a channel drilled both wide -and deep, while that produced by the discharge of a smaller volume -was correspondingly narrow and shallow. To-day the gorges of large -cross section have, moreover, a relatively placid surface, whereas -through the constricted sections the water of the river is unable to -pass without first raising its level at the upper end and under the -head thus produced rushing through under an increased velocity. The -best illustration of such a constricted section is the Gorge of the -Whirlpool Rapids. - -[Illustration: - -FIG. 389.—Sketch map of the greater portion of the Niagara Gorge to -show the changes in cross section in their relations to Niagara history -(based upon a map by Taylor).] - -Our reading of the history should begin at the site of the present -cataract, since the records of later events are so much the more -complete and legible, and it should ever be our plan to proceed from -the clearly written pages to those half effaced and illegible. - -As we have learned, the most abrupt change in the cross section of the -gorge is found a little above the railroad bridges, where the Upper -Great Gorge is joined to the Gorge of the Whirlpool Rapids (Fig. 389). -In view of the remarkably uniform cross section of the Upper Great -Gorge, there is no reason to doubt that it has been drilled throughout -under essentially the same volume of water, and that its lower limit -marks the position of the former cataract when the waters from the -upper lakes were transferred from the “North Bay Outlet” into the -present or “Port Huron Outlet” and Lake Erie. As the upper limit of the -Gorge of the Whirlpool Rapids thus corresponds to the closing of the -“North Bay Outlet” and the extinction of the Nipissing Great Lakes, so -its lower limit doubtless corresponds to the opening of that outlet and -the termination of the preceding Algonquin stage; for in the stage of -the Nipissing lakes the water of the upper lakes, as we have learned, -reached the ocean through the northern outlet. - -Mr. Frank Taylor, who has given much study to the problem of Niagaran -history, believes that the Middle Great Gorge, comprising the Eddy -Basin and the Cove section, represents the gorge drilling which -occurred during the later stage of Lake Algonquin after the “Trent -Outlet” had been closed and the waters of the upper lakes had been -turned into the Erie Basin. - -Summarizing, then, the episodes of the lake and the gorge history are -to be correlated as follows:— - - GLACIAL LAKE NIAGARA GORGE - - Early Lakes Iroquois and Algonquin. Drilling of the gorge from the - Lewiston Escarpment to the Cove - section above the Wintergreen - Flats. - - Later Lakes Iroquois and Algonquin Drilling of Middle Great Gorge. - with upper lakes discharging - into Erie basin. - - Nipissing Great Lakes with the Drilling of the narrow Gorge of - upper lake waters diverted from the Whirlpool Rapids. - Lake Erie. - - Recent St. Lawrence drainage Drilling of Upper Great Gorge to - since the waters of the upper lakes the present cataract. - were discharged into Lake Erie - through occupation of the Port - Huron Outlet. - - -=Time measures of the Niagara clock.=—In primitive civilizations time -has sometimes been measured by the lapse necessary to accomplish a -certain task, such, for example, as walking the distance between two -points; and the natural clock of Niagara has been of this type. But men -possess differences in strength and speed, and the same man is at some -times more vigorous than at others, and so does not work at a uniform -rate. The cataract of Niagara, charged with the pent-up energy of the -waters of all the Great Lakes, can rush its work as it is clearly -unable to do at times when the greater part of this energy has been -diverted. Units of distance measured along the gorge are therefore too -unreliable for our use, with the unique exception of the stretch from -the railroad bridges to the site of the present cataract, within which -stretch the gorge cross sections are so nearly uniform as to indicate -an approximation to continued application of uniform energy. This -energy we may actually measure in the existing cataract, and so fix -upon a unit of time that can be translated into years. - -In order to secure the normal rate of recession of this Upper Great -Gorge, we should add to the volume of water in the Canadian Fall that -now passing over the American; and for the reason that the blocks -which fall from the cataract cornice and are the tools of the drilling -instrument approximate to a definite size fixed by their joint planes, -the effect of this added energy it is not easy to estimate. We may be -sure, however, that the drilling action would be somewhat increased by -the junction of the two Falls, and thus are assured that the average -rate of recession within the Upper Great Gorge has been somewhat in -excess of the five feet per year determined by Gilbert for the present -Canadian Fall. The Upper Great Gorge is about two miles in length, -and its beginning may thus be dated near the dawning of the Christian -Era. The Whirlpool Gorge was cut when the ice vacated the North Bay -Outlet in Canada, and still lay as a broad mantle over all northeastern -Canada. For the earlier gorge and lake stages, the time estimates are -hardly more than guesses, and we need not now concern ourselves with -them. - - -=The horologe of late glacial time in Scandinavia.=—A glacial -timepiece of somewhat different construction and of greater refinement -has been made use of in Scandinavia to derive the “geochronology -of the last 12,000 years.” Instead of retreating over the land and -impounding the drainage as it did so, the latest continental glacier -of Scandinavia ended below sea level, and as it retired, its great -subglacial river laid down a giant esker known as the Stockholm Os, -which was bordered by a delta and fringed on either side by water-laid -moraines of the block type. These recessional moraines are upon the -average less than 1000 feet apart, and are believed to have each been -formed in a single season. The delta deposits which surround the esker -are of thin-banded clay, and as an additional uppermost band is found -outside every moraine, these bands are also believed to represent each -the delta deposit of a single year. In studies extending over many -years, Baron de Geer, with the aid of a large body of student helpers, -has succeeded in completing a count of moraines and clay layers, and so -in determining the time to be 12,000 years since the ice front of the -latest continental glacier lay across southern Sweden. The fertility -of conception and the thoroughness of execution of this epoch-making -investigation recommend its conclusion to the scientific reader. - - -READING REFERENCES FOR CHAPTER XXV - - G. K. GILBERT. Niagara Falls and their History, Nat. Geogr. Soc. Mon., - vol. 1, No. 7, 1895, pp. 203-236. - - F. B. TAYLOR. Origin of the Gorge of the Whirlpool Rapids at Niagara, - Bull. Geol. Soc. Am., vol. 9, 1898, pp. 59-84. - - A. W. GRABAU. Guide to the Geology and Paleontology of Niagara Falls - and Vicinity, Bull. N. Y. State Mus., vol. 9, No. 45, 1901, pp. 1-85, - pls. 1-11. - - J. W. SPENCER. The Falls of Niagara, etc. Dept. of Mines, Geol. Surv. - Branch, Canada, 1907, pp. 490, pls. 43. - - G. K. GILBERT. Rate of Recession of Niagara Falls, etc. Bull. 306, U. - S. Geol. Surv., 1907, pp. 31, pls. 11. - - G. DE GEER. Quaternary Sea Bottoms of Western Sweden. Paper 23, Livret - Guide Cong. Géol. Intern., 1910, pp. 57, pls. 3. - - - - -CHAPTER XXVI - -LAND SCULPTURE BY MOUNTAIN GLACIERS - - -=Contrasted sculpturing of continental and mountain glaciers.=—In -discussing in a previous chapter the rock pavement lately uncovered by -the Greenland glacier, we learned that this surface had been lowered -by the processes of plucking and abrasion, the combined effect of -which is always to reduce the irregularities of the surface, soften -its outlines, and from sharply projecting masses to develop rounded -shoulders of rock—_roches moutonnées_. - -Though the same processes act in much the same manner beneath -mountain glaciers, though here upon all parts of the bed, they are, -in the earlier stages at least, subordinated to a third process more -important than the two acting together. Sculpture by mountain glaciers, -instead of reducing surface irregularities and softening outlines, -increases the accent of the relief and produces the most sharply -rugged topography that is known. In nearly all places where Alpinists -resort for difficult rock climbing, mountain glaciers are to be seen, -or the evidence for their former presence may be read in unmistakable -characters. - - -=Wind distribution of the snow which falls in mountains.=—Until quite -recently students of glaciation have concerned themselves but little -with the work of the wind in lifting and redistributing the snow after -it has fallen. We have already seen that, for the continental glaciers, -wind appears to be the chief transporting agent, if we except the -marginal lobes where glacier flow assumes large importance. In the case -of mountain glaciers, also, we are to find that for the earlier stages -particularly wind is of the first importance as a redistributing agent. -In the higher levels snow is swept up from the ground by all high -winds, and does not find a resting place until it is dropped beneath an -eddy in some irregularity of the surface; and if the inherited surface -be relatively smooth, this will be found in most cases upon the lee of -the mountain crest. - -In normal cases at least the inherited irregularities of the higher -zones of mountain upland are the gentle depressions which develop at -the heads of streams. These become, then, the sites of snowdrifts that -are augmented in size from year to year, though at first they melt away -in the late summer. - - -[Illustration: FIG. 390.—Snowdrift hollowing its bed by nivation and -building a delta (at the left). Quadrant Mountain, Yellowstone National -Park.] - -=The niches which form on snowdrift sites.=—Wherever a drift is -formed, a process is set in operation, the effect of which is to hollow -out and lower the ground beneath it, a process which has been called -_nivation_. The drift shown in Fig. 390 was photographed in late summer -at an elevation of some 9000 feet in the Yellowstone National Park. -The very gently sloping surface surrounding the drift is covered with -grass, but within a zone a few feet in width on the borders of the -drift no grass is growing, and in its place is found a fine brown soil -which is fast becoming the prey of the moving water derived by melting -of the drift. This is explained by the water permeating the crevices of -the rock and being rent by the nightly freezing. Farther from the drift -the ground is dry, and no such action is possible. With each succeeding -spring the augmented drift as it melts carries all finely comminuted -rock material down slopes beneath the snow to emerge at the lowest -margin and be there deposited in the form of a delta. By the operation -of this process of nivation the higher parts of the drift site are -lowered as deposition goes on upon the lower. The combined effect is -thus to produce a _niche_ or faintly etched amphitheater upon the slope -of the mountain (Fig. 391). - -[Illustration: FIG. 391.—Amphitheater formed on a drift site in -northern Lapland (after a photograph by G. von Zahn).] - - -=The augmented snowdrift moves down the valley—birth of the -glacier.=—In still lower air temperatures the drifts enlarge with -each succeeding year until they endure throughout the summer season. -From this stage on, an increment of snow is left from each succeeding -season. No longer entirely wasted by melting, the time soon comes when -the upper snow layers will by their weight compress the lower into ice, -and the mass will begin to creep down the slope along the course of the -inherited valley. The enlarged snowdrift which feeds this ice stream is -called the _névé_ or _firn_. - -Against the sloping cliff which had been shaped by nivation at the -upper margin of the snowdrift, that snow which is not of sufficient -depth to begin a movement towards the valley separates from the moving -portion, opening as it does so a cleft or crevasse parallel to the -wall. This crack in the snow is called by its German name _Bergschrund_ -or _Randspalte_, and may perhaps be referred to as the marginal -crevasse (Fig. 392). - -[Illustration: - -FIG. 392.—The marginal crevasse or Bergschrund on the highest margin -of a glacier (after Gilbert).] - - -=The excavation of the glacial amphitheater or cirque.=—It has been -found that the marginal crevasse plays a most important rôle in the -sculpture of mountains by glaciers, for the great amphitheater which -is everywhere the collecting basin for the nourishment of mountain -glaciers is not an inherited feature, but the handiwork of the ice -itself. This was the discovery of Mr. W. D. Johnson, an American -topographer and geologist, who, in order to solve the problem of the -amphitheater allowed himself to be lowered into such a crevasse upon -the Mount Lyell glacier of the Sierra Nevadas in California. - -[Illustration: - -FIG. 393.—Niches and cirques in the same vicinity in the Bighorn -Mountains of Wyoming. _A, A_, unmodified valleys; _B, B_, niches on -drift sites; _C, C_, cirques on small glacier sites (after map by F. E. -Mathes, U. S. G. S.).] - -Let down a distance of a hundred and fifty feet, he reached the bottom -of the crack, and in a drizzling rain of thaw water stood upon a floor -composed of rock masses in part dislodged from a wall which extended -some twenty feet upwards upon the cliff side of the crevasse. It was -evident that the warm air of the day produced the thaw water which was -constantly dripping and which filled every crack and cranny of the rock -surface. With the sinking of the sun below the peaks the sudden chill, -so characteristic of the end of the day in high mountains, causes this -water to freeze and thus rend the rock along its planes of jointing. -Broad and thin plates of ice, loosened by melting at the walls, could -be extracted from the crevices of the rock as mute witnesses to the -powerful stresses developed by this most vigorous of weathering -processes. - -[Illustration: - -FIG. 394.—Subordinate small cirques in the amphitheater on the west -face of the Wannehorn above the Great Aletsch Glacier of Switzerland.] - -In short, the rock wall above the glacier, which in its initial stage -was the upper wall of the niche hollowed beneath the snowdrift, is -first steepened and later continually both recessed and deepened by -an intensive frost rending which is in operation at the base of the -marginal crevasse. The same process does not go on as rapidly above the -surface of the névé for the reason that _the necessary wetting of the -rock surface does not there so generally result from the daily summer -thaw_. At the bottom of the marginal crevasse alone is this condition -fully realized. Intensive frost action _where the rock is wet with thaw -water daily_ is thus a fundamental cause, both of the hollowing of -the early drift site to form the niche, and of the later enlargement -of this niche into an amphitheater or cirque when the drift has been -transformed into the névé of a glacier. Inasmuch as the crevasse forms -where the snow and ice pull away from the rock toward the middle of the -depression, the cirque wall in its early stage has the outline of a -semicircle. In the Bighorn Mountains of Wyoming, all stages, from the -unmodified valley heads to the full-formed cirque, may be seen near -one another (Fig. 393). It will be noted that wherever a glacier has -formed, as indicated by the cirque, there is a series of lakes which -have developed in the valley below (see p. 412). - - -[Illustration: FIG. 395.—“Biscuit cutting” effect of glacial sculpture -in the Uinta Mountains of Wyoming (after Atwood).] - - -[Illustration: - -FIG. 396.—Two intersecting inverted cones representing glacial cirques -of different sizes, to show that their intersection is the arc of a -hyperbola, the curve to which the col approximates.] - -=Life history of the cirque.=—In its earliest stage the cirque is more -or less uniformly supplied with snow from all sides, and so it enlarges -by recession in a manner to retain its early semicircular outline. In -a later stage a larger proportion of the snow reaches the cirque at -its sides so that its further enlargement causes it to broaden and to -flatten somewhat that part of its outline which represents the head of -the valley (Fig. 389, p. 364). As the territory of the upland is still -further invested by the cirques, their nourishment becomes still more -irregular, and the circular outline gives place to a scalloped border, -as the amphitheater becomes differentiated into subordinate smaller -cirques, each of which corresponds to a scallop of the outline (Fig. -398 and Fig. 394). - -=Grooved and fretted uplands.=—The partial investment by cirques of a -mountain upland yields a type of topography quite unlike that produced -by any other geological process. The irregularly connected remnants -of the inherited upland resemble nothing so much as a layer of dough -from which biscuits have been cut (Fig. 395). The surface as a whole, -furrowed as it is below the cirques, -may be described as a _grooved upland_ (plate 19 A). A further -continuation of the process removes all traces of the earlier upland, -for the cirques intersect from opposite sides and thus yield palisades -of sharp rock pinnacles which rise on precipitous walls from a terraced -floor. This ultimate product of cirque sculpture by glaciers is called -a _fretted upland_ (plate 18 A and 19 B). - -┌─────────────────────────────────────────────────────────────────────┐ -│ PLATE 18. │ -│ │ -│ [Illustration: _A._ Fretted upland of the Alps seen from the summit │ -│ of Mount Blanc.] │ -│ │ -│ [Illustration: _B._ Model of the Malaspina Glacier and the fretted │ -│ upland above it (after model by L. Martin). │ -└─────────────────────────────────────────────────────────────────────┘ - -┌──────────────────────────────────────────────────────────────────┐ -│ PLATE 19. │ -│ │ -│ [Illustration: _A._ Contour map of a grooved upland, Bighorn │ -│ Mountains, Wyoming (U. S. Geol. Survey).] │ -│ │ -│ [Illustration: _B._ Contour map of a fretted upland, Philipsburg │ -│ Quadrangle, Montana (U. S. Geol. Survey).] │ -└──────────────────────────────────────────────────────────────────┘ - - -=The features carved above the glacier.=—The ranges of pinnacles -carved out by mountain glaciers have become known by various names of -foreign derivation, such as _arête_, _grat_, _aiguille_ mountains, -“files of _gendarmes_”, etc. They may, perhaps, be best referred to as -_comb ridges_, and according to their position they are differentiated -into main and lateral comb ridges, as will be clear from the second map -of plate 19. - -[Illustration: FIG. 397.—A col shaped like a hyperbola between Mount -Sir Donald and Yogo Peak in the Selkirks (after a plate by the Keystone -Plate Co.).] - -With the gradual invasion of the upland upon which the cirques have -made their attack, the area from which winds may gather up the snow -is steadily diminished, and hence cirque recession is correspondingly -retarded. Cirques which have approached each other from opposite sides -of the ridge until they have become tangent at one point may, however, -still receive nourishment at the sides and so continue to cut down the -intervening rock wall to form a pass or _col_. The theoretical curve -which results from this intersection is that known as the hyperbola, of -which an illustration is afforded by Fig. 396. An approximation to this -form is clearly furnished by most of the mountain passes in glaciated -mountain districts, and a particularly good illustration is furnished -from the vicinity of Glacier on the line of the Canadian Pacific -Railway (Fig. 397). - -[Illustration: - -FIG. 398.—Diagrams to illustrate the progressive investment of an -upland by cirques with the formation of comb ridges, cols, and horns. -I, early stage, youth; II, intermediate stage; III, late stage, -maturity.] - -Upon either side of the col the land mass is left in high relief, -rising from a more or less triangular base (Fig. 398, III) into a sharp -horn or tooth. An illustration of such a _horn_ is furnished by the -Matterhorn in the Swiss Alps, or by Mount Sir Donald in the Selkirks, -though less noteworthy examples may be found in every maturely -glaciated mountain district. - - -=The features shaped beneath the glacier.=—Those features which are -carved above the glacier—the comb ridge, the col, and the horn—are -all shaped as a result of intensive weathering upon the cirque wall. -The shaping at lower levels is accomplished by processes in operation -below the glacier surface, where weathering is excluded and where -plucking and abrasion work together to tear away and grind off the rock -surface. By their joint action the valley is both deepened and widened, -directly to the height of the glacier surface, and indirectly through -undermining as far up as rock extends. Thus the valley is transformed -into one of broad and flat bed and precipitous side walls—the U-shaped -section illustrated by valleys of the Swiss Alps and in fact in all -districts which have been strongly glaciated by mountain glaciers (Fig. -399). - -[Illustration: - -FIG. 399.—The U-shaped Kern valley in the Sierra Nevadas of California -(after W. B. Scott).] - -As high up in the valley as it has been occupied by the glacier, -the bed is rounded, smoothed, and polished, and marked by the -characteristic glacial scorings or striæ which point down the valley. -Above the level of the glacier’s upper surface, on the other hand, -erosion is accomplished through undermining or sapping, a process which -always leaves precipitous slopes of ragged surface made up of the joint -planes on which the fallen blocks have separated from the cliff. Thus -there is found a sharp line which separates the smoothly rounded rock -surface below from the jagged and precipitous one above (Fig. 400). -Inasmuch as this boundary usually separates the scalable from the -inaccessible slopes above, snow is apt to lodge at this level and make -it strikingly apparent. - -[Illustration: - -FIG. 400.—Glaciated valley wall in the Sierra Nevadas of California, -showing the sharp line which separates the abraded from the undermined -rock surface (after a photograph by Fairbanks).] - -[Illustration: - -FIG. 401.—View of the Vale of Chamonix from the séracs of the Glacier -des Bossons. The alb of the opposite side is well brought out.] - -If uplift of the land occurs while glaciers occupy the valleys of -mountains, an increased capacity for deepening the valley is imparted -to these ice streams, and we find, as a result, a deep central valley -of U cross section excavated within a relatively broad trough visible -above the shoulder on either side of the later furrow. Save only for -its characteristic curves, such a valley bears close resemblance to -a mature stream valley which has been rejuvenated (see p. 173). The -remnants of the earlier glacier-carved valley are, as already stated, -gently curving high terraces so common in Switzerland, where they -are known as _albs_ or high mountain meadows. These albs may be seen -to special advantage on the sides of the Chamonix valley (Fig. 401), -the Lauterbrunnen valley, or in fact almost any of the larger Alpine -valleys. - - -=The cascade stairway in glacier-carved valleys.=—If now, instead of -giving our attention to the cross section, we follow the course of the -valley that has been occupied by a glacier, we find that it descends -by a series of steps or terraces having many backwardly directed -treads (plate 19), whereas a normal and well-established river valley -has only forward grades. Because of these backward grades the stream -waters are impounded, and so lakes are found strung along the valley -in chains as the larger beads are found in a rosary, and these are the -characteristic _rock basin lakes_ sometimes referred to as “Paternoster -Lakes” (see p. 412 and Fig. 402). - -┌───────────────────────────────────────────────────────────────────┐ -│ PLATE 20. │ -│ │ -│ [Illustration: Map of the surface modeled by mountain glaciers in │ -│ the Sierra Nevadas of California (after I. C. Russell).] │ -└───────────────────────────────────────────────────────────────────┘ - -When the backward grades upon the valley floor are especially steep, -the rock step becomes a _rock bar_, or _Riegel_, of which nearly every -Alpine valley has its examples. In a walk from the Grimsel to Meiringen -many such bars are passed. Carrying in suspension the sharp rock sand -from the glacier deposits along its bed, the stream which succeeds -to the glacier as it vacates its valley saws its way through these -obstructions with a rapidity that is amazing, thus producing narrow -defiles, of which the Gorge of the Aar near Meiringen and that of the -Gorner near Zermatt are such well-known examples (Fig. 403). - -[Illustration: - -FIG. 402.—Map of an area near the continental divide in Colorado, -showing an unglaciated surface to the west of the divide, where the -westerly winds have cleared the ground of snow, and the glacier-carved -country to the eastward. Note the regular forms of the youthful cirque, -the glacier stairway, and the rock basin lakes (U. S. G. S.).] - -[Illustration: - -FIG. 403.—Gorge of the Albula River near Berkum in the Engadine, cut -through a rock bar by the river which has succeeded to the earlier -glacier.] - -[Illustration: - -FIG. 404.—Idealistic sketch showing both glaciated and nonglaciated -side valleys tributary to a glaciated main valley (after Davis).] - -It is characteristic of rivers that the tributaries cut their valleys -more rapidly than does the main stream within the neighboring section, -though they cannot cut lower than their outlets—the side streams enter -_accordantly_. This is easily explained because the grades of the -tributary streams are the steeper, and, as we well know, the corrosion -of a valley is augmented at a most amazing rate for each increase of -its grade. No such law controls the processes of plucking and abrasion -by which the glacier lowers its floor, for these processes appear to -depend for their efficiency upon the depth of the ice, and the supply -of cutting tools, quite as much as upon the grade of the bed. To apply -a homely illustration, the hollowing of flagstones upon our walks is -dependent more upon the number of persons that pass over them, and upon -their size and the number of protruding nails in their boot heels, than -upon the grades upon which they are placed. At all events we find that -the main glacier valleys are cut deeper than the side valleys, so that -the latter become _hanging valleys_—they enter the main valley, not -upon its bed, but some distance above it (Fig. 404). - -The U-shaped hanging valleys, like the main valley, are much too large -for the streams which now fill them, and these diminutive side streams -plunge over the steep wall of the main valley in ribbon-like falls so -thin that the wind turns them aside and disperses the water in the -spray of a “bridal veil.” Such falls are found by the hundred in every -glaciated mountain district, imparting to it one of the greatest of its -scenic charms. - -[Illustration: FIG. 405.—Character profiles in landscapes sculptured -by mountain glaciers.] - -[Illustration: FIG. 406.—Flat dome shaped under the margin of -a Norwegian ice cap with projecting rock knobs and moraines in -foreground.] - - -=The character profiles which result from sculpture by mountain -glaciers.=—The lines which are repeated in landscapes carved by -mountain glaciers are easy to recognize (Fig. 405). The highest -horizon lines are the outlines of horns which are separated by cols. -Minaret-like palisades, or “files of _gendarmes_”, often run for long -distances as the characteristic comb ridges. Lower down and lacking -the lighter background of the sky, we make out with less distinctness -the U-valley, either with or without the albs to show that the -sculpturing process has been interrupted by uplift. - -[Illustration: FIG. 407.—Two views illustrating successive stages in -the shaping of tinds or “beehive” mountains.] - - -=The sculpture accomplished by ice caps.=—In the case of ice caps, -the only rock exposed is found in the neighborhood of the margin—the -projecting islands known as nunataks. It is essential for the existence -of the ice cap that the rock base should have relatively slight -irregularities compared to the dimensions of the cap itself. Except -in very high latitudes this base must be somewhat elevated, for like -mountain glaciers ice caps are nourished by the surface air currents, -and their snows are deposited above the snow line. - - -=The Norwegian tind or beehive mountain.=—Within temperate or tropical -climes the snow line lies so high that only the loftier mountains -are able to support glaciers. It follows that those which are formed -flow upon relatively high grades with correspondingly high rate of -movement and increased cutting power. Within high latitudes the snow -is found nearer the sea level, and glaciers are for the most part -correspondingly sluggish in their movements as well as less active -denuding agents. - -To this condition characteristic of high latitude glaciers, there is -added in Norway another in the peculiar shape of the basement beneath -the recent and the still existing glaciers. The plateau of Norway is -intersected by a network of deep and steep walled fjords, and the -glaciers have developed as small ice caps perched upon veritable -pedestals of rock, over the margins of which their outlet tongues of -ice descend on steep slopes into the fjord. The tops of the pedestals -thus come to be shaped by the plucking and abrading processes into -flat domes (Fig. 406), while the knobs of rock, which as nunataks -reach above the surface of the ice, divide the outflowing ice tongues -at the margin of the pedestal. These tongues being much more active -denuding agents, because of their steep gradients, continually lower -their beds, thus transforming the earlier knobs of rock into high and -steep mountains of more or less circular base. Such “beehive” mountains -upon the margins of the fjords are the characteristic Norwegian _tinds_ -(Fig. 407). - - -READING REFERENCES FOR CHAPTER XXVI - - I. C. RUSSELL. Quaternary History of Mono Valley, California, 8th Ann. - Rept. U. S. Geol. Surv., 1889, pp. 329-371, pls. 27-37. - - F. E. MATTHES. Glacial Sculpture of the Bighorn Mountains, Wyoming, - 21st Ann. Rept. U. S. Geol. Surv., 1900, Pt. ii, pp. 179-185, pl. 23. - - W. D. JOHNSON. Maturity in Alpine Glacial Erosion, Jour. Geol., vol. - 12, 1904, pp. 569-578. - - G. K. GILBERT. Systematic Asymmetry of Crest Lines in the High Sierras - of California, _ibid._, pp. 579-588. - - EMM. DE MARTONNE. Sur la Formation des Cirques, Ann. de Géogr., vol. - 10, 1901, pp. 10-16. - - W. M. DAVIS. Glacial Erosion in North Wales, Quart. Jour. Geol. Soc. - Lond., vol. 65, 1909, pp. 281-350, pl. 14. - - ED. BRÜCKNER. Die Glazialen Züge im Antlitz der Alpen, Naturw. - Wochenschr., N. F., vol. 8, 1909. - - WILLIAM H. HOBBS. Characteristics of Existing Glaciers, pp. 1-96. - - - - -CHAPTER XXVII - -SUCCESSIVE GLACIER TYPES OF A WANING GLACIATION - - -=Transition from the ice cap to the mountain glacier.=—A study of -existing glaciers leads inevitably to the conclusion that although -subject to short period advances and retreats, yet, broadly speaking, -glaciers are now gradually wasting away, surrounded by wide areas upon -which are the evidences of their recent occupation. We are thus living -in a receding hemicycle of glaciation. - -[Illustration: FIG. 408.—Schematic diagram to show the relationships -of glacier types formed in succession during a receding hemicycle of -glaciation.] - -Many mountain districts which now support small glaciers only, or -none at all, were once nearly or quite submerged beneath snow and -ice. If once covered by an ice carapace or cap, our present interest -in them begins at that stage of the receding hemicycle _when the rock -surface has made its reappearance above the surface of the snow-ice -mass_. At this stage intensive frostwork, the characteristic high -level weathering, begins, and cirques develop above the scars of those -earlier amphitheaters formed in the advancing hemicycle. - - -=The piedmont glacier.=—In this early stage of transition from the -ice cap to the mountain glacier, the ice flows outward to the mountain -front in ill-defined streams divided by the projecting ridges, and -upon reaching the mountain front these streams deploy upon it so as -to coalesce in a great stagnant ice apron whose upper surface slopes -gently forward at an angle of a few degrees at the most (Fig. 408, -stage I). This is the _piedmont glacier_, a type found to-day in the -high latitudes of Alaska and in the southern Andes (Fig. 409 and pl. 18 -B). - -[Illustration: FIG. 409.—Map of the Malaspina glacier of Alaska, the -best known of existing piedmont glaciers (after Russell).] - -During this stage the cirques may be but poorly defined, and ice flows -in both directions from rock divides so that the streams transect the -range, and later, after the glaciers have disappeared, may expose a -pass smoothed and polished upon its floor and with striæ directed in -opposite directions from the highest point. The pass of the Grimsel -in Switzerland furnishes an excellent illustration of such earlier -transection of the range. - - -[Illustration: - -FIG. 410.—Map of the Baltoro glacier of the Himalayas, a typical -glacier of the dendritic type.] - -=The expanded-foot glacier.=—As air temperatures continue to become -milder, the glacier streams within the mountains are less deep and -hence more clearly defined, and instead of coalescing upon the mountain -foreland, they now issue from the mountains to form individual aprons -and are described as _expanded-foot glaciers_ (Fig. 408, stage II, and -Fig. 292, p. 264). - - -[Illustration: - -FIG. 411.—The Triest glacier, a hanging glacieret separated from the -Great Aletsch glacier to which it was lately a tributary.] - -=The dendritic glacier.=—Still later in the hemicycle nourishment of -the glaciers is diminished as depletion from melting increases, so that -the glacier streams no longer reach to the mountain front. Branches -continue to enter the main valley from the several side valleys like -the short branches of a tall tree, and because of this arrangement such -a glacier may be described as a _dendritic glacier_ (Fig. 408, stage -III, and Fig. 410). - -Inasmuch as the depletion from melting increases at a rapid rate in -descending to lower levels, the tributary glacier valleys “hanging” -above the main valley in the lower stretches become separated, and may -continue to exist as series of hanging glacierets upon either side -of the main valley below the glacier front (Fig. 408, stage III, and -Fig. 411). It must be clear from this that any attempt to name each -separated ice stream without regard to its relationship must lead to -endless confusion, for glacier size is in such sensitive adjustment -to air temperature that a fall or rise of a few degrees only in the -average annual temperature of the district may prove sufficient to fuse -many glaciers into one or separate one ice mass into many smaller ones. - -When in high latitudes a dendritic glacier descends in fjords to -below the level of the sea, it is attacked by the water in the same -manner as are the outlets of Greenland glaciers, and is then known as -a “tidewater glacier”, which may thus be a subtype or variety of the -dendritic glacier (Fig. 412). - -[Illustration: - -FIG. 412.—The Harriman fjord glacier of Alaska, a tidewater variety of -dendritic glacier (after a map by Gannett).] - - -=The radiating (Alpine) glacier.=—In the progressive wastings of -dendritic glaciers, there comes a time when their dendritic outlines -give place to radiating ones. Attention has already been called to the -division of the cirque into subordinate basins separated by small rock -arêtes and yielding a markedly scalloped border (Fig. 394, p. 371). -When the ice front retires from the main valley into one of these -mature cirques, the now wasted ice stream is broken up into subordinate -glacierets, each of which occupies one of the basins within the larger -cirque, and these ice streams flow together to produce a glacier whose -component elements radiate like the sticks within a lady’s fan (Fig. -408, stage IV, and Fig. 413). - -[Illustration: - -FIG. 413.—Map of the Rotmoos glacier, a radiating glacier of -Switzerland (after Sonklar).] - - -=The horseshoe glacier.=—As the glacier draws near to its final -extinction, it is crowded hard against the wall of the amphitheater in -which it has so long been nourished. Up to this stage it has offered -a swelling front outwardly convex as a direct consequence of the laws -controlling its flow. No longer amply nourished, for the first time -its front is hollowed, and it awaits its final dissolution curled up -against the cirque wall (Fig. 408, stage V, and Fig. 414). Practically -all the glaciers of the United States and southern Canada are of this -type. - -The above classification is one depending directly upon glacier -nourishment, and hence also upon size, and upon the stage of the -glacial hemicycle. In order to determine the type of any glacier it is -necessary to know the outlines of the mountain valley—its divide—and -those of the glacier or glaciers within it. It is likely that the -types of the advancing hemicycle of glaciation would be much the same, -save only for the _new-born_ or _nivation glacier_, which would be as -different as possible from the horseshoe type, to which in size it -corresponds. Upon the continent of Antarctica, where the absence of any -general melting of the ice, even in the summer season and near the sea -level, introduces special conditions, some additional glacier types are -found, which, however, it is not necessary that we consider here. - -[Illustration: FIG. 414.—Outline map of the Asulkan glacier in the -Selkirks, a typical horseshoe glacier.] - - -=The inherited-basin glacier.=—It may be, however, that glaciers have -developed, not upon mountains shaped in a cycle of river erosion, nor -yet in succession to an ice cap, as in the normal cases which we have -considered. On the contrary, glaciers may develop where basins of -one sort or another have been inherited from the preceding period. -In such cases inherited depressions may become more important than -the auto-sculpture of the glacier. Glaciers which develop under such -conditions may be described as _inherited-basin glaciers_. - -[Illustration: FIG. 415.—Outline map of the Illecillewaet glacier, an -inherited-basin glacier in the Selkirks.] - -A partly closed basin between ridges may supply a collecting ground for -snows carried from neighboring slopes by the wind, and so may yield a -broad névé, approaching in size a small ice cap, yet without developing -definite ice streams except upon its border. Such a glacier is the -Illecillewaet glacier of the Selkirks (Fig. 415). - -Again in low latitudes the high and pointed volcanic peaks may push -up beyond the snow line into the upper atmosphere, and so become -snow-capped. Definite cirques do not develop well under these -circumstances, and the loose materials of which such peaks are always -composed are attacked in somewhat irregular fashion from the different -sides. This is the case of Mount Rainier and similar peaks of the -Cascade range of North America. - - -=Summary of types of mountain glacier.=—In tabular form the various -types of mountain glacier may be arranged as follows:— - -MOUNTAIN GLACIERS - - _Piedmont glacier._ Mountain valleys entirely occupied and largely - submerged, with overflow upon the foreland to form a common ice apron - through coalescence of neighboring streams. - - _Expanded-foot glacier._ Valley entirely occupied and an overflow upon - the foreland sufficient to produce individual ice apron. - - _Dendritic glacier._ Valley not completely occupied but with tributary - ice streams ranged along the sides of the main stream, and with - hanging glacierets separated near the glacier foot. - - _Radiating glacier._ Glacier largely included in a cirque with - subordinate glacierets converging below like the sticks in a lady’s - fan. - - _Horseshoe glacier._ Small glacier remnants hugging the cirque wall - and having an incurving front. - - _Inherited-basin glacier._ Of form dependent on a basin inherited and - not shaped by the glacier itself. - - -READING REFERENCE FOR CHAPTER XXVII - - WILLIAM H. HOBBS. The Cycle of Mountain Glaciation, Geogr. Jour., vol. - 37, 1910, pp. 268-284. - - - - -CHAPTER XXVIII - -THE GLACIER’S SURFACE FEATURES AND THE DEPOSITS UPON ITS BED - - -=The glacier flow.=—The downward flow of the ice within a mountain -glacier has been the subject of many investigations and the topic of -many heated discussions since the time when Louis Agassiz and his -companions set a line of stakes across the Aar glacier and numbered the -surface bowlders in preparation for repeated observations. Their first -observation was that the line of stakes, which had run straight across -the glacier, was distorted into a curve which was convex downstream -(Fig. 416, A´), thus showing that the surface layers have more rapid -motion in proportion as they are distant from the side margins. -Summarizing these and later studies, it may be stated that the glacier -increases its rate of motion from its side margin towards its center -line, from its bed upwards towards its surface, and below the névé the -velocity is greatest where the fall is greatest and also wherever the -cross section diminishes. In all these particulars, then, the ice of -the glacier behaves like a stream of water. The average rate of flow of -Alpine glaciers varies from a few inches to a few feet per day, and is -greater during the warm summer season. The Muir glacier of Alaska has -been shown to move at the rate of about seven feet per day. - -[Illustration: - -FIG. 416.—Diagram to illustrate the migrations of lines of stakes -crossing a glacier, due to its surface movement, _A_, original position -of lines; _A´_, later positions; _a_ and _a´_, original and distorted -forms of a square section of the glacier surface near its margin; _r_, -_r´_, diagonal crevasses.] - -In traveling from the névé downward to the glacier foot, the snow not -only changes into ice, but it undergoes a granulating process with -continued increase in the size of the nodules until at the foot of -the glacier these may be picked out of the partially melted ice as -articulating balls the size of the fist or larger. Glacier ice has -therefore a structure quite different from that of lake ice, since the -latter is developed in parallel needles perpendicular to the freezing -surface. - - -=Crevasses and séracs.=—Prominent surface indications of glacier -movement are found in the open cracks or _crevasses_, which are the -marks of its yielding to tensional stresses. Crevasses are apt to run -either directly across the glacier, wherever there is a steep descent -upon its bed, or diagonally, running in from the margin and directed -up-glacier (_r_, _r_, _r_, of Fig. 416), though they occasionally run -longitudinally with the glacier when there is a rock terrace at the -side of the valley beneath the ice. The diagonal crevasses at the -glacier margin are due to the more sluggish movement where the ice is -held back by friction upon the walls of the valley, as will be clear -from Fig. 416. The square _a_ has by this movement been distorted into -the lozenge _a´_, so that the line _xy_ has been extended into _x´y´_, -with the obvious tendency to open cracks in the direction _ss_. - -Every glacier surface below its névé is marked by steps or terraces, -which are well understood to overlie corresponding steps of the cascade -stairway to be seen in all vacated glacier valleys (plate 19). The -steep risers of these steps are usually marked by parallel crevasses -which cross the glacier. Under the rays of the sun, which strike them -more from one side than from the other, the slices into which the ice -is divided are transformed into sharpened blades and needles which are -known as _séracs_ (Fig. 401, p. 376, and Fig. 417). - -[Illustration: - -FIG. 417.—Transverse crevasses at the fall below a glacier step -transformed by unsymmetrical melting into séracs.] - -The numerous crevasses tell us that the ice is many times wrenched -apart during its journey down the glacier. This has been illustrated by -somewhat grewsome incidents connected with accidents to Alpinists, but -as they illustrate in some measure both the mode and the rate of motion -of Swiss glaciers, they are worthy of our consideration. - - -=Bodies given up by the Glacier _des Bossons_.=—In the year 1820, -during one of the earlier ascents of Mont Blanc, three guides were -buried beneath an avalanche near the _Rochers Rouges_ in the névé -of the Glacier des Bossons (Fig. 418). In 1858 Dr. Forbes, who had -measured the rate of flow of a number of Alpine glaciers, predicted -that the bodies of the victims of this accident would be given up by -the glacier after being entombed from thirty-five to forty years. In -the year 1861, or forty-one years after the disaster, the heads of -the three guides, separated from their bodies, with some hands and -fragments of clothing, appeared at the foot of the Glacier des Bossons, -and in such a state of preservation that they were easily recognized -by a guide who had known them in life. Inasmuch as these fragments of -the bodies had required forty-one years to travel in the ice the three -thousand meters which separate the place of the accident from the foot -of the glacier, the rate of movement was twenty centimeters, or eight -inches, per day. - -[Illustration: - -FIG. 418.—View of the _Glacier des Bossons_ upon the slopes of Mont -Blanc showing the position of accidents to Alpinists and the place of -reappearance of their bodies.] - -[Illustration: FIG. 419.—Lines of flow upon the surface of the -Hintereisferner glacier in the Alps (after Hess).] - -Various separated parts of the body of Captain Arkwright, who had been -lost in 1866 upon the névé of the same glacier, reappeared at its foot -after entombment in the ice for a period of thirty-one years. To-day -the time of reappearance of portions of the bodies of persons lost -upon Mont Blanc is rather accurately predicted, so that friends repair -to Chamonix to await the giving up of its victims by the Glacier des -Bossons. - - -[Illustration: - -FIG. 420.—Lateral and medial moraines of the _Mer de glace_ and its -tributary ice streams.] - -=The moraines.=—The horns and comb ridges which rise above the glacier -surface are continually subject to frost weathering, and from time to -time drop their separated fragments upon the glacier. Falling as these -do from considerable heights, they reach the ice under a high velocity, -and rebounding, sometimes travel well out upon its surface before -coming to a temporary rest. Upon a fresh snow surface of the névé -their tracks may sometimes be followed with the eye for considerable -distances, and their fall is a constant menace to Alpine climbers. -Below the névé the larger number of such fragments remain near the -cliff, and the lines of flow of the ice within the glacier surface are -such that blocks which reach points farther out upon the glacier are -later gathered in beneath the cliff at the side (Fig. 419). The ridge -of angular rock débris which thus forms at the side of the glacier is -called a _lateral moraine_ (see Fig. 411, p. 385, and Fig. 420). - -At the junction of two glacier streams, the lateral moraines are -joined, and there move out upon the ice surface of the resultant -glacier as a _medial moraine_. Thus from the number of medial moraines -upon a glacier surface it is possible to say that the important -tributary glaciers number one more (Fig. 420). - -[Illustration: FIG. 421.—Ideal cross-section of a mountain glacier to -show the position of moraines and other peculiarities characteristic of -the surface of the bed.] - -The plucking and abrading processes in operation beneath the glacier, -quarry the rock upon its bed, and after shaping and smoothing the -separated rock fragments, these are incorporated within the lower -layers of the ice as _englacial_ rock débris. In spaces favorable for -its accumulation, a portion of this material, together with much finer -débris and rock flour, is left behind as a ground moraine upon the bed -of the glacier (see Fig. 421). - -[Illustration: FIG. 422.—Fragments of rock of different sizes, to -bring out their different effects upon the melting of the glacier -surface.] - -At the foot of the glacier the relatively angular rock débris, which -has been carried upon the surface, and the soled and polished englacial -material from near the bottom, are alike deposited in a common marginal -ridge known as the _terminal_ or _end moraine_ (plate 21 B). - - -┌──────────────────────────────────────────────────────────────────────┐ -│ PLATE 21. │ -│ │ -│ [Illustration: _A._ View of the Harvard Glacier, Alaska, showing the │ -│ characteristic terraces (after U. S. Grant).] │ -│ │ -│ [Illustration: _B._ The terminal moraine at the foot of a mountain │ -│ glacier (after George Kinney).] │ -└──────────────────────────────────────────────────────────────────────┘ - -=Selective melting upon the glacier surface.=—The white surface of the -glacier generally reflects a large proportion of the sun’s rays which -reach it, and its more rapid melting is largely accomplished through -the agency of rock fragments spread upon its surface. Such fragments, -however, promote or retard the melting process in inverse proportion to -their size up to a certain limit, and above that size their action is -always to protect the glacier from the sun. This nice adjustment to the -size of the rock fragments will be clear from examination of Fig. 422, -for rock is a poor conductor of heat, and in even the longest summer -day a thin outer layer only is appreciably warmed. Large rock blocks, -grouped in the medial and lateral moraines, hold back the process of -lowering the glacier surface during the summer, so that late in the -season these moraines stand fifty feet or more above the glacier as -armored ice ridges. - -[Illustration: - -FIG. 423.—Small glacier table upon the surface of the Great Aletsch -glacier in 1908.] - -Isolated and large rock slabs, as the season advances, may come to form -the capping of an ice pedestal which they overhang and are known as -_glacier tables_ (Fig. 423). Such tables the sun attacks more upon one -side than upon the other, so that the slab inclines more and more to -the south and may eventually slip down until its edges rest against the -glacier surface. Rounded bowlders, which less frequently become perched -upon ice pedestals, may, from a similar process, slide down upon the -southern side and leave a pyramid of ice furrowed upon this side and -known as an _ice pyramid_. - -Fine dirt when scattered over the glacier surface is, on the other -hand, most effective in lowering its level by melting. Use was made -of this knowledge to lower the great drifts of snow which had to be -removed each season during the construction of the new Bergen railway -of southern Norway. Each dirt particle, being warmed throughout by -the sun’s rays, melts its way rapidly into the glacier surface until -the _dust well_ which it has formed is so deep that the slanting -rays of the sun no longer reach it. When the dirt particles are near -together, the thin walls which separate the dust wells are attacked -from the sides in the warm air of summer days, thus producing from a -patch of dirt upon the glacier surface a _bath tub_ (Fig. 424 _d_). At -night the water which fills these basins is frozen to form a lining -of ice needles projecting inward from the wall, and this, repeated in -succeeding nights, may entirely close the basin with water ice and -produce the familiar _glacier star_ (Fig. 424 _c_). - -[Illustration: - -FIG. 424.—Effects of differential melting and subsequent refreezing -upon the glacier surface. _a_, dust wells; _b_, glacier _tub_ produced -by melting about a group of scattered dust particles; _c_, glacier star -produced when the inclosed water of the glacier well has frozen in -successive nights; _d_, “bath tub.”] - -If the dirt upon the glacier surface, instead of being scattered, is -so disposed as to make a patch completely covering the ice to the -thickness of an inch or more, the effect is altogether different. -Protecting as it now does the ice below, a local ice hillock rises upon -its site as the surrounding surface is lowered, and as this grows in -height its declivities increase and a portion of the dirt slides down -the side. The final product of this shaping is an almost perfectly -conical ice hill encased in dirt and known as a _débris_, _sand_, or -_dirt cone_ (Fig. 425). The novice in glacier study is apt to assume -that these black cones contain only dirt, but is rudely awakened to the -reality when he attempts to kick them to pieces. Both glacier tubs and -débris cones may assume large dimensions; as, for example, in Alaska, -where they may be properly described as lakes and hills. - -[Illustration: FIG. 425.—Dirt cone and one with its casing in part -removed. Victoria glacier (after Sherzer).] - -A patch of hard and dense snow which is less easily melted than that -upon which it rests may lead to the formation of snow cones upon -the glacier surface similar in size and shape to the better known -débris cones. Such cones of snow have, with doubtful propriety, been -designated “penitents”, for it is pretty clear that the interesting -bowed snow figures, which really resemble penitents and which were -first described from the southern Andes under the name of _nieves -penitentes_, are of somewhat different character. - -[Illustration: - -FIG. 426.—Schematic diagram to show the manner of formation of glacier -cornices.] - -One further ice feature shaped by differential melting around rock -particles remains to be mentioned. Wherever the seasonal snowfalls -of the névé are exposed in crevasses, they are generally found to be -separated by layers of dirt, and lines of pebbles similarly separate -those ice layers which are revealed at the foot of the glacier. In -either case, if the sun’s rays can reach these layers in an opened -crevasse, the half-buried rock fragments are warmed by the sun upon -their exposed surfaces and slowly melt their way down the ice surface, -thus removing from it a thin layer of snow or ice and causing that part -above the pebble layer to project like a cornice. This process will go -on until the overhanging cornice protects the pebbles from any further -warming by the sun, but each lower pebble layer that is reached by the -sun will produce an additional cornice, so that the original surface -may at the bottom have been retired by the process a number of inches. -These features are described as _glacier cornices_ (Fig. 426). - - -[Illustration: FIG. 427.—Superglacial stream upon the Great Aletsch -glacier.] - -=Glacier drainage.=—Already in the early morning of every warm summer -day, active melting has begun upon the surface of the Swiss glaciers. -Rills of icy water soon make their way along depressions upon the -surface, and are joined to one another so that they sometimes form -brooks of considerable size (Fig. 427). Such streams continue their -serpentine courses until these are intersected by a crevasse down which -the waters plunge in a whirling vortex which soon develops a vertical -shaft of circular section within the ice. Such shafts with their -descending columns of whirling water are the well-known _moulins_, or -“_mills_”, which may be detected from a distance by their gurgling -sounds. The first plunge of the water may not reach to the bottom of -the glacier, in which case the stream finds a passageway below the -surface but above the floor until another crevasse is encountered and -a new plunge made, here perhaps to the bottom. Once upon the valley -floor the stream is joined by others, and pursues its course within an -ice tunnel of its own making (Fig. 421, p. 394) until it issues at the -glacier front. - -The coarser of the rock débris which was gathered up by the stream -upon the glacier surface is deposited within the tunnel in imperfect -assortment (gravel and sand), while all finer material and that lifted -from the floor (rock flour) is retained in suspension and gives to the -escaping stream its opaque white appearance. This _glacier milk_ may -generally be traced far down the valleys or out upon the foreland, -and is often the traveler’s first indication that a range which he is -approaching supports glaciers. - - -[Illustration: - -FIG. 428.—Ideal form of the surface left on the site of the apron of -a piedmont glacier. _M_, moraine; _T_, outwash; _C_, basin usually -occupied by a lake; _D_, drumlins (after Penck).] - -=Deposits within the vacated valley.=—For every excavation of the -higher portions of the upland through glacial sculpture, there is a -corresponding deposit of the excavated materials in lower levels. So -far as these materials are deposited directly by the ice, they form -the lateral, medial, ground, and terminal moraines already described. -A considerable proportion of them are, however, deposited by the -water outside the terminal moraine; but as with the shrinking glacier -the ice front retires in halting movements over the area earlier -ice-covered, the terminal moraines are ranged along the vacated valley -as _recessional moraines_, each with a _valley train_ of outwash below. -About the apron of the piedmont glacier, such deposits are particularly -heavy (Fig. 428). During the “ice age” the Swiss glaciers extended down -the valleys below the existing ice remnants and spread upon the Swiss -foreland as great piedmont glaciers such as may now be seen in Alaska. -To-day we find there moraines and glacial outwash, a lake in the middle -of the apron site, and sometimes a group of radiating drumlins like -those found within the ice lobes of the continental glacier in southern -Wisconsin (Fig. 429, and Fig. 344, p. 317). - -[Illustration: - -FIG. 429.—Moraines and drumlins about Lake Constance upon the site of -the earlier piedmont glacier of the Upper Rhine. The white area outside -the outermost moraine is buried in glacial outwash (after Penck and -Brückner).] - -Behind the recessional moraines within the glaciated valley are found -the valley moraine lakes (Fig. 448, p. 413), in association with the -rock basin lakes due to glacial sculpture (Fig. 447, p. 412). After -the glacier has vacated its valley, the precipitous side walls become -the prey of frostwork and are the scenes of disastrous avalanches or -landslides. Within the cirques, drifts of snow are nourished long after -the ice has disappeared, and as a consequence the amphitheater walls -succumb to the process of solifluxion (p. 153). - -Diversions and reversals of drainage, which are so characteristic of -the work of continental glaciers, are hardly less common to glaciated -mountain districts. Many of our most beautiful waterfalls have resulted -from either the temporary or permanent obstruction of earlier valleys -above the falls. The famous Yosemite Falls offers an interesting -illustration of the shifting of an earlier waterfall, itself no doubt -due to ice blocking in a still earlier glaciation (plate 22 B). - - -=Marks of the earlier occupation of mountains by glaciers.=—It is -well that we should now bring together within a small compass those -evidences by which the existence of earlier mountain glaciers may be -proven in any district. These marks are so deeply stamped upon the -landscape that no one need err in their interpretation. - - -MARKS OF MOUNTAIN GLACIERS - - _High-level sculpture._ The grooved upland with its cirques, or the - fretted upland with its cirques, cols, horns, and comb ridges. - - _Low-level sculpture._ The U-shaped main valley, the hanging side - valleys with their ribbon falls, the glacier staircase with its rock - bars and gorges, the rounded, polished, and striated rock floor. - - _Deposits._ The recessional moraines of till and the valley trains of - sand and gravel, the soled erratic blocks derived always from higher - levels of the valley. - - _Lakes._ The valley moraine lakes and the chains of rock basin lakes. - - -READING REFERENCES FOR CHAPTER XXVIII - - Glacier movement:— - - L. AGASSIZ. Nouvelles Études et Expériences sur les Glaciers Actuels, - etc., Paris, 1847, pp. 435-539. - - H. HESS. Die Gletscher, Braunschweig, 1904, pp. 115-150. - - H. F. REID. The Mechanics of Glaciers, Jour. Geol., vol. 4, 1896, pp. - 912-928; Glacier Bay and Its Glaciers, 16th Ann. Rept. U. S Geol. - Surv., Pt. i, 1898, pp. 445-448. - -┌─────────────────────────────────────────────────────────────────────┐ -│ PLATE 22. │ -│ │ -│ [Illustration: _A._ Model of the vicinity of Chicago, showing the │ -│ position of the ancient beaches and the outlet of the former Lake │ -│ Chicago.] │ -│ │ -│ [Illustration: _B._ Map of Yosemite Falls and its earlier site near │ -│ Eagle Peak (after F. E. Matthes).] │ -└─────────────────────────────────────────────────────────────────────┘ - - - - -CHAPTER XXIX - -A STUDY OF LAKE BASINS - - -=Freshwater and saline lakes.=—Lakes require for their existence a -basin within which water may be impounded, and a supply of water more -than sufficient to meet the losses from seepage and evaporation. If -there is a surplus beyond what is needed to meet these losses, lakes -have outlets and remain fresh; their content of mineral matter is -then too slight to be detected by the palate. If, on the other hand, -supply is insufficient for overflow, continued evaporation results in a -concentration of the mineral content of the water, subject as it is to -continual augmentation from the inflowing streams. - -As we have seen, there are in areas of small rainfall special -weathering processes which tend to bring out the salts from the -interior of rock masses, these concentrated salts generally first -appearing as a surface efflorescence which is ultimately transferred -through the agency of wind and cloudburst to the characteristically -saline desert lakes. - -Lake basins may be formed in many ways. Depressions of the land surface -may result from tectonic movements of the crust; they may be formed by -excavating processes; but in by far the greater number of instances -they result from the obstruction in some manner of valleys which were -before characterized by uniformly forward grades. In relatively few -cases loose materials are heaped up in such a manner as to produce -fairly symmetrical basins. - - -[Illustration: - -FIG. 430.—Map and diagram to bring out the characteristics of newland -lakes.] - -=Newland lakes.=—On land recently elevated from the sea, basins of -lakes may be merely the inherited slight irregularities of the earlier -sea floor, in which case they may be assumed to be largely the result -of an irregular distribution of deposits derived from the land. Lakes -of this type are especially well exhibited in Florida, and are known as -newland lakes (Fig. 430). Such lakes are exceptionally shallow, and are -apt to have irregular outlines and extremely low banks. Under these -circumstances, they are soon filled with a rank growth of vegetation, -so that it is sometimes difficult to properly distinguish lake and -marsh. - - -[Illustration: FIG. 431.—View of the Warner Lakes, Oregon (after -Russell).] - -[Illustration: FIG. 432.—Schematic diagrams to illustrate the -characteristics of basin-range lakes.] - -=Basin-range lakes.=—Newland lakes may be said to have their origin in -an uplift of the land and sea floor near their common margin. A lake -type dependent upon movements of the earth’s crust but within interior -areas has been described as the basin-range type and is exemplified -by the Warner Lakes of Oregon. In this district great rectangular -blocks of the earth’s crust, which in their upper portions at least -are composed of basaltic lavas, have undergone vertical adjustments -in level and have been tilted so that the corresponding corners of -neighboring blocks have been given a similar degree of down-tilt (Fig. -431). Lakes formed in this way are of triangular outline, are bounded -on the two shorter sides by cliffs, but have extremely flat shores on -their longest side. From this shore the water increases gradually in -depth and attains a maximum depth at or near the opposite angle. Such -lakes naturally betray a tendency to appear in series (Fig. 432), and -are unfortunately much too often illustrated on a small scale after a -shower by the tilted blocks of imperfectly made cement sidewalks. - -[Illustration: - -FIG. 433.—Schematic diagrams of rift-valley lakes, and the rift valley -of the Jordan with the Dead Sea and the Sea of Galilee as remnants of a -larger lake in which their basins were included.] - - -=Rift-valley lakes.=—Another type of lake basin which has its origin -in faulted block movements is known as the rift-valley lake, and is -best exemplified by the great lakes of east Central Africa. In this -type a strip of crust, many times as long as it is wide, has been -relatively sunk between the blocks on either side so as to produce a -deep rift, or what in Germany is known as a _Graben_ (trench). Such -a basin when occupied by water yields a lake which is long, straight, -deep, and narrow, and is in addition bounded on the sides by steep rock -cliffs. At the ends the shores are generally by contrast decidedly -low. If the hard rock at the bottom of the lake could be examined, it -would be found to be of the same type as that exposed near the top of -the side cliffs. The valley of the Jordan in Palestine is a rift of -this character and was at one time occupied by a long and narrow lake -of which the Dead Sea and the Sea of Galilee are the existing remnants -(Fig. 433). - -[Illustration: - -FIG. 434.—Map showing the rift valley lakes of east Central Africa.] - -One of the most striking examples of a rift valley lake is Lake -Tanganyika, while Albert Nyanza, Nyassa, and Rudolf in the same region -are similar (Fig. 434). - -[Illustration: - -FIG. 435.—Earthquake lakes which were formed in the flood plain of the -lower Mississippi during the earthquake of 1811 (after Humphreys).] - - -=Earthquake lakes.=—The complex adjustments in level of the surface -of the ground at the time of sensible earthquakes are many of them -made apparent in no other way than by the derangements of the surface -water. This is at such times impounded either in pools or in broad -lakes, which inasmuch as they date from known earthquakes have been -called “earthquake lakes”, even though in a strict sense any lake -which has originated in earth movements might properly be regarded as -an earthquake lake. To avoid unnecessary confusion, the term must, -however, be restricted to those lakes which are known to have been -formed at the time of definite earthquakes (Fig. 435). Reelfoot Lake -in Tennessee, which in late years has acquired undesirable notoriety -because of the feuds between the fishermen of the district and the -constituted authorities, is a lake more than twenty miles across -and came into existence during the great earthquake of the lower -Mississippi valley in 1811. - - -=Crater lakes.=—The craters of volcanic mountains are natural basins -in which surface waters are certain to be collected, provided only -the supply is sufficient and seepage into the loose materials is -not excessive. Some craters, still visibly more or less active, are -occupied by lakes (Fig. 436). - -[Illustration: - -FIG. 436.—View of lake in Poas Crater in Costa Rica, a volcanic crater -more than half a mile across and with walls 800 feet deep. At intervals -there is an ejection of steam mixed with mud and ash after the manner -of a geyser (after H. Pittier).] - -In the larger number of cases in which craters become occupied by -lakes, the evidence of continued activity is lacking, and it would -appear in such cases that the lava of the chimney had consolidated into -a volcanic plug, closing the bottom of the crater. Notable groups of -crater lakes are the _Caldera_ of the Roman Campagna (Fig. 437) and -the so-called _maare_ of the Eifel about the Lower Rhine. Crater lakes -are easy to recognize by their circular plan, their steep walls of -volcanic materials, and their considerable depth with a maximum near -the center. - -One of the most remarkable of these water-filled basins is Crater Lake -in Oregon, which has a diameter of about six miles and is believed to -have resulted from the incaving of a great volcanic cone in the latest -stage of its activity. This remarkable feature has now been made a -national park and will soon be conveniently reached by tourists and -counted one of the greatest nature wonders of the Pacific slope. - -[Illustration: - -FIG. 437.—Diagrams to illustrate the characteristics of crater lakes. -The Roman Campagna is a plain formed of volcanic ash, with the crater -lakes of Bracciano, Vico, and Bolseno arranged on a line traversing it.] - - -=Coulée lakes.=—Far more important as lakes are those volcanic basins -which arise from the flow of a stream of lava across the valley of a -river so as to impound its waters (Fig. 438). - -At the time of the great eruption under Skaptár Jökull in 1783 the -river Skaptár and many of its tributaries were blocked by the flow of -lava, which it is estimated exceeded in bulk the mass of Mont Blanc. - -[Illustration: - -FIG. 438.—View of Snag Lake, a _coulée_ lake with lava dam shown in -middle distance (after Fairbanks).] - - -=Morainal lakes.=—As we have learned, the obstruction of drainage, -due to the distribution of rock débris by continental glaciers, has -yielded lakes in almost countless numbers. Probably ninety per cent -or more of the known lakes have had this origin, and the type is so -common within the once glaciated regions that it forms perhaps the -best distinguishing mark of former glaciation. The hummocky surface -of morainal deposits is so characteristic that the lakes of this type -are never very large and are correspondingly irregular in outline. -They have often numerous islands, and their banks are formed of the -combination of rock flour and ice-worn materials known as till (Fig. -439). The smallest of the morainal lakes are mere kettles on the -marginal moraine, and these rapidly become replaced by peat bogs. In -contrast with pit lakes, morainal lakes lack the steep surrounding -slopes and the encircling plain. - -[Illustration: FIG. 439.—Diagrams to illustrate the characteristics -of morainal lakes, and a sample map of such lakes from the glaciated -region of North America.] - - -=Pit lakes.=—The so-called pit lakes have their origin in continental -glaciation, and are found in groups within broad plains of glacial -outwash (mainly sand and gravel), which are for this reason described -as “pitted plains” (see p. 314). Those areas which lay between -neighboring lobes of the ice sheet were subject to particularly heavy -deposits of outwash material, and are, in consequence, particularly -likely to be occupied by pit lakes. As has been pointed out in an -earlier section, the water derived from surface melting within the -marginal portions of a continental glacier descends to the bottom -in the crevasses and thereafter flows in an ice tunnel under the -same conditions as water flowing in a pipe. Having in most cases a -considerable head at the outer margin of the ice, this water may rise -and issue well above the lower ice layers and so cover a portion of the -ice margin beneath sand and gravel (Fig. 440). Separated blocks, often -of massive proportions, are thus buried beneath nonconducting materials -by which they are long protected from further melting. Eventually, -however, with the approach of still milder climates they disappear, -thus causing the overlying sand and gravel to descend and form a pit of -steep walls similar to the sawdust pits over melted ice blocks within -our storehouses. - -[Illustration: FIG. 440.—Diagram to show the manner of formation of -pit lakes.] - -Pit lakes are thus easily recognized by their occurrence usually in -groups within a plain of glacial outwash and by their characteristic -banks inclined at the angle of repose of such materials (Fig. 441). - -[Illustration: FIG. 441.—Diagrams to illustrate the characteristics of -pit lakes and a sample map from the glaciated region of North America.] - - -=Glint or colk lakes.=—It has been found to be true of existing -continental glaciers that where their mass has been held back by -a mountain wall, their current at the portals within this rampart -becomes greatly accelerated. Though the upper layers of the glacier -in the vicinity may move forward with a velocity of but an inch per -day, the current within the outlet may be as much as seven hundred -or a thousand times as great. In many respects these conditions are -similar to those about the raceway of a reservoir where the near-by -surface of the water is lowered by the indraught of the outlet and the -current in the raceway is so accelerated that, unless protected, the -bottom of the race is carried away and a basin excavated which extends -a short distance both above and below the position of the dam. In -Holland such basins hollowed out beneath breaks in the dykes are known -as colks. Basins which were excavated beneath the glacier outlets by -a similar process would not be open to our inspection until after the -ice had disappeared from the region; but it is most significant that -in Scandinavia, where the Pleistocene continental glacier, advancing -westward from the Baltic, was held in check by the escarpment at the -Norwegian boundary (the _glint_), lake basins have been excavated -in hard rock whose walls show the abrading and polishing which are -characteristic of glacial sculpture, and whose positions are such that -they lie beneath the former outlets partly above and in part below the -line of the escarpment. Their position in reference to the rampart and -to the former outlets is brought out in Fig. 442. The largest of the -glint lakes of this series is Torneträsk in northern Lapland (see p. -277 and Fig. 443). - -[Illustration: - -FIG. 442.—Diagram to show the manner of formation of glint or outlet -lakes where the continental glacier of Scandinavia issued from the -Baltic depression through portals in its mountain rampart.] - -[Illustration: - -FIG. 443.—Map showing a series of glint lakes which lie across the -international boundary of Sweden and Norway.] - - -=Ice-dam lakes.=—Whenever a continental glacier, either in advancing -its front or in retiring, lies across the lines of drainage upon their -downstream side, water is impounded along the ice front so as to form -ice-dam lakes. Such lakes are found to-day in Greenland and in the -southern Andes, and similar bodies of water of far greater size and -importance came into existence in Pleistocene times each time that the -continental glaciers of northern North America and Europe advanced -upon or retired from suitably directed river systems. Thus above the -Baltic depression, when the ice front lay to the eastward of the main -watershed, each easterly sloping valley was obstructed by the ice and -occupied by an ice-dam lake (Fig. 444), the beaches of which may all be -traced to-day (Fig. 445). - -[Illustration: - -FIG. 444.—Ice-dam lakes (in black) between the front of the late -Pleistocene glacier of northern Europe and the divide near the -Norwegian boundary (after G. de Geer).] - -One side of each ice-dam lake is formed by an ice cliff at the glacier -front, and if the region is relatively flat, the remaining shores -are likely to be formed by a marginal moraine which the glacier has -abandoned in its retreat. In their smaller stages, therefore, ice-dam -lakes on prairie country have the form of a crescent, which is the more -pronounced because the waves by their attack upon the ice front flatten -the curvature of its outline (see Fig. 360, p. 330). - -The life of an ice-dam lake is begun and ended in important changes -of glacier outline, and after the draining of lakes by this process -the land shores may be traced in beaches, and the ice margin by a -water-laid moraine of low relief (Fig. 359, p. 330). - -A much smaller but in many respects similar ice-dam lake is to-day to -be seen at the side of the Great Aletsch glacier, a mountain glacier of -Switzerland. The traveler who makes the easy ascent of the Eggishorn -may look directly down upon this crescent-shaped lake with its ice -cliff on the glacier side (see Fig. 446). - -[Illustration: - -FIG. 445.—Wave-cut terrace at an elevation of 177.5 meters above sea -on the southern slope of the northern Dala valley north of Baggedalen -in Sweden. To the right in the foreground is a peat bog (after -Munthe).] - -[Illustration: - -FIG. 446.—View of the Márjelen Lake at the side of the Great Aletsch -glacier, seen looking directly down from the summit of the Eggishorn -(after a photograph by I. D. Scott).] - - -=Glacier lobe lakes.=—Upon the sites of the former lobes of the -Pleistocene glacier of North America are found the basins of the -Laurentian River system, the largest freshwater lakes in the world. -There has been much controversy concerning the manner of formation -of these lakes, but the view which has seemed to have the largest -following is that they were excavated by the eroding action of the -continental glacier over the drainage basins of former rivers. It is -but one phase of the long controversy between opposing schools, which -have advocated on the one hand the efficiency of glacier ice as an -eroding agent, and upon the other its supposed protection from the -weathering processes. The positions and the outlines of the several -lakes of the series sufficiently proclaim their connection with the -former glacial lobes, and the name which we have adopted leaves the -exact manner of their formation a still open question. The recognition -of the importance of the glacial anticyclone, in giving shape to the -glacier surface and in effecting a transfer of snow from the central to -the marginal portions, has had the effect of emphasizing the relative -importance of erosion under the marginal and lobate portions. Thus -the importance of ice lobes has been greatly accentuated, though this -applies only to the shaping of the basins and not in any important -way to the impounding of the present waters. The present Laurentian -Lakes owe their existence to the elevation by successive uplifts of -the country to the northward and eastward, since the glacier retired -from the lake region. When the ice front lay to the northward of the -Ottawa River, the discharge of the upper lakes was by a channel through -Nipissing River and Lake and thence down the Ottawa River to a gulf in -the lower St. Lawrence. The uplift of the land has had the effect of -raising a barrier where the former outlet existed, and diverting the -waters to a roundabout channel by way of Detroit and Lake Erie (see -Fig. 365, p. 335). - - -[Illustration: - -FIG. 447.—Diagrams to illustrate the arrangement and the characters of -rock-basin lakes, together with a map of such lakes from the Bighorn -Mountains in Wyoming.] - -=Rock-basin lakes.=—The reversed grades which develop in a valley -deepened by mountain glaciers—the back-tilted treads of the cascade -stairway (see p. 376)—furnish a series of basins hollowed in rock which -are strung along the course of the valley like pearls upon a thread, -or, far better, like the larger beads in a rosary (Fig. 447). This -characteristic arrangement accounts for the name “Paternoster Lakes” -which has sometimes been applied to them in Europe. Their positions in -series within U-shaped mountain valleys, and their rock shores with -characteristically smoothed and striated surfaces, make them easy of -determination. In the higher portions of the valley, where the treads -of the cascade stairway are relatively narrow, such lakes are often -approximately circular in outline, but in the lower levels and upon -wider treads they may be ribbon-like, though lakes of this type are to -a large extent replaced in the lower levels by the valley moraine type -or a combination of the two. - - -[Illustration: FIG. 448.—Convict Lake, a lake behind a moraine dam -within a glaciated valley of the Sierra Nevadas, California (after a -photograph by Fairbanks).] - -=Valley moraine lakes.=—The recessional moraines which mark the halting -stations of mountain glaciers, while retiring up their valleys, -form dams in the later river and so produce a type of lake which is -in contrast with the morainal lakes which result from continental -glaciation. They may, therefore, be distinguished by the name _valley -moraine lakes_. Their positions on the bed of a U-shaped mountain -valley, and the glacial materials which compose the dams, are -sufficient for their identification (Fig. 448). Moraine Lake and Lake -Louise in the Canadian Rockies are typical examples. Rock basin and -valley moraine lakes may occur in alternation or combined in mountain -valleys. - - -[Illustration: - -FIG. 449.—Lake basins produced by successive slides from the steep -walls of a glaciated mountain valley (after Russell).] - -=Landslide lakes.=—The sheer-walled valleys which are carved by -mountain glaciers are too steep to long retain their perpendicularity -when the support of the glacier has been removed. Aided by the ever -present joint planes, which admit water to the rock, they succumb to -frost action, and further give way in avalanches whenever the rock -is of sufficiently porous material to become saturated with water. -Landslides sometimes occur successively until the original valley wall -has been replaced by a terraced slope. The treads of the steps in -this terrace have generally a backward-sloping grade, so that basins -are formed to be filled by relatively long and narrow lakes or by -successions of small pools (Fig. 449 and plate 23 B). - -[Illustration: - -FIG. 450.—Lake Garda, a border lake upon the site of a piedmont apron -at the margin of the Alpine highland (after Penck and Brückner).] - -When the avalanched material is so disposed as to dam the valley, much -larger lakes of this type come into existence. During an earthquake -which occurred on January 25, 1348, there was a landslide within the -valley of the Gail, Carinthia, which destroyed seventeen villages and -produced a lake which even to-day is represented by a great marsh. - - -=Border lakes.=—Whenever mountain glaciers push out their fronts -beyond the borders of the mountain range by which they are nourished, -they spread upon the foreland in broad aprons about which morainic -accumulations are particularly heavy. This elevation of morainal -walls about the margins of the aprons yields natural basins that are -occupied by lakes so soon as the glacier retires its front within the -valley. Because such lakes are found at the borders of upland districts -they have been called _border lakes_. The beautiful Lakes Constance, -Lucerne, Maggiore, Lugano, Como, and Garda (Fig. 450), on the borders -of the Alpine highland, are all of this type. - -┌───────────────────────────────────────────────────────────────────┐ -│ PLATE 23. │ -│ │ -│ [Illustration: _A._ View of the American Fall at Niagara, showing │ -│ the accumulation of rocks beneath (after Grabau).] │ -│ │ -│ [Illustration: _B._ Crystal Lake, a landslide lake in Colorado. │ -│ (_Photograph by Howland Bancroft._)] │ -└───────────────────────────────────────────────────────────────────┘ - - -=Ox-bow lakes.=—The cutting off of a meander within the flood plain of -a river yields a lake which is of horseshoe (ox-bow) outline and lies -generally with low banks within a plain composed of river silt. Before -separating from the parent stream the meander had begun to silt up, -especially at the ends. Ox-bow lakes are, however, relatively deep near -the convex shore and correspondingly shallow toward the concave margin -(Fig. 451). - -[Illustration: FIG. 451.—Diagrams to bring out the characteristics of -ox-bow lakes, together with a map of such lakes from the flood plain of -the Arkansas River.] - -[Illustration: - -FIG. 452.—Diagrammatic section to illustrate the formation of -saucer-like basins between the levees of streams flowing in a flood -plain.] - - -=Saucer lakes.=—As we have learned, a river meandering in its flood -plain has banks which are higher than the average level of the plain, -for the reason that at flood time the main current of the stream still -persists in the channel, thus allowing the burden of sediment to be -dropped in the relatively slack water upon its margin. Because of these -natural embankments or levees, tributary streams are often compelled -to flow long distances in nearly parallel direction before effecting -a junction. Between the trunk stream and its tributaries, likewise -bounded by levees, and between streams and the valley walls, there thus -exist low basins which are more or less saucer-shaped (Fig. 452). At -flood time, when the levees are overflowed or crevassed, water enters -these depressions, and an additional supply may be derived from the -walls of the valley. Good illustrations of such lakes are furnished -by the flood plain of the former river Warren near the banks of the -present Minnesota River (Fig. 453). - -[Illustration: FIG. 453.—Saucer lakes upon the bed of the former river -Warren (from the Minneapolis sheet, U. S. G. S.).] - - -=Crescentic levee lakes.=—As we approach the delta of a river, the -size and importance of the levee increases, and here a new type of -levee lake may develop in series (Fig. 454). At flood time the levee -is breached near the point of sharpest curvature on the convex side -(Fig. 454 _a_). When the waters are subsiding, the current is kept -away from the old channel by the rising grade of the levee as well as -by the inertia of the current, and an entrance to the old channel is -first found below the next change in curvature of the meander, since -here scour becomes effective in cutting through the levee. The new -channel is thus established in the form of a loop inclosing the old -one, and the process of levee building now erects a wall about the -territory newly acquired by the meander. This territory has the form -of a crescent, and when occupied by water produces a crescentic levee -lake often joined to its neighbors in series. The abandoned channel now -closed at both ends by levees may be occupied by water to produce a -subordinate ribbon type of curving trench (Fig. 454 _b_, _c_). - -The importance of levees in obstructing drainage to form lakes is only -beginning to be appreciated. It has quite recently been shown that -when trunk streams are greatly swollen and burdened with sediment -while flowing from a receding continental glacier, they may build such -high levees as to aggrade their tributary streams above the junctions, -even producing reversed grades and so impounding the waters to form -extensive lakes. During the “ice age” lakes of this type were formed -in Illinois and Kentucky rivers just above their junctions with the -Ohio. The old lake floor with its eastern shore line and its protruding -islands is easily made out upon the new topographic maps of Kentucky. - -[Illustration: - -FIG. 454.—Levee lakes developed concentrically in series within -meanders of a stream tributary to the Mississippi and flowing upon its -delta plain. _b_ and _c_ are examples of the ribbon type of levee lake -due to occupation of the abandoned river channel. The larger number of -lakes, of which Sip Lake and Texas Lake are examples, have the form of -crescents and lie between abandoned levees (from recent map of U. S. G. -S.).] - - -=Raft lakes.=—Within humid regions the flood plains of our larger -rivers are generally forested, and as the river swings from side to -side in its perpetual meanderings, the timber which grows upon the -convex side of each meander is progressively undermined by the river -and felled upon its bank. The prostrate trees remain upon the banks -during the low water of the summer season, to be gathered up at the -time of flood in the next spring season. It is log jams thus acquired -which so generally block the main channel of a river and turn the -current across the neck of the meander when cut-offs occur with the -formation of ox-bow lakes. When the mass of timber thus gathered up -by the river is excessive, as, for example, within the flood plain of -the Red River of Arkansas and Louisiana, huge log rafts are produced -which dam up the river so effectively as to produce temporary lakes. -The impounded waters soon find an outlet over the levee at some point -higher up the river, and the waters flowing off through the timbered -bottom lands, other logs are caught by the standing timber as in a -weir. A second dam is thus formed which is separated from the initial -one by open water, and in this way the driftwood dam acquires enormous -proportions as it gradually moves up the river. After a period of -perhaps a century or more, the lower sections of the jam become decayed -and dislodged so as to float down the river. - -[Illustration: - -FIG. 455.—Raft lakes along the banks of the Red River in Arkansas and -Louisiana at their fullest recorded development (after A. C. Veatch, U. -S. G. S.).] - -In the lower Red River a great raft of alternating jams and open water -reached a length of about one hundred and sixty miles and moved up -the river at the average rate of something less than a mile per year. -Within the limits of the dam all tributary streams were blocked, so -that secondary lakes were formed in a double fringe about the main -river (Fig. 455). The great raft which formed here in the latter part -of the fifteenth century has now at the beginning of the twentieth -been largely removed and measures have been adopted to prevent its -re-formation. - - -[Illustration: - -FIG. 456.—The Swiss lakes Thun and Brienz, formed by deltas at the -junction of streams tributary to a steep-walled valley.] - -=Side-delta lakes.=—It is characteristic of river drainage that the -tributary streams enter the main valley on steeper gradients than the -trunk stream at the point of junction. Wherever the difference in -velocity of the two streams at the junction is large, and the side -stream is charged with sediment, a delta will be formed at the mouth -of the tributary stream. Such deltas push out from the shore and -may eventually block the main channel so as to form a more or less -sausage-shaped expansion of the river—a side-delta lake. Traverse and -Big Stone Lakes in the valley of the Warren River in Minnesota have -been formed in this way (Fig. 354, p. 326). Lakes Thun and Brienz in -the Swiss Alps are of similar origin, the beautiful city of Interlaken -being built upon the delta plain over the valley of the earlier river -(Fig. 456). The Mississippi has similarly been expanded to form Lake -Pepin above the delta at the mouth of the Chippewa River. - - -[Illustration: - -FIG. 457.—Delta lakes formed at the mouth of the Mississippi through -the junction of the levees of radiating distributaries with the shore -of the estuary (after Berghaus).] - -=Delta lakes.=—A somewhat different type of delta lake has been -formed in Louisiana, where the “father of waters” discharges into the -gulf. Here the various distributaries radiate from the main channel -to produce the “bird-foot” delta type and the toes in this foot by -their junction with the banks which outline the ancient estuary, have -separated in succession a series of basins that before were in direct -connection with the sea (Fig. 457). Lake Pontchartrain is the largest -of this series, while the so-called Lake Borgne is in process of -separation. - -Where large deltas push out from the shore into the open sea, the -levees which border the individual distributaries are attacked by the -waves and their materials are transported by the shore currents and -built into barriers. These barriers cut off the re-entrants between -neighboring distributaries so as to produce lagoons or lakes (Fig. 458). - -[Illustration: - -FIG. 458.—A type of delta lakes formed by levees in part destroyed -and built into barriers on the margin of the delta of the Nile (after -Supan).] - -A type of delta lake, which more resembles the side-delta lake above -described, has formed at the mouth of the Colorado River, where it -enters the Gulf of Lower California. The Imperial Valley lying to the -north of this delta is the desiccated floor of the earlier Gulf of -Lower California which has been captured from the sea by the delta -of the Colorado. The rampart of mountains, by which this valley is -surrounded, has cut it off from any water supply derived from clouds, -and its waters being no longer renewed from the sea, the region has -passed through a period of desiccation which has left the Salton -Sink as the only existing remnant of the earlier lagoon. It will be -remembered that careless operations in diverting distributaries of the -Colorado recently reversed this process so that the waters rose in the -valley, and expensive emergency operations were necessary in order to -again turn the waters of the Colorado into their accustomed channels. - -[Illustration: FIG. 459.—Diagrams to illustrate the characteristics of -barrier lakes, with an example from the southern coast of the Island of -Nantucket.] - - -=Barrier lakes.=—The Salton Sink illustrates a type of lake which is -formed at the border of the sea through the erection of some kind of -barrier which captures a small area of the ocean’s surface. Though such -lakes may be properly described as strand lakes, it is usually at the -mouth of a river that the process becomes effective. The common type -of _barrier lakes_ is found developed on most ragged coast lines where -the shore currents have formed first bars and later barriers at the -mouths of the estuaries (Fig. 459). Such embankments are usually gently -curving or crescent shaped and are composed of sand or shingle which -presents a steep landward and a gradual seaward slope. - - -[Illustration: - -FIG. 460.—Dune lakes on the coast of France (after Berghaus).] - -=Dune lakes.=—Within the narrow strips of shore in which all the fine -soil that could be available for plant life has been washed away by -the waves, beach sand is exposed to the direct action of the winds. In -time of storm the sand is picked up and after drifting in the wind is -collected in long ridges parallel to the shore. Constantly traveling -along shore, these dunes block the mouths of rivers and thus produce a -series of lakes such as are indicated in Fig. 460. - - -=Sink lakes.=—Another class of lakes are due either directly or -indirectly to the work of underground waters. In districts which are -underlain by limestone, the surface water descending along the joints -of the limestone may widen these passageways through solution of the -rock and at lower levels flow on the floors of caverns eaten out by the -same process on bedding planes of the formation. At the intersections -of joints, more or less circular shafts known as “swallow-holes” go -down to the caves from the surface. Locally, also the cavern roofs -give way so as to choke the galleries with rubble and leave a basin at -the surface which has an irregular but generally a more or less oval -outline. If sufficiently clogged at the bottom by finer rock débris, -these basins become occupied by small lakes which are known as sinks, -and constitute one of the best surface indications of a limestone -country. - - -[Illustration: FIG. 461.—Sink lakes in Florida, with a schematic -diagram to illustrate the manner of their formation (map from U. S. G. -S.).] - -=Karst lakes—poljen.=—In the limestone country to the north and -east of the Adriatic Sea—the so-called Karst region—there are many -interesting features which are directly traceable to the solution -of the country rock. Here all the surface water descends in certain -districts along the widened joint planes so that the drainage is -largely subterranean. The so-called _dolines_ or sinks of very regular -and symmetrical forms resembling deep bowls cover a large part of the -surface. - -The entire country is, moreover, faulted in the most intricate fashion -into many rift valleys. The drainage being so largely subterranean, -these downthrown blocks of crust, the so-called _poljen_, become -flooded at certain seasons of the year when the subterranean passages -become choked or are too small to carry away all the water. A seasonal -lake of this character is the Zirknitz Lake (p. 189). - - -=Playa lakes.=—It is the law of the desert that the arid region be -walled in by mountains. This encircling rampart forces the clouds to -rise, and by robbing them of their moisture leaves the desert dry and -barren. Those waters which fall upon the inner margin of the ranges -drain toward the interior of this pan-like depression and are not -returned to the sea—the desert is without an outlet. Infrequent though -they be, the desert rains are of the cloudburst type and in the hills -develop torrents whose waters, emerging upon the desert floor, develop -lakes in the space of a few minutes or at most hours. In the hot and -dry atmosphere the waters of these shallow basins may be sucked up in -the space of a few hours but reappear in the same basins at the time -of the next succeeding cloudburst. Such ephemeral lakes are known as -playas. - - -=Salines.=—Desert lakes more favored in their supply of water may be -relatively long lived and persist for periods measured in years or -centuries. Such lakes are, however, extremely sensitive to climatic -changes (see p. 198). - -For the reason that they have no outlet the waters of desert lakes -become salt through continued evaporation. They are, therefore, spoken -of as _salines_. Lake Bonneville, so long as it discharged its waters -over the sill of the Red Rock Pass, must have remained fresh; but when -the level of its waters had fallen below this outlet, its waters became -salt and the content increased as the volume diminished. - -The shallow basins upon the floors of desert lakes may have come into -existence in various ways; but it would appear that the irregular -removal of the soil by the winds, modified as this is by differences -in composition of the rock materials and by vegetable growth, and the -deposition of sand by the same agent, are by far the most important. -Many of the types of tectonic and volcanic lakes which have been -described are characteristic of humid and arid regions alike. - - -=Alluvial-dam lakes.=—Within the mountains upon the desert borders, -the alluvial fans which form at the mouths of valleys, because of the -characteristic cloudburst, sometimes obstruct a main valley at the -junction with its tributaries. By this process the waters of the main -river are impounded in essentially the same manner as are the rivers of -humid regions by the deltas of their tributaries. - - -=Résumé.=—The types of lakes which we have now considered are arranged -below in tabular form so as to show their relationship to important -geological processes. While not complete, the list includes the more -important classes, as well as others which, while not of common -occurrence, are yet of interest in giving further illustration to the -processes which have been treated in earlier chapters. - -By giving careful attention to criteria which have been above -suggested, it should be possible in the greater number of instances at -least to determine whether any lake which is visited has had its origin -in one or another of the processes described. - - -CLASSIFICATION OF LAKES - - -_Tectonic Lakes_ _Volcanic Lakes_ - - Newland lakes Crater lakes - Basin-range lakes Coulée lakes - Rift-valley lakes - Earthquake lakes - -_Continental Glaciation Lakes_ _Mountain Glaciation Lakes_ - - Morainal lakes Rock-basin lakes - Pit lakes Valley moraine lakes - Glint or colk lakes Landslide lakes - Ice-dam lakes Border lakes - Glacier-lobe lakes - - -_River Lakes_ _Strand Lakes_ - - Ox-bow lakes Barrier lakes - Saucer lakes Dune lakes - Crescentic levee lakes - Raft lakes - Side-delta lakes - Delta lakes - -_Ground Water Lakes_ _Desert Lakes_ - - Sink lakes Playa lakes - Karst lakes—_poljen_ Salines - Alluvial dam lakes. - - -READING REFERENCES FOR CHAPTER XXIX - - General:— - - I. C. RUSSELL. Lakes of North America. Boston, 1895, pp. 125, pls. 23. - - A. P. BRIGHAM. Lakes, A Study for Teachers, Jour. Sch. Geogr., vol. 1, - 1897, pp. 65-72. - - N. M. FENNEMAN. The Lakes of Southeastern Wisconsin, Bul. 8, Wis. - Geol. and Nat. Hist. Surv., 1902 (Rev. Ed., 1910), pp. 188, pls. 37. - - A. DELEBECQUE. Les Lacs Français (with Atlas). Paris, 1898. (Work - crowned by the Society of Geology of Paris.) - - H. R. MILL. Bathymetrical Survey of the English Lakes, Geogr. Jour., - vol. 6, 1895, pp. 46-73, 135-166. - - A. SUPAN. Grundzüge der Physischen Erdkunde. Leipzig, 1896, pp. - 531-548. - - H. BERGHAUS. Atlas der Hydrographie. Gotha, 1891, pl. 3. - - R. D. SALISBURY. Physiography. 1907, pp. 292-327. - - CHARLES RABOT. Revue de limnologie, La Géographie, Vol. 4, 1901, pp. - 110-119, 172, 189. - - I. C. RUSSELL. A Geological Reconnaissance in Southern Oregon, 4th - Ann. Rept. U. S. Geol. Surv., 1884, pp. 442-447. (Basin range lakes.) - - ED. SUESS. The Face of the Earth, vol. 4, 1909, pp. 268-286. (Rift - valley lakes.) - - J. S. DILLER. Crater Lake, Nat. Geogr. Mag., vol. 8, 1897, pp. 33-48, - pl. 1; Geology of Lassen Peak Quadrangle, California, Geol. Fol. 15, - U. S. Geol. Surv., 1895. (Coulée lakes.) - - N. M. FENNEMAN. Lakes of Southeastern Wisconsin, _l.c._, pp. 4-6. (Pit - lakes.) - - ED. SUESS. The Face of the Earth, vol. 2, 1906, pp. 340-346, pl. 7. - (Glint lakes.) - - I. C. RUSSELL. A Preliminary Paper on the Geology of the Cascade - Mountains in Northern Washington, 20th Ann. Rept. U. S. Geol. Surv. - Pt. ii, 1900, pl. 14. (View of a rock-basin lake.) - - E. W. SHAW. Preliminary Statement concerning a New System of - Quaternary Lakes in the Mississippi Basin, Jour. Geol., 1911, pp. - 481-491. (New type of levee lakes.) - - A. C. VEATCH. Formation and Destruction of the Lakes of the Red River - Valley, Prof. Pap. No. 46, U. S. Geol. Surv., pp. 60-62, pls. 29-33. - (Raft lakes.) - - M. NEUMEYER. Erdgeschichte, vol. 1, pp. 595-596. (Poljen.) - - - - -CHAPTER XXX - -THE EPHEMERAL EXISTENCE OF LAKES - - -=Lakes as settling basins.=—Of all the processes which conspire to -blot out the lakes with which our northern landscapes are dotted, -the one of greatest importance is in most cases a process of filling -by the sediments brought in by tributary streams. The carrying of -sediment in suspension depends, as we know, upon the velocity of the -current, and as this is checked where it reaches the lake margin, all -coarser material is at once deposited to form a delta, while the finer -sediments are held longer in suspension and finally settle in thin -layers over the entire bottom of the lake. Clay deposits surrounded by -coarser sediments are thus characteristic of filled lake basins. - -[Illustration: FIG. 462.—Map of the Arve and the upper Rhone to show -the importance of Lake Geneva as a settling basin of the larger stream.] - -How waters are clarified by their passage through a lake is indicated -by a comparison of a river system such as the St. Lawrence, with -a river like the Missouri and Mississippi. Not only are the lower -stretches of the St. Lawrence in striking contrast with the muddy -floods of the Missouri and Mississippi; but the delta, which is so -remarkable a feature in the Mississippi, has no counterpart in the -northern river. - -The most noteworthy examples of settling are, however, furnished by -the lakes of Switzerland, for the reason that SWISS rivers are heavily -charged with rock flour produced beneath the numerous glaciers at the -valley heads, and, further, because these rivers descend with turbulent -currents to near the borders of the larger lakes. To look out upon the -murky waters of the upper Rhone, where they enter Lake Geneva near -Villeneuve, and then to watch the flood of crystal water which issues -from the lake and passes under the bridge at Geneva, is an object -lesson which no traveling student should miss (Fig. 462). Yet even more -instructive is a visit to the _Bois de la Bâtie_ at the junction of -this clear stream with the Arve, a half hour’s walk only below Geneva. -The waters of the Arve have come on a steep descent directly from the -glaciers of the Mont Blanc district, and as they meet the cleared -waters of the Rhone, they flow beside them down the common valley -without mingling. Dull and opaque, the Arve waters can be discerned for -a long distance as a white belt against the left bank of the river, -sharply defined against the blue reflecting surface of the Rhone waters -(Fig. 463). Upon the banks of the Arve, just above its junction, a -cement manufactory has been established to utilize the clays which are -here deposited. - -[Illustration: - -FIG. 463.—View looking upstream across the opaque waters of the Arve -to the clear reflecting surface of the Rhone. To the right across the -Arve is seen the cement works for recovering the Arve sediments.] - -Wherever lakes are contained in long and narrow valleys, the greater -part of the tributary drainage enters at the upper end, and the delta -which there forms extends from bank to bank. As it continues to advance -into the lake, the earlier water basin is gradually transformed into a -level plain of delta deposit, a feature so common as to be deserving -of a special name. The Scottish lochs, which are lakes of this type, -are each extended in a longer or shorter delta plain described as a -_strath_, and this local term may well be given a general application -(frontispiece). The city of Ithaca, the seat of Cornell University, is -built upon a strath at the head of Lake Cayuga, and numberless Scottish -and Swiss hamlets have been located upon such fertile plains (Fig. 464). - -[Illustration: - -FIG. 464.—The village of Poschiavo in eastern Switzerland, built upon -a strath at the head of Lake Poschiavo.] - - -=Drawing off of water by erosion of outlet.=—Next in importance to -the filling up of lake basins as a factor in their early extinction is -the cutting down of their channels of outflow. Whenever the walls of -the outlet are cut in rock, this draining process is apt to be slow, -for the reason that the outlet stream is of filtered water and so -lacks the necessary cutting tools. By far the larger number of lakes -are, however, held back by dams of loose drift deposits laid down by -the earlier continental glaciers; and so the very clarity of the water -promotes the erosion of the outlet by allowing the stream’s full burden -of sediment to be lifted and then removed from the channel. - - -=The pulling in of headlands and the cutting off of bays.=—The removal -of projecting headlands by wave action, though it increases the area of -the lake, yet it decreases directly the volume of lake water through -formation of the built terrace, and indirectly in far larger measure -through the transformation of bays into quiet lagoons within which the -extinguishing process of peat growth is set in operation. - - -=Lake extinction by peat growth.=—The first condition for the growth -of lake vegetation is quiet water. Within small lakes, such as the -kettle basins upon moraines, aquatic vegetation develops rapidly, -and bogs of peat might almost be included among the most important -distinguishing marks of a glaciated country. Within larger lakes it is -only after barrier beaches have been thrown across the mouths of the -bays to form natural breakwaters for the waves that this process of -lake extinction by peat growth can become effective. - -[Illustration: - -FIG. 465.—View of the floating bog and surrounding zones of vegetation -in a small glacial lake of the Yellowstone National Park (after a -photograph by Fairbanks).] - -Many erroneous notions are still held concerning the prime importance -of sphagnum in peat formation, owing to the peculiar local conditions -under which the early studies were made. Within the glaciated districts -of the United States, the formation of peat involves the successive -growths of a number of zones of vegetation and the formation of a -floating bog which advances into the lake from the shores, followed -in turn by belts of low shrubs, tamaracks, and lastly deciduous trees -(Fig. 465). - -In most cases the first plants to develop in a quiet lake are the water -lilies, though these are sometimes preceded by chara and floating -bladderwort. Next behind the water lilies come the sedges, which form a -mat of floating bog by their grasslike stems sinking down in the water -and being there interwoven with the rhizomes below. This mat of sedge -is often so firm that cattle may advance upon it to the water’s edge, -but it is separated by a layer of water from the bed of growing peat -at the bottom of the lake (Fig. 466). This bed of peat appears to grow -upward toward the surface and become joined to the shore end of the -floating bog by decaying vegetation which is dropped from the bottom of -the mat above. - -[Illustration: FIG. 466.—Diagram to show how small lakes are -transformed into peat bogs (after C. A. Davis).] - -In order behind the floating bog come the advanced plants of the -conifer group, with sphagnum and low shrub here upon a peat base -extending to the lake bottom. Behind the belt of shrubs arise the -tamaracks and spruces, and lastly, toward the shore, come the deciduous -trees and especially poplars, maples, and marginal willows. Upon the -margin of the basin there is usually a low trench, or “fosse”, filled -with water during wet seasons, as a result, no doubt, of seasonal -inwash that does not reach the residual lake toward the center of the -basin. - - -=Extinction of lakes in desert regions.=—In arid regions there are -special causes of lake extinction. Thus the blowing in of sand and dust -carried for long distances in the air, a by no means negligible factor -even in humid regions, here assumes large importance. The now exposed -basins of extinct desert lakes afford the evidence, however, of an even -greater factor of extinction, in climatic change. The clouds, which -at one time found their way into the drainage basin of a lake, may -later through the rise of a mountain barrier be cut off, and so with -reduced water supply a period of lake desiccation is begun. When, in -this process of drying up, the lake level has fallen below that of the -outlet, the saline content of the waters begins to increase, and later -a stage is reached, as in Great Salt Lake, when the sodium salts are -precipitated. When the lake has become extinct, these deposits remain -as a witness to the changed climatic condition. - - -=The rôle of lakes in the economy of nature.=—It is natural, in -considering the extinction of lakes, to give some attention to the -rôle which they play in the economy of nature. That lakes filter the -water of rivers, and prevent the formation of important delta deposits, -has already been noticed. A curious exception to this general rule is -furnished by the great delta at the head of Lake St. Clair, just below -the outlet of Lake Huron. This anomaly is, however, explained by the -peculiar currents of Lake Huron, which are so directed as to sweep the -beach sand into the swift current of the outlet, to be deposited in the -quiet waters of Lake St. Clair (Fig. 467). - -[Illustration: - -FIG. 467.—Map to show anomalous position of the delta in Lake St. -Clair, due to the peculiar currents in Lake Huron (after maps by Cole).] - -As regulators of the flow of rivers, lakes perform an important -function. Such disastrous floods as are characteristic of the spring -season within the basin of the lower Mississippi could not occur in the -lower St. Lawrence, for the reason that the great basins of the lakes -serve as distributing reservoirs. The annual floods, upon which the -agriculture of Egypt depends, are explained by the flood waters from -the high mountains of Abyssinia entering the Nile _below_ the lakes of -its upper basin. - -In one further respect large inland bodies of water have an important -function as regulators. It is the property of water to respond but -slowly to the variations in the quantity of heat which reaches the -earth’s surface from the sun. A larger quantity of heat must be -added to or abstracted from a body of water, in order to change its -temperature by one degree, than would be required for a like change in -the same bulk of earth or rock. Thus bodies of water by more slowly -acquiring the summer’s heat retard the coming spring, and by storing -up this energy and carrying it over into the autumn the warm season is -prolonged and early frosts prevented. The fruit belts about the lower -Great Lakes are thus dependent upon this regulating property of the -lake waters. The discomfort of the long spring of raw weather is thus -compensated by an unusually salubrious harvest season. - - -=Ice ramparts on lake shores.=—Small ridges known as ice ramparts are -formed upon lake shores by the action of lake ice, though subject to -so many qualifying conditions that the range of their occurrence is -somewhat limited. Within districts where a winter ice cover of some -thickness is formed, the shores of lakes are apt to present ridges of -bowlders parallel to and near the water’s edge, and such lakes have -sometimes become known as “wall lakes” (Fig. 468). - -[Illustration: - -FIG. 468.—A bowlder wall upon the shore of a small lake in the -Adirondacks of New York.] - -In many cases these small ridges have been formed at the time of the -spring “break up” of the ice; for the ice cover, when once loosened, -is drifted in great rafts first against one shore, and later, with a -change of wind direction, against another. Under the impact of such -heavy rafts, the half-submerged bowlders near the shore are forced up -the beach until they lie in a ridge or bowlder wall. - -At other times such bowlder walls, and far more interesting ridges as -well, result from a kind of ice shove independent of the wind, but -caused by expansion within the ice itself during a sudden rise of -temperature of the surrounding air. Such ice ramparts require for their -explanation a consideration of the sequence of events from the time the -ice cover closes the lakes. - -[Illustration: FIG. 469.—Diagrams to show the effect of ice shove in -producing ice ramparts upon the shores of lakes (after Buckley with a -slight modification).] - -The first lake ice of early winter forms in most cases with air -temperatures a few degrees only below the freezing point of the water. -When later a severe “cold wave” arrives, the ice cover is contracted -and becomes too small for the lake surface. To this contraction it -yields and opens cracks up which the water rises, and in the prevailing -low temperature this water is quickly frozen and the lake cover again -made complete. Skaters are familiar with the opening of these cracks -and the loud “roaring” which accompanies it on cold mornings, the sharp -skate runners sometimes starting a crack in the strained ice, as does a -light scratch upon glass that is in a similar strained condition. - -[Illustration: FIG. 470.—Various forms of ice ramparts (after -Buckley).] - -The original ice cover of the lake, which was formed at near-freezing -temperatures, has now received a number of inserted wedges of new ice -at a time when its contracted volume has made this possible. If now -a “warm wave” succeeds to the “cold wave” in the air, the ice cover -expands at a rate corresponding to its rate of contraction, so that a -strong pressure is exerted against the shore (Fig. 469). Sliding up -the sloping surface of the cut and built terrace, the force of this -shove may be deflected upward against the cliff, and if this is of -loose materials, the effect may be to ram bowlders into the bank, to -push up ramparts or ridges, to overturn trees, etc. (Fig. 470). In -marsh land the frozen surface layer may slide over its unfrozen base -and be forced up into broken folds (lower diagram of Figs. 469 and 470). - -[Illustration: - -FIG. 471.—Map of Lake Mendota at Madison, Wisconsin, showing the -position of the ridge which forms from ice expansion, and the ice -ramparts about the shores of the bays (based on Buckley’s map).] - -In order that ice ramparts may be formed, it is necessary that -the winter climate of the district be severe and characterized by -alternating cold and warm waves, involving considerable range of air -temperature below the freezing point. If the lake is small, the push of -the ice will be through so small a distance as not to yield appreciable -ramparts. If, on the other hand, the lake is too large, the ice cover -is not rigid enough to transmit the push to the distant shore, but, -like a long beam employed in the same manner to transmit a compressive -stress, it is bent out of a straight line and later broken. Thus in a -broad lake, with the coming of a “warm wave”, the ice cover opens in a -crack from shore to shore and finds relief from the stress by pushing -up a ridge above the crack. On such lakes ice ramparts are found only -about the shores of bays whose expanse does not greatly exceed a mile -(Fig. 471). - -When there is heavy snowfall, ice ramparts either do not form or are -of smaller dimensions, probably in part because the ice is blanketed -by the snow and so prevented from sudden elevation of temperature -during the “warm wave”, but even more because the ice cover is sensibly -bowed down under its load and so rendered incompetent to transmit the -developed stresses to the shores. - - -READING REFERENCES FOR CHAPTER XXX - - Lake extinction by peat growth: - - C. A. DAVIS. Peat, Essays on its Origin, Uses, and Distribution in - Michigan, Ann. Rept. Mich. Geol. Surv. for 1906, 1907, pp. 105-182; - Peat Deposits as Geological Records, 10th Rept. Mich. Acad. Sci., - 1908, pp. 107-112. - - G. P. BURNS. Bog Studies. Ann Arbor, 1906, pp. 13. - -Ice ramparts: - - C. H. HITCHCOCK. Shore Ramparts in Vermont, Proc. Am. Assoc. Adv. - Sci., vol. 13, 1869, pp. 335-337. - - G. K. GILBERT. Lake Bonneville, Mon. 1, U. S. Geol. Surv., 1890, pp. - 71-72. - - E. R. BUCKLEY. Ice Ramparts, Trans. Wis. Acad. Sci., etc., vol. 13, - 1900, pp. 141-162, pls. 1-18. - - WILLIAM H. HOBBS. Requisite Conditions for the Formation of Ice - Ramparts, Jour. Geol., vol. 19, 1911, pp. 157-160. - - - - -CHAPTER XXXI - -THE ORIGIN AND THE FORMS OF MOUNTAINS - - -=A mountain defined.=—As ordinarily understood, mountains are -elevations upon the earth’s surface which rise above the general level -of the country. Their summits need not be at great heights above the -sea, but it is essential that they project above the average level -of the surrounding country by at least a quarter of a mile. Lower -elevations are described as hills. On the other hand, the elevation of -a plateau like the “High Plains” of the western United States may be as -much as a mile, but the vast expanse of nearly level surface precludes -the use of the term “mountain.” The word is thus applied to a feature -of the earth and not merely to an elevated tract. - -In a collective sense, though more often in the plural form, the -term is properly applied to groups of similar features which have a -common origin in local uplift of the land. The origin of mountains -used in this sense of mountain complexes is thus connected with some -essentially local uplift of the earth’s surface. This may take place -by the processes of folding and superincumbent fault displacement, -by volcanic extravasations or ejections, or by a deeper seated and -essentially hydrostatic elevation of rock beds over molten rock -material. - -The existing _forms_ of mountains, as we are to see, are largely shaped -by the erosional processes which are set in operation by the uplift -itself, though often completed long subsequent to it. - - -=The festoons of mountain arcs.=—From our earliest studies of school -geographies, we have become familiar with the arrangement of the more -important mountains in long chains or systems. Comparatively few -persons have given any further attention to the arrangement of the -chains, though over large areas of the earth’s surface the distribution -of mountain ranges is deeply significant. The map of Asia in particular -presents a series of great sweeping arcs or crescents which are grouped -as though hung upon the map in festoons with knots or vertexes to -separate neighboring groups (Fig. 474, p. 438, and Fig. 472). - -[Illustration: - -FIG. 472.—The great multiple mountain arc of Sewestan, British India -(after de Saint Martin and Schrader).] - -The significance of these mountain groupings in the evolution of the -earth’s surface has been pointed out by the great Viennese geologist -Suess, to whom we are indebted for focusing upon the plan of the -earth an amount of attention which before had been largely given to -the preparation of hypothetical sections of strata which were largely -buried from sight beneath the earth’s surface. Broadly speaking, -the mountain arcs may be said to be grouped about those shields of -older rock which geological studies have shown to be the oldest land -masses upon the globe. Within the northern hemisphere these original -continents are represented by the areas of crystalline rock centered -over Hudson Bay, the Baltic Sea, and an area in northeastern Siberia -known to geologists as Angara Land. In our study of the figure of -the earth (Chapter II) it was found that these shields represent the -truncated angles of the rounded tetrahedral form toward which the -planet is tending (Fig. 3, p. 12). - - -=Theories of origin of the mountain arcs.=—The mountain arcs, when -studied in detail, are found to be composed of closely folded rock -strata, the flexures of which are generally so overturned that their -axial planes dip toward the center of the arc (Fig. 473). It was the -view of Suess that these arcs are to be explained by a pushing outward -of the rock strata from the center of the arc toward its periphery, -thus causing a wrinkling of the surface strata and an overriding of the -surrounding formations, which upon this hypothesis opposed a greater -resistance to the sliding movement. The folding together of the strata -due to the sliding naturally involves a very considerable diminution -of the surface area presented by the strata (Fig. 22, p. 42). In the -case of the Alpine chains it has been estimated that a flat land area, -four hundred to eight hundred miles across, has by the folding process -been reduced to a width of only about one hundred miles, or from a -fourth to an eighth of its former width. - -[Illustration: - -FIG. 473.—_a_, diagram to illustrate the Suess’ theory of the origin -of mountain arcs; _b_, the author’s modification of this view.] - -The weakness of Professor Suess’ theory lies in the fact that such -compression as it implies is assumed to be due to an _outward_ -movement of the relatively small area of the earth’s outer shell -which is included _within_ the arc. It must be obvious that such a -movement, being from a center toward three sides at once, would for -this circumscribed area involve enormous proportionate reduction in -superficial area of the strata and could only result in a hiatus near -the center of the arc. No such gap is to be found, and one would, -moreover, be difficult to account for upon any plausible hypothesis. -On the other hand, the general contraction of the planet as a whole, -involving as it does reduction of surface over large areas, is a -well-recognized fact; and if it be true that the shields formed by the -older continents are less subject to contraction than the remaining -portions of the surface, it is easy to understand why the earth’s outer -skin should be wrinkled by _underfolding_ and thrusting about these -continental margins. The contrast of this view with that of Professor -Suess is expressed in the diagrams of Fig. 473. - -[Illustration: - -FIG. 474.—Festoons of mountain arcs about the borders of the Pacific -Ocean—Pacific type of coast (based upon Suess).] - -We may illustrate this conception by a stretched sheet of rubber cloth -such as is in common use by dentists, upon which a thin layer of hot -Canada balsam has been spread. This substance congeals upon cooling to -near-normal temperatures, and if a small local area of the balsam layer -be chilled and the tension upon the rubber then released, the viscous -balsam of the unchilled portion of the layer is thrown into wrinkles -about the cooled and more resistant areas. These more resistant -portions of the stratum may thus represent the ancient continental -shields of our planet. - - -=The Atlantic and Pacific coasts contrasted.=—In his studies of -mountain arcs in their relation to the plan of the earth, Professor -Suess has shown how the arrangements of the mountain chains about the -two larger oceans represent two strongly contrasted types. Whereas -about the Pacific margin the mountain arcs are, as it were, strung in -festoons which trend parallel to and are convex toward the coast, or -else lie in fringing garlands of islands in the same attitude (Fig. -474); the mountain chains about the Atlantic become sharply truncated -as they reach the coast, and thus indicate that the basin of this ocean -has been produced by an inthrow or depression between great marginal -displacements in some period subsequent to the formation of the -mountains. - -[Illustration: - -FIG. 475.—The interrupted system of the Armorican Mountains common to -western Europe and eastern North America (after Arldt).] - -Thus the mountain folds of the Appalachian system are in Newfoundland -cut off abruptly at the coast line, and the same beds, similarly -truncated, are encountered again across the expanse of ocean in the -folds at the coast of western Europe (Fig. 475). In discontinuous -remnants this ancient mountain chain may be traced in an east and -west direction across western and central Europe. We have thus here -to do with a single mountain system which extends from central Europe -to northern Alabama, out of which a great link has been taken by the -subsequent sinking in of the basin of the Atlantic Ocean. - -[Illustration: - -FIG. 476.—Schematic representation of a “zone of diverse displacement” -in the Great Basin of the western United States (after Powell).] - -=The block type of mountain.=—The inclusion of most elevations in -mountain chains and arcs is one of the most obvious facts to any one -who has examined world atlases with this subject in mind. Such chains -are almost invariably composed of folded rocks, thus indicating that -erosion has removed great superincumbent masses of strata since the -crustal compression produced the folds at considerable depths below the -then surface. - -There are, however, large elevated tracts upon the earth’s surface -which are intersected by deep valleys, but where no arrangement of the -elevated portions within chains or ranges is to be detected. In such -cases the distribution of mountain and valley may bear a resemblance to -a mosaic of disturbed parts which stand at different levels (Fig. 476). - -[Illustration: FIG. 477.—Section of an East African block mountain -(after J. W. Gregory).] - -Such block mountain districts are to be found in many parts of the -earth’s surface, but notably within the Great Basin of the western -United States, and in the land area which borders the Indian Ocean -upon the west and northwest. In contrast with the mountain arcs, so -strikingly exemplified by the continent of Asia as a whole, its extreme -southwestern portion is made up of an alternation of plateau and -rift valley separated from each other by great displacements. Though -modified to some extent by erosion, the elevations seem generally to -represent the displaced crust blocks which in mutual adjustments have -been left at the highest levels (Fig. 477). The valley of the Jordan, -with the mountains of Lebanon rising above it, is near the northern -extremity of this faulted mountain region (Fig. 434, p. 404), while -the Great Rift valley, crossing east Central Africa, and the many -neighboring rifts to the east and west, are graven in lines so deep -that an observer upon a neighboring planet might perhaps detect them. - -It is not necessary in all cases to assume that the block mountains of -a faulted district represent the blocks which in the adjustments were -left the highest. Erosion in the course of time accomplishes marvels of -transformation, and it may result that heavy masses of more resistant -rock eventually project the highest, even though they may represent the -downthrown blocks in the fault mosaic (Fig. 43, p. 60). - -[Illustration: FIG. 478.—Tilted crust blocks in the Queantoweap -valley.] - -Where in addition to undergoing changes of level the earth blocks -have been tilted, the features long since described from our western -interior basin as “Basin Range structure” are developed. Here the -upper surface of the disturbed earth blocks betrays the evidence of a -definite tilt in some one direction (Fig. 478, and Fig. 431, p. 402). - - -=Mountains of outflow or upheap.=—An important type of mountain, -generally described as volcanic, may be due either to the outflow of -lava at the earth’s surface, or to accumulations of separated fragments -of lava, first thrown into the air, and then deposited by gravity or -admixed with water as volcanic mud. Such mountains, both before and -after modification by erosion, assume the strikingly characteristic -forms which have been fully discussed in Chapters IX and X. The -dominant types are the lava dome and the puy, the cinder cone, and -the more complex composite cone. Excepting only the surface produced -by the few great fissure eruptions and the semivolcanic mesa type, the -individual mountains of volcanic origin develop features with notably -circular bases. - -[Illustration: - -FIG. 479.—Pen drawing of the laccolite of the Carriso Mountain by W. -H. Holmes, which shows the jagged surface of the igneous rock core and -the sloping tables which still remain of the roof of sedimentary rocks -(after Cross).] - -[Illustration: - -FIG. 480.—Map of laccolitic mountains. A portion of the Judith -Mountains, Montana. The intrusive igneous rock is shown in black (after -Weed).] - - -=Domed mountains of uplift—laccolites.=—At a considerable number -of widely separated localities upon the earth’s surface, mountainous -regions are encountered, the central areas or cores of which are -composed of intrusive igneous rock such as granite, and about this -core the sediments dip away in all directions as though they had once -formed a continuous roof above it and had been forced into this dome by -hydrostatic pressure of the once viscous material beneath (Fig. 152, p. -143, and Figs. 479 and 480). Examples of such domed mountains of uplift -were first described by Gilbert from the Henry Mountains of Utah, but -instances are furnished by many elevated tracts, especially within -the western United States. Such mountains are known as _laccolites_, -but when one margin at least of the igneous core corresponds to a -displacement, the mountain is described as a _bysmalite_ (Fig. 481). - -[Illustration: FIG. 481.—Ideal sections of laccolite and bysmalite.] - -When subjected to long-continued erosion, the generally fissured -granitic core of the laccolite weathers in a wholly different manner -from the bedded sediments which surround and still in part mount -over it. The former usually presents a more or less jagged surface -which contrasts sharply with the gently sloping tables of the latter -(Fig. 479). About the high granite core of the mountain, the several -strata of the uptilted formations present each a steep slope toward -this higher land, and a gentler slope in the opposite direction. -Such unsymmetrical ridges which surround the mountain area are often -referred to as “hog backs” (plate 12 B). The arrangement of the -strata in the hog backs thus presents an overlapping series like the -shingles upon a roof, except that the overlapping is here from the -bottom instead of the top, and the exposed ends thus face toward the -crest. Unlike a shingle roof the hog backs do not shed the water which -descends to them from the higher levels, but, on the contrary, they -cause it to flow in troughs parallel to the base of the slope except -where outlets are found through them. - - -=Mountains carved from plateaus.=—In the mountain types thus far -discussed, the local uplifting of the land has itself developed -features which in the aggregate may be referred to as mountains, even -though the characters of the original surface are soon destroyed by -erosive processes of one sort or the other. Erosive processes are, -however, quite competent to produce mountain forms from a featureless -plateau, and particularly through the incision by streams of running -water, the best studied process of mountain sculpture (see Chapters -XI-XIII). This process of throwing valleys about an elevated section -of the earth’s surface, and so carving out mountains, is sometimes -described as _circumvallation_; and if the term “mountain” be applied -in its ordinary sense to describe an individual feature, it is clear -that most mountains have been formed in this way. - -To discuss the characteristic shapes of such mountains would be largely -to review the contents of this book, and especially those portions -which discuss the character profiles resulting from the action of each -sculpturing or molding agent. The work of frost and other weathering -agencies, of running water, of mountain and of continental glacier, -would all have to be considered in order to evolve the history of each -mountain. - -In addition to discovering the agents which were chiefly responsible -for the shaping of the mountain, we may, further, in many cases -determine at what stage the work of one agent has been succeeded by -that of another, and at least at what stage of its complete cycle of -activity the latest agent is now at work. - -[Illustration: - -FIG. 482.—The gabled façade so largely developed in desert landscapes -and sharply contrasted with the recurring curves in the landscapes of -humid districts (from a painting of the Grand Cañon of the Colorado by -Moran).] - - -=The climatic conditions of the mountain sculpture.=—Since the -different geological agencies operate either in a different manner -or with differences in vigor according to the varying climatic -conditions, the mountains of arid regions may in most cases be readily -differentiated from those of the more habitable humid sections of -country. In broad lines these differences may be summed up in the -greater prevalence of the curving line within the landscapes of humid -districts. This may be largely ascribed to the influence of the -mat of vegetation, which protects the rock surface from more rapid -mechanical degeneration, and arrests the sliding movements within -the already loosened rock débris. In place of the reversed curves -of the lines of beauty, so generally observed in the landscapes of -well-watered regions, the desert lands present ever a repetition of the -vertical cliff alternating with a sort of many gabled façade which is -occasionally due to truncation of mountain spurs by the waves of former -lakes, but far more often the outlines of débris cones built up beneath -each prominent joint of the cliff walls (Fig. 482). - - -=The effect of the resistant stratum.=—In a striking manner mountain -landscapes may disclose the influence of the diversified rock materials -and of the rock structures as well. After prolonged erosion there -is likely to be little correspondence between the positions of the -anticlinal folds and the crests of the higher mountains. Such mountains -are, in fact, much more likely to rise over synclines than upon the -site of anticlines. The traveler who enters the Alps by any of the -several railways, or who journeys by steamer over the beautiful lake -of Lucerne, has a most favorable opportunity to study the position -of the rock folds in the mountain sections that are unrolled in -succession before him. Rarely indeed will he find a definite anticline -in correspondence with a mountain peak, for the layers which are most -resistant have developed the peaks, and it is because the outer layers -of the anticlines open by local tension (see Fig. 26, p. 45) that -they were first cut away by erosion, so that the hard layers within -the synclines are likely to constitute the peaks within the existing -surface. - -[Illustration: - -FIG. 483.—The Mythen, composed of Jurassic and Cretaceous sediments, -and resting upon softer Tertiary formations. View from a balloon (after -a photograph by C. Schmidt).] - -When, as sometimes happens, an older and likewise more resistant bed -has been folded back upon younger and softer formations, an isolated -remnant may be found “unrooted” to its base, upon which it appears as -though floating within a billowy sea of the softer formations (Fig. -483). - - -=The mark of the rift in the eroded mountains.=—Applying the term -“mountain” in its collective sense for a circumscribed area of uplifted -crust, whether represented to-day by a folded or a faulted complex, -a lava mass, or a granite dome; the period of uplift has marked the -beginning of the activity of sculpturing agencies. By these the mass -is pared down as it is shaped into a more or less intricate design of -component and essentially repeating units. In the vernacular the word -“mountain” is applied to these units into which the larger mountain -mass is subdivided. - -[Illustration: - -FIG. 484.—The battlement type of erosion mountains. Die Drei Zinnen -(Three Battlements) in the Dolomites (after Marr).] - -It has been one of the main objects of this work to point out that the -peculiar shapes of these elementary mountains are each characteristic -of the erosive agents which produced them, and that each surface has -marks which may be recognized in those lines of profile which recur -within the landscape—the character profiles. In the subdivision -of the larger mass—the _genetical_ mountain—to form the numerous -smaller masses—the _erosional_ or _circumvallational_ mountains—there -is disclosed a pattern of fractures which has guided the erosional -agents in their incisional operations (see Chapter XVII). In high -altitudes, where the action of frost is so potent in prying at the -wider fractures, this subdivision of the mass may be revealed by the -sculpturing of squared towers or battlements (Fig. 484). - -[Illustration: FIG. 485.—Symmetrically formed low islands repeated in -ranks upon Temagami Lake, Ontario.] - -For other examples in which the sculptured surface is largely the -handiwork of a single erosional agent, as over vast areas in the -Canadian wilderness, the revelation of the fracture design is no less -apparent. Here a series of crystalline rocks underlie broad expanses -of territory and are without noteworthy variations of hardness and -almost bare of surface débris. Sculptured beneath a mantling ice sheet, -excavation has naturally been concentrated above the more widely -gaping fissures of the joint-fault system, doubtless already marked out -in the river network which the glacier overrode. The result has been a -division of the surface into a series of low, oval ridges or hummocks, -which over vast areas are repeated with monotonous regularity. Wherever -the lower levels have been flooded, symmetrical low islands of nearly -uniform elevation rise from the expanse of water and may be counted -by thousands. Though the smaller islands have notably regular shore -lines, the larger ones disclose their composition from smaller units -by the breaking of their shores into similar bays spaced with regular -intervals (Fig. 485, and Figs. 243 and 245, p. 229). - -The ever repeating fracture design of the earth’s crust is not -restricted to the mountain masses which it has broken up, and the -unity of which it has done so much to conceal. It extends far outside -the margin of these masses, and is in fact common to whole continents -and perhaps even to the planet as a whole. The part played by this -design of fractures in the control of the sculpture of landscapes it -would be hard to overestimate. Through its influence the striking -features molded by one agent have been merged in the contrasted shapes -developed by another. It is the great outline blender in the creation -of nature’s masterpieces of form and color. Thus the lines of this -mysterious fracture network, though stamped in indelible characters -upon our landscapes, are generally lost in the ensemble effect and may -long remain undiscovered. Like a moss-grown inscription upon a slab of -marble, though veiled, it may yet be deciphered; and if the veil be -withdrawn, the runic characters are disclosed, and one of nature’s laws -lies open before us. - - -READING REFERENCES FOR CHAPTER XXXI - - Mountain arcs or festoons:— - - ED. SUESS. The Face of the Earth, vol. 2, 1906, pp. 201-207; vol. 4, - 1909, pp. 498-542. - -Block mountains:— - - G. K. GILBERT. Surveys West of the 100th Meridian (Wheeler), vol. 3, - Geology, Washington, 1875, Pt. 1, pp. 19 _et seq._, 48. - - J. W. POWELL. Report on the Geology of the Eastern Portion of the - Uinta Mountains and a Region of Country Adjacent thereto, U. S. Geol. - and Geogr. Surv. Ter., II Div. Washington, 1876, pp. 218. - - JOHN W. GREGORY. The Great Rift Valley. London, 1896, pp. 422. - -Laccolites and bysmalites:— - - G. K. GILBERT. Report on the Geology of the Henry Mountains, U. S. - Geol. and Geogr. Surv. Ter., 1877, pp. 18-98. - - WHITMAN CROSS. The Laccolitic Mountain Groups of Colorado, Utah, and - Arizona, 14th Ann. Rept. U. S. Geol. Surv., 1895, pp. 157-241, pls. - 7-16. - - W. H. WEED and L. V. PIRSSON. Geology and Mineral Resources of the - Judith Mountains of Montana, 18th Ann. Rept. U. S. Geol. Surv., Pt. - iii, 1898, pp. 485-556, pl. 75. - - W. H. WEED. Geology of the Little Belt Mountains, Montana, etc., 20th - Ann. Rept. U. S. Geol. Surv., Pt. iii, 1900, pp. 387-400. - - VERA DE DERWIES. Recherches géologiques et pétrographiques sur les - loccolithes des environs de Piatigorsk (Caucase du Nord). Geneva, - 1905, pp. 84, pls. 3. - - R. A. DALY. The Mechanics of Igneous Intrusion, Am. Jour. Sci. (4), - vol. 15, 1903, pp. 269-278; vol. 16, 1903, pp. 107-126. - - JOSEPH BARRELL. Geology of the Marysville Mining District, Montana. A - study of Igneous Intrusion and Contact Metamorphism. Prof. Pap. 57, U. - S. Geol. Surv., 1907, pp. 151-178. - -Climatic condition in relation to land sculpture:— - - C. E. DUTTON. Tertiary History of the Grand Canyon District, Mon. 2, - U. S. Geol. Surv., 1882, pp. 264, pls. 42. - - - - -APPENDIX A - -THE QUICK DETERMINATION OF THE COMMON MINERALS - - -Before one may gain a knowledge of rocks or the architecture of -their arrangement within the earth’s crust, it is quite essential -that some familiarity should be acquired with the appearance and -properties of the commonest minerals, and particularly those which -enter as essential constituents into the more abundant rocks. To be a -competent mineralogist, one must have a rather extended knowledge both -of inorganic chemistry and of the science of crystallography, which, -fascinating as it is to study, involves some technical knowledge of -mathematics and much laboratory experience. Though necessary to any one -who contemplates making a career as a geologist, this special study is -not essential to a cultural course like the present one. The attempt -will here be made to bring together a body of fact, from the study of -which the student may quickly learn to recognize the commonest minerals -in their usual varieties. The tests he is to apply are mainly physical, -and in place of an elaborate discussion of crystal symmetry, pictures -only can be supplied. - -To the beginner the usual textbook of mineralogy is difficult to -read intelligently, for the reason that for each mineral species it -sets before him a catalogue of each physical property in its turn, -with little indication of those data which in the individual case -have special diagnostic value. None the less, however, the student -is advised to consider the several properties of each mineral in a -definite order, and the following may serve as well as any: crystal or -other form, cleavage, fracture, luster, color, streak, transparency, -tenacity, hardness, magnetism, and specific gravity. In endeavoring to -connect the specific values of these properties with individual mineral -species, the chemical composition and the manner of occurrence are -not to be forgotten. It is well for the student to be supplied with a -small pocket lens and with a pocket knife the blade of which has been -magnetized. - -=Crystal form.=—Some mineral species generally occur in more or less -definite crystals—are bounded by definite plane surfaces developed -when the mineral was formed; others in groups of interfering crystals -or aggregates, in which case the mineral is said to be crystalline; -while still others are rarely found crystallized at all. Thus in -a given case crystal form may, or may not, be important for the -diagnosis of the substance. If a mineral species is usually to be -found in crystals, the student should be aware of the fact, and if -possible should have a mental picture of the common crystal shape or -shapes. Without an extended knowledge of crystallography, this must be -supplied him by drawings. Since crystals of most species are apt to be -distorted, owing to the fact that some planes within the same group -appear upon the crystal with a larger development than others, it is -convenient to remember that markings, such as lines or etchings upon -the crystal faces, are the same throughout the same group of planes, -and in the text figures such groups of planes are indicated by the use -of a common letter. For crystalline aggregates such terms as fibrous, -radiating, massive, or granular have their usual meanings. - -=Cleavage.=—It is characteristic of most crystals that they break -or _cleave_ along certain directions so as to leave plane or nearly -plane surfaces, and the luster of the cleaved surface measures the -perfection of the cleavage property. It is important always to note -how many such directions of cleavage are present, and, roughly at -least, at what angles they intersect—whether they are perpendicular -to each other or inclined at some other angle. Further, it should be -noted whether a given cleavage is _perfect_, that is, easy, which will -be indicated by the thinness of the plates which can be secured. An -extremely perfect cleavage is possessed by the mineral mica, whose -plates are thinner than the thinnest paper. In the case of imperfect or -interrupted cleavage, the fracture surfaces are not plane throughout, -but interrupted, the surface “jumping” from one plane to a neighboring -parallel one. It is especially important to note whether, in the case -of several cleavages possessed by a crystal, all have the same degree -of perfection, or whether they exhibit differences. - -=Fracture.=—In minerals with poorly developed cleavage, the fracture -surface is described as _fracture_. Fracture is thus perfect in -proportion as cleavage is imperfect. The fracture is described as -conchoidal when it shows waving spherical surfaces like broken glass. -For fine aggregates the fracture is described as even, uneven, earthy, -etc., names which are generally intelligible. - -=Luster.=—This term is applied especially to the manner in which light -is reflected from mineral surfaces. The most important distinction is -made between those minerals which have a _metallic_ luster and those -which have not, the former being always opaque. Other characteristic -lusters are adamantine (like oiled glass), vitreous (glassy), resinous, -waxy, etc. - -=Color.=—For minerals which possess metallic luster the color is -always practically the same, and hence it becomes a valuable diagnostic -property. Of minerals which have nonmetallic luster, the color may be -always the same and hence characteristic, but in the case of many -minerals it ranges between wide limits and sometimes runs almost the -entire gamut of hues, yet without appreciable changes in the chemical -composition of the mineral. - - -=Streak.=—This term is applied to the color of the mineral powder, and -is usually fairly constant, even when the surface color of different -specimens may vary within wide limits. In the case of fairly soft -minerals the streak is best examined by making a mark on a piece of -unglazed porcelain (streak stone). - - -=Transparency= (=diaphaneity=).—The terms “transparent”, -“translucent”, “subtranslucent”, and “opaque” are used to describe -decreasing grades of permeability by light rays. Through transparent -bodies print may be read, while translucent bodies allow the light to -be transmitted in considerable quantity through them, though without -rendering the image of objects. - - -=Tenacity.=—This comprehensive term includes such properties as -brittleness, flexibility, elasticity, malleability, etc. - - -=Hardness.=—Quite erroneous notions are held concerning the meaning -of this very common word, which properly implies a resistance offered -to abrasion. It is one of the most valuable properties for the quick -determination of minerals, since minerals range from diamond upon the -one hand—the hardest of substances—to talc and graphite, which are -so soft as to be deeply scratched by the thumb nail. For practical -purposes it is sufficient to make use of a rough scale of hardness made -up from common or well-known minerals. If we exclude the gem minerals, -this scale need include but seven numbers, which are: talc, 1; gypsum, -2; calcite, 3; fluor spar, 4; apatite, 5; feldspar, 6; and quartz, 7. -A given mineral is softer than a mineral in the scale when it can be -visibly scratched by a scale mineral, but will not leave a scratch when -the conditions are reversed. If each will scratch the other with equal -readiness, the two minerals have the same hardness. - -Since it may often be desirable to test mineral hardness when no scale -is at hand, the following substitutes may be made use of: 1, greasy -feel and easily scratched by the thumb nail; 2, takes a scratch from -the thumb nail, but much less readily; 3, scratched by a copper coin -and very easily by a pocket knife; 4, scratched without difficulty by -a knife; 5, scratched with difficulty by a knife, but easily by window -glass; 6, scratched by window glass; 7, scratches window glass with -readiness, but a grain of sand may be substituted to represent quartz -in the scale. - - -=Magnetism.=—Though nearly all minerals which contain important -quantities of the elements iron, cobalt, or nickel may be attracted to -a strong electromagnet, there are but two common minerals, and these -of widely different appearance, whose powder is lifted by a common -magnet. Others are, however, lifted after strong heating in the air -(_ignition_), and this is a valuable test. - -=Specific gravity.=—Rough tests of relative weight, or specific -gravity, may be made by lifting fair-sized specimens in the hand. -Better determinations require the use of a spring balance. - -=Treatment with acid.=—The carbonate minerals react with warm and -dilute mineral acid so as to give a boiling effect (effervescence), -since carbonic acid gas escapes into the air in the process. - - -PROPERTIES OF THE COMMON MINERALS - -The more important common minerals fall into two classes according as -they have large economic importance as ores, or enter in an important -way into the composition of rocks. - - -I. The Minerals of Economic Importance - -=Hematite.=—The sesquioxide of iron, Fe_{2}O_{3}, and by far the most -important ore of iron. Rarely in good crystals, but sometimes in thin -opaque scales bearing some resemblance to mica and known as micaceous -or specular iron ore. At other times in nodules built up from radial -needles (needle ore); in hard masses mixed with fine quartz grains -(hard hematite); or in soft reddish brown earth (soft hematite). -Color, black to cherry red. The powdered mineral always cherry red or -reddish brown, and easily lifted by the magnet after ignition. Hardness -5.5-6.5; specific gravity 5. - -=Magnetite.=—The magnetic oxide of iron, Fe_{3}O_{4}, often in -crystals like Fig. 486, ^{1-2}. Black and opaque with a metallic -luster. Streak black. Lifted by a magnet and sometimes itself capable -of lifting filings of soft iron (lodestone). Hardness 5.5-6.5. Specific -gravity 5. - -=Limonite.=—The most abundant and most valuable of the hydrated iron -ores, 2 Fe_{2}O_{3}. 3 H_{2}O. Chemical composition the same as iron -rust, with which in the earthy form it is identical. Never in crystals, -but often in mammillary or rounded pendant forms resembling icicles, -or sometimes clusters of grapes. Its yellow (rust) streak is its best -diagnostic property. Ignited it gives off water and becomes magnetic. -The streak and its notably lower specific gravity distinguish it from -certain forms of hematite which it outwardly resembles. Hardness 5-5.5. -Specific gravity 3.6-4. - -=Pyrite, iron pyrites, or “fool’s gold.”=—The sulphide of iron, -FeS_{2}. The most widely distributed sulphide mineral and now a chief -source of the great chemical reagent, sulphuric acid or vitriol. -Often, but not always, in crystals (Fig. 486, ^{3-5}) which have -peculiar striæ upon their faces. At other times the mineral is found -massive or in radiated needles. Bright metallic luster with the color -of new brass, though often tarnished or altered upon the surface to -limonite. Hard and brittle, and so distinguished from gold, which is -soft and malleable and of the color of the paler old brass (which -contained a larger percentage of zinc). Gold is, further, about four -times as heavy as pyrite. Hardness 6-6.5. Specific gravity 5. - -=Chalcopyrite, copper pyrites.=—A mixed sulphide of copper and iron. -If in crystals, like Fig. 486, ^6; otherwise massive or compact. -Luster metallic. Color orange-yellow, often with local blue and green -iridescence like a pigeon’s throat. Distinguished from pyrite by the -deeper color and lower hardness, and from gold, particularly, by its -brittleness and lower specific gravity. Hardness 3.5-4. Specific -gravity 4. - -=Galenite, galena.=—Sulphide of lead, PbS. The chief ore of lead, and, -from admixture of a silver mineral, of silver as well. Usually found in -crystals (Fig. 486, ^7). Always cleaves into blocks bounded by six very -perfect rectangular faces which, when freshly broken, show a bright -silvery luster and quickly tarnish to a peculiarly “leaden” surface. -Very heavy. Color and streak lead-gray. Hardness 2.5. Specific gravity -7.5. - -=Sphalerite, zinc blende.=—Sulphide of zinc, ZnS, usually with -considerable admixture of sulphide of iron. The great ore of zinc. -Not infrequently in crystals (Fig. 486, ^{8-9}), but more often in -cleavable crystalline aggregates. The cleavage in fine aggregates is -sometimes difficult to make out, but in coarse-grained masses it is -seen to be equally and highly perfect in six different directions, -so that a symmetrical twelve-faced form may sometimes be broken out -(dodecahedron). Luster like that of rosin (rosin jack), though when -with large iron admixture the color may approach black (black jack). -The lighter colored varieties are translucent. Hardness 3.5-4. Specific -gravity 4. - -=Malachite.=—Hydrated (basic) copper carbonate. The green copper ore -and the common surface alteration product of other copper minerals. -Usually has a microscopic structure made up of fine needle-like -crystals, but generally massive in various imitative shapes not unlike -those of the iron ores. Sometimes earthy. Its color is bright green, -and it is usually found in association with other characteristic copper -ores, such as chalcopyrite and azurite. When relatively pure and in -large masses, it is a beautiful ornamental stone. Effervesces with -acid. Hardness 3.5-4. Specific gravity 4. - -[Illustration: FIG. 486.—Forms of Crystals: 1-2, magnetite; 3-5, -pyrite; 6, chalcopyrite; 7, galenite; 8-9, sphalerite; 10-13, calcite.] - -=Azurite.=—Hydrated (basic) copper carbonate, less hydrated than -malachite, and known as the blue carbonate of copper. Generally in very -minute and quite complex crystals, but also in imitative shapes similar -to those of malachite, and at other times earthy. Slightly lighter in -weight than malachite, from which it is easily distinguished, as from -most other minerals, by its bright azure blue color and its somewhat -lighter blue streak. Effervesces with nitric acid. Hardness 3.5-4. -Specific gravity 3.7-3.8. - -=Calcite.=—Calcium carbonate, CaCO_{3}. Almost always in crystals -(Fig. 486, ^{10-13}), or in confused crystal aggregates, though -rarely fibrous or dull and earthy. Some of the forms of the crystals -are described as “dog-tooth spar”, others as “nail-head spar”, while -still others are modified hexagonal prisms. There is a beautifully -perfect cleavage of the mineral along three directions which make -angles of about 105° with each other, so that under the hammer the -substance breaks into blocks which are shaped like the crystal of Fig. -486, ^{10}. Usually white or gray, but occasionally faintly tinted. -Streak white. Effervesces with cold and dilute mineral acids. An -associate of many ores and the chief mineral of limestone. A similar -mineral—dolomite—contains in addition magnesium carbonate, has -simpler crystals (like the drawing of Fig. 486, ^{10}, but often with -rounded faces), and effervesces only when the acid is warmed. Hardness -3. Specific gravity 2.7. - -=Gypsum.=—Hydrated calcium sulphate, CaSO_{4}.2 H_{2}O, and the -source of plaster of Paris. Often in simple crystals (Fig. 487, ^1) -or else “swallow tail”, like Fig. 487, ^2; in which case the mineral -is generally either transparent or translucent and is described as -selenite. Such crystals show a cleavage approaching in perfection that -of the micas, but, unlike the mica laminæ, those produced by cleavage -in gypsum though flexible are not elastic. There are also fibrous forms -of gypsum (satin spar), a fine-grained form (alabaster), and the impure -earthy form (rock gypsum). Very soft, light in weight, and difficultly -fusible. Color usually white, gray, or pale yellow. Hardness 2. -Specific gravity 2.3. - -=Copper glance.=—A sulphide of copper, Cu_{2}S. Not usually well -crystallized, but generally massive and associated or variously admixed -with other copper ores such as chalcopyrite, malachite, etc. Fracture -conchoidal, luster metallic, color and streak blackish lead-gray, -though often tarnished blue or green from surface alterations to the -copper carbonates. Softer and heavier than chalcopyrite. Blowpipe or -chemical tests are necessary for its identification. Hardness 2.5-3. -Specific gravity 5.5-5.8. - -=Cerussite.=—The white or carbonate lead ore, PbCO_{3}, and an -important ore of silver as well. Often in crystals of considerable -complexity, though Fig. 487, ^{3-4}, shows some common shapes. Often -granular, massive, or earthy (gray carbonate ore). Very brittle and -with conchoidal fracture. The luster is adamantine or like that of -oiled glass. Color generally white or gray. Very heavy, the heaviest -of light colored and nonmetallic minerals. Dissolves in nitric acid -with effervescence. Hardness 3-3.5. Specific gravity 6.5. - -=Siderite.=—The carbonate or “spathic” ore of iron, FeCO_{3}. Either -in crystals resembling in form Fig. 486, ^{10}, but with rounded faces, -or cleavable massive to finely granular and earthy. The crystalline -varieties cleave easily into smaller blocks of the same form as those -of calcite. Color usually gray or brown and streak white. On strongly -igniting, the white powder becomes black and magnetic. Lighter in both -color and weight than the other iron ores, and unlike them siderite -effervesces with acid. Distinguished from calcite by its higher -specific gravity and its change upon being ignited. Hardness 3.5-4. -Specific gravity 3.9. - -=Smithsonite.=—Carbonate of zinc, ZnCO_{3}, and an important ore of -that metal. Seldom found in crystals except as a replacement of calcite -crystals, in which case it shows the forms characteristic of the latter -mineral. Usually kidney-shaped, stalactitic, or else in incrustations -upon other minerals. Sometimes granular or earthy. Brittle. Luster -vitreous, color white or greenish gray, though often stained yellow -with iron rust. Streak white except when the mineral is stained with -iron. Effervesces with warm acid. Hardness 5. Specific gravity 4.4. - -=Pyrolusite.=—Black oxide of manganese, MnO_{2}, though generally -impure from admixture with other manganese oxides. Usually in intricate -aggregates which may be columnar, fibrous, mammillary, earthy, etc. -Opaque, with color and streak both black. Soft and easily soils -the fingers. With hydrochloric acid gives off the choking fumes of -chlorine. Hardness 2-2.5. Specific gravity 4.8. - - -II. The Minerals important as Rock Makers - -These minerals are in most cases complex silicates of one or more of -a certain number of metals such as aluminium, calcium, magnesium, -iron, sodium, potassium, or hydroxyl (OH). For their identification an -examination of the physical properties is usually sufficient, whereas -of the typical ore minerals already considered, additional chemical -tests may be necessary. - -=Feldspars.=—A group of similar alumino-silicates of potassium, -sodium, and calcium. The most important of all rock-making minerals. -Although with wide variation in chemical composition, the feldspars are -yet broadly divided into two classes; the one striated, and the other -an unstriated potash or orthoclase variety. The pocket lens is usually -necessary in order to make out the striations upon the crystal or -cleavage surfaces. When formed in veins, feldspar appears in crystals -(Fig. 487, ^{5-6}), but as a rock constituent the mutual interference -of crystals prevents the development of bounding faces. Two cleavage -directions, nearly or quite perpendicular to each other, are notably -different in their perfection. Hard enough to scratch glass, but easily -scratched by sand. Color pink (usually orthoclase or microline), white -(often albite) to gray. Sometimes with beautiful “pigeon’s throat” -effect of iridescence (labradorite). Low specific gravity. Hardness 6. -Specific gravity 2.5-2.8. - -[Illustration: - -FIG. 487.—Forms of Crystals: 1-2, gypsum; 3-4, cerussite; 5-6, -feldspar; 7, quartz; 8, pyroxene (cross section); 9, hornblende -(cross section); 10, garnet; 11, nephelite; 12-14, staurolite; 15-16, -tourmaline (cross sections); 17, olivine.] - -=Quartz.=—Oxide of silicon or silica, SiO_{2}. Both an important vein -mineral associated with the ores and a rock maker. In the former case -particularly, often in crystals of notably simple forms (Fig. 487, ^7). -Few minerals which are not gems are so hard. Remarkable freedom from -cleavage so that the mineral breaks much like window glass—conchoidal -fracture. Wide range in both transparency and color. Transparent and -colorless crystalline variety (rock crystal), brown translucent (smoky -quartz), turbid white (milky quartz), and various colored varieties -(carnelian, jasper, jet, etc.). Insoluble in acids and infusible. -Hardness 7. Specific gravity 2.6. - -=Micas.=—Like the feldspars a group of complex silicates, but here -chiefly of potassium, magnesium, iron, and hydroxyl. Abundant as rock -makers, the micas are all characterized by the thinnest and toughest of -elastic cleavage plates, such as are generally known as isinglass. When -a needle is driven sharply through a thin scale of mica, a six-rayed -puncture star forms about the needle point. The darker common variety -of mica is rich in iron and magnesium and is called biotite, and the -lighter colored alkaline variety, muscovite. Hardness 2.5-3.1. Specific -gravity 2.7-3.1. - -=Chlorite.=—Generally an intricate mixture of more or less similar -microscopic crystals having varying and rather complex chemical -compositions and related to the micas, but all characterized by -a peculiar leaf green color. These minerals are a common product -of hydration weathering in rocks which are rich in magnesium and -iron—especially those that contain biotite, pyroxene, or hornblende -(see below). Hardness 1-2.5. Specific gravity 2.5-3. - -=Pyroxenes.=—An important group of related rock-making minerals all -of which are silicates of the bases magnesium, calcium, aluminium, -iron, and manganese. Quite generally developed either in columnar or -needle-like crystals which are uniformly shaped in cross section like -Fig. 487, ^8. Two rather imperfect cleavages are directed parallel -to the longer axis of the crystal and nearly at right angles to each -other. The colors of all but the lime varieties are dark and generally -green, dark brown, bronze, or black. The lime varieties are white, -gray, or pale green. A dark colored and common iron variety is known as -augite. Streak generally either white or lightly tinted. Hardness 5-6. -Specific gravity 3.2-3.6. - -=Amphiboles.=—A group of minerals of the same chemical composition -as the pyroxenes, with which also in most physical properties they -agree. The principal distinction is found in the shape of the cross -section and in the cleavage (Fig. 487, ^9). Whereas the cross sections -of pyroxenes are generally eight sided, those of the amphiboles have -six sides, and whereas the cleavage directions of pyroxenes are nearly -at right angles to each other (87°), the similar but much more perfect -cleavage directions of the amphiboles are inclined at an obtuse angle -(124½°). Owing to the obliquity of the amphibole cleavage, fractured -surfaces of the mineral appear splintery, which is not in the same -measure true of the pyroxenes. A fibrous variety of amphibole, and -occasionally other varieties of the mineral, is a not uncommon product -of weathering of pyroxenes. Other physical properties of the amphiboles -are in the main almost identical with those of the pyroxenes. - -=Garnet.=—Complex alumino-silicates or ferro-silicates of calcium, -magnesium, iron, or manganese, or several of these combined. Nearly -always in crystals, and usually found in mica schist (see below). -The crystals usually have twelve similar faces, each a lozenge -(dodecahedron), or else twenty-four similar faces, or the two forms -combined (Fig. 487, ^{10}). Brittle. From any but the gem minerals garnet -is easily distinguished by its hardness, which in different varieties -ranges from somewhat below to somewhat above that of quartz. The luster -is vitreous, and the color runs the gamut of reds, browns, and greens, -but with the common hue dark red to black. Streak white. Hardness -6.5-7.5. Specific gravity 3.1-4.3. - -=Nephelite= (=nephelene=).—An alumino-silicate of sodium and -potassium. In certain special provinces this mineral is developed in -abundance as an essential constituent of igneous rocks, but elsewhere -practically unknown. The rare crystals are hexagonal prisms (Fig. -487, ^{11}), but the mineral is most easily determined by its general -resemblance to feldspar, but with the differences of cleavage, luster, -and reaction with acid. Whereas the feldspars have two cleavages, -either nearly or quite perpendicular to each other and of different -degrees of perfection, nephelite has three equal cleavages inclined -60° and 120° to each other and of less perfection than either feldspar -cleavage. The luster of nephelite is perhaps the best clew to its -identity, since this is greasy and simulated by but few minerals. -The fine powder of the mineral treated for some time with strong -hydrochloric acid forms a perfect jelly of silicic acid, whereas -the feldspars do not. Though itself gray or white and unobtrusive, -nephelite is usually associated with brightly colored minerals, which -are often the first clew to its presence in a rock. Hardness 5.5-6. -Specific gravity 2.5-2.6. - -=Talc= (=soapstone=).—A silicate of magnesium and hydroxyl which is -an important alteration product through weathering of certain pyroxene -rocks especially. Usually a foliated mass, this product is occasionally -fibrous or even granular. Talc is one of the softest of minerals, -having a greasy feel and being easily scratched with the thumb nail. -The luster of the foliated varieties is apt to be pearly, and the -color apple-green to white, though sometimes stained brown from oxide -of iron. The streak of the mineral is white except when stained by -iron. Although the rocks which are composed mainly of talc (soapstone) -are exceedingly soft, they are very tough and remarkably resistant. -Hardness 1-1.5. Specific gravity 2.7-2.8. - -=Serpentine.=—Like talc, serpentine is a silicate of magnesium and -hydroxyl, and an important product of the breaking down of magnesium -minerals in the process of weathering. The mineral is usually found -as a fine web of microscopic needle-like fibers, and is best roughly -diagnosed by its color and its associated minerals. Like talc it is -usually developed within those igneous rocks from which feldspar is -lacking, but where either pyroxene or olivine is found in abundance or -was previous to alteration. The characteristic color of serpentine is -leek-green. The rock largely composed of serpentine is called by the -same name, and being exceedingly tough and unchanging is, in spite of -its softness, a valuable building and ornamental stone. A red magnesium -garnet is apt to be associated with such serpentine masses. Hardness -2.5-4, because of impurities. Specific gravity 2.5-2.6. - -=Staurolite.=—A silicate of aluminium, iron, and hydroxyl. Found -in metamorphic rocks usually in association with garnet. Always in -crystals bounded by simple forms generally crossed, as shown in Fig. -487, ^{12-14}. The color is dark reddish brown, and the streak is -colorless to grayish. The hardness is exceptional and higher than that -of quartz. Hardness 7-7.5. Specific gravity 3.6-3.7. - -=Tourmaline.=—An exceptionally complex silicate of boron and aluminium -as well as iron, magnesium, and the alkalies. Found in metamorphic -rocks and always crystallized. The crystals are columns or needles -whose cross section is the best guide to their identity, since this -is a modified triangle unlike that of any other mineral (Fig. 487, -^{15-16}). Additional diagnostic properties are the characteristic -striations which run lengthwise of the crystals upon prism faces, and -the lack of any cleavage (difference from hornblende). The hardness is -also a valuable property, since this is greater than that of quartz. -The mineral is brittle and the fracture subconchoidal. The range in -color is as great as, or greater than, that of garnet, though the -common forms are jet black. Streak uncolored. Hardness 7-7.5. Specific -gravity 3-3.2. - -=Olivine.=—A silicate of magnesium and iron and a rock-making mineral -found only in those igneous rocks which have little or no feldspar. It -easily suffers alteration by weathering and passes into serpentine, -and in fact is seldom found except when at least partially altered to -the fibrous webs of that mineral. The form of the unaltered crystals -within the rocks is shown in Fig. 487, ^{17}, and, cut in sections, the -mineral appears in more or less elongated hexagons. The hardness of -the unaltered mineral is about that of quartz. It has rather imperfect -cleavages in two rectangular directions, and is usually translucent, -with a vitreous luster and a color which is olive-green when not -stained brown by oxide of iron. Streak uncolored. Hardness 6.5-7. -Specific gravity 3.2-3.3. - - - - -APPENDIX B - -SHORT DESCRIPTIONS OF SOME COMMON ROCKS - - -In Chapter IV the classification and the structure of rocks have been -briefly discussed. Below are added brief descriptions of the more -important common rocks. For rocks as for minerals it is, however, -essential that a collection of well-chosen specimens be studied for -purposes of comparison. A small pocket lens is a valuable aid in making -out the component minerals and the textures of the finer grained rocks. - - -1. Intrusive Rocks - -=Granite.=—Of granitic texture, though sometimes porphyritic as well. -The most abundant mineral constituent is a pink or white feldspar, -usually without visible striations, with which there is usually in -subordinate quantity a white striated feldspar. Next in importance to -the feldspar is quartz, which because of its lack of cleavage shows a -peculiar gray surface resembling wet sugar. In addition to feldspar -and quartz there is generally, though not universally, a dark colored -mineral, either mica or hornblende. The mica is usually biotite, though -often associated with muscovite. - -=Syenite.=—Like granite, but without quartz, with more striated -feldspar, and generally also the rock has a darker average tint. -While biotite is the commonest dark colored constituent of granite, -hornblende is more apt to take its place in syenite. Less common than -granite, to which it is closely related in origin and in composition. - -=Gabbro.=—A dark colored rock of granitic texture composed of striated -feldspar with broad cleavage surfaces and usually an abundance of -pyroxene. In contrast to the feldspars of granite, those of gabbroes -are often dull and colored grayish yellow or greenish. The pyroxene -is often in part changed to fibrous amphibole. Magnetite may be an -abundant accessory mineral. - -=Diabase.=—In color dark like gabbro, and of similar constitution. In -diabase, however, the feldspar crystals, instead of being broad and of -irregularly interrupted outline, are relatively long (“lath-shaped”), -and the pyroxene acts as a filler of the residual space between them. - -=Peridotite.=—A heavy and dark colored rock of granitic texture which -is nearly or quite devoid of feldspar but contains olivine. When -altered, as it generally is, it is largely a mass of serpentine, talc, -and chlorite, surrounding cores, it may be, of still unaltered pyroxene -and olivine. Magnetite is an abundant constituent, and a red garnet is -apt to be present. - - -2. Extrusive Rocks - -=Obsidian.=—A rock glass rich in silica. It is usually black and -breaks with a perfect conchoidal fracture. It often passes over through -insensible gradations into pumice, which differs only in its vesicular -structure. As regards chemical composition, obsidian and pumice are not -notably different from rhyolite (below). - -=Rhyolite.=—A light colored rock of porphyritic texture, often also -with fluxion or spherulitic textures, or both combined. The porphyritic -appearance is given the rock by large crystals of a glassy, unstriated -feldspar and crystals of quartz. Rhyolite is a very siliceous lava -containing rather more silica than granite, to which of the intrusive -rocks it is most closely related, and from which it differs in its -texture and in the manner of its occurrence in nature. Whereas -granite is found in great batholites, laccolites, and bysmalites, -and consolidated in most cases beneath the earth’s surface, rhyolite -generally occurs in sheets, flows, or dikes, and consolidated either -above or in fissures near to the surface. - -=Trachyte.=—Similar to rhyolite, but usually with a peculiar gray -aspect from the greater abundance of feldspar crystals. The rock -is less siliceous than rhyolite, contains no quartz crystals, and -approaches a feldspar in its average composition. - -=Andesite.=—Similar to rhyolite in appearance and in origin, but more -basic and correspondingly dark in color. The porphyritic crystals -are of lath-shaped, striated feldspar, with which are associated -crystals of either biotite or hornblende or both. A fluxion texture is -particularly characteristic of this type of extrusive rock. - -=Basalt.=—A dark colored or black basic rock of porphyritic texture -which differs but little from diabase. It may show under the lens fine -lath-shaped crystals of striated feldspar associated with crystals -of augite, but more frequently the rock is dense and without visible -mineral constituents. It is particularly likely to occur divided up -into columns six inches to a foot in diameter and known as basaltic -columns. Especially fine examples are known from the Giant’s Causeway -and other localities in the western British Isles. - - -3. Sedimentary Rocks of Mechanical Origin - -=Conglomerate= (“=pudding stone=”).—A rock made up from pebbles which -are cemented together with sand and finer materials. The pebbles are -usually worn by work of the waves upon a shore, and may vary in size -from a pea to large bowlders. They may consist of almost any hard -mineral or rock, though the sand about them is largely quartz. - -=Sandstone.=—A rock composed of sand cemented together either by -calcareous, siliceous, or ferruginous materials. Sandstones are -described as friable when their surface grains are easily rubbed off, -or as compact when they are more firmly cemented. Sandstones are often -distinctly banded and are sometimes variously stained with oxide of -iron. Those sandstones which have been formed upon a seacoast are known -as marine sandstones, while those derived from accumulations collected -by the wind in deserts are distinguished as continental deposits. -Sandstones form much thicker formations than conglomerates, the latter -usually constituting a basal layer only of the sandstone formation -(basal conglomerate). - -=Shale.=—A consolidated mud stone which is probably the most abundant -rock formation. In large part clay admixed in varying proportions with -extremely fine sandy grains. - - -4. Sedimentary Rocks of Chemical Precipitation - -=Calcareous tufa= (=travertine=).—Not to be confused with tuff, -which is a fragmental extrusive or volcanic rock. Calcareous tufa is -formed when waters which contain carbonic acid gas and lime carbonate -in solution, give off the gas and with it the power to hold the lime -in solution. Such a liberation of the gas may occur when the stream -is dashed into spray above a cascade, and the lime is then deposited -about the site of the falls. Travertine is generally porous and formed -of more or less concentric layers or incrustations. A remarkable -illustration is furnished by the travertine deposits of Tivoli and -other localities near Rome, since here the material supplies a valuable -building stone. - -=Oölitic limestone= (=oolite=).—This rock is made up of spherical -nodules and so has the appearance of fish roe. Broken apart, each grain -reveals in its center a core of siliceous sand about which carbonate -of lime has been deposited in concentric layers. It is thought that -waters charged with carbonate of lime, in issuing from a river near -a sea beach, coat the sand grains of the latter with successive thin -films of lime carbonate due to the rhythmic ebb and flow of the tides, -evaporation of the adhering water taking place when the sands are -exposed at low tide. - - -5. Sedimentary Rocks of Organic Origin - -=Limestone.=—A generally white or gray rock composed of carbonate of -lime with varying proportions of clay, silica, and other impurities. -The lime carbonate is usually derived from the hard parts of marine -organisms, and the argillaceous and siliceous impurities from the finer -land-derived sediments which descend with them to the bottom. - -=Dolomite= (=dolomitic or magnesium limestone=).—Differs from -limestone in containing varying proportions of the mineral dolomite -(_ante_, p. 455), which is made up of equal parts of calcium and -magnesium carbonates. Difficult to distinguish from limestone unless a -chemical test is made for magnesium, though it may be said in general -that dolomite is less soluble in cold mineral acids. - -=Peat.=—An accumulation of decomposed vegetable matter within -small lakes and in lagoons separated from larger ones (_ante_, p. -429). Peat represents the first stage in the formation of coal -from vegetable matter, and differs from the coals by its larger -proportion of contained water. Because of this water its fuel value is -correspondingly small. It is usually dark brown or black and reveals -something of the structure of the plants out of which it was formed. - - -6. Metamorphic Rocks - -=Gneiss.=—A generally more or less banded (gneissic) metamorphic rock -with a mineral constitution similar to granite, and often developed -by metamorphic processes from that rock. It may at other times, by -processes not essentially different, be derived from sedimentary -formations. It usually contains as important constituents unstriated -feldspar and quartz, but in addition it may include a striated -feldspar, biotite, muscovite, or hornblende, or several of these -combined. In proportion as mica or hornblende is abundant, it has a -marked banded texture, but it differs from mica schist (see below) not -only in the presence of its feldspar, but in the smaller proportion -of mica. Biotite gneiss, hornblende gneiss, etc., are terms used to -designate varieties in which one or the other of the dark colored -constituents predominate. - -=Mica schist.=—A metamorphic rock without feldspar and mainly composed -of quartz and light colored mica (muscovite). The abundant mica lends -to the rock its characteristic schistose texture, which differs from -the usual gneissic texture. In some cases the mica is wrapped about -the grains of quartz, but at other times it forms a series of almost -continuous membranes separating layers of quartz. - -=Sericite schist.=—A variety of schist which is characterized by -an abundance of a peculiar silvery mica rich in the element group -hydroxyl. The mica scales are often microscopic and wrought into an -intricate web with the quartz constituent. - -=Talc schist.=—A schist made up largely of talc, but with varying -proportions of quartz, magnetite, etc. From the abundance of the talc -it is usually pale green or white. - -=Chlorite schist.=—A greenish, fine-grained metamorphic rock in which -chlorite is the principal mineral, but in which magnetite is a quite -characteristic accessory constituent. - -=Staurolitic garnetiferous mica schist.=—A mica schist in which garnet -and staurolite are so abundant as to be essential constituents. - -=Clay slate.=—A metamorphosed mud stone or shale. In the process of -metamorphism the rock has been hardened, given a slaty cleavage, and -innumerable minute scales of mica have developed to produce a silky -luster upon the cleavage faces. The color may be gray, green, purple, -or black. - -=Quartzite.=—A metamorphosed sandstone in which the sand grains have -become enlarged by accretion of silica. Whereas a sandstone fractures -about its constituent grains, a break in quartzite is continued through -the grains and the cement alike. In contrast to sandstones, the -quartzites derived from them are usually lighter in color and often -nearly white. - -=Marble= (=crystalline limestone=).—The result of metamorphism upon -limestones. Usually white in color but sometimes gray, blue gray, or -yellow, and sometimes variously broken or brecciated and stained with -iron oxide. Effervesces with cold dilute acid. - -=Coals.=—Under the head of peat the first stage in the formation of -coals from vegetable matter has been briefly described. Lignite, or -brown coal, represents a further stage and one in which the vegetable -structure is still recognizable. It is usually brownish black or -black in color and contains a considerable proportion of water. With -increased pressure or dynamic metamorphism, further percentages of the -volatile constituents are eliminated, and when from seventy-five to -ninety per cent of carbon remains, the material burns with a yellow -flame and is known as bituminous coal. This is the great fuel for -the production of steam. A continuation of the metamorphic processes -carries off a further proportion of the volatile matter and leaves a -dense, hard, black substance with sometimes as much as ninety-five -per cent of carbon. This is the so-called “hard coal” or anthracite -generally used for fuel in our houses, for which purpose it is so well -adapted because it burns with a production of much heat and almost -without smoke. - - - - -APPENDIX C - -THE PREPARATION OF TOPOGRAPHICAL MAPS - - -=Topographical maps a library of physiography.=—For the satisfactory -working out in detail of the geology of any region of complex -structure, an accurate topographical map is prerequisite. This is so -much the more true because nearly all complexly folded or faulted rock -masses are to be found in mountainous, or at least in hilly regions. -The making of the topographical map must, therefore, precede that of -the geological map, and in modern usage the latter is a topographical -and a geological map combined in one. - -Within certain narrow limits, predictions concerning the geological -history of a province may often be made by an expert geologist from -examination of an accurate topographical map. Just as in forecasting -the weather upon the basis of the usual weather maps, such predictions -can sometimes be made with entire confidence in their accuracy, -while at other times a guess only may be hazarded. The great value -of the modern topographical map is becoming, however, universally -acknowledged, and every highly civilized nation has either completed -or has in preparation sectional topographical maps of its domain on -such a scale as is warranted by its financial condition and its state -of development. Thus there is now being accumulated a vast library of -geographical and to some extent geological information, of which the -student of geology must be prepared to make use. - -=The nature of a contour map.=—More and more the contour map is -replacing the earlier and less scientific methods of representing -topography on the large scale sectional maps, and hence this type only -need here be considered. In the contour map, the relief of the land -is represented by a series of curving lines, each the intersection of -a particular horizontal plane with the land surface, and the several -planes separated by uniform differences of elevation. This altitude -interval is known as the contour interval. Its choice is a matter -of considerable importance, for though regions of relatively simple -topography may be adequately represented upon a map of large contour -interval, say one hundred feet, another district may require an -interval as short as five feet. A contour map with this interval may be -conceived to have been made by flooding the region which it represents -and preparing maps of the shore lines for each rise of five feet of the -water surface, and superimposing the several maps thus derived with -accurate registration one above the other. Wherever the land slopes -are steep, the shore lines of the several maps will be crowded closely -together and give the effect of a relatively dark local shade; where, -upon the other hand, the surface is relatively flat, the several shores -will be widely spaced and the effect will be to produce a white area -upon the map. Thus in contour maps dark tones indicate steep gradients -and pale tones a flatness of surface. - -=The selection of scale and contour interval.=—With the use of -the small scale in the contour map, the tones of the map will be -correspondingly dark, though the relative differences in tone will -remain the same. With the use of a closer contour interval the tones -will deepen throughout. The adjustment of scale and contour interval -to any given region is a matter requiring experience in topographical -mapping, and in addition a knowledge of the geological significance of -topographic features. Unfortunately, the element of expense and the -special commercial objects held in view, conspire to select scales -and contour intervals which are often little adapted to the districts -surveyed. - -=The method of preparing a topographical map.=—Having fixed upon the -scale and the contour interval which is to be employed, the task of -the topographical surveyor is next to fix accurately the positions and -the elevations of a sufficient number of points to _control_ the map, -and then to hang, as it were, upon these points as attachments the -design represented by the relief. Were the surface of the ground to -be represented by a flexible fabric, the map maker might raise from a -flat base a series of stout posts of the heights and in the positions -which he has determined, and upon these supports arrange the slopes of -the fabric much as drapery is adjusted. The determination of the exact -positions and the elevations of his control stations is, therefore, -a process coldly precise and formal; whereas in the shaping of the -surfaces his attention should be fixed more upon correctly reproducing -the shapes than upon fixing accurately the position of every point. -As a matter of fact, the position of the average point will be most -accurately fixed when the shapes of the features are most clearly -comprehended. To some extent, therefore, the topographer should be -familiar with the geological significance of the earth features which -he is representing. - -=Laboratory exercises in the preparation of topographical maps.=—The -principles which underlie the surveyor’s method for preparing a -topographical map may be learned in the laboratory by the use of -models and the simple device shown in plate 24 A and B. To represent -the section of country to be mapped a model in plaster of Paris is -substituted, and this is placed within a rectangular tank to which -locating carriages and altitude gauges are attached that allow the -student to fix the position and the elevation of any point upon the -surface of the model. - -┌──────────────────────────────────────────────────────────────────┐ -│ PLATE 24. │ -│ │ -│ [Illustration: _A._ Apparatus for exercise in the preparation of │ -│ topographic maps.] │ -│ │ -│ [Illustration: _B._ The same apparatus in use for testing the │ -│ contours of a map.] │ -│ │ -│ [Illustration: _C._ Modeling apparatus in use.] │ -└──────────────────────────────────────────────────────────────────┘ - -Upon each model the student “locates”, or fixes, the position of a -sufficient number of points for the control of his map, entering upon -an appropriate map base for each position the altitude which was read -from the gauges. Now _with the map always before him_ he “sketches in” -the forms of the surface by means of contour lines. For this purpose -it is often desirable to fix roughly the direction of the steepest -slope at a number of places, and noting the differences in elevation -between control stations, divide up the distance in accordance with the -curves of slope and start the contours at right angles to the slope. -Afterwards such sections are connected by sketching in with the model -always in view for control (Fig. 488). - -[Illustration: FIG. 488.—A student’s map prepared from a model by the -use of the contour apparatus represented in plate 24 A.] - -=The verification of the map.=—The map prepared, its accuracy may be -tested by a simple method which is denied the topographer who has to -do with the actual surface of the ground. The locating carriages and -altitude gauges are removed from the tank, which is next filled with -water and leveled by means of guide marks upon the interior. A few -drops of milk or of ordinary clothes blueing are added to the water to -render it opaque, and it is then drawn off at the faucet in successive -installments, so that the surface drops by layers corresponding in -thickness to the contour interval of the map, plate 24 B. As each layer -is withdrawn, that contour of the map to which the shore line should -correspond is carefully examined and corrected. By such corrections -the nature of the first errors made is soon appreciated, and the -method of procedure is thus more easily acquired. At the same time the -significance of the design of the map is more quickly learned than by a -mere examination of the standard government maps. - -The work above outlined calls for waterproofed models of suitable form -and size, and a series, each of which sets forth some typical feature -or series of features, has been designed by Mr. Irving D. Scott.[2] - -=The preparation of physiographic models.=—The apparatus used -to prepare the topographic map is adapted also for preparing a -physiographic model from a standard topographical map. For this purpose -the method is essentially reversed, though the tank is replaced to -advantage by a light metal frame elevated upon one side so as to permit -a free use of the hands in modeling the clay. - -The material used in preparing the model is artists’ modeling clay[3] -which has a base of beef suet, and hence does not dry out and crack as -does ordinary clay. Its form is, therefore, retained indefinitely, and -it may be used again and again. Most maps must be enlarged in modeling, -and the simplest way is often to photographically or by pantograph -enlarge the map to the scale of the model. The map prepared, it is -covered by a thin celluloid plate which has cut upon it a series of -crossed lines spaced in inches and larger subdivisions to correspond to -those of the locating carriages (plate 24 C). - -The enlargement of the map is not essential to experienced workers, -and the standard map may be covered in similar manner by a transparent -plate with “checkerboard” design, the squares of which bear some simple -relation in size to the larger divisions of the locating carriages -(Plate 24 C, rear). - -The method of preparing the model is comparatively simple. Beginning at -any point upon the map, the intersection of a heavy contour line with -one of the guide lines of the celluloid “position plate” is carefully -noted. Both the position and the elevation of this point are fixed by -the point of the altitude gauge of the modeling frame, and the clay -built up beneath it to that height. With the fingers the clay is now -roughly shaped in various directions from this point, the altitude -gauge is advanced by the locating carriage so as to correspond in -position to the intersection of the next heavy contour line with the -same guide line of the position plate, and the elevation for this point -similarly adjusted upon the model. As before, the surface of the clay -is roughly shaped in advance and upon the sides so as to conform to -the indications of the map; and this process is repeated until the -work is finished. Corrections for intermediate positions may be -carried to any desired degree of refinement which the scale and the -accuracy of the map permit. Models which are larger than the area of -the modeling frame are prepared by making a square foot at a time by -the above described process, and then moving the frame forward and -adjusting in a new position by means of the sharp pins in the legs -of the apparatus. - - - READING REFERENCES - - WILLIAM H. HOBBS, New Laboratory Methods for Instruction in Geography, - Journal of Geography, vol. 7, 1909, pp. 97-104. Also Scot. Geogr. - Mag., vol. 24, 1908, pp. 643-652. The Modeling of Physiographic Forms - in the Laboratory, _ibid._, vol. 8, 1910, pp. 225-228. - - - - -APPENDIX D - -LABORATORY MODELS FOR STUDY IN THE INTERPRETATION OF GEOLOGICAL MAPS - - -[Illustration: FIG. 489.—Models to represent outcrops of rock.] - -The laboratory models which have been described on page 63, and are -used to represent outcrops in the study of geological maps, are shown -in Fig. 489. The drum-shaped blocks serve to represent massive rocks -which occur in irregularly shaped masses such as batholites and flows. -The long, narrow strips are for intrusive rocks in the form of dikes, -while the larger blocks provided with a swivel joint are used for -outcrops of sedimentary rocks, and after adjustment they give the dip -and strike of the exposure. The wing bolts used in their construction -should be of bronze, because of the effect of iron upon the compass. -For the same reason tables should not be placed near iron beams or -columns. All these blocks can be made by an ordinary carpenter, and -should be available in sufficient numbers to arrange problems like -those of Figs. 47, 48, and 490. With a view to supplying suggestions -for other problems of the same general nature, the three additional -field maps of Fig. 491 have been introduced. - -[Illustration: FIG. 490.—Special laboratory table set with a problem -in geological mapping which is solved in Figs. 47 and 48.] - -[Illustration: FIG. 491.—Three field maps to be used as suggestions in -arranging laboratory tables for problems in the preparation of areal -geological maps.] - -The list of questions given below is intended to indicate the nature of -some of the problems which the student should be asked to solve in the -preparation of each map. The numbers in parentheses refer to pages in -this book where further information is given:— - - STRATIGRAPHICAL - - 1. Of the formations represented what ones are sedimentary and what - igneous (Chap. IV, App. B)? - - 2. Which formations, if any, are separated by unconformities (51-53)? - - 3. What is the order of age of the sedimentary formations (65)? - - 4. What are the _exposed_ thicknesses of each of these formations - (48-49)? - - 5. Do any of these values represent full thickness of the formation, - and if so, which ones? - - 6. What is the age in terms of the sedimentary formations of each of - the igneous rock masses (65)? - - 7. Which igneous rocks, if any, occur in batholites (143, 441)? Which, - if any, in dikes (140)? - - - STRUCTURAL - - 8. What formations, if any, have monoclinal dip (42)? - - 9. Indicate upon the map by dashed lines the crests of all anticlines - and the trough lines of synclines. - - 10. Indicate by arrows the direction of pitch of all plunging - anticlines and synclines wherever disclosed by changes of dip and - strike (43). - - 11. Indicate the approximate position of all faults whose position - is disclosed (58-61), and, if possible, state which limb is the one - downthrown. - - 12. Prepare suitable geological sections. - - - READING REFERENCE - - WILLIAM H. HOBBS. Apparatus for Instruction in Geography and - Structural Geology. III. The Interpretation of Geologic Maps. School - Science and Mathematics, vol. 9, 1909, pp. 644-653. - - - - -APPENDIX E - -SUGGESTED ITINERARIES FOR PILGRIMAGES TO STUDY EARTH FEATURES - - -The chief value of the laboratory studies discussed in the preceding -appendices is as a preparation for observations made in the field—the -laboratory _par excellence_ of the geologist. The pilgrimages whose -itineraries are here suggested have been planned especially for -impressing by observation the lessons of this book. Such journeys are -best interrupted at a relatively small number of localities which, -because already studied in some detail, are specially adapted to serve -as centers for local excursions. These localities will in most cases -be the great scenic places to which tourists resort, or the seats of -universities near which specially detailed explorations have been often -made. - -Within the United States a few local geological guides have been -published, and the Geologic Folios published by the United States -Geological Survey are already available for a number of such centers. -For one long geological pilgrimage we are fortunate in having a -carefully prepared guide, namely, from New York to the Yellowstone -National Park and back, with a side trip to the Grand Cañon of the -Colorado. Except for the side trip this route, in large measure, -corresponds with one here chosen, and for the return journey especially -the student is referred to it for information (Geological Guide Book of -the Rocky Mountain Excursion, edited by Samuel Franklin Emmons. Comte -Rendu de la Congrés Géologique Internationale, 5me Session, Washington, -1891, 1893, pp. 253-487, map and plates 13, figs. 32). - -Our journey is begun at New York City, which is built about the deeply -submerged channels of an estuary choked with glacial deposits, though -the channel may be followed as a deep cañon across the continental -shelf to its margin (252,[4] pl. 17 B). New York City is also upon the -margin of the glaciated area, the outer terminal moraine of which is -well represented on Long Island (298). Across the Hudson in New Jersey -is the great Coastal Plain which meets the oldland in a well-defined -margin (159, 246, 247). A local geological guide of the vicinity of the -metropolis has been written by Gratacap (Geology of the City of New -York, Greater New York. Brentanos, New York, 1904, pp. 119, pls. and -map). - -Traveling by the New York Central Railway, we follow up the Mohawk -outlet of the glacial lakes Iroquois and Algonquin (334), first -skirting upon the east the great sills of intrusive basalt known as the -Palisades, with their markedly columnar jointing and intersections by -numerous faults. Above Peekskill we enter the picturesque narrows of -the river (174), cut in the hard crystalline rocks of the Highlands. -Entering the Mohawk Valley, we pass Syracuse with limestone caverns -and well-oriented joints widened by solution through the agency of the -descending ground water (181, pl. 6 B). A branch line to the southwest -reaches the vicinity of Cayuga Lake and Ithaca, where are well-oriented -joints which have controlled the drainage directions, and there is also -a typical strath (55, 87, 428). - -To Niagara Falls at least a day should be allotted for the “gorge ride” -by trolley car, thus making the complete circuit of the brink of the -gorge with interruptions and local studies at all important points -(352-366, pl. 23 A). From Niagara Falls over the Michigan Central -Railway we reach Detroit on the present outlet of the upper Great Lakes -as well as of the later Lake Algonquin (334). From this city as a -center a trip is made by electric railway to Ypsilanti and Ann Arbor, -across the bottoms of the early glacial lakes from the first Maumee to -Warren (330-333). The strong Whittlesey beach is encountered at the -little station of Ridge Road, and one of the Maumee beaches on Summer -Street in Ypsilanti. The city of Ypsilanti is built upon a terrace -(165) of the Huron River, and another terrace in the same series is -crossed by the electric line. In an excursion of a few miles down the -river, passing meanders (164-165) and ox-bow lakes (165, 415), is -found an interesting case of stream capture near the little village of -Rawsonville (175. See Isaiah Bowman, Jour. Geol., Vol. 12, 1904, pp. -326-334). - -Continuing our journey from Ypsilanti over a high moraine (312), Ann -Arbor is reached, built upon the level plain of outwash with fosses -sometimes separating it from the moraine (281, 314). Upon the campus of -the university are great bowlders of jasper conglomerate and jaspilite, -which were transported from the north by the continental glacier (305). -Across the river from the Michigan Central station and behind the -little church is a delta formed in one of the glacial lakes Maumee and -here opened in section (168). West of the city is a great valley which -was the former course of the Huron River when thus diverted by the -continental glacier lying to the eastward of Ann Arbor—border drainage -(see Ann Arbor folio by the U. S. G. S., and, further, R. C. Allen and -I. D. Scott, An Aid to Geological Field Studies in the Vicinity of Ann -Arbor, George Wahr, publisher, Ann Arbor). - -Returning to Detroit (M. C. Ry.), the great Sibley quarries in -limestone near Trenton may be visited. They display perfect jointing, -numerous fossils, and especially well-glaciated surfaces interrupted -by deep troughs and showing striæ of several glaciations (304). From -Detroit the journey is continued by steamer to Mackinac Island in the -strait connecting Lakes Michigan and Huron, passing on the way through -the peculiar delta of the St. Clair River (431), and coming in view of -the notched headlands, which are a monument to the post-glacial uplift -of the glaciated area (250, 341). A day is spent at Mackinac Island -and St. Ignace in order to study with some care these uplifted strands -of the late glacial lakes (341-344). Chicago may now be reached either -by steamer or by rail, and in its vicinity we may see the elevated -beaches and the ancient outlet of Lake Chicago (331-332, 347, pl. 22 -A. See Chicago Folio, U. S. G. S.). By the Chicago and Northwestern -Railway the area of recessional moraines and intermediate outwash -plains, and later that of the drumlins, are crossed in journeying to -Madison, Wisconsin. By examination of the maps on pages 308 and 317 -in connection with the larger scale atlas sheets of the United States -Geological Survey (Janesville, Evansville, and Madison sheets), this -car journey can be made most instructive in gaining familiarity with -the characteristic glacial features, and this study is continued to -special advantage in excursions about Madison as a center (316-317, -407). This is the more true since at numerous localities in the -vicinity of Madison the well-striated glacier pavement is exposed for -comparison of the striæ as regards direction with the axes of the -several types of glacial features. - -An especially instructive excursion may be made by carriage in a single -day to the “driftless area” some twelve miles west of the city. Before -reaching it we cross in alternation a series of recessional terminal -moraines (pl. 17 C) and outwash plains, and near Cross Plains encounter -the partially dissected upland with its arborescent drainage and -even sky line (298, 300-301, 312-313, pl. 16 A and B). Typical shore -formations (233, 241, 242) are studied to advantage about Lake Mendota -in a walking trip to and beyond Picnic Point, where are found the best -ice ramparts (431-434. See Buckley, Trans. Wis. Acad. Sci., Vol. 13, -pp. 141-162, pls. 18). - -Our journey is now continued over the Chicago and Northwestern Railway -to Devils Lake near Baraboo, where we cross a salient of the driftless -area, within which lies Devils Lake, imprisoned in a former valley of -the Wisconsin River, since diverted to another course as a result of -the glacial invasion (312-313). The valley here is a former narrows in -hard quartzite (466), which towers above the lake in unstable chimneys -(300), such as the Devils Tower, but such remnants are not found on -the other side of the moraine, being there replaced by rounded rock -shoulders. Just north of the lake the marginal moraine which blocks -the valley is so characteristic as to merit special study (pl. 17 -C). Only a few miles northward along the railway from Devils Lake is -Ableman, where, exposed in a high cliff, the hard purple quartzite -with beautiful ripple marks to reveal its plane of sedimentation -(pl. 11 A) dips vertically, and is overlain by horizontally bedded -yellow sandstone. The marked angular unconformity which is thus -displayed is further made evident by a basal layer of conglomerate -(463) in the sandstone (51-53). Here also are deposits of loess along -the river, which display their vertical joint surfaces (207). An -excellent geological guide to this interesting district and that of -the neighboring “Dalles” of the Wisconsin River has been written by -Salisbury and Atwood (The Geography of the Region about Devils Lake and -the Dalles of the Wisconsin, etc., Bull. 5, Wis. Geol. and Nat. Hist. -Surv., 1900, pp. 151, pls. 38, figs. 47). - -If we have taken a conveyance at Devils Lake for Ableman, we may -continue in the same manner to Kilbourn, where begin the picturesque -Dalles of the Wisconsin River—here a young gorge cut in sandstone, -because the Wisconsin was diverted from its old valley to border -drainage at the edge of the driftless area (300, 321). The side cañons -of the river, through their abrupt zigzags, reveal the control of their -courses by the joint system (224). In the journey up the rapids by -steamer to inspect the Dalles, we observe many beautiful examples of -cross bedding in the sandstone (37). - -From Kilbourn we continue our journey to Minneapolis over the Chicago, -Milwaukee, and St. Paul Railway, and near Camp Douglas are over a -peneplain, out of which rise prominent monadnocks (171). At La Crosse -the Mississippi River is reached, flowing beneath bluffs of sandstone -which are capped by loess (207). The meanderings and the numerous -cut-offs of the Mississippi may be observed to the left (415). Lake -Pepin is a side-delta lake blocked by the deposits of the Chippewa -River (419). - -From Minneapolis an excursion is made to Fort Snelling to view the -young gorge of the Mississippi, cut by the Falls of St. Anthony for a -distance of about eight miles in manner similar to that of the seven -miles of Niagara gorge (354), and to compare this narrow gorge with -the broad valley of the Warren River which drained Lake Agassiz (327). -Somewhat farther up the Warren River are examples of saucer lakes (416). - -From Minneapolis the journey may be continued by the Great Northern -Railway to Livingston, Montana, thus crossing between the stations of -Muscoda and Buffalo the bed of Lake Agassiz and its marginal beaches -(325-328. For local geology of Minnesota consult C. W. Hall, Geology of -Minnesota, Vol. 1, Minneapolis, 1903). - -The Yellowstone Park is entered from Livingston (Livingston Geological -Folio, U. S. G. S.) and departure from it made at the relatively -new Union Pacific terminal at the southwest margin. The regular -trip through the Park includes visits to the several geyser basins -(191-194), Obsidian Cliff (33, 463), the Cañon of the Yellowstone, etc. -Good climbers can make a side trip from near the Mammoth Hot Springs -to the top of Quadrant Mountain, the remnant of a “biscuit cut” upland -(372), and there study the nivation process (368, Yellowstone National -Park Folio, U. S. G. S.). - -The trip from the Park to Salt Lake City, over the Union Pacific -Railway, passes through the Red Rock Pass, the former outlet of Lake -Bonneville (423), into the desert of the Great Basin (Chaps. XV and -XVI). Great Salt Lake is a saline lake or sink with an interesting -record of climatic changes (198, 401). The front of the Wasatch Range, -in view and easily reached from Salt Lake City, is deeply scored by -the horizontal shore terraces of Lake Bonneville (198, 199), and -these terraces are extended at every reëntrant by barrier beaches of -great perfection. In the Pleistocene period mountain glaciers in part -occupied the valleys of this range, though they did not always extend -as far as the mountain front. Big Cottonwood Cañon, which realizes this -condition, and the neighboring Little Cottonwood Cañon, from whose -front its glacier spread into an expanded foot (264), thus show for -comparison in a single view the V and the low U sections respectively -(172, 376). Here are also alluvial fans (213) and recent faults which -intersect them. - -From Salt Lake City the return to New York may be made by the Denver -and Rio Grande Railway across deserts and through the Royal Gorge, -the cañon of the Arkansas River. A full itinerary of the points of -geological interest along this route, and continued to Chicago, -Washington, and New York, is supplied in much detail in the guide of -the geological excursion to the Rocky Mountains above cited. This -the traveling geologist should not fail to study. Some references to -points along this journey will be found on preceding pages of this -book (219-220, High Plains; 170, Allegheny Plateau in West Virginia; -176, water gap of Harper’s Ferry; 176-177, 184-186, side trip up the -Shenandoah Valley to Luray Caverns and Snickers Gap; 251, Chesapeake -Bay). - -Instead of returning directly from Salt Lake City, the traveler, -if he has sufficient time at his disposal, may extend his journey -southwestward across the Great Basin to Los Angeles. A branch line -from this route leaves the Vegas Valley and passes within reach of the -famous Death Valley (201) to Tonopah (79) and the Owens Valley (77-78, -92), where are many surface faults dating from the earthquake of 1872 -and other less recent disturbances. Returning to the junction point, -the route continues across the Colorado and Mohave deserts to Los -Angeles. From Los Angeles as a center the exceptionally interesting -terraces, caves, and stacks of an uplifted coast are to be seen -to best advantage near Pt. Harford (Chap. XIX). The islands of San -Clemente and Santa Catalina may also be reached from Los Angeles (239, -248, 249, 250, 256, 257, pls. 5 B, 7 A, 12 A). The return to the East, -if made by the Santa Fe Railway, permits of a visit to the Grand Cañon -(174, 443) from the station of Williams. From that point eastward the -geology of the route is fully covered in Emmons’ Guide to the Rocky -Mountain Excursion already cited. - - * * * * * - -For the benefit of those who are privileged to travel in Europe, and -the number increases yearly, a pilgrimage is suggested which may easily -be made to correspond with plans laid out on the basis of historical, -artistic, and scenic points of interest. The only popular guide of a -general nature written for geologists traveling abroad appears to be -a brief but valuable little paper by Professor Lane (The Geological -Tourist in Europe, Popular Science Monthly, Vol. 33, 1888, pp. -216-229). The publishing house of Gebrüder Bornträger in Berlin is -now publishing a quite valuable series of geological guides dealing -with special districts and written by well-known authorities (Sammlung -Geologischer Führer). Of this series some thirteen numbers have already -been issued. Many other valuable local guides of a geological nature -are the Livrets Guides of the International Geological and Geographical -Congresses, and the similar pamphlets supplied in connection with -annual meetings of national or provincial geological societies. - -Passengers on steamships sailing from the harbor of New York pass -out over a deeply submerged cañon (252) largely filled with glacial -deposits, through the Narrows (174), and in sight of Sandy Hook, a -modified spit (238, 240). To the left are seen the great morainic -accumulations at the border of the glaciated area on Long Island (298). -In the course of the trans-Atlantic voyage a much-rounded iceberg may -be encountered (291), though this is much more apt to occur upon the -northern routes from Quebec, and late in the season. Upon entering the -English Channel the land on both coasts rises in steep cliffs, where -are found all the common shore features well developed (Chap. XVIII). -The German steamships pass in sight of Heligoland, that last remnant of -wave erosion (236). - -While traveling in Europe, the student should consult a map of the -glaciated area (299), and so learn to recognize its peculiarities, and -carefully mark its marginal moraine (311) and other strongly marked -features. - -If the British Isles are visited and the more rugged areas are -selected, one may study the cirques and other characteristic features -due to the presence of mountain glaciers about Snowdon (Chap. XXVI). -More mature stages of the same processes are to be found in the -Scottish Highlands and the Inner Hebrides, but especially upon the -Island of Skye (Fig. 492). A very valuable aid to excursions in -this district is Baddeley’s Scotland (part I, Dulau, London) and -Sir Archibald Geikie’s Explanatory Notes to accompany Bartholomew’s -Geological Map of Scotland (map and notes in cover, Edinburgh, 1892, -pp. 23). - -[Illustration: FIG. 492.—Sketch map of Western Scotland and the -Inner Hebrides to show location of some points of special geological -interest.] - -It is from Oban, the “Charing Cross of the Highlands”, that one should -start out upon the summer steamers in order to reach both Skye and -Staffa, the latter with fine basaltic columns (463), and Fingal’s -Cave. In sailing to Skye one passes upon either shore of the narrow -fjords many relics left in the dissection of volcanoes (139-143 and -Sir A. Geikie, Ancient Volcanoes of Great Britain, Vol. II); also -rocky islands and skerries marking submergence (252), and the coast -terraces which register a later uplift (250). Skye is a complex of -many intrusive and volcanic rocks of such markedly different colors -as to appear as tints in the landscape. In the Cuchillin Hills of dark -green rises the massive gabbro (462) cut by cirques into the jagged -pinnacles of horns and comb ridges (373); while lower down and to the -east are rounded domes of rhyolite (463) abraded beneath the glaciers -and of a delicate salmon tint. Still lower and to the westward are -flat mesas composed of horizontal layers of black basalt under a rich -carpeting of the brightest verdure. Eastward across the channel are -seen the purplish walls of an ancient sandstone. The jagged gabbro -core of the island thus represents a fretted upland (372) and is now -the training ground of the Alpinist (Abraham, Rock Climbing in Skye, -Longmans, London, 1908), while nestled in one of the bottoms of a -U-valley is Loch Coruisk, a typical rock-basin lake (412), its shores -of hard rock planed and scored. - -From Skye we may go to study the remarkable thrusts (45) on the north -shore of Loch Maree, a marked lineament, and one directed at right -angles to that on the course of the Caledonian Canal connecting -Loch Linne with Loch Ness. This northeast wall of Loch Maree is a -strikingly rectilinear fault represented by an escarpment, up which -we climb to find at the top the crushed and fluted thrust planes of -movement dipping southeastward at a flat angle. Here also are beautiful -rock-basin lakes, lying in hollows molded beneath the continental -glacier. On our way from Skye we have passed up Loch Carron, a sea loch -or fjord (252), and along the strath at its head known as Strathcarron -(428). - -Returning now to Oban, it is but a short trip by steamer up Loch Linne -to Fort William along the striking lineament (226) which continues to -Loch Ness and beyond (Fig. 492), and thence by rail to Glen Roy and the -neighboring glens of Lochaber (322-325). - -From Paris as a starting point, we may visit in a most picturesque -region the beautifully preserved craters of extinct volcanoes in the -Auvergne of Central France (105, 124, 145), which district is entered -from Clermont-Ferrand. Here are found the characteristic puys, steep -lava domes of viscous lava (105), which figured largely in the early -controversies of geologists concerning the origin of rocks. - -[Illustration: FIG. 493.—Outline map of a geological pilgrimage across -the continent of Europe.] - -The rest of our pilgrimage will be so planned as to enter the noble -river Rhine at its mouth (Fig. 493), ascend its course to its -birthplace in the snows of Switzerland, and after further exploration -of the features of this fretted upland, traverse northern and central -Italy so as to make our departure for America by the southern route. -Entering then upon this course in the Low Countries, we have first -the opportunity of observing the characteristics of a great delta -with natural levees artificially strengthened as dikes (165-168). -Here also are found dunes of beach material which has been raised by -the wind into a great rampart near the shore (209-211). Such a wall -of dune sand is well displayed at the bathing resort at Scheveningen -near the Hague (421). The flood plain of the Rhine (162-165) may be -studied in a journey up the river to the university town of Bonn, from -whence a day’s excursion should be devoted to the relics of volcanoes -known as the Seven Mountains (H. von Dechen, Geognostischer Führer in -das Siebengebirge, Bonn, 1861). As a preparation for this trip and -others in the volcanic Eifel higher up the river, a visit should be -made to the mineral and rock collections of the Poppelsdorfer Schloss -at the University. In the volcanic Eifel are found some of the most -interesting of crater lakes (405), the largest being Lake Laach -with its somewhat peculiar volcanic ejectamenta and its picturesque -abbey (see von Dechen, Geognostischer Führer zu der Vulkanreihe der -Vorder-Eifel, etc., Bonn, 1886. Consult also Lane, A Geological Tourist -in Europe, _l.c._). - -Continuing our course up the river from Bonn, we soon enter the gorge -of the Rhine cut in an uplifted peneplain (169, 171, 174). From -Coblenz, where the Moselle enters the Rhine, a side trip may be made up -this tributary river past Zell with its entrenched meanders (173) to -the ancient Roman city of Treves. Above Bingen on the Rhine we leave -behind us the narrow gorge and rapid current of the river and continue -over the broad floor at the bottom of a rift valley (403), lying -between the forest of Odin and the Black Forest on the east and the -“Blue Alsatian Mountains” far away to the west. At the margins of this -plain are beds of loess with their characteristic joint structures and -inclusions (207), and in the higher hills on either hand a wealth of -intrusive igneous rocks. - -At the entrance of the Neckar River to this broad plain is nestled the -picturesque castle and university town of Heidelberg, a convenient -center for excursions (Julius Ruska, Geologische Streifzüge in -Heidelbergs Umgebung, etc., Nägele, Leipzig, 1908, pp. 208, map). -At Strassburg (Schwarzwaldstrasse 12) is located the German Chief -Station for Earthquake Study, with a particularly large set of modern -seismographs. In the university cabinet is also one of the largest and -most representative mineral collections in Europe. For excursions in -the neighborhood consult Benecke, Sammlung Geognostische Führer, Vol. -5, Elsass, 1900. - -From Strassburg we may go by the Black Forest Railway to the Hegau with -its volcanic plugs (140), each surmounted by a picturesque castle. -We enter next the broadly extended piedmont apron site, above which -Lake Constance still remains as a border lake (399). Outwash aprons -(314), moraines (311), and drumlins (317) are each in turn encountered. -Still continuing our course up the Rhine from Bregenz, we enter the -fretted upland (372) of the Alps, mountains composed of great folds and -thrusts about a core of intrusive rock (Rothpletz, Sammlung Geologische -Führer, Vol. 10, 1902, Thrusts in the Alps between Lake Constance and -the Engadine). Some fourteen miles above Chur we pass the terrace -produced by successive landslides (414), known far and wide as the -Flimser Bergstürz. The further assent of the cascade stairway of this -glacier-carved valley brings us to the Furka Pass, from which point -magnificent views of the fretted upland are obtained. At the Känzli, a -mile from the hotel, one may view the névé of the Rhone Glacier, which -may also be easily visited. - -We have now followed a great river from its mouth in the sands of -Holland to its source in the snows of the higher Alps. Passing over -the divide and descending to Gletsch, we may observe the lower end, -or foot, of the Rhone glacier and the crevasses and séracs (391) on -the steep descent of this radiating glacier (383, 386). The response -which glaciers make to climatic changes is here well illustrated by the -recession of the glacier front from near the hotel (its position in the -’50s of the nineteenth century) to its present position about a mile -farther up the valley. - -The characteristics of a glaciated mountain valley may be further -illustrated by climbing to the Grimsel Pass, which is scratched and -striated (377, 385), and then descending the valley of the Aar to -Meyringen (377). Near the Grimsel Hospice are the characteristic rock -basin lakes (412), and upon the Aar Glacier to our left were carried -out the epoch-making researches of Louis Agassiz, the founder of the -glacial theory for explaining the drift. We encounter some thirteen -rock bars (377). Just before reaching Meyringen we pass the last of -these, the Gorge of the Aar, cut by the stream through limestone. - -Interlaken (419) may be made the center for additional excursions up -the Lauterbrunnen Valley, with its prominent albs (376) and its ribbon -fall of the Staubbach (378). By the Jungfrau Mountain railway we may -now ascend partly in tunnels of the rock to the Ewigeismeer, and look -down upon the névé and bergschrunds of the Great Aletsch Glacier (370, -see Baltzer, Sammlung Geologische Führer, Vol. 10, Bernese Oberland, -1906). Returning to Interlaken by way of Grindelwald, one may study the -foot of a radiating glacier, the Untergrindelwald glacier, with its -tunnel and its milky and braided stream. - -Crossing now the Alpine foreland to Villeneuve at the upper end of -Lake Geneva and upon a well-developed strath (426, 428), we may look -out upon the turbid waters extending far from the shore of the lake. -Journeying to Geneva by steamer we note the gradual clearing of the -water until at the outlet of the lake it is as clear as crystal. A -walking trip from Geneva takes us to the Bois de la Bâtie, where the -Arve with turbid waters meets this clear stream (427). - -The railroad to Chamonix ascends another cascade stairway (376), -affords views of complexly folded sedimentary rocks (43), and at -Chamonix itself the mer de glace supplies opportunities for the study -of moraines (386, 393) and glacial movement (390-392). To experienced -Alpinists the summit of Mount Blanc offers a remarkably extended -outlook over the fretted upland of the Alps (pl. 18 A). From the -station of LeFayet below Chamonix, one may ascend to the Désert de la -Platé, where are Schratten in limestone due to solution (188). - -Crossing by one of the passes to the valley of the Rhone at Martigny -we may reach Zermatt, to-day the climbing center of the Alps. From -the subordinate cirques surrounding this village descend the Gorner, -Findelen, St. Theodul, and other components of this radiating glacier. -A black tooth of rock, the Matterhorn, towers above the other peaks and -shows to greatest advantage this feature of glacial sculpture (374), -while the Gorge of the Gorner is a severed rock bar like that of the -Aar (377). Either on foot or over the mountain railway we may ascend to -the Gorner Grat, a subordinate comb ridge (373) which affords one of -the most magnificent and instructive views of radiating glaciers. - -From Brig, farther up the Rhone Valley, an excursion is made to the -Eggishorn Hotel, a center for study on and about the Great Aletsch -Glacier (329, 371, 385, 388, 395, 410). The easy ascent of the -Eggishorn is rewarded by a view almost directly downward upon the -ice-dammed Márjelen Lake (329, 411). - -From Brig one may make his entry into Italy, either over the -picturesque Simplon route afoot or by diligence, or else beneath it -through the railway tunnel. By an alternation of short steamboat and -rail trips the journey is continued in a direction transverse to the -longer axes of the border lakes Maggiore, Lugano, and Como, and later -southward to Milan. In leaving the village of Como we pass over heavy -morainic deposits on the apron borders of the expanded-foot glacier -(383, 385) which once occupied the valley above. On the journey from -Milan to Venice, over the fertile plains of Lombardy, the similar -accumulations about Lake Garda (414) are first encountered at the -little station of Lonato and left behind at Somma Campagna (Tornquist, -Sammlung Geologische Führer, Vol. 9, Northern Italy, 1902). - -The city of Venice is built upon pile foundations in the lagoon behind -the barrier beach known as the Lido (242, 428-429). From here we may -reach the Karst country by way of Trieste, some of the more interesting -and typical features being found near Divača (187-189, 422, pl. 6 A). -In a different direction from Venice by way of Belluno we enter the -Dolomites with their patterned relief and battlemented towers (228, -445). - -Additional centers for geological excursions on the route to our point -of departure from Italy are Rome and Naples. At the Italian capitol and -in its neighborhood we may study the volcanic Campagna with its beds -of tuff (105) and its crater lakes (405. See Sir A. Geikie, The Roman -Campagna, Landscape in History and other Essays, Macmillan, 1905, pp. -308-352; also Deecke, Sammlung Geologische Führer, Vol. 8, Campagna, -1901). From Rome it is an easy journey to the cataract of Tivoli with -its deposits of travertine (184). In the opposite direction from Rome -across the Campagna rise the Alban Hills, ruins of a composite cone -with several crater lakes on the sites of former vents. On the summit -of the encircling crater rim, like the Monte Somma of the Vesuvian -Mountain now a crescent only, is located the chief Italian station for -earthquake study. - -From Naples we may reach in short excursions and study with some -care still active volcanic mountains. To the east is Mount Vesuvius -(94, 97, 122, 124, 127-137), which was in grand eruption in April, -1906. Westward from Naples are the Campi Phlegraeii, or burning -fields, with many craters. Of these Astroni offers a fine example of -a large-cratered cinder cone (105). In the same vicinity are Monte -Nuovo (96) and the Solfatara (97), the latter a type of volcano which -no longer erupts lava, but in its place emits carbon dioxide and other -gaseous emanations (Grotto del Cane). The starting point for excursions -in the Phlegræan fields is Pozzuoli with its Temple of Jupiter Serapis -(254-255), reached from Naples by an electric line which pierces the -wall of an immense crater (Posilippo) composed of fine yellow volcanic -ash known as Pozzuolan. - -From Naples steamers make short excursions to Sorrento with its deep -ash deposits, and to Capri with its blue grotto (257-258). Herculaneum -(139) and Pompeii (122), buried during the eruption of 79 A.D., are on -the line of the Circum-Vesuvian Railway. - -Steamships to New York from Naples call at Gibraltar, the land-tied -island _par excellence_ (241). Most steamships of the southern route -pass through or near the volcanic islands of the Azores, and certain -boats touch at Algiers, from which a line of railway gives access to -Biskra on the borders of the Desert of Sahara. - -Throughout these pilgrimages the traveler should be on the alert to -note not only the agent responsible for the features which come under -his observation, but, especially where this is the common sculpturing -agent of running water, he should not fail to notice the stage of the -erosion cycle which is represented (Chapter XIII). - - - - -INDEX - - - Abrasion, beneath glaciers, 275. - - Abyssinia, fissure eruptions in, 101. - - Accordance, of tributary valleys, 162. - - Adiabatic refrigeration, in relation to glaciers, 262. - - Adolescence, in cycle of erosion, 169. - - Advancing hemicycle of glaciation, 263-266. - - Advective zone, of atmosphere, 270. - - Aftershocks, of earthquakes, 83. - - Agassiz, glacial lake, 325-328. - - Agassiz, Louis, cited, 339, 400. - - Age, of strata, 38, 52. - - Aggradation, 162. - - Aktian deposits, 36. - - Alaskan coast, map of, 79. - - Albs, 376. - - Alden, W. C., cited, 316, 318, 319. - - Algæ, growth of, in hot springs, 194. - - “Alkali” in deserts, 201. - - Alluvial bench, 214. - - Alluvial cone, 213. - - Alluvial-dam lakes, 423. - - Alluvial fan, 213. - - Alpine glaciers, 383, 386. - - Alterations of minerals, 27. - - Altitude, of different parts of lithosphere, 18. - - American Falls, future extinction of, 357. - - Amphiboles, 459. - - Amphitheaters, formed on drift sites, 369. - - Amundsen, R., cited, 23. - - Analysis, of folds, 54. - - Anderson, Tempest, cited, 146, 147. - - Andersson, J. G., cited, 157, 295. - - Andesite, 463. - - Angular unconformity, 53. - - Antarctica, 154, 281. - - Antarctic protuberance, 17. - - Antarctic shelf ice, 289, 290. - - Anticlinal folds, 42. - - Anticlines, 42; - tension in, 45. - - Anticyclone, glacial, 284. - - Ants, factor in rock decomposition, 156. - - Apron, alluvial, 213. - - Aprons, outwash, 280, 281. - - Arbenz, P., cited, 195. - - Arches, of folded strata, 42; - sea, 233, 234. - - Architecture, of fractured earth superstructure, 55. - - Arctic depression, 17. - - Areal geological map, 62. - - Arêtes, 373. - - Arldt, Theodore, cited, 11, 19, 438. - - Arnold, Ralph, cited, 157. - - Arrangement of oceans and continents, 10. - - Artesian wells, 190, 191, 196. - - Ash, volcanic, 122. - - Askja, eruption of, in 1875, 101. - - Assmann, R., cited, 294. - - Astronomical _vs._ geodetic observations, 12. - - Atlantis, North, 16. - - Atmosphere, compressibility of, 8. - - Attack, of the weather, 149. - - Atwood, W. W., cited, 7, 160, 298, 300, 313, 372. - - Axial plane, of folds, 42. - - Axis, of folds, 42. - - Azurite, 453. - - - Bacteria, part taken in weathering, 156. - - “Bad Lands”, control of relief in, 223, 224. - - “Bad Land” topography, 214. - - _Bajir_, 216. - - Balance, between degradation and aggradation, 161. - - Bandai-san, dissection of, 141. - - Barchans, 211. - - Barrancoes, 139. - - Barrell, J., cited, 221, 447. - - Barrier beaches, 240; - sections of, 242; - uplifted, 249, 250. - - Barrier lakes, 420. - - Barriers, 240; - mountain, in relation to glaciers, 262. - - Bars, 240. - - Basal conglomerate, 37, 53. - - Basalt, 463; - faulted blocks of, 58; - of Hawaii, 105. - - Base level, 159. - - Basin-range lakes, 402, 403. - - Basin Range structure, 440. - - Basins, flat bottomed, separating dunes, 216; - of exudation, 272; - of sedimentation, earlier, 38. - - Bastin, E. S., cited, 210. - - Batholites, 143. - - “Bath tubs”, 395. - - Beach pebbles, 239. - - Beach sand, 206, 238. - - Beaches, remaining from ice-dam lakes, 410; - shingle, 239; - storm, 240; - uplifted, “feathering out” of, 344. - - Bedded structure of rocks, 31. - - Beede, J. W., cited, 195. - - “Bee-hive” mountains, 380, 381. - - _Belgica_ expedition, 289. - - Belt of sea which divides land masses, 11. - - Berghaus, H., cited, 424. - - Bergschrund, 370. - - Berson, A., cited, 294. - - Berthaut, General, cited, 7. - - “Bird-foot” delta, 167. - - “Biscuit cutting” effect of glacial sculpture, 372. - - Blackwelder, E., cited, 318. - - Block mountains, 446. - - Blocks, orographic, 58. - - _Bocchi_, 125. - - Bog, floating, 429. - - Bogs, of peat, 429, 430. - - Bonney, T. G., cited, 146. - - Borax deposits, in deserts, 201. - - Border drainage, about glaciers, 316, 320, 321. - - Border lakes, 399, 414. - - Bosses, 143. - - “Bottoms”, from entrenched meanders, 173. - - “Bowlder clay”, 310. - - “Bowlder pavement”, 237. - - Bowlders, faceted, 310; - glacial, 298; - “soled”, 276, 310; - thrown up during earthquakes, 69. - - Bowlder trains, 306. - - Bowman, Isaiah, cited, 179. - - Box cañons, 214. - - Braided streams, 280. - - Branner, J. C., cited, 6, 91. - - “Bread-crust” lava projectiles, 119. - - Breakers, 232. - - Breccia, fault, 60. - - Bridges, nature of damage to, during earthquakes, 75, 76. - - Brigham, A. P., cited, 424. - - Brögger, W. C., cited, 66. - - Bruce, W. S., cited, 290, 382, 399, 414. - - Bryant, H. G., cited, 289. - - Buckley, E. R., cited, 433, 434. - - Built terraces, 235. - - Bunsen, cited, 192. - - Burns, G. P., cited, 434. - - Burton, W. K., cited, 92. - - Buttes, 216. - - Bysmalite, 442, 447. - - - Calcareous ooze, 36. - - Calcareous sinter, 184. - - Calcareous tufa, 464. - - Calcite, 455. - - Caldera, 405, of composite volcanic cones, 126. - - Camiguin volcano, birth of, 96, 97. - - Campbell, M. R., cited, 178. - - Cañons, 160; - box, 214. - - Capri, blue grotto of, 257, 258. - - Capture, river, 175, 176, 179. - - Carbonization, 151. - - Cascade Mountains, fissure eruptions of, 102. - - Cascade stairway, 376. - - Caspian Depression, 14. - - Cauliflower cloud, 130. - - Caverns, galleries directed by joints, 182; - of limestone, 182, 195; - refuge of predatory animals, 185. - - Caves, sea, 234. - - Cellular structure, of lava domes, 112. - - Centers of dispersion, of North American Pleistocene glaciers, 298. - - Centrosphere, 8. - - Cerussite, 455. - - Chaix, A., cited, 195. - - Chaix, E., cited, 195. - - Chalcopyrite, 453. - - Challenger expedition, 38, 96, 97, 293. - - Chamberlin, T. C., cited, 29, 156, 191, 196, 205, 221, 222, 293, 295, - 318, 319, 337, 339. - - Character profiles, coast, due to uplift or depression, 259; - composite, 229; - directly due to volcanic agencies, 145, 146; - from stream erosion in humid climates, 177; - of arid lands, 220; - of shore features, 243; - referable to continental glaciers, 318; - referable to mountain glaciers, 379. - - “Checkerboard topography”, 226. - - Chemical sediments, 34. - - Chicago outlet, 331. - - Chimneys, in “driftless area”, 300. - - Chimneys, shore feature, 234. - - China, loess of, 207. - - Chlorite, 458. - - Chlorite schist, 465. - - Cicatrice, from dissection of volcanoes, 142. - - Cinder cones, 105; - corrugations upon, 138; - diameter of crater in relation to violence of explosions, 123; - grander eruptions of, 117; - profiles of, 123; - secondary, 111. - - Cinder eruptions, artificially simulated, 122. - - Cirques, 371; - life history of, 371; - subordinate, 371. - - Cities, destruction of, by drifting sand, 218. - - Clastic rocks, 30. - - Clay slate, 466. - - Cleavage, mineral, 27, 450; - rock, 44. - - Clefts, volcanic, in Iceland, 99. - - Cliffs, notched, 233. - - Climatic conditions, in relation to mountain sculpture, 443. - - Clinometer, 48. - - Cloudbursts, in deserts, 201, 212. - - Cloud zones, 268, 269, 294. - - Coals, 466. - - Coast, Dalmatian, grottoes of, 258. - - Coast, elevation of, during earthquakes, 80; - submergences of, during earthquakes, 80. - - Coastal plains, 246; - belted, 247. - - Coast lines, even, 246; - indicative of uplift or submergence, 245, 246; - ragged, 246. - - Coast records, 245. - - Coasts, Atlantic and Pacific contrasted, 438; - embayed, 251. - - Coast terraces, 80, 250, 241; - uplift, effect of, on sediments, 38. - - Coats Land, shelf ice of, 290. - - Cobalt, in meteorites, 23. - - Cobb, Collier, cited, 179. - - Coigns, of earth’s tetrahedral figure, 15. - - Coleman, A. P., cited, 318. - - Colk lakes, 408, 409. - - Colks, scape, 277. - - Collet, L. W., cited, 39. - - Colorado desert, 74. - - Color, of minerals, 450. - - Cols, 374; - origin of in cirque intersection, 372. - - Comb ridges, 373. - - Compass, geologist’s, 47, 48. - - Competent layer, 42; - in relation to lava reservoirs, 144. - - Composite cones, _caldera_ of, 126, 127. - - Composite groups of joints, 57. - - Composite volcanic cones, 105. - - Composition of earth, 29. - - Composition of the earth’s core, 21. - - Compression of a district during earthquakes, 76. - - Cones, alluvial, 213; - cinder, 105; - composite volcanic, 105. - - Conformable series, 51. - - Conglomerate, 34, 463; - basal, 37, 53. - - Constructional topography, 309. - - Construction of buildings, in earthquake regions, 89-91. - - Continental glacier, behind rampart, 281; - in Victoria Land, 280-285; - of Antarctica, literature of, 295; - of Greenland, 271; - of Greenland, melting on margin of, 278; - of Greenland, literature, 295. - - Continental glaciers, contrasted with mountain glaciers, 266-268; - defined, 266-267; - of “ice age”, 297; - of ice age, cross section of, 302; - nourishment of, 283, 286, 295; - profiles of, 267. - - Continental platform, 19. - - Continental shelves, 18, 19; - origin, 232. - - Continents, arrangement of, 10; - development of, 14; - increase in area of, through wave action, 241; - past history of, 14. - - Contortions of the strata, 40. - - Contours, of topographic maps, 62. - - Contraction of earth’s surface, during earthquakes, 74. - - Contrary movements upon coasts, 254, 257. - - Convective zone, of atmosphere, 270. - - Conway, W. M., cited, 294. - - Copernicus, cited, 10. - - Copper glance, 455. - - Coquina, 35. - - Cornish, Vaughan, cited, 211, 222, 244. - - Corrasion, 162. - - Corrosion, of rocks, 156. - - Coulée lakes, 406. - - Coves, 233, 234. - - Cracks, earthquake, 74. - - Crater, evolution of form of, 128. - - Crater lakes, 405, 406. - - Craterlets, 84; - sections of, 85. - - Craters, mechanics of explosions in, 115. - - Crater, volcanic, 95. - - Credner, G. R., cited, 179. - - Crescentic levee lakes, 416, 417. - - Crestline, of an anticline, 42. - - Crevasse, marginal, on mountain glaciers, 370. - - Crevasses, in connection with river cut-offs, 164; - on glaciers, 391. - - Cross, Whitman, cited, 216, 441, 447. - - Cross-bedded structure, 37. - - “Crystal cellars”, 27. - - Crystal form, of minerals, 449. - - Crystals, behavior under special treatment, 24, 25; - essential nature of, 23; - forms of, 454, 457; - individuality of, 24; - mutilated, later growth of, 26; - symmetry of form of, 23. - - Crustal shortening, 42. - - Cuestas, 246, 247; - south of Lake Ontario, 361, 362. - - Cut and built terrace, on steep shore of loose materials, 237. - - Cut-offs, of meanders, 164. - - Cut rock terraces, 235. - - Cuvier, cited, 199. - - Cvijić, J., cited, 195. - - Cycle of glaciation, 263, 294. - - Cycles, of glaciation, Pleistocene, 297; - of stream meanders, 163. - - - Dana, J. D., cited, 6, 104, 106, 109, 111, 146, 147. - - Dana, E. S., cited, 29. - - Daly, R. A., cited, 447. - - Dante, cited, 9. - - Darton, N. H., cited, 179. - - Darwin, Charles, cited, 199, 322, 323, 339. - - Daubrée, A., cited, 54. - - David, T. W. E., cited, 23. - - Davis, C. A., cited, 434. - - Davis, W. M., cited, 7, 178, 179, 221, 247, 276, 317-319, 378, 382. - - Deceptive unconformity, 53. - - Decomposition, 149, 156; - mechanical results of, 150. - - Débris cones, 395. - - Deep sea deposits, 36, 38. - - Deflation, 204. - - Deforestation, in relation to agriculture, 156; - of Karst region, 188; - relation to erosion, 157. - - Degeneration, 149. - - De Geer, G., cited, 351, 366, 410. - - Degradation, 161, 162. - - Dekkan, fissure eruptions of, 101. - - Delebecque, A., cited, 424. - - De Lorenzo, cited, 125, 132. - - Delta, “Bird-foot”, 167; - bottom-set beds, 167; - dry, 213; - of Mississippi River, rate of growth of, 168. - - Delta deposits, manner of growth of, 167. - - Delta lakes, 419, 420. - - Delta region, of a river, 35. - - Deltas, abnormal, below outlets of lakes, 431; - in relation to agriculture, 166; - in relation to population, 166; - lake, 428; - of rivers, 165, 166, 179; - sections of, 168. - - Dendritic glaciers, 383, 385, 386. - - Deniston, cited, 121. - - Deposition, in zones about desert, 216, 217. - - Deposits, aktian, 36; - chemical, 34; - continental, 37; - deep sea, 36, 38; - delta, manner of growth of, 167; - fluviatile, 35; - fluvio-glacial, 31, 310; - in valley vacated by glacier, 398; - glacial, 31; - lacustrine, 35, 217; - littoral, 36; - marine, 35; - mechanical, 34; - organic, 34; - salt, 217; - shoal water, 26; - sinter, 184; - terrigenous, 36. - - Derangement of water flow, during earthquakes, 83, 84. - - Derwies, V. de, cited, 447. - - Descent of ground water, 180. - - Desert, due to deforestation, 156; - erosion in, 214, 222; - law of, 197. - - Desert lakes, 423. - - Desert landscapes, features in, 209. - - Desert rains, 212. - - Desert rocks, red color of, 222. - - Desert varnish, 201, 222. - - Deserts, former shore lines in, 198; - self-registering gauge of past climates, 198. - - Destructional topography, 309. - - Detection of plunging folds, 49, 50. - - Detonations, during Vulcanian eruptions, 131. - - Device, to simulate building of cinder cones, 122. - - Diabase, 462. - - Diagram, to illustrate formation of lava reservoirs, 143. - - Diagrams for comparison of fold types, 42; - to show the effect of spheroidal weathering, 150. - - Diamonds, in the drift, 307. - - Diffission, 204. - - Dikes, hollow, 140; - in China, 167; - in Holland, 166; - from volcanic dissection, 140. - - Diller, J. S., cited, 39, 425. - - “Diluvium”, 305. - - Dimples, on margin of continental glaciers, 272. - - Dip, 46. - - Dirt cones, 396. - - Disintegration, 156; - of rocks in deserts, 202; - through root expansion, 154; - through tree growth, 154, 155. - - Dislocations, marginal, about deserts, 212. - - Dispersion of the drift, 304-309, 319. - - Displacement, total, on faults, 59. - - Dissection of volcanoes, 139. - - Distributaries, on alluvial fans, 213, 220. - - Divides, 170; - migration of, 175. - - Dolines, of Karst region, 187, 422. - - Dolomite, 465. - - Dolomites, 203, 228, 445. - - Domed mountains of uplift, 441. - - Dome structure, of granite masses, 152, 157. - - Domes, lava, 105. - - Dovetailing, of sea and land, 11, 17. - - Drainage, changes of, due to glaciation, 336-338; - haphazard, of glaciated area, 301; - interference of glaciers with, 320; - of glaciers, 397; - reversals of, due to glaciation, 337, 338; - trellis, 175. - - Drainage lines, control of, by fractures, 224. - - Drainage networks, controlled by fractures, 225, 226; - repeating pattern in, 225. - - Drake, Sir Francis, circumnavigation of the globe, 10. - - _Dreikanten_, 205. - - Driblet cones, 104, 125; - of Kilauea, 107. - - “Drift”, 305. - - Drift, assorted, 309; - dispersion of, 304-309; - englacial, 277, 278; - unassorted, 309. - - “Driftless area”, 300, 313, 318. - - Driftless area, map of, 298. - - Drift sites, 368, 369. - - Drowned rivers, 251. - - Drumlins, 311, 316, 317, 399. - - Dry deltas, 213. - - Drygalski, E. von, cited, 273, 279, 295, 296. - - Dry weathering, in deserts, 201. - - Dune, war with oasis, 216. - - Dune lakes, 421. - - Dunes, 222; - forms of, 210, 211; - in relation to obstructions, 209, 210; - stopped by vegetation, 211; - wandering, 209, 211. - - Dust, carried out of desert, 206, 222; - volcanic, 122. - - Dust wells, 395. - - Dutton, C. E., cited, 85, 92, 178, 200, 222, 447. - - - Earlier figures of the earth, 14. - - Earth, a magnet, 23; - composition of, 20; - oblateness of, 10; - rigidity of, 20, 21, 29; - scale of its elevations, 10, 11; - theories of origin of, 20, 29; - surface shell, chemical constitution of, 23; - surface shell, response to load, 340. - - Earth features, shaped by running water, 169. - - Earth figure, evolution of ideas concerning, 9. - - Earthquake cracks, 74. - - Earthquake fountains, 190. - - Earthquake lakes, 404. - - Earthquake, of Alaska, 1899, 72, 77, 79, 80, 81; - of Assam, 1897, 72, 77; - of California, 1906, 70, 72, 73, 74, 90, 91; - of Casamicciola, 1883, 87; - of Costa Rica, 1910, 68; - of India, 1819, 84; - of Jamaica, 1692, 80; - of Jamaica, 1907, 80; - of Japan, 1891, 72, 75; - of lower Mississippi Valley, 1811, 83; - of Messina, 1908, 68; - of Owens Valley, California, 1872, 73, 77, 78, 79; - of Servia, 1904, 84; - of South Carolina, 1886, 85. - - Earthquake shocks, heavy over loose foundations, 88. - - Earthquakes, aftershocks of, 83; - associated with growing mountains, 86; - changes in earth’s surface during, 71; - connected with lines of fracture, 86; - descriptive reports upon, 92; - due to adjustments between blocks of shell, 78, 79; - faults and fissures, 71; - focused at fault intersections, 87; - fountains during, 83, 86; - localized at corners of earth blocks, 87; - manifestations of changes in level, 68; - nature of shocks, 67; - of Ischia, localization of, 87; - shown by coast terraces, 250; - special lines of heavy shock, 86; - in unstable areas of earth’s crust, 86; - wave motions of, 68; - zones in distribution of, 86. - - Earth relief, repeating patterns in, 223. - - Eckert, cited, 188. - - Effect of contraction upon a spherical body, 13. - - Egg-spinning demonstration of earth rigidity, 20. - - “Elevation-crater” theory of volcanoes, 95, 139. - - Embankments, shore, 240. - - Embayed coasts, 251. - - Emerson, B. K., cited, 19. - - End moraines, 394. - - Engell, M. C., cited, 296. - - Englacial débris, 393. - - Englacial drift, 277, 278. - - _Entonnoirs_, 182. - - Entrenchment of meanders, 172, 173, 179. - - Eolian sand, 206. - - Eolian sediments, 30. - - Erosional unconformity, 53. - - Erosion cycle, 159. - - Erosion, effect of, in adding curves to landscape, 65; - glacial, in contrast with normal weathering, 377; - in desert, 214; - shadow, 206; - stream, as modified by resistant rocks, 174. - - “Erratic blocks”, 304. - - Eruptions, Strombolian, 117; - Vulcanian, 117, 125. - - Escarpments, from faults, 59. - - Eskers, 311, 315, 316, 363. - - Estes, L. A., cited, 93. - - Estuaries, 251. - - Etna, eruption of 1669, 122. - - Evolution, doctrine of, in connection with fossils, 38. - - Evolution of ideas concerning the earth’s figure, 9. - - Exfoliation, 151, 203. - - Expanded foot glaciers, 383, 385. - - Experiment, to illustrate relation of earthquake shocks to - foundations, 88. - - Experiments, on fracture and flow, 40, 41; - for demonstration of earthquakes, 81, 82. - - Exposures, rock, 46. - - Extrusive rocks, 463. - - - Fairbanks, H. W., cited, 155, 170, 174, 201, 205, 214, 224, 248, 249, - 250, 260, 302, 375, 406, 413, 429. - - Fairchild, H. L., cited, 339. - - Falls, “Bridal veil”, 378. - - Falls, ribbon, 378. - - Fan, alluvial, 213. - - Farrington, O. C., cited, 29. - - Fault, drag upon, 60. - - Fault breccia, 60. - - Fault topography, 65. - - Faults, 58, 440; - during earthquakes, 71; - earthquake, change in throw upon, 76, 77, 78; - earthquake, disappear in loose materials, 73; - earthquake, of small displacements, 74; - earthquake, plan of, 76, 78; - illusory nature of, 59; - methods of detecting, 59; - post-glacial, 74; - relation of escarpments to, 60; - shown by changes in strike and dip, 61; - shown by offsets, 61. - - Feldspars, 456. - - Fenneman, N. M., cited, 424, 425. - - Festoons of mountain arcs, 435, 436. - - Field ice, 286. - - Field map, geological, 62, 63. - - Figure of the earth, the, 8. - - Figures, earlier, of the earth, 14; - earth, evolution of, 15. - - Figure toward which the earth is tending, 12. - - “Fire girdle” of the Pacific, 98. - - Firn, 369. - - Fissure eruptions, of volcanoes, 101. - - Fissures, during earthquakes, 71; - earthquake, 74; - in connection with volcanoes, 99-101. - - Fissure springs, 61, 190, 195. - - Fjords, 290, 340. - - “Float copper”, 305. - - Flooded portions of continents, 18. - - Flood plain, 178; - manner of grading of, 162. - - Floors of hydrosphere and atmosphere, 18. - - Flow, experiments on, 41; - zone of, 40. - - Flow texture, of extrusive rocks, 33. - - Fluviatile deposits, 35. - - Fluvio-glacial deposits, 31. - - Fluxion texture, of extrusive rocks, 33. - - Folds, analysis of, 54; - comparison of shapes of, 44; - mutilated, restoration of, 45; - pitching, 43; - secondary, 44; - shapes of, 43. - - Fold topography, 65. - - Forbes, J. D., cited, 294. - - Fore-set beds, 167. - - Forest, destruction of, in relation to agriculture, 156. - - Formation of lava reservoirs, 143. - - Formations, measurement of thickness of, 48, 49. - - Fort Snelling, on Warren River, 327, 331. - - Fosses, glacial, 281, 314; - in connection with peat bogs, 430. - - Fracture control, of drainage lines, 224. - - Fracture, experiments on, 41; - of minerals, 450; - zone of, 40, 46. - - Fractures, in rocks, shown by rectilinear lines on map, 65; - system of, 55. - - Free, E. E., cited, 222. - - Free waves, 232. - - Fretted upland, 372, 373. - - Frost, prying work of, 152. - - Frost action, 223. - - Frost snow, 285. - - Fuller, M. L., cited, 157, 195. - - Fumeroles, 97. - - - Gabbro, 462. - - Gabled façade, in desert landscapes, 221, 443. - - Galenite, 453. - - Gannett, Henry, cited, 178, 386. - - Gaps, water, 176; - wind, 176. - - Garnet, 459. - - Gautier, E. F., cited, 221. - - Geikie, A., cited, 6, 7, 148, 178, 244, 318. - - Geikie, James, cited, 6, 318. - - Geoid, departure from spherical surface of, 10. - - Geological map, 46, 54; - areal, 62, 63; - base of, 61; - field, 62, 63. - - Geological section, 46, 47. - - Geology, defined, 1. - - Geyserite, 194. - - Geysers, 191-194; - effect of plugging with sod, 193; - in relation to drainage lines, 191; - soaping of, 194. - - _Geysir_, 192. - - Gilbert, G. K., cited, 93, 148, 157, 178, 179, 198, 221, 224, 240, - 244, 294, 344, 345, 347, 350, 355, 356, 357, - 358, 359, 362, 366, 370, 381, 434, 446, 447. - - _Gjás_, volcano fissures in Iceland, 99. - - Glacial anticyclone, 284. - - Glacial deposits, 30, 31. - - Glacial fringe, of Grant Land, 285. - - Glacial Lake Agassiz, 325-328, 339. - - Glacial lakes, at close of ice age, 320; - of St. Lawrence Valley, 329. - - Glaciated regions, aspects of, 302; - characteristics of, 301; - contrasted with nonglaciated, 299, 309. - - Glaciation, conditions essential to, 261; - cycle of, 263; - Permo-Carboniferous, 298. - - Glaciations, following changes in earth’s figure, 15; - previous to “ice age”, literature of, 318. - - Glacier broom, over continental ice, 285. - - Glacier cornices, 397. - - Glacier deposits, upon its bed, 390. - - Glacier drainage, 397. - - Glacier flow, 390, 400; - data from accidents to Alpinists, 392. - - Glacier gravings, 301, 319; - multiple records, 304. - - Glacier lobe lakes, 411. - - Glacier milk, 398. - - Glacier mills, 278. - - Glacier pavement, 276. - - Glaciers, birth of, 369; - crevasses on, 391; - dendritic, 383, 385, 386; - grinding tools of, 276; - horseshoe, 383, 386, 387; - inherited basin, 387-389; - initiation of, 262; - in relation to wind direction, 262; - main types of, 266; - mountain, cross sections of, 394; - mountain, expanded-foot type, 264; - mountain, land sculpture by, 367; - mountain, successive stages, 383; - nivation, 387; - nourishment of, 268-270; - piedmont, 383, 384; - radiating, 383, 386; - sensitiveness to temperature changes, 263; - séracs, 391; - surface features of, 390; - tide water, 290, 386. - - Glacier stars, 395. - - Glacier tables, 395. - - Glacier types, successive, during waning glaciation, 383. - - Glacier wells, 278. - - Glassy texture, of extrusive rocks, 32. - - Glen Roy, 322, 339. - - Glint, 409. - - Glint lakes, 408, 409. - - Gneiss, 465. - - Gneiss banding, 31. - - Goethe, cited on volcano structure, 139. - - Gold, E., cited, 294. - - Goldthwait, J. W., cited, 259, 320, 341, 345, 351. - - Gondwana Land, 16. - - Gorges, through rock bars, 378. - - Grabau, A. W., cited, 361, 366. - - Grading of flood plain, 162. - - Grand Cañon of the Colorado, 146, 169, 174, 215, 443. - - Grand River outlet, 333. - - Granite, 462; - dome structure in, 152, 157. - - Granite domes, 221. - - Granitic texture, of igneous rocks, 33. - - _Grats_, 373. - - Gravel, kame, 310. - - “Gravel piedmont”, 214. - - Great Basin, 190, 198, 439. - - Great Lakes, probable future of, 347, 348; - submergence of certain shores of, 349, 350. - - Great Ross Barrier, 282. - - Great Salt Lake, 199; - fluctuations of level of, 198. - - Green, W. Lowthian, cited, 19. - - Gregory, J. W., cited, 11, 19, 439, 446. - - Grooved upland, 372, 373. - - Gross, H., cited, 294. - - Grossman, cited, 268. - - Grottoes, sea, colors of, 258. - - Ground water, 180; - descent of, in relation to joints, 181. - - Ground water lakes, 424. - - Grund, A., cited, 195. - - Gullies, early stages of, 160. - - Gulliver, F. P., cited, 244, 319. - - Gullying process, started by deforestation, 156. - - Gypsum, 455. - - - Hade, on faults, 59. - - Hague, Arnold, cited, 196. - - Halemaumau, Kilauea, 107, 108. - - Hamilton, Sir William, cited, 128. - - Hanging valleys, 378. - - Hardness, of minerals, 451. - - Harwood, W. A., cited, 294. - - Haug, E., cited, 7, 133, 211. - - Haughton, Samuel, cited, 56. - - Hawaii, lava domes of, 105; - lava surfaces of, 113; - map of, 106; - section through, 106. - - Hayes, C. W., cited, 156. - - Headlands, notched, 341. - - Heave, of faults, 59. - - Hebrews, conception of the universe, 9. - - Hedin, Sven, cited, 221. - - Heilprin, A., cited, 148. - - Heim, A., cited, 54. - - Heligoland, 236. - - Helland, A., cited, 99. - - Hematite, 452. - - Hemicycles, of glaciation, 263, 264. - - Herculaneum, buried beneath mud flows, 139. - - Hess, H., cited, 267, 272, 294, 393, 400. - - High plains, 435; - origin of, 219. - - Hilgard, E., cited, 222. - - Hinge lines, of uptilt, 344-347. - - Hitchcock, C. H., cited, 106, 147, 434. - - Hobson, B., cited, 120. - - Hogarth, William, cited, 170. - - Hogarthian line of beauty, in landscapes, 170-171. - - “Hog backs”, 442. - - Holmes, W. H., cited, 441. - - Horns, 374. - - Horseshoe glaciers, 383, 386, 387. - - Hot springs, 191; - colors in, due to algæ, 194. - - Hovey, E. O., cited, 136, 137, 148. - - Hovey, H. C., cited, 183, 195. - - Howchin, W., cited, 298. - - Howe, E., cited, 140. - - Howell, cited, 325. - - Hudson River, narrows of, 174. - - Hudsonian channel, 252. - - Hummocks, on pack ice, 286. - - Humphrey, R. L., cited, 90, 93. - - Humphreys, cited, 404. - - Humus, in relation to weathering, 156. - - Huntington, Ellsworth, cited, 216, 217, 221, 222. - - Hus, H. T. A. de L., cited, 183. - - Hydration, 151. - - Hydrosphere, 8. - - Hypothesis, the value of, 6; - Laplacian, of the universe, 20. - - - Icebergs, 296; - Antarctic, 292, 293; - Antarctic, formation of, 292; - blue, 292; - manner of formation of, 291, 292; - northern, 291. - - Ice caps, profiles of, 267, 268; - sculpture, 380. - - Ice-dammed lakes, 321, 323, 410, 411; - in St. Lawrence Valley, 339; - of Scottish glens, 322. - - Ice floes, 287. - - Iceland, fissure eruptions of, 102. - - Ice pyramids, 395. - - Ice ramparts, 431-434; - manner of formation of, 433. - - Igneous rocks, 30; - textures of, 32. - - Imlay outlet, 332. - - Inbreak, of lava surface, 107. - - Incised topography, 301. - - Inherited basin glacier, 387-389. - - Interlobate moraines, 314. - - Inter-pluvial periods, 198. - - Intricate pattern of river etchings, 158. - - Intrusive rocks, 32, 462. - - Islands, land-tied, 241; - steep rocky, due to submergence, 252. - - Isobases, 347. - - Isoclinal folds, 42. - - Isothermal zone of atmosphere, 270. - - - Jagger, T. A., Jr., cited, 148. - - Jamieson, T. F., cited, 221, 322, 339. - - Jeannette exploring expedition, 287, 295. - - Jensen, H. I., cited, 110, 113, 147. - - Johnson, D. W., cited, 7, 148. - - Johnson, W. D., cited, 77, 213, 219, 220, 222, 370, 381. - - Johnston-Lavis, H. J., cited, 87, 131, 132, 134, 138, 147, 148. - - Joint blocks, in Niagara limestone, 353. - - Joint plane, seat of frost action, 370. - - Joints, 56; - effect on surface features, 57; - closed during earthquakes, 76; - composite nature of, 58; - composite groups of, 57; - disorderly, 57; - displacements upon, 58; - master, 56; - space intervals of, 58; - sets of, 55; - system of, 55. - - Joint series, combinations of, 56. - - Joint systems, 66. - - Jorullo, birth of, 96. - - Judd, John W., cited, 116, 118, 139, 148. - - Julien, A. A., 156. - - Jura Mountains, 46. - - - Kame gravel, 310. - - Kames, 311, 314. - - Kammerbühl, 139. - - _Karrenfelder_, 188. - - Karst, characters of, 186-187; - once forested, 188. - - Karst conditions, 195. - - Karst lakes, 422. - - _Katavothren_, 188. - - Katzer, F., cited, 195. - - Kearney, Th. H., cited, 222. - - Kelvin, Lord, cited, 20, 29. - - “Kettle moraines”, 311-314. - - “Kettles” on moraines, 312. - - Kikuchi, Y., cited, 148. - - Kilauea, 101, 106; - draining of lava in crater of, 108; - eruption of 1840, 109, 111, 112; - lava movements in, 106, 107; - moving platform in crater, 107; - range in height of lava in, 107. - - King, F. H., cited, 157, 195. - - Knebel, W. von, cited, 185, 195, 258, 260. - - “Knob and basin” topography, 314. - - Knott, C. G., cited, 92. - - Kopisch, August, cited, 258. - - Kotô, B., cited, 92. - - Krakatoa, dissected by eruption, 142. - - Krakatoa, eruption of 1883, 141, 142. - - _Kuppen_, 105. - - Kurische Nehrung, wandering dunes of, 210. - - - Laboratory apparatus, for simulation of cinder eruptions, 122. - - Laboratory models, for study of geological maps, 63. - - Laccolites, 143, 441, 442, 447. - - Lacroix, A., cited, 148. - - Lacustrine deposits, 35. - - Lake Agassiz, glacial, 325-328. - - Lake Algonquin, 334, 342. - - Lake Arkona, 332, 333. - - Lake basins, study of, 401. - - Lake Bonneville, 199. - - Lake Chicago, 330, 332, 333. - - Lake Eulalie, draining of, during earthquake, 83. - - Lake Iroquois, 334, 335. - - Lake Maumee, 330, 331, 332, 345. - - Lake Ojibway, glacial, 338. - - Lake stages, in St. Lawrence Valley, 336. - - Lake Warren, 333, 334. - - Lake Whittlesey, 332, 333. - - Lakes, alluvial dam, 423; - as regulators of air temperature, 431; - as regulators of river flow, 431; - as settling basins, 426-428; - barrier, 420; - basin range, 402, 403; - become extinct through wave action, 428; - border, 399, 414; - classification of, 424; - colk, 408, 409; - continental glaciation, 424; - coulée, 406; - crater, 405, 406; - crescentic, 329, 330; - crescentic levee, 416, 417; - currents in, 431; - delta, 419, 420; - desert, 424; - drained by cutting down of outlet, 428; - dune, 421; - drained during earthquakes, explanation of, 83; - earthquake, 404; - ephemeral existence of, 426; - extinction by peat growth, 429-430; - extinction of, in desert regions, 430; - fresh water, 401; - glacier lobe, 411; - glint, 408, 409; - ground water, 424; - ice dam, 410, 411; - intramorainal, about continental glaciers, 279, 280; - karst, 422; - landslide, 414; - morainal, 315, 406, 407; - mountain glaciation, 424; - newland, 401, 402; - ox-bow, 165, 415; - pit, 315, 407, 408; - playa, 422; - raft, 417, 418; - rift-valley, 403, 404; - river, 424; - rock basin, 376, 377, 400, 412; - rock basin about continental glaciers, 279; - rôle of, in economy of nature, 430; - saline, 401; - salines, 423; - saucer, 415, 416; - seasonal, 189, 422; - side delta, 326, 327, 418, 419; - sink, 421; - strand, 424; - tectonic, 424; - valley moraine, 400, 413; - volcanic, 424; - “wall”, 432. - - Laki, eruption in 1783, 99. - - Laminated structure, of rocks, 31. - - Lamplugh, G. W., cited, 225. - - Land, growth of, from volcanic outflow, 113, 114; - sliced during earthquake, 80; - uptilt of, at close of ice age, 340. - - Land areas, concentration of, in northern hemisphere, 11. - - Land sculpture, by mountain glaciers, 367; - in relation to climatic conditions, 443; - referable to ice caps, 380. - - Land shields, 15. - - Landslide lakes, 414. - - Land-tied islands, 241. - - Lane, A. C., cited, 148. - - Lankester, E. Ray, cited, 260. - - La Noe, G. de, cited, 7. - - _Lapilli_, 119, 122. - - Laplacian hypothesis of the universe, 20. - - Lateral moraines, 393. - - Lateral movements, deep seated, during earthquakes, 81. - - Lava, 32; - block, 113; - composition and properties of, 103; - discharging from tunnel, 111; - fluidity of basic, 103; - movements, in caldron of Kilauea, 107; - probable origin from shale, 144; - ropy, 113; - viscosity of siliceous, 103. - - Lava domes, probable structure of walls of, 112; - slopes of, 103, 104, 105. - - Lava projectiles, pear-shaped type, 121. - - Lava reservoirs, formation of, 143. - - Lava streams, appearance of, 133, 134. - - Lava surface, 113, 124. - - Law of the desert, 197. - - Lawson, A. C., cited, 92, 260, 351. - - Leads, in pack ice, 286. - - Le Conte, Joseph, cited, 6. - - Leffingwell crater, California, 104. - - Levees, 166. - - Leverett, Frank, cited, 6, 104, 166, 312, 318, 321, 330, 332, 333, - 334, 337, 339, 344, 345. - - Lewiston escarpment, at Niagara, shaping of, 360-362. - - Libbey, W., cited, 274. - - Life histories, of rivers, 158. - - Light figure, from surface of crystal, 25. - - Lightning, in connection with volcanic eruptions, 130. - - Limbs of faults, 59; - of folds, 43. - - Limestone, 464; - origin of, 36; - sinks, 182. - - Limestone, caverns of, 182. - - Limonite, 452. - - Linck, G., cited, 122. - - Lindenkohl, A., cited, 260. - - Lineaments, 87, 226, 227. - - Line of beauty, Hogarthian, in landscapes, 170, 171. - - _Lithodomus_, borings of, in records of oscillation, 254. - - Lithosphere, a complex of interlocking crystals, 25; - and its envelopes, 8. - - Littoral deposits, 36. - - Loess, 35, 207; - erosion of, 208. - - Loessmännchen, 208. - - Lubbock, Sir John, cited, 7. - - Luray caverns, Virginia, 186. - - Luster, of minerals, 450. - - Lyell, Sir Charles, cited, 7, 96, 146, 199, 259, 260, 304. - - - _Maare_, 405. - - McGee, W. J., cited, 157, 259. - - Mackinac Island, records of uplift of, 341-344. - - Madison, Wisconsin, 233, 237, 241, 317, 434. - - Magellan, circumnavigation of globe, 9. - - Magma, defined, 30. - - Magnetism, of minerals, 451. - - Magnetite, 452. - - Malachite, 453. - - _Mamelons_, 105. - - Mammoth Cave, 182, 183. - - Mantle, rock, 155. - - Map, contour, nature of, 467; - of Armorican mountains, 438; - of barrier beaches, 242-243; - of bowlder train from Iron Hill, 306; - of cirques and niches, in Bighorn Mountains, 371; - of coast lines, 246; - geological, 54, 61; - geological, method of preparing, 46, 63; - of continental divide in Colorado, 377; - of continental glacier in Victoria Land, 282; - of Dalager’s nunataks, 277; - of expanded foot glaciers, 264; - of front of Green Bay lobe, 317; - of glacial features, Southern Finland, 315; - of glacial Lake Agassiz, 325, 326, 328; - of glaciated area, Europe, 299; - of glaciated area, North America, 298; - of ice ramparts on Lake Mendota, 434; - of inner Sandusky Bay, 350; - of Kilauea and neighboring slopes, 109; - of Lake Chicago and later Lake Maumee, 332; - of Lake Maumee, 330; - of Lakes Whittlesey and Saginaw, 333; - of lava outflows on Vesuvius, 1906, 131; - of lava streams on Mauna Loa, 126; - of marginal moraines, 312; - of mountain arcs of Eastern Asia, 438; - of mountain arc of Sewestan, 436; - of North Polar regions, 288; - of part of “fire girdle” of the Pacific, 98; - of Scottish glens, 322-324; - of Volcano, 118; - of volcano belts, 98; - of Warren River, 326, 327; - topographical, 61; - topographical, preparation of, 467, 468; - topographical, verification of, 469; - to show dispersion of diamonds in Lake region, 308; - to show dispersion of peculiar rocks, 305; - to show distribution of existing glaciers, 263; - to show formation of shore features, 238; - to show glaciated areas of Pleistocene period, 297; - to show reciprocal relation of land and sea, 11. - - Marble, 466. - - Margerie, Emm. de, cited, 7, 54. - - Marginal moraines, 278-280, 311-314. - - Marine clays, as marks of uplift, 253. - - Marine deposits, 35. - - Märjelen Lake, 329, 411. - - Marks, of origin of rocks, 30; - of uplift, on coasts, 245. - - Marr, John E., cited, 7, 445. - - Martel, E. A., cited, 181, 187, 195. - - Martin, Lawrence, cited, 77, 92, 260, 280, 351. - - Martonne, E. de, cited, 7, 195, 222, 382. - - Massive structure, of rocks, 31. - - Master joints, 56. - - Matavanu, eruption in 1906, 110, 113, 147. - - Mat of vegetation, shield to lithosphere, 155. - - Matthes, F. E., cited, 7, 371, 381. - - Maturity, of upland, 170. - - Mauna Loa, 106; - eruptions of, 109. - - Meander scars, 165. - - Meanders, entrenchment of, 172, 173, 179; - stream, 163; - stream, undermining by, 164. - - Measurement of thickness, of formations, 48, 49. - - Mechanical sediments, 34. - - Medial moraines, 393; - from nunataks, 274. - - Mediterranean seas, 14. - - Melting, selective, on glacier surface, 394. - - Melville, G. W., cited, 289. - - Mercalli, G., cited, 89, 117, 119, 147. - - Merrill, George P., cited, 156. - - Mesa, 215, 216; - origin of, 112. - - Metamorphic rocks, 30, 31, 465. - - Meteorites, compared with earth, 22; - composition of, 21, 23. - - Mica, 458. - - Mica schist, 465. - - Michailovitch, J., cited, 84. - - Microscopical petrography, 27. - - Migration, of divides, 175. - - Mill, H. R., cited, 424. - - Mills, glacier, 398. - - Milne, John, cited, 75, 92, 93. - - Mineral fragments, possibility of growth of, 24. - - Minerals, alterations of, 27, 28; - common, properties of, 452-461; - of economic importance, 452-456; - important as rock makers, 456-461; - properties of, 26, 27; - quick determination of, 449. - - Mississippi River, 167. - - Mitchell, G. E., cited, 157. - - Moats, about nunataks, 273, 274. - - Models, laboratory, for study of geological maps, 63. - - Mojsisovics von Mojsvár, E., cited, 228. - - Mokuaweoweo, crater of, 106. - - “Mole-hill” effect, after earthquakes, 73. - - Molten rock, rise to earth’s surface, 94. - - Monadnocks, 172. - - Monte Nuovo, 96. - - Monte Somma, _caldera_ of, 127. - - Montessus de Ballore, de F., cited, 92, 93. - - Monti Rossi, crystal rain from, 122; - parasitic cones of, 125. - - Mont Pelé, post-eruption stage of, 135-138; - spine of, 136, 137, 138. - - Moore, W. H., cited, 294. - - Morainal lakes, 315, 406, 407. - - Moraines, interlobate, 314; - lateral, 393; - marginal, 278-280; - medial, 393; - medial, from nunataks, 274; - of mountain glaciers, 393, 394; - recessional, 399; - surface, 277; - terminal, 311-314, 394; - water-laid, 330. - - Moreno, F. P., cited, 235. - - Moseley, E. L., cited, 350, 351. - - Moselle River, with entrenched meanders, 173. - - Motive power, of rivers, 158. - - Moulins, 398. - - Mountain arcs, festoons of, 435, 436; - theories of origin of, 436, 437. - - Mountain glaciation lakes, 424. - - Mountain glaciers, contrasted with continental glaciers, 266-268; - defined, 266-268; - dendritic, 383, 385, 386; - expanded-foot type, 264; - horseshoe, 383, 386, 387; - land sculpture by, 367; - marks of, 400; - piedmont, 383, 384; - profiles of, 267; - radiating, 383, 386; - studies of special districts, 294; - summary of types of, 389. - - Mountain ramparts, about continental glaciers, 271. - - Mountains, battlement type, 228, 445; - block type, 439; - carved from plateaux, 442; - of circumvallation, 442, 445; - defined, 435; - domed, of uplift, 441; - erosional, 445; - evidence for occupation by mountain glaciers, 400; - genetical, 445; - largely shaped by erosion, 435; - of outflow and upheap, 440; - origin and forms of, 435; - truncated at coast lines, 438. - - Mt. Etna, 125, 126. - - Mt. Vesuvius, 94; - appearance of, from Naples at night, 129; - ash curtain, during eruption, 132; - ash-fall over, 1906, 133; - “cauliflower” cloud over, 133; - changed appearance after eruption of 1906, 132; - eruption of 79 A.D., 97; - eruption of 1872, 124; - eruption of 1906, 127-137; - history of, 97; - lavas of, 32. - - Mud cones, 84; - aligned upon a fissure, 84. - - Mud-crack structure, 37. - - Mud, flocculent calcareous, of Florida, 36. - - Mud flows, which destroyed Herculaneum, 139. - - Mud veneer, from eruption of Taal, 121. - - Muir, John, cited, 7. - - Munthe, H., cited, 313, 351, 410. - - Murray, Sir John, cited, 39, 293. - - “Mushroom rocks”, 205. - - - Nansen, F., cited, 17, 260, 271, 272, 287, 295. - - Narrows, river, 174, 327. - - Natural Bridge, near Lexington, Virginia, 184. - - Natural bridges, 184. - - Natural sand blast, 204. - - Nature of materials in the lithosphere, 20. - - Necks, volcanic, 140. - - Nephelite, 459. - - Neumayr, Melchior, cited, 7, 146, 195, 196, 222, 425. - - Névé, 369. - - Newborn glacier, 387. - - Newland, 159, 247. - - Newland lakes, 401, 402. - - New Madrid earthquake, 83. - - New River, of Cumberland plateau, 173. - - Niagara Falls, 352-366; - episodes in history of, 362-365; - the clock of recent geological time, 364. - - Niagara gorge, 352-366; - drilling of, 353, 355; - episodes in history of, in connection with glacial lakes, 364; - plan and section of, 355; - rate of recession of, 356. - - Niches, 371; - beneath snowdrift sites, 368, 369. - - Nickel, in meteorites, 23. - - _Nieves penitentes_, 397. - - Nipissing Great Lakes, 335, 342. - - Nipissing outlet, 335, 336. - - Nippur, sand mounds over, 218. - - Nivation, 368. - - Nivation glacier, 387. - - Noble, F. H., cited, 147. - - Nordenskiöld, Otto, cited, 154, 157, 295. - - North Atlantis, 16. - - North Bay outlet, 335. - - Northwest Highlands of Scotland, thrusts of, 45. - - Norway, repeating patterns of, 229. - - Notched cliffs, 233; - elevated, 248. - - Nourishment of continental glaciers, 295. - - Nunataks, 272, 274, 277. - - Nussbaum, F., cited, 161. - - - Oasis, 216. - - Oblateness, of the earth, 10. - - Observational geology _vs._ speculative philosophy, 5. - - Obsidian, 463. - - Obsidian Cliff, 33. - - Ocean of Tethys, 16. - - Oceanic platform, 19. - - Oceans, arrangement of, 10. - - Oldham, R. D., cited, 72, 76, 92. - - Oldland, 159, 247. - - Olivine, 461. - - Omori, F., cited, 147. - - Oölite, 464. - - Oölitic limestone, 464. - - Ooze, calcareous, 36; - composition of, 39. - - Optical mineralogy, 27. - - Order of deposition, during marine transgression, 37. - - Order of superposition, of strata, 52. - - Organic sediments, 34. - - _Orgeln_, 182. - - Orleans, Duc d’, cited, 286. - - Orographic blocks, 58. - - Osar, 311, 315, 316. - - Oscillations of movement, on coasts, 253. - - Outcrop blocks, for study of maps, 63. - - Outcroppings, 46. - - Outlets, from continental glaciers, 271; - of glacial lakes, 326, 327. - - Outwash plains, 280, 281, 311, 313, 314, 399, 408. - - Overthrust, 45. - - Owens Valley, California, map of earthquake faults in, 78. - - “Ox-bow”, of river, 165. - - Ox-bow lakes, 165, 415. - - - Pack, drift of, 287; - the, 286. - - Pack ice, 286. - - Pagination, of the earth record, 38. - - _Pahoehoe_ type of lava surface, 113. - - Pan form of deserts, 197. - - Panum crater, _caldera_ of, 126. - - “Parallel roads”, of Scottish glens, 322-325, 328, 339. - - Partially dissected upland, 160. - - Passarge, S., cited, 221, 222. - - “Paternoster lakes”, 376. - - Pattern, of river etchings, 158. - - Patterns, repeating, 223. - - Pavement, bowlder, 237; - glacier, 276; - tessellated from soil flow, 154. - - Pavlow, A. P., cited, 108. - - Peale, A. C., cited, 195, 196. - - Peary, R. E., cited, 17, 283, 289, 295, 296. - - Peat, 465; - formation of, 429, 430. - - Peat bogs, 429. - - “Pelé’s Hair”, 107. - - Pelé, spine of, 148. - - Penck, A., cited, 294, 399, 414. - - Peneplain, 171, 179. - - “Penitents”, 397. - - “Perched bowlders”, 306. - - Peridotite, 462. - - Periods, interpluvial, 198; - pluvial, 198. - - Peripheral granulation, 31. - - Perret, F. A., cited, 148. - - Philippi, E., cited, 295. - - Phillips, John, cited, 56. - - Physiographic models, preparation, of, 470. - - Piedmont glaciers, 383, 384. - - _Pino_, 119, 130. - - Pipes, volcanic, 140. - - Piracy, river, 175, 176. - - Pirsson, L. V., cited, 39, 447. - - Pitch, 43. - - Pitching folds, 43. - - Pit lakes, 315, 407, 408. - - Pitted plains, 314, 407, 408. - - Pittier, H., cited, 405. - - Plains, flood, 178; - coastal, 246; - outwash, 280, 281; - pitted, 314, 407, 408. - - Platform, continental, 18, 19; - oceanic, 18, 19. - - Playa lakes, 422. - - Playfair, Sir John, cited, 178. - - Plucking, beneath glaciers, 275. - - Plugs, volcanic, 140. - - Plunge and flow structure, 37. - - Plunging folds, 43; - detection of, 49, 50. - - Pluvial periods, 198. - - Pocket rocks, in desert, 200, 201, 202. - - Poles, wind, of the earth, 263; - earlier, 297. - - _Poljen_, 189, 422. - - Pompeii, destruction of, 97; - volcanic materials over, 122. - - _Ponores_, 188. - - Porphyritic texture, of certain igneous rocks, 32. - - Portals, in mountain rampart, surrounding continental glaciers, 271. - - Potato shape, of earth, 7. - - _Pourquoi-Pas_ expedition, 289. - - Powell, J. W., cited, 178, 439, 446. - - Pratt, W. E., cited, 147. - - Precipitation, in relation to glaciation, 261. - - Pressure ridges, on pack ice, 286. - - Prinz, cited, 14, 19, 54, 133, 148. - - Processes by which rocks are formed, 30. - - Profile, cut by waves on steep rocky shore, 236. - - Profiles, character, 177, 318; - character, directly due to volcanic agencies, 145, 146; - character, coast, due to uplift or depression, 259; - character, of arid lands, 220; - character, of shore features, 243; - character, referable to mountain glaciers, 379; - of cinder cones, 123. - - Projectiles, lava, “bread-crust” type, 119; - volcanic, 121. - - Prying work of frost, 152. - - “Pudding stone”, 463. - - Pumiceous texture, of extrusive rocks, 32. - - Pumpelly, Raphael, cited, 222. - - Pumpelly, R. W., cited, 212. - - _Puys_, 105. - - _Puys_ of Auvergne, 124. - - Pyrite, 452. - - Pyrolusite, 456. - - Pyroxenes, 458. - - - Quartz, 458. - - Quartzite, 466. - - _Quebradas_, 75. - - - Rabot, C., cited, 424. - - Radiating glaciers, 383, 386. - - Raft lakes, 417, 418. - - Rafts, log, in Red River, 418. - - Railway tracks, buckled, during earthquakes, 75. - - Rain erosion, 214. - - Rainfall, infrequent in deserts, 197. - - Raised beaches, 326, 328. - - Ramparts, ice, 431-434. - - _Randspalte_, 370. - - Rapids, in Rhine gorge, 169. - - _Rapilli_, 122. - - Rath, G. vom, cited, 147. - - Reaction rims, about minerals, 28. - - Receding hemicycle of glaciation, 264. - - Recessional moraines, 399. - - Reciprocal relation, of land and sea, map to show, 11. - - Réclus, E., cited, 147. - - Records, of rise or fall of land, 245. - - Red clay, of the deep sea, 39. - - Red color, of desert rocks, 202. - - Reid, H. F., cited, 294, 296, 400. - - Rejuvenated rivers, 173, 174. - - Relief forms, carved by waves, 213. - - Relief patterns, dividing lines of, 226. - - Repeating patterns, in earth relief, 223; - composite, 227. - - Reservoirs, of lava, local, 95. - - Residual rocks, 30. - - Resistant rocks, in relation to erosion, 174. - - Rhine, gorge of, 169. - - Rhyolite, 463. - - Ribbon falls, 378. - - Richter, E., cited, 294. - - Richtofen, Freiherr von, cited, 207, 222. - - “Ridge roads”, 328. - - _Riegel_, 377. - - Rifting, in eroded mountains, 444. - - Rift-valley lakes, 403, 404. - - Rift valleys, 440. - - Rigidity of the earth, 20, 29. - - Ripple markings, 36. - - River, zone of the dwindling, 213. - - River capture, 175. - - River deltas, 179. - - River etchings, intricate pattern of, 158. - - River lakes, 424. - - River narrows, 174, 327. - - River networks, in relation to precipitation, 161; - in relation to rock architecture, 161; - meshes of, 161. - - Rivers, braided, 280; - cross sections of, in successive stages, 172; - drowned, 251, 340; - early aspects of, 159; - life begun in uplift, 159; - life histories of, 158; - motive power of, 158; - rejuvenated, 173, 174; - submerged channels of, 252; - swollen during melting of continental glaciers, 320; - tributary, accordant, 377; - young, 159, 160. - - River terraces, 165, 178. - - River valley, longitudinal section of, 161. - - _Roches moutonnées_, 276, 301, 367. - - Rock bars, 377; - cut through by gorges, 378. - - Rock basin lakes, 376, 377, 400, 412. - - Rock cleavage, 44. - - “Rock glaciers”, 153. - - “Rocking stones”, 306. - - Rock mantle, 155; - relation to topography, 156. - - Rock pedestals, 381. - - Rock terraces, 215. - - Rocks, clastic, 30; - corrosion of, 156; - description of some common, 462-466; - extrusive, 32, 463; - igneous, 30; - igneous, textures of, 32; - igneous, massive structure of, 31; - intrusive, 32, 462, 463; - laminated structure of, 31; - marks of origin of, 30; - metamorphic, 30, 31, 465; - residual, 30; - sedimentary, 30; - sedimentary, of chemical precipitation, 464; - sedimentary, of mechanical origin, 463; - sedimentary, of organic origin, 464; - sedimentary, rounded grains of, 31; - volcanic, 32. - - Ross Barrier, 282. - - Rudolph, E., cited, 92. - - Rudski, M. P., cited, 19. - - Russell, I. C., cited, 126, 147, 148, 175, 178, 222, 293, 294, 296, - 381, 384, 414, 424, 425. - - - St. Anthony Falls, recession of, 327, 354. - - St. David’s gorge, near Niagara, 352, 359, 360, 363. - - St. Goars, on Rhine, 169. - - Saint Martin, cited, 436. - - St. Paul’s rocks, a dissected volcano, 141. - - Salients, of newly incised upland, 169. - - Salines, 423. - - Salisbury, R. D., cited, 156, 160, 205, 222, 293, 295, 298, 300, 305, - 313, 318, 319, 339, 424. - - Salton sink, 420. - - Sand, beach, 206; - eolian, 206; - volcanic, 122. - - Sand blast, natural, 204. - - Sand cones, 84. - - “Sand devils”, 209. - - Sandstone, 464. - - Sand storms, 209. - - Santa Catalina, 239, 257. - - Sapper, K., cited, 111, 147, 148. - - Sarasin, P. and F., cited, 248. - - Sardeson, F. W., cited, 327, 339. - - Saucer lakes, 415, 416. - - Sawa Lake, of Persian desert, 199. - - Scaling, 151. - - Scape colks, 277. - - Scars, from dissection of volcanoes, 142; - meander, 165. - - Schist, chlorite, 465; - mica, 465; - sericite, 465; - talc, 465. - - Schistosity, 31. - - Schrader, cited, 436. - - _Schratten_, 188. - - Scidmore, E. R., cited, 70. - - Scoriaceous texture, of extrusive rocks, 32. - - Scott, I. D., cited, 411, 470. - - Scott, R. F., cited, 282, 295. - - Scott, W. B., cited, 6, 60, 72, 259, 274, 375. - - “Scree”, 152. - - Scrope, P., cited, 96, 124, 146. - - Sea caves, 234; - elevated, 248. - - Sea coves, 233. - - Sea ice, 286, 292. - - Seaquakes, 69; - distribution of, 70; - downward movement of sea floor during, 81; - number and magnitude of, 81. - - Seasonal lakes, 189, 422. - - Section, geological, 46, 47; - across mountain wall about desert, 212. - - Sederholm, J. J., cited, 315. - - Sedimentary rocks, 30; - of chemical precipitation, 464; - of mechanical origin, 463; - of organic origin, 464. - - Seismic sea wave, 69; - Japan, 1896, 70. - - Seismotectonic lines, 87. - - Sekiya, S., cited, 141, 148. - - Séracs, 391. - - Serapeum, at Pozzuoli, 254. - - Sericite schist, 465. - - Series, conformable, 51; - unconformable, 51. - - Serpentine, 460. - - Shackleton, Sir Ernest, cited, 17, 282, 283, 292, 295. - - Shadow erosion, 206. - - Shadow weathering, 203. - - Shale, 464. - - Shaler, N. S., cited, 7, 157, 244, 306, 317, 319. - - Shapes of rock folds, 43. - - Shaw, E. W., cited, 425. - - Shearing, in folds, 45. - - “Sheep backs”, 276. - - Shelf, continental, 18, 19. - - Shelf ice, 281, 282, 283; - Antarctic, 289, 290; - of ice age, 317. - - Sherzer, W. H., cited, 294. - - Shields, of lithosphere, 436. - - Shingle, 239. - - Shoal water deposits, 36. - - Shore current, work of, 237, 238. - - Shore lines, elevated, 340; - migration of landward with uplift, 251. - - Side delta lakes, 418, 419. - - Siderite, 456. - - Sieberg, A., cited, 92. - - Sieger, R., cited, 259. - - Siliceous lava, viscous, 103. - - Siliceous sinter, 194. - - Sills, 142. - - Sinclair, W. J., cited, 152. - - Sink lakes, 421. - - Sinks, in limestone, 182. - - Sinter, calcareous, 184; - siliceous, 194. - - Sinter columns, formation of, 185. - - Sinter deposits, 184. - - Sjögren, Otto, cited, 225. - - Skaptár fissure in Iceland, 99. - - Skyline, straight, of mature upland, 170. - - Slate, clay, 466. - - Slichter, C. S., cited, 195. - - Slickensides, on fault, 60. - - Smith, George Otis, cited, 173. - - Smithsonite, 456. - - “Smoke” of volcanoes, nature of, 128. - - Smyth, C. H., Jr., cited, 157. - - Snake river, Idaho, lava plains of, 102. - - Snickers Gap, 177. - - Snow, B. W., cited, 193. - - Snowbergs, 292, 293. - - Snowdrift sites, 368. - - Snow line, 261. - - Soil flow, 153, 157. - - Soil striping, 154. - - Solfatara condition of volcanoes, 97. - - Solger, F., cited, 222. - - Solifluxion, 153, 157. - - Sonklar, cited, 386. - - Spallanzani, cited, 115. - - Spatter cones, 104. - - Speculative philosophy _vs._ observational geology, 5. - - Spencer, J. W., cited, 260, 344, 350, 353, 366. - - Spethmann, H., cited, 267. - - Sphalerite, 453. - - Spherulites, 33. - - Spherulitic texture, of igneous rocks, 33. - - Sphinx, erosion by natural sand blast, 205. - - Spits, 240. - - Spitzbergen, 154. - - Springs, fissure, 190, 195; - surface, 181; - thermal, 190. - - Stability, not the order of nature, 4. - - Stacks, 233; - elevated, 249, 343. - - Stage of adolescence, 169, 170. - - Stairway, cascade, 376. - - Stalactites, growth of, 184. - - Stalagmites, formation of, 185. - - Staurolite, 460. - - Steppes, 215. - - Still river, of Connecticut, history of, 338. - - Stone, G. H., cited, 253, 260, 315, 319. - - “Stone ginger”, 208. - - “Stone lattice”, 205, 206. - - “Stone rivers”, 153. - - Strahan, A., cited, 318. - - Strand lakes, 424. - - Strata, conformable, 51; - contortions of, 40. - - Straths, 428. - - Streak, of minerals, 451. - - Stream capture, 179. - - Stream, meandering, cross section of, 163; - braided, 280; - intermittent, 180. - - Stream velocity, determined by gradient, 158. - - Strike, 46. - - Striped ground, 154. - - _Strokr_, 193. - - Strombolian eruptions, 117. - - Stromboli, cinder cone of, 115; - excentric crater of, 115; - explanation of eruptions in, 116, 117. - - Structure, cross-bedded, 37. - - Submerged channels, of rivers, 252. - - Submergence of land, during earthquakes, 80. - - Suess, E., cited, 19, 142, 259, 277, 425, 436, 437, 438, 446. - - _Suffioni_, arrangement on faults, 87. - - Supan, A., 420, 424. - - Surface moraines, 277. - - Surface springs, 181. - - “Swallow holes”, 182, 422. - - Swamp lands, drained during earthquakes, 83. - - Sweinfurth, G., cited, 222. - - Syenite, 462. - - Symbols, T., to express strike and dip, 48. - - Synclinal folds, 42. - - Synclines, 42. - - System of fractures, 55. - - - Taal volcano, double explosive eruption of 1911, 120, 121. - - Table mountains, origin of, 112. - - _Takyr_, 216. - - Talc, 460. - - Talc schist, 465. - - Talmage, J. E., cited, 221. - - Talus, 152, 153, 215. - - Tangier-Smith, W. S., cited, 260. - - Tarr, R. S., cited, 77, 92, 233, 260, 295, 301. - - Taylor, F. B., cited, 259, 330, 339, 342, 343, 346, 350, 355, 366. - - Tectonic lakes, 424. - - Temperature, diurnal changes of, in deserts, 202. - - Temple of Jupiter Serapis, oscillations of level of, 254, 255. - - Terminal moraine, of Pleistocene glaciations, 298, 299. - - Terminal moraines, of mountain glaciers, 394. - - Terraced valleys, 320, 321. - - Terraces, built, 235; - coast, 80, 235, 341; - river, 165, 178, 320, 321; - rock, 215. - - Terra Rossa, of Karst region, 188. - - Tessellated pavement, from soil flow, 154. - - Tethys, ocean of, 16. - - Tetrahedron, reciprocal relations of antipodal parts, 13; - truncated, toward which earth is tending, 12. - - Tetrahedrons, twin, 16. - - Thaw water, soil flow in presence of, 153. - - Theory, evolved from working hypothesis, 6; - mixture with observation, on maps, 63. - - Thermal springs, 190. - - Thickness of formations, 65. - - Thompson, Bertha, cited, 155. - - Thomson and Tait, cited, 29. - - Thomson, Wyville, cited, 296. - - Thoroddsen, Th., cited, 103, 123, 147, 267. - - Throw, on faults, 59. - - Thrusts, 45. - - “Tidal waves”, 70. - - Tides, effect on a fluid earth, 20. - - Tidewater glaciers, 290, 386. - - Till, 31, 310. - - Tillite, 31. - - Till plains, 311. - - Tinds, 380, 381. - - Tivoli, travertine of, 184. - - Tombolas, 241. - - Tongues, ice, on margin of continental glaciers, 272. - - Topographic maps, 61; - preparation of, 467. - - Topography, built up, 301; - constructional, 309; - destructional, 309; - fault, 65; - fold, 65; - incised, 301; - knob and basin, 314. - - Top-set beds, 167. - - Tourmaline, 460. - - Tower, W. S., cited, 178. - - Trachyte, 463. - - Transgression, of the sea, 37. - - Transparency, of minerals, 451. - - Travertine, 184, 464. - - Trees, how affected by advancing lava, 133; - undermined on stream meanders, 164. - - “Trellis drainage”, 175. - - Troughline, of a syncline, 42. - - Trunk channels of descending water, 181. - - Tsunamis, 70. - - T symbols, to express strike and dip, 48. - - Tufa, calcareous, 464. - - Tunnels, lava, 111, 112, 125. - - Twin tetrahedrons, 16. - - Tyndall, John, cited, 192, 196. - - - Udden, J. A., cited, 222. - - Unconformable series, 51. - - Unconformity, 65; - episodes in history of, 52; - meaning of, 51. - - Underfolding, of earth’s shell, 437. - - Underground water, 180. - - Undertow, 236. - - Unstable erosion remnants, in “driftless area”, 300. - - Upham, Warren, cited, 325, 327, 339, 344, 350. - - Upland, fretted, 372, 373; - grooved, 372, 373; - maturely dissected, 170; - mature, unfavorable to commercial development, 171; - newly incised, 169; - partially dissected, 160; - progressive investment of, by cirques, 374. - - Uplift, marks of, on coasts, 245; - sudden, of coasts, 247. - - Upraised cliffs, 249. - - Uptilt, in basin of Lake Agassiz, 350; - of glaciated area, evidence that it continues, 348-350; - of glaciated area, supposed nature of, 344-347. - - U-shaped valleys, 374. - - Usu-san (New Mountain), birth of, 96. - - - Valley moraine lakes, 400, 413. - - Valleys, hanging, 378; - of V-form, 172; - U-shaped, 374. - - Valley trains, 311, 399. - - Van Hise, C. R., cited, 54. - - Varnish, desert, 201. - - Veatch, A. C., cited, 418, 425. - - Verbeek, R. D. M., cited, 100, 142, 147, 148. - - Vesicular texture, of extrusive rocks, 32. - - Victoria Falls, 225. - - Vincentius of Beauvais, cited, 9. - - Volcanic ash, 122. - - “Volcanic bombs”, 121. - - Volcanic dust, 122. - - Volcanic eruptions, during changes in earth’s figure, 15. - - Volcanic lakes, 424. - - Volcanic mountains, of ejected materials, 115; - of exudation, 94. - - Volcanic necks, 140. - - Volcanic pipes, 140. - - Volcanic plugs, 139, 140. - - Volcanic projectiles, 121. - - Volcanic rocks, 32. - - Volcanic sand, 122. - - Volcano belts, of the earth, 98. - - Volcano, definition of, 95. - - Volcano, eruption in 1888, 118, 120, 147; - history of, 118, 119. - - Volcanoes, active, 97; - arrangement over fissures, 99; - birth of, 96; - cone-producing period of, 127; - convulsive eruptions of, 105; - crater-producing period of, 128; - dissection of, 139, 148; - dormant, 97; - early views concerning, 95; - “elevation-crater” theory of, 95; - explosive eruptions of, 105; - extinct, 97; - fissure eruptions of, 101; - location at fissure intersections, 100; - map of, in Java, 100; - migration of vent along fissure, 101, 124; - misconceptions concerning, 94; - mud flows after eruptions, 138; - of Gulf of Guinea, 101; - regarded as retaining walls, 124, 125; - relation to mountain ranges, 144; - sequence of events within chimney of, during eruption, 134, 135; - solfataric activity of, 97; - three types of, 105. - - V-shaped valley, 172. - - Vulcanello, 119. - - Vulcanian eruptions, 117, 125. - - - Waltershausen, S. von, cited, 148. - - Walther, Johannes, cited, 201, 202, 203, 204, 205, 206, 211, 215, 221. - - Wandering dunes, 209. - - Warren river, 416. - - “Washes”, 213. - - Water, derangement of flow during earthquakes, 83; - ground, 180; - percolating, rôle of, 149; - running, earth features shaped by, 169; - shot up in sheets during earthquake, 83; - thaw, soil flow in presence of, 153. - - Water gaps, 176. - - Water pipes, buckled in ground, during earthquakes, 75. - - Water table, 180; - extreme depth of, 201, 203. - - Water wave, effect of breaking on shore, 233; - free, 232; - motion of, 231. - - Watson, T. L., cited, 259. - - Wave, water, the motion of, 231. - - Wave base, 232. - - Wave length, 231. - - Weathering, carbonization, 151; - chemical, 149; - chemical agents of, 149; - dry, 201; - exfoliation, 151; - frost action, 152; - hydration, 151; - in relation to climate, 150; - internal, in deserts, 201; - mechanical, 149; - of lithosphere surface, 29; - shadow, 203; - spheroidal, 150, 151; - two contrasted processes of, 149. - - _Wed_ (_Wadi_), 212, 213, 214. - - Weed, W. H., cited, 196, 441, 447. - - West Indies, seismotectonic lines of, 88. - - Wheeler, W. H., cited, 244. - - Whirlpool basin, at Niagara, 359; - excavation of, 360. - - Whitbeck, R. H., cited, 319. - - White, David, cited, 318. - - Willis, Bailey, cited, 45, 54, 157, 260, 318. - - Winchell, N. H., cited, 354. - - Wind, in relation to location of glaciers, 377; - in relation to mountain glaciers, 367. - - Wind distribution of snow, 367. - - Wind gaps, 176. - - _Windkanten_, 205. - - Wind poles, of the earth, 263; - of earth, earlier, 297. - - Wintergreen Flats, site of captured fall, 358. - - Wisconsin diamonds, 307, 308. - - Woodworth, J. B., cited, 74, 351. - - Worcester, Dean C., cited, 96. - - Working hypothesis, 6. - - Workman, Fanny Bullock, cited, 294. - - Workman, W. H., cited, 294. - - Wright, F. E., cited, 351. - - - Yellowstone National Park, 33, 191, 193, 194. - - Yosemite Valley, 59, 152. - - Young rivers, 159, 160. - - - Zahn, G. W. von, cited, 244. - - Zigzag ranges, due to plunging folds, 51. - - Zittel, K. v., cited, 19. - - Zone of diverse displacement, 439. - - Zone of flow, 40, 143. - - Zone of fracture, 40, 46. - - Zones, of deposition, surrounding desert, 216, 217; - upper and lower cloud, 268, 269. - - -Printed in the United States of America. - - - - - FOOTNOTES: - -[1] Italian for mouth; plural _bocchi_. - -[2] These models and the contouring apparatus are now manufactured for -the use of schools and colleges by Eberbach and Son, Ann Arbor, Mich. - -[3] This clay is manufactured by the A. H. Abbott Company, art dealers, -Wabash Avenue, Chicago. - -[4] Numbers in parenthesis refer to pages in this book, where further -information is to be found. - - - - - * * * * * * - - - - -Transcriber’s note: - -—Obvious typographical errors have been silently corrected. 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- padding-left: 1em; - margin-bottom: 0em; - text-indent: -1em; - width: 32em; - margin: auto;} - -.ch450 {margin-top: 0.2em; - font-size: 90%; - line-height: 1em; - text-align: justify; - padding-left: 1em; - margin-bottom: 0em; - text-indent: -1em; - width: 35em; - margin: auto;} - -.footnotes {border: dashed 1px;} - -.label {position: absolute; right: 84%; text-align: right;} - -.fnanchor {vertical-align: super; - font-size: .8em; - text-decoration: none; - font-style: normal; - font-weight: normal;} - -.transnote {background-color: #E6E6FA; - color: black; - font-size:smaller; - padding:0.5em; - margin-bottom:5em; - font-family:sans-serif, serif; } - - hr.full { width: 100%; - margin-top: 3em; - margin-bottom: 0em; - margin-left: auto; - margin-right: auto; - height: 4px; - border-width: 4px 0 0 0; /* remove all borders except the top one */ - border-style: solid; - border-color: #000000; - clear: both; } - </style> -</head> -<body> -<h1>The Project Gutenberg eBook, Earth Features and Their Meaning, by William -Herbert Hobbs</h1> -<p>This eBook is for the use of anyone anywhere in the United States -and most other parts of the world at no cost and with almost no -restrictions whatsoever. You may copy it, give it away or re-use it -under the terms of the Project Gutenberg License included with this -eBook or online at <a -href="http://www.gutenberg.org">www.gutenberg.org</a>. If you are not -located in the United States, you'll have to check the laws of the -country where you are located before using this ebook.</p> -<p>Title: Earth Features and Their Meaning</p> -<p> An Introduction to Geology for the Student and the General Reader</p> -<p>Author: William Herbert Hobbs</p> -<p>Release Date: December 12, 2015 [eBook #50671]</p> -<p>Language: English</p> -<p>Character set encoding: UTF-8</p> -<p>***START OF THE PROJECT GUTENBERG EBOOK EARTH FEATURES AND THEIR MEANING***</p> -<p> </p> -<h4>E-text prepared by Giovanni Fini<br /> - and the Online Distributed Proofreading Team<br /> - (<a href="http://www.pgdp.net">http://www.pgdp.net</a>)<br /> - from page images generously made available by<br /> - Internet Archive<br /> - (<a href="https://archive.org">https://archive.org</a>)</h4> -<p> </p> -<table border="0" style="background-color: #ccccff;margin: 0 auto;" cellpadding="10"> - <tr> - <td valign="top"> - Note: - </td> - <td> - Images of the original pages are available through - Internet Archive. See - <a href="https://archive.org/details/cu31924004975763"> - https://archive.org/details/cu31924004975763</a> - </td> - </tr> -</table> -<p> </p> -<hr class="full" /> -<p> </p> -<p> </p> -<p> </p> - -<div class="limit"> - -<div class="chapter"> - -<div class="figcenter"> - <img src="images/cover.jpg" width="350" height="500" alt="" /> -</div> - - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_ii" id="Page_ii">[ii]</a></span></p> - -<div class="chapter"> - -<p class="pc4 large">EARTH FEATURES AND THEIR MEANING</p> - -<p><span class="pagenum"><a name="Page_iii" id="Page_iii">[iii]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-002.jpg" width="200" height="80" - alt="" - title="" /> -</div> - -<p class="pc reduct">THE MACMILLAN COMPANY</p> -<p class="pc small">NEW YORK · BOSTON · CHICAGO -DALLAS · SAN FRANCISCO</p> -<p class="pc1 reduct">MACMILLAN & CO., <span class="smcap">Limited</span></p> -<p class="pc small">LONDON · BOMBAY · CALCUTTA -MELBOURNE</p> -<p class="pc1 reduct">THE MACMILLAN CO. OF CANADA, <span class="smcap">Ltd.</span></p> -<p class="pc small">TORONTO</p> - -<p><span class="pagenum"><a name="Page_iv" id="Page_iv">[iv]</a></span></p> - -<div class="bord p4"> -<p class="pr5"><span class="smcap">Plate 1.</span></p> -<div class="figcenter"> - <img src="images/ill-003.jpg" width="400" height="576" id="p1" - alt="" - title="" /> - <div class="caption"><p class="pc">Mount Balfour and the Balfour Glacier in the Selkirks.</p> -</div></div> -</div> - -<p><span class="pagenum"><a name="Page_v" id="Page_v">[v]</a></span></p> - - -<h1 class="p4">EARTH FEATURES -<span class="little">AND</span><br /> -<span class="reduct">THEIR MEANING</span></h1> - -<p class="pc4 large">AN INTRODUCTION TO GEOLOGY</p> -<p class="pc2 mid">FOR THE STUDENT AND THE GENERAL READER</p> - -<p class="pc4 lmid">BY</p> -<p class="pc large">WILLIAM HERBERT HOBBS</p> -<p class="pc2 reduct">PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN<br /> -AUTHOR OF “EARTHQUAKES. AN INTRODUCTION TO<br /> -SEISMIC GEOLOGY”; “CHARACTERISTICS OF<br /> -EXISTING GLACIERS”; ETC.</p> - -<p class="pc4 lmid">New York -THE MACMILLAN COMPANY -1921</p> - -<p class="pc2 reduct"><i>All rights reserved</i></p> - -<p><span class="pagenum"><a name="Page_vi" id="Page_vi">[vi]</a></span></p> - -<p class="pc4 reduct"><span class="smcap">Copyright</span>, 1912, -<span class="smcap">By</span> THE MACMILLAN COMPANY.</p> - -<p class="pc4 small">Norwood Press -J. S. Cushing Co.—Berwick & Smith Co. -Norwood, Mass., U.S.A.</p> - -<p><span class="pagenum"><a name="Page_vii" id="Page_vii">[vii]</a></span></p> - -<p class="pc4 lmid">TO THE MEMORY</p> -<p class="pc">OF</p> -<p class="pc lmid">GEORGE HUNTINGTON WILLIAMS</p> - -</div> - -<hr class="chap" /> - -<p><span class="pagenum"><a name="Page_viii" id="Page_viii">[viii]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">PREFACE</h2> - -<p><span class="smcap">The</span> series of readings contained in the present volume give in -somewhat expanded form the substance of a course of illustrated -lectures which has now for several years been delivered each -semester at the University of Michigan. The keynote of the course -may be found in the dominant characteristics of the different earth -features and the geological processes which have been betrayed -in the shaping of them. Such a geological examination of landscape -is replete with fascinating revelations, and it lends to the -study of Nature a deep meaning which cannot but enhance the -enjoyment of her varied aspects.</p> - -<p>That there is a real place for such a cultural study of geology -within the University is believed to be shown by the increasing -number of students who have elected the work. Even more than -in former years the American travels afar by car or steamship, and -the earth’s surface features in all their manifold diversity are thus -one after the other unrolled before him. The thousands who each -year cross the Atlantic to roam over European countries may by -historical, literary, or artistic studies prepare themselves to derive -an exquisite pleasure as they visit places identified with past -achievement of one form or another. Yet the Channel coast, the -gorge of the Rhine, the glaciers of Switzerland, and the wild scenery -of Norway or Scotland have each their fascinating story to tell -of a history far more remote and varied. To read this history, the -runic characters in which it is written must first of all be mastered; -for in every landscape there are strong individual lines of character -such as the pen artist would skillfully extract for an outline -sketch. Such <i>character profiles</i> are often many times repeated in -each landscape, and in them we have a key to the historical record.</p> - -<p>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<span class="pagenum"><a name="Page_ix" id="Page_ix">[ix]</a></span> -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.</p> - -<p>Regarded as a textbook of geology, the present work offers some -departures from existing examples. Though it has been customary -to combine in a single text historical with dynamical and structural -geology, a tendency has already become apparent to treat the historical -division apart from the others. Again, a desire to treat the -science of geology comprehensively has led some authors into including -so many subjects as to render their texts unnecessarily -encyclopedic and correspondingly uninteresting to the general -reader. It is the author’s belief that there is a real need for a book -which may be read intelligently by the general public, and it must -be recognized that the beginner in the subject cannot cover the -entire field by a single course of readings. The present work has, -therefore, been prepared with a view to selecting for study those -dominant geological processes which are best illustrated by features -in northern North America and Europe. It is this desire to illustrate -the readings by travels afield, which accounts for the prominence -given to the subject of glaciation; for the larger number of -colleges and universities in both America and Europe are surrounded -by the heavy accumulations that have resulted from former glaciations.</p> - -<p>Emphasis has also been placed upon the dependence of the dominant -geological processes of any region upon existing climatic conditions, -a fact to which too little attention has generally been given. -This explains the rather full treatment of desert regions, of which, -in our own country particularly, much may be illustrated upon the -transcontinental railway journeys.</p> - -<p>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.<span class="pagenum"><a name="Page_x" id="Page_x">[x]</a></span> -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.</p> - -<p>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.</p> - -<p>The scope of the book precludes the possibility of furnishing the -reader with the sources for the body of fact and theory which is -presented, although much may be inferred from the names which -appear beneath the illustrations, and more definite knowledge will -be found in the references to literature supplied at the ends of -chapters. A large amount of original and unpublished material -is for a similar reason unlabeled, and it has been left for the professional -geologist to detect these new strands which have been -drawn into the web.</p> - -<p class="pr2">WILLIAM HERBERT HOBBS.</p> - -<p class="pcl"><span class="smcap">Ann Arbor, Michigan</span>, -October 25, 1911.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_xi" id="Page_xi">[xi]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CONTENTS</h2> - -<table id="toc" summary="contents"> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER I</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Compilation of Earth History</span></td> - </tr> - - <tr> - <td> </td> - <td class="tdrl"><span class="small">PAGE</span></td> - </tr> - - <tr> - <td class="tdt1">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 <i>versus</i> speculative philosophy—The scientific -attitude and temper—The value of the hypothesis—Heading references</td> - <td class="tdrl"><a href="#Page_1">1</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER II</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Figure of the Earth</span></td> - </tr> - - <tr> - <td class="tdt1">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 which the earth is tending—Astronomical -<i>versus</i> geodetic observations—Changes of figure during -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 hydrosphere -and atmosphere—Reading references</td> - <td class="tdrl"><a href="#Page_8">8</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER III</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Nature of the Materials in the Lithosphere</span></td> - </tr> - - <tr> - <td class="tdt1">The rigid quality of our planet—Probable composition of the earth’s core—The -earth a magnet—The chemical constitution of the earth’s surface -shell—The essential nature of crystals—The lithosphere a complex -of interlocking crystals—Some properties of natural crystals, -minerals—The alterations of minerals—Reading references</td> - <td class="tdrl"><a href="#Page_20">20</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER IV</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Rocks of the Earth’s Surface Shell</span></td> - </tr> - - <tr> - <td class="tdt1">The processes by which rocks are formed—The marks of origin—The -metamorphic rocks—Characteristic textures of the igneous rocks—The -<span class="pagenum"><a name="Page_xii" id="Page_xii">[xii]</a></span>classification of rocks—Subdivisions of the sedimentary rocks—The -different deposits of ocean, lake, and river—Special marks of -littoral deposits—The order of deposition during a transgression of -the sea—The basins of deposition of earlier ages—The deposits of the -deep sea—Reading references</td> - <td class="tdrl"><a href="#Page_30">30</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER V</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Contortions of the Strata within the Zone of Flow</span></td> - </tr> - - <tr> - <td class="tdt1">The zones of fracture and flow—Experiments which illustrate the fracture -and flow of solid bodies—The arches and troughs of the folded -strata—The elements of folds—The shapes of rock folds—The overthrust -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 references</td> - <td class="tdrl"><a href="#Page_40">40</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER VI</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Architecture of the Fractured Superstructure</span></td> - </tr> - - <tr> - <td class="tdt1">The system of the fractures—The space intervals of joints—The displacements -upon joints: faults—Methods of detecting faults—The -base of the geological map—The field map and the areal geological -map—Laboratory models for study of geological maps—The method -of preparing the map—Fold <i>vs.</i> fault topography—Reading references</td> - <td class="tdrl"><a href="#Page_55">55</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER VII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Interrupted Character of Earth Movements: Earthquakes -and Seaquakes</span></td> - </tr> - - <tr> - <td class="tdt1">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</td> - <td class="tdrl"><a href="#Page_67">67</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER VIII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Interrupted Character of Earth Movements: Earthquakes -and Seaquakes</span> (<i>concluded</i>)</td> - </tr> - - <tr> - <td class="tdt1">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</td> - <td class="tdrl"><a href="#Page_81">81</a><span class="pagenum"><a name="Page_xiii" id="Page_xiii">[xiii]</a></span></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER IX</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Rise of Molten Rock to the Earth’s Surface; Volcanic -Mountains of Exudation</span></td> - </tr> - - <tr> - <td class="tdt1">Prevalent misconceptions about volcanoes—Early views concerning volcanic -mountains—The birth of volcanoes—Active and extinct volcanoes—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</td> - <td class="tdrl"><a href="#Page_94">94</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER X</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Rise of Molten Rock to the Earth’s Surface; Volcanic -Mountains of Ejected Materials</span></td> - </tr> - - <tr> - <td class="tdt1">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 cinder -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 Pelé—The -aftermath of mud flows—The dissection of volcanoes—The -formation of lava reservoirs—Character profiles—Reading references</td> - <td class="tdrl"><a href="#Page_115">115</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XI</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Attack of the Weather</span></td> - </tr> - - <tr> - <td class="tdt1">The two contrasted processes of weathering—The rôle 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 mantle -and its shield in the mat of vegetation—Reading references</td> - <td class="tdrl"><a href="#Page_149">149</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Life Histories of Rivers</span></td> - </tr> - - <tr> - <td class="tdt1">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 contrasted—The -balance between degradation and aggradation—The -<span class="pagenum"><a name="Page_xiv" id="Page_xiv">[xiv]</a></span>accordance of tributary valleys—The grading of the flood plain—The -cycles of stream meanders—The cut-off of the meander—Meander -scars—River terraces—The delta of the river—The levee—The -sections of delta deposits</td> - <td class="tdrl"><a href="#Page_158">158</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XIII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Earth Features shaped by Running Water</span></td> - </tr> - - <tr> - <td class="tdt1">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 rocks—The capture of one river by -another—Water and wind gaps—Character profiles—Reading references</td> - <td class="tdrl"><a href="#Page_169">169</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XIV</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Travels of the Underground Water</span></td> - </tr> - - <tr> - <td class="tdt1">The descent within the unsaturated zone—The trunk channels of descending -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</td> - <td class="tdrl"><a href="#Page_180">180</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XV</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Sun and Wind in the Lands of Infrequent Rains</span></td> - </tr> - - <tr> - <td class="tdt1">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</td> - <td class="tdrl"><a href="#Page_197">197</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XVI</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Features in Desert Landscapes</span></td> - </tr> - - <tr> - <td class="tdt1">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</td> - <td class="tdrl"><a href="#Page_209">209</a><span class="pagenum"><a name="Page_xv" id="Page_xv">[xv]</a></span></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XVII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Repeating Patterns in the Earth Relief</span></td> - </tr> - - <tr> - <td class="tdt1">The weathering processes under control of the fracture system—The -fracture control of the drainage lines—The repeating pattern in drainage -networks—The dividing lines of the relief patterns: lineaments—The -composite repeating patterns of the higher orders—Reading -references</td> - <td class="tdrl"><a href="#Page_223">223</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XVIII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Forms carved and molded by Waves</span></td> - </tr> - - <tr> - <td class="tdt1">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 barrier—The -land-tied island—A barrier series—Character profiles—Reading -references</td> - <td class="tdrl"><a href="#Page_231">231</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XIX</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Coast Records of the Rise or Fall of the Land</span></td> - </tr> - - <tr> - <td class="tdt1">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 cuesta—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 movements -upon a coast—The contrasted islands of San Clemente and -Santa Catalina—The Blue Grotto of Capri—Character profiles—Reading -references</td> - <td class="tdrl"><a href="#Page_245">245</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XX</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Glaciers of Mountain and Continent</span></td> - </tr> - - <tr> - <td class="tdt1">Conditions essential to glaciation—The snow-line—Importance of mountain -barriers in initiating glaciers—Sensitiveness of glaciers to temperature -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</td> - <td class="tdrl"><a href="#Page_261">261</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXI</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Continental Glaciers of Polar Regions</span></td> - </tr> - - <tr> - <td class="tdt1">The inland ice of Greenland—The mountain rampart and its portals—The -<span class="pagenum"><a name="Page_xvi" id="Page_xvi">[xvi]</a></span>marginal rock islands—Rock fragments which travel with the -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</td> - <td class="tdrl"><a href="#Page_271">271</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Continental Glaciers of the “Ice Age”</span></td> - </tr> - - <tr> - <td class="tdt1">Earlier cycles of glaciation—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</td> - <td class="tdrl"><a href="#Page_297">297</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXIII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Glacial Lakes which marked the Decline of the Last Ice Age</span></td> - </tr> - - <tr> - <td class="tdt1">Interference of glaciers with drainage—Temporary lakes due to ice blocking—The -“parallel roads” of the Scottish glens—The glacial Lake -Agassiz—Episodes of the glacial lake history within the St. Lawrence -Valley—The crescentic 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 Hudson’s -Bay drainage basin—Reading references</td> - <td class="tdrl"><a href="#Page_320">320</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXIV</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Uptilt of the Land at the Close of the Ice Age</span></td> - </tr> - - <tr> - <td class="tdt1">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 references</td> - <td class="tdrl"><a href="#Page_340">340</a><span class="pagenum"><a name="Page_xvii" id="Page_xvii">[xvii]</a></span></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXV</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Niagara Falls a Clock of Recent Geological Time</span></td> - </tr> - - <tr> - <td class="tdt1">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 correlation -with those of the glacial lakes—Time measures of the Niagara -clock—The horologe of late glacial time in Scandinavia—Reading -references</td> - <td class="tdrl"><a href="#Page_352">352</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXVI</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Land Sculpture by Mountain Glaciers</span></td> - </tr> - - <tr> - <td class="tdt1">Contrasted sculpturing of continental and mountain glaciers—Wind distribution -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 amphitheater -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</td> - <td class="tdrl"><a href="#Page_367">367</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXVII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">Successive Glacier Types of a Waning Glaciation</span></td> - </tr> - - <tr> - <td class="tdt1">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</td> - <td class="tdrl"><a href="#Page_383">383</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXVIII</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Glacier’s Surface Features and the Deposits upon its Bed</span></td> - </tr> - - <tr> - <td class="tdt1">The glacier flow—Crevasses and séracs—Bodies given up by the <i>Glacier -des Bossons</i>—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</td> - <td class="tdrl"><a href="#Page_390">390</a><span class="pagenum"><a name="Page_xviii" id="Page_xviii">[xviii]</a></span></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXIX</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">A Study of Lake Basins</span></td> - </tr> - - <tr> - <td class="tdt1">Fresh water and saline lakes—Newland lakes—Basin-range lakes—Rift-valley -lakes—Earthquake lakes—Crater lakes—Coulée lakes—Morainal -lakes—Pit lakes—Glint or colk lakes—Ice-dam lakes—Glacier-lobe -lakes—Rock-basin lakes—Valley moraine lakes—Landslide -lakes—Border lakes—Ox-bow lakes—Saucer lakes—Crescentic -levee lakes—Raft lakes—Side-delta lakes—Delta lakes—Barrier -lakes—Dune lakes—Sink lakes—Karst lakes: <i>poljen</i>—Playa lakes—Salines—Alluvial-dam -lakes—Résumé—Reading references</td> - <td class="tdrl"><a href="#Page_401">401</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXX</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Ephemeral Existence of Lakes</span></td> - </tr> - - <tr> - <td class="tdt1">Lakes as settling basins—Drawing off of water by erosion of outlet—The -pulling in of headlands and the cutting off of bays—Lake extinction -by peat growth—Extinction of lakes in desert regions—The rôle of -lakes in the economy of nature—Ice ramparts on lake shores—Reading -references</td> - <td class="tdrl"><a href="#Page_426">426</a></td> - </tr> - - <tr> - <td class="tdc1" colspan="2"><span class="smcap">CHAPTER XXXI</span></td> - </tr> - - <tr> - <td class="tdc2" colspan="2"><span class="smcap">The Origin and the Forms of Mountains</span></td> - </tr> - - <tr> - <td class="tdt1">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 -effect of the resistant stratum—The mark of the rift in the eroded -mountains—Reading references</td> - <td class="tdrl"><a href="#Page_435">435</a></td> - </tr> - - - <tr> - <td class="tdc1" colspan="2">APPENDICES</td> - </tr> - - <tr> - <td class="tdt1">A. The quick determination of the common minerals</td> - <td class="tdrl"><a href="#Page_449">449</a></td> - </tr> - - <tr> - <td class="tdt1">B. Short descriptions of some common rocks</td> - <td class="tdrl"><a href="#Page_462">462</a></td> - </tr> - - <tr> - <td class="tdt1">C. The preparation of topographical maps</td> - <td class="tdrl"><a href="#Page_467">467</a></td> - </tr> - - <tr> - <td class="tdt1">D. Laboratory models for study in the interpretation of geological maps</td> - <td class="tdrl"><a href="#Page_472">472</a></td> - </tr> - - <tr> - <td class="tdt1">E. Suggested itineraries for pilgrimages to study earth features</td> - <td class="tdrl"><a href="#Page_475">475</a></td> - </tr> - - <tr> - <td class="tdt1"><span class="smcap">Index</span></td> - <td class="tdrl"><a href="#Page_489">489</a></td> - </tr> - -</table> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_xix" id="Page_xix">[xix]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">LIST OF PLATES</h2> - -<table id="top" summary="plates"> - - <tr> - <td class="tdrh"><span class="small">PLATE</span></td> - </tr> - - <tr> - <td class="tdrh">1.</td> - <td class="tdt2" colspan="2">Mount Balfour and the Balfour Glacier in the Selkirks</td> - <td class="tdrl"><a href="#p1"><span class="reduct"><i>Frontispiece</i></span></a></td> - </tr> - - <tr> - <td class="tdrl" colspan="4"><span class="little">FACING PAGE</span></td> - </tr> - - <tr> - <td class="tdrh" rowspan="3">2.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Layers compressed in experiments and showing the effect of a competent<br /> -layer in the process of folding</td> - <td class="tdrl"><a href="#p2a">44</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Experimental production of a series of parallel thrusts within<br /> -closely folded strata</td> - <td class="tdrl"><a href="#p2b">44</a></td> - </tr> - - <tr> - <td class="tdt2">C.</td> - <td class="tdt1">Apparatus to illustrate shearing action within the overturned limb<br /> -of a fold</td> - <td class="tdrl"><a href="#p2c">44</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">3.</td> - <td class="tdt2">A.</td> - <td class="tdt1">An earthquake fault opened in Formosa in 1906 with vertical and -lateral displacements combined</td> - <td class="tdrl"><a href="#p3a">72</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Earthquake faults opened in Alaska in 1889 on which vertical -slices of the earth’s shell have undergone individual adjustments</td> - <td class="tdrl"><a href="#p3b">72</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="3">4.</td> - <td class="tdt2">A.</td> - <td class="tdt1">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</td> - <td class="tdrl"><a href="#p4a">82</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">The same apparatus after a sudden adjustment</td> - <td class="tdrl"><a href="#p4b">82</a></td> - </tr> - - <tr> - <td class="tdt2">C.</td> - <td class="tdt1">Model to illustrate a block displacement in rocks which are intersected -by master joints</td> - <td class="tdrl"><a href="#p4c">82</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">5.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Once wooded region in China now reduced to desert through deforestation</td> - <td class="tdrl"><a href="#p5a">156</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">“Bad Lands” in the Colorado Desert</td> - <td class="tdrl"><a href="#p5b">156</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">6.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Barren Karst landscape near the famous Adelsberg grottoes</td> - <td class="tdrl"><a href="#p6a">188</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Surface of a limestone ledge where joints have been widened through -solution</td> - <td class="tdrl"><a href="#p6b">188</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">7.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Ranges of dunes upon the margin of the Colorado Desert</td> - <td class="tdrl"><a href="#p7a">210</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Sand dunes encroaching upon the oasis of Oued Souf, Algeria</td> - <td class="tdrl"><a href="#p7b">210</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">8.</td> - <td class="tdt2">A.</td> - <td class="tdt1">The granite needles of Harney Peak in the Black Hills of South -Dakota</td> - <td class="tdrl"><a href="#p8a">216</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Castellated erosion chimneys in El Cobra Cañon, New Mexico</td> - <td class="tdrl"><a href="#p8b">216</a></td> - </tr> - - <tr> - <td class="tdrh">9.</td> - <td class="tdt2" colspan="2">Map of the High Plains at the eastern front of the Rocky Mountains</td> - <td class="tdrl"><a href="#p9">220</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">10.</td> - <td class="tdt2">A.</td> - <td class="tdt1">View in Spitzbergen to illustrate the disintegration of rock under -the control of joints</td> - <td class="tdrl"><a href="#p10a">228</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Composite pattern of the joint structures within recent alluvial -deposits of the Syrian Desert</td> - <td class="tdrl"><a href="#p10b">228</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">11.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Ripple markings within an ancient sandstone</td> - <td class="tdrl"><a href="#p11a">232</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Wave breaking as it approaches the shore</td> - <td class="tdrl"><a href="#p11b">232</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">12.<span class="pagenum"><a name="Page_xx" id="Page_xx">[xx]</a></span></td> - <td class="tdt2">A.</td> - <td class="tdt1">V-shaped cañon cut in an upland recently elevated from the sea, -San Clemente Island, California</td> - <td class="tdrl"><a href="#p12a">256</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">A “hogback” at the base of the Bighorn Mountains, Wyoming</td> - <td class="tdrl"><a href="#p12b">256</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">13.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Precipitous front of the Bryant Glacier outlet of the Greenland -inland ice</td> - <td class="tdrl"><a href="#p13a">272</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Lateral stream beside the Benedict Glacier outlet, Greenland</td> - <td class="tdrl"><a href="#p13b">272</a></td> - </tr> - - <tr> - <td class="tdrh">14.</td> - <td class="tdt2" colspan="2">View of the margin of the Antarctic continental glacier in Kaiser -Wilhelm Land</td> - <td class="tdrl"><a href="#p14">282</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">15.</td> - <td class="tdt2">A.</td> - <td class="tdt1">An Antarctic ice foot with boat party landing</td> - <td class="tdrl"><a href="#p15a">290</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">A near view of the front of the Great Ross Barrier, - Antarctica</td> - <td class="tdrl"><a href="#p15b">290</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">16.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Incised topography within the “driftless area”</td> - <td class="tdrl"><a href="#p16a">300</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Built-up topography within the glaciated region</td> - <td class="tdrl"><a href="#p16b">300</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="3">17.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Soled glacial bowlders which show differently directed striæ upon -the same facet</td> - <td class="tdrl"><a href="#p17a">306</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Perched bowlder upon a striated ledge of different rock -type, Bronx Park, New York</td> - <td class="tdrl"><a href="#p17b">306</a></td> - </tr> - - <tr> - <td class="tdt2">C.</td> - <td class="tdt1">Characteristic knob and basin surface of a moraine</td> - <td class="tdrl"><a href="#p17c">306</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">18.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Fretted upland of the Alps seen from the summit of Mount Blanc</td> - <td class="tdrl"><a href="#p18a">372</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Model of the Malaspina Glacier and the fretted upland above it</td> - <td class="tdrl"><a href="#p18b">372</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">19.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Contour map of a grooved upland, Bighorn Mountains, Wyoming</td> - <td class="tdrl"><a href="#p19a">372</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Contour map of a fretted upland, Philipsburg Quadrangle, Montana</td> - <td class="tdrl"><a href="#p19b">372</a></td> - </tr> - - <tr> - <td class="tdrh">20.</td> - <td class="tdt2" colspan="2">Map of the surface modeled by mountain glaciers in the Sierra Nevadas -of California</td> - <td class="tdrl"><a href="#p20">376</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">21.</td> - <td class="tdt2">A.</td> - <td class="tdt1">View of the Harvard Glacier, Alaska, showing the characteristic -terraces</td> - <td class="tdrl"><a href="#p21a">394</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">The terminal moraine at the foot of a mountain glacier</td> - <td class="tdrl"><a href="#p21b">394</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">22.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Model of the vicinity of Chicago, showing the position of the -outlet of the former Lake Chicago</td> - <td class="tdrl"><a href="#p22a">400</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Map of Yosemite Falls and its earlier site near Eagle Peak</td> - <td class="tdrl"><a href="#p22b">400</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="2">23.</td> - <td class="tdt2">A.</td> - <td class="tdt1">View of the American Fall at Niagara, showing the accumulation -of blocks beneath</td> - <td class="tdrl"><a href="#p23a">414</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">Crystal Lake, a landslide lake in Colorado</td> - <td class="tdrl"><a href="#p23b">414</a></td> - </tr> - - <tr> - <td class="tdrh" rowspan="3">24.</td> - <td class="tdt2">A.</td> - <td class="tdt1">Apparatus for exercise in the preparation of topographic maps</td> - <td class="tdrl"><a href="#p24a">468</a></td> - </tr> - - <tr> - <td class="tdt2">B.</td> - <td class="tdt1">The same apparatus in use for testing the contours of a map</td> - <td class="tdrl"><a href="#p24b">468</a></td> - </tr> - - <tr> - <td class="tdt2">C.</td> - <td class="tdt1">Modeling apparatus in use</td> - <td class="tdrl"><a href="#p24c">468</a></td> - </tr> - -</table> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_xxi" id="Page_xxi">[xxi]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">ILLUSTRATIONS IN THE TEXT</h2> - -<table id="toi" summary="illustrations"> - - <tr> - <td class="tdrl"><span class="small">FIG.</span></td> - <td> </td> - <td class="tdrl"><span class="small">PAGE</span></td> - </tr> - - <tr> - <td class="tdrh">1.</td> - <td class="tdt2">Diagram to show the measure of the earth’s surface irregularities</td> - <td class="tdrl"><a href="#f1">11</a></td> - </tr> - - <tr> - <td class="tdrh">2.</td> - <td class="tdt2">Map to show the reciprocal relation of areas of land and sea</td> - <td class="tdrl"><a href="#f2">11</a></td> - </tr> - - <tr> - <td class="tdrh">3.</td> - <td class="tdt2">The tetrahedral form toward which the earth is tending</td> - <td class="tdrl"><a href="#f3">12</a></td> - </tr> - - <tr> - <td class="tdrh">4.</td> - <td class="tdt2">A truncated tetrahedron to show the reciprocal relation of projection -and depression upon the surface</td> - <td class="tdrl"><a href="#f4">13</a></td> - </tr> - - <tr> - <td class="tdrh">5.</td> - <td class="tdt2">Approximations to earlier and present figures of the earth</td> - <td class="tdrl"><a href="#f5">15</a></td> - </tr> - - <tr> - <td class="tdrh">6.</td> - <td class="tdt2">Diagrams for comparison of coasts upon an upright and upon an inverted -tetrahedron</td> - <td class="tdrl"><a href="#f6">17</a></td> - </tr> - - <tr> - <td class="tdrh">7.</td> - <td class="tdt2">The continents, including submerged portions</td> - <td class="tdrl"><a href="#f7">18</a></td> - </tr> - - <tr> - <td class="tdrh">8.</td> - <td class="tdt2">Diagram to indicate the altitude of different parts of the lithosphere -surface</td> - <td class="tdrl"><a href="#f8">18</a></td> - </tr> - - <tr> - <td class="tdrh">9.</td> - <td class="tdt2">Diagram to show how the terrestrial rocks grade into the meteorites</td> - <td class="tdrl"><a href="#f9">22</a></td> - </tr> - - <tr> - <td class="tdrh">10.</td> - <td class="tdt2">Comparison of a crystalline with an amorphous substance</td> - <td class="tdrl"><a href="#f10">24</a></td> - </tr> - - <tr> - <td class="tdrh">11.</td> - <td class="tdt2">“Light figure” seen upon etched surface of calcite</td> - <td class="tdrl"><a href="#f11">25</a></td> - </tr> - - <tr> - <td class="tdrh">12.</td> - <td class="tdt2">Battered sand grains which have developed crystal faces</td> - <td class="tdrl"><a href="#f12">26</a></td> - </tr> - - <tr> - <td class="tdrh">13.</td> - <td class="tdt2">Unassimilated grains of quartz within a garnet crystal</td> - <td class="tdrl"><a href="#f13">28</a></td> - </tr> - - <tr> - <td class="tdrh">14.</td> - <td class="tdt2">New minerals developed about the core of an augite crystal</td> - <td class="tdrl"><a href="#f14">28</a></td> - </tr> - - <tr> - <td class="tdrh">15.</td> - <td class="tdt2">A common rim of new mineral developed by reaction where earlier -minerals come into contact</td> - <td class="tdrl"><a href="#f15">28</a></td> - </tr> - - <tr> - <td class="tdrh">16.</td> - <td class="tdt2">Laminated structure of a sedimentary rock</td> - <td class="tdrl"><a href="#f16">30</a></td> - </tr> - - <tr> - <td class="tdrh">17.</td> - <td class="tdt2">Characteristic textures of igneous rocks</td> - <td class="tdrl"><a href="#f17">33</a></td> - </tr> - - <tr> - <td class="tdrh">18.</td> - <td class="tdt2">Diagram to show the order of sediments laid down during a transgression -of the sea</td> - <td class="tdrl"><a href="#f18">37</a></td> - </tr> - - <tr> - <td class="tdrh">19.</td> - <td class="tdt2">Fractures produced by compression of a block of molder’s wax</td> - <td class="tdrl"><a href="#f19">41</a></td> - </tr> - - <tr> - <td class="tdrh">20.</td> - <td class="tdt2">Apparatus to illustrate the folding of strata</td> - <td class="tdrl"><a href="#f20">41</a></td> - </tr> - - <tr> - <td class="tdrh">21.</td> - <td class="tdt2">Diagrams of fold types</td> - <td class="tdrl"><a href="#f21">42</a></td> - </tr> - - <tr> - <td class="tdrh">22.</td> - <td class="tdt2">Diagrams to illustrate crustal shortening</td> - <td class="tdrl"><a href="#f22">42</a></td> - </tr> - - <tr> - <td class="tdrh">23.</td> - <td class="tdt2">Anticlinal and synclinal folds</td> - <td class="tdrl"><a href="#f23">43</a></td> - </tr> - - <tr> - <td class="tdrh">24.</td> - <td class="tdt2">Diagrams to illustrate the shapes of rock folds</td> - <td class="tdrl"><a href="#f24">44</a></td> - </tr> - - <tr> - <td class="tdrh">25.</td> - <td class="tdt2">Secondary and tertiary flexures superimposed upon the primary ones</td> - <td class="tdrl"><a href="#f25">44</a></td> - </tr> - - <tr> - <td class="tdrh">26.</td> - <td class="tdt2">A bent stratum to illustrate tension and compression upon opposite -sides</td> - <td class="tdrl"><a href="#f26">45</a></td> - </tr> - - <tr> - <td class="tdrh">27.</td> - <td class="tdt2">A geological section with truncated arches restored</td> - <td class="tdrl"><a href="#f27">47</a></td> - </tr> - - <tr> - <td class="tdrh">28.</td> - <td class="tdt2">Diagram to illustrate the nature of strike and dip</td> - <td class="tdrl"><a href="#f28">47</a></td> - </tr> - - <tr> - <td class="tdrh">29.</td> - <td class="tdt2">Diagram to show the use of T symbols for strike and dip observation</td> - <td class="tdrl"><a href="#f29">48</a></td> - </tr> - - <tr> - <td class="tdrh">30.</td> - <td class="tdt2">Diagram to show how the thickness of a formation is determined</td> - <td class="tdrl"><a href="#f30">49</a></td> - </tr> - - <tr> - <td class="tdrh">31.</td> - <td class="tdt2">A plunging anticline</td> - <td class="tdrl"><a href="#f31">50</a></td> - </tr> - - <tr> - <td class="tdrh">32.<span class="pagenum"><a name="Page_xxii" id="Page_xxii">[xxii]</a></span></td> - <td class="tdt2">A plunging syncline</td> - <td class="tdrl"><a href="#f32">50</a></td> - </tr> - - <tr> - <td class="tdrh">33.</td> - <td class="tdt2">An unconformity upon the coast of California</td> - <td class="tdrl"><a href="#f33">51</a></td> - </tr> - - <tr> - <td class="tdrh">34.</td> - <td class="tdt2">Series of diagrams to illustrate the episodes involved in the production -of an angular unconformity</td> - <td class="tdrl"><a href="#f34">52</a></td> - </tr> - - <tr> - <td class="tdrh">35.</td> - <td class="tdt2">Types of deceptive or erosional unconformities</td> - <td class="tdrl"><a href="#f35">53</a></td> - </tr> - - <tr> - <td class="tdrh">36.</td> - <td class="tdt2">A set of master joints in shale</td> - <td class="tdrl"><a href="#f36">55</a></td> - </tr> - - <tr> - <td class="tdrh">37.</td> - <td class="tdt2">Diagram to show the manner of replacement of one set of joints by -another</td> - <td class="tdrl"><a href="#f37">56</a></td> - </tr> - - <tr> - <td class="tdrh">38.</td> - <td class="tdt2">Diagram to show the different combinations of joint series</td> - <td class="tdrl"><a href="#f38">56</a></td> - </tr> - - <tr> - <td class="tdrh">39.</td> - <td class="tdt2">View of the shore in West Greenland</td> - <td class="tdrl"><a href="#f39">57</a></td> - </tr> - - <tr> - <td class="tdrh">40.</td> - <td class="tdt2">View in Iceland which shows joint intervals of more than one order</td> - <td class="tdrl"><a href="#f40">57</a></td> - </tr> - - <tr> - <td class="tdrh">41.</td> - <td class="tdt2">Faulted blocks of basalt near Woodbury, Connecticut</td> - <td class="tdrl"><a href="#f41">58</a></td> - </tr> - - <tr> - <td class="tdrh">42.</td> - <td class="tdt2">A fault in previously disturbed strata</td> - <td class="tdrl"><a href="#f42">59</a></td> - </tr> - - <tr> - <td class="tdrh">43.</td> - <td class="tdt2">Diagram to show the effect of erosion upon a fault</td> - <td class="tdrl"><a href="#f43">60</a></td> - </tr> - - <tr> - <td class="tdrh">44.</td> - <td class="tdt2">A fault plane exhibiting drag</td> - <td class="tdrl"><a href="#f44">60</a></td> - </tr> - - <tr> - <td class="tdrh">45.</td> - <td class="tdt2">Map to show how a fault may be indicated by abrupt changes in strike -and dip</td> - <td class="tdrl"><a href="#f45">61</a></td> - </tr> - - <tr> - <td class="tdrh">46.</td> - <td class="tdt2">A series of parallel faults revealed by offsets</td> - <td class="tdrl"><a href="#f46">61</a></td> - </tr> - - <tr> - <td class="tdrh">47.</td> - <td class="tdt2">Field map prepared from the laboratory table</td> - <td class="tdrl"><a href="#f47">64</a></td> - </tr> - - <tr> - <td class="tdrh">48.</td> - <td class="tdt2">Areal geological map based upon the field map</td> - <td class="tdrl"><a href="#f48">64</a></td> - </tr> - - <tr> - <td class="tdrh">49.</td> - <td class="tdt2">A portion of the ruins of Messina</td> - <td class="tdrl"><a href="#f49">67</a></td> - </tr> - - <tr> - <td class="tdrh">50.</td> - <td class="tdt2">Ruins of the Carnegie Palace of Peace at Cartaga, Costa Rica</td> - <td class="tdrl"><a href="#f50">68</a></td> - </tr> - - <tr> - <td class="tdrh">51.</td> - <td class="tdt2">Overturned bowlders from Assam earthquake of 1897</td> - <td class="tdrl"><a href="#f51">69</a></td> - </tr> - - <tr> - <td class="tdrh">52.</td> - <td class="tdt2">Post sunk into ground during Charleston earthquake</td> - <td class="tdrl"><a href="#f52">69</a></td> - </tr> - - <tr> - <td class="tdrh">53.</td> - <td class="tdt2">Map showing localities where shocks have been reported at sea off -Cape Mendocino, California</td> - <td class="tdrl"><a href="#f53">70</a></td> - </tr> - - <tr> - <td class="tdrh">54.</td> - <td class="tdt2">Effect of seismic water wave in Japan</td> - <td class="tdrl"><a href="#f54">70</a></td> - </tr> - - <tr> - <td class="tdrh">55.</td> - <td class="tdt2">A fault of vertical displacement</td> - <td class="tdrl"><a href="#f55">71</a></td> - </tr> - - <tr> - <td class="tdrh">56.</td> - <td class="tdt2">Escarpment produced by an earthquake fault in India</td> - <td class="tdrl"><a href="#f56">72</a></td> - </tr> - - <tr> - <td class="tdrh">57.</td> - <td class="tdt2">A fault of lateral displacement</td> - <td class="tdrl"><a href="#f57">72</a></td> - </tr> - - <tr> - <td class="tdrh">58.</td> - <td class="tdt2">Fence parted and displaced by lateral displacement on fault during -California earthquake</td> - <td class="tdrl"><a href="#f58">72</a></td> - </tr> - - <tr> - <td class="tdrh">59.</td> - <td class="tdt2">Fault with vertical and lateral displacements combined</td> - <td class="tdrl"><a href="#f59">72</a></td> - </tr> - - <tr> - <td class="tdrh">60.</td> - <td class="tdt2">Diagram to show how small faults may be masked at the earth’s surface</td> - <td class="tdrl"><a href="#f60">73</a></td> - </tr> - - <tr> - <td class="tdrh">61.</td> - <td class="tdt2">“Mole hill” effect above buried earthquake fault</td> - <td class="tdrl"><a href="#f61">73</a></td> - </tr> - - <tr> - <td class="tdrh">62.</td> - <td class="tdt2">Post-glacial earthquake faults</td> - <td class="tdrl"><a href="#f62">74</a></td> - </tr> - - <tr> - <td class="tdrh">63.</td> - <td class="tdt2">Earthquake cracks in Colorado desert</td> - <td class="tdrl"><a href="#f63">74</a></td> - </tr> - - <tr> - <td class="tdrh">64.</td> - <td class="tdt2">Railway tracks broken or buckled at time of earthquake</td> - <td class="tdrl"><a href="#f64">75</a></td> - </tr> - - <tr> - <td class="tdrh">65.</td> - <td class="tdt2">Railroad bridge in Japan damaged by earthquake</td> - <td class="tdrl"><a href="#f65">75</a></td> - </tr> - - <tr> - <td class="tdrh">66.</td> - <td class="tdt2">Diagrams to show contraction of earth’s crust during an earthquake</td> - <td class="tdrl"><a href="#f66">76</a></td> - </tr> - - <tr> - <td class="tdrh">67.</td> - <td class="tdt2">Map of the Chedrang fault of India</td> - <td class="tdrl"><a href="#f67">76</a></td> - </tr> - - <tr> - <td class="tdrh">68.</td> - <td class="tdt2">Displacements along earthquake fault in Alaska</td> - <td class="tdrl"><a href="#f68">77</a></td> - </tr> - - <tr> - <td class="tdrh">69.</td> - <td class="tdt2">Abrupt change in direction of throw upon an earthquake fault</td> - <td class="tdrl"><a href="#f69">77</a></td> - </tr> - - <tr> - <td class="tdrh">70.</td> - <td class="tdt2">Map of faults in the Owens Valley, California, formed during earthquake -of 1872</td> - <td class="tdrl"><a href="#f70">78</a></td> - </tr> - - <tr> - <td class="tdrh">71.<span class="pagenum"><a name="Page_xxiii" id="Page_xxiii">[xxiii]</a></span></td> - <td class="tdt2">Marquetry of the rock floor in the Tonopah district, Nevada</td> - <td class="tdrl"><a href="#f71">79</a></td> - </tr> - - <tr> - <td class="tdrh">72.</td> - <td class="tdt2">Map of Alaskan coast to show adjustments of level during an earthquake</td> - <td class="tdrl"><a href="#f72">79</a></td> - </tr> - - <tr> - <td class="tdrh">73.</td> - <td class="tdt2">An Alaskan shore elevated seventeen feet during the earthquake of -1899</td> - <td class="tdrl"><a href="#f73">80</a></td> - </tr> - - <tr> - <td class="tdrh">74.</td> - <td class="tdt2">Partially submerged forest from depression of shore in Alaska during -earthquake</td> - <td class="tdrl"><a href="#f74">80</a></td> - </tr> - - <tr> - <td class="tdrh">75.</td> - <td class="tdt2">Effect of settlement of the shore at Port Royal during earthquake of -1907</td> - <td class="tdrl"><a href="#f75">80</a></td> - </tr> - - <tr> - <td class="tdrh">76.</td> - <td class="tdt2">Diagrams to illustrate the draining of lakes during earthquakes</td> - <td class="tdrl"><a href="#f76">83</a></td> - </tr> - - <tr> - <td class="tdrh">77.</td> - <td class="tdt2">Diagram to illustrate the derangements of water flow during an -earthquake</td> - <td class="tdrl"><a href="#f77">84</a></td> - </tr> - - <tr> - <td class="tdrh">78.</td> - <td class="tdt2">Mud cones aligned upon an earthquake fissure in Servia</td> - <td class="tdrl"><a href="#f78">84</a></td> - </tr> - - <tr> - <td class="tdrh">79.</td> - <td class="tdt2">Craterlet formed near Charleston, South Carolina, during the earthquake -of 1886</td> - <td class="tdrl"><a href="#f79">85</a></td> - </tr> - - <tr> - <td class="tdrh">80.</td> - <td class="tdt2">Cross section of a craterlet</td> - <td class="tdrl"><a href="#f80">85</a></td> - </tr> - - <tr> - <td class="tdrh">81.</td> - <td class="tdt2">Map of the island of Ischia to show the concentration of earthquake -shocks</td> - <td class="tdrl"><a href="#f81">87</a></td> - </tr> - - <tr> - <td class="tdrh">82.</td> - <td class="tdt2">A line of earth fracture revealed in the plan of the relief</td> - <td class="tdrl"><a href="#f82">87</a></td> - </tr> - - <tr> - <td class="tdrh">83.</td> - <td class="tdt2">Seismotectonic lines of the West Indies</td> - <td class="tdrl"><a href="#f83">88</a></td> - </tr> - - <tr> - <td class="tdrh">84.</td> - <td class="tdt2">Device to illustrate the different effects of earthquakes in firm rock -and in loose materials</td> - <td class="tdrl"><a href="#f84">88</a></td> - </tr> - - <tr> - <td class="tdrh">85.</td> - <td class="tdt2">House wrecked in San Francisco earthquake</td> - <td class="tdrl"><a href="#f85">90</a></td> - </tr> - - <tr> - <td class="tdrh">86.</td> - <td class="tdt2">Building wrecked in California earthquake by roof and upper floor -battering down the upper walls</td> - <td class="tdrl"><a href="#f86">91</a></td> - </tr> - - <tr> - <td class="tdrh">87.</td> - <td class="tdt2">Breached volcanic cone in New Zealand showing the bending down -of the strata near the vent</td> - <td class="tdrl"><a href="#f87">96</a></td> - </tr> - - <tr> - <td class="tdrh">88.</td> - <td class="tdt2">View of the new Camiguin volcano formed in 1871 in the Philippines</td> - <td class="tdrl"><a href="#f88">97</a></td> - </tr> - - <tr> - <td class="tdrh">89.</td> - <td class="tdt2">Map to show the belts of active volcanoes</td> - <td class="tdrl"><a href="#f89">98</a></td> - </tr> - - <tr> - <td class="tdrh">90.</td> - <td class="tdt2">A portion of the “fire girdle” of the Pacific</td> - <td class="tdrl"><a href="#f90">98</a></td> - </tr> - - <tr> - <td class="tdrh">91.</td> - <td class="tdt2">Volcanic cones formed in 1783 above the Skaptár fissure in Iceland</td> - <td class="tdrl"><a href="#f91">99</a></td> - </tr> - - <tr> - <td class="tdrh">92.</td> - <td class="tdt2">Diagrams to illustrate the location of volcanic vents upon fissure lines</td> - <td class="tdrl"><a href="#f92">100</a></td> - </tr> - - <tr> - <td class="tdrh">93.</td> - <td class="tdt2">Outline map showing the arrangement of volcanic vents upon the -island of Java</td> - <td class="tdrl"><a href="#f93">100</a></td> - </tr> - - <tr> - <td class="tdrh">94.</td> - <td class="tdt2">Map showing the migration of volcanoes along a fissure</td> - <td class="tdrl"><a href="#f94">101</a></td> - </tr> - - <tr> - <td class="tdrh">95.</td> - <td class="tdt2">Basaltic plateau of the northwestern United States due to fissure -eruptions of lava</td> - <td class="tdrl"><a href="#f95">102</a></td> - </tr> - - <tr> - <td class="tdrh">96.</td> - <td class="tdt2">Lava plains about the Snake River in Idaho</td> - <td class="tdrl"><a href="#f96">102</a></td> - </tr> - - <tr> - <td class="tdrh">97.</td> - <td class="tdt2">Characteristic profiles of lava volcanoes</td> - <td class="tdrl"><a href="#f97">103</a></td> - </tr> - - <tr> - <td class="tdrh">98.</td> - <td class="tdt2">A driblet cone</td> - <td class="tdrl"><a href="#f98">104</a></td> - </tr> - - <tr> - <td class="tdrh">99.</td> - <td class="tdt2">Leffingwell Crater, a cinder cone in the Owens Valley, California</td> - <td class="tdrl"><a href="#f99">104</a></td> - </tr> - - <tr> - <td class="tdrh">100.</td> - <td class="tdt2">Map of Hawaii and its lava volcanoes</td> - <td class="tdrl"><a href="#f100">106</a></td> - </tr> - - <tr> - <td class="tdrh">101.</td> - <td class="tdt2">Section through Mauna Loa and Kilauea</td> - <td class="tdrl"><a href="#f101">106</a></td> - </tr> - - <tr> - <td class="tdrh">102.</td> - <td class="tdt2">Schematic diagram to illustrate the moving platform in the crater of -Kilauea</td> - <td class="tdrl"><a href="#f102">107</a></td> - </tr> - - <tr> - <td class="tdrh">103.</td> - <td class="tdt2">View of the open lava lake of Halemaumau</td> - <td class="tdrl"><a href="#f103">108</a></td> - </tr> - - <tr> - <td class="tdrh">104.<span class="pagenum"><a name="Page_xxiv" id="Page_xxiv">[xxiv]</a></span></td> - <td class="tdt2">Map to show the manner of outflow of the lava from Kilauea in the -eruption of 1840</td> - <td class="tdrl"><a href="#f104">109</a></td> - </tr> - - <tr> - <td class="tdrh">105.</td> - <td class="tdt2">Lava of Matavanu flowing down to the sea during the eruption of -1906</td> - <td class="tdrl"><a href="#f105">110</a></td> - </tr> - - <tr> - <td class="tdrh">106.</td> - <td class="tdt2">Lava stream discharging into the sea from a lava tunnel</td> - <td class="tdrl"><a href="#f106">111</a></td> - </tr> - - <tr> - <td class="tdrh">107.</td> - <td class="tdt2">Diagrammatic representation of the structure of lava volcanoes as a -result of the draining of frozen lava streams</td> - <td class="tdrl"><a href="#f107">112</a></td> - </tr> - - <tr> - <td class="tdrh">108.</td> - <td class="tdt2">Diagram to show the formation of mesas by outflow of lava in valleys -and subsequent erosion</td> - <td class="tdrl"><a href="#f108">112</a></td> - </tr> - - <tr> - <td class="tdrh">109.</td> - <td class="tdt2">Surface of lava of the Pahoehoe type</td> - <td class="tdrl"><a href="#f109">113</a></td> - </tr> - - <tr> - <td class="tdrh">110.</td> - <td class="tdt2">Three successive views to show the growth of the island of Savaii, -from lava outflow in 1906</td> - <td class="tdrl"><a href="#f110">113</a></td> - </tr> - - <tr> - <td class="tdrh">111.</td> - <td class="tdt2">View of the volcano of Stromboli showing the excentric position of -the crater</td> - <td class="tdrl"><a href="#f111">116</a></td> - </tr> - - <tr> - <td class="tdrh">112.</td> - <td class="tdt2">Diagrams to illustrate the eruptions within the crater of Stromboli</td> - <td class="tdrl"><a href="#f112">117</a></td> - </tr> - - <tr> - <td class="tdrh">113.</td> - <td class="tdt2">Map of Volcano in the Æolian Islands</td> - <td class="tdrl"><a href="#f113">118</a></td> - </tr> - - <tr> - <td class="tdrh">114.</td> - <td class="tdt2">“Bread-crust” lava projectile from the eruption of Volcano in 1888</td> - <td class="tdrl"><a href="#f114">119</a></td> - </tr> - - <tr> - <td class="tdrh">115.</td> - <td class="tdt2">“Cauliflower cloud” of steam and ash rising above the cinder cone -of Volcano</td> - <td class="tdrl"><a href="#f115">120</a></td> - </tr> - - <tr> - <td class="tdrh">116.</td> - <td class="tdt2">Eruption of Taal volcano in 1911 seen from a distance of six miles</td> - <td class="tdrl"><a href="#f116">120</a></td> - </tr> - - <tr> - <td class="tdrh">117.</td> - <td class="tdt2">The thick mud veneer upon the island of Taal (after a photograph -by Deniston)</td> - <td class="tdrl"><a href="#f117">121</a></td> - </tr> - - <tr> - <td class="tdrh">118.</td> - <td class="tdt2">A pear-shaped lava projectile</td> - <td class="tdrl"><a href="#f118">121</a></td> - </tr> - - <tr> - <td class="tdrh">119.</td> - <td class="tdt2">Artificial production of a cinder cone</td> - <td class="tdrl"><a href="#f119">122</a></td> - </tr> - - <tr> - <td class="tdrh">120.</td> - <td class="tdt2">Diagram to show the contrast between a lava dome and a cinder cone</td> - <td class="tdrl"><a href="#f120">123</a></td> - </tr> - - <tr> - <td class="tdrh">121.</td> - <td class="tdt2">Mayon volcano on the island of Luzon, Philippine Islands</td> - <td class="tdrl"><a href="#f121">123</a></td> - </tr> - - <tr> - <td class="tdrh">122.</td> - <td class="tdt2">A series of breached cinder cones due to migration of the eruption -along a fissure</td> - <td class="tdrl"><a href="#f122">124</a></td> - </tr> - - <tr> - <td class="tdrh">123.</td> - <td class="tdt2">The mouth upon the inner cone of Mount Vesuvius from which flowed -the lava of 1872</td> - <td class="tdrl"><a href="#f123">124</a></td> - </tr> - - <tr> - <td class="tdrh">124.</td> - <td class="tdt2">A row of parasitic cones raised above a fissure opened on the flanks -of Etna in 1892</td> - <td class="tdrl"><a href="#f124">125</a></td> - </tr> - - <tr> - <td class="tdrh">125.</td> - <td class="tdt2">View of Etna, showing the parasitic cones upon its flanks</td> - <td class="tdrl"><a href="#f125">125</a></td> - </tr> - - <tr> - <td class="tdrh">126.</td> - <td class="tdt2">Sketch map of Etna to show the areas covered by lava and tuff respectively</td> - <td class="tdrl"><a href="#f126">126</a></td> - </tr> - - <tr> - <td class="tdrh">127.</td> - <td class="tdt2">Panum crater showing the caldera</td> - <td class="tdrl"><a href="#f127">126</a></td> - </tr> - - <tr> - <td class="tdrh">128.</td> - <td class="tdt2">View of Mount Vesuvius before the eruption of 1906</td> - <td class="tdrl"><a href="#f128">127</a></td> - </tr> - - <tr> - <td class="tdrh">129.</td> - <td class="tdt2">Sketches of the summit of the Vesuvian cone to bring out the changes -in its outline</td> - <td class="tdrl"><a href="#f129">128</a></td> - </tr> - - <tr> - <td class="tdrh">130.</td> - <td class="tdt2">Night view of Vesuvius from Naples before the outbreak of 1906, -showing a small lava stream descending the central cone</td> - <td class="tdrl"><a href="#f130">129</a></td> - </tr> - - <tr> - <td class="tdrh">131.</td> - <td class="tdt2">Scoriaceous lava encroaching upon the tracks of the Vesuvian railway</td> - <td class="tdrl"><a href="#f131">130</a></td> - </tr> - - <tr> - <td class="tdrh">132.</td> - <td class="tdt2">Map of Vesuvius, showing the position of the lava mouths opened -upon its flanks during the eruption of 1906</td> - <td class="tdrl"><a href="#f132">131</a></td> - </tr> - - <tr> - <td class="tdrh">133.</td> - <td class="tdt2">The ash curtain over Vesuvius lifting and disclosing the outlines of -the mountain</td> - <td class="tdrl"><a href="#f133">132</a></td> - </tr> - - <tr> - <td class="tdrh">134.<span class="pagenum"><a name="Page_xxv" id="Page_xxv">[xxv]</a></span></td> - <td class="tdt2">The central cone of Vesuvius as it appeared after the eruption of 1906</td> - <td class="tdrl"><a href="#f134">132</a></td> - </tr> - - <tr> - <td class="tdrh">135.</td> - <td class="tdt2">A sunken road upon Vesuvius filled with indrifted ash</td> - <td class="tdrl"><a href="#f135">133</a></td> - </tr> - - <tr> - <td class="tdrh">136.</td> - <td class="tdt2">View of Vesuvius from the southwest during the waning stages of -the eruption</td> - <td class="tdrl"><a href="#f136">133</a></td> - </tr> - - <tr> - <td class="tdrh">137.</td> - <td class="tdt2">The main lava stream advancing upon Boscotrecase</td> - <td class="tdrl"><a href="#f137">133</a></td> - </tr> - - <tr> - <td class="tdrh">138.</td> - <td class="tdt2">A pine snapped off by the lava and carried forward upon its surface</td> - <td class="tdrl"><a href="#f138">133</a></td> - </tr> - - <tr> - <td class="tdrh">139.</td> - <td class="tdt2">Lava front pushing over and running around a wall in its path</td> - <td class="tdrl"><a href="#f139">134</a></td> - </tr> - - <tr> - <td class="tdrh">140.</td> - <td class="tdt2">One of the ruined villas in Boscotrecase</td> - <td class="tdrl"><a href="#f140">134</a></td> - </tr> - - <tr> - <td class="tdrh">141.</td> - <td class="tdt2">Three diagrams to illustrate the sequence of events during the cone-building -and crater-producing periods</td> - <td class="tdrl"><a href="#f141">135</a></td> - </tr> - - <tr> - <td class="tdrh">142.</td> - <td class="tdt2">The spine of Pelé rising above the chimney of the volcano after the -eruption of 1902</td> - <td class="tdrl"><a href="#f142">136</a></td> - </tr> - - <tr> - <td class="tdrh">143.</td> - <td class="tdt2">Successive outlines of the Pelé spine</td> - <td class="tdrl"><a href="#f143">137</a></td> - </tr> - - <tr> - <td class="tdrh">144.</td> - <td class="tdt2">Corrugated surface of the Vesuvian cone due to the mud flows which -followed the eruption of 1906</td> - <td class="tdrl"><a href="#f144">138</a></td> - </tr> - - <tr> - <td class="tdrh">145.</td> - <td class="tdt2">View of the Kammerbühl near Eger in Bohemia</td> - <td class="tdrl"><a href="#f145">139</a></td> - </tr> - - <tr> - <td class="tdrh">146.</td> - <td class="tdt2">Volcanic plug exposed by natural dissection of a volcanic cone in -Colorado</td> - <td class="tdrl"><a href="#f146">140</a></td> - </tr> - - <tr> - <td class="tdrh">147.</td> - <td class="tdt2">A dike cutting beds of tuff in a partly dissected volcano of southwestern -Colorado</td> - <td class="tdrl"><a href="#f147">140</a></td> - </tr> - - <tr> - <td class="tdrh">148.</td> - <td class="tdt2">Map and general view of St. Paul’s rocks, a volcanic cone dissected -by waves</td> - <td class="tdrl"><a href="#f148">141</a></td> - </tr> - - <tr> - <td class="tdrh">149.</td> - <td class="tdt2">Dissection by explosion of Little Bandai-san in 1888</td> - <td class="tdrl"><a href="#f149">141</a></td> - </tr> - - <tr> - <td class="tdrh">150.</td> - <td class="tdt2">The half-submerged volcano of Krakatoa before and after the eruption -of 1883</td> - <td class="tdrl"><a href="#f150">142</a></td> - </tr> - - <tr> - <td class="tdrh">151.</td> - <td class="tdt2">The cicatrice of the Banat</td> - <td class="tdrl"><a href="#f151">142</a></td> - </tr> - - <tr> - <td class="tdrh">152.</td> - <td class="tdt2">Diagram to illustrate a probable cause of formation of lava reservoirs -and the connection with volcanoes upon the surface</td> - <td class="tdrl"><a href="#f152">143</a></td> - </tr> - - <tr> - <td class="tdrh">153.</td> - <td class="tdt2">Effect of relief of load upon rocks by arching of a competent formation</td> - <td class="tdrl"><a href="#f153">144</a></td> - </tr> - - <tr> - <td class="tdrh">154.</td> - <td class="tdt2">Character profiles connected with volcanoes</td> - <td class="tdrl"><a href="#f154">146</a></td> - </tr> - - <tr> - <td class="tdrh">155.</td> - <td class="tdt2">Diagrams to show the effect of decomposition in producing spheroidal -bowlders</td> - <td class="tdrl"><a href="#f155">150</a></td> - </tr> - - <tr> - <td class="tdrh">156.</td> - <td class="tdt2">Spheroidal weathering of an igneous rock</td> - <td class="tdrl"><a href="#f156">151</a></td> - </tr> - - <tr> - <td class="tdrh">157.</td> - <td class="tdt2">Dome structure in granite mass</td> - <td class="tdrl"><a href="#f157">152</a></td> - </tr> - - <tr> - <td class="tdrh">158.</td> - <td class="tdt2">Talus slope beneath a cliff</td> - <td class="tdrl"><a href="#f158">153</a></td> - </tr> - - <tr> - <td class="tdrh">159.</td> - <td class="tdt2">Striped ground from soil flow</td> - <td class="tdrl"><a href="#f159">154</a></td> - </tr> - - <tr> - <td class="tdrh">160.</td> - <td class="tdt2">Pavement of horizontal surface due to soil flow</td> - <td class="tdrl"><a href="#f160">154</a></td> - </tr> - - <tr> - <td class="tdrh">161.</td> - <td class="tdt2">Tree roots prying rock apart on fissure</td> - <td class="tdrl"><a href="#f161">154</a></td> - </tr> - - <tr> - <td class="tdrh">162.</td> - <td class="tdt2">Bowlder split by a growing tree</td> - <td class="tdrl"><a href="#f162">155</a></td> - </tr> - - <tr> - <td class="tdrh">163.</td> - <td class="tdt2">Rock mantle beneath soil and vegetable mat</td> - <td class="tdrl"><a href="#f163">155</a></td> - </tr> - - <tr> - <td class="tdrh">164.</td> - <td class="tdt2">Diagram to show the varying thickness of mantle rock upon the -different portions of a hill surface</td> - <td class="tdrl"><a href="#f164">156</a></td> - </tr> - - <tr> - <td class="tdrh">165.</td> - <td class="tdt2">Gullies from earliest stage of a river’s life</td> - <td class="tdrl"><a href="#f165">160</a></td> - </tr> - - <tr> - <td class="tdrh">166.</td> - <td class="tdt2">Partially dissected upland</td> - <td class="tdrl"><a href="#f166">160</a></td> - </tr> - - <tr> - <td class="tdrh">167.</td> - <td class="tdt2">Longitudinal sections of upper portion of a river valley</td> - <td class="tdrl"><a href="#f167">161</a></td> - </tr> - - <tr> - <td class="tdrh">168.<span class="pagenum"><a name="Page_xxvi" id="Page_xxvi">[xxvi]</a></span></td> - <td class="tdt2">Map and sections of a stream meander</td> - <td class="tdrl"><a href="#f168">163</a></td> - </tr> - - <tr> - <td class="tdrh">169.</td> - <td class="tdt2">Tree undermined on the outer bank of a meander</td> - <td class="tdrl"><a href="#f169">164</a></td> - </tr> - - <tr> - <td class="tdrh">170.</td> - <td class="tdt2">Diagrams to show the successive positions of stream meanders</td> - <td class="tdrl"><a href="#f170">164</a></td> - </tr> - - <tr> - <td class="tdrh">171.</td> - <td class="tdt2">An ox-bow lake in the flood plain of a river</td> - <td class="tdrl"><a href="#f171">165</a></td> - </tr> - - <tr> - <td class="tdrh">172.</td> - <td class="tdt2">Schematic representation of a series of river terraces</td> - <td class="tdrl"><a href="#f172">165</a></td> - </tr> - - <tr> - <td class="tdrh">173.</td> - <td class="tdt2">“Bird-foot” delta of the Mississippi River</td> - <td class="tdrl"><a href="#f173">167</a></td> - </tr> - - <tr> - <td class="tdrh">174.</td> - <td class="tdt2">Diagrams to show the nature of delta deposits as exhibited in sections</td> - <td class="tdrl"><a href="#f174">168</a></td> - </tr> - - <tr> - <td class="tdrh">175.</td> - <td class="tdt2">Gorge of the River Rhine near St. Goars</td> - <td class="tdrl"><a href="#f175">169</a></td> - </tr> - - <tr> - <td class="tdrh">176.</td> - <td class="tdt2">Valley with rounded shoulders characteristic of the stage of adolescence</td> - <td class="tdrl"><a href="#f176">170</a></td> - </tr> - - <tr> - <td class="tdrh">177.</td> - <td class="tdt2">View of a maturely dissected upland</td> - <td class="tdrl"><a href="#f177">170</a></td> - </tr> - - <tr> - <td class="tdrh">178.</td> - <td class="tdt2">Hogarth’s line of beauty</td> - <td class="tdrl"><a href="#f178">171</a></td> - </tr> - - <tr> - <td class="tdrh">179.</td> - <td class="tdt2">View of the oldland of New England, with Mount Monadnock rising -in the distance</td> - <td class="tdrl"><a href="#f179">171</a></td> - </tr> - - <tr> - <td class="tdrh">180.</td> - <td class="tdt2">Comparison of the cross sections of river valleys of different stages</td> - <td class="tdrl"><a href="#f180">172</a></td> - </tr> - - <tr> - <td class="tdrh">181.</td> - <td class="tdt2">The Beavertail Bend of the Yakima River</td> - <td class="tdrl"><a href="#f181">173</a></td> - </tr> - - <tr> - <td class="tdrh">182.</td> - <td class="tdt2">A rejuvenated river valley</td> - <td class="tdrl"><a href="#f182">174</a></td> - </tr> - - <tr> - <td class="tdrh">183.</td> - <td class="tdt2">Plan of a river narrows</td> - <td class="tdrl"><a href="#f183">174</a></td> - </tr> - - <tr> - <td class="tdrh">184.</td> - <td class="tdt2">Successive diagrams to illustrate the origin of “trellis drainage”</td> - <td class="tdrl"><a href="#f184">175</a></td> - </tr> - - <tr> - <td class="tdrh">185.</td> - <td class="tdt2">Sketch maps to show the earlier and present drainage near Harper’s -Ferry</td> - <td class="tdrl"><a href="#f185">176</a></td> - </tr> - - <tr> - <td class="tdrh">186.</td> - <td class="tdt2">Section to illustrate the history of Snickers Gap</td> - <td class="tdrl"><a href="#f186">177</a></td> - </tr> - - <tr> - <td class="tdrh">187.</td> - <td class="tdt2">Character profiles of landscapes shaped by stream erosion in humid -climates</td> - <td class="tdrl"><a href="#f187">177</a></td> - </tr> - - <tr> - <td class="tdrh">188.</td> - <td class="tdt2">Diagram to show the seasonal range in the position of the water table</td> - <td class="tdrl"><a href="#f188">180</a></td> - </tr> - - <tr> - <td class="tdrh">189.</td> - <td class="tdt2">Diagram to show the effect of an impervious layer upon the descending -water</td> - <td class="tdrl"><a href="#f189">181</a></td> - </tr> - - <tr> - <td class="tdrh">190.</td> - <td class="tdt2">Sketch map to illustrate corrosion of limestone along two series of -vertical joints</td> - <td class="tdrl"><a href="#f190">181</a></td> - </tr> - - <tr> - <td class="tdrh">191.</td> - <td class="tdt2">Diagram to show the relation of limestone caverns to the river system -of the district</td> - <td class="tdrl"><a href="#f191">182</a></td> - </tr> - - <tr> - <td class="tdrh">192.</td> - <td class="tdt2">Plan of a portion of Mammoth Cave, Kentucky</td> - <td class="tdrl"><a href="#f192">183</a></td> - </tr> - - <tr> - <td class="tdrh">193.</td> - <td class="tdt2">Trees and shrubs growing upon the bottoms of limestone sinks</td> - <td class="tdrl"><a href="#f193">183</a></td> - </tr> - - <tr> - <td class="tdrh">194.</td> - <td class="tdt2">Diagrams to show the manner of formation of stalactites and stalagmites</td> - <td class="tdrl"><a href="#f194">185</a></td> - </tr> - - <tr> - <td class="tdrh">195.</td> - <td class="tdt2">Sinter formations in the Luray caverns</td> - <td class="tdrl"><a href="#f195">186</a></td> - </tr> - - <tr> - <td class="tdrh">196.</td> - <td class="tdt2">Map of the dolines of the Karst region</td> - <td class="tdrl"><a href="#f196">187</a></td> - </tr> - - <tr> - <td class="tdrh">197.</td> - <td class="tdt2">Cross section of a doline formed by inbreak</td> - <td class="tdrl"><a href="#f197">187</a></td> - </tr> - - <tr> - <td class="tdrh">198.</td> - <td class="tdt2">Sharp Karren of the Ifenplatte</td> - <td class="tdrl"><a href="#f198">188</a></td> - </tr> - - <tr> - <td class="tdrh">199.</td> - <td class="tdt2">The Zirknitz seasonal lake</td> - <td class="tdrl"><a href="#f199">189</a></td> - </tr> - - <tr> - <td class="tdrh">200.</td> - <td class="tdt2">Fissure springs arranged at intersections of rock fractures</td> - <td class="tdrl"><a href="#f200">190</a></td> - </tr> - - <tr> - <td class="tdrh">201.</td> - <td class="tdt2">Schematic diagrams to illustrate the different types of artesian wells</td> - <td class="tdrl"><a href="#f201">191</a></td> - </tr> - - <tr> - <td class="tdrh">202.</td> - <td class="tdt2">Cross section of Geysir, Iceland</td> - <td class="tdrl"><a href="#f202">192</a></td> - </tr> - - <tr> - <td class="tdrh">203.</td> - <td class="tdt2">Apparatus for simulating geyser action</td> - <td class="tdrl"><a href="#f203">193</a></td> - </tr> - - <tr> - <td class="tdrh">204.</td> - <td class="tdt2">Cone of siliceous sinter about the Lone Star Geyser</td> - <td class="tdrl"><a href="#f204">194</a></td> - </tr> - - <tr> - <td class="tdrh">205.<span class="pagenum"><a name="Page_xxvii" id="Page_xxvii">[xxvii]</a></span></td> - <td class="tdt2">Former shore lines in the Great Basin</td> - <td class="tdrl"><a href="#f205">198</a></td> - </tr> - - <tr> - <td class="tdrh">206.</td> - <td class="tdt2">Map of the former Lake Bonneville</td> - <td class="tdrl"><a href="#f206">199</a></td> - </tr> - - <tr> - <td class="tdrh">207.</td> - <td class="tdt2">Borax deposits in Death Valley, California</td> - <td class="tdrl"><a href="#f207">201</a></td> - </tr> - - <tr> - <td class="tdrh">208.</td> - <td class="tdt2">Hollowed forms of weathered granite in a desert of Central Asia</td> - <td class="tdrl"><a href="#f208">201</a></td> - </tr> - - <tr> - <td class="tdrh">209.</td> - <td class="tdt2">Hollow hewn blocks in a wall in the Wadi Guerraui</td> - <td class="tdrl"><a href="#f209">202</a></td> - </tr> - - <tr> - <td class="tdrh">210.</td> - <td class="tdt2">Smooth granite domes shaped by exfoliation</td> - <td class="tdrl"><a href="#f210">203</a></td> - </tr> - - <tr> - <td class="tdrh">211.</td> - <td class="tdt2">Granite blocks rent by diffission</td> - <td class="tdrl"><a href="#f211">204</a></td> - </tr> - - <tr> - <td class="tdrh">212.</td> - <td class="tdt2">“Mushroom Rock” from a desert in Wyoming</td> - <td class="tdrl"><a href="#f212">205</a></td> - </tr> - - <tr> - <td class="tdrh">213.</td> - <td class="tdt2">Windkanten shaped by sand blast in the desert</td> - <td class="tdrl"><a href="#f213">205</a></td> - </tr> - - <tr> - <td class="tdrh">214.</td> - <td class="tdt2">The “stone lattice” of the desert</td> - <td class="tdrl"><a href="#f214">206</a></td> - </tr> - - <tr> - <td class="tdrh">215.</td> - <td class="tdt2">Shadow erosion in the desert</td> - <td class="tdrl"><a href="#f215">206</a></td> - </tr> - - <tr> - <td class="tdrh">216.</td> - <td class="tdt2">Cliffs in loess with characteristic vertical jointing</td> - <td class="tdrl"><a href="#f216">207</a></td> - </tr> - - <tr> - <td class="tdrh">217.</td> - <td class="tdt2">A cañon in loess worn by traffic and wind</td> - <td class="tdrl"><a href="#f217">207</a></td> - </tr> - - <tr> - <td class="tdrh">218.</td> - <td class="tdt2">Diagrams to illustrate the effects of obstructions in arresting wind-driven -sand</td> - <td class="tdrl"><a href="#f218">209</a></td> - </tr> - - <tr> - <td class="tdrh">219.</td> - <td class="tdt2">Sand accumulating on either side of a firm and impenetrable obstruction</td> - <td class="tdrl"><a href="#f219">210</a></td> - </tr> - - <tr> - <td class="tdrh">220.</td> - <td class="tdt2">Successive diagrams to illustrate the history of the town of Kunzen -upon the Kurische Nehrung</td> - <td class="tdrl"><a href="#f220">210</a></td> - </tr> - - <tr> - <td class="tdrh">221.</td> - <td class="tdt2">View of desert barchans</td> - <td class="tdrl"><a href="#f221">211</a></td> - </tr> - - <tr> - <td class="tdrh">222.</td> - <td class="tdt2">Diagrams to show the relationships of dunes to sand supply and wind -direction</td> - <td class="tdrl"><a href="#f222">211</a></td> - </tr> - - <tr> - <td class="tdrh">223.</td> - <td class="tdt2">Ideal section showing the rising mountain wall about a desert and -the neighboring slope</td> - <td class="tdrl"><a href="#f223">212</a></td> - </tr> - - <tr> - <td class="tdrh">224.</td> - <td class="tdt2">Dry delta at the foot of a range upon the borders of a desert</td> - <td class="tdrl"><a href="#f224">213</a></td> - </tr> - - <tr> - <td class="tdrh">225.</td> - <td class="tdt2">Map of distributaries of streams which issue at the western base of -the Sierra Nevadas</td> - <td class="tdrl"><a href="#f225">213</a></td> - </tr> - - <tr> - <td class="tdrh">226.</td> - <td class="tdt2">A group of “demoiselles” in the “bad lands”</td> - <td class="tdrl"><a href="#f226">214</a></td> - </tr> - - <tr> - <td class="tdrh">227.</td> - <td class="tdt2">Amphitheater at the head of the Wadi Beni Sur</td> - <td class="tdrl"><a href="#f227">215</a></td> - </tr> - - <tr> - <td class="tdrh">228.</td> - <td class="tdt2">Mesa and outlier in the Leucite Hills of Wyoming</td> - <td class="tdrl"><a href="#f228">216</a></td> - </tr> - - <tr> - <td class="tdrh">229.</td> - <td class="tdt2">Flat-bottomed basin separating dunes</td> - <td class="tdrl"><a href="#f229">216</a></td> - </tr> - - <tr> - <td class="tdrh">230.</td> - <td class="tdt2">Billowy surface of the salt crust on the central sink of the desert of -Lop</td> - <td class="tdrl"><a href="#f230">217</a></td> - </tr> - - <tr> - <td class="tdrh">231.</td> - <td class="tdt2">Schematic diagram to show the zones of deposition in their order -from the margin to the center of a desert</td> - <td class="tdrl"><a href="#f231">217</a></td> - </tr> - - <tr> - <td class="tdrh">232.</td> - <td class="tdt2">Mounds upon the site of the buried city of Nippur</td> - <td class="tdrl"><a href="#f232">218</a></td> - </tr> - - <tr> - <td class="tdrh">233.</td> - <td class="tdt2">Exhumed structures in the buried city of Nippur</td> - <td class="tdrl"><a href="#f233">218</a></td> - </tr> - - <tr> - <td class="tdrh">234.</td> - <td class="tdt2">Section across the High Plains</td> - <td class="tdrl"><a href="#f234">219</a></td> - </tr> - - <tr> - <td class="tdrh">235.</td> - <td class="tdt2">Section across the lenticular threads of alluvial deposits of the High -Plains</td> - <td class="tdrl"><a href="#f235">220</a></td> - </tr> - - <tr> - <td class="tdrh">236.</td> - <td class="tdt2">Distributaries of the foot hills superimposed upon an earlier series</td> - <td class="tdrl"><a href="#f236">220</a></td> - </tr> - - <tr> - <td class="tdrh">237.</td> - <td class="tdt2">Character profiles in the landscapes of arid lands</td> - <td class="tdrl"><a href="#f237">220</a></td> - </tr> - - <tr> - <td class="tdrh">238.</td> - <td class="tdt2">Rain sculpturing under control by joints</td> - <td class="tdrl"><a href="#f238">224</a></td> - </tr> - - <tr> - <td class="tdrh">239.</td> - <td class="tdt2">Sagging of limestone above joints</td> - <td class="tdrl"><a href="#f239">224</a></td> - </tr> - - <tr> - <td class="tdrh">240.</td> - <td class="tdt2">Map of the joint-controlled Abisko Cañon in Northern Lapland</td> - <td class="tdrl"><a href="#f240">225</a></td> - </tr> - - <tr> - <td class="tdrh">241.</td> - <td class="tdt2">Map of the gorge of the Zambesi River below Victoria Falls</td> - <td class="tdrl"><a href="#f241">225</a></td> - </tr> - - <tr> - <td class="tdrh">242.<span class="pagenum"><a name="Page_xxviii" id="Page_xxviii">[xxviii]</a></span></td> - <td class="tdt2">Controlled drainage network of the Shepaug River in Connecticut</td> - <td class="tdrl"><a href="#f242">226</a></td> - </tr> - - <tr> - <td class="tdrh">243.</td> - <td class="tdt2">A river network of repeating rectangular pattern</td> - <td class="tdrl"><a href="#f243">226</a></td> - </tr> - - <tr> - <td class="tdrh">244.</td> - <td class="tdt2">Squared mountain masses which reveal a distribution of joints in -block patterns of different orders</td> - <td class="tdrl"><a href="#f244">228</a></td> - </tr> - - <tr> - <td class="tdrh">245.</td> - <td class="tdt2">Island groups of the Lofoten Archipelago</td> - <td class="tdrl"><a href="#f245">229</a></td> - </tr> - - <tr> - <td class="tdrh">246.</td> - <td class="tdt2">Diagrams to illustrate the composite profiles of the islands on the -Norwegian coast</td> - <td class="tdrl"><a href="#f246">229</a></td> - </tr> - - <tr> - <td class="tdrh">247.</td> - <td class="tdt2">Diagram to show the nature of the motions within a free water wave</td> - <td class="tdrl"><a href="#f247">231</a></td> - </tr> - - <tr> - <td class="tdrh">248.</td> - <td class="tdt2">Diagram to illustrate the transformation of a free wave into a breaker</td> - <td class="tdrl"><a href="#f248">232</a></td> - </tr> - - <tr> - <td class="tdrh">249.</td> - <td class="tdt2">Notched rock cliff and fallen blocks</td> - <td class="tdrl"><a href="#f249">233</a></td> - </tr> - - <tr> - <td class="tdrh">250.</td> - <td class="tdt2">A wave-cut chasm under control by joints</td> - <td class="tdrl"><a href="#f250">233</a></td> - </tr> - - <tr> - <td class="tdrh">251.</td> - <td class="tdt2">Grand Arch upon one of the Apostle Islands in Lake Superior</td> - <td class="tdrl"><a href="#f251">234</a></td> - </tr> - - <tr> - <td class="tdrh">252.</td> - <td class="tdt2">Stack near the shore of Lake Superior</td> - <td class="tdrl"><a href="#f252">234</a></td> - </tr> - - <tr> - <td class="tdrh">253.</td> - <td class="tdt2">The Marble Islands, stacks in a lake of the southern Andes</td> - <td class="tdrl"><a href="#f253">235</a></td> - </tr> - - <tr> - <td class="tdrh">254.</td> - <td class="tdt2">Squared stacks revealing the position of the joint planes on which -they were carved</td> - <td class="tdrl"><a href="#f254">235</a></td> - </tr> - - <tr> - <td class="tdrh">255.</td> - <td class="tdt2">Ideal section cut by waves upon a steep rocky shore</td> - <td class="tdrl"><a href="#f255">236</a></td> - </tr> - - <tr> - <td class="tdrh">256.</td> - <td class="tdt2">Map showing the outlines of the island of Heligoland at different -stages in its history</td> - <td class="tdrl"><a href="#f256">236</a></td> - </tr> - - <tr> - <td class="tdrh">257.</td> - <td class="tdt2">Ideal section carved by waves upon a steep shore of loose materials</td> - <td class="tdrl"><a href="#f257">237</a></td> - </tr> - - <tr> - <td class="tdrh">258.</td> - <td class="tdt2">Sloping cliff and boulder pavement at Scituate, Massachusetts</td> - <td class="tdrl"><a href="#f258">237</a></td> - </tr> - - <tr> - <td class="tdrh">259.</td> - <td class="tdt2">Map to show the nature of the shore current and the forms which are -molded by it</td> - <td class="tdrl"><a href="#f259">238</a></td> - </tr> - - <tr> - <td class="tdrh">260.</td> - <td class="tdt2">Crescent-shaped beach in the lee of a headland</td> - <td class="tdrl"><a href="#f260">239</a></td> - </tr> - - <tr> - <td class="tdrh">261.</td> - <td class="tdt2">Cross section of a beach pebble</td> - <td class="tdrl"><a href="#f261">239</a></td> - </tr> - - <tr> - <td class="tdrh">262.</td> - <td class="tdt2">A storm beach on the northeast shore of Green Bay</td> - <td class="tdrl"><a href="#f262">240</a></td> - </tr> - - <tr> - <td class="tdrh">263.</td> - <td class="tdt2">Spit of shingle on Au Train Island, Lake Superior</td> - <td class="tdrl"><a href="#f263">240</a></td> - </tr> - - <tr> - <td class="tdrh">264.</td> - <td class="tdt2">Barrier beach in front of a lagoon</td> - <td class="tdrl"><a href="#f264">241</a></td> - </tr> - - <tr> - <td class="tdrh">265.</td> - <td class="tdt2">Cross section of a barrier beach with lagoon in its rear</td> - <td class="tdrl"><a href="#f265">242</a></td> - </tr> - - <tr> - <td class="tdrh">266.</td> - <td class="tdt2">Cross section of a series of barriers and an outer bar</td> - <td class="tdrl"><a href="#f266">242</a></td> - </tr> - - <tr> - <td class="tdrh">267.</td> - <td class="tdt2">A barrier series and an outer bar on Lake Mendota at Madison, -Wisconsin</td> - <td class="tdrl"><a href="#f267">242</a></td> - </tr> - - <tr> - <td class="tdrh">268.</td> - <td class="tdt2">Series of barriers at the western end of Lake Superior</td> - <td class="tdrl"><a href="#f268">243</a></td> - </tr> - - <tr> - <td class="tdrh">269.</td> - <td class="tdt2">Character profiles resulting from wave action upon shores</td> - <td class="tdrl"><a href="#f269">243</a></td> - </tr> - - <tr> - <td class="tdrh">270.</td> - <td class="tdt2">The even shore line of a raised coast</td> - <td class="tdrl"><a href="#f270">246</a></td> - </tr> - - <tr> - <td class="tdrh">271.</td> - <td class="tdt2">The ragged coast line produced by subsidence</td> - <td class="tdrl"><a href="#f271">246</a></td> - </tr> - - <tr> - <td class="tdrh">272.</td> - <td class="tdt2">Portion of the Atlantic coastal plain at the base of the oldland</td> - <td class="tdrl"><a href="#f272">246</a></td> - </tr> - - <tr> - <td class="tdrh">273.</td> - <td class="tdt2">Ideal form of cuestas and intermediate lowlands carved from a coastal -plain</td> - <td class="tdrl"><a href="#f273">247</a></td> - </tr> - - <tr> - <td class="tdrh">274.</td> - <td class="tdt2">Uplifted sea cave on the coast of California</td> - <td class="tdrl"><a href="#f274">248</a></td> - </tr> - - <tr> - <td class="tdrh">275.</td> - <td class="tdt2">Double-notched cliff near Cape Tiro, Celebes</td> - <td class="tdrl"><a href="#f275">248</a></td> - </tr> - - <tr> - <td class="tdrh">276.</td> - <td class="tdt2">Uplifted stacks on the coast of California</td> - <td class="tdrl"><a href="#f276">249</a></td> - </tr> - - <tr> - <td class="tdrh">277.</td> - <td class="tdt2">Uplifted shingle beach across the entrance to a former bay upon the -coast of California</td> - <td class="tdrl"><a href="#f277">250</a></td> - </tr> - - <tr> - <td class="tdrh">278.</td> - <td class="tdt2">Raised beach terraces near Elie, Fife, Scotland</td> - <td class="tdrl"><a href="#f278">250</a></td> - </tr> - - <tr> - <td class="tdrh">279.</td> - <td class="tdt2">Uplifted sea cliffs and terraces on the Alaskan coast</td> - <td class="tdrl"><a href="#f279">250</a></td> - </tr> - - <tr> - <td class="tdrh">280.<span class="pagenum"><a name="Page_xxix" id="Page_xxix">[xxix]</a></span></td> - <td class="tdt2">Diagrams to show how excessive sinking upon the sea floor will cause -the shore to migrate landward</td> - <td class="tdrl"><a href="#f280">251</a></td> - </tr> - - <tr> - <td class="tdrh">281.</td> - <td class="tdt2">A drowned river mouth or estuary upon a coastal plain</td> - <td class="tdrl"><a href="#f281">251</a></td> - </tr> - - <tr> - <td class="tdrh">282.</td> - <td class="tdt2">Archipelago of steep rocky islets due to submergence</td> - <td class="tdrl"><a href="#f282">252</a></td> - </tr> - - <tr> - <td class="tdrh">283.</td> - <td class="tdt2">The submerged Hudsonian channel which continues the Hudson -River across the continental shelf</td> - <td class="tdrl"><a href="#f283">252</a></td> - </tr> - - <tr> - <td class="tdrh">284.</td> - <td class="tdt2">Marine clay deposits near the mouths of the Maine rivers which preserve -a record of earlier subsidence and later elevation</td> - <td class="tdrl"><a href="#f284">253</a></td> - </tr> - - <tr> - <td class="tdrh">285.</td> - <td class="tdt2">View of the three standing columns of the Temple of Jupiter Serapis, -at Pozzuoli</td> - <td class="tdrl"><a href="#f285">254</a></td> - </tr> - - <tr> - <td class="tdrh">286.</td> - <td class="tdt2">Three successive views to set forth the recent oscillations of level on -the northern shore of the Bay of Naples</td> - <td class="tdrl"><a href="#f286">255</a></td> - </tr> - - <tr> - <td class="tdrh">287.</td> - <td class="tdt2">Relief map of San Clemente Island, California</td> - <td class="tdrl"><a href="#f287">256</a></td> - </tr> - - <tr> - <td class="tdrh">288.</td> - <td class="tdt2">Relief map of Santa Catalina Island, California</td> - <td class="tdrl"><a href="#f288">257</a></td> - </tr> - - <tr> - <td class="tdrh">289.</td> - <td class="tdt2">Cross section of the Blue Grotto, on the island of Capri</td> - <td class="tdrl"><a href="#f289">258</a></td> - </tr> - - <tr> - <td class="tdrh">290.</td> - <td class="tdt2">Character profiles of coast elevation and subsidence</td> - <td class="tdrl"><a href="#f290">259</a></td> - </tr> - - <tr> - <td class="tdrh">291.</td> - <td class="tdt2">Map showing the distribution of existing glaciers and the two important -wind poles of the earth</td> - <td class="tdrl"><a href="#f291">263</a></td> - </tr> - - <tr> - <td class="tdrh">292.</td> - <td class="tdt2">An Alaskan glacier spreading out at the foot of the range which -nourishes it</td> - <td class="tdrl"><a href="#f292">264</a></td> - </tr> - - <tr> - <td class="tdrh">293.</td> - <td class="tdt2">Surface of a glacier whose upper layers spread with but slight restraint -from retaining walls</td> - <td class="tdrl"><a href="#f293">265</a></td> - </tr> - - <tr> - <td class="tdrh">294.</td> - <td class="tdt2">Section through a mountain glacier</td> - <td class="tdrl"><a href="#f294">267</a></td> - </tr> - - <tr> - <td class="tdrh">295.</td> - <td class="tdt2">Profile across the largest of the Icelandic ice caps</td> - <td class="tdrl"><a href="#f295">267</a></td> - </tr> - - <tr> - <td class="tdrh">296.</td> - <td class="tdt2">Ideal section across a continental glacier</td> - <td class="tdrl"><a href="#f296">267</a></td> - </tr> - - <tr> - <td class="tdrh">297.</td> - <td class="tdt2">View of the Eyriks Jökull, an ice cap of Iceland</td> - <td class="tdrl"><a href="#f297">268</a></td> - </tr> - - <tr> - <td class="tdrh">298.</td> - <td class="tdt2">The zones of the lower atmosphere as revealed by recent kite and -balloon exploration</td> - <td class="tdrl"><a href="#f298">269</a></td> - </tr> - - <tr> - <td class="tdrh">299.</td> - <td class="tdt2">Map of Greenland, showing the area of inland ice and the routes of -explorers</td> - <td class="tdrl"><a href="#f299">271</a></td> - </tr> - - <tr> - <td class="tdrh">300.</td> - <td class="tdt2">Profile in natural proportions across the southern end of the continental -glacier of Greenland</td> - <td class="tdrl"><a href="#f300">272</a></td> - </tr> - - <tr> - <td class="tdrh">301.</td> - <td class="tdt2">Map of a glacier tongue with dimple above</td> - <td class="tdrl"><a href="#f301">273</a></td> - </tr> - - <tr> - <td class="tdrh">302.</td> - <td class="tdt2">Edge of the Greenland inland ice, showing the nunataks diminishing -in size toward the interior</td> - <td class="tdrl"><a href="#f302">274</a></td> - </tr> - - <tr> - <td class="tdrh">303.</td> - <td class="tdt2">Moat surrounding a nunatak in Victoria Land</td> - <td class="tdrl"><a href="#f303">274</a></td> - </tr> - - <tr> - <td class="tdrh">304.</td> - <td class="tdt2">A glacier pavement of Permo-Carboniferous age in South Africa</td> - <td class="tdrl"><a href="#f304">276</a></td> - </tr> - - <tr> - <td class="tdrh">305.</td> - <td class="tdt2">Diagrams to illustrate the manner of formation of scape colks</td> - <td class="tdrl"><a href="#f305">277</a></td> - </tr> - - <tr> - <td class="tdrh">306.</td> - <td class="tdt2">Marginal moraine now forming at the edge of the continental glacier -of Greenland</td> - <td class="tdrl"><a href="#f306">279</a></td> - </tr> - - <tr> - <td class="tdrh">307.</td> - <td class="tdt2">Small lake between the ice front and a moraine which it has recently -built</td> - <td class="tdrl"><a href="#f307">279</a></td> - </tr> - - <tr> - <td class="tdrh">308.</td> - <td class="tdt2">View of a drained lake bottom between the ice front and an abandoned -moraine</td> - <td class="tdrl"><a href="#f308">280</a></td> - </tr> - - <tr> - <td class="tdrh">309.</td> - <td class="tdt2">Diagrams to show the manner of formation and the structure of an -outwash plain and fosse</td> - <td class="tdrl"><a href="#f309">280</a></td> - </tr> - - <tr> - <td class="tdrh">310.<span class="pagenum"><a name="Page_xxx" id="Page_xxx">[xxx]</a></span></td> - <td class="tdt2">Map of the ice masses of Victoria Land, Antarctica</td> - <td class="tdrl"><a href="#f310">282</a></td> - </tr> - - <tr> - <td class="tdrh">311.</td> - <td class="tdt2">Sections across the inland ice and the shelf ice of Antarctica</td> - <td class="tdrl"><a href="#f311">283</a></td> - </tr> - - <tr> - <td class="tdrh">312.</td> - <td class="tdt2">Diagram to show the nature of the fixed glacial anticyclone above -continental glaciers</td> - <td class="tdrl"><a href="#f312">284</a></td> - </tr> - - <tr> - <td class="tdrh">313.</td> - <td class="tdt2">Snow deltas about the margins of a glacier tongue in Greenland</td> - <td class="tdrl"><a href="#f313">285</a></td> - </tr> - - <tr> - <td class="tdrh">314.</td> - <td class="tdt2">View of the sea ice of the Arctic region</td> - <td class="tdrl"><a href="#f315">286</a></td> - </tr> - - <tr> - <td class="tdrh">315.</td> - <td class="tdt2">Map of the north polar regions, showing the area of drift ice and the -tracks of the <i>Jeannette</i> and the <i>Fram</i></td> - <td class="tdrl"><a href="#f315">288</a></td> - </tr> - - <tr> - <td class="tdrh">316.</td> - <td class="tdt2">The shelf ice of Coats Land with surrounding pack ice</td> - <td class="tdrl"><a href="#f316">290</a></td> - </tr> - - <tr> - <td class="tdrh">317.</td> - <td class="tdt2">Tidewater cliff on a glacier tongue from which icebergs are born</td> - <td class="tdrl"><a href="#f317">290</a></td> - </tr> - - <tr> - <td class="tdrh">318.</td> - <td class="tdt2">A Greenlandic iceberg after a long journey in warm latitudes</td> - <td class="tdrl"><a href="#f318">291</a></td> - </tr> - - <tr> - <td class="tdrh">319.</td> - <td class="tdt2">Diagram showing one way in which northern icebergs are born from -the glacier tongue</td> - <td class="tdrl"><a href="#f319">291</a></td> - </tr> - - <tr> - <td class="tdrh">320.</td> - <td class="tdt2">A northern iceberg surrounded by sea ice</td> - <td class="tdrl"><a href="#f320">292</a></td> - </tr> - - <tr> - <td class="tdrh">321.</td> - <td class="tdt2">Tabular Antarctic iceberg separating from the shelf ice</td> - <td class="tdrl"><a href="#f321">293</a></td> - </tr> - - <tr> - <td class="tdrh">322.</td> - <td class="tdt2">Map of the globe, showing the areas covered by continental glaciers -during the “ice age”</td> - <td class="tdrl"><a href="#f322">297</a></td> - </tr> - - <tr> - <td class="tdrh">323.</td> - <td class="tdt2">Glaciated granite bowlder weathered out of a moraine of Permo-Carboniferous -age, South Australia</td> - <td class="tdrl"><a href="#f323">298</a></td> - </tr> - - <tr> - <td class="tdrh">324.</td> - <td class="tdt2">Map to show the glaciated and nonglaciated regions of North -America</td> - <td class="tdrl"><a href="#f324">298</a></td> - </tr> - - <tr> - <td class="tdrh">325.</td> - <td class="tdt2">Map of the glaciated and nonglaciated areas of northern Europe</td> - <td class="tdrl"><a href="#f325">299</a></td> - </tr> - - <tr> - <td class="tdrh">326.</td> - <td class="tdt2">An unstable erosion remnant characteristic of the “driftless area”</td> - <td class="tdrl"><a href="#f326">300</a></td> - </tr> - - <tr> - <td class="tdrh">327.</td> - <td class="tdt2">Diagram showing the manner in which a continental glacier obliterates -existing valleys</td> - <td class="tdrl"><a href="#f327">301</a></td> - </tr> - - <tr> - <td class="tdrh">328.</td> - <td class="tdt2">Lake and marsh district in northern Wisconsin</td> - <td class="tdrl"><a href="#f328">302</a></td> - </tr> - - <tr> - <td class="tdrh">329.</td> - <td class="tdt2">Cross section in natural proportion of the latest North American -continental glacier</td> - <td class="tdrl"><a href="#f329">303</a></td> - </tr> - - <tr> - <td class="tdrh">330.</td> - <td class="tdt2">Diagram showing the earlier and the later glacier records together -upon the same limestone surface</td> - <td class="tdrl"><a href="#f330">304</a></td> - </tr> - - <tr> - <td class="tdrh">331.</td> - <td class="tdt2">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</td> - <td class="tdrl"><a href="#f331">305</a></td> - </tr> - - <tr> - <td class="tdrh">332.</td> - <td class="tdt2">Map of the “bowlder train” from Iron Hill, Rhode Island</td> - <td class="tdrl"><a href="#f332">306</a></td> - </tr> - - <tr> - <td class="tdrh">333.</td> - <td class="tdt2">Shapes and approximate natural sizes of some of the diamonds from -the Great Lakes region</td> - <td class="tdrl"><a href="#f333">307</a></td> - </tr> - - <tr> - <td class="tdrh">334.</td> - <td class="tdt2">Glacial map of a portion of the Great Lakes region</td> - <td class="tdrl"><a href="#f334">308</a></td> - </tr> - - <tr> - <td class="tdrh">335.</td> - <td class="tdt2">Section in coarse till</td> - <td class="tdrl"><a href="#f335">310</a></td> - </tr> - - <tr> - <td class="tdrh">336.</td> - <td class="tdt2">Sketch map of portions of Michigan, Ohio, and Indiana, showing the -distribution of moraines</td> - <td class="tdrl"><a href="#f336">312</a></td> - </tr> - - <tr> - <td class="tdrh">337.</td> - <td class="tdt2">Map of the vicinity of Devil’s Lake, Wisconsin, partly covered by -the continental glacier</td> - <td class="tdrl"><a href="#f337">313</a></td> - </tr> - - <tr> - <td class="tdrh">338.</td> - <td class="tdt2">Moraine with outwash apron in front</td> - <td class="tdrl"><a href="#f338">313</a></td> - </tr> - - <tr> - <td class="tdrh">339.</td> - <td class="tdt2">Fosse between an outwash plain and a moraine</td> - <td class="tdrl"><a href="#f339">314</a></td> - </tr> - - <tr> - <td class="tdrh">340.</td> - <td class="tdt2">View along an esker in southern Maine</td> - <td class="tdrl"><a href="#f340">315</a></td> - </tr> - - <tr> - <td class="tdrh">341.</td> - <td class="tdt2">Outline map of moraines and eskers in Finland</td> - <td class="tdrl"><a href="#f341">315</a></td> - </tr> - - <tr> - <td class="tdrh">342.<span class="pagenum"><a name="Page_xxxi" id="Page_xxxi">[xxxi]</a></span></td> - <td class="tdt2">Sketch maps showing the relationships of drumlins and eskers</td> - <td class="tdrl"><a href="#f342">316</a></td> - </tr> - - <tr> - <td class="tdrh">343.</td> - <td class="tdt2">View of a drumlin, showing an opening in the till</td> - <td class="tdrl"><a href="#f343">317</a></td> - </tr> - - <tr> - <td class="tdrh">344.</td> - <td class="tdt2">Outline map of the front of the Green Bay lobe to show the relationships -of drumlins, moraines, outwash plains, and ground moraine</td> - <td class="tdrl"><a href="#f344">317</a></td> - </tr> - - <tr> - <td class="tdrh">345.</td> - <td class="tdt2">Character profiles referable to continental glacier</td> - <td class="tdrl"><a href="#f345">318</a></td> - </tr> - - <tr> - <td class="tdrh">346.</td> - <td class="tdt2">View of the flood plain of the ancient Illinois River near Peoria</td> - <td class="tdrl"><a href="#f346">320</a></td> - </tr> - - <tr> - <td class="tdrh">347.</td> - <td class="tdt2">Broadly terraced valleys which mark the floods that once issued from -the continental glacier of North America</td> - <td class="tdrl"><a href="#f347">321</a></td> - </tr> - - <tr> - <td class="tdrh">348.</td> - <td class="tdt2">Border drainage about the retreating ice front south of Lake Erie</td> - <td class="tdrl"><a href="#f348">321</a></td> - </tr> - - <tr> - <td class="tdrh">349.</td> - <td class="tdt2">The “parallel roads” of Glen Roy in the Scottish Highlands</td> - <td class="tdrl"><a href="#f349">322</a></td> - </tr> - - <tr> - <td class="tdrh">350.</td> - <td class="tdt2">Map of Glen Roy and neighboring valleys of the Scottish Highlands</td> - <td class="tdrl"><a href="#f350">322</a></td> - </tr> - - <tr> - <td class="tdrh">351.</td> - <td class="tdt2">Three successive diagrams to set forth the late glacial lake history of -the Scottish glens</td> - <td class="tdrl"><a href="#f351">324</a></td> - </tr> - - <tr> - <td class="tdrh">352.</td> - <td class="tdt2">Harvesting time on the fertile floor of the glacial Lake Agassiz</td> - <td class="tdrl"><a href="#f352">325</a></td> - </tr> - - <tr> - <td class="tdrh">353.</td> - <td class="tdt2">Map of Lake Agassiz</td> - <td class="tdrl"><a href="#f353">325</a></td> - </tr> - - <tr> - <td class="tdrh">354.</td> - <td class="tdt2">Map showing some of the beaches of Lake Agassiz and its outlet</td> - <td class="tdrl"><a href="#f354">326</a></td> - </tr> - - <tr> - <td class="tdrh">355.</td> - <td class="tdt2">Narrows of the Warren River where it passed between jaws of granite -and gneiss</td> - <td class="tdrl"><a href="#f355">327</a></td> - </tr> - - <tr> - <td class="tdrh">356.</td> - <td class="tdt2">Map of the valley of the Warren River near Minneapolis</td> - <td class="tdrl"><a href="#f356">327</a></td> - </tr> - - <tr> - <td class="tdrh">357.</td> - <td class="tdt2">Portion of the Herman beach on the shore of the former Lake Agassiz</td> - <td class="tdrl"><a href="#f357">328</a></td> - </tr> - - <tr> - <td class="tdrh">358.</td> - <td class="tdt2">Map of the continental glacier of North America when it covered the -entire St. Lawrence basin</td> - <td class="tdrl"><a href="#f358">329</a></td> - </tr> - - <tr> - <td class="tdrh">359.</td> - <td class="tdt2">Outline map of the early Lake Maumee</td> - <td class="tdrl"><a href="#f359">330</a></td> - </tr> - - <tr> - <td class="tdrh">360.</td> - <td class="tdt2">Map to show the first stages of the ice-dammed lakes within the -St. Lawrence basin</td> - <td class="tdrl"><a href="#f360">330</a></td> - </tr> - - <tr> - <td class="tdrh">361.</td> - <td class="tdt2">Outline map of the later Lake Maumee and its outlet</td> - <td class="tdrl"><a href="#f361">332</a></td> - </tr> - - <tr> - <td class="tdrh">362.</td> - <td class="tdt2">Outline map of lakes Whittlesey and Saginaw</td> - <td class="tdrl"><a href="#f362">333</a></td> - </tr> - - <tr> - <td class="tdrh">363.</td> - <td class="tdt2">Map of the glacial Lake Warren</td> - <td class="tdrl"><a href="#f363">333</a></td> - </tr> - - <tr> - <td class="tdrh">364.</td> - <td class="tdt2">Map of the glacial Lake Algonquin</td> - <td class="tdrl"><a href="#f364">334</a></td> - </tr> - - <tr> - <td class="tdrh">365.</td> - <td class="tdt2">Outline map of the Nipissing Great Lakes</td> - <td class="tdrl"><a href="#f365">335</a></td> - </tr> - - <tr> - <td class="tdrh">366.</td> - <td class="tdt2">Probable preglacial drainage of the upper Ohio region</td> - <td class="tdrl"><a href="#f366">337</a></td> - </tr> - - <tr> - <td class="tdrh">367.</td> - <td class="tdt2">Diagrams to illustrate the episodes in the recent history of a Connecticut -river</td> - <td class="tdrl"><a href="#f367">338</a></td> - </tr> - - <tr> - <td class="tdrh">368.</td> - <td class="tdt2">The notched rock headland of Boyer Bluff on Lake Michigan</td> - <td class="tdrl"><a href="#f368">341</a></td> - </tr> - - <tr> - <td class="tdrh">369.</td> - <td class="tdt2">View of Mackinac Island from the direction of St. Ignace</td> - <td class="tdrl"><a href="#f369">342</a></td> - </tr> - - <tr> - <td class="tdrh">370.</td> - <td class="tdt2">The “Sugar Loaf”, a stack of Lake Algonquin upon Mackinac Island</td> - <td class="tdrl"><a href="#f370">342</a></td> - </tr> - - <tr> - <td class="tdrh">371.</td> - <td class="tdt2">Beach ridges in series on Mackinac Island</td> - <td class="tdrl"><a href="#f371">343</a></td> - </tr> - - <tr> - <td class="tdrh">372.</td> - <td class="tdt2">Notched stack of the Nipissing Great Lakes at St. Ignace</td> - <td class="tdrl"><a href="#f372">343</a></td> - </tr> - - <tr> - <td class="tdrh">373.</td> - <td class="tdt2">Series of diagrams to illustrate the evolution of ideas concerning the -uplift of the lake region since the Ice Age</td> - <td class="tdrl"><a href="#f373">344</a></td> - </tr> - - <tr> - <td class="tdrh">374.</td> - <td class="tdt2">Map of the Great Lakes region to show the isobases and hinge lines of -uptilt</td> - <td class="tdrl"><a href="#f374">345</a></td> - </tr> - - <tr> - <td class="tdrh">375.</td> - <td class="tdt2">Series of diagrams to indicate the nature of the recovery of the crust -by uplift when unloaded of an ice mantle</td> - <td class="tdrl"><a href="#f375">346</a></td> - </tr> - - <tr> - <td class="tdrh">376.</td> - <td class="tdt2">Portion of the Inner Sandusky Bay, for comparison of the shore line -of 1820 with that of to-day</td> - <td class="tdrl"><a href="#f376">350</a></td> - </tr> - - <tr> - <td class="tdrh">377.<span class="pagenum"><a name="Page_xxxii" id="Page_xxxii">[xxxii]</a></span></td> - <td class="tdt2">Ideal cross section of the Niagara Gorge to show the marginal terrace</td> - <td class="tdrl"><a href="#f377">353</a></td> - </tr> - - <tr> - <td class="tdrh">378.</td> - <td class="tdt2">View of the bed of the Niagara River above the cataract where water -has been drained off</td> - <td class="tdrl"><a href="#f378">353</a></td> - </tr> - - <tr> - <td class="tdrh">379.</td> - <td class="tdt2">View of the Falls of St. Anthony in 1851</td> - <td class="tdrl"><a href="#f379">354</a></td> - </tr> - - <tr> - <td class="tdrh">380.</td> - <td class="tdt2">Ideal section to show the nature of the drilling process beneath the -cataract</td> - <td class="tdrl"><a href="#f380">355</a></td> - </tr> - - <tr> - <td class="tdrh">381.</td> - <td class="tdt2">Plan and section of the gorge, showing how the depth is proportional -to the width</td> - <td class="tdrl"><a href="#f381">355</a></td> - </tr> - - <tr> - <td class="tdrh">382.</td> - <td class="tdt2">Comparative views of the Canadian Falls in 1827 and 1895</td> - <td class="tdrl"><a href="#f382">356</a></td> - </tr> - - <tr> - <td class="tdrh">383.</td> - <td class="tdt2">Map to show the recession of the Canadian Fall</td> - <td class="tdrl"><a href="#f383">357</a></td> - </tr> - - <tr> - <td class="tdrh">384.</td> - <td class="tdt2">Comparison of the present with the future falls</td> - <td class="tdrl"><a href="#f384">358</a></td> - </tr> - - <tr> - <td class="tdrh">385.</td> - <td class="tdt2">Bird’s-eye view of the captured Canadian Fall at Wintergreen Flats</td> - <td class="tdrl"><a href="#f385">358</a></td> - </tr> - - <tr> - <td class="tdrh">386.</td> - <td class="tdt2">Map of the Whirlpool Basin</td> - <td class="tdrl"><a href="#f386">360</a></td> - </tr> - - <tr> - <td class="tdrh">387.</td> - <td class="tdt2">Map of the cuestas which have played so important a part in fixing -the boundaries of the lake basins</td> - <td class="tdrl"><a href="#f387">361</a></td> - </tr> - - <tr> - <td class="tdrh">388.</td> - <td class="tdt2">Bird’s-eye view of the cuestas south of Lakes Ontario and Erie</td> - <td class="tdrl"><a href="#f388">362</a></td> - </tr> - - <tr> - <td class="tdrh">389.</td> - <td class="tdt2">Sketch map of the greater portion of the Niagara Gorge to illustrate -Niagara history</td> - <td class="tdrl"><a href="#f389">363</a></td> - </tr> - - <tr> - <td class="tdrh">390.</td> - <td class="tdt2">Snowdrift hollowing its bed by nivation</td> - <td class="tdrl"><a href="#f390">368</a></td> - </tr> - - <tr> - <td class="tdrh">391.</td> - <td class="tdt2">Amphitheater formed upon a drift site in northern Lapland</td> - <td class="tdrl"><a href="#f391">369</a></td> - </tr> - - <tr> - <td class="tdrh">392.</td> - <td class="tdt2">The marginal crevasse on the highest margin of a glacier</td> - <td class="tdrl"><a href="#f392">370</a></td> - </tr> - - <tr> - <td class="tdrh">393.</td> - <td class="tdt2">Niches and cirques in the Bighorn Mountains of Wyoming</td> - <td class="tdrl"><a href="#f393">371</a></td> - </tr> - - <tr> - <td class="tdrh">394.</td> - <td class="tdt2">Subordinate cirques in the amphitheater on the west face of the -Wannehorn</td> - <td class="tdrl"><a href="#f394">371</a></td> - </tr> - - <tr> - <td class="tdrh">395.</td> - <td class="tdt2">“Biscuit cutting” effect of glacial sculpture in the Uinta Mountains -of Wyoming</td> - <td class="tdrl"><a href="#f395">372</a></td> - </tr> - - <tr> - <td class="tdrh">396.</td> - <td class="tdt2">Diagram to show the cause of the hyperbolic curve of cols</td> - <td class="tdrl"><a href="#f396">372</a></td> - </tr> - - <tr> - <td class="tdrh">397.</td> - <td class="tdt2">A col in the Selkirks</td> - <td class="tdrl"><a href="#f397">373</a></td> - </tr> - - <tr> - <td class="tdrh">398.</td> - <td class="tdt2">Diagrams to illustrate the formation of comb ridges, cols, and horns</td> - <td class="tdrl"><a href="#f398">374</a></td> - </tr> - - <tr> - <td class="tdrh">399.</td> - <td class="tdt2">The <span class="font reduct"><b>U</b></span>-shaped -Kern Valley in the Sierra Nevadas of California</td> - <td class="tdrl"><a href="#f399">375</a></td> - </tr> - - <tr> - <td class="tdrh">400.</td> - <td class="tdt2">Glaciated valley wall, showing the sharp line which separates the -abraded from the undermined rock surface</td> - <td class="tdrl"><a href="#f400">375</a></td> - </tr> - - <tr> - <td class="tdrh">401.</td> - <td class="tdt2">View of the Vale of Chamonix from the séracs of the <i>Glacier des -Bossons</i></td> - <td class="tdrl"><a href="#f401">376</a></td> - </tr> - - <tr> - <td class="tdrh">402.</td> - <td class="tdt2">Map of an area near the continental divide in Colorado</td> - <td class="tdrl"><a href="#f402">377</a></td> - </tr> - - <tr> - <td class="tdrh">403.</td> - <td class="tdt2">Gorge of the Albula River in the Engadine cut through a rock bar</td> - <td class="tdrl"><a href="#f403">378</a></td> - </tr> - - <tr> - <td class="tdrh">404.</td> - <td class="tdt2">Idealistic sketch, showing glaciated and nonglaciated side valleys</td> - <td class="tdrl"><a href="#f404">378</a></td> - </tr> - - <tr> - <td class="tdrh">405.</td> - <td class="tdt2">Character profiles sculptured by mountain glaciers</td> - <td class="tdrl"><a href="#f405">379</a></td> - </tr> - - <tr> - <td class="tdrh">406.</td> - <td class="tdt2">Flat dome shaped under the margin of a Norwegian ice cap</td> - <td class="tdrl"><a href="#f406">379</a></td> - </tr> - - <tr> - <td class="tdrh">407.</td> - <td class="tdt2">Two views which illustrate successive stages in the shaping of tinds</td> - <td class="tdrl"><a href="#f407">380</a></td> - </tr> - - <tr> - <td class="tdrh">408.</td> - <td class="tdt2">Schematic diagram to bring out the relationships of the various types -of mountain glaciers</td> - <td class="tdrl"><a href="#f408">383</a></td> - </tr> - - <tr> - <td class="tdrh">409.</td> - <td class="tdt2">Map of the Malaspina Glacier of Alaska</td> - <td class="tdrl"><a href="#f409">384</a></td> - </tr> - - <tr> - <td class="tdrh">410.</td> - <td class="tdt2">Map of the Baltoro Glacier of the Himalayas</td> - <td class="tdrl"><a href="#f410">385</a></td> - </tr> - - <tr> - <td class="tdrh">411.</td> - <td class="tdt2">View of the Triest Glacier, a hanging glacieret</td> - <td class="tdrl"><a href="#f411">385</a></td> - </tr> - - <tr> - <td class="tdrh">412.</td> - <td class="tdt2">Map of the Harriman Fjord Glacier of Alaska</td> - <td class="tdrl"><a href="#f412">386</a></td> - </tr> - - <tr> - <td class="tdrh">413.<span class="pagenum"><a name="Page_xxxiii" id="Page_xxxiii">[xxxiii]</a></span></td> - <td class="tdt2">Map of the Rotmoos Glacier, a radiating glacier of Switzerland</td> - <td class="tdrl"><a href="#f413">386</a></td> - </tr> - - <tr> - <td class="tdrh">414.</td> - <td class="tdt2">Outline map of the Asulkan Glacier in the Selkirks, a horseshoe -glacier</td> - <td class="tdrl"><a href="#f414">387</a></td> - </tr> - - <tr> - <td class="tdrh">415.</td> - <td class="tdt2">Outline map of the Illecillewaet Glacier of the Selkirks, an inherited-basin -glacier</td> - <td class="tdrl"><a href="#f415">388</a></td> - </tr> - - <tr> - <td class="tdrh">416.</td> - <td class="tdt2">Diagram to illustrate the surface flow of glaciers</td> - <td class="tdrl"><a href="#f416">390</a></td> - </tr> - - <tr> - <td class="tdrh">417.</td> - <td class="tdt2">Diagram to show the transformation of crevasses into séracs</td> - <td class="tdrl"><a href="#f417">391</a></td> - </tr> - - <tr> - <td class="tdrh">418.</td> - <td class="tdt2">View of the <i>Glacier des Bossons</i>, showing the position of accidents -to Alpinists</td> - <td class="tdrl"><a href="#f418">392</a></td> - </tr> - - <tr> - <td class="tdrh">419.</td> - <td class="tdt2">Lines of flow upon the surface of the <i>Hintereisferner</i> Glacier in the -Alps</td> - <td class="tdrl"><a href="#f419">393</a></td> - </tr> - - <tr> - <td class="tdrh">420.</td> - <td class="tdt2">Lateral and medial moraines of the <i>Mer de Glace</i> and its tributaries</td> - <td class="tdrl"><a href="#f420">393</a></td> - </tr> - - <tr> - <td class="tdrh">421.</td> - <td class="tdt2">Ideal cross section of a mountain glacier</td> - <td class="tdrl"><a href="#f421">394</a></td> - </tr> - - <tr> - <td class="tdrh">422.</td> - <td class="tdt2">Diagrams to illustrate the melting effects upon glacier ice of rock -fragments of different sizes</td> - <td class="tdrl"><a href="#f422">394</a></td> - </tr> - - <tr> - <td class="tdrh">423.</td> - <td class="tdt2">Small glacier table upon the Great Aletsch Glacier</td> - <td class="tdrl"><a href="#f423">395</a></td> - </tr> - - <tr> - <td class="tdrh">424.</td> - <td class="tdt2">Effects of differential melting and subsequent refreezing upon a glacier -surface</td> - <td class="tdrl"><a href="#f424">396</a></td> - </tr> - - <tr> - <td class="tdrh">425.</td> - <td class="tdt2">Dirt cone with its casing in part removed</td> - <td class="tdrl"><a href="#f425">396</a></td> - </tr> - - <tr> - <td class="tdrh">426.</td> - <td class="tdt2">Schematic diagram to show the manner of formation of glacier cornices</td> - <td class="tdrl"><a href="#f426">397</a></td> - </tr> - - <tr> - <td class="tdrh">427.</td> - <td class="tdt2">Superglacial stream upon the Great Aletsch Glacier</td> - <td class="tdrl"><a href="#f427">398</a></td> - </tr> - - <tr> - <td class="tdrh">428.</td> - <td class="tdt2">Ideal form of the surface left on the site of a piedmont glacier apron</td> - <td class="tdrl"><a href="#f428">399</a></td> - </tr> - - <tr> - <td class="tdrh">429.</td> - <td class="tdt2">Map of the site of the earlier piedmont glacier of the Upper Rhine</td> - <td class="tdrl"><a href="#f429">399</a></td> - </tr> - - <tr> - <td class="tdrh">430.</td> - <td class="tdt2">Diagram and map to bring out the characteristics of newland lakes</td> - <td class="tdrl"><a href="#f430">402</a></td> - </tr> - - <tr> - <td class="tdrh">431.</td> - <td class="tdt2">View of the Warner Lakes, Oregon</td> - <td class="tdrl"><a href="#f431">402</a></td> - </tr> - - <tr> - <td class="tdrh">432.</td> - <td class="tdt2">Schematic diagram to illustrate the characteristics of basin-range lakes</td> - <td class="tdrl"><a href="#f432">403</a></td> - </tr> - - <tr> - <td class="tdrh">433.</td> - <td class="tdt2">Schematic diagram of rift-valley lakes and the valley of the Jordan</td> - <td class="tdrl"><a href="#f433">403</a></td> - </tr> - - <tr> - <td class="tdrh">434.</td> - <td class="tdt2">Map of the rift-valley lakes of East Central Africa</td> - <td class="tdrl"><a href="#f434">404</a></td> - </tr> - - <tr> - <td class="tdrh">435.</td> - <td class="tdt2">Earthquake lakes formed in 1811 in the flood plain of the Lower -Mississippi</td> - <td class="tdrl"><a href="#f435">404</a></td> - </tr> - - <tr> - <td class="tdrh">436.</td> - <td class="tdt2">View of a crater lake in Costa Rica</td> - <td class="tdrl"><a href="#f436">405</a></td> - </tr> - - <tr> - <td class="tdrh">437.</td> - <td class="tdt2">Diagrams to illustrate the characteristics of crater lakes</td> - <td class="tdrl"><a href="#f437">406</a></td> - </tr> - - <tr> - <td class="tdrh">438.</td> - <td class="tdt2">View of Snag Lake, a coulée lake in California</td> - <td class="tdrl"><a href="#f438">406</a></td> - </tr> - - <tr> - <td class="tdrh">439.</td> - <td class="tdt2">Diagrams to illustrate the characteristics of morainal lakes</td> - <td class="tdrl"><a href="#f439">407</a></td> - </tr> - - <tr> - <td class="tdrh">440.</td> - <td class="tdt2">Diagram to show the manner of formation of pit lakes</td> - <td class="tdrl"><a href="#f440">408</a></td> - </tr> - - <tr> - <td class="tdrh">441.</td> - <td class="tdt2">Diagrams to illustrate the characteristics of pit lakes</td> - <td class="tdrl"><a href="#f441">408</a></td> - </tr> - - <tr> - <td class="tdrh">442.</td> - <td class="tdt2">Diagram to show the manner of formation of glint lakes</td> - <td class="tdrl"><a href="#f442">409</a></td> - </tr> - - <tr> - <td class="tdrh">443.</td> - <td class="tdt2">Map of a series of glint lakes on the boundary of Sweden and Norway</td> - <td class="tdrl"><a href="#f443">409</a></td> - </tr> - - <tr> - <td class="tdrh">444.</td> - <td class="tdt2">Map of ice-dam lakes near the Norwegian boundary of Sweden</td> - <td class="tdrl"><a href="#f444">410</a></td> - </tr> - - <tr> - <td class="tdrh">445.</td> - <td class="tdt2">Wave-cut terrace of a former ice-dam lake in Sweden</td> - <td class="tdrl"><a href="#f445">410</a></td> - </tr> - - <tr> - <td class="tdrh">446.</td> - <td class="tdt2">View of the Márjelen Lake from the summit of the Eggishorn</td> - <td class="tdrl"><a href="#f446">411</a></td> - </tr> - - <tr> - <td class="tdrh">447.</td> - <td class="tdt2">Diagrams to illustrate the arrangement and the characters of rock-basin -lakes</td> - <td class="tdrl"><a href="#f447">412</a></td> - </tr> - - <tr> - <td class="tdrh">448.</td> - <td class="tdt2">Convict Lake, a valley-moraine lake of California</td> - <td class="tdrl"><a href="#f448">413</a></td> - </tr> - - <tr> - <td class="tdrh">449.</td> - <td class="tdt2">Lake basins produced by successive slides from the steep walls of a -glaciated mountain valley</td> - <td class="tdrl"><a href="#f449">414</a></td> - </tr> - - <tr> - <td class="tdrh">450.<span class="pagenum"><a name="Page_xxxiv" id="Page_xxxiv">[xxxiv]</a></span></td> - <td class="tdt2">Lake Garda, a border lake upon the site of a piedmont apron</td> - <td class="tdrl"><a href="#f450">414</a></td> - </tr> - - <tr> - <td class="tdrh">451.</td> - <td class="tdt2">Diagrams to bring out the characteristics of ox-bow lakes</td> - <td class="tdrl"><a href="#f451">415</a></td> - </tr> - - <tr> - <td class="tdrh">452.</td> - <td class="tdt2">Diagrammatic section to illustrate the formation of saucer-like basins -between the levees of streams on a flood plain</td> - <td class="tdrl"><a href="#f452">415</a></td> - </tr> - - <tr> - <td class="tdrh">453.</td> - <td class="tdt2">Saucer lakes upon the bed of the former river Warren</td> - <td class="tdrl"><a href="#f453">416</a></td> - </tr> - - <tr> - <td class="tdrh">454.</td> - <td class="tdt2">Levee lakes developed in series within meanders in a delta plain</td> - <td class="tdrl"><a href="#f454">417</a></td> - </tr> - - <tr> - <td class="tdrh">455.</td> - <td class="tdt2">Raft lakes along the banks of the Red River in Arkansas and Louisiana</td> - <td class="tdrl"><a href="#f455">418</a></td> - </tr> - - <tr> - <td class="tdrh">456.</td> - <td class="tdt2">Map of the Swiss lakes Thun and Brienz</td> - <td class="tdrl"><a href="#f456">419</a></td> - </tr> - - <tr> - <td class="tdrh">457.</td> - <td class="tdt2">Delta lakes formed at the mouth of the Mississippi</td> - <td class="tdrl"><a href="#f457">419</a></td> - </tr> - - <tr> - <td class="tdrh">458.</td> - <td class="tdt2">Delta lakes at the margin of the Nile delta</td> - <td class="tdrl"><a href="#f458">420</a></td> - </tr> - - <tr> - <td class="tdrh">459.</td> - <td class="tdt2">Diagrams to illustrate the characteristics of barrier lakes</td> - <td class="tdrl"><a href="#f459">420</a></td> - </tr> - - <tr> - <td class="tdrh">460.</td> - <td class="tdt2">Dune lakes on the coast of France</td> - <td class="tdrl"><a href="#f460">421</a></td> - </tr> - - <tr> - <td class="tdrh">461.</td> - <td class="tdt2">Sink lakes in Florida, with a schematic diagram to illustrate the -manner of their formation</td> - <td class="tdrl"><a href="#f461">421</a></td> - </tr> - - <tr> - <td class="tdrh">462.</td> - <td class="tdt2">Map of the Arve and the Upper Rhone</td> - <td class="tdrl"><a href="#f462">426</a></td> - </tr> - - <tr> - <td class="tdrh">463.</td> - <td class="tdt2">View of the Arve and the Rhone at their junction</td> - <td class="tdrl"><a href="#f463">427</a></td> - </tr> - - <tr> - <td class="tdrh">464.</td> - <td class="tdt2">A village in Switzerland built upon a strath at the head of Lake -Poschiavo</td> - <td class="tdrl"><a href="#f464">428</a></td> - </tr> - - <tr> - <td class="tdrh">465.</td> - <td class="tdt2">View of the floating bog and surrounding zones of vegetation in a -small glacial lake</td> - <td class="tdrl"><a href="#f465">429</a></td> - </tr> - - <tr> - <td class="tdrh">466.</td> - <td class="tdt2">Diagram to show how small lakes are transformed into peat bogs</td> - <td class="tdrl"><a href="#f466">430</a></td> - </tr> - - <tr> - <td class="tdrh">467.</td> - <td class="tdt2">Map to show the anomalous position of the delta in Lake St. Clair</td> - <td class="tdrl"><a href="#f467">431</a></td> - </tr> - - <tr> - <td class="tdrh">468.</td> - <td class="tdt2">A bowlder wall upon the shore of a small lake</td> - <td class="tdrl"><a href="#f468">432</a></td> - </tr> - - <tr> - <td class="tdrh">469.</td> - <td class="tdt2">Diagrams to show the effect of ice shove in producing ice ramparts -upon the shores of lakes</td> - <td class="tdrl"><a href="#f469">433</a></td> - </tr> - - <tr> - <td class="tdrh">470.</td> - <td class="tdt2">Various forms of ice ramparts</td> - <td class="tdrl"><a href="#f470a">433</a></td> - </tr> - - <tr> - <td class="tdrh">471.</td> - <td class="tdt2">Map of Lake Mendota, showing the position of the ridge which forms -from ice expansion and the ice ramparts upon the shores</td> - <td class="tdrl"><a href="#f471">434</a></td> - </tr> - - <tr> - <td class="tdrh">472.</td> - <td class="tdt2">The great multiple mountain arc of Sewestan, British India</td> - <td class="tdrl"><a href="#f472">436</a></td> - </tr> - - <tr> - <td class="tdrh">473.</td> - <td class="tdt2">Diagrams to illustrate the theories of origin of mountain arcs</td> - <td class="tdrl"><a href="#f473">437</a></td> - </tr> - - <tr> - <td class="tdrh">474.</td> - <td class="tdt2">Festoons of mountain arcs about the borders of the Pacific Ocean</td> - <td class="tdrl"><a href="#f474">438</a></td> - </tr> - - <tr> - <td class="tdrh">475.</td> - <td class="tdt2">The interrupted Armorican Mountains common to western Europe -and eastern North America</td> - <td class="tdrl"><a href="#f475">438</a></td> - </tr> - - <tr> - <td class="tdrh">476.</td> - <td class="tdt2">A zone of diverse displacement in the western United States</td> - <td class="tdrl"><a href="#f476">439</a></td> - </tr> - - <tr> - <td class="tdrh">477.</td> - <td class="tdt2">Section of an East African block mountain</td> - <td class="tdrl"><a href="#f477">439</a></td> - </tr> - - <tr> - <td class="tdrh">478.</td> - <td class="tdt2">Tilted crust blocks in the Queantoweap valley</td> - <td class="tdrl"><a href="#f478">440</a></td> - </tr> - - <tr> - <td class="tdrh">479.</td> - <td class="tdt2">View of the laccolite of the Carriso Mountain</td> - <td class="tdrl"><a href="#f479">441</a></td> - </tr> - - <tr> - <td class="tdrh">480.</td> - <td class="tdt2">Map of laccolitic mountains</td> - <td class="tdrl"><a href="#f480">441</a></td> - </tr> - - <tr> - <td class="tdrh">481.</td> - <td class="tdt2">Ideal sections of laccolite and bysmalite</td> - <td class="tdrl"><a href="#f481">442</a></td> - </tr> - - <tr> - <td class="tdrh">482.</td> - <td class="tdt2">The gabled façade largely developed in desert landscapes</td> - <td class="tdrl"><a href="#f482">443</a></td> - </tr> - - <tr> - <td class="tdrh">483.</td> - <td class="tdt2">Balloon view of the Mythen in Switzerland</td> - <td class="tdrl"><a href="#f483">444</a></td> - </tr> - - <tr> - <td class="tdrh">484.</td> - <td class="tdt2">The battlement type of erosion mountain</td> - <td class="tdrl"><a href="#f484">445</a></td> - </tr> - - <tr> - <td class="tdrh">485.</td> - <td class="tdt2">Symmetrically formed low islands repeated in ranks upon Temagami -Lake, Ontario</td> - <td class="tdrl"><a href="#f485">445</a></td> - </tr> - - <tr> - <td class="tdrh">486.</td> - <td class="tdt2">Forms of crystals of a number of minerals</td> - <td class="tdrl"><a href="#f486">454</a></td> - </tr> - - <tr> - <td class="tdrh">487.</td> - <td class="tdt2">Forms of crystals of a number of minerals</td> - <td class="tdrl"><a href="#f487">457</a></td> - </tr> - - <tr> - <td class="tdrh">488.<span class="pagenum"><a name="Page_xxxv" id="Page_xxxv">[xxxv]</a></span></td> - <td class="tdt2">A student’s contour map</td> - <td class="tdrl"><a href="#f488">469</a></td> - </tr> - - <tr> - <td class="tdrh">489.</td> - <td class="tdt2">Models to represent outcrops of rock</td> - <td class="tdrl"><a href="#f489">472</a></td> - </tr> - - <tr> - <td class="tdrh">490.</td> - <td class="tdt2">Special laboratory table set with a problem in geological mapping -which is solved in <a href="#f47">Figs. 47</a> and <a href="#f48">48</a></td> - <td class="tdrl"><a href="#f490">472</a></td> - </tr> - - <tr> - <td class="tdrh">491.</td> - <td class="tdt2">Three field maps to be used as suggestions in arranging laboratory -table for problems in the preparation of areal geological maps</td> - <td class="tdrl"><a href="#f491">473</a></td> - </tr> - - <tr> - <td class="tdrh">492.</td> - <td class="tdt2">Sketch map of Western Scotland and the Inner Hebrides to -show location of some points of special geological interest</td> - <td class="tdrl"><a href="#f492">481</a></td> - </tr> - - <tr> - <td class="tdrh">493.</td> - <td class="tdt2">Outline map of a geological pilgrimage across the continent of Europe</td> - <td class="tdrl"><a href="#f493">483</a></td> - </tr> - -</table> - -<p><span class="pagenum"><a name="Page_xxxvi" id="Page_xxxvi">[xxxvi]</a></span></p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_xxxvii" id="Page_xxxvii">[xxxvii]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">EXPLANATORY LIST OF ABBREVIATIONS FOR -JOURNAL NAMES IN READING REFERENCES</h2> - -<p class="pex p2">Am. Geol.: American Geologist.</p> - -<p class="pex">Am. Jour. Sci.: American Journal of Science, New Haven.</p> - -<p class="pex">Ann. de Géogr.: Annales de Géographie, Paris.</p> - -<p class="pex">Ann. Rept. Geol. and Geogr. Surv. Ter.: Annual Report of the Geological and -Geographical Survey of the Territories (Hayden), Washington.</p> - -<p class="pex">Ann. Rept. Geol. and Nat. Hist. Surv. Minn.: Annual Report of the Geological -and Natural History Survey of Minnesota, Minneapolis.</p> - -<p class="pex">Ann. Rept. Mich. Geol. Surv.: Annual Report of the Michigan Geological Survey, -Lansing.</p> - -<p class="pex">Ann. Rept. U. S. Geol. Surv.: Annual Report of the United States Geological -Survey, Washington.</p> - -<p class="pex">Bull. Am. Geogr. Soc.: Bulletin of the American Geographical Society, New -York.</p> - -<p class="pex">Bull. Earthq. Inv. Com. Japan: Bulletin of the Earthquake Investigation Committee -of Japan, Tokyo.</p> - -<p class="pex">Bull. Geogr. Soc. Philadelphia: Bulletin of the Geographical Society of Philadelphia.</p> - -<p class="pex">Bull. Geol. Soc. Am.: Bulletin of the Geological Society of America.</p> - -<p class="pex">Bull. Mus. Comp. Zoöl.: Bulletin of the Museum of Comparative Zoölogy, -Harvard College, Cambridge.</p> - -<p class="pex">Bull. N. Y. State Mus.: Bulletin of the New York State Museum, Albany.</p> - -<p class="pex">Bull. Soc. Belge d’Astronomie: Bulletin de la Société Belge d’Astronomie, -Brussels.</p> - -<p class="pex">Bull. Soc. Belge Géol.: Bulletin de la Société Belge de Géologie, Brussels.</p> - -<p class="pex">Bull. Soc. Sc. Nat. Neuchâtel: Bulletin de la Société des Sciences Naturelles de -Neuchâtel.</p> - -<p class="pex">Bull. Univ. Calif. Dept. Geol.: Bulletin of the University of California, Department -of Geology, Berkeley.</p> - -<p class="pex">Bull. U. S. Geol. Surv.: Bulletin of the United States Geological Survey, -Washington.</p> - -<p class="pex">Bull. Wis. Geol. and Nat. Hist. Surv.: Bulletin of the Wisconsin Geological and -Natural History Survey, Madison.</p> - -<p class="pex">C. R. Cong. Géol. Intern.: Comptes Rendus de la Congrès Géologique Internationale.</p> - -<p class="pex">Dept. of Mines, Geol. Surv. Branch, Canada: Department of Mines, Geological -Survey Branch, Canada.</p> - -<p class="pex"><span class="pagenum"><a name="Page_xxxviii" id="Page_xxxviii">[xxxviii]</a></span></p> - -<p class="pex">Geogr. Abh.: Geographische Abhandlungen.</p> - -<p class="pex">Geogr. Jour.: Geographical Journal, London.</p> - -<p class="pex">Geol. Folio U. S. Geol. Surv.: Geological Folio of the United States Geological -Survey.</p> - -<p class="pex">Geol. Mag.: Geological Magazine, London (sections designated by decades).</p> - -<p class="pex">Jour. Am. Geogr. Soc.: Journal of the American Geographical Society, New -York.</p> - -<p class="pex">Jour. Coll. Sci. Imp. Univ. Tokyo: Journal of the College of Science of the -Imperial University, Tokyo, Japan.</p> - -<p class="pex">Jour. Geol.: Journal of Geology, Chicago.</p> - -<p class="pex">Jour. Sch. Geogr.: Journal of School Geography.</p> - -<p class="pex">Livret Guide Cong. Géol. Intern.: Livret Guide Congrès Géologique Internationale.</p> - -<p class="pex">Mem. Geol. Surv. India: Memoirs of the Geological Survey of India, Calcutta.</p> - -<p class="pex">Mitt. Geogr. Ges. Hamb.: Mitteilungen der Geographische Gesellschaft, Hamburg.</p> - -<p class="pex">Mon. U. S. Geol. Surv.: Monograph of the United States Geological Survey, -Washington.</p> - -<p class="pex">Nat. Geogr. Mag.: National Geographic Magazine, Washington.</p> - -<p class="pex">Nat. Geogr. Mon.: National Geographic Monographs, American Book Company, -New York.</p> - -<p class="pex">Naturw. Wochenschr.: Naturwissenschaftliche Wochenschrift.</p> - -<p class="pex">Pet. Mitt.: Petermanns Mittheilungen aus Justus Perthes’ Geographischer -Anstalt, Gotha.</p> - -<p class="pex">Pet. Mitt., Ergänzungsh. or Erg.: Petermanns Mittheilungen, Gotha (Ergänzungsheft -or Supplementary Paper).</p> - -<p class="pex">Phil. Jour. Sci.: Philippine Journal of Science, Manila.</p> - -<p class="pex">Phil. Trans.: Philosophical Transactions of the Royal Society, London.</p> - -<p class="pex">Proc. Am. Acad. Arts and Sci.: Proceedings of the American Academy of Arts -and Sciences.</p> - -<p class="pex">Proc. Am. Assoc. Adv. Sci.: Proceedings of the American Association for the -Advancement of Science.</p> - -<p class="pex">Proc. Am. Phil. Soc.: Proceedings of the American Philosophical Society, -Philadelphia.</p> - -<p class="pex">Proc. Bost. Soc. Nat. Hist.: Proceedings of the Boston Society of Natural -History, Boston.</p> - -<p class="pex">Proc. Ind. Acad. Sci.: Proceedings of the Indiana Academy of Science.</p> - -<p class="pex">Proc. Linn. Soc. New South Wales: Proceedings of the Linnean Society of -New South Wales.</p> - -<p class="pex">Proc. Ohio State Acad. Sci.: Proceedings of the Ohio State Academy of Science.</p> - -<p class="pex">Prof. Pap. U. S. Geol. Surv.: Professional Paper of the United States Geological -Survey, Washington.</p> - -<p class="pex">Pub. Carneg. Inst.: Publication of the Carnegie Institution of Washington.</p> - -<p class="pex">Pub. Mich. Geol. and Biol. Surv.: Publication of the Michigan Geological and -Biological Survey, Lansing.</p> - -<p class="pex">Quart. Jour. Geol. Soc. Lond.: Quarterly Journal of the Geological Society, -London.</p> - -<p class="pex"><span class="pagenum"><a name="Page_xxxix" id="Page_xxxix">[xxxix]</a></span></p> - -<p class="pex">Rept. Brit. Assoc. Adv. Sci.: Report of the British Association for the Advancement -of Science.</p> - -<p class="pex">Rept. Geol. Surv. Mich.: Report of the Geological Survey of Michigan, Lansing.</p> - -<p class="pex">Rept. Mich. Acad. Sci.: Report of the Michigan Academy of Science, Lansing.</p> - -<p class="pex">Rept. Nat. Conserv. Com.: Report of the National Conservation Commission, -Washington.</p> - -<p class="pex">Rept. Smithson. Inst.: Report of the Smithsonian Institution, Washington.</p> - -<p class="pex">Sci. Bull. Brooklyn Inst. Arts and Sci.: Science Bulletin of the Brooklyn Institute -of Arts and Sciences.</p> - -<p class="pex">Scot. Geogr. Mag.: Scottish Geographic Magazine, Edinburgh.</p> - -<p class="pex">Smith. Cont. to Knowl.: Smithsonian Contributions to Knowledge, Washington.</p> - -<p class="pex">Tech. Quart.: Technology Quarterly of the Massachusetts Institute of Technology, -Boston.</p> - -<p class="pex">Trans. Am. Inst. Min. Eng.: Transactions of the American Institute of Mining -Engineers, New York.</p> - -<p class="pex">Trans. Roy. Dublin Soc.: Transactions of the Royal Dublin Society.</p> - -<p class="pex">Trans. Seis. Soc. Japan: Transactions of the Seismological Society of Japan, -Tokyo.</p> - -<p class="pex">Trans. Wis. Acad. Sci.: Transactions of the Wisconsin Academy of Sciences, -Arts, and Letters, Madison.</p> - -<p class="pex">U. S. Geogr. and Geol. Surv. Rocky Mt. Region: United States Geographical -and Geological Survey of the Rocky Mountain Region (Powell), Washington.</p> - -<p class="pex">Zeit. d. Gesell. f. Erdk. z. Berlin: Zeitschrift der Gesellschaft für Erdkunde -zu Berlin.</p> - -<p class="pex">Zeit. f. Gletscherk: Zeitschrift für Gletscherkunde, Berlin.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_xl" id="Page_xl">[xl]</a><br /><a name="Page_1" id="Page_1">[1]</a></span></p> - -<div class="chapter"> - -<p class="pc4 elarge">EARTH FEATURES AND THEIR MEANING</p> - -<h2 class="p4">CHAPTER I</h2> - -<p class="pch">THE COMPILATION OF EARTH HISTORY</p> - -<p><b>The sources of the history.</b>—The science which deals with the -chapters of earth history that antedate the earliest human writings -is geology. The pages of the record are the layers of rock -which make up the outer shell of our world. Here as in old -manuscripts pages are sometimes found to be missing, and on -others the writing is largely effaced so as to be indistinct or even -illegible. An intelligent interpretation of this record requires a -knowledge of the materials and the structure of the earth, as -well as a proper conception of the agencies which have caused -change and so developed the history. These agencies in operation -are physical and chemical processes, and so the sciences of -physics and chemistry are fundamental in any extended study of -geology. Not only is geology, so to speak, founded upon chemistry -and physics, but its field overlaps that of many other important -sciences. The earliest earth history has to do with the -form, size, and physical condition of a minor planet in the solar -system. The earliest portion of the story belongs therefore to -astronomy, and no sharp line can be drawn to separate this chapter -from those later ones which are more clearly within the domain -of geology.</p> - - -<p><b>Subdivisions of geology.</b>—The terms “cosmic geology” and -“astronomic geology” have sometimes been used to cover the -astronomy of the earth planet. The later earth history develops, -among other things, the varied forms of animal and vegetable life -which have had a definite order of appearance. Their study is -to a large extent zoölogy and botany, though here considered -from an essentially different viewpoint. This subdivision of our -science is called paleontological geology or paleontology, which<span class="pagenum"><a name="Page_2" id="Page_2">[2]</a></span> -in common usage includes the plant as well as the animal world, -or what is sometimes called paleobotany. In order to fix the -order of events in geological history, these biological studies are -necessary, for the pages of the record have many of them been -misplaced as a result of the vicissitudes of earth history, and the -remains of life in the rock layers supply a pagination from which -it is possible to correctly rearrange the misplaced pages. As compiled -into a consecutive history of the earth since life appeared -upon it, we have the division of historical geology; though this -differs but little from stratigraphical geology, the emphasis in the -case of the former being placed on the history itself and in the -latter upon the arrangement of events—the pagination of the -record.</p> - -<p>So far as they are known to us, the materials of which the -earth is composed are minerals grouped into various characteristic -aggregates known as rocks. Here the science is founded upon -mineralogy as well as chemistry, and a study of the rock materials -of the earth is designated petrographical geology or petrography. -The various rocks which enter into the composition of the earth’s -outer shell—the only portion known to us from direct observation—are -built into it in an architecture which, when carefully -studied, discloses important events in the earth’s history. The -division of the science which is concerned with earth architecture -is geotectonic or structural geology.</p> - - -<p><b>The study of earth features and their significance.</b>—The -features upon the surface of the earth have all their deep significance, -and if properly understood, a flood of light is thrown, -not only upon present conditions, but upon many chapters of the -earth’s earlier history. Here the relation of our study to topography -and geography is very close, so that the lines of separation -are but ill defined. The terms “physiographical geology”, -“physiography”, and “geomorphology” are concerned with the -configuration of the earth’s surface—its physiognomy—and with -the genesis of its individual surface features. It is this genetical -side of physiography which separates it from topography -and lends it an absorbing interest, though it causes it to largely -overlap the division of dynamical geology or the study of -geological processes. In fact, the difference between dynamical -geology and physiography is largely one of emphasis, the stress<span class="pagenum"><a name="Page_3" id="Page_3">[3]</a></span> -being laid upon the processes in the former and upon the resultant -features in the latter.</p> - -<p>Under dynamical geology are included important subdivisions, -such as seismic geology, or the study of earthquakes, and vulcanology, -or the study of volcanoes. Another large subject, -glacial geology, belongs within the broad frontier common to both -dynamical geology and physiography. A relatively new subdivision -of geological science is orientational geology, which is -concerned with the trend of earth features, and is closely related -both to physiography and to dynamical and structural geology.</p> - -<p><b>Tabular recapitulation.</b>—In a slightly different arrangement -from the above order of mention, the subdivisions of geology are -as follows:—</p> - -<p class="pc2 reduct"><i>Subdivisions of Geology</i></p> - -<table id="t01" summary="t01"> - - <tr> - <td class="tred"><i>Petrographical Geology.</i></td> - <td class="tspace" rowspan="6"> </td> - <td class="tdt3">Materials of the earth.</td> - </tr> - - <tr> - <td class="tred"><i>Geotectonic Geology.</i></td> - <td class="tdt3">Architecture of the earth’s outer shell.</td> - </tr> - - <tr> - <td class="tred"><i>Dynamical Geology.</i></td> - <td class="tdt3">Earth processes.</td> - </tr> - - <tr> - <td class="tdt2">Seismic Geology—earthquakes. -Vulcanology—volcanoes. Glacial -Geology—glaciers, etc.</td> - </tr> - - <tr> - <td class="tred"><i>Physiographical Geology.</i></td> - <td class="tdt3">Earth physiognomy and its genesis.</td> - </tr> - - <tr> - <td class="tred"><i>Orientational Geology.</i></td> - <td class="tdt3">The arrangement and the trend of earth features.</td> - </tr> - -</table> - -<p class="p1">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:—</p> - -<table id="t02" summary="t02"> - - <tr> - <td class="tred"><i>Astronomic Geology.</i></td> - <td class="tdt3">Planetary history of the earth.</td> - </tr> - - <tr> - <td class="tred"><i>Statigraphic Geology.</i></td> - <td class="tdt3">The pagination of earth records.</td> - </tr> - - <tr> - <td class="tred"><i>Historical Geology.</i></td> - <td class="tdt3">The compiled record and its interpretation.</td> - </tr> - - <tr> - <td class="tred"><i>Paleontological Geology.</i></td> - <td class="tdt3">The evolution of life upon the earth.</td> - </tr> - -</table> - -<p class="p1">In every attempt at systematic arrangement difficulties are -encountered, usually because no one consideration can be used -throughout as the basis of classification. Such terms as “economic<span class="pagenum"><a name="Page_4" id="Page_4">[4]</a></span> -geology” and “mining geology” have either a pedagogical -or a commercial significance, and so would hardly fit into the -system which we have outlined.</p> - - -<p><b>Geological processes not universal.</b>—It is inevitable that the -geology of regions which are easily accessible for study should -have absorbed the larger measure of attention; but it should not -be forgotten that geology is concerned with the history of the -entire world, and that perspective will be lost and erroneous -conclusions drawn if local conditions are kept too often before -the eyes. To illustrate by a single instance, the best studied -regions of the globe are those in which fairly abundant precipitation -in the form of rain has fitted the land for easy conditions of -life, and has thus permitted the development of a high civilization. -In degree, and to some extent also in kind, geologic processes -are markedly different within those widely extended regions which, -because either arid or cold, have been but ill fitted for human -habitation. Yet in the historical development of the earth, those -geologic processes which obtain in desert or polar regions are none -the less important because less often and less carefully observed.</p> - - -<p><b>Change, and not stability, the order of nature.</b>—Man is ever -prone to emphasize the importance of apparent facts to the disadvantage -of those less clearly revealed though equally potent. -The ancient notion of the <i>terra firma</i>, the safe and solid ground, -arose because of its contrast with the far more mobile bodies of -water; but this illusion is quickly dispelled with the sudden quaking -of the ground. Experience has clearly shown that, both upon -and beneath the earth’s surface, chemical and physical changes -are going on, subject to but little interruption. “The hills rock-ribbed -and ancient as the sun” is a poetical metaphor; for the -Himalayas, the loftiest mountains upon the globe, were, to speak -in geological terms, raised from the sea but yesterday. Even -to-day they are pushing up their heads, only to be relentlessly -planed down through the action of the atmosphere, of ice, and of -running water. Even more than has generally been supposed, the -earth suffers change. Often within the space of a few seconds, -to the accompaniment of a heavy earthquake, many square miles -of territory are bodily uplifted, while neighboring areas may be -relatively depressed. Thus change, and not stability, is the order -of nature.</p> - -<p><span class="pagenum"><a name="Page_5" id="Page_5">[5]</a></span></p> - - -<p><b>Observational geology <i>versus</i> speculative philosophy.</b>—There -appears to be a more or less prevalent notion that the views which -are held by scientists in one generation are abandoned by those -of the next; and this is apt to lead to the belief that little is really -known and that much is largely guessed. Some ground there -undoubtedly is for such skepticism, though much of it may be -accounted for by a general failure among scientists, as well as -others, to clearly differentiate that which is essentially speculative -from what is based broadly upon observed facts. Even with -extended observation, the possibility of explaining the facts in -more than one way is not excluded; but the line is nevertheless -a broad one which separates this entire field of observation from -what is essentially speculative philosophy. To illustrate: the -mechanics of the action which goes on within volcanic craters is -now fairly well understood as a result of many and extended -observations, and it is little likely that future generations of -geologists will discredit the main conclusions which have been -reached. The cause of the rise of the lava to the earth’s surface -is, on the other hand, much less clearly demonstrated, and the -views which are held express rather the differing opinions than -any clear deductions from observation. Again, and similarly, the -physical history of the great continental glaciers of the so-called -“ice age” is far more thoroughly known than that of any existing -glacier of the same type; but the cause of the climatic changes -which brought on the glaciation is still largely a matter for speculation.</p> - -<p>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.</p> - - -<p><b>The scientific attitude and temper.</b>—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<span class="pagenum"><a name="Page_6" id="Page_6">[6]</a></span> -be subject to review so soon as his own horizon has been sufficiently -enlarged. False theories could hardly have endured so long in the -past, had not too great respect been given to authorities, and individual -reasoning processes been held too long in subjection.</p> - - -<p><b>The value of the hypothesis.</b>—Because all the facts necessary -for a full interpretation of observed phenomena are not at one’s -hand, this should not be made to stand in the way of provisional -explanations. If science is to advance, the use of hypothesis is -absolutely essential; but the particular hypothesis adopted should -be regarded as temporary and as indicating a line of observation -or of experimentation which is to be followed in testing it. Thus -regarded with an open mind, inadequate hypotheses are eventually -found to be untenable, whereas correct explanations of the -facts by the same process are confirmed. Most hypotheses of -science are but partially correct, for we now “see through a glass -darkly”; but even so, if properly tested, the false elements in the -hypothesis are one after the other eliminated as the embodied -truth is confirmed and enlarged. Thus “working hypothesis” -passes into theory and becomes an integral part of science.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter I</span></p> - -<p>The most comprehensive of general geological texts written in English is -Chamberlin and Salisbury’s “Geology” in three volumes (Henry Holt, -1904-1906), the first volume of which is devoted exclusively to geological -processes and their results. An abridged one-volume edition of the work -intended for use as a college text was issued in 1906 (College Geology, -Henry Holt). Other standard texts are:—</p> - -<p class="pex p1"><span class="smcap">Sir Archibald Geikie.</span> Text-book of Geology, 4th ed. 2 vols. London, -1902, pp. 1472.</p> - -<p class="pex"><span class="smcap">W. B. Scott.</span> An Introduction to Geology. 2d ed. Macmillan, 1907, -pp. 816.</p> - -<p class="pex"><span class="smcap">J. D. Dana.</span> Manual of Geology. New edition. American Book Company, -1895, pp. 1087.</p> - -<p class="pex"><span class="smcap">Joseph LeConte.</span> Elements of Geology. (Revised by Fairchild.) -Appleton, 1905, pp. 667.</p> - -<p class="p1">A very valuable guide to the recent literature of dynamical and structural -geology is Branner’s “Syllabus of a Course of Lectures on Elementary -Geology” (Stanford University, 1908).</p> - -<p>On the relation of geology to landscape, a number of interesting books -have been written:—</p> - -<p class="pex p1"><span class="smcap">James Geikie.</span> Earth Sculpture or the Origin of Land-Forms. New -York and London, 1896, pp. 397.</p> - -<p class="pex"><span class="pagenum"><a name="Page_7" id="Page_7">[7]</a></span></p> - -<p class="pex"><span class="smcap">John E. Marr.</span> The Scientific Study of Scenery. Methuen, London, -1900, pp. 368.</p> - -<p class="pex"><span class="smcap">Sir A. Geikie.</span> The Scenery of Scotland. 3d ed. Macmillan, London, -1901, pp. 540.</p> - -<p class="pex"><span class="smcap">Sir John Lubbock.</span> The Scenery of Switzerland and the Causes to which -it is Due. Macmillan, London, 1896, pp. 480.</p> - -<p class="pex"><span class="smcap">Lord Avebury.</span> The Scenery of England. Macmillan, London, 1902, -pp. 534.</p> - -<p class="pex"><span class="smcap">Sir A. Geikie.</span> Landscape in History, and Other Essays. Macmillan, -London, 1905, pp. 352.</p> - -<p class="pex"><span class="smcap">N. S. Shaler.</span> Aspects of the Earth. Scribners, New York, 1889, pp. 344.</p> - -<p class="pex"><span class="smcap">G. de La Noe et Emm. de Margerie.</span> Les Formes du Terrain, Service -Géographique de l’Armée. Paris, 1888, pp. 205, pls. 48.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> Practical Exercises in Physical Geography, with Accompanying -Atlas. Ginn and Co., Boston, 1908, pp. 148, pls. 45.</p> - -<p class="pex"><span class="smcap">John Muir.</span> The Mountains of California. Unwin, London, 1894, pp. 381.</p> - -<p class="p1">Upon the use and interpretation of topographic maps in illustration of -characteristic earth features, the following are recommended:—</p> - -<p class="pex p1"><span class="smcap">R. D. Salisbury</span> and <span class="smcap">W. W. Atwood</span>. The Interpretation of Topographic -Maps, Prof. Pap., 60 U.S. Geol. Surv., pp. 84, pls. 170.</p> - -<p class="pex"><span class="smcap">D. W. Johnson</span> and <span class="smcap">F. E. Matthes</span>. The Relation of Geology to -Topography, in Breed and Hosmer’s Principles and Practice of Surveying, -vol. 2. Wiley, New York, 1908.</p> - -<p class="pex"><span class="smcap">Général Berthaut.</span> Topologie, Étude du Terrain, Service Géographique -de l’Armée. Paris, 1909, 2 vols., pp. 330 and 674, pls. 265.</p> - -<p class="p1">The United States Geological Survey issues free of charge a list of -100 topographic atlas sheets which illustrate the more important physiographic -types. In his “Traité de Géographie Physique”, Professor E. de -Martonne has given at the end of each chapter the important foreign -maps which illustrate the physiographic types there described.</p> - -<p>“The Principles of Geology”, by Sir Charles Lyell, published first in three -volumes, appeared in the years 1830-1833, and may be said to mark the -beginning of modern geology. Later reduced to two volumes, an eleventh -edition of the work was issued in 1872 (Appleton) and may be profitably -read and studied to-day by all students of geology. Those familiar with -the German language will derive both pleasure and profit from a perusal -of Neumayr’s “Erdgeschichte” (2d ed. revised by Uhlig. Leipzig and -Vienna, 2 vols., 1895-1897), and especially the first volume, “Allgemeine -Geologie.” A recent French work to be recommended is Haug’s “Traité -de Géologie” (Paris, 1907).</p> - -<p>Some texts of physical geography may well be consulted, especially -Emm. de Martonne’s “Traité de Géographie Physique.” Colin, Paris, -1909, pp. 910, pls. 48, and figs. 396.</p> - -<p class="p2"><span class="smcap">Note.</span> An explanatory list of abbreviations used in the reading references -follows the List of Illustrations.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_8" id="Page_8">[8]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER II</h2> - -<p class="pch">THE FIGURE OF THE EARTH</p> - -<p><b>The lithosphere and its envelopes.</b>—The stony part of the -earth is known as the <i>lithosphere</i>, of which only a thin surface -shell is known to us from direct observation. The relatively unknown -central portion, or “core”, is sometimes referred to as the -centrosphere. Inclosing the lithosphere is a water envelope, the -<i>hydrosphere</i>, which comprises the oceans and inland bodies of -water, and has a mass <sup>1</sup>/<sub>4540</sub> that of the lithosphere. If uniformly -distributed, the hydrosphere would cover the lithosphere to the -depth of about two miles, instead of being collected in basins as it -now is. Though apparently not continuous, if we take into account -the zone of underground water upon the continents, the hydrosphere -may properly be considered as a continuous film about the -lithosphere. It is a fact of much significance that all the ocean -basins are connected, so that the levels are adjusted to furnish a -common record of deposits over the entire surface that is sea-covered.</p> - -<p>Enveloping the hydrosphere is the gaseous envelope, the <i>atmosphere</i>, -with a mass <sup>1</sup>/<sub>1200000</sub> that of the lithosphere. The atmosphere -is a mixture of the gases oxygen and nitrogen in parts by -volume of one of the former to four of the latter, with a relatively -small percentage of carbon dioxide. Locally, and at special -seasons, the atmosphere may be charged with relatively large -percentages of water vapor; and we shall see that both the carbon -dioxide and the vapor contents are of the utmost importance in -geological processes and in the influence upon climate. Unlike -the water which composes the hydrosphere, the gases of the -atmosphere are compressible. Forced down by the weight of -superincumbent gas, the layers of the atmosphere at the level of -the sea sustain a pressure of about fifteen pounds to the square -inch; but this pressure steadily decreases in ascending to higher -levels. From direct instrumental observation, the air has now<span class="pagenum"><a name="Page_9" id="Page_9">[9]</a></span> -been investigated to a height of more than twelve miles from the -earth’s surface.</p> - - -<p><b>The evolution of ideas concerning the earth’s figure.</b>—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.</p> - -<p>The ancient Hebrews and the Babylonians were dwellers of the -desert, and in the mountains which bounded their horizon they -saw the confines of the earth. Pushing at last westward beyond -the mountains, they found the Mediterranean, and thus arrived at -the view that the earth was a disk with a rim of mountains which -was floated upon water. The rare but violent rainfalls to which -they were accustomed—the desert cloudburst—further led them -to the belief that the mountain rim was continued upward in a -dome or firmament of transparent crystal upon which the heavenly -bodies were hung and from which out of “windows of heaven” -the water “which is above the earth” was poured out upon the -earth’s surface. Fantastic as this theory may seem to-day, it -was founded upon observation, and it well illustrates the dangers -of reasoning from observation within too limited a field.</p> - -<p>As soon as men began to sail the sea, it was noticed that the -water surface is convex, for the masts of ships were found to remain -visible long after their hulls had disappeared below the horizon. -It is difficult to say how soon the idea of the earth’s rotundity was -acquired, but it is certainly of great antiquity. The Dominican -monk Vincentius of Beauvais, in a work completed in 1244, declared -that the surfaces of the earth and the sea were both spherical. -The poet Dante made it clear that these surfaces were one, and -in his famous address upon “The Water and the Land”, which -was delivered in Verona on the 20th of January, 1320, he added -a statement that the continents rise higher than the ocean. His -explanation of this was that the continents are pulled up by the -attraction of the fixed stars after the manner of attraction of -magnets, thus giving an early hint of the force of gravitation.</p> - -<p>The earth’s rotundity may be said to have been first proven -when Magellan’s ships in 1521 had accomplished the circumnavigation -of the globe. Circumnavigation, soon after again carried<span class="pagenum"><a name="Page_10" id="Page_10">[10]</a></span> -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.</p> - -<p><b>The oblateness of the earth.</b>—Every schoolboy is to-day familiar -with the fact that the earth departs from a perfect spherical -figure by being flattened at the ends of its axis of rotation. The -polar diameter is usually given as <sup>1</sup>/<sub>299</sub> 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.</p> - -<div class="floatright"> - <img src="images/ill-050a.jpg" width="250" height="385" id="f1" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 1.</span>—Diagrams to afford -a correct impression of the -measure of the inequalities -upon the earth’s surface compared -to the earth’s radius. -The shell represented in <i>b</i> is -<sup>1</sup>/<sub>100</sub> of the earth’s radius, and -in <i>a</i> this zone is magnified -for comparison with surface -inequalities.</p> -</div></div> - -<p>The oblateness of the geoid is well understood from accepted -hypotheses to be the result of the once more rapid rotation of the -planet when its materials were more plastic, and hence more responsive -to deformation. An elastic hoop rotating rapidly about -an axis in its plane appears to the eye as a solid, and becomes -flattened at the ends of its axis in proportion as the velocity of -rotation is increased. Like the earth, the other planets in the -solar system are similarly oblate and by amounts dependent on the -relative velocities of rotation.</p> - -<p>The departure of the geoid from the spherical surface, owing to -its oblateness, is so small that in the figures which we shall use for -illustration it would be less than the thickness of a line. Since it -is well recognized and not important in our present consideration, -we shall for the time being speak of the figure of the earth in terms -of departures from a standard spherical surface.</p> - -<p><b>The arrangement of oceans and continents.</b>—There are other -departures from a spherical surface than the oblateness just referred -to, and these departures, while not large, are believed to be -full of significance. Lest the reader should gain a wrong impression -of their magnitude, it may be well to introduce a diagram -drawn to scale and representing prominent elevations and depressions -of the earth (<a href="#f1">Fig. 1</a>).</p> - -<p>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<span class="pagenum"><a name="Page_11" id="Page_11">[11]</a></span> -are considering is the naked surface of the rock. In a broad way, -the earth’s shape will be given by the arrangement of the oceans -and the continents. As -soon as we take up the -study of this arrangement, -we find that quite significant -facts of distribution -are disclosed.</p> - -<div class="floatleft"> - <img src="images/ill-050b.jpg" width="250" height="155" id="f2" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 2.</span>—Map on Mercator’s projection to show the -reciprocal relation of the land and sea areas (after -Gregory and Arldt).</p> -</div></div> - -<p>One of the most significant -facts involved in the -distribution of land and -sea, is a concentration of -the land areas within the -northern and the seas -within the southern hemisphere. -The noteworthy -exception is the occurrence -of the great and high -Antarctic continent centered -near the earth’s -south pole; and there are extensions of the northern -continent as narrowing land masses to the southward -of the equator. Hardly less significant than the existence -of land and water hemispheres is the reciprocal -or antipodal distribution of land and sea (<a href="#f2">Fig. 2</a>). -A third fact of significance is a dovetailing together of sea and -land along an east-and-west -direction. -While the seas are -generally <span class="font">A</span>-shaped -and narrow northward, -the land masses -are <span class="font">V</span>-shaped and narrow -southward, <i>but -this occurs mainly in -the southern hemisphere</i>. -Lastly, there -is some indication of -a belt of sea dividing<span class="pagenum"><a name="Page_12" id="Page_12">[12]</a></span> -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.</p> - -<div class="floatleft"> - <img src="images/ill-051.jpg" width="250" height="199" id="f3" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 3.</span>—The form toward which the figure of the earth -is tending, a tetrahedron with symmetrically truncated -angles.</p> -</div></div> - -<p><b>The figure toward which the earth is tending.</b>—Thus far in -our discussion of the earth’s figure we have been guided entirely -by the present distribution -of land and -water. There are, -however, depressions -upon the surface -of the land, in -some cases extending -below the level -of the sea, which are -not to-day occupied -by water. By far -the most notable of -these is the great -Caspian Depression, -which with its extension -divides central -and eastern Asia -upon the east from Africa and Europe upon the west. This -depression was quite recently occupied by the sea, and when -added to the present ocean basins to indicate depressions of the -lithosphere, it shows that the earth’s figure departs from the -standard spheroid <i>in the direction</i> of the form represented in -<a href="#f3">Fig. 3</a>. This form approximates to a tetrahedron, a figure bounded -by four equal triangular faces, here with symmetrically truncated -angles. Of all regular figures with plane surfaces the tetrahedron -has the smallest volume for a given surface, and it presents moreover -a reciprocal relation of projection to depression. Every -line passing through its center thus finds the surface nearer than -the average distance upon one side and correspondingly farther -upon the other (<a href="#f4">Fig. 4</a>).</p> - -<p><b>Astronomical <i>versus</i> geodetic observations.</b>—Confirmation of -the conclusions arrived at from the arrangement of oceans and<span class="pagenum"><a name="Page_13" id="Page_13">[13]</a></span> -continents has been secured in other fields. It was pointed out -that the earth’s oblateness was proven by comparison of the -measured degrees of latitude upon the earth’s surface in lower and -higher latitudes, the degree being found longer as the pole is -approached. Any variation from the spherical surface must obviously -increase the size of the measured degree of latitude in -proportion to the departure from the standard form, and so -the tetrahedral figure with one of its angles at the south pole -will require that the degrees -of latitude be longer in the -southern than they are in the -northern hemisphere. This -has been found by measurement -to be the case, and the -result is further confirmed by -pendulum studies upon the -distribution of the earth’s attraction -or gravity. If less of -the mass of the earth is concentrated -in the southern -hemisphere, its attraction as -measured in vibrations of -the pendulum should be correspondingly -smaller.</p> - -<div class="floatright"> - <img src="images/ill-052.jpg" width="250" height="248" id="f4" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 4.</span>—A truncated tetrahedron, showing -how the depression upon one side of the center -is balanced by the opposite projection.</p> -</div></div> - -<p>Other confirmations of the tetrahedral figure of the earth have -been derived from a comparison of astronomical data, which assume -the earth to be a perfect spheroid, with geodetic measurements, -which are based upon direct measurements. Thus the arc measured -in an east-and-west direction across Europe revealed a different -curvature near the angle of the tetrahedral figure from what -was found farther to the eastward.</p> - - -<p><b>Changes of figure during contraction of a spherical body.</b>—If -we inquire why the earth in cooling should tend to approach the -tetrahedral figure, an answer is easily found. When formed, -the earth appears to have been a but slightly oblate spheroid, -or practically a sphere—the shape which of all incloses the -most space for a given surface. Cooled and solidified at the surface -to the temperature of the surrounding air, and the core -still hot and continuing to lose heat, the core must continue to<span class="pagenum"><a name="Page_14" id="Page_14">[14]</a></span> -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.</p> - -<p>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.</p> - -<p>The earth’s departure from the spheroid in the direction of the -modified tetrahedron is, as we have seen, no hypothesis, but observed -fact revealed in (1) the concentration of the land about -a central ocean in the northern hemisphere; in (2) the antipodal -relation of the land to the water areas, and in (3) the threefold -subdivision of the surface into north and south belts by the two -greater oceans and the Caspian Depression.</p> - -<p><b>The earlier figures of the earth.</b>—The manner in which continent -and ocean are dovetailed into each other in an east-and-west -direction has been generally adduced as additional evidence for -the tetrahedral figure as above described. Closer examination -shows that instead of being in harmony with this figure, it indicates -a departure from it, and, as we shall see, a significant departure -which undoubtedly has its origin in the earlier history of -the planet. The mediterranean seas of both the eastern and the -western hemispheres likewise interfere with the perfection of the -tetrahedral figure and require an explanation.</p> - -<p>Let us then examine in outline the past history of the world -with reference especially to the evolution of the continents and -to the times and the manners of surface change. It is now well -known that there have been three major periods of great deformation -of the earth’s shell. The first of these of which we have<span class="pagenum"><a name="Page_15" id="Page_15">[15]</a></span> -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.</p> - -<p>Before the earliest of these great changes, the earth appears to -have approximated in its figure somewhat closely to the ideal -spheroid, for it was everywhere enveloped in the hydrosphere as a -universal ocean. Toward the close of this period came the adjustments -which brought the lithosphere to protrude through the -hydrosphere in shield-like continents whose distribution, as shown -by the rocks of this period, is of great significance. Within the -northern hemisphere rose three land shields spaced at nearly -equal intervals and at nearly equal distances from the northern -pole. One of these was centered where now is Hudson Bay, -another about the present Baltic Sea, and the relics of the third -are found in northeastern Siberia. These earliest continents -have been referred to as the Laurentian, Baltic, and Angara shields. -Within the southern hemisphere shields appear to have developed -in somewhat similar grouping, namely, in South America, in South -Africa, and in Australia (<a href="#f3">Figs. 3</a> and <a href="#f5">5</a>).</p> - -<div class="figcenter"> - <img src="images/ill-054.jpg" width="400" height="138" id="f5" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 5.</span>—Approximations to earlier and present figures of the earth.</p> -</div></div> - -<p>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<span class="pagenum"><a name="Page_16" id="Page_16">[16]</a></span> -elevation and wrinkling of these deposits into marginal mountain -ranges.</p> - - -<p><b>The continents and oceans which arose at the close of the -Paleozoic era.</b>—At the close of the second great era in the recorded -history of the earth, the now somewhat enlarged continents were -profoundly altered during a series of convulsive movements within -the surface shell of the lithosphere. When these convulsions were -over, there was a new disposition of land and sea, but one quite -different from the present arrangement. Instead of being extended -in north-south belts, as they are at present, the continents -stretched out in broad east-west zones, one in the northern and -the other in the southern hemisphere. To the broad southern -continent of which so little now remains, the name “Gondwana -Land” has been given, and to the sea which divided the northern -from the southern continent the name “Ocean of Tethys.” The -northern continent stretched across the site of the present Atlantic -Ocean as the “North Atlantis”, its northern shore to the westward -being somewhat farther south than the present northern -coast of North America, since life forms migrated in the northern -ocean from the site of Behring Sea to that of the present -North Atlantic.</p> - -<p>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 <a href="#f5">Fig. 5</a> (middle). In this figure two truncated tetrahedrons -are joined in a common plane of contact which may be -described as the twin plane. This medial depression upon the -lithosphere was occupied by the intercontinental sea, the Ocean of -Tethys.</p> - -<p>Near the close of this second great era of the earth’s continental -history, crustal convulsions, which were perhaps the most -remarkable in the entire record, resulted in the almost complete -disappearance of the southern continent and a concentration of -the land within the northern hemisphere as a somewhat interrupted -belt surrounding a central polar ocean (<a href="#f3">Figs. 3</a> and <a href="#f5">5</a>).</p> - -<p>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<span class="pagenum"><a name="Page_17" id="Page_17">[17]</a></span> -seas, are accounted for. The <span class="font">V</span>-shaped continent extensions -and the <span class="font">A</span>-shaped oceans of the southern hemisphere (<a href="#f2">Fig. 2</a>) may -likewise be considered as relics of the now largely submerged tetrahedron -of the southern hemisphere, since this had its apex to the -northward (<a href="#f6">Fig. 6</a>).</p> - -<div class="floatright"> - <img src="images/ill-056.jpg" width="250" height="206" id="f6" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 6.</span>—Diagrams for comparison of shore lines upon -tetrahedrons which have an angle, the first at the south -and the second at the north.</p> -</div></div> - -<p>Thus we see that the lithosphere can scarcely be regarded as a -perfect spheroid, since in the course of geologic ages it has undergone -successive departures -from this -original form. In -its present state it -has been described -as tetrahedral, -though we must -keep in mind that -the sharp angles -of that figure are -deeply truncated. -The soundings -first by Nansen -and more recently -by Peary in the -Arctic basin, far -to the north of the -continental border, -showed that this depression is characterized by profound -depths, and so have afforded confirmation of the tetrahedral figure. -To match this depression at the northern extremity of the -earth’s axis, a high continent reaching to elevations in excess of -10,000 feet has been penetrated by Sir Ernest Shackleton at the -opposite extremity of this polar diameter. Considering its size -and its elevation, the Antarctic continent with its glacier mantle -is the largest protuberance upon the surface of the lithosphere.</p> - -<p>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<span class="pagenum"><a name="Page_18" id="Page_18">[18]</a></span> -outline perpendicular to this face is imperfectly heart-shaped or -like a flattened “peg top.”</p> - -<div class="floatleft"> - <img src="images/ill-057a.jpg" width="250" height="153" id="f7" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 7.</span>—The continents with submerged portions -added (after Gilbert).</p> -</div></div> - -<p><b>The flooded portions of the present continents.</b>—We are accustomed -to think of the continents as ending at the shores of the -oceans. If, however, we -are to regard them as -platforms which rise -from the ocean depressions, -their margins -should be considerably -extended, for a submerged -shelf now practically -surrounds all the -continents to a nearly -uniform depth of 100 -fathoms or 600 feet. -The oceans thus more than fill their basins and may be thought of -as spilling over upon the continents. In <a href="#f7">Fig. 7</a>, the submerged portions -of the continents have been joined to those usually represented, -thus adding about 10,000,000 square miles to their area, and giving -them one third, instead of one fourth, of the lithosphere surface.</p> - -<div class="figcenter"> - <img src="images/ill-057b.jpg" width="400" height="178" id="f8" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 8.</span>—Diagram to indicate the altitude of different parts of the -lithosphere surface.</p> -</div></div> - -<p><b>The floors of the hydrosphere and atmosphere.</b>—The highest -altitudes upon the continents and the profoundest deeps of the -ocean are each removed about 30,000 feet, or nearly 6 miles, -from the level of the sea. In comparison with the entire surface -of the lithosphere, these extremes of elevation represent such -small areas as to be almost inappreciable. Only about <sup>1</sup>/<sub>80</sub> of the<span class="pagenum"><a name="Page_19" id="Page_19">[19]</a></span> -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 (<a href="#f8">Fig. 8</a>). 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.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter II</span></p> - -<p>On the evolution of ideas concerning the earth’s figure:—</p> - -<p class="pex"><span class="smcap">Suess.</span> The Face of the Earth (Clarendon Press, 1906), vol. 2, Chapter 1.</p> - -<p class="pex"><span class="smcap">v. Zittel.</span> History of Geology and Paleontology (Walter Scott, London, -1901), Chapters 1-2.</p> - -<p class="p1">The departure of the spheroid toward the tetrahedron:—</p> - -<p class="pex"><span class="smcap">W. Lowthian Green.</span> Vestiges of the Molten Globe, Part 1. London, 1875. -(Now a rare work, but it contains the original statement of the idea.)</p> - -<p class="pex"><span class="smcap">J. W. Gregory.</span> The Plan of the Earth and Its Causes, Geogr. Jour., -vol. 13, 1899, pp. 225-251 (the best general statement of the arguments -for a tetrahedral form).</p> - -<p class="pex"><span class="smcap">W. Prinz.</span> L’échelle reduite des expériences géologiques, Bull. Soc. Belge -d’Astronomie, 1899.</p> - -<p class="pex"><span class="smcap">B. K. Emerson.</span> The Tetrahedral Earth and Zone of the Intercontinental -Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pls. 9-14.</p> - -<p class="pex"><span class="smcap">M. P. Rudski.</span> Physik der Erde (Tauchnitz, Leipzig, 1911), Chapters -1-3 (the best discussion of the geoid from the purely mathematical -standpoint, so far as the spheroid is concerned).</p> - -<p class="p1">The earlier figures of the earth:—</p> - -<p class="pex"><span class="smcap">Th. Arldt.</span> Die Entwicklung der Kontinente und ihrer Lebewelt. Engelmann, -Leipzig, 1907. (Contains a valuable series of map plates, -showing the probable boundaries of the continents in the different -geological periods).</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_20" id="Page_20">[20]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER III</h2> - -<p class="pch">THE NATURE OF THE MATERIALS IN THE LITHOSPHERE</p> - -<p><b>The rigid quality of our planet.</b>—For a long time it was supposed -that the solid earth constituted a crust only which was -floated upon a liquid interior. This notion was clearly an outgrowth -of the then generally accepted Laplacian hypothesis of -the origin of the universe, which assumed fluid interiors for the -planets, the crust being suggested by the winter crust of frozen -water upon the surface of our inland lakes. To-day the nebular -hypothesis in the Laplacian form is fast giving place to quite -different conceptions, in which solid particles, and not gaseous -ones, are conceived to have built up the lithosphere. The analogy -with frozen water has likewise been abandoned with the discovery -that frozen rock, instead of floating, sinks in its molten equivalent.</p> - -<p>Yet even more cogent arguments have been brought forward -to show that whatever may be the state of aggregation within the -earth’s core—and it may be different from any now known to -us—it nevertheless has many of the properties recognized as -belonging to solid and rigid bodies. Provisionally, therefore, we -may regard the earth’s core as rigid and essentially solid. It was -long ago pointed out by the late Lord Kelvin that if our lithosphere -were not more rigid than a ball of glass of the same size, it -would be constantly passing through periodic six-hourly distortions -of great amplitude in response to the varying attractions of the -moon. An equally striking argument emanating from the same -high authority is furnished by the well-known egg-spinning demonstration. -For illustration, Kelvin was accustomed to take two -eggs, one boiled and the other raw, and attempt to spin them -upon their ends. For the boiled, and essentially solid, egg this is -easily accomplished, but internal friction of the liquid contents of -the raw egg quickly stops any rotary motion which is imparted to -it. Upon the same grounds it is argued that had the earth’s -interior possessed the properties of a liquid, rotation must long -since have ceased.</p> - -<p><span class="pagenum"><a name="Page_21" id="Page_21">[21]</a></span></p> - -<p>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.</p> - - -<p><b>Probable composition of the earth’s core.</b>—In deriving views -concerning the nature of the earth’s interior we are greatly aided -by astronomical studies. The common origin long ago indicated -for the planets of the solar system and the sun has been confirmed -by the analysis of light with the aid of the spectroscope. It has -thus been found that the same chemical elements which we find in -the earth are present also in the sun and in the other stellar bodies. -Again, the group of planets of the solar system which are nearest -to the sun—Mercury, Venus, the Earth, and Mars—have each -a high density, all except Mars, the most distant, having specific -gravities very closely 5½, that of Mars being about 4. This -average specific gravity is also that of the solid bodies, the so-called -meteorites, which reach the surface of our planet from the surrounding -space. Yet though the earth as a whole is thus found -to have a specific gravity five and a half times that of water, its -surface shell has an average density of less than half this value, -or 2.7.</p> - -<p>The study of meteorites has given us a possible clew to the -nature of the earth’s interior; for when both terrestrial and -celestial rock types are classified and placed in orderly arrangement, -it is found that the chemical elements which compose the -two groups are identical, and that these are united according to -the same physical and chemical laws. No new element has been -discovered in the one group that has not been found in the other, -and though some compounds of these elements, the minerals, occur -in the earth’s crust that have not been found in meteorites,<span class="pagenum"><a name="Page_22" id="Page_22">[22]</a></span> -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 (<a href="#f9">Fig. 9</a>). It is found, however, -that the commonest of the minerals in the earth’s shell are -absent from meteorites, as the commoner constituents of meteorites -are wanting in the earth’s crust. This observation would go -far to show that we may in the two cases be examining different -portions of quite similar bodies; and this view is strikingly confirmed -when the rocks of the two groups are arranged in the order -of their densities (<a href="#f9">Fig. 9</a>).</p> - -<div class="figcenter"> - <img src="images/ill-061.jpg" width="400" height="349" id="f9" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 9.</span>—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.; <i>a</i>, granites and rhyolites; <i>b</i>, syenites and trachytes; -<i>c</i>, diorites and andesites; <i>d</i>, gabbros and basalts; <i>e</i>, ultra-basic rocks; <i>f</i>, basic -inclosures in basalt, etc.; <i>g</i>, iron basalts of west Greenland; <i>h</i>, iron masses of -Ovifak, west Greenland; <i>a’-d’</i>, meteorites in order of density (after Judd).</p> -</div></div> - -<p>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.</p> - -<p><span class="pagenum"><a name="Page_23" id="Page_23">[23]</a></span></p> - - -<p><b>The earth a magnet.</b>—The denser, and likewise the more -common, of the meteorite rocks—the so-called meteoric irons—are -composed almost entirely of the elements iron, nickel, and -cobalt. Such aggregates are not known as yet from terrestrial -sources, although transitional types appear to exist upon the -island of Disco off the west coast of Greenland. If it were possible -to explore the earth’s interior, would such combinations of -the iron minerals be encountered? Apart from the surprising -velocity of transmission of earthquake waves, the strongest argument -for an iron core to the lithosphere is found in the magnetic -property of the earth as a whole. The only magnetic elements -known to us are those of the heavy meteorites—iron, nickel, -and cobalt,—and the earth is, as we know, a great magnet whose -northern pole in British America and whose southern pole in -Antarctica have at last been visited by Amundsen and David, -respectively. The specific gravity of iron is 7.15, and those of -nickel and cobalt, which in the meteorites are present in relatively -small amounts, are 7.8 and 7.5, respectively. Considering that -the surface shell of the earth has a specific gravity of 2.7, these -values must be regarded as agreeing well with the determined -density of the earth (5.6) and the other planets of its group (Mercury -5.7, Venus 5.4, Mars 4).</p> - - -<p><b>The chemical constitution of the earth’s surface shell.</b>—The -number of the so-called chemical elements which enter into the -earth’s composition is more than eighty, but few of these figure -as important constituents of the portion known to us. Nearly -one half of the mass of this shell is oxygen, and more than a quarter -is silicon. The remaining quarter is largely made up of aluminium, -iron, calcium, magnesium, and the alkalies sodium and potassium, -in the order named. These eight constituent elements are thus -the only ones which play any important rôle in the composition -of the earth’s surface shell. They are not found there in the free -condition, but combined in the definite proportions characteristic -of chemical compounds, and as such they are known as <i>minerals</i>.</p> - - -<p><b>The essential nature of crystals.</b>—A crystal we are accustomed -to think of as something transparent bounded by sharp -edges and angles, our ideas having been obtained largely from the -gem minerals. This outward symmetry of form is, however, but -an expression of a power which resides, so to speak, in the heart<span class="pagenum"><a name="Page_24" id="Page_24">[24]</a></span> -or soul of the crystal individual—it has its own structural make-up, -its individuality. No more correct estimates of the comparison -of crystal individualities would be obtained by the study of -outward forms alone of two minerals than would be gained by a -judgment of persons from the cut of their clothing. Too often -this outward dress tells only of the conditions by which both men -and crystals have been surrounded, and but little of the power -inherent in the individual. Many a battered mineral fragment -with little beauty to recommend it, when placed under suitable -conditions for its development, has grown into a marvel of beauty. -Few minerals are so mean that they have not within them this -inherent power of individuality which lifts them above the world -of the amorphous and shapeless.</p> - -<div class="floatleft"> - <img src="images/ill-063.jpg" width="250" height="378" id="f10" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 10.</span>—Comparison of a crystalline -with an amorphous substance when expanded -by heat and when attacked by -acid.</p> -</div></div> - -<p>Just as the real nature of a person is first disclosed by his -behavior under trying circumstances, -so of a crystal it is its -conduct under stress of one sort -or another which brings out -its real character. By way of -illustration let us prepare a -sphere from the mineral quartz—it -matters not whether we -destroy the beautiful outlines of -the crystal or employ a battered -fragment—and then prepare -a sphere of similar size and -shape from a noncrystalline or -amorphous substance like glass. -If now these two spheres be introduced -into a bath of oil and -raised to a higher temperature, -the glass globe undergoes an -enlargement without change of -its form; but the crystal ball -reveals its individuality by expanding -into a spheroid in -which each new dimension is nicely adjusted to this more complex -figure (<a href="#f10">Fig. 10</a>).</p> - -<p>We may, instead of submitting the two balls to the “trial by<span class="pagenum"><a name="Page_25" id="Page_25">[25]</a></span> -fire”, allow each to be attacked by the powerful reagent, hydrofluoric -acid. The common glass under the attack of the acid -remains as it was before, a sphere, but with shrunken dimensions. -The crystal, on the other hand, is able to control the action of the -solvent, and in so doing its individuality is again revealed in a -beautifully etched figure having many curving outlines—it is as -though the crystal had possessed a soul which under this trial has -been revealed. This glimpse into the nature of the crystal, so as -to reveal its structural beauty, is still more surprising when the -crystal is taken from the acid in the -early stages of the action and held -close beneath the eye. Now the little -etchings upon the surface display -each the individuality of the substance, -and joining with their neighbors -they send out a beautifully -symmetrical and entirely characteristic -picture (<a href="#f11">Fig. 11</a>).</p> - -<div class="floatright"> - <img src="images/ill-064.jpg" width="200" height="202" id="f11" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 11.</span>—“Light figure” seen upon -an etched surface of a crystal of -calcite (after Goldschmidt and Wright).</p> -</div></div> - -<p><b>The lithosphere a complex of -interlocking crystals.</b>—To the layman the crystal is something rare -and expensive, to be obtained from -a jeweler or to be seen displayed in -the show cases of the great museums. -Yet the one most striking quality of the lithosphere which -separates it from the hydrosphere and the atmosphere is its crystalline -structure,—a structure belonging also to the meteorite, and -with little doubt to all the planets of the earth group. A snowflake -caught during its fall from the sky reveals all the delicate tracery -of crystal boundary; collected from a thick layer lying upon the -ground, it appears as an intricate aggregate of broken fragments -more or less firmly cemented together. And so it is of the lithosphere, -for the myriads of individuals are either the ruins of former -crystals, or they are grown together in such a manner that crystal -facets had no opportunity to develop.</p> - -<p>Such mineral individuals as once possessed the crystal form may -have been broken and their surfaces ground away by mutual attrition -under the rhythmic beating of the waves upon a shore or in -the continuous rolling of pebbles on a stream bed, until as battered<span class="pagenum"><a name="Page_26" id="Page_26">[26]</a></span> -relics they are piled away together in a bed of sand. Yet -no amount of such rough handling is sufficient to destroy the crystal -individuality, and if they are now surrounded with conditions -which are suitable for their growth, their individual nature again -becomes revealed in new crystal outlines. Many of our sandstones -when turned in the bright sunlight send out flashes of light -to rival a bank of snow in early spring. These bright flashes -proceed from the facets of minute crystals formed about each -rounded grain of the sand, and if we examine them under a lens, -we may note the beauty of line formed with such exactness that -the most delicate instruments can detect no difference between -the similar angles of neighboring crystals (<a href="#f12">Fig. 12</a>).</p> - -<div class="figcenter"> - <img src="images/ill-065.jpg" width="400" height="195" id="f12" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 12.</span>—Battered sand grains which have taken on a new lease of life and have -developed a crystal form. <i>a</i>, a single grain grown into an individual crystal; <i>b</i>, -a parallel growth about a single grain; <i>c</i>, growth of neighboring grains until they -have mutually interfered and so destroyed the crystal facets—the common condition -within the mass of a rock (after Irving and Van Hise).</p> -</div></div> - -<p>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.</p> - - -<p><b>Some properties of natural crystals, minerals.</b>—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,<span class="pagenum"><a name="Page_27" id="Page_27">[27]</a></span> -in the vast number of instances it is the behavior, and not the -appearance, of the mineral substance which is made use of for identification.</p> - -<p>When a mineral is broken under the blow of a hammer, instead -of yielding an irregular fracture, like that of glass, it generally -tends to part along one or more directions so as to leave plane -surfaces. This property of <i>cleavage</i> is strikingly illustrated for -a single direction in the mineral mica, for two directions in feldspar, -and for three directions in calcite or Iceland spar. Other -properties of minerals, such as hardness, specific gravity, luster, -color, fusibility, etc., are all made use of in rough determinations -of the minerals. Far more delicate methods depend upon the -behavior of minerals when observed in polarized light, and such -behavior is the basis of those branches of geological science known -as optical mineralogy and as microscopical petrography. An outline -description of some of the common minerals and the means -for identifying them will be found in appendix A.</p> - - -<p><b>The alterations of minerals.</b>—By far the larger number of -minerals have been formed in the cooling and consequent consolidation -of molten rock material such as during a volcanic eruption -reaches the earth’s surface as lava. Beginning their growth -at many points within the viscous mass, the individual crystals -eventually may grow together and so prevent a development of -their crystal faces.</p> - -<p>Another class of minerals are deposited from solution in water -within the cavities and fissures of the rocks; and if this process -ceases before the cavities have been completely closed, the minerals -are found projecting from the walls in a beautiful lining of crystal—the -<i>Krystallkeller</i> or “crystal cellar.” It is from such -pockets or veins within the rocks that the valuable ores are obtained, -as are the crystals which are displayed in our mineral -cabinets.</p> - -<div class="floatleft"> - <img src="images/ill-067a.jpg" width="150" height="189" id="f13" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 13.</span>—Crystal -of garnet -developed in -a schist with -grains of -quartz included -because -not assimilated.</p> -</div></div> - -<p>There is, however, a third process by which minerals are formed, -and minerals of this class are produced within the solid rock as -a product of the alteration of preëxisting minerals. Under the -enormous pressures of the rocks deep below the earth’s surface, -they are as permeable to the percolating waters as is a sponge -at the surface. Under these conditions certain minerals are -dissolved and their material redeposited after traveling in the<span class="pagenum"><a name="Page_28" id="Page_28">[28]</a></span> -solution, or solution and redeposition of mineral matter may go -on together within the mass of the same rock. One new mineral -may have been produced from the dissolved materials of a number -of earlier species, or several new minerals may -be the result of the alteration of a preëxisting mineral -with a more complex chemical structure. Where -the new mineral has been formed “in place”, it has -sometimes been able to utilize the materials of all -the minerals which before existed there, or it may -have been obliged to inclose within itself those earlier -constituents which it could not assimilate in its own -structure (<a href="#f13">Fig. 13</a>).</p> - -<div class="floatright"> - <img src="images/ill-067b.jpg" width="150" height="208" id="f14" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 14.</span>—A -crystal of augite -within the -mass of a rock -altered in part -to form a rim -of the minerals -hornblende -and -magnetite. -Note the original -outline of -the augite -crystal.</p> -</div></div> - -<p>At other times a crystal which is imbedded in -rock has been attacked upon its surface by the percolating -solutions, and the dissolved -materials have been deposited in place -as a crown of new minerals which steadily widens its -zone until the center is reached and the original -crystal has been entirely transformed (<a href="#f14">Fig. 14</a>). It -is sometimes possible to say -that the action by which -these changes have been -brought about has involved -a nice adjustment of supply -of the chemical constituents -necessary to the formation -of the new mineral or minerals. -In rocks which are -aggregates of several mineral -species, a newly formed -mineral may appear only at -the common margin of certain -of these species, thus showing that -they supply those chemical elements which -were necessary to the formation of the -new substance (<a href="#f15">Fig. 15</a>). 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.</p> - -<div class="floatleft"> - <img src="images/ill-067c.jpg" width="200" height="216" id="f15" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 15.</span>—A new mineral -(hornblende) forming as an -intermediate “reaction -rim” between the mineral -having irregular fractures -(olivine) and the dusty -white mineral (lime-soda -feldspar).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_29" id="Page_29">[29]</a></span></p> - -<p>Near the earth’s surface the carbon dioxide and the moisture -which are present in the atmosphere are constantly changing -the exposed portions of the lithosphere into carbonates, hydrates, -and oxides. These compounds are more soluble than are the -minerals out of which they were formed, and they are also more -bulky and so tend to crack off from the parent mass on which -they were formed. As we are to see, for both of these reasons -the surface rocks of the lithosphere succumb to this attack from -the atmosphere.</p> - -<p>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 <span class="font">S</span>-shaped forms, or they are -parted into one or more fragments which remain imbedded within -the rock.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter III</span></p> - -<p>Theories of origin of the earth:—</p> - -<p class="pex p1"><span class="smcap">Thomson</span> and <span class="smcap">Tait</span>. Natural Philosophy. 2d ed. Cambridge, 1883, -pp. 422.</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin.</span> Chamberlin and Salisbury’s Geology, vol. 2, pp. 1-81.</p> - -<p class="p1">Rigidity of the earth:—</p> - -<p class="pex p1"><span class="smcap">Lord Kelvin.</span> 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, <i>ibid.</i>, pp. 238-272.</p> - -<p class="pex"><span class="smcap">Hobbs.</span> Earthquakes (Appleton, New York, 1907), Chapters xvi and -xvii.</p> - -<p class="p1">Composition of the earth’s core and shell:—</p> - -<p class="pex p1"><span class="smcap">O. C. Farrington.</span> The Preterrestrial History of Meteorites, Jour. -Geol., vol. 9, 1901, pp. 623-236.</p> - -<p class="pex"><span class="smcap">E. S. Dana.</span> Minerals and How to Study Them (a book for beginners -in mineralogy). Wiley, New York, 1895.</p> - -<p class="p1">On the nature of crystals:—</p> - -<p class="pex"><span class="smcap">Victor Goldschmidt.</span> Ueber das Wesen der Krystalle, Ostwalds Annalen -der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_30" id="Page_30">[30]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER IV</h2> - -<p class="pch">THE ROCKS OF THE EARTH’S SURFACE SHELL</p> - -<p><b>The processes by which rocks are formed.</b>—Rocks may be -formed in any one of several ways. When a portion of the molten -lithosphere, so-called <i>magma</i>, cools and consolidates, the product -is <i>igneous</i> rock. Either igneous or other rock may become disintegrated -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 sediment. -Such sediments, whether cemented into a coherent mass -or not, are described as <i>sedimentary</i> or <i>clastic</i> rocks. If the fluid -from which they were deposited was the atmosphere, they are -known as <i>subaërial</i> or <i>eolian</i> sediments; but if water, they are -known as <i>subaqueous</i> deposits. Still another class are ice-deposited -and are known as <i>glacial</i> deposits.</p> - -<div class="floatleft"> - <img src="images/ill-069.jpg" width="250" height="168" id="f16" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 16.</span>—Laminated structure of sedimentary rock, -Western Kansas (after a photograph by E. S. -Tucker).</p> -</div></div> - -<p>But, as we have learned, rocks may undergo transformations -through mineral alteration, in which case they are known as -<i>metamorphic</i> rocks. -When these changes -consist chiefly in the -production of more -soluble minerals at -the surface, accompanied -by thorough -disintegration, due -to the direct attack -of the atmosphere, -the resulting rocks -are called <i>residual</i> -rocks.</p> - -<p><b>The marks of origin.</b>—Each -of the -three great classes of rocks, the igneous, sedimentary, and metamorphic, -is characterized by both coarser and finer structures, in -the examination of which they may be identified. The igneous<span class="pagenum"><a name="Page_31" id="Page_31">[31]</a></span> -rocks having been produced from magmas, which are essentially -homogeneous, are usually without definite directional structures -due to an arrangement of their constituents, and are said to have -a <i>massive</i> 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 fragments -during intermediate periods. They are therefore more or -less banded, and are said to have a <i>bedded</i> or <i>laminated</i> structure -(<a href="#f16">Fig. 16</a>).</p> - -<p>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 transportation -and deposition of sediments, for it is characteristic of the -sedimentary rocks that their constituent grains are well rounded. -Eolian sediments have usually more perfectly rounded grains than -subaqueous deposits.</p> - -<p>Glacial deposits, if laid down directly by the ice, are unstratified, -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 <i>fluvio-glacial</i> deposits.</p> - - -<p><b>The metamorphic rocks.</b>—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 -constituent grains become rounded off by a sort of peripheral -crushing or granulation. Metamorphic rocks are therefore -characterized by an imperfectly banded structure described as -<i>schistosity</i> or <i>gneiss banding</i>, 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 determine,<span class="pagenum"><a name="Page_32" id="Page_32">[32]</a></span> -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.</p> - - -<p><b>Characteristic textures of the igneous rocks.</b>—In addition to -the massiveness of their general aspect and the angular boundaries -of their constituents, there are many additional textures -which are characteristic of the igneous rocks. While those that -have consolidated below the earth’s surface, the <i>intrusive</i> 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 surface. -Magmas which thus reach the surface of the earth are described -as <i>lavas</i>, and the rocks produced by their consolidation -are <i>extrusive</i> or <i>volcanic</i> 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 <i>vesicular</i>, <i>scoriaceous</i>, or <i>pumiceous</i> -texture.</p> - -<p>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 consolidated, -these earlier crystals lie scattered within a <i>groundmass</i> -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 <i>porphyritic</i> (<a href="#f17">Fig. 17 <i>b</i></a>). Should the lava arrive at -the surface before any crystals have been generated and consolidate -rapidly as a rock glass, its texture is described as <i>glassy</i> -(<a href="#f17">Fig. 17 <i>c</i></a>).</p> - -<p>When the crystals of the earlier generation are numerous and<span class="pagenum"><a name="Page_33" id="Page_33">[33]</a></span> -needle-like in form, as is very often the case, they arrange themselves -“end on” during the rock flow, so that when consolidation -has occurred, the rock has a kind of puckered lamination which -is the characteristic of the <i>fluxion</i> or <i>flow</i> texture. This texture -has sometimes been confused with the lamination of the sedimentary -rocks, so that wrong conclusions have been reached -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 -<i>spherulitic</i> texture, which is nowhere better displayed than in the -beautiful glassy lavas of Obsidian Cliff in the Yellowstone National -Park.</p> - -<div class="figcenter"> - <img src="images/ill-072.jpg" width="400" height="179" id="f17" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 17.</span>—Characteristic textures of igneous rocks. <i>a</i>, granitic texture characteristic -of the deep-seated intrusive rocks; <i>b</i>, porphyritic texture characteristic of the extrusive -and of the near-surface intrusive rocks; <i>c</i>, glassy texture of an extrusive rock.</p> -</div></div> - -<p>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 <i>granitic</i> texture -(<a href="#f17">Fig. 17 <i>a</i></a>).</p> - - -<p><b>Classification of rocks.</b>—In tabular form rocks may thus be -classified as follows:—</p> - -<table id="t03" summary="t03"> - - <tr> - <td class="tdt4" rowspan="2"><i>Igneous.</i> Massive and with sharply angular grains.</td> - <td rowspan="2"><div class="ftable"> - <img src="images/b70.jpg" width="20" height="70" - alt="" - title="" /> -</div></td> - <td class="tdt4"><i>Intrusive.</i> Granitic or porphyritic texture.</td> - </tr> - - <tr> - <td class="tdt4"><i>Extrusive.</i> Glassy or porphyritic texture; -often also with vesicular, scoriaceous, pumiceous, fluxion, or spherulitic textures.</td> - </tr> - - <tr> - <td class="tdt4" rowspan="4"><i>Sedimentary.</i> Laminate -and with rounded grains.</td> - <td rowspan="4"><div class="ftable"> - <img src="images/b120.jpg" width="20" height="120" - alt="" - title="" /> -</div></td> - <td class="tdt4"><span class="pagenum"><a name="Page_34" id="Page_34">[34]</a></span><i>Subaërial.</i> Sands and loess.</td> - </tr> - - <tr> - <td class="tdt4"><i>Subaqueous.</i> (See below.)</td> - </tr> - - <tr> - <td class="tdt4"><i>Glacial.</i> Coarse, unstratified -deposits with faceted pebbles. Till and tillite.</td> - </tr> - - <tr> - <td class="tdt4"><i>Fluvio-glacial.</i> Stratified sands - and gravels with “worked over” glacial characters.</td> - </tr> - - <tr> - <td class="tdt4" rowspan="2"><i>Metamorphic.</i> Schistose -and with grains either angular or rounded.</td> - <td rowspan="2"><div class="ftable"> - <img src="images/b50.jpg" width="20" height="50" - alt="" - title="" /> -</div></td> - <td class="tdt4"><i>Metamorphic proper.</i> Due to below surface changes.</td> - </tr> - - <tr> - <td class="tdt4"><i>Residual.</i> Disintegrated at or near surface.</td> - </tr> - -</table> - -<p class="p1"><b>Subdivisions of the sedimentary rocks.</b>—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 -<i>chemical</i> deposits, and those which have played a part in the life -process as <i>organic</i> deposits. The organic deposits from vegetable -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:—</p> - -<p class="pc1"><i>Classification of Sediments.</i></p> - -<table id="t04" summary="t04"> - - <tr> - <td class="tdt4" rowspan="4"><i>Mechanical</i></td> - <td rowspan="4"><div class="ftable"> - <img src="images/b140.jpg" width="20" height="140" - alt="" - title="" /> -</div></td> - <td class="tdt3"><i>Subaqueous</i><br />Deposited by water.</td> - <td class="tdt3">Conglomerate, sandstone and shale.</td> - </tr> - - <tr> - <td class="tdt3"><i>Subaërial</i> or <i>Eolian<br />Deposited by wind.</i></td> - <td class="tdt3">Sandstone and loess.</td> - </tr> - - <tr> - <td class="tdt3"><i>Glacial</i><br />Deposited by ice.</td> - <td class="tdt3">Till and tillite.</td> - </tr> - - <tr> - <td class="tdt3"><i>Fluvio-glacial</i><br />Glacier-water deposits.</td> - <td class="tdt3">Sands and gravels.</td> - </tr> - - <tr> - <td class="tdt4" rowspan="2"><i>Chemical</i></td> - <td rowspan="2"><div class="ftable"> - <img src="images/b50.jpg" width="20" height="50" - alt="" - title="" /> -</div></td> - <td class="tdt3">Calcareous tufa</td> - <td class="tdt3">Deposited in springs and rivers.</td> - </tr> - - <tr> - <td class="tdt3">Oölitic limestone</td> - <td class="tdt3">Deposited at the mouths of rivers -between high and low tide.</td> - </tr> - - <tr> - <td class="tdt4" rowspan="2"><i>Organic</i></td> - <td rowspan="2"><div class="ftable"> - <img src="images/b50.jpg" width="20" height="50" - alt="" - title="" /> -</div></td> - <td class="tdt3">Formed of plant remains.</td> - <td class="tdt3">Peats and coals.</td> - </tr> - - <tr> - <td class="tdt3">Formed of animal remains.</td> - <td class="tdt3">Limestones and marls.</td> - </tr> - -</table> - -<p><span class="pagenum"><a name="Page_35" id="Page_35">[35]</a></span></p> - -<p>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 deserts -as the so-called <i>loess</i> (<a href="#f216">Fig. 216</a>), 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.</p> - - -<p><b>The different deposits of ocean, lake, and river.</b>—Of the subaqueous -sediments, there are three distinct types resulting: -(1) from sedimentation in rivers, the <i>fluviatile</i> deposits; (2) from -sedimentation in lakes, the <i>lacustrine</i> deposits; and (3) from sedimentation -in the ocean, <i>marine</i> 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.</p> - -<p>Lakes and oceans have this in common that both are bodies -of standing as contrasted with flowing water; and both are subject -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.</p> - -<p>As regards the limestones, their origin is involved in considerable -uncertainty. Some, like the shell limestone or coquina -of the Florida coast, are an aggregation of remains of mollusks<span class="pagenum"><a name="Page_36" id="Page_36">[36]</a></span> -which live near the border of the sea. Other limestones are deposited -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.</p> - -<p>The marine sediments which are derived from the continents, -the so-called <i>terrigenous</i> 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) <i>littoral</i> or alongshore deposits, which are laid down between -high and low tide levels; (2) <i>shoal water</i> 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 continental -slope. The littoral and shoal water deposits are mainly -gravels and sands, while the offshore deposits are principally -muds or lime deposits.</p> - - -<p><b>Special marks of littoral deposits.</b>—The marks of ripples are -often left in the sand of a beach, and may be preserved in the sandstone -which results from the cementation of such deposits (pl. 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 distinguish -the upper from the lower surface.</p> - -<p>In the finer sands and muds of sheltered tidal flats may be preserved -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.</p> - -<p>Between the tides muds may dry out and crack in intersecting<span class="pagenum"><a name="Page_37" id="Page_37">[37]</a></span> -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 deposits -from the subaërial or continental deposits.</p> - -<p>A variation in the direction of winds of successive storms -may be responsible for the piling up of the beach sand in a peculiar -“plunge and flow” or “cross-bedded” structure, a structure -which is extremely common in littoral deposits, though simulated -in rocks of eolian origin.</p> - - -<p><b>The order of deposition during a transgression of the sea.</b>—Many -shore lines of the continents are almost constantly migrating -either landward or seaward. When the shore line advances -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 <i>transgression</i>. -Though at any moment the littoral, shoal water, and offshore -deposits are each being laid down in a particular zone, it is evident -that each must advance in turn in the direction of the shore -and so be 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 (<a href="#f18">Fig. 18</a>). 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 -<i>basal conglomerate</i>. 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.</p> - -<div class="figcenter"> - <img src="images/ill-076.jpg" width="400" height="66" id="f18" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 18.</span>—Diagram to show the order of the sediments laid down during a transgression -of the sea.</p> -</div></div> - -<p>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.<span class="pagenum"><a name="Page_38" id="Page_38">[38]</a></span> -Later, when cemented together, these become in order, coarser -and finer sandstones, shales, and limestones. The order of superposition, -reading from the bottom to the top, thus gives the order -of decreasing age of the formations.</p> - -<p>A subsequent uplift 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 <i>fossils</i> 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 doctrine -of evolution.</p> - - -<p><b>The basins of earlier ages.</b>—It was the great Viennese geologist, -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 formations -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 mountain -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 uplift.</p> - - -<p><b>The deposits of the deep sea.</b>—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 <i>Challenger</i> expedition -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,<span class="pagenum"><a name="Page_39" id="Page_39">[39]</a></span> -these deposits are the so-called “ooze”, composed of the calcareous -or chitinous parts of algæ 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 countless -myriads. The hard parts of both plant and animal organisms -descend to the bottom and there form the ooze in which are sometimes -found the ear bones of whales and the teeth of sharks.</p> - -<p>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.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter IV</span></p> - - -<p class="pex"><span class="smcap">J. S. Diller.</span> The Educational Series of Rock Specimens collected and -distributed by the United States Geological Survey, Bull. 150 -U. S. Geol. Surv., 1898, pp. 1-400.</p> - -<p class="pex"><span class="smcap">L. V. Pirsson.</span> Rocks and Rock Minerals. Wiley, New York, 1908.</p> - -<p class="pex"><span class="smcap">Sir John Murray.</span> Deep-sea Deposits, Reports of the <i>Challenger</i> -expedition, Chapter iii.</p> - -<p class="pex"><span class="smcap">L. W. Collet.</span> Les dépôts marins. Doin, Paris, 1907 (Encyclopédie -Scientifique).</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_40" id="Page_40">[40]</a></span></p> - -<div class="chapter"> - -<h2>CHAPTER V</h2> - -<p class="pch">CONTORTIONS OF THE STRATA WITHIN THE ZONE OF -FLOW</p> - - -<p><b>The zones of fracture and flow.</b>—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 represented -by the length of such a column, rocks cannot yield to pressure -by fracture, for the opening of a crack implies that the rock -upon either side is strong enough to prevent the walls from closing. -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 <i>zone of flow</i>.</p> - -<div class="floatright"> - <img src="images/ill-080a.jpg" width="200" height="460" id="f19" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 19.</span>—Two intersecting -parallel series -of fractures produced -upon each free surface -of a prismatic -block of stiff molders’ -wax when broken by -compression from the -ends (after Daubrée -and Tresca).</p> -</div></div> - -<p>In contrast, the near-surface zone is called the <i>zone of fracture</i>. -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 <i>zone of fracture and -flow</i>.</p> - - -<p><b>Experiments which illustrate the fracture and flow of solid -bodies.</b>—A prismatic block prepared from stiff molders’ wax, -if crushed between the jaws of a testing machine, yields a system<span class="pagenum"><a name="Page_41" id="Page_41">[41]</a></span> -of intersecting fractures which are perpendicular to the free surfaces -of the block and take two directions each inclined by half -of a right angle to the direction of compression -(<a href="#f19">Fig. 19</a>). 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.</p> - -<p>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 experiment. -The experiment is best devised as -in <a href="#f20">Fig. 20</a>. Here a series of layers having -varying degrees of rigidity is prepared from -beeswax as a base, either stiffened by admixture -of varying proportions of plaster of -Paris, or weakened by the use of Venice turpentine. -Such a series of layers may represent -rocks of as widely different characters as limestone -and shale. The load which is to represent -superincumbent rock is supplied in the -experiment by a deep layer of shot.</p> - -<div class="figcenter"> - <img src="images/ill-080b.jpg" width="400" height="155" id="f20" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 20.</span>—Apparatus to illustrate the folding of strata within the zone of flow -(after Willis).</p> -</div></div> - -<p>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<span class="pagenum"><a name="Page_42" id="Page_42">[42]</a></span> -latter have been protected under an arch of the more competent -layer (pl. 2 A).</p> - - -<p><b>The arches and troughs of the folded strata.</b>—Every series -of folds is made up of alternating arches and troughs. The arches -of the strata the geologist calls <i>anticlines</i> or <i>anticlinal folds</i>, and -the troughs he calls <i>synclines</i> or <i>synclinal folds</i> (<a href="#f21">Fig. 21</a>). 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 <i>monocline</i>.</p> - -<div class="floatleft"> - <img src="images/ill-081a.jpg" width="250" height="109" id="f21" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 21.</span>—Diagrams representing <i>a</i>, an -anticline; <i>b</i>, a syncline; and <i>c</i>, a monocline.</p> -</div></div> - -<p>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 shortening -is comparatively small, and the folds are described as <i>open</i> -(<a href="#f22">Fig. 22 <i>b</i></a>). 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 <i>close</i> (<a href="#f22">Fig. 22 <i>c</i></a>). This -closing up of the folds may continue -until their sides have practically the -same slope, in which case they are said -to be <i>isoclinal</i> (<a href="#f22">Fig. 22 <i>d</i></a>).</p> - -<div class="floatright"> - <img src="images/ill-081b.jpg" width="200" height="227" id="f22" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 22.</span>—A comparison of -folds to express increasing -degrees of crustal shortening -or progressive deformation -within the zone of flow: <i>a</i>, -stratum before folding; <i>b</i>, -open folds; <i>c</i>, close folds; -<i>d</i>, isoclinal folds.</p> -</div></div> - -<p><b>The elements of folds.</b>—Folds must -always be thought of as having extension -in each of the three dimensions -of space (<a href="#f23">Fig. 23</a>), 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 <i>axial plane</i>. The line in which this plane intersects -the arch or the trough is the <i>axis</i>, which may be called the -<i>crestline</i> in an anticline, and the <i>troughline</i> in a syncline.</p> - -<p>In the case of many open folds the axis is practically horizontal,<span class="pagenum"><a name="Page_43" id="Page_43">[43]</a></span> -but in more complexly folded regions this is seldom true. -The departure of the axis from the horizontal is called the <i>pitch</i>, -and folds of this type are described as <i>pitching folds</i> or <i>plunging -folds</i>. 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.</p> - -<div class="figcenter"> - <img src="images/ill-082.jpg" width="400" height="303" id="f23" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 23.</span>—Anticlinal and synclinal folds in strata (after Willis).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-083a.jpg" width="200" height="394" id="f24" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 24.</span>—Diagrams to illustrate -the different shapes of rock folds.</p> -</div></div> - -<p><b>The shapes of rock folds.</b>—By the axial plane each fold is -divided into two parts which are called its <i>limbs</i>, 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 <i>upright</i> or <i>symmetrical folds</i>. The axial plane of the -symmetrical fold is vertical (<a href="#f24">Fig. 24</a>). If this plane is inclined to -the vertical, the folds are <i>unsymmetrical</i>. 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 <i>overturned</i>. -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 <i>recumbent</i>. 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,<span class="pagenum"><a name="Page_44" id="Page_44">[44]</a></span> -though generally less easily, in passing through the Appalachian -Mountains.</p> - -<div class="floatright"> - <img src="images/ill-083b.jpg" width="250" height="181" id="f25" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 25.</span>—Secondary and tertiary flexures superimposed -upon the primary ones.</p> -</div></div> - -<p>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 possess -the shapes of the larger flexures, -and much is therefore to be learned -from their careful study (<a href="#f25">Fig. 25</a>). -It is also quite generally discovered -that parallel planes of ready parting, -which are described as <i>rock cleavage</i>, -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.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 2.</span></p> - -<div class="figcenter"> - <img src="images/ill-084a.jpg" width="400" height="163" id="p2a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Layers compressed in experiments and showing the effect of a competent layer -in the process of folding (after Willis).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-084b.jpg" width="400" height="160" id="p2b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Experimental production of a series of parallel thrusts within closely folded strata -(after Willis).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-084c.jpg" width="400" height="204" id="p2c" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>C.</i> Apparatus to illustrate shearing action within the overturned limb of a fold.</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_45" id="Page_45">[45]</a></span></p> - -<p><b>The overthrust fold.</b>—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 concave -side are crowded -closer together—there -is tension in the former -case and compression -in the latter (<a href="#f26">Fig. 26</a>). -Within an unsymmetrical -or an overturned -fold, the peculiar distortions -in the different -sections of the stratum -are less simple and are -best illustrated by -<a href="#p2c">pl. 2 C</a>. 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 -that those rock particles -which before the bending -were included in the circle -have been moved past each -other in the manner of the -blades of a pair of shears. -Such extreme “shearing” action is thus localized in the underturned -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 probably -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 <i>thrust</i> or <i>overthrust</i>.</p> - -<div class="floatleft"> - <img src="images/ill-086.jpg" width="250" height="93" id="f26" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 26.</span>—A bent stratum to illustrate tension -upon the convex and compression upon the -concave side (after Van Hise).</p> -</div></div> - -<p>In the long series of experiments conducted by Mr. Bailey -Willis of the United States Geological Survey, all the stages between -the overturned fold and the overthrust fold were reproduced. -Where a series of folds was closely compressed, a parallel series of -thrusts developed (<a href="#p2b">pl. 2 B</a>), 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 Scotland, -where the overriding is measured in miles. Near the thrust -planes the rocks show a crushing of the grains, and the planes themselves -are sometimes corrugated and polished by the movement.</p> - - -<p><b>Restoration of mutilated folds.</b>—Since flexuring of the rocks -takes place within the zone of flow at a distance of several miles<span class="pagenum"><a name="Page_46" id="Page_46">[46]</a></span> -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 superincumbent -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.</p> - -<p>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.</p> - - -<p><b>The geological map and section.</b>—The earth’s surface is in -most regions in large part covered with soil or with other incoherent -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 <i>outcropping</i> -or <i>exposure</i>. In a study of the region each such exposure 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 <i>strike</i>—and the inclination of its beds—the -<i>dip</i>. If the outcroppings are sufficiently numerous, and rock -type, strike and dip, may all be determined, the folds of the district -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 <i>geological section</i> (<a href="#f27">Fig. 27</a>). A -map upon which the data have been entered in their correct locations, -either with or without assumptions concerning the covered -areas, is known as a <i>geological map</i>.</p> - -<div class="figcenter"> - <img src="images/ill-088a.jpg" width="400" height="189" id="f27" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 27.</span>—A geological section based upon observations at outcrops, but with -the truncated arches restored.</p> -</div></div> - -<p>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 surface, -will give the directions of continuation of the individual -strata. This strike direction is usually determined at each exposure<span class="pagenum"><a name="Page_47" id="Page_47">[47]</a></span> -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 -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 <i>vice versa</i> (<a href="#f28">Fig. 28 <i>a</i></a>). Upon -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 (<a href="#f28">Fig. 28 <i>b</i></a>).</p> - -<div class="figcenter"> - <img src="images/ill-088b.jpg" width="300" height="140" id="f28" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 28.</span>—Diagram to illustrate the manner of determining the strike of rock beds -at an outcropping. <i>a</i>, a compass which has the cardinal directions in their -natural positions; <i>b</i>, a compass with the east and west initials reversed upon the -dial; <i>c</i>, home-made clinometer in position to determine the dip.</p> -</div></div> - -<p>By the dip is meant the inclination of the stratum at any exposure, -and this must obviously be measured in a vertical plane<span class="pagenum"><a name="Page_48" id="Page_48">[48]</a></span> -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 -<i>clinometer</i>, is merely a simple pendulum which serves as an indicator -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 (<a href="#f28">Fig. 28 <i>c</i></a>), but the geologist’s -compass is always provided with a clinometer attachment -to the dial.</p> - -<div class="floatleft"> - <img src="images/ill-089.jpg" width="250" height="201" id="f29" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 29.</span>—Diagram to show the use -of <span class="font reduct">T</span> symbols to indicate the dip and -strike of outcroppings.</p> -</div></div> - -<p>Since the strike is the intersection of the bedding plane with a -horizontal surface, and the dip is the intersection with that particular -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 <span class="font">T</span> -symbols, the top of the <span class="font">T</span> 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 <span class="font">T</span> 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 (<a href="#f29">Fig. 29</a>). 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 exposures. -To illustrate, in <a href="#f29">Fig. 29</a>, -the symbol marked <i>a</i> represents -an outcrop of limestone, the strike -of which is 50° east of north (N. -50° E.), and the dip of which is -45° southeast. In the same figure -<i>b</i> represents a shale outcrop in horizontal -beds, which have in consequence -a universal strike and a dip of 0°. An exposure of limestone -in vertical beds which strike N. 60° E. is shown at <i>c</i>, etc.</p> - -<div class="floatright"> - <img src="images/ill-090.jpg" width="250" height="145" id="f30" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 30.</span>—Diagram to show how the thickness -of a formation may be obtained from the -angle of the dip and the width of the exposures.</p> -</div></div> - -<p><b>Measurement of the thickness of formations.</b>—When formations -still lie in horizontal beds, we may sometimes learn their<span class="pagenum"><a name="Page_49" id="Page_49">[49]</a></span> -thickness directly either from the depth of borings to the underlying -rock, or by measurements upon steep cañon 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 vertical, -the thickness must be -obtained indirectly from the -width of the exposures and -the angle of the dip. The -factor by which the exposure -width must be multiplied -is known as the sine -of the dip angle (<a href="#f30">Fig. 30</a>), -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.</p> - -<p class="pc1"><i>Natural Sines</i></p> - -<table id="t05" summary="t05"> - - <tr> - <td class="ts10"> </td> - <td class="ts10"> </td> - <td class="ts20" rowspan="8"> </td> - <td class="ts10"> </td> - <td class="ts10"> </td> - <td class="ts20" rowspan="8"> </td> - <td class="ts10"> </td> - <td class="ts10"> </td> - </tr> - - <tr> - <td class="tdr">0°</td> - <td class="tdr">.00</td> - <td class="tdr">35°</td> - <td class="tdr">.57</td> - <td class="tdr">70°</td> - <td class="tdr">.94</td> -</tr> - - <tr> - <td class="tdr">5°</td> - <td class="tdr">.09</td> - <td class="tdr">40°</td> - <td class="tdr">.64</td> - <td class="tdr">75°</td> - <td class="tdr">.97</td> -</tr> - - <tr> - <td class="tdr">10°</td> - <td class="tdr">.17</td> - <td class="tdr">45°</td> - <td class="tdr">.71</td> - <td class="tdr">80°</td> - <td class="tdr">.98</td> -</tr> - - <tr> - <td class="tdr">15°</td> - <td class="tdr">.26</td> - <td class="tdr">50°</td> - <td class="tdr">.77</td> - <td class="tdr">85°</td> - <td class="tdr">1.00</td> -</tr> - - <tr> - <td class="tdr">20°</td> - <td class="tdr">.34</td> - <td class="tdr">55°</td> - <td class="tdr">.82</td> - <td class="tdr">90°</td> - <td class="tdr">1.00</td> -</tr> - - <tr> - <td class="tdr">25°</td> - <td class="tdr">.42</td> - <td class="tdr">60°</td> - <td class="tdr">.87</td> -</tr> - - <tr> - <td class="tdr">30°</td> - <td class="tdr">.50</td> - <td class="tdr">65°</td> - <td class="tdr">.91</td> -</tr> - -</table> - -<div class="figcenter"> - <img src="images/ill-091a.jpg" width="400" height="298" id="f31" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 31.</span>—Combined surface and sectional views of a plunging anticline (after Willis).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_50" id="Page_50">[50]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-091b.jpg" width="400" height="296" id="f32" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 32.</span>—Combined surface and sectional views of a plunging syncline (after Willis).</p> -</div></div> - -<p><b>The detection of plunging folds.</b>—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 <a href="#f31">Figs. 31</a> and <a href="#f32">32</a>, -the first representing a pitching anticline and the second a pitching -syncline. In both these reciprocal cases the strikes of the -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<span class="pagenum"><a name="Page_51" id="Page_51">[51]</a></span> -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 Appalachian -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.</p> - -<div class="figcenter"> - <img src="images/ill-092.jpg" width="400" height="292" id="f33" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 33.</span>—Unconformity between a lower and an upper series of beds upon the coast -of California. Note how the hard layer stands in relief upon the connecting -surface (after Fairbanks).</p> -</div></div> - -<p><b>The meaning of an unconformity.</b>—The rock beds, which are -deposited one above the other during a transgression of the sea, -are usually parallel and thus represent a continuous process of -deposition. Such beds are said to be <i>conformable</i>. 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 <i>unconformable</i> -series, and the break or surface of junction is an <i>unconformity</i> -(<a href="#f33">Fig. 33</a>).</p> - -<p><span class="pagenum"><a name="Page_52" id="Page_52">[52]</a></span></p> - -<p>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 delimits the lower series from the upper is somewhat -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.</p> - -<div class="floatleft"> - <img src="images/ill-093.jpg" width="250" height="236" id="f34" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 34.</span>—Series of diagrams to illustrate in succession the -episodes involved in the historical development of an -angular unconformity. The vertical arrows indicate -direction of movement of the land, and the horizontal -arrows the direction of shore migration.</p> -</div></div> - -<p>In reality, an unconformity between formations must be interpreted -to mean that the lower series is not only 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 -<a href="#f33">Fig. 33</a> may be read as follows (see <a href="#f34">Fig. 34 <i>a-e</i></a>):—</p> - -<p>(<i>a</i>) Deposition -of the lower series -during a transgression -of the sea.</p> - -<p>(<i>b</i>) Continued -subsidence and -burial of the lower -series beneath -overlying sediments, -and flexuring -in the zone of -flow.</p> - -<p>(<i>c</i>) Elevation of -the combined deposits -to and far -above sea level and -removal by erosion -of vast thicknesses -of the upper sediments.</p> - -<p>(<i>d</i>) A new subsidence -of the truncated lower series and deposition of the upper -series across its eroded surface.</p> - -<p>(<i>e</i>) A new elevation of the double series to its present position -above sea level.</p> - -<p><span class="pagenum"><a name="Page_53" id="Page_53">[53]</a></span></p> - -<div class="floatright"> - <img src="images/ill-094.jpg" width="200" height="317" id="f35" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 35.</span>—Types of deceptive or erosional -unconformities.</p> -</div></div> - -<p>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 repeated -operations is so vast as at first to stagger the mind in contemplating -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 <i>angular unconformity</i>. It may be, however, that the lower -series was not so far depressed as to enter the zone of flow, and -that its beds meet those of the upper series with apparent conformity. -Such an unconformity is often extremely difficult to -recognize, and it is described as a <i>deceptive</i> or <i>erosional unconformity</i>.</p> - -<p>With a deceptive unconformity the clew to its real nature is -usually some fact which indicates that the lower series of sediments -had been raised above the -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 furnished -by a basal conglomerate -in the upper series, or some indication -of different resistance of -different rocks of the lower series -to attacks of the atmosphere -upon them (<a href="#f33">Figs. 33</a> and <a href="#f35">35 <i>a-c</i></a>).</p> - -<p>In most cases, at least, the -lowest member of the upper -series will be a different type of -rock from the uppermost member -of the lower series, hence the -frequent occurrence of the discordant -cross bedding in sandstone -should not deceive even the novice into the assumption -of an unconformity.</p> - -<p><span class="pagenum"><a name="Page_54" id="Page_54">[54]</a></span></p> - -<p class="prr"><span class="smcap">Reading References to Chapter V</span></p> - -<p>The zones of fracture and flow:—</p> - -<p class="pex p1"><span class="smcap">C. R. Van Hise.</span> Principles of North American Precambrian Geology, -16th Ann. Rept. U.S. Geol. Surv., 1895, Pt. I, pp. 581-603.</p> - -<p class="pex"><span class="smcap">Bailey Willis.</span> Mechanics of Appalachian Structure, 13th Ann. Rept. -U.S. Geol. Surv., 1893, Pt. II, pp. 217-253.</p> - -<p class="pex"><span class="smcap">A. Daubrée.</span> Études Synthétiques de Géologie Expérimentale. Paris, -1879, pp. 306-328, pl. II.</p> - -<p class="pex"><span class="smcap">W. Prinz.</span> Quelques remarques générales à propos de l’essai de carte -tectonique de la belgique, etc., Bull. Soc. Belge Geol., vol. 18, 1904, -p. 143, pl. V.</p> - -<p class="p1">Analysis of folds:—</p> - -<p class="pex p1"><span class="smcap">Van Hise</span> and <span class="smcap">Willis</span> as above; <span class="smcap">de Margerie</span> et <span class="smcap">Heim</span>; Les dislocations -de l’écorce terrestre (in French and German languages). Zurich, -1888.</p> - -<p class="p1">Geological maps:—</p> - -<p class="pex p1"><span class="smcap">Wm. H. Hobbs.</span> The Mapping of the Crystalline Schists, Jour. Geol., -vol. 10, 1902, pp. 780-792, 858-890.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_55" id="Page_55">[55]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER VI</h2> - -<p class="pch">THE ARCHITECTURE OF THE FRACTURED SUPERSTRUCTURE</p> - -<div class="floatleft"> - <img src="images/ill-096.jpg" width="280" height="353" id="f36" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 36.</span>—A set of master joints developed in shale -upon the shores of Cayuga Lake near Ithaca, -New York (after U. S. G. S.).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-097a.jpg" width="280" height="307" id="f37" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 37.</span>—Diagram to show how sets of master joints -differing in direction by half a right angle may -abruptly replace each other.</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-097b.jpg" width="280" height="343" id="f38" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 38.</span>—Diagram to show the different -combinations of the series composing two -double sets of master joints, and in <i>a</i>, <i>a</i>, <i>a</i> -additional disorderly fractures.</p> -</div></div> - -<p><b>The system of the fractures.</b>—In referring to experiments made -upon the fracture of solid blocks under compression (<a href="#Page_41">p. 41</a>), 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 -of a right angle to the -direction of compression, -and thus perpendicular -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 compression. -From mechanical -considerations involving the resolution of forces with reference -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<span class="pagenum"><a name="Page_56" id="Page_56">[56]</a></span> -attitudes, they are, in -fact, seen to be intersected -by two parallel -series of fractures -which are perpendicular -to the earth’s surface -and to each other -and are described as -<i>joints</i>. 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 -omnipresent double series or -<i>set</i> of joints is the well-known -set of <i>master joints</i>, and very -often it is found developed -practically alone (<a href="#f36">Fig. 36</a>). -Over large areas, the direction -of the set of master joints -may remain practically constant, -or this set may quite -suddenly give place to a similar -set which is, however, -turned through half a right -angle from the first (<a href="#f37">Fig. 37</a>). Not infrequently two -such sets of master joints -are found together bisecting -each other’s angles within the -same rocks, and to them<span class="pagenum"><a name="Page_57" id="Page_57">[57]</a></span> -are sometimes added additional though less perfect series of joint -planes.</p> - -<p>Studied throughout a considerable district, the various series -which make up these two sets of master joints may be seen locally -developed in different combinations as well as in association with -additional fissure planes which are not easily reduced to any simple -law of arrangement -(<a href="#f38">Fig. 38 <i>a</i>, <i>a</i>, <i>a</i></a>). -Only rarely are regular -joint series observed -which do not -stand perpendicular -to the original attitude -of the rock -beds. In a few localities, -however, rectangular -joint sets -have been discovered -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<span class="pagenum"><a name="Page_58" id="Page_58">[58]</a></span> -joints may be such as to defy all attempts at orderly arrangement.</p> - -<div class="figcenter"> - <img src="images/ill-098a.jpg" width="400" height="219" id="f39" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 39.</span>—View on the shore at Holstensborg, West Greenland, to show the subequal -spacing of the joints (after Kornerup).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-098b.jpg" width="250" height="187" id="f40" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 40.</span>—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).</p> -</div></div> - -<p><b>The space intervals of joints.</b>—The same kind of subequal spacing -which characterizes the fractures near the surface of the block -in Daubrée’s experiment (<a href="#f19">Fig. 19</a>, <a href="#Page_41">p. 41</a>) is found simulated by the -rock joints (<a href="#f39">Fig. 39</a>). Such unit intervals between fractures may -be grouped together into larger units which are separated by fractures -of unusual perfection. We may think of such larger space -units as having the smaller ones superimposed upon them (<a href="#f40">Fig. 40</a>).</p> - - -<p><b>The displacements upon joints—faults.</b>—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 without -apparent displacement. Joints are therefore planes of disjunction -only, and not planes of displacement.</p> - -<div class="figcenter"> - <img src="images/ill-099.jpg" width="400" height="147" id="f41" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 41.</span>—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 preparing -the sketch from a photograph.</p> -</div></div> - -<p>Within many districts, however, a displacement may be seen -to have occurred upon certain of the joint planes, and these are -then described as <i>faults</i>. Such displacements of necessity imply -a differential movement of sections or blocks of the earth’s crust, -the so-called <i>orographic blocks</i>, which are bounded by the joint -planes and play individual rôles in the movement. A simple case -of such displacements in rocks intersected by a single set of master -joints is represented in the model of plate 4 C. The most prominent -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<span class="pagenum"><a name="Page_59" id="Page_59">[59]</a></span> -the fault is called its <i>throw</i>. 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 <a href="#f41">Fig. 41</a>, a simple type of -faulting which has taken place in rocks but slightly disturbed from -their original attitude, but intersected by a relatively simple system -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 additional -technical terms (<a href="#f42">Fig. 42</a>). -The inclination of a sloping fault -plane measured against the vertical -is called the <i>hade</i> of the fault. -The <i>total displacement</i> 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.</p> - -<div class="floatright"> - <img src="images/ill-100.jpg" width="200" height="135" id="f42" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 42.</span>—A fault in previously disturbed strata. <i>AB</i>, displacement; -<i>AC</i>, throw; <i>BD</i>, stratigraphic throw; -<i>BC</i>, heave; angle <i>CAB</i>, hade.</p> -</div></div> - -<p><b>Methods of detecting faults.</b>—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 escarpment 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 necessarily -planes of faulting. In many cases the actual displacements -lie buried under loose rock débris near to and paralleling the escarpment, -and in some cases as a result of the erosional processes -working upon alternately hard and soft layers of rock, the escarpment -may later appear upon the downthrown side or limb of the -fault (<a href="#f43">Fig. 43</a>). As an illustration of a fault escarpment, the -façade of El Capitan and many other rock faces of the Yosemite -valley may be instanced.</p> - -<p><span class="pagenum"><a name="Page_60" id="Page_60">[60]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-101a.jpg" width="280" height="353" id="f43" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 43.</span>—Diagrams to show how -an escarpment originally on the -upthrown side of the fault may, -through erosion, appear upon the -downthrown side.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-101b.jpg" width="280" height="325" id="f44" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 44.</span>—A fault plane exhibiting “drag.” -The opening is artificial (after Scott).</p> -</div></div> - -<p>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, whereas -fold structures will long remain -in evidence. Many faults will thus -be overlooked, and too great weight -is likely to be ascribed to the folds -in accounting for the existing attitudes -and positions of the rock -masses. Faults must therefore be -sought out if mistakes of interpretation -are to be avoided.</p> - -<p>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 (<a href="#f42">Fig. 42</a>, <a href="#Page_59">p. 59</a>). 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 <i>fault breccia</i>. 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 produces -little fracture of the -walls, but they slide over each -other in such a manner as to -yield either a smoothly corrugated -or an evenly polished -surface which is described as -“slickensides.” It may be, -however, that during the movement<span class="pagenum"><a name="Page_61" id="Page_61">[61]</a></span> -either one or both of the walls have “dragged”, and so are -curled back in the immediate neighborhood of the fault plane -(<a href="#f44">Fig. 44</a>).</p> - -<p>When, as is quite generally the case, the actual plane of displacement -of a fault is not open to inspection, the movement may -be proven by the observation of -abrupt, as contrasted with gradual, -changes in the strikes and dips -of neighboring exposures (<a href="#f45">Fig. 45</a>); -or by noting that some easily recognized -formation has been -sharply offset in its outcrops (<a href="#f46">Fig. 46</a>).</p> - -<div class="floatright"> - <img src="images/ill-102a.jpg" width="280" height="378" id="f45" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 45.</span>—Map to show how a fault -may be indicated in abrupt changes -of the strike and dip of neighboring -exposures.</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-102b.jpg" width="280" height="193" id="f46" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 46.</span>—A series of parallel -faults indicated by successive -offsets in the course of an -easily recognizable rock formation.</p> -</div></div> - -<p>There are in addition many indications -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 neighboring -formations may terminate -abruptly upon a straight line 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 preëxisting fractures.</p> - - -<p><b>The base of the geological map.</b>—Modern -topographic maps form an important -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 publishing -the results in the form of an atlas of quadrangles. Thus<span class="pagenum"><a name="Page_62" id="Page_62">[62]</a></span> -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 peculiarity -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.</p> - -<p>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 -relief 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.</p> - - -<p><b>The field map and the areal geological map.</b>—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 individual -hue joined to each other like the parts of a child’s picture -puzzle.</p> - -<p>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 judgment<span class="pagenum"><a name="Page_63" id="Page_63">[63]</a></span> -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. Experience -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 geological -map, and every student should be advised to follow this by -practical study of some small area where rocks are found in outcrop.</p> - -<p>Though the published <i>areal geological map</i> represents both fact -and theory, the map maker retains an unpublished <i>field map</i> 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.</p> - - -<p><b>Laboratory models for the study of geological maps.</b>—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 numbered -like the township sections of the national domain (<a href="#f47">Fig. 47</a>). -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 sedimentary -rock types are so constructed as to show the strike and -dip of the beds. (See <a href="#Page_472">Appendix D</a>.)</p> - -<div class="figcenter"> - <img src="images/ill-105a.jpg" width="400" height="205" id="f47" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 47.</span>—Field map prepared from a laboratory table.</p> -</div></div> - -<p><b>The method of preparing the map.</b>—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<span class="pagenum"><a name="Page_64" id="Page_64">[64]</a></span> -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 (<a href="#f47">Fig. 47</a>), which thus represents the facts only, unless -there have been uncertainties in the correlation of exposures or -in determining the position of the bedding plane.</p> - -<div class="figcenter"> - <img src="images/ill-105b.jpg" width="400" height="231" id="f48" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 48.</span>—Areal geological map constructed from the field map of <a href="#f47">Fig. 47</a>, with two -selected geological sections.</p> -</div></div> - -<p>To prepare the areal geological map from the field map, it is -first necessary to fix the <i>boundaries</i> which separate formations at -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<span class="pagenum"><a name="Page_65" id="Page_65">[65]</a></span> -of them—where the facts are more nearly adequate—may correspond. -In <a href="#f48">Fig. 48</a> is represented the areal geological map constructed -from the field map, with the doubtful area at one side left -blank.</p> - -<p>Some conclusions from this map may now be profitably considered. -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 formations -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.</p> - -<p>The dashed lines upon the map (<i>AB</i> and <i>CD</i>) have been selected -as appropriate lines along which to construct geological sections -(<a href="#f48">Fig. 48</a>, below map), and from these sections the <i>exposed</i> thicknesses -of the different formations may be calculated. In one instance -only, that of the conglomerate, can we be sure that this -exposed thickness measures the entire formation.</p> - - -<p><b>Fold <i>versus</i> fault topography.</b>—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 -generally long and straight ridges. More complex flexures, since -they generally 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 fracture<span class="pagenum"><a name="Page_66" id="Page_66">[66]</a></span> -structures are for this reason much more likely to be overlooked, -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.</p> - -<p class="prr"><span class="smcap">Reading References to Chapter VI</span></p> - -<p>Joint systems:—</p> - -<p class="pex p1"><span class="smcap">John Phillips.</span> 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 District. -London, 1836, pp. 90-98.</p> - -<p class="pex"><span class="smcap">Samuel Haughton.</span> On the Physical Structure of the Old Red Sandstone -of the County of Waterford, considered with reference to cleavage, -joint surfaces, and faults, Trans. Roy. Soc. London, vol. 148, -1858, pp. 333-348.</p> - -<p class="pex"><span class="smcap">W. C. Brögger.</span> Spaltenverwerfungen in der Gegend Langesund-Skien, -Nyt Magazin for Naturvidernskaberne, vol. 28, 1884, pp. 253-419.</p> - -<p class="pex"><span class="smcap">Wm. H. Hobbs.</span> The Newark System of the Pomperaug Valley, Connecticut, -21st Ann. Rept. U. S. Geol. Surv., Pt. III, 1901, pp. 85-143.</p> - -<p class="p1">Geological map:—</p> - -<p class="pex p1"><span class="smcap">Wm. H. Hobbs.</span> The Interpretation of Geological Maps, School Science -and Mathematics, vol. 9, 1909, pp. 644-653.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_67" id="Page_67">[67]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER VII</h2> - -<p class="pch">THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: -EARTHQUAKES AND SEAQUAKES</p> - -<p><b>Nature of earthquake shocks.</b>—Man’s belief in the stability of -Mother Earth—the <i>terra firma</i>—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 -disillusionment from the most fundamental of his beliefs. Were -he better advised, the long periods of quiet which separate earthquakes, -and not the lighter shocks which follow all grander disturbances, -would occasion him concern.</p> - -<div class="figcenter"> - <img src="images/ill-108.jpg" width="400" height="312" id="f49" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 49.</span>—View of a portion of the ruins of Messina after the earthquake of -December 28, 1908.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_68" id="Page_68">[68]</a></span></p> - -<p>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.</p> - -<p>During the grander or catastrophic earthquakes, the changes -are indeed terrifying, and have usually been accompanied by losses -to life 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 (<a href="#f49">Fig. 49</a>). Two years later a heavy earthquake resulted -in great damage to cities in Costa Rica (<a href="#f50">Fig. 50</a>), while two years -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.</p> - -<div class="figcenter"> - <img src="images/ill-109.jpg" width="400" height="232" id="f50" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 50.</span>—Ruins of the Carnegie Palace of Peace at Cartago, Costa Rica, destroyed -when almost completed by the great earthquake of May 4, 1910 (after -a photograph by Rear-Admiral Singer, U.S.N.).</p> -</div></div> - -<p>Earthquakes are usually preceded for a brief instant by subterranean -rumblings whose intensity appears to bear no relation -to the shocks which follow. The ground then rocks in wavelike<span class="pagenum"><a name="Page_69" id="Page_69">[69]</a></span> -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 sometimes -thrown up from the ground (<a href="#f51">Fig. 51</a>), and at other times -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 (<a href="#f52">Fig. 52</a>). -When bodies are thrown upward, it -would imply that a quick upward -movement of the ground had been -suddenly arrested, while the burial -of heavy bodies in the earth is probably -due to a movement which -begins suddenly and is less abruptly terminated.</p> - -<div class="figcenter"> - <img src="images/ill-110a.jpg" width="400" height="288" id="f51" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 51.</span>—Bowlders thrown into the air and overturned during the Assam -earthquake of 1897 (after R. D. Oldham).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-110b.jpg" width="200" height="82" id="f52" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 52.</span>—Heavy post sunk deeper -into the ground during the -Charleston earthquake of August -31, 1886 (after Dutton).</p> -</div></div> - -<p><b>Seaquakes and seismic sea waves.</b>—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<span class="pagenum"><a name="Page_70" id="Page_70">[70]</a></span> -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 (<a href="#f53">Fig. 53</a>). The same tendency of under-sea -disturbances to be localized upon definite -straight lines has been often illustrated -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.</p> - -<div class="floatleft"> - <img src="images/ill-111a.jpg" width="250" height="212" id="f53" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 53.</span>—Map showing the localities -at which shocks have -been reported at sea off Cape -Mendocino, California.</p> -</div></div> - -<p>Far grander disturbances upon the -floor of the ocean have been revealed -by the great sea waves—the so-called -“tidal waves”, properly referred to as <i>tsunamis</i>—which recur in -those sea districts which adjoin the special earthquake zones upon -the continents (p. 86). The forerunner of such a sea wave approaching -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 country<span class="pagenum"><a name="Page_71" id="Page_71">[71]</a></span> -and encountered a mountain wall. Such seismic waves have -been especially common upon the Pacific shore of South America -and upon the Japanese littoral (<a href="#f54">Fig. 54</a>). These waves proceed -from above the great deeps upon the ocean bottom, and clearly -result from the grander earth movements to which these depressions -owe their exceptional depth. The withdrawal of the water -from neighboring shores may be presumed to be connected with -a descent of the floor of the depression and the consequent drawing-in -of the ocean surface above. The later high wave would -thus represent the dispersion of the mountain of water which is -raised by the meeting of the waters from the different sides of the -depression.</p> - -<div class="figcenter"> - <img src="images/ill-111b.jpg" width="400" height="228" id="f54" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 54.</span>—Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after E. R. -Scidmore).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-112.jpg" width="200" height="160" id="f55" - alt="" - title="" /> - <div class="cf"><p class="pc200"><span class="smcap">Fig. 55.</span>—A fault of vertical -displacement.</p> -</div></div> - -<p><b>The grander and the lesser earth movements.</b>—Upon the -land the grander and so-called catastrophic earthquakes are -usually the accompaniment of important changes in the surface -of the ground that will be discussed in later sections. -Those shocks which do little damage to structures produce no -visible changes in the earth’s surface, except, it may be, to shake -down some water-soaked masses of earth upon the steeper slopes. -Still other movements, and these too slight to be felt even in -the night when the animal world is at rest, may yet be distinguished -by their sounds, the unmistakable rumblings which are -characteristic alike of the heaviest and the lightest of earthquake -shocks.</p> - -<div class="floatleft"> - <img src="images/ill-113a.jpg" width="200" height="142" id="f56" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 56.</span>—Escarpment produced by an -earthquake fault of vertical displacement -which cut across the Chedrang -River and thus produced a waterfall, -Assam earthquake of 1897 (after R. D. -Oldham).</p> -</div></div> - -<p><b>Changes in the earth’s surface during earthquakes—faults and -fissures.</b>—Each of the grander among historic earthquakes has -been accompanied by noteworthy changes in the configuration of -the earth’s surface within the district -where the shocks were most intense. -A section of the ground is usually -found to have moved with reference to -another upon the other side of a vertical -plane which is usually to be seen; -we have here to do with the actual -making of a fault or displacement such -as we find the fossil examples of within -the rocks. The displacement, or throw, -upon the fault plane may be either upward or downward or -laterally in one direction or the other, or these movements may be<span class="pagenum"><a name="Page_72" id="Page_72">[72]</a></span> -combined. A movement of adjacent sections of the ground -upward or downward with reference -to each other (<a href="#f55">Fig. 55</a>) has -been often observed, notably -at Midori after the great Japanese -earthquake of 1891, and -in the Chedrang valley of Assam -after the earthquake of 1897 -(<a href="#f56">Fig. 56</a>).</p> - -<div class="floatleft"> - <img src="images/ill-113b.jpg" width="230" height="135" id="f57" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 57.</span>—A fault of lateral displacement.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-113c.jpg" width="230" height="135" id="f58" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 58.</span>—Fence parted and displaced -fifteen feet by a transverse fault -formed during the California earthquake -of 1906 (after W. B. Scott).</p> -</div></div> - -<p class="vh">—————</p> - -<div class="floatright"> - <img src="images/ill-113d.jpg" width="200" height="225" id="f59" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 59.</span>—Fault with vertical -and lateral displacements -combined.</p> -</div></div> - -<p>A lateral throw, unaccompanied -by appreciable vertical -displacement (<a href="#f57">Fig. 57</a>), is especially -well illustrated by the -fault in California which was -formed during the earthquake -of 1906 (<a href="#f58">Fig. 58</a>). A combination of the two types of displacement -in one (<a href="#f59">Fig. 59</a>) is exemplified -by the Baishiko fault of -Formosa at the place shown in -plate 3 A.</p> - -<p><span class="pagenum"><a name="Page_73" id="Page_73">[73]</a></span></p> - -<p><b>The measure of displacement.</b>—To -afford some measure of the displacements -which have been observed upon earthquake -faults, it may be stated that the -maximum vertical throw measured upon -the fault in the Neo valley of Japan (1891) -was 18 feet, in the Chedrang valley of -Assam (1897) 35 feet, and of the Alaskan -coast (1899) 47 feet. Large sections of -land were bodily uplifted in these cases -within the space of a few seconds, or -at most a few minutes, by the amounts given. The largest recorded -lateral displacement measured upon an earthquake fault -is about 21 feet upon the California -rift after the earthquake of 1906; -though an amount only slightly less -than this is indicated in the shifting -of roads and arroyas dating from the -earthquake of 1872 in the Owens valley, -California. Fault lines once established -are planes of special weakness and -become later the seat of repeated -movements of the same kind.</p> - -<div class="bord p4"> - -<p class="pr5"><span class="smcap">Plate 3.</span></p> - -<div class="figcenter"> - <img src="images/ill-114a.jpg" width="400" height="312" id="p3a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> An earthquake fault opened in Formosa in 1906, with vertical and lateral displacements -combined (after Omori).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-114b.jpg" width="400" height="281" id="p3b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Earthquake faults opened in Alaska in 1889, on which vertical slices of the -earth’s shell have undergone individual adjustments (after Tarr and Martin).</p> -</div></div> - -</div> - -<div class="floatright"> - <img src="images/ill-116a.jpg" width="250" height="299" id="f60" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 60.</span>—Diagram to show how -small faults in the rock basement -may be masked at the -surface through adjustments -within the loose rock mantle.</p> -</div></div> - -<p>The greater number of earthquake -faults are found in the loose rock cover -which so generally mantles the firmer -rock basement, and it is almost certain -that the throws within the solid rock -are considerably larger than those -which are here measured at the surface, owing to the adjustments -which so readily take place in the looser materials. Those lighter -shocks of earthquake which are accompanied by no visible displacements -at the surface do, -however, in some instances affect -in a measure the flow of water -upon the surface, and thus indicate -that small changes of surface -level have occurred without -breaks sufficiently sharp to be -perceived (<a href="#f60">Fig. 60</a>). Intermediate -between the steep escarpment -and the masked displacement -just described is the so-called -“mole-hill” effect,—a rounded -and variously cracked slope or -ridge above the position of a -buried fault (<a href="#f61">Fig. 61</a>).</p> - -<div class="floatleft"> - <img src="images/ill-116b.jpg" width="250" height="248" id="f61" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 61.</span>—Diagram to show the appearance -of a “mole hill” above a buried -earthquake fault (after Kotô).</p> -</div></div> - -<p>The escarpments due to earthquake faults in loose materials -at the earth’s surface can obviously retain their steepness for a -few years or decades at the most; for because of their verticality<span class="pagenum"><a name="Page_74" id="Page_74">[74]</a></span> -they must gradually disappear in rounded slopes under the action -of the elements. Smaller displacements within a rock which -rapidly disintegrates under -the action of frost and sun -will likewise before long be -effaced. In those exceptional -instances where a -resistant rock type has had -all altered upper layers -planed away until a fresh -and hard surface is exposed, -and has further -been protected from the -frost and sun beneath a -thin layer of soil, its original -surface may be retained -unaltered for many centuries. Upon such a surface the -lightest of sensible shocks, or even the smaller earth movements -which are not perceived at the time, may leave an almost indelible -record. Such records particularly -show that the movements -which they register occur upon -the planes of jointing within the -rock, and that these ready -formed cracks have probably -been the seats of repeated and -cumulative adjustments (<a href="#f62">Fig. 62</a>).</p> - -<div class="floatleft"> - <img src="images/ill-117a.jpg" width="230" height="161" id="f62" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 62.</span>—Post-glacial earthquake faults of small -but cumulative displacement, eastern New -York (after Woodworth).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-117b.jpg" width="230" height="289" id="f63" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 63.</span>—Earthquake cracks in Colorado -desert (after a photograph by Sauerven).</p> -</div></div> - -<p class="vh">————</p> - -<p><b>Contraction of the earth’s -surface during earthquakes.</b>—The -wide variations in the -amount of the lateral displacement -upon earthquake faults, -like those opened in California -in 1906, show that at the time of -a heavy earthquake there must be -large local changes in the -density of the surface materials. Literally, thousands of fissures -may appear in the lowlands, many of them no doubt a<span class="pagenum"><a name="Page_75" id="Page_75">[75]</a></span> -secondary effect of the shaking, but others, like the <i>quebradas</i> of -the southern Andes or the “earthquake cracks” in the Colorado -desert (<a href="#f63">Fig. 63</a>), may have a deeper-seated origin. Many facts -go to show, however, that though local expansion does occur in -some localities, a surface contraction is a far more general consequence -of earth movement. In civilized countries of high industrial -development, where lines of metal of one kind or another run -for long distances beneath or upon the surface of the ground, such -general contraction of the surface may be easily proven. Comparatively -seldom are lines of metal pulled apart in such a way -as to show an expansion of the surface; whereas bucklings and -kinkings of the lines appear in many places to prove that the area -within which they are found has, as a whole, been reduced.</p> - -<div class="figcenter"> - <img src="images/ill-118a.jpg" width="400" height="135" id="f64" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 64.</span>—Diagrams to show how railway tracks are either broken or buckled -locally within the district visited by an earthquake.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-118b.jpg" width="400" height="192" id="f65" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 65.</span>—The Biwajima railroad bridge in Japan after the earthquake of 1891 -(after Milne and Burton).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-119a.jpg" width="250" height="101" id="f66" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 66.</span>—Diagrams to show how the compression -of a district and its consequent contraction -during an earthquake may close up the joint -spaces within the rock basement and concentrate -the contraction of the overlying mantle -where this is partially cut through and so -weakened in the valley sections.</p> -</div></div> - -<p>Water pipes laid in the ground at a depth of some feet may be -bowed up into an arch which appears above the surface; lines of<span class="pagenum"><a name="Page_76" id="Page_76">[76]</a></span> -curbing are raised into broken arches, and the tracks of railways -are thrown into local loops and kinks which imply a very considerable -local contraction of the surface (<a href="#f64">Fig. 64</a>). With unvarying -regularity railway or other bridges which cross rivers or ravines, -if the structures are seriously damaged, indicate that the river -banks have drawn nearer together at the time of the disturbance. -In such cases, whenever -the bridge girder has remained -in place upon its -abutments, these have -either been broken or back-tilted -as a whole in such a -manner as to indicate an -approach of the foundations -which was prevented -at the top by the stiffness -of the girder (<a href="#f65">Fig. 65</a>).</p> - -<div class="figcenter"> - <img src="images/ill-119b.jpg" width="400" height="103" id="f67" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 67.</span>—Map of the Chedrang fault which made its appearance during the Assam -earthquake of 1897. The figures give the amounts of the local vertical displacement -measured in feet (after R. D. Oldham).</p> -</div></div> - -<p>The simplest explanation -of such an approach of the banks at the sides of the valleys -cut in loose surface material is to be found in a general closing up -of the joint spaces within the underlying rock, and an adjustment -of the mantle upon the floor mainly in the valley sections -(<a href="#f66">Fig. 66</a>).</p> - -<div class="floatright"> - <img src="images/ill-120a.jpg" width="200" height="524" id="f68" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 68.</span>—Map giving the -displacements in feet -measured along an earthquake -fault formed in -Alaska in 1899 (after Tarr -and Martin).</p> -</div></div> - -<p><b>The plan of an earthquake fault.</b>—In our consideration of earthquake -faults we have thus far given our attention to the displacement -as viewed at a single locality only. Such displacements are, -however, continued for many miles, and sometimes for hundreds -of miles; and when now we examine a map or plan of such a line -of faulting, new facts of large significance make their appearance. -This may be well illustrated by a study of the plan of the Chedrang<span class="pagenum"><a name="Page_77" id="Page_77">[77]</a></span> -fault which appeared at the time of the Assam earthquake of -1897 (<a href="#f67">Fig. 67</a>). From this map it will be noticed that the upward -or downward displacement upon the perpendicular plane of the -fault is not uniform, but is subject to large and <i>sudden</i> changes. -Thus in order the measurements in feet -are 32, 0, 18, 35, 0, 8, 25, 12, 8, 2, 0. -The fault formed in 1899 upon the -shores of Russell Fjord in Alaska (<a href="#f68">Fig. 68</a>) -reveals similar sudden changes of -throw, only that here the direction of -the movement is often reversed; or, -otherwise expressed, the upthrow is -suddenly transferred from one side of -the fault to the other. Such abrupt -changes in the direction of the displacement -have been observed upon -many earthquake faults, and a particularly striking one is represented -in <a href="#f69">Fig. 69</a>.</p> - -<div class="floatleft"> - <img src="images/ill-120b.jpg" width="200" height="122" id="f69" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 69.</span>—Abrupt change in the direction -of throw upon an earthquake fault which -was formed in the Owens valley, California, -in 1872. The observer looks directly -along the course of the fault from the left -foreground to the cliff beyond and to the -left of the impounded water (after a -photograph by W. D. Johnson).</p> -</div></div> - -<p><b>The block movements of the disturbed district.</b>—The displacements -upon earthquake faults are thus seen to be subdivided into -sections, each of which differs from its neighbors upon either side -and is sharply separated from them, at least in many instances. -These points of abrupt change of displacement are, in many cases -at least, the intersection points with transverse faults (<a href="#f69">Fig. 69</a>).<span class="pagenum"><a name="Page_78" id="Page_78">[78]</a></span> -Such points of abrupt change in the degree or in the direction of -the displacement may be, when looked at from above, abrupt -turning points in the direction -of extension of the fault, whose -course upon the map appears as -a zigzag line made up of straight -sections connected by sharp -elbows (<a href="#f70">Fig. 70</a>).</p> - -<div class="floatleft"> - <img src="images/ill-121.jpg" width="230" height="557" id="f70" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 70.</span>—Map of the faults within an area -of the Owens valley, California, formed -in part during the earthquake of 1872, -and in part due to early disturbances, -In the western portions the displacements -cut across firm rock and alluvial -deposits alike without deviation of direction -(after a map by W. D. Johnson).</p> -</div></div> - -<p>Such a grouping of surface -faults as are represented upon -the map is evidence that the -area of the earth’s shell, which -is included, has at the time of -the earthquake been subject to -adjustments as a series of separate -units or blocks, certain of -the boundaries of which are the -fault lines represented. The -changes in displacement measured -upon the larger faults -make it clear that the observed -faults can represent but a fraction -of the total number of -lines of displacement, the others -being masked by variations in -the compactness of the loose -mantling deposits. Could we -but have this mantle removed, -we should doubtless find a rock -floor separated into parts like -an ancient Pompeiian pavement, -the individual blocks in which -have been thrown, some upward -and some downward, by varying -amounts. Less than a -hundred miles away to the eastward -from the Owens Valley, a -portion of this pavement has -been uncovered in the extensive<span class="pagenum"><a name="Page_79" id="Page_79">[79]</a></span> -operations of the Tonapah Mining -District, so that there we -may study in all its detail the -elaborate pattern of earth marquetry -(<a href="#f71">Fig. 71</a>) which for the -floor of the Owens valley is as -yet denied us.</p> - -<div class="floatright"> - <img src="images/ill-122a.jpg" width="280" height="283" id="f71" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 71.</span>—Marquetry of the rock floor -of the Tonapah Mining District, -Nevada (after Spurr).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-122b.jpg" width="230" height="338" id="f72" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 72.</span>—Map of a portion of the Alaskan coast to -show the adjustments in level during the earthquake -of 1899 (after Tarr and Martin).</p> -</div></div> - -<p><b>The earth blocks adjusted -during the Alaskan earthquake -of 1899.</b>—For a study of the -adjustments which take place -between neighboring earth blocks -during a great earthquake, the -recent Alaskan disturbance has -offered the advantage -that the most affected -district was upon the -seacoast, where changes -of level could be referred -to the datum of the sea’s -surface. Here a great -island and large sections -of the neighboring shore -underwent movements -both as a whole in large -blocks and in adjustments -of their subordinate -parts among themselves -(<a href="#f72">Fig. 72</a>). Some -sections of the coast were -here elevated by as much -as 47 feet, while neighboring -sections were uplifted -by smaller amounts -(<a href="#f73">Fig. 73</a>), and certain -smaller sections were -even dropped below the -level of the sea.</p> - -<div class="floatright"> - <img src="images/ill-123a.jpg" width="200" height="144" id="f73" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 73.</span>—View on Haencke Island, Disenchantment -Bay, Alaska, revealing the shore -that rose seventeen feet above the sea during -the earthquake of 1899, and was found with -barnacles still clinging to the rock (after -Tarr and Martin).</p> -</div></div> - -<p>The amount of such subsidence<span class="pagenum"><a name="Page_80" id="Page_80">[80]</a></span> -is, however, difficult to ascertain, for the reason that the -former shore features are now covered with water and thus removed -from observation. In favorable -localities the minimum -amount of submergence may -sometimes be measured upon -forest trees which are now -flooded with sea water. In -<a href="#f74">Fig. 74</a> a portion of the -coast is represented where -the beach sand is now extended -back into the spruce -forest, a distance of a hundred -feet or more, and where -sedgy beach grass is growing -among trees whose roots are -now laved in salt water. -At the front of this forest the great storm waves overturn the -trees and pile the wreckage in front of those that still remain -standing.</p> - -<div class="floatleft"> - <img src="images/ill-123b.jpg" width="230" height="132" id="f74" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 74.</span>—Partially submerged forest -upon the shore of Knight Island, Alaska, -due to the sinking of a section of the -coast during the earthquake of 1899 -(after Tarr and Martin).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-123c.jpg" width="230" height="132" id="f75" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 75.</span>—Settlement of a section of the -shore at Port Royal, Jamaica, during -the earthquake of January 14, 1907, -adjacent to a similar but larger settlement -of the near shore during the -earthquake of 1692 (after a photograph -by Brown).</p> -</div></div> - -<p class="vh">————</p> - -<p>Upon the glaciated rock surfaces of the Alaskan coast, exceptionally -favorable opportunities are found for study of the intricate -pattern of the earth mosaic which is under adjustment at the time -of an earthquake. Upon Gannett Nunatak the surface was found -divided by parallel faults into distinct slices which individually -underwent small changes of level (<a href="#p3b">plate 3 B</a>).</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_81" id="Page_81">[81]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER VIII</h2> - -<p class="pch">THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: -EARTHQUAKES AND SEAQUAKES (Concluded)</p> - -<p><b>Experimental demonstration of earth movements.</b>—The study -of the Alaskan earthquake of 1899 showed that during this adjustment -within the earth’s shell some of the local blocks moved upward -and by larger amounts than their neighbors, and that still -others were actually depressed so that the sea flowed over them. -It must be evident that such differential vertical movements of -neighboring blocks at the earth’s surface can only take place -if lateral transfers of material are made beneath it. From under -those strips of coast land which were depressed, material must -have been moved so as to fill the void which would otherwise have -formed beneath the sections that were uplifted. If we take into -consideration much larger fractions upon the surface of our planet, -we are taught by the great seaquakes which are now registered -upon earthquake instruments at distant stations that large <i>downward</i> -movements are to-day in progress beneath the sea much more -than sufficient to compensate all extensions of the earth’s surface -within those districts where the land is rising in mountains. From -under the offshore deeps of the ocean to beneath the growing -mountains upon the shore, a transfer of earth material must be -assumed to take place when disturbances are registered.</p> - -<p>Within the time interval that separates the sudden adjustments -of the surface which are manifested in earthquakes, the condition -of strain which brings them about is steadily accumulating, due, -as we generally assume, to earth contraction through loss of its -heat. It seems probable that the resistance to an immediate adjustment -is found in the rigidity of the shell because of the compression -to which it is subjected. To illustrate: a row of blocks -well fitted to each other may be held firmly as a bridge between -the jaws of a vice, because so soon as each block starts to fall a -large resistance from friction upon its surface is called into existence, -a force which increases with the degree of compression.</p> - -<p><span class="pagenum"><a name="Page_82" id="Page_82">[82]</a></span></p> - -<p>It is thus possible upon this assumption crudely to demonstrate -the adjustment of earth blocks by the simple device represented in -<a href="#p4a">plate 4 A</a>. The construction of this experimental tank is so simple -that little explanation is necessary. Wooden blocks of different -heights are supported in water within a tank having a glass front, -and are kept in a strained condition at other than their natural -positions of flotation by the compression of a simple vice at the -top. Held firmly in this position, they may thus represent the -neighboring blocks within the earth’s outer shell which are supported -upon relatively yielding materials beneath, and prevented -from at once adjusting themselves to their natural positions through -the compression to which they are subjected. Held as they now -are, the water near the ends of the tank is forced up beneath the -blocks to higher than its natural level, and thus tends to flow from -both ends toward the center. Such a movement would permit -the end blocks to drop and force the middle ones to rise. The end -blocks are, let us say, the sections of Alaskan coast line which sunk -during the earthquake, as the center blocks are the sections which -rose the full measure of 47 feet. Upon a larger scale the end blocks -may equally well be considered as the floor of the great deeps off -the Alaskan coast, whose sinking at the time of the earthquake -was the cause of the great sea wave. Upon this assumption the -center blocks would represent the Alaskan coast regarded as a -whole, which underwent a general uplift.</p> - -<p>Though we may not, in our experiment, vary the tendency to -adjustment by any contractional changes in either the water or -the blocks, we may reduce the compression of the vice, which leads -to the same general result. As the compression of the vice is -slowly relaxed, a point is at last reached at which friction upon -the block surfaces is no longer sufficient to prevent an adjustment -taking place, and this now suddenly occurs with the result shown in -<a href="#p4b">plate 4 B</a>. In the case of the earth blocks, this sudden adjustment -is accompanied by mass movements of the ground separated by -faults, and these movements produce successional vibrations that -are particularly large near the block margins, and other frictional -vibrations of such small measure as to be generally appreciated by -sounds only. The jolt of the adjustments has thrown some blocks -beyond their natural position of rest, and these sink and rise subsequently -in order to readjust themselves with lighter vibrations, -which may be repeated and continued for some time. In the case -of the earth these later adjustments are the so-called <i>aftershocks</i> -which usually continue throughout a considerable period following -every great earthquake. Gradually they fall off in intensity -and frequency until they can no longer be felt, and are thereafter -continued for a time as rumblings only.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 4.</span></p> - -<div class="figcenter"> - <img src="images/ill-126a.jpg" width="400" height="167" id="p4a" - alt="" - title="" /> - <div class="caption"><p class="ch400"><i>A.</i> 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.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-126b.jpg" width="400" height="164" id="p4b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> The same apparatus after a sudden adjustment.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-126c.jpg" width="400" height="183" id="p4c" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>C.</i> Model to illustrate a block displacement in rocks which are intersected -by master joints.</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_83" id="Page_83">[83]</a></span></p> - -<p><b>Derangement of water flow by earth movement.</b>—The water -which supported the blocks in our experiment has represented -the more mobile portion of the earth’s substance beneath its outer -zone of fracture. The surface water layers in the tank may, however, -be considered in a different -way, since their behavior is remarkably -like that of the water within -and upon the earth’s surface during -an earth adjustment. At the instant -when adjustment takes place in the -tank, water frequently spurts upward -from the cracks between the sinking -end blocks; and if in place of one -of the higher center blocks we insert -one whose top is below the level of -the water in the tank, a “lake” will -be formed above it. When the adjustment -occurs, this lake is immediately -drained by outflow of the water at its bottom along -one of the cracks between the blocks (<a href="#f76">Fig. 76</a>).</p> - -<div class="floatright"> - <img src="images/ill-128.jpg" width="250" height="228" id="f76" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 76.</span>—Diagrams to illustrate -the draining of lakes during -earthquakes.</p> -</div></div> - -<p>Such derangements of water flow as have been illustrated by -the experiment are among the commonest of the phenomena -which accompany earthquakes. Lakes and swamp lands have -during earthquakes been suddenly drained, fountains of water -have been seen to shoot up from the surface and have played for -some minutes or hours before their sudden disappearance in a sucking -down of the water with later readjustment. During the great -earthquake of the lower Mississippi valley in 1811, known as the -New Madrid earthquake, the earlier Lake Eulalie was completely -drained, and upon the now exposed bed there appeared parallel -fissures on which were ranged funnel-like openings down which -the water had been sucked. In other sections of the affected -region the water shot up in sheets along fissures to the tops of high<span class="pagenum"><a name="Page_84" id="Page_84">[84]</a></span> -trees. Areas where such spurting up of the water has been observed -have in most cases been shown to correspond to areas of -depression, and such areas have sometimes been left flooded with -water. During the Indian earthquake of 1819 an area of some -200 square miles suddenly sank and was transformed into a lake.</p> - -<div class="figcenter"> - <img src="images/ill-129a.jpg" width="400" height="230" id="f77" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 77.</span>—Diagram to illustrate-the derangements of flow of water at the time of -an earthquake; water issuing at the surface over downthrown rocks, and being -sucked down in upthrown blocks.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-129b.jpg" width="400" height="145" id="f78" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 78.</span>—Mud cones aligned upon a fissure opened at Moraza, Servia, during -the earthquake of April 4, 1904 (after Michailovitch).</p> -</div></div> - -<p><b>Sand or mud cones and craterlets.</b>—From a very moderate -depth below the surface to that of several miles, all pore spaces -and all larger openings within the rock are completely filled with -water, the “trunk lines” of whose circulation is by way of the -joints or along the bedding planes of the rocks. The principal -reservoirs, so to speak, of this water inclosed within the rock are -the porous sand formations. When, now, during an earthquake a -block of the earth’s shell is suddenly sunk and as suddenly arrested<span class="pagenum"><a name="Page_85" id="Page_85">[85]</a></span> -in its downward movement, the effect is to compress the porous -layers and so force the contained water upward along the joints to -the surface, carrying with it large quantities of the sand (<a href="#f77">Fig. 77</a>).</p> - -<div class="figcenter"> - <img src="images/ill-130a.jpg" width="400" height="262" id="f79" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 79.</span>—One of the many craterlets formed near Charleston, South Carolina, -during the earthquake of August 31, 1886. The opening is twenty feet across, -and the leaves about it are encased in sand as were those upon the branches -of the overhanging trees to a height of some twenty feet (after Dutton).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-130b.jpg" width="250" height="148" id="f80" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 80.</span>—Cross section of a craterlet to show the -trumpet-like form of the sand column.</p> -</div></div> - -<p>Ejected at the surface this water appears in fountains usually -arranged in line over joints, or even in continuous sheets, and the -sand collecting about -the jets builds up lines -of <i>sand</i> or <i>mud cones</i> -sometimes described as -“mud volcanoes” (<a href="#f78">Fig. 78</a>). -The amount of -sand thus poured out -is sometimes so great -that blankets of quicksand -are spread over -large sections of the -country. Most frequently, -however, the sand is not built above the general level -of the surface, but forms a series of <i>craterlets</i> which are largely<span class="pagenum"><a name="Page_86" id="Page_86">[86]</a></span> -shaped as the water is sucked down at the time of the readjustment -with which the play of such earthquake fountains is terminated -(<a href="#f79">Fig. 79</a>). Subsequent excavations made about such craterlets -have shown them to have the form of a trumpet, and that in the -sand which so largely fills them there are generally found scales of -mica and such light bodies as would be picked out from the heterogeneous -materials of the sand layers and carried upward in the -rush of water to the surface (<a href="#f80">Fig. 80</a>).</p> - - -<p><b>The earth’s zones of heavy earthquake.</b>—Since earthquakes -give notice of a change of level of the ground, the special danger -zones from this source are the growing mountain systems which -are usually found near the borders of the sea. Such lines of mountains -are to-day rising where for long periods in the past were the -basins of deposition of former seas. They thus represent the -zones upon the earth’s surface which are the most unstable—which -in the recent period have undergone the greatest changes -of level.</p> - -<div class="floatright"> - <img src="images/ill-132a.jpg" width="200" height="214" id="f81" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 81.</span>—Map of the island of -Ischia to show how the shocks -of recent earthquakes have been -concentrated at the crossing -point of two fractures (after -Mercalli and Johnston-Lavis).</p> -</div></div> - -<p>By far the most unstable belt upon the earth’s surface is the -rim surrounding the Pacific Ocean, within which margin it has -been estimated that about 54 per cent of the recorded shocks of -earthquake have occurred. Next in importance for seismic instability -is the zone which borders both the Mediterranean Sea -and the Caribbean—the American Mediterranean—and is extended -across central Asia through the Himalayas into Malaysia. -Both zones approximate to great circles upon the earth’s surface -and intersect each other at an angle of about 67°. It has been -estimated that about 95 per cent of the recorded continental earthquakes -have emanated from these belts.</p> - -<div class="floatleft"> - <img src="images/ill-132b.jpg" width="150" height="392" id="f82" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 82.</span>—A line of earth -fracture indicated in -the plan of the relief, -which may at any time -become the seat of -movement and resultant -shock.</p> -</div></div> - -<p><b>The special lines of heavy shock.</b>—Within any earthquake -district the shocks are not felt with equal severity at all places, -but there are, on the contrary, definite lines which the disturbance -seems to search out for special damage. From their relations to -the relief of the land these lines would appear to be lines of fracture -upon the boundaries of those sections of the crust that play individual -rôles in the block adjustment which takes place. More -or less masked as these lines are beneath the rounded curves of -the landscape, they are given an altogether unenviable prominence -with each succeeding earthquake. At such times we may think -of the earth’s surface as specially sensitized for laying bare its<span class="pagenum"><a name="Page_87" id="Page_87">[87]</a></span> -hidden structure, as is the sensitized plate under the magical influence -of the X rays.</p> - -<p>When, at the time of an earthquake, blocks are adjusted with -reference to their neighbors, the movements of oscillation are -greatest in those marginal portions -of direct contact. Corners of blocks—the -intersecting points of the important -faults—should for the same -reason be shaken with a double -violence, and this assumption appears -to be confirmed by observation. -Upon the island -of Ischia, off the -Bay of Naples, -the shocks from -recent earthquakes -have -been strangely -concentrated -near the town of -Casamicciola, -which was last destroyed in 1883. This unfortunate -city lies at the crossing point of -important fractures whose course upon the -island is marked by numerous springs and -<i>suffioni</i> (<a href="#f81">Fig. 81</a>).</p> - -<p><b>Seismotectonic lines.</b>—The lines of important -earth fractures, as will be more clearly -shown in the sequel (<a href="#Page_227">p. 227</a>), are often indicated -with some clearness by straight lines in -the plan of the surface relief (<a href="#f82">Fig. 82</a>). Lines -of this nature are easily made out upon the -map of the West Indies, and if we represent -upon it by circles of different diameters the -combined intensities of the recorded earthquakes -in the various cities, it appears that -the heavily shaken localities are ranged upon -lines stamped out in the relief, with the most severely damaged -places at their intersections (<a href="#f83">Fig. 83</a>). These lines of exceptional<span class="pagenum"><a name="Page_88" id="Page_88">[88]</a></span> -instability are known as <i>seismotectonic lines</i>—earthquake structure -lines.</p> - -<div class="figcenter"> - <img src="images/ill-133a.jpg" width="400" height="114" id="f83" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 83.</span>—Seismotectonic lines of the West Indies.</p> -</div></div> - -<p><b>The heavy shocks above loose foundations.</b>—It is characteristic -of faults that they soon bury themselves from sight under -loose materials, and are thus made difficult of inspection. The -escarpment which is the direct consequence of a vertical displacement -upon a fault tends to migrate from the place of its formation, -rounding the surface as it does so and burying the fault line beneath -its deposits (<a href="#f43">Fig. 43</a>, <a href="#Page_60">p. 60</a>).</p> - -<p>This is not, however, the sole reason why loose foundations -should be places of special danger at the time of earth shocks, for -the reason that earthquake waves are sent out in all directions -from the surfaces of displacement through the medium of the underlying -rock. These waves travel -within the firm rock for considerable -distances with only a gradual dissipation -of their energy, but with their -entry into the loose surface deposits -their energy is quickly used up in -local vibrations of large amplitude, -and hence destructive to buildings.</p> - -<div class="floatleft"> - <img src="images/ill-133b.jpg" width="200" height="159" id="f84" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 84.</span>—Device to illustrate the -different effects upon the transmission -and the character of -shocks which are produced by -firm rock and by loose materials.</p> -</div></div> - -<p>The essential difference between -firm rock and such loose materials as -are found upon a river bottom or in -the “made land” about our cities -may be illustrated by the simple -device which is represented in <a href="#f84">Fig. 84</a>. Two similar metal pans -are suspended from a firm support by bands of steel and “elastic” -braid of similar size and shape, and carry each a small block of -wood standing upon its end. Similar light blows are now administered -directly to the pans with the effect of upsetting that block<span class="pagenum"><a name="Page_89" id="Page_89">[89]</a></span> -which is supported by the loose braid because of the large range -or amplitude of movement that is imparted to the pan. The -“elastic” braid, because of these large vibrations of which it is -susceptible, may represent the loose materials when an earthquake -wave passes into them. In the case of the steel support, the -energy of the blow, instead of being dissipated in local swingings -of the pan, is to a large extent transmitted through the elastic -metal to materials beyond. The steel thus resembles in its high -elasticity the firmer rock basement, which receives and transmits -the earthquake shocks, but except when ruptured in a fault is -subject to vibrations of small amplitude only.</p> - - -<p><b>Construction in earthquake regions.</b>—Wherever earthquakes -have been felt, they are certain to occur again; and wherever -mountains are growing or changes of level are in progress, there -no record of past earthquakes is required in order to forecast the -future seismic history. Although the future earthquakes may be -predicted, the time of their coming is, fortunately or unfortunately, -still hidden from us. If one’s lot is to be cast in an earthquake -country, the only sane course to pursue is to build with due regard -to future contingencies.</p> - -<p>The danger, from destructive fires may to-day be largely met -by methods of construction which levy an additional burden of -cost. Though the danger from seismic disturbances can hardly -be met as fully as that from fire, yet it is true that buildings may -be so constructed as to withstand all save those heaviest shocks -in the immediate vicinity of the lines of large displacement. Here, -also, a considerable additional expense is involved in the method -of construction, in the case of residences particularly.</p> - -<p>From what has been said, it is obvious that much of the danger -from earthquakes can be met by a choice of site away from lines -of important fracture and from areas of relatively loose foundation. -The choice of building materials is next of importance. Those -buildings which succumb to earthquakes are in most cases racked -or shaken apart, and thus they become a prey to their own inherent -properties of inertia. Each part of a structure may be regarded -as a weight which is balanced upon a stiff rod and pivoted upon -the ground. When shocks arrive, each part tends to be thrown -into vibration after the manner of an inverted pendulum. In -proportion, therefore, as the weights are large and rest upon long<span class="pagenum"><a name="Page_90" id="Page_90">[90]</a></span> -supports, the danger of overthrow and of tearing apart is increased. -In general, structures are best constructed of light materials whose -weight is concentrated near the ground. Masonry structures, -and especially high ones, are, therefore, the least suited for resisting -earthquakes, of which the late complete destruction of the city -of Messina is a grewsome reminder. Despite repeated warnings -in the past, the buildings of that stricken city were generally constructed -of heavy rubble, which in addition had been poorly cemented -(<a href="#f49">Fig. 49</a>, <a href="#Page_67">p. 67</a>). Such structures are usually first ruptured -at the edges and corners, since here the vibrations which tend to -tear the building asunder are resisted by no supports and are -reënforced from neighboring walls.</p> - -<div class="figcenter"> - <img src="images/ill-135.jpg" width="400" height="223" id="f85" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 85.</span>—House wrecked in San Francisco earthquake of 1906 because the floors -and partitions were not securely fastened to the walls (after R. L. Humphrey).</p> -</div></div> - -<p>An advantage of the first importance is evidently secured if the -rods of the pendulum, of which the building is conceived to be composed, -have sufficient elasticity to be considerably distorted without -rupture and to again recover their original position. This is -the supreme advantage of structural steel for all large buildings, -which is coupled, however, with the disadvantage that the -riveted fastenings are apt to be quickly sheered off under the -vibrations. Large and high buildings, when sufficiently elastic, -have fortunately the property of destroying the earth waves -by interference before they have traveled above the lower -stories.</p> - -<p>For large structures in which wood cannot be used, strongly<span class="pagenum"><a name="Page_91" id="Page_91">[91]</a></span> -reënforced concrete is well adapted, for it has in general the same -advantages as steel with somewhat reduced elasticity, but with a -more effective binding together of the parts. This requirement -of thorough bracing and tying together of the several parts of a -building causes it to vibrate, not as many pendulums, but as one -body. If met, it removes largely the danger from racking strains, -and for small structures particularly it is the requirement which -is most easily complied with. For such buildings it is therefore -necessary that the framework should be built in a close network -with every joint firmly braced and with all parts securely tied together. -Especial attention should be given to the fastenings of -floor and partition ends. The house shown in <a href="#f85">Fig. 85</a> could not -have been subjected to heavy shocks, for though the walls are -thrown down, the floors and partitions have been left near their -original positions.</p> - -<div class="figcenter"> - <img src="images/ill-136.jpg" width="400" height="230" id="f86" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 86.</span>—Building wrecked at San Mateo, California, during the late earthquake. -The heavy roof and upper floor, acting as a unit, have battered down -the upper walls (after J. C. Branner).</p> -</div></div> - -<p>This tendency of the walls, floors, partitions, and roof to act -as individual units in the vibration, is one that must be reckoned -with and be met by specially effective bracing and tying at the -junctions. Otherwise these larger parts of the structure may act -like battering rams to throw over the walls or portions of them -(<a href="#f86">Fig. 86</a>).</p> - -<p><span class="pagenum"><a name="Page_92" id="Page_92">[92]</a></span></p> - -<p class="prr"><span class="smcap">Reading References for Chapters VII and VIII</span></p> - -<p class="p1">General works:—</p> - -<p class="pex"><span class="smcap">John Milne.</span> Seismology. London, 1898, pp. 320.</p> - -<p class="pex"><span class="smcap">C. E. Dutton.</span> Earthquakes in the Light of the New Seismology. Putnam, -New York, 1904, pp. 314.</p> - -<p class="pex"><span class="smcap">A. Sieberg.</span> Handbuch der Erdbebenkunde. Braunschweig, 1904, pp. -362.</p> - -<p class="pex"><span class="smcap">Count F. de Montessus de Ballore.</span> Les Tremblements de Terre, -Géographie Séismologique. Paris, 1906, pp. 475; La Science Séismologique. -Paris, 1907, pp. 579.</p> - -<p class="pex"><span class="smcap">William Herbert Hobbs.</span> Earthquakes, an Introduction to Seismic -Geology. Appleton, New York, 1907, pp. 336.</p> - -<p class="pex"><span class="smcap">C. G. Knott.</span> The Physics of Earthquake Phenomena. Clarendon Press, -Oxford, 1908, pp. 283.</p> - -<p class="pex"><span class="smcap">E. Rudolph.</span> Ueber Submarine Erdbeben und Eruptionen, Beiträge zur -Geophysik, vol. 1, 1887, pp. 133-365; vol. 2, 1895, pp. 537-666; vol. 3, -1898, pp. 273-536.</p> - -<p class="p1">Descriptive reports of some important earthquakes:—</p> - -<p class="pex"><span class="smcap">C. E. Dutton.</span> The Charleston Earthquake of August 31, 1886, 9th -Ann. Rept. U. S. Geol. Surv., 1889, pp. 203-528.</p> - -<p class="pex"><span class="smcap">B. Kotô.</span> On the Cause of the Great Earthquake in Central Japan, 1891, -Jour. Coll. Sci. Imp. Univ., Tokyo, Japan, vol. 5, 1893, pp. 295-353, -pls. 28-35.</p> - -<p class="pex"><span class="smcap">John Milne</span> and <span class="smcap">W. K. Burton</span>. The Great Earthquake of Central -Japan. 1891, pp. 10, pls. 30.</p> - -<p class="pex"><span class="smcap">R. D. Oldham.</span> Report on the Great Earthquake of 12th June, 1897, -Mem. Geol. Surv. India. Vol. 29, 1899, pp. 379, pls. 42.</p> - -<p class="pex"><span class="smcap">A. C. Lawson</span>, and others. The California Earthquake of April 18, 1906, -Report of the State Earthquake Investigation Commission, three -quarto vols. (Carnegie Institution of Washington); many plates and -figures.</p> - -<p class="pex"><i>Italian Photographic Society</i>, Messina and Reggio before and after the -Earthquake of December 28, 1908 (an interesting collection of pictures). -Florence, 1909.</p> - -<p class="pex"><span class="smcap">R. S. Tarr</span> and <span class="smcap">L. Martin</span>. Recent Changes of Level in the Yakutat -Bay Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, -pls. 12-23.</p> - -<p class="pex"><span class="smcap">William Herbert Hobbs.</span> The Earthquake of 1872 in the Owens -Valley, California, Beiträge zur Geophysik, vol. 10, 1910, pp. 352-385, -pls, 10-23.</p> - -<p class="p1">Faults in connection with earthquakes:—</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> On Some Principles of Seismic Geology, Beiträge zur -Geophysik, vol. 8, 1907, Chapters iv-v.</p> - -<p><span class="pagenum"><a name="Page_93" id="Page_93">[93]</a></span></p> - -<p class="p1">Expansion or contraction of the earth’s surface during earthquakes:—</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> A Study of the Damage to Bridges during Earthquakes, -Jour. Geol., vol. 16, 1908, pp. 636-653; The Evolution and -the Outlook of Seismic Geology, Proc. Am. Phil. Soc., vol. 48, 1909, -pp. 27-29.</p> - -<p class="p1">Earthquake construction:—</p> - -<p class="pex"><span class="smcap">John Milne.</span> Construction in Earthquake Countries, Trans. Seis. Soc., -Japan, vol. 14, 1889-1890, pp. 1-246.</p> - -<p class="pex"><span class="smcap">F. de Montessus de Ballore.</span> L’art de bâtir dans les pays à tremblements -de terre (34th Congress of French Architects), L’Architecture, -193 Année, 1906, pp. 1-31.</p> - -<p class="pex"><span class="smcap">Gilbert</span>, <span class="smcap">Humphrey</span>, <span class="smcap">Sewell</span>, and <span class="smcap">Soulé</span>. The San Francisco Earthquake -and Fire of April 18, 1906, and their Effects on Structures and -Structural Materials, Bull. 324, U. S. Geol. Surv., 1907, pp. 1-170, -pls. 1-57.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Construction in Earthquake Countries, The Engineering -Magazine, vol. 37, 1909, pp. 1-19.</p> - -<p class="pex"><span class="smcap">Lewis Alden Estes.</span> Earthquake-proof Construction, a discussion of the -effects of earthquakes on building construction with special reference -to structures of reënforced concrete, published by Trussed Concrete -Steel Company. Detroit, 1911, pp. 46.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_94" id="Page_94">[94]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER IX</h2> - -<p class="pch">THE RISE OF MOLTEN ROCK TO THE EARTH’S -SURFACE</p> - -<p class="psh">VOLCANIC MOUNTAINS OF EXUDATION</p> - -<p><b>Prevalent misconceptions about volcanoes.</b>—The more or less -common impression that a volcano is a “burning mountain” -or a “smoking mountain” has been much fostered by the school -texts in physical geography in use during an earlier period. The -best introduction to a discussion of volcanoes is, therefore, a disillusionment -from this notion. Far from being burning or smoking, -there is normally no combustion whatever in connection with a -volcanic eruption. The unsophisticated tourist who, looking out -from Naples, sees the steam cap which overhangs the Vesuvian -crater tinged with brown, easily receives the impression that the -material of the cloud is smoke. Even more at night, when a bright -glow is reflected to his eye and soon fades away, only to again glow -brightly after a few moments have passed, is it difficult to remove -the impression that one is watching an intermittent combustion -within the crater. The cloud which floats away from the crest of -the mountain is in reality composed of steam with which is admixed -a larger or smaller proportion of fine rock powder which -gives to the cloud its brownish tone. The glow observed at night -is only a reflection from molten lava within the crater, and the -variation of its brightness is explained by the alternating rise and -fall of the lava surface by a process presently to be explained.</p> - -<p>Not only is there no combustion in connection with volcanic -eruptions, but so far as the volcano is a mountain it is a product -of its own action. The grandest of volcanic eruptions have produced -no mountains whatever, but only vast plains or plateaus -of consolidated molten rock, and every volcanic mountain at -some time in its history has risen out of a relatively level surface.</p> - -<p>When the traditional notions about volcanoes grew up, it was -supposed that the solid earth was merely a “crust” enveloping -still molten material. As has already been pointed out in an earlier<span class="pagenum"><a name="Page_95" id="Page_95">[95]</a></span> -chapter, this view is no longer tenable, for we now know that the -condition of matter within the earth’s interior, while perhaps not -directly comparable to any that is known, yet has properties most -resembling known matter in a solid state; it is much more rigid -than the best tool steel. While there must be reservoirs of molten -rock beneath active volcanoes, it is none the less clear that they -are small, local, and temporary. This is shown by the comparative -study of volcanic outlets within any circumscribed district.</p> - -<p>It is perhaps not easy to frame a definition of a volcano, but -its essential part, instead of being a mountain, is rather a vent or -channel which opens up connection between a subsurface reservoir -of molten rock and the surface of the earth. An eruption occurs -whenever there is a rise of this material, together with more or less -steam and admixed gases, to the surface. Such molten rock arriving -at the surface is designated <i>lava</i>. The changes in pressure -upon this material during its elevation induce secondary phenomena -as the surface is approached, and these manifestations are -often most awe inspiring. While often locally destructive, the -geological importance of such phenomena is by reason of their -terrifying aspect likely to be greatly exaggerated.</p> - - -<p><b>Early views concerning volcanic mountains.</b>—As already pointed -out, a volcano at its birth is not a mountain at all, but only, so to -speak, a shaft or channel of communication between the surface -and a subterranean reservoir of molten rock. By bringing this -melted rock to the surface there is built up a local elevation which -may be designated a mountain, except where the volume of the -material is so large and is spread to such distances as to produce a -plain (see fissure eruptions below).</p> - -<p>In the early history of geology it was the view of the great German -geologist von Buch and his friend and colleague von Humboldt, -that a volcanic mountain was produced in much the same -manner as is a blister upon the body. The fluids which push up -the cuticle in the blister were here replaced by fluid rock which -elevated the sedimentary rock layers at the surface into a dome or -mound which was open at the top—the so-called <i>crater</i>. This -“elevation-crater” theory of volcanoes long held the stage in -geological science, although it ignored the very patent fact that -the layers on the flanks of volcanic cones are not of sedimentary -rock at all, but, on the contrary, of the volcanic materials which<span class="pagenum"><a name="Page_96" id="Page_96">[96]</a></span> -are brought up to the surface during the eruption. The observational -phase of science was, however, dawning, and the English -geologists Scrope and Lyell -were able to show by study of -volcanic mountains that the -mound about the volcanic vent -was due to the accumulation -of once molten rock which had -been either exuded or ejected. -Making use of data derived -from New Zealand, Scrope -showed that, instead of being -elevated during the formation -of a volcanic mountain, the sedimentary strata of the vicinity -may be depressed near the volcanic vent (<a href="#f87">Fig. 87</a>).</p> - -<div class="floatleft"> - <img src="images/ill-141.jpg" width="250" height="114" id="f87" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 87.</span>—Breached volcanic cone near -Auckland, New Zealand, showing the -bending down of the sedimentary strata -in the neighborhood of the vent (after -Heaphy and Scrope).</p> -</div></div> - -<p><b>The birth of volcanoes.</b>—To confirm the impression that the -formation of the volcanic mountain is in reality a secondary phenomenon -connected with eruptions, we may cite the observed birth -of a number of volcanoes. On the 20th of September, 1538, a -new volcano, since known as Monte Nuovo (new mountain), rose -on the border of the ancient Lake Lucrinus to the westward of -Naples. This small mountain attained a height of 440 feet, and -is still to be seen on the shore of the bay of Naples. From Mexico -have been recorded the births of several new volcanoes: Jorullo -in 1759, Pochutla in 1870, and in 1881 a new volcano in the Ajusco -Mountains about midway between the Gulf of Mexico and the -Pacific Ocean. The latest of new volcanoes is that raised in Japan -on November 9, 1910, in connection with the eruption of Usu-san. -This “New Mountain” reached an elevation of 690 feet.</p> - -<div class="floatleft"> - <img src="images/ill-142.jpg" width="250" height="103" id="f88" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 88.</span>—View of the new Camiguin volcano -from the sea. It was formed in 1871 over -a nearly level plain. The town of Catarman -appears at the right near the shore (after an -unpublished photograph by Professor Dean -C. Worcester).</p> -</div></div> - -<p>As described by von Humboldt, Jorullo rose in the night of the -28th of September, 1759, from a fissure which opened in a broad -plain at a point 35 miles distant from any then existing volcano. -The most remarkable of new volcanoes rose in 1871 on the island -of Camiguin northward from Mindanao in the Philippine archipelago. -This mountain was visited by the <i>Challenger</i> expedition -in 1875, and was first ascended and studied thirty years later -by a party under the leadership of Professor Dean C. Worcester, -the Secretary of the Interior of the Philippine Islands, to whom -the writer is indebted for this description and the accompanying<span class="pagenum"><a name="Page_97" id="Page_97">[97]</a></span> -illustration of this largest and most interesting of new-born volcanoes. -As in the case of Jorullo, the eruption began with the -formation of a fissure in a -level plain, some 400 yards -distant from the town of -Catarman (<a href="#f88">Fig. 88</a>). The -eruption continued for four -years, at the end of which -time the height of the summit -was estimated by the -<i>Challenger</i> expedition to be -1900 feet. At the time of -the first ascent in 1905, -the height was determined by aneroid as 1750 feet, with sharp -rock pinnacles projecting some 50 or 75 feet higher.</p> - -<p><b>Active and extinct volcanoes.</b>—The terms “active” and -“extinct” have come into more or less common use to describe -respectively those volcanoes which show signs of eruptive activity, -and those which are not at the time active. The term “dormant” -is applied to volcanoes recently active and supposed to be in a -doubtfully extinct condition. From a well-known volcano in -the vicinity of Naples, volcanoes which no longer erupt lava or -cinder, but show gaseous emanations (<i>fumeroles</i>) are said to be in -the <i>solfatara</i> condition, or to show <i>solfataric</i> activity.</p> - -<p>Experience shows that the term “extinct”, while useful, must -always be interpreted to mean apparently extinct. This may be -illustrated by the history of Mount Vesuvius, which before the -Christian era was forested in the crater and showed no signs of -activity; and in fact it is known that for several centuries no eruption -of the volcano had taken place. Following a premonitory -earthquake felt in the year 63, the mountain burst out in grand -explosive eruption in 79 <span class="smcap">A.D.</span> This eruption profoundly altered -the aspect of the mountain and buried the cities of Pompeii, Stabeii, -and Herculaneum from sight. Once more, this time during the -middle ages, for nearly five centuries (1139 to 1631) there was -complete inactivity, if we except a light ash eruption in the year -1500. During this period of rest the crater was again forested, -but the repose was suddenly terminated by one of the grandest -eruptions in the mountain’s history.</p> - -<p><span class="pagenum"><a name="Page_98" id="Page_98">[98]</a></span></p> - - -<div class="figcenter"> - <img src="images/ill-143a.jpg" width="400" height="207" id="f89" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 89.</span>—Map showing the location of the belts of active volcanoes.</p> -</div></div> - -<p><b>The earth’s volcano belts.</b>—The distribution of volcanoes is -not uniform, but, on the contrary, volcanic vents appear in definite -zones or belts, either upon the margins of the continents or included -within the oceanic areas (<a href="#f89">Fig. 89</a>). The most important of these -belts girdles the Pacific Ocean, and is represented either by chains -or by more widely spaced volcanic mountains throughout the -Cordilleran Mountain system of South and Central America and -Mexico, by the volcanoes of the Coast and Cascade ranges of North -America, the festooned volcanic chain of the Aleutian Islands, and -the similar island arcs off the eastern coast of the Eurasian continent. -The belt is further continued through the islands of -Malaysia to New Zealand, and on the Pacific’s southern margin -are found the volcanoes of Victoria Land, King Edward Land, -and West Antarctica.</p> - -<div class="figcenter"> - <img src="images/ill-143b.jpg" width="450" height="156" id="f90" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 90.</span>—A portion of the “fire girdle” of the Pacific, showing the relation of the -chains of volcanic mountains to the deeps of the neighboring ocean floor.</p> -</div></div> - -<p>This volcano girdle is by no means a perfect one, for in addition -to the principal festoons of the western border there are many<span class="pagenum"><a name="Page_99" id="Page_99">[99]</a></span> -secondary ones, and still other arcs are found well toward the -center of the oceanic area. Another broad belt of volcanoes borders -the Mediterranean Sea, and is extended westward into the -Atlantic Ocean. Narrower belts are found in both the northern -and southern portions of the Atlantic Ocean, on the margins of -the Caribbean Sea, etc. The fact of greatest significance in the -distribution seems to be that bands of active volcanoes are to be -found wherever mountain ranges are paralleled by deeps on the -neighboring ocean floor (<a href="#f90">Fig. 90</a>). As has been already pointed -out in the chapter upon earthquakes, it is just such places as these -which are the seat of earthquakes; these are zones of the earth’s -crust which are undergoing the most rapid changes of level at the -present time. Thus the rise of the land in mountains is proceeding -simultaneously with the sinking of the sea floor to form the neighboring -deeps.</p> - -<div class="figcenter"> - <img src="images/ill-144.jpg" width="400" height="32" id="f91" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 91.</span>—Volcanic cones formed in 1783 above the Skaptár fissure in Iceland -(after Helland).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-145a.jpg" width="200" height="116" id="f92" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 92.</span>—Diagrams to illustrate -the location of volcanic vents -upon fissure lines, <i>a</i>, openings -caused by lateral movement of -fissure walls; <i>b</i>, openings formed -at fissure intersections.</p> -</div></div> - -<p><b>Arrangement of volcanic vents along fissures and especially at -their intersections.</b>—Within those districts in which volcanoes -are widely separated from their neighbors, the law of their arrangement -is difficult to decipher, but the view that volcanic vents are -aligned over fissures is now supported by so much evidence that -illustrations may be supplied from many regions. An exceptionally -perfect line of small cones is found along the Skaptár -cleft in Iceland, upon which stands the large volcano of Laki. -This fissure reopened in 1783, and great volumes of lava were -exuded. Over the cleft there was left a long line of volcanic -cones (<a href="#f91">Fig. 91</a>). There are in Iceland two dominating series of -parallel fissures of the same character which take their directions -respectively northeast-southwest and north-south. Many such -fissures are traceable at the surface as deep and nearly straight -clefts or <i>gjás</i>, usually a few yards in width, but extending for many -miles. The Eldgjá has a length of more than 18 English miles -and a depth varying from 400 to 600 feet. On some of these -fissures no lava has risen to the surface, whereas others have at -numerous points exuded molten rock. Sometimes one end only -of a fissure, the more widely gaping portion, has supplied the<span class="pagenum"><a name="Page_100" id="Page_100">[100]</a></span> -conduits for the molten lava. This is well illustrated by the -cratered monticules raised by the common ant over the cracks -which separate the blocks of cement -sidewalk, the hillocks being located -where the most favorable channel was -found for the elevation of the materials.</p> - -<div class="figcenter"> - <img src="images/ill-145b.jpg" width="400" height="182" id="f93" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 93.</span>—Outline map of the eastern portion of the island of Java, displaying the arrangement -of volcanic vents in alignment upon fissures with the larger mountains -at fissure intersections (after Verbeek).</p> -</div></div> - -<p>Those places upon fissures which become -lava conduits appear to be the -ones where the cleft gapes widest so as -to furnish the widest channel. Wherever -a differential lateral movement of -the walls has occurred, openings will -be found in the neighborhood of each minor variation from a -straight line (<a href="#f92">Fig. 92 <i>a</i></a>). Wherever there are two or more series -of fissures, and this would appear to be the normal condition, -places favorable for lava conduits occur at fissure intersections. -Within such veritable volcano gardens as are to be found in Malaysia, -the law of volcano distribution became apparent so soon as -accurate maps had been prepared. Thus the outline map of a portion -of the island of Java (<a href="#f93">Fig. 93</a>) shows us that while the volcanoes -of the island present at first sight a more or less irregular -band or zone, there are a number of fissures intersecting in a network, -and that the volcanoes are aligned upon the fissures with -the larger cones located at the intersections. So also in Iceland,<span class="pagenum"><a name="Page_101" id="Page_101">[101]</a></span> -the great eruption of Askja in 1875 occurred at the intersection -of two lines of fissure.</p> - -<p>Outside these closely packed volcanic regions, similar though -less marked networks are indicated; as, for example, in and near -the Gulf of Guinea. If now, instead of reducing the scale of our -volcano maps, we increase it, the same law of distribution is no -less clearly brought out. The monticules or small volcanic cones -which form upon the flanks of larger volcanic mountains are likewise -built up over fissures which on numerous occasions have -been observed to open and the cones to form upon them.</p> - -<div class="floatright"> - <img src="images/ill-146.jpg" width="250" height="156" id="f94" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 94.</span>—Map of the Puy Pariou in the -Auvergne of central France. The seat of -eruption has migrated along the fissure -upon which the earlier cone had been -built up (after Scrope).</p> -</div></div> - -<p>Still further reducing now -the area of our studies and -considering for the moment the -“frozen” surface of the boiling -lava within the caldron of -Kilauea, this when observed at -night reveals in great perfection -the sudden formation of -fissures in the crust with the -appearance of miniature volcanoes -rising successively at -more or less regular intervals -along them.</p> - -<p>It not infrequently happens that after a volcanic vent has -become established above some conduit in a fissure, the conduit -migrates along the fissure, thus establishing a new cone with more -or less complete destruction of the old one (<a href="#f94">Fig. 94</a>).</p> - - -<p><b>The so-called fissure eruptions.</b>—The grandest of all volcanic -eruptions have been those in which the entire length and breadth -of the fissures have been the passageway for the upwelling lava. -Such grander eruptions have been for the most part prehistoric, -and in later geologic history have occurred chiefly in India, in -Abyssinia, in northwestern Europe, and in the northwestern -United States. In western India the singularly horizontal plateaus -of basaltic lava, the Dekkan traps, cover some 200,000 -square miles and are more than a mile in depth. The underlying -basement where it appears about the margins of the basalt is -in many places intersected by dikes or fissure fillings of the same -material. No cones or definite vents have been found.</p> - -<p><span class="pagenum"><a name="Page_102" id="Page_102">[102]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-147a.jpg" width="200" height="165" id="f95" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 95.</span>—Basaltic plateau of the -northwestern United States due -to fissure eruptions of lava.</p> -</div></div> - -<p>The larger portion of the northwestern British Isles would -appear to have been at one time similarly blanketed by nearly -horizontal beds of basaltic lava, which beds extended northwestward -across the sea through the Orkney and Faroe islands -to Iceland. Remnants of this vast plateau are to-day found in all -the island groups as well as in large areas of northeastern Ireland, -and fissure fillings of the same material occur throughout large -areas of the British Isles. In many cases these dikes represent -once molten rock which may never -have communicated with the surface -at the time of the lava outpouring, yet -they well illustrate what we might expect -to find if the basalt sheets of -Iceland or Ireland were to be removed.</p> - -<p>The floods of basaltic lava which in -the northwestern United States have -yielded the barren plateau of the Cascade -Mountains (<a href="#f95">Fig. 95</a>) would appear -to offer another example of fissure eruption, -though cones appear upon the -surface and perhaps indicate the position of lava outlets during the -later phases of the eruptive period. The barrenness and desolation -of these lava plains is suggested by <a href="#f96">Fig. 96</a>.</p> - -<div class="figcenter"> - <img src="images/ill-147b.jpg" width="400" height="183" id="f96" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 96.</span>—Lava plains about the Snake River in Idaho.</p> -</div></div> - -<p>Though the greater effusions of lava have occurred in prehistoric -times, and the manner of extrusion has necessarily been -largely inferred from the immense volume of the exuded materials -and the existence of basaltic dikes in neighboring regions, yet in<span class="pagenum"><a name="Page_103" id="Page_103">[103]</a></span> -Iceland we are able to observe the connection between the dikes -and the lava outflows. Professor Thoroddsen has stated that in -the great basaltic plateau of Iceland, lava has welled out quietly -from the whole length of fissures and often on both sides without -giving rise to the formation of cones. At three wider portions of -the great Eld cleft, lava welled out quietly without the formation -of cones, though here in the southern prolongation of the fissure, -where it was narrower, a row of low slag cones appeared. Where -the lava outwellings occurred, an area of 270 square miles was -flooded.</p> - -<div class="figcenter"> - <img src="images/ill-148.jpg" width="400" height="174" id="f97" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 97.</span>—Characteristic profiles of lava volcanoes. 1, basaltic lava mountain; -2, mountain of siliceous lava (after Judd).</p> -</div></div> - -<p><b>The composition and the properties of lava.</b>—In our study of -igneous rocks (Chapter IV) it was learned that they are composed -for the most part of silicate minerals, and that in their -chemical composition they represent various proportions of silica, -alumina, iron, magnesia, lime, potash, and soda. The more -abundant of these constituents is silica, which varies from 35 to -70 per cent of the whole. Whenever the content of silica is relatively -low,—basic or basaltic lava,—the cooled rock is dark in -color and relatively heavy. It melts at a relatively low temperature, -and is in consequence relatively fluid at the temperatures -which lavas usually have on reaching the earth’s surface. Furthermore, -from being more fluid, the water which is nearly always -present in large quantity within the lava more readily makes its -escape upon reaching the surface. Eruptions of such lava are -for this reason without the violent aspects which belong to extrusions -of more siliceous (more “acidic”) lavas. For the same<span class="pagenum"><a name="Page_104" id="Page_104">[104]</a></span> -reason, also, basaltic lava flows more freely and can spread much -farther before it has cooled sufficiently to consolidate. This is -equivalent to saying that its surface will assume a flatter angle of -slope, which in the case of basaltic lava seldom exceeds ten degrees -and may be less than one degree (<a href="#f97">Fig. 97</a>).</p> - -<div class="floatleft"> - <img src="images/ill-149a.jpg" width="200" height="174" id="f98" - alt="" - title="" /> - <div class="cf"><p class="pc200"><span class="smcap">Fig. 98.</span>—A driblet cone -(after J. D. Dana).</p> -</div></div> - -<p>Siliceous lavas, on the other hand, are, when consolidated, relatively -light both in color and weight and melt at relatively high -temperatures. They are, therefore, usually but partly fused and -of a viscous consistency when they arrive at the earth’s surface. -Because of this viscosity they offer much resistance to the liberation -of the contained water, which therefore is released only to -the accompaniment of more or less violent explosions. The lava -is blown into the air and usually falls as consolidated fragments -of various degrees of coarseness.</p> - -<div class="floatright"> - <img src="images/ill-149b.jpg" width="200" height="122" id="f99" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 99.</span>—View of Leffingwell crater, a cinder -cone in the Owens valley, California (after an -unpublished photograph by W. D. Johnson).</p> -</div></div> - -<p>It must not, however, be assumed that the temperature of lava -is always the same when it arrives at the surface, and hence it -may happen that a siliceous lava is exuded -at so high a temperature that it behaves -like a normal basaltic lava. On the other -hand, basaltic lavas may be extruded at -unusually low temperatures, in which case -their behavior may resemble that of the -normal siliceous lavas. If, however, as is -generally the case, the energy of explosion -of a basaltic lava is relatively small, any -ejected portions of the liquid lava travel -to a moderate height only in the air, so that on falling they are -still sufficiently pasty to -adhere to rock surfaces -and thus build up the -remarkably steep cones -and spines known as -“spatter cones” or -“driblet cones” (<a href="#f98">Fig. 98</a>). -When, on the other -hand, the energy of explosion -is great, as is normally -the case with siliceous -lavas, the portions<span class="pagenum"><a name="Page_105" id="Page_105">[105]</a></span> -of ejected lava have been fully consolidated before their fall to the -surface, so that they build up the same type of accumulation as -would sand falling in the same manner. The structures which -they form are known as tuff, cinder, or ash cones (<a href="#f99">Fig. 99</a>).</p> - -<p>Whenever the contained water passes off from siliceous lavas -without violent explosions, the lava may flow from the vent, but -in contrast to basaltic lavas it travels a short distance only before -consolidating. The resulting mountain is in consequence proportionately -high and steep (<a href="#f97">Fig. 97</a>). Eruptions characterized by -violent explosions accompanied by a fall of cinder are described -as <i>explosive</i> eruptions. Those which are relatively quiet, and in -which the chief product is in the form of streams of flowing lava, -are spoken of as <i>convulsive</i> eruptions.</p> - - -<p><b>The three main types of volcanic mountain.</b>—If the eruptions -at a volcanic vent are exclusively of the explosive type, the material -of the mountain which results is throughout tuff or cinder, -and the volcano is described as a <i>cinder cone</i>. If, on the other -hand, the vent at every eruption exudes lava, a mountain of solid -rock results which is a <i>lava dome</i>. It is, however, the exception -for a volcano which has a long history to manifest but a single -kind of eruption. At one time exuding lava comparatively -quietly, at another the violence with which the steam is liberated -yields only cinder, and the mountain is a composite of the two -materials and is known as a <i>composite volcanic cone</i>.</p> - - -<p><b>The lava dome.</b>—When successive lava flows come from a -crater, the structure which results has the form of a more or less -perfect dome. If the lava be of the basaltic or fluid type, the -slopes are flat, seldom making an angle of as much as ten degrees -with the horizon and flatter toward the summit (<a href="#f101">Fig. 101</a>, <a href="#Page_106">p. 106</a>). -If of siliceous or viscous lava, on the other hand, the slopes are -correspondingly steep and in some cases precipitous. To this -latter class belong some of the <i>Kuppen</i> of Germany, the <i>puys</i> of -central France, and the <i>mamelons</i> of the Island of Bourbon.</p> - -<div class="floatleft"> - <img src="images/ill-151a.jpg" width="250" height="282" id="f100" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 100.</span>—Map of Hawaii and the lava -volcanoes of Mokuaweoweo (Mauna -Loa) and Kilauea (after the government -map by Alexander).</p> -</div></div> - -<p><b>The basaltic lava domes of Hawaii.</b>—At the “crossroads of -the Pacific” rises a double line of lava volcanoes which reach -from 20,000 to 30,000 feet above the floor of the ocean, some -of them among the grandest volcanic mountains that are known. -More than half the height and a much larger proportion of the -bulk of the largest of these are hidden beneath the ocean’s surface.<span class="pagenum"><a name="Page_106" id="Page_106">[106]</a></span> -The two great active vents are Mokuaweoweo (on Mauna Loa) -and Kilauea, distinct volcanoes notwithstanding the fact that their -lava extravasations have been merged in a single mass. The rim -of the crater of Mauna Loa is at an elevation of 13,675 feet above -the sea, whereas that of Kilauea -is less than 4000 feet and appears -to rest upon the flank of -the larger mountain (<a href="#f100">Figs. 100</a> -and <a href="#f101">101</a>). Although one crater -is but 20 miles distant from the -other and nearly 10,000 feet -lower, their eruptions have apparently -been unsympathetic. -Nowhere have still active lava -mountains been subjected to -such frequent observations extending -throughout a long period, -and the dynamics of their -eruptions are fairly well understood. -To put this before the -reader, it will be best to consider -both mountains, for -though they have much in common, the observations from one are -strangely complementary to those of the other. The lower crater -being easily accessible, Kilauea has been often visited, and there -exists a long series of more or less consecutive observations upon -it, which have been assembled and studied by Dana and Hitchcock. -The place of outflow of the Kilauea lavas has not generally -been visible, whereas Mokuaweoweo has slopes rising nearly 14,000 -feet above the sea and displays the records of outflow of many -eruptions, some of which were accompanied by the grandest of -volcanic phenomena.</p> - -<div class="figcenter"> - <img src="images/ill-151b.jpg" width="400" height="52" id="f101" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 101.</span>—Section through Mauna Loa and Kilauea.</p> -</div></div> - -<p><b>Lava movements within the caldron of Kilauea.</b>—The craters -of these mountains are the largest of active ones, each being in<span class="pagenum"><a name="Page_107" id="Page_107">[107]</a></span> -excess of seven miles in circumference. In shape they are irregularly -elliptical and consist of a series of steps or terraces descending -to a pit at the bottom, in which are open lakes of boiling lava. -Enough is known of the history of Kilauea to state that the steep -cliffs bounding the terraces are fault walls produced by inbreak -of a frozen lava surface. The cliff below the so-called “black -ledge” was produced by the falling in of the frozen lava surface -at the time of the outflow of 1840, the lava issuing upon the -eastern flank of the mountain and pouring into the sea near -Nanawale. Since that date the floor of the pit below the level -of this ledge has been essentially a movable platform of frozen -lava of unknown and doubtless variable thickness which has risen -and descended like the floor of an elevator car between its guiding -ways (<a href="#f102">Fig. 102</a>). The floor has, however, never been complete, -for one or more open lakes are -always to be seen, that of Halemaumau -located near the southwestern -margin having been much -the most persistent. Within the -open lakes the boiling lava is apparently -white hot at the depth -of but a few inches below the -surface, and in the overturnings of the mass these hotter portions -are brought to the surface and appear as white streaks marking -the redder surface portions. From time to time the surface -freezes over, then cracks open and erupt at favored points along -the fissures, sending up jets and fountains of lava, the material of -which falls in pasty fragments that build up driblet cones. Small -fluid clots are shot out, carrying a threadlike line of lava glass -behind them, the well-known “Pelé’s hair.” Sometimes the open -lakes build up congealed walls, rising above the general level of -the pit, and from their rim the lava spills over in cascades to -spread out upon the frozen floor, thus increasing its thickness from -above (<a href="#f103">Fig. 103</a>). At other times a great dome of lava has been -pushed up from the pit of Halemaumau under a frozen shell, the -molten lava shining red through cracks in its surface and exuding -so as to heal each widely opened fissure as it forms.</p> - -<div class="floatright"> - <img src="images/ill-152.jpg" width="200" height="78" id="f102" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 102.</span>—Schematic diagram to illustrate -the moving platform of frozen -lava which rises and falls in the crater -of Kilauea.</p> -</div></div> - -<p>At intervals of from a few years to nine or ten years the crater -has been periodically drained, at which times the moving platform<span class="pagenum"><a name="Page_108" id="Page_108">[108]</a></span> -of frozen lava has sunk more or less rapidly to levels far -below the black ledge and from 900 to 1700 feet below the crater -rim. Following this descent a slow progressive rise is inaugurated, -which has sometimes gone on at a rate of more than a hundred -feet per year, though it is usually much slower than this. When -the platform has reached a height varying from 700 to 350 feet -below the crater rim, another sudden settlement occurs which -again carries the pit floor downward a distance of from 300 to 700 -feet.</p> - -<div class="figcenter"> - <img src="images/ill-153.jpg" width="400" height="265" id="f103" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 103.</span>—View of the open lava lake of Halemaumau within the crater of -Kilauea, the molten lava shown cascading over the raised lava walls on to the -floor of the pit (after Pavlow).</p> -</div></div> - -<p><b>The draining of the lava caldrons.</b>—The changes which go on -within the crater of Mokuaweoweo, though less studied than -those of Kilauea, appear to be in some respects different. Here -every eruption seems to be preceded by a more or less rapid influx -of melted lava to the pit of the crater, this phenomenon being -observed from a distance as a brilliant light above the crater—the -reflection of the glow from overhanging vapor clouds. The -uprising of the lava has often been accompanied by the formation -of high lava fountains upon the surface, and the molten lava -sometimes appears in fissures near the crater rim at levels well -above the lava surface within the pit.</p> - -<p><span class="pagenum"><a name="Page_109" id="Page_109">[109]</a></span></p> - -<p>Although in many cases the lava which has thus flooded the -crater has suddenly drained away without again becoming visible, -it is probable that in such cases an outlet has been found to some -submarine exit, since under-ocean discharge effects have been -observed in connection with eruptions of each of the volcanoes.</p> - -<div class="floatright"> - <img src="images/ill-154.jpg" width="250" height="178" id="f104" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 104.</span>—Map showing the manner of outflow of -lava from Kilauea during the eruption of 1840. -The outflowing lava made its appearance successively -at the points <i>A</i>, <i>B</i>, <i>C</i>, <i>m</i>, <i>n</i>, and finally at a -point below <i>n</i>, from whence it issued in volume and -flowed down to the sea at Nanawale (after J. D. -Dana).</p> -</div></div> - -<p>Inasmuch as no earthquakes are felt in connection with such -outflows as have been described, it is probable that the hot lava -fuses a passageway for itself into some open channel underneath -the flanks of the mountain. Such a course is well illustrated by -the outflow of Kilauea -in 1840, when, it will -be remembered, occurred -the great down-plunge -of the crater -that yielded the pit -below the black ledge. -At this time the lava -first made its appearance -upon the flanks -of the mountain at the -bottom of a small pit -or inbreak crater -which opened five -miles southeast of the -main crater of Kilauea -(<a href="#f104">Fig. 104</a>). Within -this new crater the -lava rose, and small ejections soon followed from fissures formed in -its neighborhood. Some time after, the lava sank in the first new -crater, only to reappear successively at other small openings (<a href="#f104">Fig. 104, <i>B</i>, <i>C</i>, <i>m</i>, <i>n</i></a>) and finally to issue in volume at a point eleven -miles from the shore and flow thereafter <i>upon the surface</i> of the -mountain until it had reached the sea. Only the slightest earth -tremors were felt, and as no rumblings were heard, it is evident -that the lava fused its way along a buried channel largely open at -the time (see below, <a href="#Page_112">p. 112</a>).</p> - -<p>In a majority of the eruptions of Mokuaweoweo, when the -outflowing lavas have become visible, the molten rock has apparently -fused its way out to the surface of the mountain at<span class="pagenum"><a name="Page_110" id="Page_110">[110]</a></span> -points from 1000 to 3000 feet below the bottom of the crater, -and this discharge has corresponded in time to the lowering -of the lava surface within the crater. There are, however, -three instances upon record in which the lava issued from definite -rents which were formed upon the mountain flanks at comparatively -low levels. In contrast to the formation of fused outlets, -these ruptures of a portion of the mountain’s flank were always -accompanied by vigorous local earthquakes of short duration. In -one instance (the eruption of 1851) such a rent appeared under -the same conditions but at an elevation of 12,500 feet, or near -the level of the lava in the crater.</p> - - -<p><b>The outflow of the lava floods.</b>—In order to properly comprehend -these and many otherwise puzzling phenomena connected -with volcanoes, it is necessary to keep ever in mind the quite -remarkable heat-insulating property of congealed lava. So soon -as a thin crust has formed upon the surface of molten rock, the -heat of the underlying fluid mass is given off with extreme slowness, -so that lava streams no longer connected with their internal -lava reservoirs may remain molten for decades.</p> - -<div class="figcenter"> - <img src="images/ill-155.jpg" width="400" height="208" id="f105" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 105.</span>—Lava of Matavanu upon the Island of Savaii flowing down to the -sea during the eruption of 1906. The course may be followed by the jets of steam -escaping from the surface down to the great steam cloud which rises where the -fluid lava discharges into the sea (after H. I. Jensen).</p> -</div></div> - -<p>We have seen that for Mokuaweoweo and Kilauea, lava either -quietly melts its way to the surface at the time of outflow, or -else produces a rent for its egress to the accompaniment of vigorous -local earthquakes. In either case if the lava issues at a point<span class="pagenum"><a name="Page_111" id="Page_111">[111]</a></span> -far below the crater, gigantic lava fountains arise at the point of -outflow, the fluid rock shooting up to heights which range from -250 to 600 or more feet above the surface. A certain proportion -of this fluid lava is sufficiently cooled to consolidate while traveling -in the air, and falling, it builds up a cinder cone which is left -as a location monument for the place of discharge. From this -outlet the molten lava begins its journey down the slope of the -mountain, and quickly freezes over to produce a tunnel, beneath -the roof of which the fluid lava flows with comparatively slow -further loss of heat. Save for occasional steam jets issuing from -its surface, it may give little indication of its presence until it has -reached the sea (<a href="#f105">Fig. 105</a>).</p> - -<div class="figcenter"> - <img src="images/ill-156.jpg" width="400" height="315" id="f106" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 106.</span>—Lava stream discharging into the sea from beneath the frozen roof of -a lava tunnel. Eruption of Matavanu on Savaii in 1906 (after Sapper).</p> -</div></div> - -<p>If sufficient in volume and the shore be not too distant, the -stream of lava arrives at the sea, where, discharging from the -mouth of its tunnel, it throws up vast volumes of steam and induces -ebullition of the water over a wide area (<a href="#f106">Fig. 106</a>). Professor -Dana, who visited Hawaii a few months only after the -great outflow of 1840, states that the lava, upon reaching the<span class="pagenum"><a name="Page_112" id="Page_112">[112]</a></span> -ocean, was shivered like melted glass and thrown up in millions of -particles which darkened the sky and fell like hail over the surrounding -country. The light was so bright that at a distance of -forty miles fine print could be read at midnight.</p> - -<div class="floatleft"> - <img src="images/ill-157a.jpg" width="250" height="65" id="f107" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 107.</span>—Diagrammatic representation -of the structure of the -flanks of lava volcanoes as a result -of the draining of frozen lava -streams.</p> -</div></div> - -<p>Protected from any extensive consolidation by its congealed -cover, the lava within a stream may all drain away, leaving behind -an empty lava tunnel, which in the -case of the Hawaiian volcanoes -sometimes has its roof hung with -beautiful lava stalactites and its -floor studded with thin lava spines. -Later lava outflows over the same -or neighboring courses bury such -tunnels beneath others of similar -nature, giving to the mountain flanks an elongated cellular structure -illustrated schematically in <a href="#f107">Fig. 107</a>. These buried channels -may in the future be again utilized for outflows similar in character -to that of Kilauea in 1840.</p> - -<div class="floatright"> - <img src="images/ill-157b.jpg" width="250" height="64" id="f108" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 108.</span>—Diagram to show the manner of formation -of mesas or table mountains by the outflow -of lava in valleys and the subsequent more rapid -erosion of the intervening ridges. <i>R</i>, earlier river -valley; <i>R’R’</i>, later valleys.</p> -</div></div> - -<p>While the formation of lava stalactites of such perfection and -beauty is peculiar to the Hawaiian lava tunnels, the formation of -the tunnel in connection with lava outflow is the rule wherever a dissipation -at the end has permitted of drainage. A few hours only -after the flow has begun, the frozen surface has usually a thickness -of a few inches, and this cover may be walked over with the lava still -molten below. At first in part supported by the molten lava, the -tunnel roof sometimes caves in so soon as drainage has occurred.</p> - -<div class="figcenter"> - <img src="images/ill-158a.jpg" width="400" height="196" id="f109" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 109.</span>—Surface of lava of the Pahoehoe type.</p> -</div></div> - -<p>Wherever basaltic lava has spread out in valleys on the surface -of more easily eroded -material, either cinder -or sedimentary formations, -the softer intervening -ridges are first -carried away by the -eroding agencies, leaving -the lava as cappings -upon residual elevations. -Thus are derived a type of table mountain or <i>mesa</i> of the -sort well illustrated upon the western slopes of the Sierra Nevadas -in California (<a href="#f108">Fig. 108</a>).</p> - -<p><span class="pagenum"><a name="Page_113" id="Page_113">[113]</a></span></p> - -<div class="floatright"> - <img src="images/ill-158b.jpg" width="250" height="334" id="f110" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 110.</span>—Three successive views to -illustrate the growth of the Island of -Savaii from the outflow of lava at -Matavanu in the year 1906. <i>a</i>, near the -beginning of the outflow; <i>b</i>, some weeks -later than <i>a</i>; <i>c</i>, some weeks later than -<i>b</i> (after H. I. Jensen).</p> -</div></div> - -<p>The surface which flowing lava assumes, while subject to considerable -variation, may yet be classified into two rather distinct -types. On the one hand there is the billowy surface in which -ellipsoidal or kidney-shaped masses, each with dimensions of from -one to several feet, lie merged -in one another, not unlike an -irregular collection of sofa -pillows. This type of lava has -become known as the <i>Pahoehoe</i>, -from the Hawaiian occurrence -(<a href="#f109">Fig. 109</a>). A variation from -this type is the “corded” or -“ropy” lava, the surface of -which much resembles rope as -it is coiled along the deck of -a vessel, the coils being here the -lines of scum or scoriæ arranged -in this manner by the currents -at the surface of the stream -(<a href="#f123">Fig. 123</a>, <a href="#Page_124">p. 124</a>). A quite -different type is the block lava -(<i>Aa</i> type) which usually has a -ragged scoriaceous surface and -consists of more or less separate -fragments of cooled lava (<a href="#f131">Fig. 131</a>, <a href="#Page_130">p. 130</a>).</p> - -<p><span class="pagenum"><a name="Page_114" id="Page_114">[114]</a></span></p> - -<p>Wherever lava flows into the sea in quantity, it extends the -margin of the shore, often by considerable areas. The outflow of -Kilauea in 1840 extended the shore of Hawaii outward for the -distance of a quarter of a mile, and a more recent illustration of -such extension of land masses is furnished by <a href="#f110">Fig. 110</a>.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_115" id="Page_115">[115]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER X</h2> - -<p class="pch">THE RISE OF MOLTEN ROCK TO THE EARTH’S -SURFACE</p> - -<p class="psh">VOLCANIC MOUNTAINS OF EJECTED MATERIALS</p> - -<p><b>The mechanics of crater explosions.</b>—If we now turn from -the lava volcano to the active cinder cone, we encounter an entire -change of scene. In place of the quiet flow and convulsive movements -of the molten lava, we here meet with repeated explosions -of greater or less violence. If we are to profitably study the -manner of the explosions, considering the volcanic vent as a great -experimental apparatus, it would be well to select for our purpose -a volcano which is in a not too violent mood. The well-known -cinder cone of Stromboli in the Eolian group of islands north of -Sicily has, with short and unimportant interruptions, remained in -a state of light explosive activity since the beginning of the Christian -era. Rising as it does some three thousand feet directly out -of the Mediterranean, and displaying by day a white steam cap -and an intermittent glow by night, its summit can be seen for a -distance of a hundred miles at sea and it has justly been called -the “Lighthouse of the Mediterranean.” The “flash” interval -of this beacon may vary from one to twenty minutes, and it may -show, furthermore, considerable variation of intensity.</p> - -<p>For the reason that the crater of the mountain is located at -one side and at a considerable distance below the actual summit, -the opportunity here afforded of looking into the crater is most -favorable whenever the direction of the wind is such as to push -aside the overhanging steam cloud (<a href="#f111">Fig. 111</a>). Long ago the -Italian vulcanologist Spallanzani undertook to make observations -from above the crater, and many others since his day have profited -by his example.</p> - -<p>Within the crater of the volcano there is seen a lava surface -lightly frozen over and traversed by many cracks from which -vapor jets are issuing. Here, as in the Kilauea crater, there are -open pools of boiling lava. From some of these, lava is seen<span class="pagenum"><a name="Page_116" id="Page_116">[116]</a></span> -welling out to overflow the frozen surface; from others, steam is -ejected in puffs as though from the stack of a locomotive. Within -others lava is seen heaving up and down in violent ebullition, and -at intervals a great bubble of steam is ejected with explosive violence, -carrying up with it a considerable quantity of the still -molten lava, together with its scumlike surface, to fall outside the -crater and rattle down the mountain’s slope into the sea. Following -this explosion the lava surface in the pool is lowered and -the agitation is renewed, to culminate after the further lapse of a -few minutes in a second explosion of the same nature. The rise -of the lava which -precedes the ejection -appears at night as a -brighter reflection or -glow from the overhanging -steam cloud—the -flash seen by -the mariner from his -vessel.</p> - -<div class="floatleft"> - <img src="images/ill-161.jpg" width="250" height="168" id="f111" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 111.</span>—The volcano of Stromboli, showing the -excentric position of the crater (after a sketch by -Judd).</p> -</div></div> - -<p>What is going on -within the crater of -Stromboli we may -perhaps best illustrate -by the boiling -of a stiff porridge -over a hot fire. Any one who has made corn mush over a hot -camp fire is fully aware that in proportion as the mush becomes -thicker by the addition of the meal, it is necessary to stir the -mass with redoubled vigor if anything is to be retained within the -kettle. The thickening of the mush increases its viscosity to such -an extent that the steam which is generated within it is unable to -make its escape unless aided by openings continually made for it -by the stirring spoon. If the stirring motion be stopped for a -moment, the steam expands to form great bubbles which soon -eject the pasty mass from the kettle.</p> - -<p>For the crater of Stromboli this process is illustrated by the -series of diagrams in <a href="#f112">Fig. 112</a>. As the lava rises toward the -surface, presumably as a result of convectional currents within -the chimney of the volcano, the contained steam is relieved from<span class="pagenum"><a name="Page_117" id="Page_117">[117]</a></span> -pressure, so that at some depth below the surface it begins to -separate out in minute vesicles or bubbles, which, expanding as -they rise, acquire a rapidly accelerating velocity. Soon they flow -together with a quite sudden increase of their expansive energy, -and now shooting upward with further accelerated velocity, a -layer of liquid lava with its cover of scum is raised on the surface -of a gigantic bubble and thrown high into the air. Cooled during -their flight, the quickly congealed lava masses become the tuff or -volcanic ash which is the material of the cinder cone.</p> - -<div class="figcenter"> - <img src="images/ill-162.jpg" width="400" height="242" id="f112" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 112.</span>—Diagrams to illustrate the nature of eruptions within the crater of -Stromboli.</p> -</div></div> - -<p><b>Grander volcanic eruptions of cinder cones.</b>—Most cinder and -composite cones, in the intervals between their grander eruptions, -if not entirely quiescent, lapse into a period, of light activity -during which their crater eruptions appear to be in all essential -respects like the habitual explosions within the Strombolian -crater. This phase of activity is, therefore, described as <i>Strombolian</i>. -By contrast, the occasional grander eruptions which have -punctuated the history of all larger volcanoes are described in -the language of Mercalli as <i>Vulcanian</i> eruptions, from the best -studied example.</p> - -<p>Just what it is that at intervals brings on the grander Vulcanian -outburst within a volcano is not known with certainty; -but it is important to note that there is an approach to periodicity<span class="pagenum"><a name="Page_118" id="Page_118">[118]</a></span> -in the grander eruptions. It is generally possible to distinguish -eruptions of at least two orders of intensity greater than the -Strombolian phase; a grander one, the examples of which may -be separated by centuries, and one or more orders of relatively -moderate intensity which recur at intervals perhaps of decades, -their time intervals subdividing the larger periods marked off by -the eruptions of the first order.</p> - -<div class="floatleft"> - <img src="images/ill-163.jpg" width="200" height="305" id="f113" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 113.</span>—Map of Volcano -in the Eolian group -of islands. The smaller -craters partially dissected -by the waves belong to -Vulcanello (after Judd).</p> -</div></div> - -<p><b>The eruption of Volcano in 1888.</b>—In the Eolian Islands to -the north of Sicily was located the mythical forge of Vulcan. -From this locality has come our word “volcano”, and both the -island and the mountain bear no other -name to-day (<a href="#f113">Fig. 113</a>). There is in the -structure of the island the record of a -somewhat complex volcanic history, but -the form of the large central cinder cone -was, according to Scrope, acquired during -the eruption of 1786, at which time the -crater is reported to have vomited ash for -a period of fifteen days. Passing after -this eruption into the solfatara condition, -with the exception of a light eruption in -1873, the volcano remained quiet until -1886. So active had been the fumeroles -within the crater during the latter part of -this period that an extensive plant had -been established there for the collection -especially of boracic acid. In 1886 occurred -a slight eruption, sufficient to clear out the bottom of the crater, -though not seriously to disturb the English planter whose vineyards -and fig orchards were in the valley or <i>atrio</i> near the point <i>d</i> -upon the map (<a href="#f113">Fig. 113</a>), nearly a mile from the crater rim. On -the 3d of August, 1888, came the opening discharge of an eruption, -which, while not of the first order of magnitude, was yet the greatest -in more than a century of the mountain’s history, and may serve us -to illustrate the Vulcanian phase of activity within a cinder cone. -During the day, to the accompaniment of explosions of considerable -violence, projectiles fell outside the crater rim and rolled -down the steep slopes toward the <i>atrio</i>. These explosions were -repeated at intervals of from twenty to thirty minutes, each<span class="pagenum"><a name="Page_119" id="Page_119">[119]</a></span> -beginning in a great upward rush of steam and ash, accompanied -by a low rumbling sound. During the following night the eruptions -increased in violence, and the anxious planter remained on -watch in his villa a mile from the crater. Falling asleep toward -morning, he was rudely awakened by a rain of projectiles falling -upon his roof. Hastily snatching up his two children he ran -toward the door just as a red hot projectile, some two feet in diameter, -descended through the roof, ceiling, and floor of the drawing -room, setting fire to the building. A second projectile similar to -the first was smashed into fragments at his feet as he was emerging -from the house, burning one of the children. Making his -escape to Vulcanello at the extremity of the island, the remainder -of the night and the following day, until rescue came from Lipari, -were spent just beyond the range -of the falling masses.</p> - -<div class="floatright"> - <img src="images/ill-164.jpg" width="250" height="138" id="f114" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 114.</span>—“Bread-crust” lava projectile -from the eruption of Volcano -in 1888 (after Mercalli).</p> -</div></div> - -<p>When the writer visited the -island some months later, the -eruption was still so vigorous that -the crater could not be reached. -The ruined villa, smashed and -charred, stood with its walls half -buried in ash and lapilli, among -which were partly smashed pumiceous -lava projectiles. The entire <i>atrio</i> about the mountain lay -buried in cinder to the depth of several feet and was strewn with -projectiles which varied in size from a man’s fist to several feet -in diameter (<a href="#f114">Fig. 114</a>). The larger of these exhibited the peculiar -“bread-crust” surface and had generally been smashed by the -force of their fall after the manner of a pumpkin which has been -thrown hard against the ground. One of these projectiles fully -three feet in diameter was found at the distance of a mile and a -half from the crater. Though diminished considerably in intensity, -the rhythmic explosions within the crater still recurred at -intervals varying from four minutes to half an hour, and were -accompanied by a dull roar easily heard at Lipari on a neighboring -island six miles away. Simultaneously, a dark cloud of “smoke”, -the peculiar “cauliflower cloud” or <i>pino</i> mounted for a couple -of miles above the crater (<a href="#f115">Fig. 115</a>), and the rise was succeeded -by a rain of small lava fragments or <i>lapilli</i> outside the crater rim.</p> - -<p><span class="pagenum"><a name="Page_120" id="Page_120">[120]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-165a.jpg" width="200" height="157" id="f115" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 115.</span>—Peculiar “cauliflower cloud” -or <i>pino</i> composed of steam and ash, -rising above the cinder cone of Volcano -during the waning phases of the explosive -eruption of 1888 (after a photograph -by B. Hobson).</p> -</div></div> - -<p>There seems to be no good -reason to doubt that Vulcanian -cinder eruptions of this type -differ chiefly in magnitude from -the rhythmic explosion within -the crater of Stromboli, if we -except the elevation of a considerable -quantity of accessory -and older tuff which is -derived from the inner walls -of the crater and carried upward -into the air together -with the pasty cakes of fresh -lava derived from the chimney. -It is this accessory material -which gives to the <i>pino</i> its dark or even black appearance.</p> - -<div class="floatright"> - <img src="images/ill-165b.jpg" width="200" height="310" id="f116" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 116.</span>—Double explosive eruption of -Taal volcano on the morning of January -30, 1911.</p> -</div></div> - -<p><b>The eruption of Taal volcano on January 30, 1911.</b>—The -recent eruption of the cinder -cone known as Taal volcano -is of interest, not only because -so fresh in mind, but because -two neighboring vents erupted -simultaneously with explosions -of nearly equal violence (<a href="#f116">Fig. 116</a>). -This Philippine volcano -lies near the center of a -lake some fifteen miles in -diameter and about fifty miles -south of the city of Manila. -After a period of rest extending -over one hundred and fifty -years, the symptoms of the -coming eruption developed -rapidly, and on the morning -of January 30 grand explosions -of steam and ash occurred -simultaneously in the -neighboring craters, and the -condensed moisture brought<span class="pagenum"><a name="Page_121" id="Page_121">[121]</a></span> -down the ash in an avalanche of scalding mud which buried the -entire island. Almost the entire population of the island, numbering -several hundreds, -was literally buried in the -blistering mud (<a href="#f117">Fig. 117</a>); -and the gases from the explosions -carried to the distant -shores of the lake -added to this number many -hundred victims.</p> - -<div class="floatright"> - <img src="images/ill-166a.jpg" width="280" height="380" id="f117" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 117.</span>—The thick mud veneer upon the -island of Taal (after a photograph by -Deniston).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-166b.jpg" width="280" height="193" id="f118" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 118.</span>—A pear-shaped lava projectile.</p> -</div></div> - -<p>The shocks which accompanied -the explosions raised -a great wave upon the surface -of the lake, which, advancing -upon the shores, -washed away structures for -a distance of nearly a half -mile.</p> - -<p><b>The materials and the -structure of cinder cones.</b>—Obviously -the materials -which compose cinder cones -are the cooled lava fragments -of various degrees of -coarseness which have been ejected from the crater. If larger -than a finger joint, such fragments are referred to as <i>volcanic -projectiles</i>, or, incorrectly, as “volcanic bombs.” Of the larger -masses it is often true that the force of expulsion has not been -applied opposite the center -of mass of the body. -Thus it follows that they -undergo complex whirling -motions during their -flight, and being still -semiliquid, they develop -curious pear-shaped or -less regular forms (<a href="#f118">Fig. 118</a>). -When crystals -have already separated<span class="pagenum"><a name="Page_122" id="Page_122">[122]</a></span> -out in the lava before its rise in the chimney of the volcano, the -surrounding fluid lava may be blown to finely divided volcanic -dust which floats away upon the wind, thus leaving the crystals -intact to descend as a crystal rain about the crater. Such a -shower occurred in connection with the eruption of Etna in 1669, -and the black augite crystals may to-day be gathered by the -handful from the slopes of the Monti Rossi (<a href="#f125">Fig. 125</a>, <a href="#Page_125">p. 125</a>).</p> - -<div class="floatleft"> - <img src="images/ill-167.jpg" width="250" height="283" id="f119" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 119.</span>—Artificial production of the structure of -a cinder cone with use of colored sands carried up -in alternation by a current of air (after G. Linck).</p> -</div></div> - -<p>The term <i>lapilli</i>, or sometimes <i>rapilli</i>, is applied to the ejected -lava fragments when of the average size of a finger joint. This is -the material which still -partially covers the unexhumed -portions of the -city of Pompeii. Volcanic -<i>sand</i>, <i>ash</i>, and <i>dust</i> -are terms applied in -order to increasingly -fine particles of the -ejected lava. The finest -material, the volcanic -dust, is often carried -for hundreds and sometimes -even for thousands -of miles from the -crater in the high-level -currents of the atmosphere. -Inasmuch as -this material is deposited -far from the -crater and in layers -more or less horizontal, -such material plays a small rôle in the formation of the cinder -cone. The coarser sands and ash, on the other hand, are the -materials from which the cinder cone is largely constructed.</p> - -<p>The manner of formation and the structure of cinder cones -may be illustrated by use of a simple laboratory apparatus (<a href="#f119">Fig. 119</a>). -Through an opening in a board, first white and then -colored sand is sent up in a light current of air or gas supplied -from suitable apparatus. The alternating layers of the sand -form in the attitudes shown; that is to say, dipping inward or<span class="pagenum"><a name="Page_123" id="Page_123">[123]</a></span> -toward the chimney of the volcano at all points within the crater -rim, and outward or away from it at all points outside (<a href="#f119">Fig. 119</a>). -If the experiment is carried so far that at its termination sand -slides down the crater walls into the chimney below, the inward -dipping layers will be truncated, or even removed entirely, as -shown in <a href="#f119">Fig. 119 <i>b</i></a>.</p> - - -<div class="figcenter"> - <img src="images/ill-168a.jpg" width="400" height="83" id="f120" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 120.</span>—Diagram to show the contrast between a lava dome and a cinder cone. -<i>AAA</i>, cinder cone; <i>BabC</i>, lava dome; <i>DE</i>, line of low cinder cones above a fissure -(after Thoroddsen).</p> -</div></div> - -<p><b>The profile lines of cinder cones.</b>—The shapes of cinder cones -are notably different from those of lava mountains. While the -latter are domes, the mountains constructed of cinder are conical -and have curves of profile that are concave upward instead of -convex (<a href="#f120">Fig. 120</a>). In the earlier stages of its growth the cinder -cone has a crater which in proportion to the height of the mountain -is relatively broad (<a href="#f99">Fig. 99</a>, <a href="#Page_104">p. 104</a>).</p> - -<div class="floatright"> - <img src="images/ill-168b.jpg" width="250" height="185" id="f121" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 121.</span>—Mayon volcano on the island -of Luzon, P.I. A remarkably perfect -high cinder cone.</p> -</div></div> - -<p>Speaking broadly, the diameter of the crater is a measure of -the violence of the explosions within the chimney. A single series -of short and violent explosive -eruptions builds a low and -broad cinder cone. A long-continued -succession of moderately -violent explosions, on the -other hand, builds a high cone -with crater diameter small if -compared with the mountain’s -altitude, and the profile afforded -is a remarkably beautiful sweeping -curve (<a href="#f121">Fig. 121</a>). Toward -the summit of such a cone the -loose materials of which it is composed are at as steep an angle -as they can lie, the so-called angle of repose of the material; -whereas lower down the flatter slopes have been determined by the -distribution of the cinder during its fall from the air. When one<span class="pagenum"><a name="Page_124" id="Page_124">[124]</a></span> -makes the ascent of such a mountain, he encounters continually -steeper grades, with the most difficult slope just below the crest.</p> - -<div class="floatleft"> - <img src="images/ill-169a.jpg" width="200" height="62" id="f122" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 122.</span>—A series of breached -cinder cones where the place of -eruption has migrated along the -underlying fissure. The Puys -Noir, Solas, and La Vache in the -Mont Dore Province of central -France (after Scrope).</p> -</div></div> - -<p><b>The composite cone.</b>—The life -histories of volcanoes are generally -so varied that lava domes and the -pure types of cinder cones are less -common than volcanoes in which -paroxysmal eruptions have alternated -with explosions, and where, therefore, -the structure of the mountain represents -a composite of lava and cinder. -Such composite cones possess a skeleton -of solid rock upon which have been built up alternate sloping -layers of cinder and lava. In most respects such cones stand in -an intermediate position between -lava domes and cinder -cones.</p> - -<div class="floatright"> - <img src="images/ill-169b.jpg" width="200" height="292" id="f123" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 123.</span>—The <i>bocca</i> or mouth upon the -inner cone of Mount Vesuvius from which -flowed the lava stream of 1872. This -lava stream appears in the foreground -with its characteristic “ropy” surface.</p> -</div></div> - -<p>Regarded as a retaining wall -for the lava which mounts in -the chimney, the cinder cone -is obviously the weakest of -all. Should lava rise in a -cinder cone without an explosion -occurring, the cone is -at once broken through upon -one side by the outwelling -of the lava near the base. -Thus arises the characteristic -<i>breached</i> cone of horseshoe -form (<a href="#f122">Fig. 122</a>).</p> - -<div class="floatleft"> - <img src="images/ill-170a.jpg" width="200" height="136" id="f124" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 124.</span>—A row of parasitic cones raised -above a fissure which was opened upon -the flanks of Mount Etna during the -eruption of 1892 (after De Lorenzo).</p> -</div></div> - -<p>Quite in contrast with the -weak cinder cone is the lava -dome with its rock walls and -relatively flat slopes. Considered -as a retaining wall for -lava it is much the strongest -type of volcanic mountain, -and it is likely that the hydrostatic pressure of the lava within -the crater would seldom suffice to rupture the walls, were it not<span class="pagenum"><a name="Page_125" id="Page_125">[125]</a></span> -that the molten rock first fuses its way into old stream tunnels -buried under the mountain slopes (see <i>ante</i>, p. 112). Composite -cones have a strength as retaining walls for lava which is intermediate -between that of the -other types. Their Vulcanian -eruptions of the convulsive -type are initiated by the formation -of a rent or fissure upon -the mountain flanks at elevations -well above the base, the -opening of the fissure being -generally accompanied by a -local earthquake of greater or -less violence.</p> - -<p>From one or -more such fissures the lava -issues usually with sufficient -violence at the place of outflow to build up over it either an -enlarged type of driblet cone, referred to as a “mouth”, or <i>bocca</i><a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> -(<a href="#f123">Fig. 123</a>), or one or more cinder cones which from their position -upon the flanks of the larger volcano are referred to as <i>parasitic -cones</i> (<a href="#f124">Fig. 124</a>). The lava of Vesuvius more frequently yields -<i>bocchi</i> at the place of outflow, whereas the flanks of Etna are -pimpled with great numbers of parasitic cinder cones, each the -monument to some earlier eruption (<a href="#f125">Fig. 125</a>).</p> - -<p><span class="pagenum"><a name="Page_126" id="Page_126">[126]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-170b.jpg" width="400" height="221" id="f125" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 125.</span>—View looking toward the summit of Etna from a position upon the -southern flank near the village of Nicolosi. The two breached parasitic cones -seen behind this village are the Monti Rossi which were thrown up in 1669 and -from which flowed the lava which overran Catania (after a photograph by -Sommer).</p> -</div></div> - -<p>It is generally the case that a single -eruption makes but a relatively small -contribution to the bulk of the mountain. -From each new cone or <i>bocca</i> there proceeds -a stream of lava spread in a relatively -narrow stream extending down the -slopes (<a href="#f126">Fig. 126</a>).</p> - -<div class="floatleft"> - <img src="images/ill-171a.jpg" width="200" height="204" id="f126" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 126.</span>—Sketch map of -Etna, showing the individual -surface lava streams -(in black) and the tuff -covered surface (stippled).</p> -</div></div> - -<p><b>The caldera of composite cones.</b>—Because -of the varied episodes in the -history of composite cones, they lack the -regular lines characteristic of the two -simpler types. The larger number of the -more important composite cones have -been built up within an outer crater of -relatively large diameter, the Somma cone or <i>caldera</i>, which -surrounds them like a gigantic ruff or collar. This caldera is -clearly in most cases at least the relic of an earlier explosive -crater, after which successive eruptions of lesser violence have -built a more sharply conical structure. This can only be interpreted -to mean that most larger and long-active volcanoes have -been born in the grandest throes of their life history, and that a -larger or smaller lateral migration of the vent has been responsible -for the partial destruction of the explosion crater. Upon Vesuvius<span class="pagenum"><a name="Page_127" id="Page_127">[127]</a></span> -we find the crescent-like rim of Monte Somma; on Etna it -is the Val del Bove, etc. It is this caldera of composite cones -which gave rise to the theory of the “elevation crater” of von -Buch (see <i>ante</i>, <a href="#Page_95">p. 95</a>, and <a href="#f127">Fig. 127</a>).</p> - -<div class="figcenter"> - <img src="images/ill-171b.jpg" width="400" height="175" id="f127" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 127.</span>—Panum crater, showing the caldera and the later interior cones -(after Russell).</p> -</div></div> - -<p><b>The eruption of Vesuvius in 1906.</b>—The volcano Vesuvius -rises on the shores of the beautiful bay of Naples only about ten -miles distant from the city of Naples. The mountain consists of -the remnant of an earlier broad-mouthed explosion crater, the -Monte Somma, and an inner, more conical elevation, the Monte -Vesuvio. Before the eruption of 1906 this central cone was sharply -conical and rose to -a height of about -4300 feet above -the surface of the -bay, or above the -highest point of the -ancient caldera. -The base of this -inner cone is at an -elevation of something -less than half -that of the entire -mass, and is separated -from the encircling -ring wall of the old crater by the <i>atrio</i>, to which corresponds -in height a perceptible shelf or <i>piano</i> upon the slope toward -the bay of Naples (<a href="#f128">Fig. 128</a>).</p> - -<div class="floatright"> - <img src="images/ill-172.jpg" width="250" height="147" id="f128" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 128.</span>—View of Mount Vesuvius as it appeared from -the Bay of Naples shortly before the eruption of 1906. -The horn to the left is Monte Somma.</p> -</div></div> - -<p>An active composite cone like that of Vesuvius is for the greater -part of the time in the Strombolian condition; that is to say, light -crater explosions continue with varying intensity and interval, -except when the mountain has been excited to the periodic Vulcanian -outbreaks with which its history has been punctuated. -The Strombolian explosions have sufficient violence to eject small -fragments of hot lava, which, falling about the crater, slowly built -up a rather sharp cone. The period of Strombolian activity has, -therefore, been called the <i>cone-producing period</i>. Just before each -new outbreak of the Vulcanian type, the altitude of the mountain -has, therefore, reached a maximum, and since the larger explosive -eruptions remove portions of this cone at the same time that<span class="pagenum"><a name="Page_128" id="Page_128">[128]</a></span> -they increase the dimensions of the crater, the Vulcanian stage in -contrast to the other has been called the <i>crater-producing period</i>. -In this period, then, the material ejected during the explosions does -not consist solely of fresh lava cakes, but in part of the older débris -derived from the crater walls, whence it is avalanched upon the -chimney after each larger explosion. The overhanging -cloud, which during the Strombolian -period has consisted largely of steam and is -noticeably white, now assumes a darker tone, -the “smoke” which characterizes the Vulcanian -eruption.</p> - -<div class="floatleft"> - <img src="images/ill-173.jpg" width="150" height="515" id="f129" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 129.</span>—A series -of consecutive -sketches of the -summit of the -Vesuvian cone, -showing the modifications -in its outline -(after Sir William -Hamilton).</p> -</div></div> - -<p>On several historical occasions the cone of -Vesuvius has been lowered by several hundred -feet, the greatest of relatively recent truncations -having occurred in 1822 and in 1906. Between -Vulcanian eruptions the Strombolian activity is -by no means uniform, and so the upward growth -of the cone is subject to lesser interruptions and -truncations (<a href="#f129">Fig. 129</a>).</p> - -<p>The Vesuvian eruption of 1906 has been -selected as a type of the larger Vulcanian eruption -of composite cones, because it combined the -explosive and paroxysmal elements, and because -it has been observed and studied with greater -thoroughness than any other. The latest previous -eruption of the Vulcanian order had -occurred in 1872. Some two years later the -period of active cone building began and proceeded -with such rapidity that by 1880 the new -cone began to appear above the rim of the crater -of 1872. From this time on occasional light -eruptions interrupted the upbuilding process, -and as the repairs were not in all cases completed -before a new interruption, a nest of cones, each smaller -than the last, arose in series like the outdrawn sections of an old-time -spyglass. At one time no less than five concentric craters -were to be seen.</p> - -<p>For a brief period in the fall of 1904 Vesuvius had been in almost -absolute repose, but soon thereafter the Strombolian crater explosions<span class="pagenum"><a name="Page_129" id="Page_129">[129]</a></span> -were resumed. On May 25, 1905, a small stream of lava -began to issue from a fissure high up upon the central cone, and -from this time on the lava continued to flow down to the valley or -<i>atrio</i>, separating the inner cone from the caldera remnant of Monte -Somma. Seen in the night, this stream of lava appeared from -Naples like a red hot wire laid against the mountain’s side (<a href="#f130">Fig. 130</a>). -With gradual augmentation of Strombolian explosions -and increase in volume of the flowing lava stream, the same condition -continued until the first days of April in 1906. The flowing -lava had then overrun the tracks of the mountain railway and -accumulated in considerable quantity within the <i>atrio</i> (<a href="#f131">Fig. 131</a>).</p> - -<div class="figcenter"> - <img src="images/ill-174.jpg" width="350" height="469" id="f130" - alt="" - title="" /> - <div class="caption"><p class="ch350"><span class="smcap">Fig. 130.</span>—Night view of Vesuvius from Naples before -the outbreak of 1906. A small lava stream is seen -descending from a high point upon the central cone -(after Mercalli).</p> -</div></div> - -<p>On the morning of April 4, a preliminary stage of the eruption -was inaugurated by the opening of a new radial fissure about 500<span class="pagenum"><a name="Page_130" id="Page_130">[130]</a></span> -feet below the summit of the cone (<a href="#f132">Fig. 132 <i>a</i></a>), and by early afternoon -the cone-destroying stage began with the rise of a dark “cauliflower -cloud” or <i>pino</i> to replace the lighter colored steam cloud. -The cone was beginning to fall into the crater, and old lava débris -was mingled in the ejections with the lava clots blown from the -still fluid material within the chimney. From now on short and -snappy lightning flashes played about the black cloud, giving out -a sharp staccato “tack-a-tack.” The volume and density of the -cloud and the intensity of the crater explosions continued to increase -until the culmination on April 7. On April 5 at midnight a -new lava mouth appeared upon the same fissure which had opened -near the summit, but now some 300 feet lower (<a href="#f132">Fig. 132 <i>b</i></a>). The -lava now welled out in larger volume corresponding to its greater -head, and the stream which for ten months had been flowing from -the highest outlet upon the cone now ceased to flow. The next -morning, April 6, at about 8 o’clock, lava broke out at several -points some distance east of the opening <i>b</i>, and evidently upon -another fissure transverse to the first (<a href="#f132">Fig. 132 <i>c</i></a>). The lava surface -within the chimney must still have remained near its old -level,—effective draining had not yet begun,—since early upon -the following morning a small outflow began nearly at the top of -the cone upon the opposite side and at least a thousand feet higher.</p> - -<div class="figcenter"> - <img src="images/ill-175.jpg" width="400" height="249" id="f131" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 131.</span>—Scoriaceous lava encroaching upon the tracks of the Vesuvian railway -(after a photograph by Sommer).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_131" id="Page_131">[131]</a></span></p> - -<div class="floatright"> - <img src="images/ill-176.jpg" width="200" height="367" id="f132" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 132.</span>—Map of Vesuvius, showing the position -and order of formation of the lava mouths upon -its flanks during the eruption of 1906 (after -Johnston-Lavis).</p> -</div></div> - -<p>The culmination of the eruption came in the evening of April 7, -when, to the accompaniment of light earthquakes felt as far as -Naples, lava issued for the first time in great volume from a mouth -more than halfway -down the mountain side -(<a href="#f132">Fig. 132 <i>f</i></a>), and thus -began the drainage of -the chimney. At about -the same time with loud -detonations a huge -black cloud rose above -the crater in connection -with heavy explosions, -and a rain of cinder was -general in the region -about the mountain but -especially within the -northeast quadrant. -Those who were so fortunate -as to be in Pompeii -had a clear view of -the mountain’s summit -where red hot masses of -lava were thrown far -into the air. The direction -of these projections -was reported to have -been not directly upward, -but inclined -toward the northeast -quadrant of the mountain; -but since with a -northeast surface wind -the heaviest deposit of -ash and dust should -have been upon the southwestern quadrant of the mountain, it -is evident that the material was carried upward until it reached -the contrary upper currents of the atmosphere, to be by them distributed.</p> - -<p><span class="pagenum"><a name="Page_132" id="Page_132">[132]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-177a.jpg" width="400" height="233" id="f133" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 133.</span>—The ash curtain which had overhung Vesuvius lifting and disclosing -the outlines of the mountain on April 10, 1911 (after De Lorenzo).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-177b.jpg" width="250" height="154" id="f134" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 134.</span>—The central cone of Vesuvius as it -appeared after the eruption of 1906, but with -the earlier profile indicated. The truncation -represents a lowering of the summit by some -five hundred feet, with corresponding increase in -the diameter of the crater (after Johnston-Lavis).</p> -</div></div> - -<p>When the heavy curtain of ash, which now for a number of -succeeding days overhung all the circum-Vesuvian country, began -to lift (<a href="#f133">Fig. 133</a>), it was seen that the summit of the cone had been -truncated an average of some 500 feet (<a href="#f134">Fig. 134</a>). All the slopes -and much of the surrounding country had the aspect of being -buried beneath a cocoa-colored -snow of a depth -to the northeastward of -several feet, where it had -drifted into all the hollow -ways so as almost to -efface them (<a href="#f135">Fig. 135</a>). -More than thrice as -heavy as water, the weak -roof timbers of the houses -at the base of the mountain -gave way beneath -the added load upon -them, thus making many -victims. Inasmuch, however, -as the ash-fall partakes -of the same general characters as in eruptions from cinder -cones, we may here give our attention especially to the streams of<span class="pagenum"><a name="Page_133" id="Page_133">[133]</a></span> -lava which issued upon the -opposite flank of the mountain -(<a href="#f136">Fig. 136</a>).</p> - -<div class="floatleft"> - <img src="images/ill-178a.jpg" width="230" height="161" id="f135" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 135.</span>—A sunken road filled with indrifted -cocoa-colored ash from the Vesuvian -eruption of 1906.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-178b.jpg" width="230" height="287" id="f136" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 136.</span>—View of Vesuvius taken from the -southwest during the waning stages of the -eruption of 1906. In the middle distance -may be discerned the several lava mouths -aligned upon a fissure, and the courses of -the streams which descend from them. In -the foreground is the main lava stream with -scoriaceous surface (after W. Prinz).</p> -</div></div> - -<p class="vh">————</p> - -<p>The main lava stream -descended the first steep -slopes with the velocity of a -mile in twenty-five minutes, -about the strolling speed of -a pedestrian, but this rate -was gradually reduced as -the stream advanced farther -from the mouth. Taking -advantage of each depression of the surface, the black stream -advanced slowly but relentlessly toward the cities at the southwest -base of the mountain. With a motion not unlike that of a -heap of coal falling over itself down a slope, the block lava<span class="pagenum"><a name="Page_134" id="Page_134">[134]</a></span> -advances without burning -the objects in its path -(<a href="#f137">Fig. 137</a>).</p> - -<div class="floatleft"> - <img src="images/ill-178c.jpg" width="230" height="149" id="f137" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 137.</span>—The main lava stream of -1906 advancing upon the village of -Boscotrecase.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-178d.jpg" width="230" height="172" id="f138" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 138.</span>—An Italian pine snapped off -by the lava and carried forward upon -its surface as a passenger (after Haug).</p> -</div></div> - -<p class="vh">————</p> - -<p>The beautiful -pines are merely charred -where snapped off and -are carried forward upon -the surface of the stream -(<a href="#f138">Fig. 138</a>). When a real -obstruction, such as a -bridge or a villa, is encountered, -the stream is -at first halted, but the -rear crowding upon the -van, unless a passage is found at the side, the lava front rises -higher and higher until by its weight the obstruction is forced to -give way (<a href="#f139">Figs. 139</a> and <a href="#f140">140</a>).</p> - -<div class="floatleft"> - <img src="images/ill-179a.jpg" width="280" height="188" id="f139" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 139.</span>—Lava front both pushing over and -running around a wall which lies athwart its -course (after Johnston-Lavis).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-179b.jpg" width="280" height="197" id="f140" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 140.</span>—One of the villas in Boscotrecase -which was ruined by the Vesuvian lava flow -of 1906. The fragments of masonry from -the ruined walls traveled upon the lava -current, where they sometimes became -incased in lava.</p> -</div></div> - -<p><b>The sequence of events within the chimney.</b>—The thorough -study of this Vesuvian eruption has placed us in a position to infer -with some confidence in our conclusions the sequence of events -within the chimney and -crater of the volcano, both -before and during the eruption. -Anticipating some -conclusions derived from the -observed dissection of volcanoes, -which will be discussed -below, it may be -stated that what might be -termed the core of the composite -cone—the chimney—is -a more or less cylindrical -plug of cooled lava -which during the active -period of the vent has an -interior bore of probably variable caliber. This plug in its -lower section appears in solid black in all the diagrams of <a href="#f141">Fig. 141</a>. -During the cone-building period (<a href="#f141">Fig. 141 <i>a</i> and <i>b</i></a>) the plug -is obviously built upward along with the cone, for lava often flows -out at a level a few hundred feet only below the crater rim. By<span class="pagenum"><a name="Page_135" id="Page_135">[135]</a></span> -what process this chimney -building goes on is not well -understood, though some light -is thrown upon it by the post-eruption -stage of Mont Pelé in -1902-1903 (see below).</p> - -<div class="floatleft"> - <img src="images/ill-180.jpg" width="200" height="475" id="f141" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 141.</span>—Three diagrams to illustrate -the sequence of events within the crater -of a composite cone during the cone-building -and crater-producing periods. -<i>a</i> and <i>b</i>, two successive stages of the -cone building or Strombolian period; -<i>c</i>, enlargement of the crater, truncation -of the cone, and destruction of the upper -chimney during the relatively brief -crater-producing or Vulcanian period.</p> -</div></div> - -<p>Both the older and newer -sections of this plug or chimney -are furnished some support -against the outward pressure -of the contained lava by the -surrounding wall of tuff; and -they are, therefore, in a condition -not unlike that of the -inner barrel of a great gun over -which sleeves of metal have -been shrunk so as to give support -against bursting pressures. -On the other hand, when not -sustaining the hydrostatic pressure -of the liquid lava within, -the chimney would tend to be -crushed in by the pressure -of the surrounding tuff. Its -strength to withstand bursting -pressures is dependent not -alone upon the thickness of its -rock walls, but also upon its -internal diameter or caliber. -A steam cylinder of given -thickness of wall, as is well -known, can resist bursting -pressures in proportion as its -internal diameter is small. So -in the volcanic chimney, any -tendency to remelt from within -the chimney walls must weaken -them in a twofold ratio.</p> - -<p><span class="pagenum"><a name="Page_136" id="Page_136">[136]</a></span></p> - -<p>We are yet without accurate -temperature observations upon the lava in volcanic chimneys, -but it seems almost certain that these temperatures rise as the -Vulcanian stage is approaching, and such elevation of temperature -must be followed by a greater or less re-fusion of the chimney -walls. The sequence of events during the late Vesuvian eruption -is, then, naturally explained by progressive re-fusion and consequent -weakening of the chimney walls, thus permitting a radial -fissure to open near the top and gradually extend downwards. -Thus at first small and high outlets were opened insufficient to -drain the chimney, but later, on April 7, after this fissure had<span class="pagenum"><a name="Page_137" id="Page_137">[137]</a></span> -been much extended and a new and larger one had opened at a -lower level, the draining began and the surface of lava commenced -rapidly to sink.</p> - -<div class="figcenter"> - <img src="images/ill-181.jpg" width="400" height="489" id="f142" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 142.</span>—The spine of Pelé rising above the chimney of the volcano after -the eruption of 1902 (after Hovey).</p> -</div></div> - -<p>When the rapid sinking of the lava surface occurred, the lower -lava layers were almost immediately relieved of pressure, thus -causing a sudden expansion of the contained steam and resulting -in grand crater explosions. The partially refused and fissured -upper chimney, now unable to withstand the inward pressure of -the surrounding tuff walls, since outward pressures no longer -existed, crushed in and contributed its materials and those of -the surrounding tuff to the fragments of fresh lava rising in -volume in the grand explosions (<a href="#f141">Fig. 141 <i>c</i></a>). In outline, then, -these seem to be the conditions which are indicated by the -sequence of observed events in connection with the late Vesuvian -outbreak.</p> - -<div class="figcenter"> - <img src="images/ill-182.jpg" width="400" height="302" id="f143" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 143.</span>—Outlines of the Pelé spine upon successive dates. The full line represents -its outline on December 26, 1902; the dotted-dashed line is a profile of -January 3, 1902; while the dotted line is that of January 9, 1903. The dark -line is a fissure (after E. O. Hovey).</p> -</div></div> - -<p><b>The spine of Pelé.</b>—The disastrous eruption of Mont Pelé -upon Martinique in the year 1902 is of importance in connection -with the interesting problem of the upward growth of volcanic -chimneys during the cone-building period of a volcano. After -the conclusion of this great Vulcanian eruption, a spine of lava<span class="pagenum"><a name="Page_138" id="Page_138">[138]</a></span> -grew upward from the chimney of the main crater until it had -reached an elevation of more then a thousand feet above its base, -a figure of the same order of magnitude as the probable height of -the upper section of the Vesuvian chimney previous to the eruption -of 1906. The Pelé spine (<a href="#f142">Fig. 142</a>) did not grow at a uniform -rate, but was subject to smaller or larger truncations, but for a -period of 18 days the upward growth was at the rate of about 41 -feet per day. Later, the mass split upon a vertical plane revealing -a concave inner surface, and was somewhat rapidly reduced in -altitude to 600 feet (<a href="#f143">Fig. 143</a>), only to rise again to its full height -of about 1000 feet some three months later.</p> - -<p>While apparently unique as an observed phenomenon, and not -free from uncertainty as to its interpretation, the growth of this -obelisk has at least shown us that a mass of rock can push its way -up above the chimney of an active volcano even when there are no -walls of tuff about it to sustain its outward pressures.</p> - -<div class="floatleft"> - <img src="images/ill-183.jpg" width="250" height="121" id="f144" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 144.</span>—Corrugated surface of the Vesuvian cone -after the mud flows which followed the eruption in 1906 -(after Johnston-Lavis).</p> -</div></div> - -<p><b>The aftermath of mud flows.</b>—When the late Vulcanian explosions -of Vesuvius had come to an end, all slopes of the mountain, -but especially -the higher ones, -were buried in -thick deposits of -the cocoa-colored -ash, included in -which were larger -and smaller projectiles. -As this -material is extremely -porous, it -greedily sucks up -the water which falls during the first succeeding rains. When -nearly saturated, it begins to descend the slopes of the mountain -and soon develops a velocity quite in contrast with that of the -slow-moving lava. The upper slopes are thus denuded, while -the fields and even the houses about the base are invaded by these -torrents of mud (<i>lava d’acqua</i>). Inasmuch as these mud flows are -the inevitable aftermath of all grander explosive eruptions, the -Italian government has of late spent large sums of money in the -construction of dikes intended to arrest their progress in the future.<span class="pagenum"><a name="Page_139" id="Page_139">[139]</a></span> -It was streams of this sort that buried the city of Herculaneum -after the explosive eruption of 79 <span class="smcap">A.D.</span></p> - -<p>After the mud flows have occurred, the Vesuvian cone, like all -similar volcanic cones under the same conditions, is found with -deep radial corrugations (<a href="#f144">Fig. 144</a>), such as were long ago described -as “barrancoes” and supposed to support the “elevation -crater” theory of volcano formation.</p> - - -<p><b>The dissection of volcanoes.</b>—To the uninitiated it might appear -a hopeless undertaking to attempt to learn by observation -the internal structure of a volcano, and especially of a complex -volcano of the composite type. The earliest successful attempt -appears to have been made by Count Caspar von Sternberg in -order to prove the correctness -of the theory -of his friend, the poet -Goethe. Goethe had -claimed that a little -hill in the vicinity of -Eger, on the borders -of Bohemia, was an extinct -volcano, though -the foremost geologist -of the time the famous -Werner, had promulgated -the doctrine -that this hill, in common with others of similar aspect, originated -in the combustion of a bed of coal. The elevation in question, -which is known as the Kammerbühl, consists mainly of cinder, -and Goethe had maintained that if a tunnel were to be driven -horizontally into the mountain from one of its slopes, a core or -plug of lava would be encountered beneath the summit. The -excavations, which were completed in 1837, fully verified the -poet’s view, for a lava plug was found to occupy the center of -the mass and to connect with a small lava stream upon the side -of the hill (<a href="#f145">Fig. 145</a>).</p> - -<div class="floatright"> - <img src="images/ill-184.jpg" width="250" height="150" id="f145" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 145.</span>—The Kammerbühl near Eger, showing -the tunnel completed in 1837 which proved the -volcanic nature of the mountain (after Judd).</p> -</div></div> - -<p>It is not, however, to such expensive projects that reference -is here made, but rather to processes which are continually going -on in nature, and on a far grander scale. The most important -dissecting agent for our purpose is running water, which is continually<span class="pagenum"><a name="Page_140" id="Page_140">[140]</a></span> -paring down the earth’s surface and disclosing its buried -structures. How much more convincing than any results of -artificial excavation, as evidence of the internal structure of a -volcano, is the monument represented in <a href="#f146">Fig. 146</a>, since here the -lava plug stands in relief like a -gigantic thumb still surrounded by -a remnant of cinder deposits. Such -exposed chimneys of former volcanoes -are found in many regions, and have -become known as volcanic <i>necks</i>, -<i>pipes</i>, or <i>plugs</i>.</p> - -<div class="figcenter"> - <img src="images/ill-185a.jpg" width="400" height="293" id="f146" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 146.</span>—Volcanic plug exposed by natural dissection of a -volcanic cone in Colorado (U. S. G. S.).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-185b.jpg" width="250" height="319" id="f147" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 147.</span>—A dike cutting beds of -tuff in a partly dissected volcano -of southwestern Colorado (after -Howe, U. S. G. S.).</p> -</div></div> - -<p>Not infrequently the beds of tuff -composing the flanks of the volcano, -upon dissection by the same process, -bring to light walls of cooled lava -standing in relief (<a href="#f147">Fig. 147</a>)—the -filling of the fissure which gave outlet -to the flanks of the mountain at the -time of the eruption. Study of exposed -dikes formed in connection -with recent eruptions of Vesuvius -has shown that in many instances they are still hollow, the lava -having drained from them before complete consolidation.</p> - -<p><span class="pagenum"><a name="Page_141" id="Page_141">[141]</a></span></p> - -<p>Another agent which is effective in uncovering the buried structures -of volcanoes is the action of waves on shores. Always a -relatively vigorous erosive agency, the softer structures of volcanic -cones are removed with especial facility by this agent. On -the shores of the island of Volcano, the little -cone of Vulcanello has been nearly half -carried away by the waves, so as to reveal -with especial perfection the structure of the -cinder beds as well as the internal rock -skeleton of the mass. Here the characteristic -dips of lava streams, intercalated as -they now are between tuff deposits and the -lava which consolidated in fissures, are both -revealed.</p> - -<div class="floatright"> - <img src="images/ill-186a.jpg" width="150" height="235" id="f148" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 148.</span>—Map and general -view of St. Paul’s -Rocks, a volcanic cone -dissected by waves.</p> -</div></div> - -<p>In mid-Atlantic a quite perfect crater, the -St. Paul’s Rocks, has been cut nearly in -half so as to produce a natural harbor -(<a href="#f148">Fig. 148</a>).</p> - -<p>In still other instances we may thank the -volcano itself for opening up the interior of -the mountain for our inspection. The eruption in 1888 of the -Japanese volcano of Bandai-san, by removing a considerable part -of the ancient cone, has afforded us a section completely through -the mountain. The summit and one side of the small Bandai was -carried completely away, and there was substituted a yawning -crater eccentric to the former mountain and having its highest -wall no less than 1500 -feet in height (<a href="#f149">Fig. 149</a>). -In two hours from the -first warning of the explosion -the catastrophe -was complete and the -eruption over.</p> - -<div class="floatleft"> - <img src="images/ill-186b.jpg" width="250" height="81" id="f149" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 149.</span>—Dissection by explosion of Little -Bandai-san in 1888 (after Sekiya).</p> -</div></div> - -<p>The eruption of Krakatoa -in 1883, probably -the grandest observed volcanic explosion in historic times, left -a volcanic cone divided almost in half and open to inspection -(<a href="#f150">Fig. 150</a>). Rakata, Danan, and Perbuatan had before constituted -a line of cones built up round individual craters subsequent<span class="pagenum"><a name="Page_142" id="Page_142">[142]</a></span> -to the partial destruction of an earlier caldera, portions -of which were still existent in the islands Verlaten and Lang. -By the eruption of 1883 all the exposed parts and considerable -submerged portions of the two smaller cones were entirely destroyed, -and the larger one, known as Rakata, was divided just -outside the plug so as to leave a precipitous wall rising directly -from the sea and -showing lava streams -in alternation with -somewhat thicker -tuff layers, the whole -knit together by numerous -lava dikes.</p> - -<div class="floatleft"> - <img src="images/ill-187a.jpg" width="250" height="89" id="f150" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 150.</span>—The half-submerged volcano of Krakatoa -in the Sunda Straits before and after the eruption of -1883 (after Verbeek).</p> -</div></div> - -<p>In order to carry -our dissecting process -down to levels -below the base of the volcanic mountain, it is usually necessary to -inspect the results of erosion by running water. Here the plug or -chimney, instead of being surrounded by tuff, is inclosed by the -country rock of the region, which is commonly a sedimentary -formation. Such exposed lower sections of volcanic chimneys are -numerous along the northwestern shores of the British Isles. -Where aligned upon -a dislocation or noteworthy -fissure in the -rocks, the group of -plugs has been referred -to as a scar or -<i>cicatrice</i> (<a href="#f151">Fig. 151</a>). -Associated with the -plugs of the cicatrice -are not infrequently -dikes, or, it may be, sheets of lava extended between layers of -sediment and known as <i>sills</i>.</p> - -<div class="floatright"> - <img src="images/ill-187b.jpg" width="250" height="111" id="f151" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 151.</span>—The cicatrice of the Banat (after Suess).</p> -</div></div> - -<p>If we are able to continue the dissection process to still greater -depths, we encounter at last igneous rock having a texture known -as granitic and indicating that the process of consolidation was -not only exceedingly slow but also uninterrupted. This rock -is found in masses of larger dimensions, and though generally of<span class="pagenum"><a name="Page_143" id="Page_143">[143]</a></span> -more or less irregular form, no one dimension is of a different order -of magnitude from the others. Such masses are commonly described -as <i>bosses</i>, or, if especially large, as <i>batholites</i> (<a href="#f152">Fig. 152</a>). -Wherever the rock beds appear as though they had been forced -up by the upward pressure of the igneous mass, the latter takes -the form of a mushroom and has been described as a <i>laccolite</i> -(<a href="#f479">Figs. 479-481</a>, <a href="#Page_441">pp. 441-442</a>). Evidence seems, however, to accumulate -that in the greater number of cases the molten rock has fused -its way upward, in part assimilating and in part inclosing the rock -which it encountered. This process -of upward fusion has been -likened to the progress of a red -hot iron burning its way through -a board.</p> - - -<p><b>The formation of lava reservoirs.</b>—The -discarding of the -earlier notion that the earth has -a liquid interior makes it proper -in discussing the subject of volcanoes -to at least touch upon -the origin of the molten rock -material. As already pointed -out, such reservoirs as exist -must be local and temporary, -or it would be difficult to see -how the existing condition of -earth rigidity could be maintained. -From the rate at which rock temperatures rise, at -increasing depths below the surface, it is clear that all rocks would -be melted at very moderate depths only, if they were not kept in a -solid state by the prodigious loads which they sustain. Any relief -from this load should at once result in fusion of the rock.</p> - -<div class="floatright"> - <img src="images/ill-188.jpg" width="250" height="273" id="f152" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 152.</span>—Diagram to illustrate a probable -cause of formation of lava reservoirs, -and to show the connection -between such reservoirs and the volcanoes -at the surface.</p> -</div></div> - -<p>Now the restriction of active volcanoes to those zones of the -earth’s surface within which mountains are rising, and where -in consequence earthquakes are felt, has furnished us at least a -clew to the origin of the lava. Regarded as a structure capable -of sustaining a load, the competency of an arch is something quite -remarkable, so that the arching up of strong rock formations into -anticlines within the upper layers of the zone of flow, or of combined<span class="pagenum"><a name="Page_144" id="Page_144">[144]</a></span> -fracture and flow, would be sufficient to remove the load -from relatively weak underlying beds, which in consequence would -be fused and form local reservoirs of lava (<a href="#f152">Figs. 152</a> and <a href="#f153">153</a>).</p> - -<p>It has been further quite generally observed that lines of volcanoes, -in so far as they betray any relation in position to neighboring -mountain ranges, tend to appear upon the rear or flatter -limb of unsymmetrical arches, or where local tension would favor -the opening of channels toward the surface. Moreover, wherever -recent block movements of surface portions of the earth’s shell -have been disclosed in the neighborhood of volcanoes, the latter -appear to be connected with downthrown blocks, as though the lava -had, so to speak, been squeezed out from -beneath the depressed block or blocks.</p> - -<div class="floatleft"> - <img src="images/ill-189.jpg" width="200" height="128" id="f153" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 153.</span>—Result of experiment -with layers of composition -to illustrate the -effect of relief of load upon -rocks by arching of competent -formation (after -Willis).</p> -</div></div> - -<p>We must not, however, forget that the -igneous rocks are greatly restricted in the -range of their chemical composition. No -igneous rock type is known which could -be formed by the fusion of any of the -carbonate rocks such as limestone or -dolomite, or of the more siliceous rocks, -such as sandstone or quartzite. There -remains only the argillaceous class of -sediments, the shales and slates, and so -soon as we examine the composition of these rocks we are struck by -the remarkable resemblance to that of the class of igneous rocks. -For purposes of comparison there is given below the composite or -average constitution of igneous rocks in parallel column, with the -average attained by combining the analyses of 56 slates and shales, -the latter recalculated with water excluded:</p> - -<table cellspacing="0" id="t06" summary="t06"> - - <tr> - <td class="tdouble1" colspan="10"> </td> - </tr> - - <tr> - <td class="t0111" rowspan="2"> </td> - <td class="t1111" colspan="6"><span class="smcap">Average Igneous Rock</span></td> - <td class="t1101" colspan="3" rowspan="2"><span class="smcap">Average Shale</span></td> -</tr> - - <tr> - <td class="t1111" colspan="3">(Clark)</td> - <td class="t1111" colspan="3">(Washington)</td> -</tr> - - <tr> - <td class="t0110">SiO<sub><span class="small">2</span></sub></td> - <td class="t1100">61.25</td> - <td class="t0110" colspan="2" rowspan="2"> </td> - <td class="t1100">61.69</td> - <td class="t0110" colspan="2" rowspan="2"> </td> - <td class="t1100">63.34</td> - <td class="t0100" colspan="2" rowspan="2"> </td> -</tr> - - <tr> - <td class="t0010">Al<sub><span class="small">2</span></sub>O<sub><span class="small">3</span></sub></td> - <td class="t1000">15.81</td> - <td class="t1000">15.94</td> - <td class="t1000">15.56</td> -</tr> - - <tr> - <td class="t0010">Fe<sub><span class="small">2</span></sub>O<sub><span class="small">3</span></sub></td> - <td class="t1000">2.70</td> - <td class="tw" rowspan="2">}</td> - <td class="t0010" rowspan="2">6.31</td> - <td class="t1000">1.88</td> - <td class="tw" rowspan="2">}</td> - <td class="t0010" rowspan="2">4.53</td> - <td class="t1000">4.41</td> - <td class="tw" rowspan="2">}</td> - <td rowspan="2">7.89</td> -</tr> - - <tr> - <td class="t0010">FeO</td> - <td class="t1000">3.61</td> - <td class="t1000">2.65</td> - <td class="t1000">3.48</td> -</tr> - - <tr> - <td class="t0010">MgO</td> - <td class="t1000">4.47</td> - <td class="t0010" colspan="2" rowspan="6"> </td> - <td class="t1000">4.90</td> - <td class="t0010" colspan="2" rowspan="6"> </td> - <td class="t1000">3.54</td> - <td colspan="2" rowspan="6"> </td> -</tr> - - <tr> - <td class="t0010">CaO</td> - <td class="t1000">5.03</td> - <td class="t1000">5.02</td> - <td class="t1000">3.33</td> -</tr> - - <tr> - <td class="t0010">Na<sub><span class="small">2</span></sub>O</td> - <td class="t1000">3.64</td> - <td class="t1000">4.09</td> - <td class="t1000">1.29</td> -</tr> - - <tr> - <td class="t0010">K<sub><span class="small">2</span></sub>O</td> - <td class="t1000">2.87</td> - <td class="t1000">3.35</td> - <td class="t1000">3.52</td> -</tr> - - <tr> - <td class="t0010">TiO<sub><span class="small">2</span></sub></td> - <td class="t1000">.62</td> - <td class="t1000">.48</td> - <td class="t1000">.53</td> -</tr> - - <tr> - <td class="t0010"> </td> - <td class="t1000">100.00</td> - <td class="t1000">100.00</td> - <td class="t1000">100.00</td> -</tr> - - <tr> - <td class="tdouble2" colspan="10"> </td> - </tr> - -</table> - -<p><span class="pagenum"><a name="Page_145" id="Page_145">[145]</a></span></p> - -<p class="p1">This close resemblance is probably of deep significance, for the -reason that shales and slates are structurally the weakest of all -rocks and for the further reason that they rather generally directly -underlie the carbonate rocks, which are by contrast the -strongest (see <i>ante</i>, <a href="#Page_37">p. 37</a>). For these reasons shales and slates are -the only rocks which are likely to be fused by relief from load -through the formation of anticlinal arches within the earth’s zone -of flow. If this view is well founded, lavas and other igneous -rocks are in large part fused argillaceous sediments formed in connection -with the process of folding, or are refused rocks of igneous -origin and similar composition.</p> - - -<p><b>Character profiles.</b>—The character profiles of features connected -in their origin with volcanoes are particularly easy to -recognize, and in a few cases in which they might be confused with -others of a different origin, an examination of the materials of -the features should lead to a definitive judgment.</p> - -<p>The lava plains which result from massive outflows of basalt -might perhaps strictly be regarded as lack of feature, so great may -be their continuous extent. Wherever definite vents exist, a -broad flat dome is the usual result of the extravasation of a basaltic -lava. The puys of France and many of the Kuppen of Germany, -being formed from less fluid lava, have afforded profiles -with relatively small radius of curvature.</p> - -<p>In its youthful stage, the cinder cone usually presents a broad -summit sag and relatively short side slopes, whereas the cone of -later stages is apt to present long sweeping and upwardly concave -curves with both the gradient and the radius of curvature increasing -rapidly toward the summit. In contrast, too, with the earlier -stage, the crest is relatively small. A marked reduction in the -high symmetry of such profiles is noted wherever a breaching by -lava outflow has occurred (<a href="#f154">Fig. 154</a>).</p> - -<p>With the composite cone, complexity and corresponding lack -of symmetry is introduced, especially in the partially ruined -caldera, and by the more or less accidental distribution of parasitic -cones, as well as by migrations of the central cone. Peculiarly -similar acuminated profiles result from spatter-cone formation, -from the formation of a superchimney spine, and by the uncovering -of the chimney through denudational processes—the volcanic -neck.</p> - -<p><span class="pagenum"><a name="Page_146" id="Page_146">[146]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-191.jpg" width="400" height="221" id="f154" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 154.</span>—Character profiles connected with volcanoes.</p> -</div></div> - -<p>Another important feature resulting from denudation is the -Mesa or table mountain with its protecting basalt cap above softer -rocks. Its profile most resembles that of table mountains due to -differential erosion of alternately strong and weak horizontally -bedded rocks, such as compose the upper portion of the section in -the Grand Cañon of the Colorado. Here, however, in place of a -single unusually strong top layer there are found several strong -layers in alternation with weaker ones so as to produce additional -steps in the profile.</p> - -<p class="prr"><span class="smcap">Reading References to Chapters IX and X</span></p> - -<p class="p1">General works:—</p> - -<p class="pex"><span class="smcap">Paulett Scrope.</span> The Geology of the Extinct Volcanoes of Central -France. John Murray, London, 1858, pp. 258. (An epoch-making -work of early date which, like the following reference, may be studied -to advantage to-day.)</p> - -<p class="pex"><span class="smcap">Sir Charles Lyell.</span> Principles of Geology, vol. 1, Chapters xxiii-xxv.</p> - -<p class="pex"><span class="smcap">Melchior Neumayr.</span> Erdgeschichte, vol. 1, Allgemeine Geologie, revised -edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable information -clearly presented).</p> - -<p class="pex"><span class="smcap">J. D. Dana.</span> Characteristics of Volcanoes, with Contributions of Facts -and Principles from the Hawaiian Islands. Dodd, Mead, and Company, -New York, 1890, pp. 397.</p> - -<p class="pex"><span class="smcap">Tempest Anderson.</span> Volcanic Studies in Many Lands, being reproductions -of photographs by the author with explanatory notes. John -Murray, London, 1903, pp. 200, pls. 105.</p> - -<p class="pex"><span class="smcap">T. G. Bonney.</span> Volcanoes, their Structure and Significance. John -Murray, London, 1899, pp. 331.</p> - -<p><span class="pagenum"><a name="Page_147" id="Page_147">[147]</a></span></p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Volcanoes of North America. Macmillan, New York, -1897, pp. 346.</p> - -<p class="pex"><span class="smcap">Elisée Réclus.</span> Les volcans de la terre, Belgian Society of Astronomy, -Meteorology, and Physics of the Globe, 1906-1910 (a valuable descriptive -geographical and bibliographical work of reference).</p> - -<p class="pex"><span class="smcap">G. Mercalli.</span> I vulcani attivi della terre. Hoepli, Milan, 1907, pp. 421. -(A most valuable work, beautifully illustrated, but in the Italian -language.)</p> - -<p class="p1">Arrangement of volcanic vents:—</p> - -<p class="pex"><span class="smcap">Th. Thoroddsen.</span> Die Bruchlinien und ihre Beziehungen zu den Vulkanen, -Pet. Mitt., vol. 51, 1905, pp. 1-5, pl. 5.</p> - -<p class="pex"><span class="smcap">R. D. M. Verbeek.</span> Various volumes and atlases of maps covering the -Dutch East Indies and fully cited in the following reference (p. 21).</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Evolution and the Outlook of Seismic Geology, -Proc. Am. Phil. Soc., vol. 48, 1909, pp. 17-27.</p> - -<p class="p1">Birth of volcanoes:—</p> - -<p class="pex"><span class="smcap">F. Omori.</span> The Usu-san Eruption and Earthquake and Elevation Phenomena, -Bull. Earthq. Inv. Com., Japan, vol. 5, No. 1, 1911, pp. 1-37, -pls. 1-13.</p> - -<p class="p1">Fissure eruptions:—</p> - -<p class="pex"><span class="smcap">Th. Thoroddsen.</span> Island, IV, Vulkane, Pet. Mitt., Ergänzungsh. 153, -1906, pp. 108-111.</p> - -<p class="pex"><span class="smcap">A. Geikie.</span> Text-book of Geology, 4th ed., pp. 342-346.</p> - -<p class="p1">Lava domes of Hawaii:—</p> - -<p class="pex"><span class="smcap">J. D. Dana.</span> Characteristics of Volcanoes (as above).</p> - -<p class="pex"><span class="smcap">C. H. Hitchcock.</span> Hawaii and Its Volcanoes. Honolulu, 1909, pp. 314.</p> - -<p class="p1">Eruption of Matavanu volcano in 1906:—</p> - -<p class="pex"><span class="smcap">Karl Sapper.</span> Der Matavanu-Ausbruch auf Savaii, 1905-1906, Zeit. -d. Gesell. f. Erdk. z. Berlin, vol. 19, 1906, pp. 686-709, 4 pls.</p> - -<p class="pex"><span class="smcap">H. J. Jensen.</span> The Geology of Samoa, and the Eruptions in Savaii, Proc. -Linn. Soc., New South Wales, vol. 31, 1906, pp. 641-672, pls. 54-64.</p> - -<p class="pex"><span class="smcap">Tempest Anderson.</span> The Volcano of Matavanu in Savaii, Quart. Jour. -Geol. Soc., London, vol. 66, 1910, pp. 621-639, pls. 45-52.</p> - -<p class="p1">Eruption of Volcano in 1888:—</p> - -<p class="pex"><span class="smcap">H. J. Johnston-Lavis.</span> The South Italian Volcanoes. Naples, 1891, -pp. 342, pls. 16.</p> - -<p class="p1">Eruption of Taal volcano in 1911:—</p> - -<p class="pex"><span class="smcap">W. E. Pratt.</span> The Eruption of Taal Volcano, January 30, 1911, Phil. -Jour. Sci., vol. 6, No. 2, Sec. A, 1911, pp. 63-86, pls. 1-14.</p> - -<p class="pex"><span class="smcap">F. H. Noble.</span> Taal Volcano, album of views of 1911 eruption, Manila, -1911, pp. 1-48.</p> - -<p class="p1">The volcano of Etna:—</p> - -<p class="pex"><span class="smcap">G. vom Rath.</span> Der Aetna. Bonn, 1872, pp. 1-33. (A beautiful piece of -descriptive writing from both the geological and scenic standpoints.)</p> - -<p class="pex"><span class="pagenum"><a name="Page_148" id="Page_148">[148]</a></span></p> - -<p class="pex"><span class="smcap">Sartorius von Waltershausen.</span> Der Aetna. Leipzig, 1880, 2 quarto -vols., pp. 371 and 548.</p> - -<p class="p1">The eruption of Vesuvius in 1906:—</p> - -<p class="pex"><span class="smcap">H. J. Johnston-Lavis.</span> Geological Map of Monte Somma and Vesuvius, -with a short and concise account, etc. Geo. Philip & Son, London, -1891.</p> - -<p class="pex"><span class="smcap">H. J. Johnston-Lavis.</span> The Eruption of Vesuvius in April, 1906, Trans. -Roy. Dublin Soc., vol. 9, 1909, Pt. VIII, pp. 139-200, pls. 3-23 (the -most authoritative work upon the subject).</p> - -<p class="pex"><span class="smcap">T. A. Jaggar, Jr.</span> The Volcano Vesuvius in 1906, Tech. Quart., vol. 19, -1906, pp. 105-115.</p> - -<p class="pex"><span class="smcap">W. Prinz.</span> L’éruption du Vesuv d’avril, 1906, Ciel et Terre, 27e Année, -1906, pp. 1-49.</p> - -<p class="pex"><span class="smcap">Frank A. Perret.</span> Notes on the Electrical Phenomena of the Vesuvian -Eruption, April, 1906, Sci. Bull., Brooklyn Inst. Arts and Sci., vol. 1, -No. 11, pp. 307-312; Vesuvius, Characteristics and Phenomena of -the Present Repose Period, Am. Jour. Sci., vol. 28, 1909, pp. 413-430.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Grand Eruption of Vesuvius in 1906, Jour. -Geol., vol. 14, 1906, pp. 636-655.</p> - -<p class="p1">The spine of Pelée:—</p> - -<p class="pex"><span class="smcap">E. O. Hovey.</span> The New Cone of Mont Pelée and the Gorge of the Rivière -Blanche, Martinique, Am. Jour. Sci., vol. 16, 1903, pp. 269-281, pls. -11-14.</p> - -<p class="pex"><span class="smcap">A. Heilprin.</span> The Tower of Pelée. Philadelphia, 1904, pp. 62, pls. 22.</p> - -<p class="pex"><span class="smcap">A. Lacroix.</span> La montagne Pelée et ses éruptions, Acad. des Sciences, -Paris, 1904, Chapter iii.</p> - -<p class="pex"><span class="smcap">Karl Sapper.</span> In den Vulkangebieten Mittelamerikas und Westindiens, -Stuttgart, 1905, pp. 172-178.</p> - -<p class="pex"><span class="smcap">A. C. Lane.</span> Absorbed Gases of Vulcanism, Science, N.S., vol. 18, 1903, -p. 760.</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> The Mechanism of the Mont Pelée Spine, <i>ibid.</i>, vol. 19, -1904, pp. 927-928.</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Pelée Obelisk once More, <i>ibid.</i>, vol. 21, 1905, pp. 924-931.</p> - -<p class="p1">The dissection of volcanoes:—</p> - -<p class="pex"><span class="smcap">J. W. Judd.</span> Volcanoes, Chapter v.</p> - -<p class="pex"><span class="smcap">S. Sekya</span> and <span class="smcap">Y. Kikuchi</span>. The Eruption of Bandai-San, Trans. Seis. -Soc., Japan, vol. 13, Pt. 2, 1890, pp. 140-222, pls. 1-9.</p> - -<p class="pex"><span class="smcap">R. D. M. Verbeek.</span> Krakatau. Batavia, 1885, pp. 557, pls. 25.</p> - -<p class="pex"><span class="smcap">Royal Society</span>. The Eruption of Krakatoa and Subsequent Phenomena. -London, 1888, pp. 494.</p> - -<p><span class="smcap">G. K. Gilbert.</span> Report on the Geology of the Henry Mountains, U.S. -Geogr. and Geol. Surv., Rocky Mt. Region, Washington, 1877, pp. -22-60.</p> - -<p class="pex"><span class="smcap">Sir A. Geikie.</span> Ancient Volcanoes of Great Britain, vol. 2 especially.</p> - -<p class="pex"><span class="smcap">D. W. Johnson.</span> Volcanic Necks of the Mount Taylor Region, New -Mexico, Bull. Geol. Soc. Am., vol. 18, 1907, pp. 303-324, pls. 25-30.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_149" id="Page_149">[149]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XI</h2> - -<p class="pch">THE ATTACK OF THE WEATHER</p> - -<p><b>The two contrasted processes of weathering.</b>—It has already -been pointed out that change and not stability is the order of -nature. Within the earth’s outer shell and upon it rock alteration -goes on continually, and from some portions of its surface the -changed material is as constantly migrating to neighboring or -even far distant regions. Before such transportation can begin -the hard rock must first be broken down and reduced to fragments -which the transporting agencies are competent to move.</p> - -<p>To accomplish this breaking down, or <i>degeneration</i>, of the rock -masses, either a wide range in temperature or chemical reaction is -essential. In the atmosphere are found such active chemical -agents as oxygen and carbon dioxide, the so-called carbonic acid -gas; and these agents in the presence of water react chemically -with the minerals of the rocks and form other minerals such as the -hydrates and carbonates, which are lighter in weight and more -soluble. This <i>chemical</i> attack upon the outer shell of the lithosphere -is described as <i>decomposition</i>.</p> - -<p>On the other hand the rock may succumb to changes which are -purely mechanical and are due either to the stresses set up by differences -between surface and interior temperatures, or to the prying -action of the frost in the crevices. Such purely mechanical degeneration -of the rocks is in contrast with decomposition and is -described as <i>disintegration</i>. The two processes of decomposition -and disintegration may, however, go on together; and the changes -of volume that are caused by decomposition may result directly -in considerable disintegration, as we are to see.</p> - - -<p><b>The rôle of the percolating water.</b>—In order to effect chemical -change or reaction, it is essential that the substances which are -to react must be brought into such intimate contact with each -other as it is seldom possible to attain except by solution. The -chemical reactions which go on between the gaseous atmosphere -and the solid lithosphere are accomplished through solution of the<span class="pagenum"><a name="Page_150" id="Page_150">[150]</a></span> -gases in water. This water, derived from rain or snow, percolates -into the ground or descends along the crevices in the rocks, carrying -with it a certain measure of dissolved air. This air differs -from that of the surrounding atmospheric envelope by containing -relatively large amounts of oxygen and -of the other active element carbon dioxide. -It follows from the important rôle -thus performed by the percolating water -that the process of decomposition will -be relatively important in humid regions -where the atmospheric precipitation -is sufficient for the purpose.</p> - -<div class="floatleft"> - <img src="images/ill-195.jpg" width="200" height="463" id="f155" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 155.</span>—Successive diagrams -to show the effect of -decomposition and resulting -disintegration upon joint -blocks so as to produce -spheroidal bowlders by -weathering.</p> -</div></div> - -<p>Within hot and dry regions there is -a larger measure of rock disintegration, -and distinct chemical changes unlike -those of humid regions take place in the -higher temperatures and with the more -concentrated saline solutions. The discussion -of such changes will be deferred -until desert conditions are treated in -another chapter.</p> - - -<p><b>Mechanical results of decomposition—spheroidal -weathering.</b>—From an -earlier chapter it has been learned that -the rocks of the earth’s outermost shell -are generally intersected by a system of -vertical fissures which at each locality -tend to divide the rock into parallel and -upright rectangular prisms. It is these -joints which offer relatively easy paths -for the descent of the water into the -rocks. In rocks of sedimentary origin -there are found, in addition to the vertical joints, planes of bedding -originally horizontal, and in the intrusive and volcanic rocks -a somewhat similar parting, likewise parallel to the surface of the -ground. The combined effect of the joints and the additional -parting planes is thus to separate the rock mass into more or -less perfect squared blocks (<a href="#f155">Fig. 155</a>, upper figure) which stand -in vertical columns.</p> - -<p><span class="pagenum"><a name="Page_151" id="Page_151">[151]</a></span></p> - -<p>The water which percolates downward upon the joints, finds -its way laterally along the parting planes, and so subjects the entire -surface of each block to simultaneous attack by its reagents. -Though all parts of the surface of each block are alike subject to -attack, it is the angles and the edges which are most vigorously -acted upon. In the narrow crevices the solutions move but sluggishly, -and as they are soon impoverished of their reagents in the -attack upon the rock, fresh solution can reach the middle of the -faces from relatively few directions. The edges are at the same -time being reached from many more directions, and the corners -from a still larger number.</p> - -<p>The minerals newly formed by these chemical processes of -hydration and carbonization are notably lighter, and hence more -bulky than the minerals from whose constituents they have been -largely formed. Strains are thus set up which tend to separate -the bulkier new material from the core of unaltered rock below. -As the process continues, distinct channels for the moving waters -are developed favorable to action at the edges and corners of the -blocks. Eventually, the squared block is by this process transformed -into a spheroidal core of still unaltered rock wrapped in -layers of decomposed material, like the outer wrappings of an onion. -These in turn are usually imbedded in more thoroughly disintegrated -material from which -the shell structure has disappeared -(<a href="#f156">Fig. 156</a>).</p> - -<div class="floatright"> - <img src="images/ill-196.jpg" width="250" height="166" id="f156" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 156.</span>—Spheroidal weathering of an -igneous rock.</p> -</div></div> - -<p><b>Exfoliation or scaling.</b>—A -fact of much importance to -geologists, but one far too -often overlooked, is that rocks -are but poor conductors for -heat. It results from this -that in the bright sun of a -summer’s day a thin skin, as -it were, upon the rock surface may be heated to a relatively high -temperature, although the layer immediately below it is practically -unaffected. The consequent expansion of the surface layer -causes stresses that tend to scale it off from the layer below, -which, uncovered in its turn, develops new strains of the same -sort. This process of exfoliation acquires exceptional importance<span class="pagenum"><a name="Page_152" id="Page_152">[152]</a></span> -in desert regions where the rock surfaces are daily elevated to -excessively high temperatures (see <a href="#Page_197">Chapter XV</a>).</p> - -<div class="floatleft"> - <img src="images/ill-197.jpg" width="250" height="190" id="f157" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 157.</span>—Dome structure in granite -mass, Yosemite valley, California -(after a photograph by Sinclair).</p> -</div></div> - -<p><b>Dome structure in granite masses.</b>—In large granite masses, -such as are to be found in the ranges of the Sierra Nevada of California, -a peculiar dome structure is sometimes found developed -upon a large scale, and has had an important influence upon the -breaking down of the rock and -upon the shaping of the mountain -(<a href="#f157">Fig. 157</a>). Such a structure, made -up as it is of prodigious layers, -can have little in common with -the veneers of weathered minerals -which are the result of exfoliation, -and it is quite likely that -the dome structure is in some -way connected with the relief of -these massive rocks from their -load—the rock which once rested -upon them, but has been carried away by erosion since the uplift -of the range.</p> - -<p><b>The prying work of frost.</b>—In all countries where winter temperatures -range below the freezing point of water, a most potent -agent of rock disintegration is the frost which pries at every crevice -and cranny of the surface rock. Important in the temperate zones, -in the polar regions it becomes almost the sole effective agent of -rock weathering. There, as elsewhere, its efficiency as a disintegrating -agent is directly dependent upon the nature of the crevices -within the rock, so that the omnipresent joints are able to exercise -a degree of control over the sculpturing of the surface features -which is hardly to be looked for elsewhere (see <a href="#p10a">plate 10 A</a>).</p> - -<div class="floatright"> - <img src="images/ill-198.jpg" width="250" height="237" id="f158" - alt="" - title="" /> - <div class="caption"><p class="pc250"><span class="smcap">Fig. 158.</span>—Talus slope beneath a cliff.</p> -</div></div> - -<p><b>Talus.</b>—Wherever the earth’s surface rises in steep cliffs, the -rock fragments derived from frost action, or by other processes of -disintegration, as they become detached either fall or slide rapidly -downward until arrested upon a flatter slope. Upon the earlier -accumulations of this kind, the later ones are deposited, until their -surface slopes up to the cliff face as steeply as the material will lie—the -angle of repose. Such débris accumulations at the base of -a cliff (<a href="#f158">Fig. 158</a>) are known as <i>talus</i>, and the slope is described as -a talus slope, or in Scotland as a “scree.”</p> - -<p><span class="pagenum"><a name="Page_153" id="Page_153">[153]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-199a.jpg" width="230" height="85" id="f159" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 159.</span>—Striped ground from soil flow -of chipped rock fragments upon a slope, -Snow Hill Island, West Antarctica (after -Otto Nordenskiöld).</p> -</div></div> - -<p><b>Soil flow in the continued presence of thaw water.</b>—So soon -as the rocks are broken down by the weathering processes, they are -easily moved, usually to lower levels. In part this transportation -may be accomplished by gravity slowly acting upon the disintegrated -rock and causing -it to creep down the slope. -Yet even in such cases -water is usually present -in quantity sufficient to fill -the spaces between the -grains, and so act as a -lubricant to facilitate the -migration.</p> - -<p>Upon a large scale rocks -which were either originally -incoherent or have -been made so by weathering, -after they have become -saturated with -water, may start into sudden motion as great landslides or avalanches, -which in the space of a few moments materially change the -face of the country, and by burying the bottom lands leave disaster -and misery in their wake.</p> - -<div class="floatright"> - <img src="images/ill-199b.jpg" width="200" height="170" id="f160" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 160.</span>—Pavement of horizontal -surface due to soil -flow, Spitzbergen (after Otto -Nordenskiöld).</p> -</div></div> - -<p>Within the subpolar regions, where a large part of the surface -is for much of the year covered with snow, the underlying rocks -are for long periods saturated with thaw water, and in alternation -are repeatedly frozen and thawed. Essentially similar conditions -are met with in the high, snow-capped mountains of temperate or -torrid regions. For the subpolar regions particularly it is now -generally recognized that somewhat special processes of soil flow, -described under the name <i>solifluction</i>, are characteristic. The -exact nature of these processes is as yet imperfectly understood, but -there can be little doubt concerning the large rôle which they have -played in the transportation of surface materials. Such soil flow -is clearly manifested under different aspects, and it is likely that -by this comprehensive term distinct processes have been brought -together.</p> - -<div class="floatleft"> - <img src="images/ill-199c.jpg" width="230" height="173" id="f161" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 161.</span>—Tree roots entering fissured rock and -prying its sections apart.</p> -</div></div> - -<p>Possibly the most striking aspect of the soil flow in subpolar -regions is furnished by the remarkable “stone rivers” and “rock<span class="pagenum"><a name="Page_154" id="Page_154">[154]</a></span> -glaciers”; though the more generally characteristic are peculiar -stripings or other markings which appear upon the surface of the -ground and thus betray the -movements of the underlying -materials. Upon slopes it is -not uncommon for the surface -to be composed of angular rock -fragments riven by the frost -and crossed by broad parallel -furrows as though a gigantic -plow had gone over it (<a href="#f159">Fig. 159</a>). -The direction of the furrows is always up and down the -slope, and the striping is marked in proportion -as the slope is steep. Where the -bottom is reached, the furrows are replaced -by a sort of mosaic pavement -of hexagonal repeating figures, each of -which may be an area of the surface six -feet or more across (<a href="#f160">Fig. 160</a>, and <a href="#f390">Fig. 390</a>, <a href="#Page_368">p. 368</a>). -The depressions which -separate the “blocks” of the pavement -are often filled with clay, while the inclosed -surfaces are made up of coarsely -chipped stone.</p> - -<p><b>The splitting wedges of roots and trees.</b>—In the mechanical -breakdown of the rocks -within humid regions a -not unimportant part is -sometimes taken by the -trees, which insinuate the -tenuous extremities of their -rootlets into the smallest -cracks and by continued -growth slowly wedge even -the firmer rocks apart (<a href="#f161">Fig. 161</a>). -In a similar manner -the small tree trunk growing -within a crevice of the -rock may in time split its parts asunder (<a href="#f162">Fig. 162</a>).</p> - -<p><span class="pagenum"><a name="Page_155" id="Page_155">[155]</a></span></p> - -<div class="floatright"> - <img src="images/ill-200a.jpg" width="200" height="240" id="f162" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 162.</span>—A large glacial bowlder -split by a growing tree near East -Lansing, Michigan (after a photograph -by Bertha Thompson).</p> -</div></div> - -<p><b>The rock mantle and its shield in the mat of vegetation.</b>—Through -the action of weathering, the rocks, as we have seen, -lose their integrity within a surface layer, which, though it may be -as much as a hundred feet or more -in thickness, must still be accounted -a mere film above the underlying bed -rock. The mechanical agents of the -breakdown operate only within a few -feet of the surface, and the agents of -rock decomposition, derived as they -are from the atmosphere, become -inert before they have descended to -any considerable depth. The surface -layer of incoherent rock is usually -referred to as the <i>rock mantle</i> (<a href="#f163">Fig. 163</a>). -Where the rock mantle is relatively -deep, as it is in the states -south of the Ohio in the eastern -United States, there is found, deep -below the outer layer of soil, a partially decomposed and disintegrated -rock, of which the unaltered minerals lie unchanged in -position but separated by the new minerals which have resulted -from the breakdown of their more -susceptible associates. While thus -in a certain sense possessing the -original structure, this altered material -is essentially incoherent and -easily succumbs to attack by the -pick and spade, so that it is only -at considerably greater depths -that the unaltered rock is encountered.</p> - -<div class="floatleft"> - <img src="images/ill-200b.jpg" width="200" height="149" id="f163" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 163.</span>—Rock mantle consisting of -broken rock, above which is soil and -a vegetable mat. Coast of California -(after a photograph by Fairbanks).</p> -</div></div> - -<p>Because of the tendency of -mantle rock to creep down upon -slopes it is generally found thicker -upon the crests and at the bases of hills and thinnest upon their -slopes (<a href="#f164">Fig. 164</a>).</p> - -<p>In the transformation of the upper portion of the mantle rock -into soil, additional chemical processes to those of weathering<span class="pagenum"><a name="Page_156" id="Page_156">[156]</a></span> -are carried through by the agency of earthworms, bacteria, and -other organisms, and by the action of humus and other acids derived -from the decomposition of vegetation. The bacteria particularly -play a part in the formation of carbonates, as they do -also in changing -the nitrogen of -the air into nitrates -which become -available -as plant food. -Within the -humid tropical -regions ants and other insects enter as a large factor in rock -decomposition, as they do also in producing not unimportant -surface irregularities.</p> - -<div class="floatleft"> - <img src="images/ill-201.jpg" width="250" height="71" id="f164" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 164.</span>—Diagram to show the varying thickness of -mantle rock upon the different portions of a hill surface -(after Chamberlin and Salisbury).</p> -</div></div> - -<p>How important is the cover of vegetation in retaining the rock -mantle and the upper soil layer in their respective positions, as -required for agricultural purposes, may be best illustrated by the -disastrous consequences of allowing it to be destroyed. Wherever, -by the destruction of forests, by the excessive grazing of animals, -or by other causes, the mat of turf has been destroyed, the surface -is opened in gullies by the first hard rain, and the fertile layer -of soil is carried from the slopes and distributed with the coarser -mantle upon the bottom lands. Thus the face of the country is -completely transformed from fertile hills into the most desolate -of deserts where no spear of grass is to be seen and no animal food -to be obtained (plate 5 A). The soil once washed away is not again -renewed, for the continuation of the gullying process now effectively -prevents its accumulation.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 5.</span></p> - -<div class="figcenter"> - <img src="images/ill-202a.jpg" width="400" height="235" id="p5a" - alt="" - title="" /> - <div class="caption"><p class="ch400"><i>A.</i> Once wooded region in China now reduced to desert through deforestation -(after Willis).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-202b.jpg" width="400" height="250" id="p5b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> “Bad Lands” in the Colorado Desert (after Mendenhall).</p> -</div></div> - -</div> - -<p class="prr"><span class="smcap">Reading References to Chapter XI</span></p> - -<p>Decomposition and disintegration:—</p> - -<p class="pex"><span class="smcap">George P. Merrill.</span> The Principles of Rock Weathering, Jour. Geol., -vol. 4, 1896, pp. 704-724, 850-871. Rocks, Rock Weathering, and -Soils. Macmillan, New York, 1897, Pt. iii, pp. 172-411.</p> - -<p class="pex"><span class="smcap">Alexis A. Julien.</span> On the Geological Action of the Humus Acids, Proc. -Am. Assoc. Adv. Sci., vol. 28, 1879, pp. 311-410.</p> - -<p class="p1">Corrosion of rocks:—</p> - -<p class="pex"><span class="smcap">C. W. Hayes.</span> Solution of Silica under Atmospheric Conditions, Bull. -Geol. Soc. Am., vol. 8, 1897, pp. 213-220, pls. 17-19.</p> - -<p><span class="pagenum"><a name="Page_157" id="Page_157">[157]</a></span></p> - -<p class="pex"><span class="smcap">M. L. Fuller.</span> Etching of Quartz in the Interior of Conglomerates, -Jour. Geol., vol. 10, 1902, pp. 815-821.</p> - -<p class="pex"><span class="smcap">C. H. Smyth, Jr.</span> Replacement of Quartz by Pyrites and Corrosion of -Quartz Pebbles, Am. Jour. Sci. (4), vol. 19, 1905, pp. 282-285.</p> - -<p class="p1">Dome structure of granite masses:—</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Domes and Dome Structure of the High Sierra, Bull. -Geol. Soc. Am., vol. 15, 1904, pp. 29-36, pls. 1-4.</p> - -<p class="pex"><span class="smcap">Ralph Arnold.</span> Dome Structure in Conglomerate, <i>ibid.</i>, vol. 18, 1907, -pp. 615-616.</p> - -<p class="p1">Soil flow:—</p> - -<p class="pex"><span class="smcap">J. Gunnar Andersson.</span> Solifluction, a Component of Subaërial Denudation, -Jour. Geol., vol. 14, 1906, pp. 91-112.</p> - -<p class="pex"><span class="smcap">Otto Nordenskiöld.</span> Die Polarwelt und ihre Nachbarländer, Leipzig, -1909, pp. 60-65.</p> - -<p class="pex"><span class="smcap">Ernest Howe.</span> Landslides in the San Juan Mountains, Colorado, etc., -Prof. Pap., 67 U. S. Geol. Surv., 1909, pp. 1-58, pls. 1-20.</p> - -<p class="pex"><span class="smcap">G. E. Mitchell.</span> Landslides and Rock Avalanches, Nat. Geogr. Mag., -vol. 21, 1910, pp. 277-287.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Soil Stripes in Cold Humid Regions and a Kindred -Phenomenon, 12th Rept. Mich. Acad. Sci., 1910, pp. 51-53, pls. 1-2.</p> - -<p class="p1">Relation of deforestation to erosion:—</p> - -<p class="pex"><span class="smcap">N. S. Shaler.</span> Origin and Nature of Soils, 12th Ann. Rept. U. S. Geol. -Surv., 1891, Pt. 1, pp. 268-287.</p> - -<p class="pex"><span class="smcap">W. J. McGee.</span> The Lafayette Formation, <i>ibid.</i>, pp. 430-448.</p> - -<p class="pex"><span class="smcap">F. H. King.</span> Soils. Macmillan, New York, 1908, pp. 50-54.</p> - -<p class="pex"><span class="smcap">Bailey Willis.</span> Water Circulation and Its Control, Rept. Nat. Conserv. -Com., 1909, vol. 2, pp. 687-710.</p> - -<p class="pex"><span class="smcap">W. J. McGee.</span> Soil erosion, Bull. 71, U. S. Bureau of Soils, 1911, pp. 60, -pls. 33.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_158" id="Page_158">[158]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XII</h2> - -<p class="pch">THE LIFE HISTORIES OF RIVERS</p> - -<p><b>The intricate pattern of river etchings.</b>—The attack of the -weather upon the solid lithosphere destroys the integrity of its -surface layer, and through reducing it to rock débris makes it the -natural prey of any agent competent to carry it along the surface. -We have seen how, for short distances, gravity unaided may pile -up the débris in accumulations of talus, and how, when assisted by -thaw water which has soaked into the material, it may accomplish -a slow migration by a peculiar type of soil flow. Yet far more -potent transporting agencies are at work, and of these the one of -first importance is running water. Only in the hearts of great -deserts or in the equally remote white deserts of the polar regions -is the sound of its murmurings never heard. Every other part of -the earth’s surface has at some time its running water coursing -in valleys which it has itself etched into the surface. It is this -etching out of the continents in an intricate pattern of anastomosing -valleys which constitutes the chief difference between the land -surface and the relatively even floor of the oceans.</p> - - -<p><b>The motive power of rivers.</b>—Every river is born in throes -of Mother Earth by which the land is uplifted and left at a higher -level than it was before. It is the difference of elevation thus -brought about between separated portions of the land areas that -makes it possible for the water which falls upon the higher portions -to descend by gravity to the lower. This natural “head” due to -differences of elevation is the motive power of the local streams, -and for each increase in elevation there is an immediate response -in renewed vigor of the streams. The elevated area off which the -rivers flow is here termed an upland.</p> - -<p>The velocity of a stream will be dependent not only upon the -difference in altitude between its source and its mouth, but upon -the distance which separates them, since this will determine the -grade. The level of the mouth being the lowest which the stream<span class="pagenum"><a name="Page_159" id="Page_159">[159]</a></span> -can reach is termed the <i>base level</i>, and the current is fixed by the -slope or declivity. The capacity to lift and transport rock débris -is augmented at a quite surprising rate with every increase in -current velocity, the law being that the weight of the heaviest -transportable fragment varies with the sixth power of the velocity -of the current. Thus if one stream flows twice as rapidly as -another, it can transport fragments which are sixty-four times as -heavy.</p> - - -<p><b>Old land and new land.</b>—The uplifts of the continents may -proceed without changes in the position of the shore lines, in -which case areas, already carved by streams but no longer actively -modified by them, are worked upon by tools freshly sharpened -and driven by greater power. The land thus subjected to active -stream cutting is described as old land, and has already had -engraved upon it the characteristic pattern of river etchings, -albeit the design has been in part effaced.</p> - -<p>If, upon the other hand, the shore line migrates seaward with -the uplift, a portion of the relatively even sea floor, or new land, -is elevated and laid under the action of the running water. -As we are to see, stream cutting is to some extent modified when -a river pattern is inherited from the uplift. The uplift, whether -of old land only or of both old land and new land, marks the -starting point of a new river history, usually described as an -<i>erosion cycle</i>.</p> - -<div class="figcenter"> - <img src="images/ill-207a.jpg" width="400" height="168" id="f165" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 165.</span>—Two successive forms of gullies from the earliest stage of a -river’s life (after Salisbury and Atwood).</p> -</div></div> - -<p><b>The earlier aspects of rivers.</b>—Though geologists have sometimes -regarded the uplift of the continents as a sort of upwarping -in a continuous curved surface, the discussions of river histories -and the pictorial illustrations of them have alike clearly assumed -that the uplift has been essentially in blocks and that the elevated -area meets the lower lying country or the sea in a more or -less definite escarpment. The first rivers to develop after the -uplift may be described as gullies shaped by the sudden downrush -of storm waters and spaced more or less regularly along the -margin of the escarpment (<a href="#f165">Fig. 165</a>). These gullies are relatively -short, straight, and steep; they have precipitous walls and few, -if any, tributaries.</p> - -<p><span class="pagenum"><a name="Page_160" id="Page_160">[160]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-207b.jpg" width="400" height="305" id="f166" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 166.</span>—Partially dissected upland (after Salisbury and -Atwood).</p> -</div></div> - -<p>With time the gully heads advance into the upland as they -take on tributaries; and so at length they in part invest it and -dissect it into numerous irregularly bounded and flat-topped -tables which are separated by cañons (<a href="#f166">Fig. 166</a>). At the same -time the grade of the channel is becoming flatter, and its precipitous -walls are being replaced by curving slopes, as will be more -fully described in the sequel. It is because of this progressive -reduction of grades with increasing age that the early stages of -a river’s life are much the most turbulent of its history. The -water then rushes down the steep grades in rapids, and is often -at times opened out in some basin to form a lake where differences -of uplift have been characteristic of neighboring sections.<span class="pagenum"><a name="Page_161" id="Page_161">[161]</a></span> -For several reasons such basins in the course of a stream are relatively -short lived (Chapter XXX), and they disappear with the -earlier stages of the river history.</p> - - -<p><b>The meshes of the river network.</b>—From the continued throwing -out of new tributaries by the streams, the meshes in the -river network draw more closely together as the stages of its history -advance. The closeness of texture which is at last developed -upon the upland is in part determined by the quantity of rainfall, -so that in New Jersey with heavy annual precipitation the meshes -in the network are much smaller than they are, for example, -upon the semiarid or arid plains of the western United States. -Its design will, however, in either case more or less clearly express -the plan of rock architecture which is hidden beneath the surface -(<a href="#Page_223">Chapter XVII</a>).</p> - - -<p><b>The upper and lower reaches of a river contrasted.</b>—From -the fact that the river progressively invades new portions of the -upland and lays the acquired sections under more and more -thorough investment, it has near its headwaters for a long time -a frontier district which may be regarded as youthful even though -the sections near its mouth have reached a somewhat advanced -stage. The newly acquired sections of river valley may thus -possess the steep grade and precipitous walls which are characteristic -of early gullies and cañons and are in contrast with -the more rounded and flat-bottomed sections below. Lateral -streams, from the fact that they are newer than the main or trunk -stream to which they are tributary, likewise descend upon somewhat -steeper grades (<a href="#f167">Fig. 167</a>).</p> - -<div class="figcenter"> - <img src="images/ill-208.jpg" width="450" height="54" id="f167" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 167.</span>—Characteristic longitudinal sections of the upper portion of a river -valley and its tributaries (after scaled sections by Nussbaum).</p> -</div></div> - -<p><b>The balance between degradation and aggradation.</b>—We have -seen that the power to transport rock fragments is augmented at -a most surprising rate with every increase in the current velocity. -While the lighter particles of rock may be carried as high up as -the surface of the water, the heavier ones are moved forward -upon the bottom with a combined rolling and hopping motion -aided by local eddies. Those particles which come in contact<span class="pagenum"><a name="Page_162" id="Page_162">[162]</a></span> -with the bottom or sides of the channel abrade its surface so as -ever to deepen and widen the valley. This cutting accomplished -by partially suspended débris in rapidly moving currents of water is -known as <i>corrasion</i> and the stream is said to be <i>incising</i> its valley.</p> - -<p>As the current is checked upon the lower and flatter grades, -some of its load of sediment, and especially the coarser portion, -will be deposited and so partially fill in the channel. A nice -balance is thus established between <i>degradation</i> and the contrasted -process known as <i>aggradation</i>. The older the river valley -the flatter become the grades at any section of its course, and -thus the point which separates the lower zone of aggradation -from the upper one of degradation moves steadily upstream with -the lapse of time.</p> - - -<p><b>The accordance of tributary valleys.</b>—It is a consequence of -the great sensitiveness of stream corrasion to current velocity -that no side stream may enter the trunk valley at a level above -that of the main stream—the tributary streams enter the trunk -stream <i>accordantly</i>. Each has carved its own valley, and any -abrupt increase in gradient of the side streams near where they -enter the main stream would have increased the local corrasion -at an accelerated rate and so have cut down the channel to the -level of the trunk stream.</p> - - -<p><b>The grading of the flood plain.</b>—All rivers are subject to -seasonal variations in the volume of their waters. Where there -are wet and dry seasons these differences are greatest, and for a -large part of the year the valleys in such regions may be empty -of water, and are in fact often utilized for thoroughfares. In the -temperate climates of middle latitudes rivers are generally flooded -in the spring when the winter snows are melted, though they -may dwindle to comparatively small streams during the late -summer. In the upper reaches of the river the current velocities -are such that the usual river channel may carry all the water of -flood time; but lower down and in the zone of aggradation, where -the current has been checked, the level of the water rises in flood -above the banks of its usual channel and spreads over the surrounding -lowlands. As a deposit of sediment is spread upon the -surface, the succession of the annual deposits from this source -raises the general level as a broad floor described as the <i>flood plain</i> -of the river.</p> - -<p><span class="pagenum"><a name="Page_163" id="Page_163">[163]</a></span></p> - - -<p><b>The cycles of stream meanders.</b>—The annual flooding with -water and simultaneous deposition of silt is not, however, the -only grading process which is in operation upon the flood plain. -It is characteristic of swift currents that their course is maintained -in relatively straight lines because of the inertia of the -rapidly moving water. In proportion as their currents become -sluggish, rivers are turned aside by the smallest of obstructions; -and once diverted from their straight course, a law of nature -becomes operative which increases the curvature of the stream -at an accelerated rate up to a critical point, when by a change, -sudden and catastrophic, a new and direct course is taken, to be -in its turn carried through a similar cycle of changes. This -so-called <i>meandering</i> of a stream is accompanied by a transfer of -sediment from one bend or meander of the river to those below -and from one bank to the other. Inasmuch as the later meanders -cross the earlier ones and in time occupy all portions of the plain -to the same average extent, a process of rough grading is accomplished -to which the annual overflow deposit is supplementary.</p> - -<div class="floatright"> - <img src="images/ill-210.jpg" width="250" height="104" id="f168" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 168.</span>—Map and sections of a stream meander. -The course of the main current is indicated by the -dashed line.</p> -</div></div> - -<p>The course of the current in consecutive meanders and the -cross sections of the channel which result directly from the meandering -process will be made clear from examination of <a href="#f168">Fig. 168</a>. -So soon as diverted from its direct course, the current, by its -inertia of motion, is -thrown against the -outer or convex side -so as to scour or -corrade that bank. -Upon the concave -or inner side of the -curve there is in consequence -an area of -slack water, and here -the silt scoured from higher meanders is deposited. The scouring -of the current upon the outer bank and the filling upon the inner -thus gives to the cross section of the stream a generally unsymmetrical -character (<a href="#f168">Fig. 168 <i>ab</i></a>). Between meanders near the -point of inflection of the curve, and there only, the current is centered -in the middle of the channel and the cross section is symmetrical -(<a href="#f168">Fig. 168 <i>cd</i></a>).</p> - -<p><span class="pagenum"><a name="Page_164" id="Page_164">[164]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-211a.jpg" width="200" height="211" id="f169" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 169.</span>—Tree in part undermined -upon the outer bank of a meander.</p> -</div></div> - -<p>The scour upon the convex side of a meander causes the river -to swing ever farther in that direction, and through invasion of -the silted flood plain to migrate across it. Trees which lie in its -path are undermined and fall outward -in the stream with tops directed -with the current (<a href="#f169">Fig. 169</a>). -Whenever the flood plain is forested, -the fallen trees may be so -numerous as to lie in ranks along -the shore, and at the time of the -next flood they are carried downstream -to jam in narrow places -along the channel and give the erroneous -impression that the flood -has itself uprooted a section of forest -(see <a href="#Page_418">p. 418</a>).</p> - -<div class="floatright"> - <img src="images/ill-211b.jpg" width="150" height="158" id="f170" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 170.</span>—Diagrams to -show the successive -positions of stream -meanders and the -relatively stationary -point near the sharpest -curvature.</p> -</div></div> - -<p><b>The cut-off of the meander.</b>—As -the meander swings toward its extreme position it becomes -more and more closely looped. Adjacent loops thus approach -nearer and nearer to each other, but in the successive positions -a nearly stationary point is established near where the river -makes its sharpest turn (<a href="#f170">Fig. 170, <i>G</i></a>, and -<a href="#f454">Fig. 454</a>, <a href="#Page_417">p. 417</a>). At length the neck of land -which separates meanders is so narrow that -in the next freshet a temporary jamming of -logs within the channel may direct the waters -across the neck, and once started in the new -direction a channel is scoured out in the -soft silt. Thus by a breaking through of -the bank of the stream, a so-called “crevasse”, -the river suddenly straightens its -course, though up to this time it has steadily -become more and more sharply serpentine. -After the cut-off has occurred, the old channel -may for a time continue to be used by the -stream in common with the new one, but the advantage in velocity -of current being with the cut-off, the old channel contains slacker -water and so begins to fill with silt both at the beginning and -the end of the loop. Eventually closed up at both ends, this loop<span class="pagenum"><a name="Page_165" id="Page_165">[165]</a></span> -or “ox-bow” is entirely separated from the new channel, and -once abandoned of the stream is transformed into an ox-bow -lake (<a href="#f171">Fig. 171</a> and <a href="#Page_415">p. 415</a>).</p> - -<div class="floatright"> - <img src="images/ill-212a.jpg" width="250" height="168" id="f171" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 171.</span>—An ox-bow lake in the flood -plain of a river.</p> -</div></div> - -<p><b>Meander scars.</b>—Swinging as it occasionally does in its -meanderings quite across the flood plain and against the bank of -the earlier degrading river in -this section, the meander at -times scours the high bank -which bounds the flood plain, -and undermining it in the same -manner, it excavates a recess -of amphitheatral form which is -known as a meander <i>scar</i> (<a href="#f172">Fig. 172</a>). -At length the entire bank -is scarred in this manner so as -to present to the stream a series of concave scallops separated by -sharp intermediate salients of cuspate form.</p> - -<div class="floatleft"> - <img src="images/ill-212b.jpg" width="250" height="84" id="f172" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 172.</span>—Schematic representation of a series -of river terraces. <i>a</i>, <i>b</i>, <i>c</i>, <i>e</i>, successive terraces -in order of age. <i>d</i>, <i>d</i>, <i>d</i>, <i>d</i>, terrace slopes formed -of meander scars.</p> -</div></div> - -<p><b>River terraces.</b>—Whenever the river’s history is interrupted -by a small uplift, or the base level is for any reason lowered, the -stream at once begins to sink its channel into the flood plain. -Once more flowing upon a low grade, it again meanders, and so -produces new walls at a lower level, but formed, like the first, of -intersecting meander scars. Thus there is produced a new flood -plain with cliff and terrace -above, which is -known as a <i>river terrace</i>. -A succession of uplifts -or of depressions of the -base level yields terraces -in series, as they appear -schematically represented -in <a href="#f172">Fig. 172</a>. Such terraces -are to be found well developed upon most of our larger -rivers to the northward of the Ohio and Missouri. The highest -terrace is obviously the remnant of the earliest flood plain, as the -lowest represents the latest.</p> - -<p><b>The delta of the river.</b>—As it approaches its mouth the river -moves more and more sluggishly over the flat grades, and swings -in broader meanders as it flows. Yet it still carries a quantity<span class="pagenum"><a name="Page_166" id="Page_166">[166]</a></span> -of silt which is only laid down after its current has been stopped -on meeting the body of standing water into which it discharges. -If this be the ocean, the salinity of the sea water greatly aids in a -quick precipitation of the finest material. This clarifying effect -upon the water of the dissolved salt may be strikingly illustrated -by taking two similar jars, the one filled with fresh and the other -with salt water, and stirring the same quantity of fine clay into -each. The clay in the salt water is deposited and the water -cleared long before the murkiness of the other has disappeared.</p> - -<p>By the laying down of the residue of its burden of sediment -where it meets the sea, the river builds up vast plains of silt and -clay which are known as deltas and which often form large local -extensions of the continents into the sea. Whereas in its upper -reaches the river with its tributary streams appears in the plan -like a tree and its branches, in the delta region the stream, by -dividing into diverging channels called distributaries (<a href="#f458">Fig. 458</a>, -<a href="#Page_420">p. 420</a>), completes the resemblance to the tree by adding the -roots. From the divergence of the distributaries upon the delta -plain the Greek capital letter Δ is suggested and has supplied the -name for these deposits. Of great fertility, the delta plains of -rivers have become the densely populated regions of the globe, -among which it is necessary to mention only the delta of the -Nile in Egypt, those of the Ganges and Brahmaputra in India, -and those of the Hoang and Yangtse rivers in China.</p> - - -<p><b>The levee.</b>—When the snows thaw upon the mountains at -the headwaters of large rivers, freshets result and the delta regions -are flooded. At such times heavily charged with sediment, a -thin deposit of fertile soil is left upon the surface of the delta -plain, and in Egypt particularly this is depended upon for the -annual enrichment of the cultivated fields. Though at this time -the waters spread broadly over the plain, the current still continues -to flow largely within the normal channel, so that the slack water -upon either side becomes the locus for the main deposit of the -sediment. There is thus built up on either side of the channel a -ridge of silt which is known as a <i>levee</i>, and this bank is steadily -increased in height from year to year (<a href="#f452">Fig. 452</a>).</p> - -<p>To prevent the danger of floods upon the inhabited plains, -artificial levees are usually raised upon the natural ones, and in a -country like Holland, such levees (dikes) involve a large expenditure<span class="pagenum"><a name="Page_167" id="Page_167">[167]</a></span> -of money and no small degree of engineering skill and experience -to construct. So important to the life of the nation is -the proper management of its dikes, that in the past history of -China each weak administration has been marked by the development -of graft in this important department and by floods which -have destroyed the lives of hundreds of thousands of people.</p> - -<div class="floatright"> - <img src="images/ill-214.jpg" width="200" height="191" id="f173" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 173.</span>—“Bird-foot” delta -of the Mississippi River.</p> -</div></div> - -<p>Wherever there has been a markedly rapid sinking upon a -delta region, and depressions are common in delta territory, no -doubt as a result of the loading down -of the crust, the river may present the -paradoxical condition of flowing at a -higher level than the surrounding country. -Between the levees of neighboring -distributaries there are peculiar saucer-shaped -depressions of the country which -easily become filled with water. At the -extremity of the delta the levee may be -the only land which shows above the -ocean surface, and so present the peculiar -“bird-foot” outline which is characteristic of the extremity -of the Mississippi delta, though other processes than the mere -sinking of the deposits may contribute to this result (<a href="#f173">Fig. 173</a>).</p> - - -<p><b>The sections of delta deposits.</b>—If now we leave the plan of -the delta to consider the section of its deposits, we find them so -characteristic as to be easily recognized. Considered broadly, -the delta advances seaward after the manner of a railroad embankment -which is being carried across a lake. Though the greater -portion of the deposit is unloaded upon a steep slope at the front, -a smaller amount of material is dropped along the way, and a -layer of extremely fine material settles in advance as the water -clears of its finely suspended particles (<a href="#f174">Fig. 174</a>). Simultaneous -deposits within a delta thus comprise a nearly horizontal layer -of coarser materials, the so-called top-set bed; the bulk of the -deposit in a forward sloping layer, the so-called fore-set bed; -and a thin film of clay which is extended far in advance, the -bottom-set bed (<a href="#f174">Fig. 174, 2</a>). If at any point a vertical section is -made through the deposits, beds deposited in different periods -are encountered; the oldest at the bottom in a horizontal position, -the next younger above them and with forward dip, and the<span class="pagenum"><a name="Page_168" id="Page_168">[168]</a></span> -youngest and coarsest upon the top in nearly horizontal position -(<a href="#f174">Fig. 174, 3</a>).</p> - -<div class="floatleft"> - <img src="images/ill-215.jpg" width="250" height="245" id="f174" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 174.</span>—Diagrams to show the nature of delta deposits -as exhibited in section.</p> -</div></div> - -<p>It has been estimated that the surface of the United States -is now being pared down by erosion at the average rate of an -inch in 760 years. -The derived material -is being -deposited in the -flood plain and -delta regions of its -principal rivers. -Some 513 million -tons of suspended -matter is in the -United States carried -to tidewater -each year, and -about half as much -more goes out to -sea as dissolved -matter. If this -material were removed -from the -Panama Canal cutting, an 85-foot sea-level canal would be excavated -in about 73 days. The Mississippi River alone carries -annually to the sea 340 million tons of suspended matter, or -two thirds of the entire amount removed from the area of the -United States as a whole. It is thus little wonder that great -deltas have extended their boundaries so rapidly and that the -crust is so generally sinking beneath the load.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_169" id="Page_169">[169]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XIII</h2> - -<p class="pch">EARTH FEATURES SHAPED BY RUNNING WATER</p> - -<p><b>The newly incised upland and its sharp salients.</b>—The successive -stages of incising, sculpturing, and finally of reducing an -uplifted land area, are each of them possessed of distinctive -characters which are all to be read either from the map or in the -lines of the landscape. Upon the newly uplifted plain the incising -by the young rivers is to be found chiefly in the neighborhood -of the margins. In this stage the valleys are described as -<span class="font">V</span>-shaped cañons, for the valley wall meets the upland surface -in sharp salients (<a href="#p12a">plate 12 A</a>), and the lines of the landscape are -throughout made up from straight elements. Though the landscapes -of this stage present the grandest scenery that is known -and may be cut out in massive proportions, often with rushing -river or placid lake to enhance the effect of crag and gorge, they -lack the softness and grace of -outline which belong only to the -maturer erosion stages. The -grand cañon of the Colorado -presents the features characteristic -of this stage in the grandest -and most sublime of all examples, -and the castled Rhine is a -gorge of rugged beauty, carved -out from the newly elevated -plateau of western Prussia, -through which the water swirls in eddying rapids (<a href="#f175">Fig. 175</a>).</p> - -<div class="floatright"> - <img src="images/ill-216.jpg" width="200" height="127" id="f175" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 175.</span>—Gorge of the River Rhine -near St. Goars, incised within an uplifted -plain which forms the hill tops.</p> -</div></div> - -<p><b>The stage of adolescence.</b>—As the upland becomes more -largely invaded as a consequence of the headward advance of -the cañons and their sending out of tributary side cañons, the -sharp angles in which the cañon walls intersect the plain become -gradually replaced by well-rounded shoulders. Thus the lines in -the landscape of this stage are a combination of the straight<span class="pagenum"><a name="Page_170" id="Page_170">[170]</a></span> -line with a simple curve convex toward the sky (<a href="#f176">Fig. 176</a>). In -this stage large sections of the original plateau remain, though -cut into small areas by the extensions -of the tributary valleys.</p> - -<div class="floatleft"> - <img src="images/ill-217a.jpg" width="200" height="95" id="f176" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 176.</span>—<span class="font">V</span>-shaped valley with well-rounded -shoulders characteristic of -the stage of adolescence. Allegheny -plateau in West Virginia.</p> -</div></div> - -<p><b>The maturely dissected upland.</b>—Continued -ramifications -by the rivers eventually divide -the entire upland area into separated -parts, and the rounding -of the shoulders of valleys proceeds -simultaneously until of the -original upland no easily recognizable -compartments are to be found. Where before were flat -hilltops are now ridges or watersheds, the well-known <i>divides</i>. -The upland is now said to be completely dissected or to have -arrived at <i>maturity</i>. The streams are still vigorous, for they -make the full descent from the upland level to base level, and -yet a critical turning point of -their history has been reached, -and from now on they are to -show a steady falling off in efficiency -as sculpturing agents.</p> - -<div class="floatright"> - <img src="images/ill-217b.jpg" width="200" height="83" id="f177" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 177.</span>—View of a maturely dissected -upland from one of its hilltops, Klamath -Mountains, California (after a -photograph by Fairbanks).</p> -</div></div> - -<p>Viewed from one of the hilltops, -the landscape of this stage -bears a marked resemblance to -a sea in which the numberless -divides are the crests of billows, and these, as distance reduces -their importance in the landscape, fade away into the even line -of the horizon (<a href="#f177">Fig. 177</a>).</p> - -<p><b>The Hogarthian line of beauty.</b>—Since the youthful stage of -the upland, when the lines of its landscape were straight, its -character rugged, and its rivers wild and turbulent, there has -been effected a complete transformation. The only straight line -to be seen is the distant horizon, for the landscape is now molded -in softened outlines, among which there is a repeated recurrence -of the line of beauty made famous by Hogarth in his “Analysis of -Beauty.” As well known to all art students, this is a sinuous -line of reversed or double curvature—a curve which passes -insensibly at a point of inflection from convex to concave (<a href="#f178">Fig. 178)</a>. -<span class="pagenum"><a name="Page_171" id="Page_171">[171]</a></span> -The curve of beauty is now found in every section of the -hills, and it imparts to the landscape a gracefulness and a measure -of restfulness as well, which are not to be found in the landscapes -of earlier stages in the erosion cycle. In the bottoms of the -valleys also the initial windings of the -rivers within their narrow flood plains -add silver beauty lines which stand -out prominently from the more somber -background of the hills.</p> - -<div class="floatright"> - <img src="images/ill-218a.jpg" width="200" height="101" id="f178" - alt="" - title="" /> - <div class="cf"><p class="pc200"><span class="smcap">Fig. 178.</span>—Hogarth’s line of -beauty.</p> -</div></div> - -<p>Considered from the commercial -viewpoint, the mature upland is one -of the least adaptable as a habitation for highly civilized man. -Direct lines of communication run up hill and down dale in -monotonous alternation, and almost the only way of carrying a -railroad through the region, without an expenditure for trestles -which would be prohibitive, is to follow the tortuous crest of a -main divide or the equally winding bed of one of the larger valleys.</p> - -<div class="floatleft"> - <img src="images/ill-218b.jpg" width="250" height="126" id="f179" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 179.</span>—View of the old land of New -England, with Mount Monadnock rising -in the distance.</p> -</div></div> - -<p><b>The final product of river sculpture—the peneplain.</b>—When -maturity has been reached in the history of a river, its energies -are devoted to a paring down of the valley slopes and crests so -as to reduce the general level. From this time on hill summits -no longer fall into a common level—that of the original upland—for -some mount notably higher than others, and with increasing -age such differences become accentuated. There is now also -a larger aggradation of the valleys to form the level floors of -flood plains, out of which at length the now slight elevations rise -upon such gentle slopes that the process of land sculpture approaches -its end. Gradually -the vigor of the stream has -faded away, and can now only -be renewed through a fresh -uplift of the land, or, what -would amount to the same -thing, a depression of the base -level. Upland and river have -reached old age together, and -the approximation to a new -plain but little elevated above base level is so marked that the -name <i>peneplain</i> is applied to it. Scattered elevations, which because<span class="pagenum"><a name="Page_172" id="Page_172">[172]</a></span> -of some favoring circumstance rise to greater heights above -the general level of the peneplain, are known as <i>monadnocks</i> after -the type example of Mount Monadnock in New Hampshire (<a href="#f179">Fig. 179</a>).</p> - -<div class="floatleft"> - <img src="images/ill-219.jpg" width="250" height="169" id="f180" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 180.</span>—Comparison of the cross sections of river -valleys for the different stages of the erosion cycle.</p> -</div></div> - -<p><b>The river cross sections of successive stages.</b>—To the successive -stages of a river’s life it has been common to carry over -the names from the well-marked periods of a human life. If -neglecting for the moment the general aspect of the upland, we -fix our attention upon -the characteristic -cross sections of the -river valley, we find -that here also there -are clearly marked -characters to distinguish -each stage of the -river’s life (<a href="#f180">Fig. 180</a>). -In infancy the steep, -narrow, and sharp-angled -cañon is a characteristic; -with youth -the wider <span class="font">V</span>-form has already developed; in adolescence the angles -of the cañon are transformed into well-rounded shoulders, and the -valley broadens so as in the lower reaches to lay down a flood -plain; in maturity the divides and the double curves of the line -of beauty appear; while in the decline of old age the valleys are -extremely broad and flat and are floored by an extended flood -plain.</p> - - -<p><b>The entrenchment of meanders with renewed uplift.</b>—Upon -the reduced grades which are characteristic of the declining stage -of a river’s life, the current has little power to modify the surface -configuration. On the old land of this stage a renewed uplift -starts the streams again into action. This infusion of driving -power into moving water, regarded as a machine capable of accomplishing -certain work, is like winding up a clock that has -run down. Once more the streams acquire a velocity sufficient -to enable them to cut their valleys into the land surface, and -so a new erosional cycle may be inaugurated upon the old land -surface—the peneplain. After such an uplift has been accomplished<span class="pagenum"><a name="Page_173" id="Page_173">[173]</a></span> -and the rivers have sunk their early valleys within the -new upland, we may look out from this now elevated surface -and the eye take in but a single horizontal line, since we view -the plain along its edge.</p> - -<div class="figcenter"> - <img src="images/ill-220.jpg" width="400" height="187" id="f181" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 181.</span>—The Beavertail Bend of the Yakima Cañon in central Washington -(after George Otis Smith).</p> -</div></div> - -<p>By the uplift the meanders of the earlier rivers may become -entrenched in the new upland, the wide lobes of the individual -meanders being now separated by mountains where before had -been plains of silt only. The New River of the Cumberland -plateau and the Yakima River of central Washington (<a href="#f181">Fig. 181</a>) -furnish excellent American examples of intrenched meanders, as -the Moselle River does in Europe. Upon the course of the latter -river near the town of Zell a tunnel of the railroad a quarter of -a mile in length pierces a mountain in the neck of a meander -lobe in which the river itself travels a distance of more than six -miles in order to make the same advance. The Kaiser Wilhelm -tunnel in the same district penetrates a larger mountain included -in a double meander of the river. Although intrenched, river -meanders are still competent to scour and so undermine the -outer bank, and with favoring conditions they may by this process -erode extended “bottoms” out of the plateau. (See Lockport -quadrangle, U. S. G. S.)</p> - -<p><b>The valley of the rejuvenated river.</b>—Whenever a new uplift -occurs before an erosional cycle has been completed, the rivers -become intrenched, not in a peneplain, but in the bottoms of -broad valleys. The sweeping curves which characterize mature<span class="pagenum"><a name="Page_174" id="Page_174">[174]</a></span> -landscapes may thus be brought into striking contrast with the -straight lines of youthful cañons which with <span class="font">V</span>-sections descend -from their lowest levels -(<a href="#f182">Fig. 182</a>). The full -cross section of such a -valley shows a central <span class="font">V</span> -whose sharp shoulders -are extended outward -and upward in the softened -curves of later erosion -stages.</p> - -<div class="floatleft"> - <img src="images/ill-221a.jpg" width="250" height="155" id="f182" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 182.</span>—A rejuvenated river valley (after a -photograph by Fairbanks).</p> -</div></div> - - -<p><b>The arrest of stream -erosion by the more resistant -rocks.</b>—The capacity -of a river to erode and carry away the rock material -that lies along its course is dependent not only upon the velocity -of the current, but also upon the hardness, the firmness -of texture, and the solubility of the material. Particularly in -arid and semiarid regions, where no mantle of vegetation is at -hand to mask the surfaces of the firmer rock masses, differences -of this kind are stamped deeply upon the landscape. The rock -terraces in the Grand Cañon of the Colorado together represent -the stronger rock formations of the region, while sloping talus -accumulations bury the weaker beds from sight.</p> - -<div class="floatright"> - <img src="images/ill-221b.jpg" width="200" height="132" id="f183" - alt="" - title="" /> - <div class="cf"><p class="pc200"><span class="smcap">Fig. 183.</span>—Plan of a river narrows.</p> -</div></div> - -<p>Each area of harder rock which rises athwart the course of a -stream causes a temporary arrest in the process of valley erosion -and is responsible for a noteworthy local contraction of the river -valley. The valley is carved less widely as well as less deeply, -and since a river can never corrade -below its base, a “temporary base -level” is for a time established -above the area of harder rock. -Owing to the contraction of the -valley under these conditions, the -locality is described as a river -<i>narrows</i> (<a href="#f183">Fig. 183</a>). The narrows -upon the Hudson River occur in -the Highlands where the river leaves a broad expanse occupied -by softer sediments to traverse an island-like area of hard crystalline<span class="pagenum"><a name="Page_175" id="Page_175">[175]</a></span> -rocks. Within the narrows of a river the steep walls, characteristic -of youth and the turbulent current as well, are often retained -long after other portions of the river have acquired the more restful -lines of river maturity. The picturesque crag and the generally -rugged character of river narrows render them points of special -interest upon every navigable river.</p> - -<div class="floatright"> - <img src="images/ill-222.jpg" width="200" height="339" id="f184" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 184.</span>—Successive diagrams -to illustrate repeated -river piracy and the development -of “trellis drainage”, -(after Russell).</p> -</div></div> - -<p><b>The capture of one river’s territory by another.</b>—The effect -of a hard layer of rock interposed in the course of a stream is -thus always to delay the advance of the erosional process at all -levels above the obstruction. When a stream in incising its -valley degrades its channel through a veneer of softer rocks into -harder materials below, it is technically described as having <i>discovered</i> -the harder layer. Where several neighboring streams flow -by similar routes to their common base level, those which discover -a harder rock will advance their headwaters less rapidly -into the upland and so will be at a disadvantage in extending -their drainage territory. A stream -which is not thus hindered will in the -course of time rob the others of a portion -of their territory, for it is able to -erode its lower reaches nearer to base -level and thus acquire for its upper -reaches, where erosion is chiefly accomplished, -an advantage in declivity. The -divide which separates its headwaters -from those of its less favored neighbor -will in consequence migrate steadily into -the neighbor’s territory. The divide -is thus a sort of boundary wall separating -the drainage basins of neighboring -streams, and any migration must extend -the territory of the one at the expense -of the other. As more and more territory -is brought under the dominion of -the more favored stream, there will come -a time when the divide in its migration -will arrive at the channel of the stream that is being robbed, and -so by a sudden act of annexation draw off all the upper waters -into its own basin. By this <i>capture</i> the stream whose territory has<span class="pagenum"><a name="Page_176" id="Page_176">[176]</a></span> -been invaded is said to have been <i>beheaded</i>. By this act of <i>piracy</i> -the stronger stream now develops exceptional activity because of -the local steep grades near the point of capture, and with this -newly acquired cutting power the invader is competent to advance -still further and enter the territory of the stream that lies -next beyond. The type of drainage network which results from -repeated captures of this kind is known as “trellis drainage” -(<a href="#f184">Fig. 184</a>), a type well illustrated by the rivers of the southern -Appalachians.</p> - -<p>In general it may be said that, other conditions being the -same, of two neighboring streams which have a common base -level, that one which takes the longest route will lose territory -to the other, since it must have the flatter average slope. Stream -capture may thus come about without the discovery of hard -rock layers which are more unfavorable to one stream than another.</p> - -<div class="floatleft"> - <img src="images/ill-223.jpg" width="250" height="149" id="f185" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 185.</span>—Sketch maps to show the earlier and the -present drainage condition about the Blue Ridge -near Harper’s Ferry.</p> -</div></div> - -<p><b>Water and wind gaps.</b>—In the Allegheny plateau rivers cross, -the range of harder rocks in deep mountain narrows which upon -the horizon appear as gateways through the barrier of the mountain -wall. Such gateways -are sometimes -referred to as “water -gaps”, of which the -Delaware Water Gap -is perhaps the best -known example, -though the Potomac -crosses the Blue -Ridge at the historic -Harper’s Ferry through -a similar portal. The -valley of the tributary -Shenandoah has been the scene of an interesting episode in the -struggle of rival streams which is typical of others in the same -upland region. The records which may be made out from the -landscapes show clearly that in an earlier but recent period, -when the general surface stood at a higher level which has been -called the Kittatinny Plain, the younger Potomac of that time -and a younger but larger ancestor of Beaverdam Creek each<span class="pagenum"><a name="Page_177" id="Page_177">[177]</a></span> -crossed the Blue Ridge of the time through similar water gaps -(<a href="#f185">Fig. 185, map</a>, and <a href="#f186">Fig. 186</a>). The Potomac of that time was, -however, the more -deeply intrenched, -and possessing an -advantage in slope -it was able to -advance the divide -at the head of its -tributary, the -Shenandoah, into -the territory of Beaverdam Creek. Thus the beheading of the -Beaverdam by the Shenandoah was accomplished (<a href="#f185">Fig. 185, second -map</a>) and its upper waters annexed to the Potomac system. -With the subsequent lowering of the general level of the country -which yielded the present Shenandoah Plain, the former water gap -of Beaverdam Creek was abandoned of its stream at a high level -in the range. Known as Snickers Gap, it may serve as a type of -the “wind gaps” of similar origin which are not altogether uncommon -in the Appalachian Mountain system (<a href="#f186">Fig. 186</a>).</p> - -<div class="floatright"> - <img src="images/ill-224a.jpg" width="250" height="97" id="f186" - alt="" - title="" /> - <div class="cf"><p class="pc200"><span class="smcap">Fig. 186.</span>—Section to illustrate the history of Snickers -Gap.</p> -</div></div> - -<p><b>Character profiles.</b>—For humid regions the landscapes possess -characters which, speaking broadly, depend upon the stage of the -erosion cycle. For the earliest stages the straight line enters -as almost the only element in the design; as the cycle advances -to adolescence the rounded forms begin to replace the angles of<span class="pagenum"><a name="Page_178" id="Page_178">[178]</a></span> -the immature stages, and with full maturity the lines of beauty -alone are characteristic. As this critical stage is passed irregularity -of feature and ever more flattened curves are found to correspond -to the decline of the river’s vital energies. There are -thus marks of senility in the work of rivers (<a href="#f187">Fig. 187</a>).</p> - -<div class="figcenter"> - <img src="images/ill-224b.jpg" width="400" height="203" id="f187" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 187.</span>—Character profiles of landscapes shaped by stream erosion in humid -climates.</p> -</div></div> - -<p class="prr"><span class="smcap">Reading References for Chapters XII and XIII</span></p> - -<p class="p1">General:—</p> - -<p class="pex"><span class="smcap">Sir John Playfair.</span> Illustrations of the Huttonian Theory of the Earth. -Edinburgh, 1802, pp. 350-371.</p> - -<p class="pex"><span class="smcap">J. W. Powell.</span> Exploration of the Colorado River of the West and its -Tributaries. Washington, 1875, pp. 149-214.</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Report on the Geology of the Henry Mountains. Washington, -1877, pp. 99-150. (A classic upon the work of rivers.)</p> - -<p class="pex"><span class="smcap">C. E. Dutton.</span> Tertiary History of the Grand Cañon District (with -atlas), Mon. 2, U. S. Geol. Surv., 1882, pp. 264.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> The Rivers and Valleys of Pennsylvania, Nat. Geogr. Mag. -vol. 1, 1889, pp. 203-219; The Triassic Formation of Connecticut, -18th Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 144-153.</p> - -<p class="pex"><span class="smcap">Sir A. Geikie.</span> The Scenery of Scotland. London, 1901, pp. 1-12.</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Rivers of North America. Putnam. New York, 1898, -pp. 327.</p> - -<p class="pex"><span class="smcap">M. R. Campbell.</span> Drainage Modifications and their Interpretation, -Jour. Geol., vol. 4, 1896, pp. 567-581, 657-678.</p> - -<p class="pex"><span class="smcap">Henry Gannett.</span> Physiographic Types, U. S. Geol. Surv., Topographic -Atlas, Folios 1-2, 1896, 1900.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> The Geographical Cycle, Geogr. Jour., vol. 14, 1899, -pp. 481-504.</p> - -<p class="p1">The flood plain:—</p> - -<p class="pex"><span class="smcap">Henry Gannett.</span> The Flood of April, 1897, in the Lower Mississippi, -Scot. Geogr. Mag., vol. 13, 1897, pp. 419-421.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> The Development of River Meanders, Geol. Mag., Decade -iv, vol. 10, 1903, pp. 145-148.</p> - -<p class="pex"><span class="smcap">W. S. Tower.</span> The Development of Cut-off Meanders, Bull. Am. Geogr. -Soc., vol. 36, 1904, pp. 589-599.</p> - -<p class="p1">River terraces:—</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> The Terraces of the Westfield River, Massachusetts, Am. -Jour. Sci., vol. 14, 1902, pp. 77-94, pl. 4; River Terraces in New England, -Bull. Mus. Comp. Zoöl., vol. 38, 1902, pp. 281-346.</p> - -<p class="p1">River deltas:—</p> - -<p class="pex"><span class="pagenum"><a name="Page_179" id="Page_179">[179]</a></span></p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> The Topographic Features of Lake Shores, 5th Ann. -Rept. U. S. Geol. Surv., 1885, pp. 104-108; Lake Bonneville, Mon. I, -U. S. Geol. Surv., 1890, pp. 153-167.</p> - -<p class="pex">Charts of Mississippi River Commission.</p> - -<p class="pex"><span class="smcap">G. R. Credner.</span> Die Deltas, ihre Morphologie, geographische Verbreitung -und Entstehungsbedingungen, Pet. Mitt. Ergh. 56, 1878, -pp. 1-74, pls. 1-3.</p> - -<p class="p1">The peneplain:—</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> Plains of Marine and Subaërial Denudation, Bull. Geol. -Soc. Am., vol. 7, 1896, pp. 377-398; The Peneplain, Am. Geol., vol. -23, 1899, pp. 207-239.</p> - -<p class="p1">Intrenchment of meanders:—</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> The Seine, the Meuse, and the Moselle, Nat. Geogr. Mag., -vol. 7, 1896, pp. 189-202.</p> - -<p class="p1">Stream capture:—</p> - -<p class="pex"><span class="smcap">N. H. Darton.</span> Examples of Stream Robbing in the Catskill Mountains, -Bull. Geol. Soc. Am., vol. 7, 1896, pp. 505-507, pl. 23.</p> - -<p class="pex"><span class="smcap">Collier Cobb.</span> A Recapture from a River Pirate, Science, vol. 22, 1893, -p. 195.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Still Rivers of Western Connecticut, Bull. -Geol. Soc. Am., vol. 13, 1902, pp. 17-22, pl. 1.</p> - -<p class="pex"><span class="smcap">Isaiah Bowman.</span> A Typical Case of Stream Capture in Michigan, Jour. -Geol., vol. 12, 1904, pp. 326-334.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_180" id="Page_180">[180]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XIV</h2> - -<p class="pch">THE TRAVELS OF THE UNDERGROUND WATER</p> - -<p><b>The descent within the unsaturated zone.</b>—Of the moisture -precipitated from the atmosphere, that portion which neither -evaporates into the air nor runs off upon the surface, sinks into -the ground and is described as the <i>ground water</i>. Here it descends -by gravity through the pores and open spaces, and at a quite -moderate depth arrives at a zone which is completely saturated -with water. The depth of the upper surface of this saturated zone -varies with the humidity of the climate, with the altitude of the -earth’s surface, and with many other similarly varying factors. -Within humid regions its depth may vary from a few feet to a few -hundred feet, while in desert areas the surface may lie as low as a -thousand feet or more.</p> - -<p>The surface of the zone of the lithosphere that is saturated -with water is called the <i>water table</i>, and though less accentuated it -conforms in general to the relief of the country (<a href="#f188">Fig. 188</a>). Its -depth at any point is found from the levels of all perennial streams -and from the levels at which water stands in wells.</p> - -<div class="figcenter"> - <img src="images/ill-227.jpg" width="400" height="109" id="f188" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 188.</span>—Diagram to show the seasonal range in the position of the water table -and the cause of intermittent streams.</p> -</div></div> - -<p>During the season of small precipitation the water table is -lowered, and if at such times it falls below the bed of a valley, -the surface stream within the valley dries up, to be revived when, -after heavier precipitation, the water table has in turn been raised. -Such streams are said to be <i>intermittent</i>, and are especially characteristic -of semiarid regions (<a href="#f188">Fig. 188</a>).</p> - -<p><span class="pagenum"><a name="Page_181" id="Page_181">[181]</a></span></p> - -<p>Wherever in descending from the surface an impervious layer, -such as clay, is encountered, the further downward progress of the -water is arrested. Now conducted in a lateral direction it issues -at the surface as a spring at the line of emergence of the upper surface -of the impervious layer (<a href="#f189">Fig. 189</a>).</p> - -<div class="figcenter"> - <img src="images/ill-228a.jpg" width="400" height="60" id="f189" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 189.</span>—Diagram to show how an impervious layer conducts the descending -water in a lateral direction to issue in surface springs.</p> -</div></div> - -<p><b>The trunk channels of descending water.</b>—While within the -unconsolidated rock materials near the surface of the earth, it is -clear that water can circulate in proportion as the materials are -porous and so relatively pervious. As the pore spaces become -minute and capillary, the difficulty of permeation through the -materials becomes very great. Thus in the noncoherent rocks -it is the coarse gravel and the layers of sand which serve as the -underground channels, while the fine clays have the effect of an -impervious wall upon the circulating waters. In coarse sand as -much as a third of the volume of the material is pore space for the -absorption and transmission of water. Even under these favorable -conditions the movement of the water is exceedingly slow -and usually less than a fifth of a mile a year.</p> - -<div class="floatright"> - <img src="images/ill-228b.jpg" width="250" height="168" id="f190" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 190.</span>—Sketch map of the Oucane de Chabrières -near Chorges in the High Alps, to illustrate the corrosion -of limestone along two series of vertical joints -(after Martel).</p> -</div></div> - -<p>Within the hard rocks it is the sandstones which have the largest -pore spaces, but in -nearly all consolidated -rocks there are additional -spaces along -certain of the bedding -planes, the joint openings -(<a href="#f190">Fig. 190</a>), and -the crushed zones of -displacement, so that -these parting planes -become the trunk -channels, so to speak, -of the circulating -water. It is along<span class="pagenum"><a name="Page_182" id="Page_182">[182]</a></span> -such crevices that in the course of time the mineral matter carried -in solution by the water is deposited to produce the ore veins -and the associated crystallized minerals.</p> - - -<p><b>The caverns of limestones.</b>—Where limestone formations have -a nearly flat upper surface, a large part of the surface water enters -the rock by way of the joint spaces, which it soon widens by solution -into broad crevices with well-rounded shoulders. At joint -intersections solution of the limestone is so favored that the water -may here descend in a sort of vertical shaft until it meets a bedding -plane extending laterally and offering more favorable conditions -for corrosion. Its journey now begins in a lateral direction, and -solution of the rock continuing, a tunnel may be etched out and -extended until another joint is encountered which is favorable to -its further descent into the formation. By this process on alternating -shafts and galleries the water descends to near the surface -of the water table by a series of steps, and is eventually discharged -into the river system of the district (<a href="#f191">Fig. 191</a>). Within the larger -caverns the water at the lowest level -usually flows as a subterranean river -to emerge later into the light from beneath -a rock arch.</p> - -<div class="floatleft"> - <img src="images/ill-229.jpg" width="200" height="84" id="f191" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 191.</span>—Diagram to show the -relation of caverns in limestone -to the river system of the district -and to the “swallow -holes” upon the surface.</p> -</div></div> - -<p>From the plan of a system of connecting -caverns it may often be observed -that the galleries of the several -levels are alike directed along two -rectangular directions which indicate -the master joint directions within the limestone formation. This -is especially clear from the map of the galleries in the explored -portions of the Mammoth Cave (<a href="#f192">Fig. 192</a>).</p> - - -<p><b>Swallow holes and limestone sinks.</b>—Above the caverns of -limestone formations there are selected points where the water -has descended in the largest volume, and here funnel-shaped -depressions have been dissolved out from the surface of the rock. -In different districts such depressions have become known as -“sinks”, “swallow holes”, <i>entonnoirs</i>, and <i>Orgeln</i>. Wherever the -depressions have a characteristic circular outline, there can be -little doubt that they are the product of solution by the descending -water, and have relatively small connections only with the -subterranean caverns. They have thus naturally collected upon<span class="pagenum"><a name="Page_183" id="Page_183">[183]</a></span> -their bottoms the insoluble clay which was contained in the impure -limestone as well as a certain amount of slope wash from the surface. -Inasmuch as the clays are impervious to water, the bottoms -of these swallow holes are better supplied with moisture than the -surrounding rock surfaces, and -by nourishing a more vigorous -plant growth are strongly impressed -upon the landscape -(<a href="#f193">Fig. 193</a>).</p> - -<div class="figcenter"> - <img src="images/ill-230a.jpg" width="300" height="361" id="f192" - alt="" - title="" /> - <div class="caption"><p class="pc300"><span class="smcap">Fig. 192.</span>—Plan of a portion of Mammoth Cave, Kentucky (after H. C. Hovey).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-230b.jpg" width="250" height="153" id="f193" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 193.</span>—Trees and shrubs growing -luxuriantly upon the bottoms of sinks -within a limestone country (after a -photograph by H. T. A. de L. Hus).</p> -</div></div> - -<p>Certain of the depressions -above caverns are, however, -less regular in outline, and their -bottoms are occupied by a -mass of limestone rubble. In -some instances, at least, these<span class="pagenum"><a name="Page_184" id="Page_184">[184]</a></span> -depressions appear to be the result of local incaving of the cavern -roofs. An incaving of this nature may close up an earlier gallery in -the cavern and divert the cave waters to a new course. The destruction -of the roofs of caverns through this process of incaving may -continue until only relatively small remnants are left. From long -subterranean tunnels the caves are thus transformed into subaërial -rock bridges that have become known as “natural bridges.” The -best-known American example is the Natural Bridge near Lexington, -Virginia. Much grander natural bridges have been formed -in sandstone by a totally different process, and must not be confused -with these limestone remnants of caverns.</p> - - -<p><b>The sinter deposits.</b>—Just as water can dissolve the calcareous -rocks with the formation of caverns, it can under other conditions -deposit the material which has thus been taken into solution. -Its power to hold carbonate of lime in solution is dependent -upon the presence of carbonic acid gas within the water. Water -charged with gas and dissolved lime carbonate is said to be “hard”, -and if the gas be driven off by boiling or otherwise, the dissolved -lime is thrown out of solution and deposited in a form well known -to all housekeepers.</p> - -<p>Hard water flowing in a surface stream, if dashed into spray -at a cascade, may deposit its lime carbonate in an ever thickening -veneer wherever the spray is dashed about the falls. This material, -when cut in section, has waving parallel layers and is known as -<i>travertine</i> or <i>calcareous sinter</i>. Some of the most remarkable deposits -of this nature may be seen at the cascade of Tivoli near -Rome, and most of the Roman buildings have been constructed -from travertine that has been quarried in the vicinity.</p> - - -<p><b>The growth of stalactites.</b>—Water, after percolating slowly -through the crevices of limestone, where it becomes charged with -the carbonic acid gas and with dissolved carbonate of lime, may -trickle from the roof of a cavern. Emerging from the narrow -crevice, it may give off some of its contained gas and is usually -subject to evaporation, with the result that the lime carbonate is -left adhering to the rock surface from which evaporation took -place. If the water collects upon the cavern roof so slowly that -it can entirely evaporate before a drop can form, the entire content -of carbonate will be left adhering to the roof. Evaporation is -most rapid near the margins and over the center of each drop as it<span class="pagenum"><a name="Page_185" id="Page_185">[185]</a></span> -develops, and the deposit which is left thus takes the form of tiny -white rings at those points upon the crevice where there is the -easiest passage for the trickling water. To the outer surface of -these rings water will first adhere and then evaporate, as it will -also slowly ooze through the passage in the ring, but here without -evaporation until it reaches the lower surface. A pendant structure -will, therefore, develop, growing outward in all directions by -the deposition of concentric layers which are thickest near the roof, -and downward into the form of a rock “icicle” through evaporation -of the water which collects near the tip. These pendant -sinter formations are known as stalactites and are thus formed of -concentric layers arranged like a series of nested cornucopias with -a perforation of nearly uniform caliber along the axis of the structure -(<a href="#f194">Fig. 194</a>).</p> - -<div class="floatright"> - <img src="images/ill-232.jpg" width="250" height="192" id="f194" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 194.</span>—Diagrams to show the -manner of formation of stalactites, -stalagmites, and sinter columns -beneath parallel crevices upon the -roofs of caverns (in part after von -Knebel).</p> -</div></div> - -<p><b>Formation of stalagmites.</b>—Wherever the water percolates -through the roof of the cavern so rapidly that it cannot entirely -evaporate upon the roof, a portion -falls to the floor, and, spattering as -it strikes, builds up a relatively -thick cone of sinter known as a -stalagmite, and this is accurately -centered beneath a stalactite upon -the roof. In proportion as the -cavern is high, the dropping water -is widely dispersed as it strikes the -floor, with the formation of a correspondingly -thick and blunt stalagmite. -As this rises by growth toward -the roof, it often develops -upon its summit a distinct crater-like -depression (<a href="#f194">Fig. 194, lower figure</a>). When the process is -long continued, stalactites and stalagmites may grow together -to form columns which may be ranged with their neighbors -like the pipes of an organ, and like them they give out clear -tones when struck lightly with a mallet. At other times the -columns are joined to their neighbors to form hangings and draperies -of the most fantastic and beautiful design (<a href="#f195">Fig. 195</a>).</p> - -<div class="figcenter"> - <img src="images/ill-233.jpg" width="400" height="314" id="f195" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 195.</span>—Sinter formations in the Luray caverns, Virginia.</p> -</div></div> - -<p>In remote antiquity limestone caverns afforded a refuge to many -species of predatory birds and animals as well as to our earliest<span class="pagenum"><a name="Page_186" id="Page_186">[186]</a></span> -ancestors. The bones of all these denizens of the caves lie entombed -within the clays and the sinter formations upon the cavern -floors, and they tell the story of a fierce and long-continued warfare -for the possession of these natural strongholds. The evidence -is clear that these cave men with their primitive weapons were -able at times to drive away the cave bears, lions, and hyenas, and -to set up in the cavern their simple hearths, only in their turn to -be conquered by the ferocity of their enemies. Some of the European -caves have yielded many wagonloads of the skeletons of -these fierce predatory animals, together with the simple weapons -of the primitive man.</p> - -<p><b>The Karst and its features.</b>—Most so-called limestones have a -large admixture of argillaceous materials (clays) and of siliceous -or sandy particles. Such impurities make up the bulk of the clays -and muds which are left behind when the soluble portions of the -limestone have been dissolved.</p> - -<div class="floatright"> - <img src="images/ill-234a.jpg" width="250" height="249" id="f196" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 196.</span>—Map of the dolines of the Karst region -near Divača.</p> -</div></div> - -<p>Swallow holes we have found to be characteristic features within -such districts. When limestones are more nearly pure, as in the<span class="pagenum"><a name="Page_187" id="Page_187">[187]</a></span> -Karst region east of the Adriatic Sea, similar features are developed, -but upon a grander scale, and certain additional forms are -encountered. In place of -the sink or swallow hole, -there appears the “karst -funnel” or <i>doline</i>, a deep, -bowl-shaped depression -having a flat bottom. -Such funnels may be 30 -to 3000 feet across and -from 6 to 300 feet in depth -(<a href="#f196">Fig. 196</a>). Though in -one or two instances -known to be the result -of the break down of -cavern roofs (<a href="#f197">Fig. 197</a>), -yet like the swallow holes -of other regions these -larger funnels appear generally -to be the work of -solution by the descending waters. Where they have been opened -in artificial cuttings along railroads or in mines, the original rock -is found intact at the bottom, with -small crevices only going down to -lower levels. Over the bottoms of -the dolines there is spread a layer -of fertile red clay, the <i>terra rossa</i>, -like that which is obtained as a -residue when a fragment of the -limestone has been dissolved in -laboratory experiments.</p> - -<div class="floatleft"> - <img src="images/ill-234b.jpg" width="200" height="127" id="f197" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 197.</span>—Cross section of the doline -formed by inbreak of a cavern -roof. The Stara Apnenka doline -in Carinthia (after Martel).</p> -</div></div> - -<p><b>A desert from the destruction of -forests.</b>—Between the dolines is -found a veritable desert with jutting limestone angles and little -if any vegetation. The water which falls upon the surface either -runs off quickly or goes down to the subterranean caverns by which -so much of the country is undermined. Hence it is that the gardens -which furnish the sustenance for the scattered population -are all included within the narrow limits of the doline bottoms.<span class="pagenum"><a name="Page_188" id="Page_188">[188]</a></span> -Although to-day so largely a barren waste, we know that the Karst -upon the Adriatic was in remote antiquity a heavily forested region -and that it supplied the myriads of wooden piles upon which -the city of Venice is supported. The vessels which brought to -this port upon the Adriatic its ancient prosperity were built from -wood brought from this tract of modern desert. In the days of -Venetian grandeur the fertile terra rossa formed a veneer upon -the rock surface of the Karst and so retained the surface waters -for the support of the luxuriant forest cover. After deforestation -this veneer of rich soil was washed by the rains into the dolines -or into the few stream courses of the region, thus leaving a barren -tract which it will be all but impossible to reclaim (<a href="#p6a">plate 6 A</a>).</p> - -<div class="floatleft"> - <img src="images/ill-235.jpg" width="250" height="174" id="f198" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 198.</span>—Sharp <i>Karren</i> of the Ifenplatte -Allgäu (after Eckert).</p> -</div></div> - -<p>Upon the steeper slopes -over the purer limestones, -the rain water runs away, -guided by the joints within -the rock. There is thus -etched out a more or less -complete network of narrow -channels (<a href="#f190">Fig. 190</a>, -<a href="#Page_181">p. 181</a>), between which the -remnants rise in sharp -blades to produce a structure -often simulated upon -the fissured surface of a -glacier that has been melted in the sun’s rays (<a href="#f401">Fig. 401</a>). These -almost impassable areas of karst country are described as <i>Schratten</i> -or <i>Karrenfelder</i> (<a href="#f198">Fig. 198</a>).</p> - - -<p><b>The ponore and the polje.</b>—To-day large areas of the Karst -are devoid of surface streams, nearly all the surface water finding -its way down the crevices of the limestone into caverns, and there -flowing in subterranean courses. The foot traveler in the Karst -country is sometimes suddenly arrested to find a precipice yawning -at his feet, and looking down a well-like opening to the depth -of a hundred feet or more, he may see at the bottom a large river -which emerges from beneath the one wall to disappear beneath -the other. These well-like shafts are in the Austrian Karst known -as <i>Ponores</i>, while to the southward in Greece they are called -<i>Katavothren</i>.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 6.</span></p> - -<div class="figcenter"> - <img src="images/ill-236a.jpg" width="400" height="240" id="p6a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Barren Karst landscape near the famous Adelsberg grottoes. -(<i>Photograph by I. D. Scott.</i>)</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-236b.jpg" width="400" height="280" id="p6b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Surface of a limestone ledge where joints have been widened through solution. -Syracuse, N.Y. -(<i>Photograph by I. D. Scott.</i>)</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_189" id="Page_189">[189]</a></span></p> - -<p>Elsewhere the karst river may emerge from its subterranean -course in a broader depressed area bounded by vertical cliffs, from -which it later disappears beneath the limestone wall. Such depressions -of the karst are known as <i>poljen</i>, and appear in most -cases to be above the downthrown blocks in the intricate fault -mosaic of the region. Some of these steeply walled inclosures -have an area of several hundred square miles, and especially at -the time of the spring snow melting they are flooded with water -and so transformed into seasonal lakes (<a href="#f199">Fig. 199</a> and <a href="#Page_422">p. 422</a>). It -appears that at such times the cave -galleries of the region with their local -narrows are not able to carry off all -the water which is conducted to them; -and in consequence there is a temporary -impounding of the flood waters in -those portions of the river’s course -which are open to the sky and more -extended. The rush of water at such -times may bring the red clay into the -subterranean channels in sufficient -quantity to clog the passages. The -Zirknitz Lake usually has high water -two or three times a year, and exceptionally the flooding has continued -for a number of years. It has thus in some districts been -necessary to afford relief to the population through the construction -of expensive drainage tunnels.</p> - -<div class="floatright"> - <img src="images/ill-238.jpg" width="200" height="187" id="f199" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 199.</span>—The Zirknitz seasonal -lake within a polje of the Karst -(after Berghaus).</p> -</div></div> - -<p>The conditions which are typified in the Karst area to the east -of the Adriatic Sea are encountered also in many other lands; as, -for example, in the Vorarlberg and Swiss Alps, in Lebanon, and -in Sicily.</p> - - -<p><b>The return of the water to the surface.</b>—Water which has descended -from the surface and been there held between impervious -layers, may be under the pressure of its own weight or “head”; -and will later find its way upward, it may be to the surface or -higher, where a perforation is discovered in its otherwise impervious -cover. Such local perforations are produced naturally by -lines of fracture or faulting (widened at their intersections), -and artificially through the sinking of deep wells. The water, -which at ordinary times reaches the surface upon fissures, is usually<span class="pagenum"><a name="Page_190" id="Page_190">[190]</a></span> -concentrated locally at the intersections of the fracture network, -where it issues in lines of fissure springs (<a href="#f200">Fig. 200</a>); but at the time -of earthquakes the water may rise above the surface in lines of -fountains (<a href="#Page_83">p. 83</a>), or occasionally as sheets of water which may -mount some tens of feet into the air.</p> - -<div class="floatleft"> - <img src="images/ill-239.jpg" width="200" height="186" id="f200" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 200.</span>—Fissure springs arranged upon -lines of rock fracture at intersections, -Pomperaug valley, Connecticut.</p> -</div></div> - -<p>In contrast to the flow of surface springs, which varies with the -season through wide ranges both in its volume and in temperature -of the water, the volume of -fissure springs is but slightly -affected by the seasonal precipitation, -and the water temperature -is maintained relatively -constant. Rock is but -a poor heat conductor, and the -seasonal temperature changes -descend a few feet only into the -ground. Thus water which -rises from depths of a few hundred -feet only is apt to be icy -cold, while from greater depths -the effect of the earth’s internal -heat is apparent in a uniform -but relatively higher temperature of the water. Such “warm” -or <i>thermal</i> springs are apt to contain considerable mineral matter -in solution, both because the water is far traveled and because its -higher temperature has considerably increased its solvent properties.</p> - -<p>It has long been recognized that lines of junction of different -rock formations at the base of mountain ranges are localities favorable -for the occurrence of thermal springs. These junction -lines are usually within zones where by movement upon fractures -the widest openings in the rock have formed, and the catchment -area of the neighboring mountain highland has supplied head for -the ground water. A map of the hot springs within the Great -Basin of the western United States would present in the main a -map of its principal faults.</p> - - -<p><b>Artesian wells.</b>—From the natural fissure spring an artesian -well differs in the artificial character of the perforation of the impervious -cover to the water layer. The water of artesian wells -may flow out at the surface under pressure, or it may require<span class="pagenum"><a name="Page_191" id="Page_191">[191]</a></span> -pumping to raise it from some lower level. Ideal conditions are -furnished where the geological structure of the district is that of a -broad basin or syncline. The water which falls in a neighboring -upland is here impounded between two parallel, saucer-like walls -and will flow under its head if the upper wall be perforated at -some low level (<a href="#f201">Fig. 201, 3</a>).</p> - -<div class="figcenter"> - <img src="images/ill-240.jpg" width="400" height="195" id="f201" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 201.</span>—Schematic diagrams to illustrate the different types of artesian wells, -(1) A non-flowing well; (2) flowing wells without basin structure caused by -clogging of the pervious formation; (3) flowing wells in an artesian basin. The -dotted lines are the water levels within the pervious layers (after Chamberlin).</p> -</div></div> - -<p>A monoclinal structure may furnish artesian conditions when -the generally pervious layer has become clogged at a low level so -as to hold back the water (<a href="#f248">Fig. 248, 2</a>). Pumping wells may be -used successfully even when such clogging does not exist, for the -slow-moving underground water flows readily in the direction of -all free outlets (<a href="#f201">Fig. 201, 1</a>).</p> - - -<p><b>Hot springs and geysers.</b>—Thermal springs whose temperature -approaches the boiling point of water are known as <i>hot springs</i>. -A <i>geyser</i> is a hot spring which intermittently ejects a column of -water and steam. Both hot springs and geysers are to be found -only in volcanic regions, and appear to be connected with uncooled -masses of siliceous lava. In two of the three known geyser regions, -Iceland and New Zealand, the volcanoes of the neighborhood are -still active, and the lavas of the Yellowstone National Park date -from the quite recent geological period which immediately preceded -the so-called “Ice Age.”</p> - -<p>Wherever found, geysers are in the low levels along lines of drainage<span class="pagenum"><a name="Page_192" id="Page_192">[192]</a></span> -where the underground water would most naturally reappear -at the surface. Their water has penetrated to considerable depths -below the surface, but has been chiefly heated by ascending steam -or other vapors. The water journey has been chiefly made along -fissures, as is shown by the cool springs which often issue near -them. Though some hot springs and geysers may disappear from -a district, others are found to be forming, and there is no good -reason to think that geysers are rapidly dying out, as was at -one time supposed.</p> - -<p>The action of a geyser was first satisfactorily explained by the -great German chemist Bunsen after he had made studies of the -Icelandic geysers, and the mechanics of the eruption was later -strikingly illustrated in the laboratory by an artificial geyser constructed -by the Irish physicist Tyndall. In many respects this -action is like that of the Strombolian eruption within a cinder -cone, since it is connected with the viscosity of the fluid and the -resistance which this opposes to the liberation of the developing -vapor. In the case of the geyser, a column of heated water stands -within a vertical tube and is heated near the bottom of the column.</p> - -<div class="floatleft"> - <img src="images/ill-241.jpg" width="250" height="221" id="f202" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 202.</span>—Cross section of Geysir, -Iceland, with simultaneously observed -temperatures recorded at the -left, and the boiling temperatures for -the same levels at the right (after -Campbell).</p> -</div></div> - -<p>Though the water may at its surface have the normal boiling -temperature and be there in quiet ebullition, the boiling point -for all lower levels is raised by the -weight of the column of superincumbent -liquid, and so for a time -the formation of steam within the -mass is prevented. In <a href="#f202">Fig. 202</a> -is shown a cross section of the -Icelandic <i>Geysir</i> from which our -name for such phenomena has been -derived, and to this section have -been added the actual observed -temperatures of the water at the -different levels as well as the temperatures -at which boiling can -take place at these levels. From -this it will be seen that at a depth -of 45 feet the water is but 2° Centigrade below its boiling point. -A slight increase of temperature at this level, due to the constantly -ascending steam, will not only carry this layer above the<span class="pagenum"><a name="Page_193" id="Page_193">[193]</a></span> -boiling point, but the expansion of the steam within the mass will -elevate the upper layers of the water into zones where the boiling -points are lower, and thus bring about a sudden and violent ebullition -of all these upper portions. Thus is explained the almost -universal observation that just before geysers erupt the hot water -rises in the bowls and generally overflows them.</p> - -<div class="floatright"> - <img src="images/ill-242.jpg" width="150" height="480" id="f203" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 203.</span>—Apparatus -for simulating geyser -action in the lecture -room (by courtesy -of Professor B. -W. Snow).</p> -</div></div> - -<p>The water ejected from the geyser is considerably cooled in the -air; and after its return to the tube must be again heated by the -ascending vapors before another eruption can -occur. The measure of the cooling, the time -necessary to fill the tube, and the supply of -rising steam, all play a part in fixing the period -which separates consecutive eruptions. If the -top of the tube be narrowed from its average -caliber, as is commonly observed to be true of -the geysers within the Yellowstone National -Park, the escape of the steam is further hindered, -and frequent geyser eruption promoted.</p> - -<p>An artificial geyser for demonstration of the -phenomenon in the lecture room is represented -in <a href="#f203">Fig. 203</a>. The cut has been prepared from -a photograph of an apparatus designed by -Professor B. W. Snow of the University of -Wisconsin. In this design the tube is contracted -so as to have a top diameter one fourth -only of what it is at the bottom, where heat is -directly applied by multiple Bunsen lamps. -The water once sufficiently heated, this artificial -geyser erupts at regular intervals of time -which are dependent upon the dimensions of -the apparatus and the quantity of heat applied.</p> - -<p>In case of natural geysers a considerable -quantity of heat escapes between eruptions in -steam which issues quietly from the bowl of -the geyser. If this heat be retained by plugging the mouth -of the tube with a barrowful of turf, as is sometimes done -with the geyser <i>Strokr</i> in Iceland, eruption is promoted and so -takes place earlier. Another method of securing the same result -is to increase the viscosity of the water through the addition of<span class="pagenum"><a name="Page_194" id="Page_194">[194]</a></span> -soap, as was accidentally discovered by a Chinaman who was utilizing -the geyser water in the Yellowstone Park for laundry operations. -After this discovery it became a common custom to -“soap” the Yellowstone geysers in order to make them play; -but this method was prohibited under heavy penalty after the disastrous -eruption of the Excelsior Geyser.</p> - - -<p><b>The deposition of siliceous sinter by plant growth.</b>—Geysers -are known only from areas of siliceous volcanic lava, and this may -perhaps have its cause in the easier -solution of the geyser tube from -such materials. The silica dissolved -in the heated waters is -<i>again</i> deposited at the surface to -form <i>siliceous sinter</i> or <i>geyserite</i>. -This material forms terraces surrounding -the geysers or is built up -into mounds which are often quite -symmetrical, such as those of the -Bee Hive and Lone Star geysers -of the Yellowstone Park (<a href="#f204">Fig. 204</a>).</p> - -<div class="floatleft"> - <img src="images/ill-243.jpg" width="250" height="252" id="f204" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 204.</span>—Cone of siliceous sinter -built up about the mouth of the -Lone Star Geyser in the Yellowstone -National Park.</p> -</div></div> - -<p>The greater part of this separation -of silica from the heated -geyser waters is due to the action -of plants or algæ that are able -to grow in the boiling waters and which produce the beautiful -colors in the linings to the hot springs. The wonderful variety -of the tints displayed is accounted for by the fact that the algæ -take on different colors at different temperatures. The silica -is deposited from the water in the gelatinous hydrated form, which, -however, dries in the sun to a white sand. The growth within the -pools goes on in a manner similar to that of a coral reef, the algæ -dying below and there becoming encased in the rock lining while -still continuing to grow upon the surface. Whereas sinter of this -nature, when deposited by evaporation alone, can produce a maximum -thickness of layer of a twentieth of an inch each year, the -growth from alga deposition within limited areas may be as much -as eight inches during the same period.</p> - -<p><span class="pagenum"><a name="Page_195" id="Page_195">[195]</a></span></p> - -<p class="prr"><span class="smcap">Reading References for Chapter XIV</span></p> - -<p class="p1">General:—</p> - -<p class="pex"><span class="smcap">F. H. King.</span> Principles and Conditions of the Movements of Ground -Water, 19th Ann. Rept. U. S. Geol. Surv., 1899, Pt. ii, pp. 59-294, -pls. 6-16.</p> - -<p class="pex"><span class="smcap">C. S. Slichter.</span> The Motions of the Underground Waters, Water Supply -Paper No. 67, U. S. Geol. Surv., 1902, pp. 1-106, pls. 1-8; Field -Measurements of the Rate of Movement of Underground Waters, -<i>ibid.</i>, No. 140, 1905, pp. 1-122, pls. 1-15.</p> - -<p class="pex"><span class="smcap">M. L. Fuller.</span> Occurrence of Underground Water, <i>ibid.</i>. No. 114, 1905, -pp. 18-40, pls. 4; Bibliographic review and index of papers relating -to underground waters published by the United States Geological -Survey, 1879-1904, <i>ibid.</i>, No. 120, 1905, pp. 1-128.</p> - -<p class="p1">Caverns:—</p> - -<p class="pex"><span class="smcap">E. A. Martel.</span> Les abimes, les eaux souterraines, les cavernes, les -sources, la spélæologie. Delagrave, Paris, pp. 578. (Lavishly illustrated.)</p> - -<p class="pex"><span class="smcap">H. C. Hovey.</span> Celebrated American Caverns. Cincinnati, 1896, pp. 228; -The Mammoth Cave of Kentucky. Louisville, 1897, pp. 111.</p> - -<p class="pex"><span class="smcap">J. W. Beede.</span> Cycle of Subterranean Drainage in the Bloomington -Quadrangle, Proc. Ind. Acad. Sci., 1910, pp. 1-31.</p> - -<p class="p1">Karst conditions:—</p> - -<p class="pex"><span class="smcap">J. Cvijic.</span> Das Karstphänomen, Geogr. Abh., vol. 5, 1893.</p> - -<p class="pex"><span class="smcap">Émile Chaix.</span> La topographie du desert de platé (Hautes Savoie), Le -Globe, vol. 34, 1895, pp. 1-44, pls. 1-16, pp. 217-330.</p> - -<p class="pex"><span class="smcap">W. v. Knebel.</span> Höhlenkunde mit Berücksichtigung der Karstphänomene. -Vieweg, Braunschweig, 1906, pp. 222.</p> - -<p class="pex"><span class="smcap">A. Grund.</span> Die Karsthydrographie, Studien aus Westbosnien, Geogr. -Abh., vol. 7, No. 3, 1903, pp. 200.</p> - -<p class="pex"><span class="smcap">Émile Chaix-du Bois</span> et <span class="smcap">André Chaix</span>. Contribution a l’étude des -lapies en Carniole et au Steinernes Meer, Le Globe, vol. 46, 1907, -pp. 17-56, pls. 26.</p> - -<p class="pex"><span class="smcap">P. Arbenz.</span> Die Karrenbildungen geschildert am Beispiele der Karrenfelder -bei der Frutt in Kanton Obwalden (Schweiz). Deutsch. Alpenzeitung, -Munich, 1909, pp. 1-9.</p> - -<p class="pex"><span class="smcap">F. Katzer.</span> Karst und Karsthydrographie. Sarejevo, 1909, pp. 95.</p> - -<p class="pex"><span class="smcap">M. Neumayr.</span> Erdgeschichte, vol. 1, pp. 500-510.</p> - -<p class="pex"><span class="smcap">E. de Martonne.</span> Traité de Géographie Physique, pp. 462-472 (excellent -summaries in this and the last reference).</p> - -<p class="pex"><span class="smcap">E. A. Martel.</span> The Land of the Causses, Appalachia, vol. 7, 1893, pp. -18-149, pls. 4-13.</p> - -<p class="p1">Fissure springs:—</p> - -<p class="pex"><span class="smcap">A. C. Peale.</span> Natural Mineral Waters of the United States, 14th Ann. -Rept. U. S. Geol. Surv., Pt. ii, 1894, pp. 49-88.</p> - -<p><span class="pagenum"><a name="Page_196" id="Page_196">[196]</a></span></p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Newark System of the Pomperaug Valley. -Connecticut, 21st Ann. Rept. U. S. Geol. Surv., Pt. iii, 1901, pp. 91-93.</p> - -<p class="p1">Artesian wells:—</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin.</span> Requisite and Qualifying Conditions of Artesian -Wells, 5th Ann. Rept. U. S. Geol. Surv., 1885, pp. 131-173.</p> - -<p class="p1">Hot springs and geysers:—</p> - -<p class="pex"><span class="smcap">A. C. Peale.</span> Yellowstone Park, Thermal Springs, 12th Ann. Rept. -Geol. and Geogr. Surv. Ter. (Hayden), Pt. ii, Sec. ii, pp. 63-454 -(many plates and maps).</p> - -<p class="pex"><span class="smcap">W. H. Weed.</span> Geysers, Rept. Smithson. Inst., 1891, pp. 163-178.</p> - -<p><span class="smcap">Arnold Hague</span> and <span class="smcap">W. H. Weed</span> (on hot springs and geysers of Yellowstone -National Park), C. R. Cong. Géol. Intern., Washington, 1891, -pp. 346-363.</p> - -<p class="pex"><span class="smcap">W. H. Weed.</span> Formation of Travertine and Siliceous Sinter by the -Vegetation of Hot Springs, 9th Ann. Rept. U. S. Geol. Surv., 1889, -pp. 613-676, pls. 78-87.</p> - -<p class="pex"><span class="smcap">M. Neumayr.</span> Erdgeschichte, vol. 1, pp. 500-510.</p> - -<p class="pex"><span class="smcap">Arnold Hague.</span> Soaping Geysers, Trans. Am. Inst. Min. Eng., vol. 17, -1889, pp. 546-553.</p> - -<p class="pex"><span class="smcap">John Tyndall.</span> Heat as a Mode of Motion, New York, 1873, pp. 115-121 -(artificial geyser).</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_197" id="Page_197">[197]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XV</h2> - -<p class="pch">SUN AND WIND IN THE LANDS OF INFREQUENT -RAINS</p> - -<p><b>The law of the desert.</b>—It is well to keep ever in mind that -there is no universal law which dominates Nature’s processes in -all the sections of her realm. Those changes which, because often -observed, are most familiar, may not be of general application, -for the reason that the areas habitually occupied by highly civilized -races together comprise but a small portion of the earth’s -surface. In the dank tropical jungle, upon the vast arid sand -plains, and in the cold white spaces near the poles, Nature has -instituted peculiar and widely different processes.</p> - -<p>The fundamental condition of the desert is aridity, and this -necessitates an exclusion from it of all save the exceptional rain -cloud. Thus deserts are walled in by mountain ranges which -serve as barriers to intercept the moisture-bringing clouds. They -are in consequence saucer-shaped depressions, often with short -mountain ranges rising out of the bottoms, and such rain as falls -within the inclosure is largely upon the borders. Of this rainfall -none flows out from the desert, for the water is largely returned -to the atmosphere through evaporation.</p> - -<p>The desert history is thus begun in isolation from the sea from -which the cloud moisture is derived, a balance being struck between -inflow and evaporation. Yet if deserts have no outlets, -it is not true that they have no rivers. These are occasionally -permanent, often periodic, but generally ephemeral and violent. -The characteristic drainage of deserts comes as the immediate -result of sudden cloudburst. As a consequence, the desert stream -flows from the mountain wall choked with sediment, and entering -the depressed basin, is for the most part either sucked down into the -floor or evaporated and returned to the atmosphere. The dissolved -material which was carried in the water is eventually left<span class="pagenum"><a name="Page_198" id="Page_198">[198]</a></span> -in saline deposits, and the great burden of sediment accumulates -in thick stratified masses which in magnitude outstrip the largest -deltas in the ocean.</p> - - -<p><b>The self-registering gauge of past climates.</b>—From the initiation -of the desert in its isolation from the lands tributary to the -sea, its history becomes an individual and independent one. An -increasing quantity of rainfall will be marked by larger inflow to -the basin, and the lakes which form in its lowest depression will, -as a consequence, rise and expand over larger areas. A contrary -climatic change will bring about a lowering of the lakes and leave -behind the marks of former shorelines above the water level (<a href="#f205">Fig. 205</a>). -Deserts are thus in a sense self-registering climatic gauges -whose records go back far beyond the historic past. From them -it is learned that there have been alternating periods of larger and -smaller precipitation, which are referred to as <i>pluvial</i> and <i>interpluvial</i> -periods.</p> - -<div class="figcenter"> - <img src="images/ill-247.jpg" width="450" height="111" id="f205" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 205.</span>—Former shore lines on the mountain wall surrounding the desert of the -Great Basin. View from the temple in Salt Lake City (after Gilbert).</p> -</div></div> - -<p>From such records it is learned that the Great Basin of the -western United States was at one time occupied by two great desert -lakes, the one in the eastern portion being known as Lake Bonneville -(<a href="#f206">Fig. 206</a>). With the desiccation which followed upon the -series of pluvial periods, which in other latitudes resulted in great -continental glaciers and has become known as the Glacial Period, -this former desert lake dried up to the limits of Great Salt Lake and -a few smaller isolated basins. Between 1850 and 1869 the waters -of Great Salt Lake were rising, while from 1876 to 1890 their level -was falling, though subject to periodic fluctuations, and in recent -years the waters of the lake have risen so high as to pass all records -since the occupation of the country. As a consequence the so-called -Salt Lake “cut-off” of the Union Pacific Railway, constructed -at great expense across a shallow portion of the lake, has<span class="pagenum"><a name="Page_199" id="Page_199">[199]</a></span> -been overflowed by its waters. The Sawa Lake in the Persian -Desert, which disappeared some five hundred years ago, again -came into existence in 1888 so as to cover the caravan route to -Teheran.</p> - -<div class="floatright"> - <img src="images/ill-248.jpg" width="200" height="404" id="f206" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 206.</span>—Map of the former -Lake Bonneville (dotted -shores), and the boundaries -of the Great Salt Lake of -1869 (smaller area) and that -of the present (after Berghaus).</p> -</div></div> - -<p>The record in the rocks of the distant past reveals the fact that -in some former deserts barriers were, in the course of time, broken -down, with the result that an invading -sea entered through the breached wall. -The result was the sudden destruction -of land life, the remains of which are -preserved in “bone beds”, now covered -by true marine deposits. A still later -episode of the history was begun when -the sea had disappeared and land animals -again roamed above the earlier -desert. Such an alternation of marine -deposits with the remains of land plants -and animals in the deposits of the Paris -Basin, led the great Cuvier to his belief -that geologic history was comprised of -a succession of cataclysms in which life -was alternately destroyed and re-created -in new forms—a view which later, under -the powerful influence of Lyell and -Darwin, gave way to that of more -gradual changes and the evolution of -life forms.</p> - - -<p><b>Some characteristics of the desert -wastes.</b>—The great stretches of the -arid lands have been often compared to -the ocean, and the Bedouin’s camel is -known as “the ship of the desert.” Though a deceptive resemblance -for the most part, the comparison is not without its value. -Both are closed basins, and it is in this respect that the desert and -the ocean may be said to most resemble each other, for none of -the water and none of the sediment is lost to either except as -boundaries are, with the progress of time, transposed or destroyed. -Flatness of surface and monotony of scenery both have in common, -and the waters and the sand are in each case salt; yet the ocean,<span class="pagenum"><a name="Page_200" id="Page_200">[200]</a></span> -from the tropics to the poles, has the same salts in essentially the -same proportions, while in the desert the widest variations are -found both in the salts which are present and in their relative -quantities.</p> - -<p>Upon the borders of the ocean are found ridges of yellow sand -heaped up by the wind, but these ramparts are small in comparison -to those which in deserts are found upon the borders (<a href="#p7a">plate 7 A</a>).</p> - -<p>The desert is a land of geographic paradoxes. As Walther has -pointed out, we have rain in the desert which does not wet, springs -which yield no brooks, rivers without mouths, forests preserved -in stone, lakes without outlets, valleys without streams, lake basins -without lakes, depressions below the level of the sea yet barren -of water, intense weathering with no mantle of disintegrated rock, -a decomposition of the rocks from within instead of from without, -and valleys which branch sometimes upstream and sometimes -down.</p> - -<p>Within the deserts curious mushroom-like remnants of erosion -afford a local relief from the searching rays of the desert sun. -Pocket-like openings large enough for a hermit’s habitation are -hollowed out by the wind from the disintegrated rock masses. -Amphitheaters open out from little erosion valleys or wadi, and -isolated outliers of the mountains stand like sentinels before their -massive fronts.</p> - -<p>Because of the general absence of clouds above a desert, no -shield such as is common in humid regions is provided against the -blinding intensity of the sun’s rays. Sun temperatures as high -as 180° Fahrenheit have been registered over the deserts of -western Africa. Every one is familiar with the fact that a -blanket of thick clouds is a prevention of frosts at night, for, with -the setting of the sun and the consequent radiation of heat from -the earth, these rays are intercepted by the clouds, returned and -re-returned in many successive exchanges. Over desert regions -the absence of any such blanket of moisture is responsible for the -remarkable falls of temperature at sunset. Though shortly before -temperatures of 100° Fahrenheit or greater may have been -measured, it is not uncommon for water to freeze during the -following night. Much the same conditions of sudden temperature -change with nightfall are experienced in high mountains when -one has ascended above the blanketing clouds.</p> - -<p><span class="pagenum"><a name="Page_201" id="Page_201">[201]</a></span></p> - -<div class="floatright"> - <img src="images/ill-250a.jpg" width="250" height="159" id="f207" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 207.</span>—Borax deposits upon the floor -of Death valley, California (after a photograph -by Fairbanks).</p> -</div></div> - -<p><b>Dry weathering—the red and brown desert varnish.</b>—In -desert lands the fierce rays of the sun suck up all the available -moisture, and the water table may be hundreds of feet below the -surface. Roots of trees a hundred feet or more in length have -been found to testify to the -fierce struggle of the desert -plant with the arid conditions. -In humid regions the meteoric -water dissolves the more -soluble sodium salts near the -surface of the rock and carries -them out to the ocean, where -they add to the saltness of the -sea. In the desert the rare -precipitations prevent an outflow, -but the sun’s strong rays suck out with the moisture the -salts from within the rock, and evaporating upon the surface, the -salts are left as a coat of “alkali”, which is in part carried away on -the wind and in part washed off in one of the rare cloudbursts. -In either case these constituents find their way to the lowest depressions -of the basin, -where they contribute -to the saline deposits -of the desert lakes (<a href="#f207">Fig. 207</a>).</p> - -<div class="floatleft"> - <img src="images/ill-250b.jpg" width="250" height="186" id="f208" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 208.</span>—Hollowed forms of weathered granite in -a desert of central Asia (after Walther).</p> -</div></div> - -<p>Certain of the saline -constituents of the -rocks, as they are thus -drawn out by the sun’s -rays, fuse with the rock -at the surface to form a -dense brown substance -with smooth surface -coat, known as <i>desert -varnish</i>. Within the interior a portion of the salts crystallize -within the capillary fissures, and like water freezing within a pipe, -they rend the walls apart. As a direct consequence of this -disintegrating process the interior of rock masses may crumble -into sand; and if the hard shell of varnish be broken at any<span class="pagenum"><a name="Page_202" id="Page_202">[202]</a></span> -point, the wind makes its entrance and removes the interior portion -so as to leave a hollow shell—the characteristic “pocket -rock” (<a href="#f208">Fig. 208</a>) of the desert. The nummulitic limestone of -Mokkatan and many of the great hewn blocks of Egyptian limestone -sound hollow under the tap of the hammer, and when -broken, they reveal a shell a few inches only in thickness (<a href="#f209">Fig. 209</a>).</p> - -<div class="floatleft"> - <img src="images/ill-251.jpg" width="250" height="142" id="f209" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 209.</span>—Hollow hewn blocks in a wall in the -Wadi Guerraui (after Walther).</p> -</div></div> - -<p>The brown desert varnish is one of the most characteristic -marks of an arid country. It is found in all deserts under much -the same conditions, and -is especially apt to be present -in sandstone. When -scratched, the surface of -the rock becomes either -cherry-red, indicating anhydrous -ferric oxide, or it -is yellowish, due to the -hydrated iron oxide which -we know as iron rust. -Thus it is seen that the -sands of deserts, in contrast to those yielded by other processes -within humid regions, have a characteristic red color, and this -may vary from brownish red upon the one hand to a rich carmine -upon the other.</p> - - -<p><b>The mechanical breakdown of the desert rocks.</b>—The chemical -changes of decomposition within desert rocks are, as we have -seen, largely due to the action of concentrated solutions of salts -at high temperatures. That there is a certain mechanical rending -of these rocks, due to the “freezing” of salts within the capillary -fissures, has been already mentioned. A further strain -effect arises in rocks like granite, which are a mixture of different -minerals. Heated to a high temperature during the day and -cooled through a considerable range at night, the different minerals -alternately expand and contract at different rates and by different -relative amounts, so that strains are set up, tending to -tear them apart. The effect of these strains is thus a surface -crumbling of rocks.</p> - -<p>But rock is, as already pointed out, a relatively poor conductor -of heat, and hence it is a relatively thin skin only which passes<span class="pagenum"><a name="Page_203" id="Page_203">[203]</a></span> -through the daily round of wide temperature range. This outer -shell when heated is expanded, and so tends to peel off, or exfoliate, -like the outer skin of an onion. The process is therefore -described as <i>exfoliation</i>. In all rocks of homogeneous texture the -continued action of this process results in convexly spherical surfaces, -the material scaled off in the process remaining as a slope -or talus which surrounds the projecting knob (<a href="#f210">Fig. 210</a>). Naked, -these projecting domes rise above the rim of débris at their bases. -Not a particle of dust adheres to the fresh rock surface—no -dirt interferes with its glaring whiteness. Yet close at hand lie -masses of débris into which wells may be carried to depths of -more than six hundred feet without encountering either solid -rock or ground water. The bare walls of granite sometimes mount -upwards for thousands of feet into the air, as steep and as inaccessible -as the squared towers of the Tyrolean Dolomites.</p> - -<div class="figcenter"> - <img src="images/ill-252.jpg" width="400" height="278" id="f210" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 210.</span>—Smooth granite domes shaped by exfoliation and surrounded by a rim of -talus. Gebel Karsala, Nubian Desert (after Walther).</p> -</div></div> - -<p>Rock is such a poor conductor of heat that special strains are -set up at the margin of sunlight and shade. This localization of -the disintegration on the margin of the shaded portions of rock -masses is known as <i>shadow weathering</i> (see <a href="#f215">Fig. 215</a>, <a href="#Page_206">p. 206</a>).</p> - -<p>There is, however, still another mechanical disintegrating -process characteristic of the desert regions, which is likewise -dependent upon the sudden changes of temperature. Rains,<span class="pagenum"><a name="Page_204" id="Page_204">[204]</a></span> -though they may not occur for a year or more, come as sudden -downpours of great volume and violence. Rock masses, which -are highly heated beneath the desert sun, if suddenly dashed -with water, may be rent apart by the differential strains set up -near the surface. That rocks may be easily rent as a result of -sudden chilling is well known to our Northern farmers, who are -accustomed to rid themselves of objectionable bowlders by first -building a fire about them and then dashing water upon their -surface. Thus split into fragments, even the larger bowlders -may be handled and so removed from the farming land. The -natural process of rock rending by the occasional cloudburst may -be described as <i>diffission</i>. Blocks as much as twenty-five feet in -diameter have been observed -in the desert of western -Texas, soon after being -broken into several fragments -at the time of a downpour -of rain (<a href="#f211">Fig. 211</a>).</p> - -<div class="floatleft"> - <img src="images/ill-253.jpg" width="250" height="166" id="f211" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 211.</span>—Granite blocks in the Sierra de -los Dolores of Texas, rent into several -fragments by the dash of rain (after -Walther).</p> -</div></div> - -<p><b>The natural sand blast.</b>—Because -of the saucer-like -shape, the vast expanse, and -the absence of wind breaks, -the potency of wind as a -geological agent is in desert -areas not easily overestimated. -While most of its work is accomplished with the aid of -tools, it has been proven that even without this help, considerable -work is done through the friction of the wind alone, particularly -when moving as powerful eddies in cracks and crannies. This -wear of the wind, unaided by cutting tools, is known as <i>deflation</i>.</p> - -<p>The greater work of the wind is, however, accomplished with -the aid of larger or smaller rock particles, the sand and dust, -with which it is so generally charged above the deserts. Unprotected -by any mat of vegetation the materials of the desert -surface are easily lifted and are constantly migrating with the -wind. The finest dust is raised high into the air, and is carried -beyond the marginal barriers, but none of the sand or coarser -materials ever passes beyond the borders.</p> - -<div class="floatright"> - <img src="images/ill-254a.jpg" width="200" height="96" id="f212" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 212.</span>—“Mushroom rock” from a -desert in Wyoming (after Fairbanks).</p> -</div></div> - -<p>The efficiency of this sand as a cutting tool when carried by the<span class="pagenum"><a name="Page_205" id="Page_205">[205]</a></span> -wind is directly proportioned to the size of the grain, since with -larger fragments a heavier blow is struck when carried at any -given velocity. These more effective grains are, however, not lifted -far above the ground, but advance with a squirming or hopping -motion, much as do the larger pebbles upon the bottom of a river -at the time of a spring freshet. To quote Professor Walther: -“Whoever has had the opportunity -to travel over a surface -of dune sand when a strong wind -is blowing has found it easy to -convince himself of the grinding -action of the wind. At such -times the ground becomes alive, -everywhere the sand is creeping -over the surface with snake-like -squirmings, and the eye quickly -tires of these writhing movements of the currents of sand and -cannot long endure the scene.”</p> - -<div class="floatleft"> - <img src="images/ill-254b.jpg" width="250" height="149" id="f213" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 213.</span>—Windkanten shaped by the -desert sand blast (after Chamberlin and -Salisbury).</p> -</div></div> - -<p>A direct consequence of this restriction of the more effective -cutting tools to the layer of air just above the ground, is the -strong tendency to cut away all projecting masses near their -bases. The “mushroom rocks”, which are so characteristic of -desert landscapes, have been shaped in this manner (<a href="#f212">Fig. 212</a>). -Another product of the desert -sand blast is the so-called <i>Windkante</i> -(wind-edge) or <i>Dreikante</i> -(three-edge), a pebble which is -usually shaped in the form of -a pyramid (<a href="#f213">Fig. 213</a>).</p> - -<p>Whenever a rock face, open -to direct attack by the drifting -sand, is constituted of parts -which have different hardness, -the blast of sand pecks away -at the softer places and leaves the harder ones in relief. Thus is -produced the well-known “stone lattice” of the desert (<a href="#f214">Fig. 214</a>). -Particularly upon the neck of the great Sphinx have the flying -sand grains, by removing the softer layers, brought the sedimentary -structures of the sandstone into strong relief.</p> - -<p><span class="pagenum"><a name="Page_206" id="Page_206">[206]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-255a.jpg" width="250" height="186" id="f214" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 214.</span>—The “stone lattice” of the -desert, the work of the natural sand -blast (after Walther).</p> -</div></div> - -<p>When guided both by planes of sedimentation and planes of jointing, -forms of a very high degree of ornamentation are developed. -Some of the most remarkable forms are due to the protection afforded -to the sun-exposed surfaces by the shell of desert varnish. -In the shaded portions of projecting masses there is no such protection, -and here the sand blast insinuates itself into every crack -and cranny. In this it is aided -by shadow weathering due to -the differential strains set up at -the border of the expanded sun-heated -surface. As a result, -projecting rock masses are sometimes -etched away beneath and -give the effect of a squatting -animal. These forms, due to -shadow erosion, have also been -likened to projecting faucets. -(<a href="#f215">Fig. 215</a>).</p> - -<div class="floatright"> - <img src="images/ill-255b.jpg" width="250" height="129" id="f215" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 215.</span>—Projecting rock carved by the drifting -sand into the form of a couchant animal as a result -of shadow weathering and erosion. Cut in granite -on the north Indian Desert (after Walther).</p> -</div></div> - -<p>Worn by its impact upon neighboring sand grains while in transport, -but much more as it is thrown against the ground or hard -rock surfaces, the wind-driven or <i>eolian</i> sand is at last worn into -smoothly rounded granules which approach the form of a sphere. -Compared to the surface -which sea sand -acquires by attrition, -this shaping process is -much the more efficient, -since in the -water the beach sand -is buoyed up and is -more effectively cushioned -against its neighboring -grains. The -grains of beach sand -when examined under -a microscope are found to be much more irregular in form and -usually display the original fracture surfaces only in part abraded.</p> - -<div class="floatright"> - <img src="images/ill-256a.jpg" width="200" height="228" id="f216" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 216.</span>—Cliffs in loess 200 feet in -height which exhibit the characteristic -vertical jointing (after von Richtofen).</p> -</div></div> - -<p><b>The dust carried out of the desert.</b>—When, standing upon the -mountain wall that surrounds a desert, the traveler gazes out to<span class="pagenum"><a name="Page_207" id="Page_207">[207]</a></span> -windward over the great depression, his field of view is generally -obscured by the yellow haze of the dust clouds moving across -the margins. Upon the mountain -flanks and extending far outside -the borders, this cloud of dust -settles as a shrouding mantle of -impalpable yellow powder, which -is known as <i>loess</i>. These deposits -are continually deepening, and -have sometimes accumulated until -they are hundreds or even thousands -of feet in thickness. Before -reaching its final resting place the -dust of this deposit may have -settled many times, and has certainly -been in part redistributed -by the streams near the desert -margin. In it are the ingredients -which are necessary for the nourishment -of plants, and it constitutes the most important of natural -soils. Continually fed by new deposits from -the desert, and refertilized from below by a -natural process so soon as the upper layers -become impoverished, it requires no artificial -fertilization. Without artificial aids the loess -of northern China has been tilled for thousands -of years without any signs of exhaustion.</p> - -<div class="floatleft"> - <img src="images/ill-256b.jpg" width="150" height="282" id="f217" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 217.</span>—A cañon -in loess worn by -traffic and wind. A -highway in northern -China (after -von Richtofen).</p> -</div></div> - -<p>Though easily pulverized between the fingers, -loess is none the less characterized by a perfect -vertical jointing and stands on vertical faces -as does the solid rock (<a href="#f216">Fig. 216</a>), but it is absolutely -devoid of layers or bedding. Its capacity -of standing in vertical cliffs the loess owes -to a never failing content of lime carbonate -which acts as a cement, and to a peculiar porous -structure caused by capillary canals that run -vertically through the mass, branching like -rootlets and lined with carbonate of lime. This texture once -destroyed, loess resolves itself into a common sticky clay.</p> - -<p><span class="pagenum"><a name="Page_208" id="Page_208">[208]</a></span></p> - -<p>By the feet of passing animals or by wheels of vehicles, the loess -is crushed, and a portion is lifted and carried away by the wind. -Thus in the course of time roadways sink deep into the mass as -steep-walled cañons (<a href="#f217">Fig. 217</a>). A portion of the now structureless -clay remaining upon the roadway is at the time of the rains -transformed into a thick mud which makes traveling all but -impossible, though before its structure has been destroyed the -loess is perfectly drained to the bottom of its deposits.</p> - -<p>The particles which compose the loess are sharply angular -quartz fragments, so fine that all but a few grains can be rubbed -into the pores of the skin. Fine scales of mica, such as are easily -lifted by the wind, are disseminated uniformly throughout the -mass. The only inclosures which are arranged in layers consist -of irregularly shaped concretions of clay. These show a striking -resemblance to ginger roots and are called by the Chinese “stone -ginger”, though they are elsewhere more generally known by -their German name of <i>Loessmännchen</i>, or loess dolls. These -concretions are so disposed in the loess that their longer axes are -vertical, and they were evidently separated from the mass and -not deposited with it.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_209" id="Page_209">[209]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XVI</h2> - -<p class="pch">THE FEATURES IN DESERT LANDSCAPES</p> - -<div class="floatright"> - <img src="images/ill-258.jpg" width="200" height="200" id="f218" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 218.</span>—Diagrams to illustrate the -effects of obstructions of different -types in arresting wind-driven sand. -<i>a</i>, An unyielding obstruction which -permits the wind to pass through it; -<i>b</i>, a flexible and perforated obstruction; -<i>c</i>, an unyielding closed barrier -(after Schulze).</p> -</div></div> - -<p><b>The wandering dunes.</b>—Over the broad expanse of the desert, -sand and dust, and occasionally gypsum from the saline deposits, -are ever migrating with the wind; on quiet days in the eddying -“sand devils”, but especially during the terrifying sand storms -such as in the windy season darken the air of northern China and -southern Manchuria. This drift of the sand is halted only when -an obstruction is encountered—a projecting rock, a bush, or a -bunch of grass, or again the buildings of a city or a town. The -manner in which the sand is arrested -by obstacles of different -kinds is of great interest and importance, -and is utilized in raising -defenses against its encroachments. -If the obstacle is unyielding but -allows some of the wind to pass -through it, no eddies are produced -and the sand is deposited both to -windward and to leeward of the -obstruction to form a fairly symmetrical -mound (<a href="#f218">Fig. 218 <i>a</i></a>). An -obstruction which yields to the -wind causes the sand to deposit -in a mound which is largely to -leeward of the obstruction (<a href="#f218">Fig. 218 <i>b</i></a>). -A solid wall, on the other -hand, by inducing eddies, is at -first protected from the sand and mounds deposit both to windward -and to leeward (<a href="#f218">Fig. 218 <i>c</i></a> and <a href="#f219">Fig. 219</a>).</p> - -<p>Except when held up by an obstruction, the drifting sand travels -to leeward in slowly migrating mounds or ridges which are known -as <i>dunes</i>. Their motion is due to the wind lifting the sand from<span class="pagenum"><a name="Page_210" id="Page_210">[210]</a></span> -the windward side and carrying it over the crest, from where it -slides down the leeward slope and assumes a surface which is -the angle of repose of the material. In contrast with this the -windward slope is -notably gradual, being -shaped in conformity -to the wind -currents.</p> - -<div class="floatleft"> - <img src="images/ill-259a.jpg" width="250" height="126" id="f219" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 219.</span>—Sand accumulating both to windward and -to leeward of a firm and impenetrable obstruction. -The wind comes from the left (after a photograph -by Bastin).</p> -</div></div> - -<p>The dunes which -are raised upon seashores, -like those of -the desert, are constantly -migrating, -those upon the shores -of the North Sea at -the average rate of -about twenty feet per year. Relentlessly they advance, and despite -all attempts to halt them, have many times overwhelmed -the villages along the coast. Upon the great barrier beach known -as the <i>Kurische Nehrung</i>, on the southeastern shore of the Baltic -Sea, such a burial of villages has more than once occurred, but -as in the course of time further migration of the dune has proceeded, -the ruins of the buried villages have been exhumed by -this natural excavating process (<a href="#f220">Fig. 220</a>).</p> - -<div class="figcenter"> - <img src="images/ill-259b.jpg" width="400" height="148" id="f220" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 220.</span>—Successive diagrams to show how the town of Kunzen was buried, and -subsequently exhumed in the continued migration of a great dune upon the -Kurische Nehrung (after Behrendt).</p> -</div></div> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 7.</span></p> - -<div class="figcenter"> - <img src="images/ill-260a.jpg" width="400" height="250" id="p7a" - alt="" - title="" /> - <div class="caption"><p class="ch400"><i>A.</i> Ranges of dunes upon the margin of the Colorado Desert (after Mendenhall).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-260b.jpg" width="400" height="236" id="p7b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Sand dunes encroaching upon the oasis of Wed Souf. Algeria (after T. H. -Kearney).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_211" id="Page_211">[211]</a></span></p> - -<p><b>The forms of dunes.</b>—The forms assumed by dunes are dependent -to a very large extent upon the strength of the wind -and the available supply of sand. With small quantities of -sand and with moderate winds, sickle-shaped dunes known as -<i>barchans</i> (<a href="#f221">Fig. 221</a>) are formed, whose convex and flatter slopes -are toward the wind and whose steep concave leeward slopes are -maintained at the angle of repose. The barchan is shaped by the -wind going both over and around the dune, constantly removing -sand from the windward side and depositing it to leeward. With -larger supplies of sand and winds which are not too violent a -series of barchans is built up, and these are arranged transversely -to the wind direction (<a href="#f222">Fig. 222 <i>b</i></a>). If the winds are more violent, -the minor depressions in the crests of the dunes become wind -channels, and the sand is then trailed out along them until the -arrangement of the ridges is parallel to the wind (<a href="#f222">Fig. 222 <i>c</i></a>). -The surfaces of dunes are -generally marked by beautiful -ripples in the sand, -which, seen from a little -distance, may give the appearance -of watered silk -(<a href="#p7a">plate 7 A</a>).</p> - -<div class="figcenter"> - <img src="images/ill-262a.jpg" width="400" height="124" id="f221" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 221.</span>—View of desert barchans (after Haug).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-262b.jpg" width="250" height="119" id="f222" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 222.</span>—Diagrams to show the relationships -in form and in orientation of dunes to the supply -of sand and to the strength of the wind. -<i>a</i>, barchans formed by small supplies of sand -and moderate winds; <i>b</i>, transverse dune ridges, -formed when supply of sand is large and winds -are moderate; <i>c</i>, dune ridges formed with large -sand supply and violent winds (after Walther -and Cornish).</p> -</div></div> - - -<p>Under normal conditions -dunes are not stationary -but continue to -wander with the prevailing -winds until they have -reached the outer edge of -the zone of vegetation -near the base of the foothills -at the margin of the desert. Here the grasses and other -desert plants arrest the first sand grains that reach them, and -they continue to grow higher as the sands accumulate. Some of<span class="pagenum"><a name="Page_212" id="Page_212">[212]</a></span> -the desert plants, like the yuccas, have so adapted themselves to -desert conditions that they may grow upward with the sand for -many feet and so keep their crowns above its surface.</p> - - -<p><b>The cloudburst in the desert.</b>—Such clouds as enter the -desert through its mountain ramparts, and those derived from -evaporation from the hot desert soil, usually precipitate their -moisture before passing out of the basin. Above the highly -heated floor the heavy rain clouds are unable to drop their burden. -The rain can sometimes be seen descending, but long -before it has reached the ground it has again passed into vapor, -and through repetition of this process the clouds become so charged -with moisture that when they encounter a mountain wall and -are thus forced to rise, there is a sudden downpour not equaled -in the humid regions. Desert rains are rare, but violent beyond -comparison. Often for a year or more there is no rainfall upon -the loose sand or porous clay, and the few plants which survive -must push their roots deep down until they have reached the -zone of ground water. When the clouds burst, each small cañon -or <i>wed</i> (pl. <i>wadi</i>) within the mountain wall is quickly occupied -by a swollen current which carries a thick paste of sediment and -drowns everything before it. Ere it has flowed a mile, it may -be that the water has disappeared entirely, leaving a layer of mud -and sand which rapidly dries out with the reappearance of the sun.</p> - -<div class="figcenter"> - <img src="images/ill-263.jpg" width="450" height="142" id="f223" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 223.</span>—Ideal section across the rising mountain wall surrounding a desert -and a part of the neighboring slope (after R. W. Pumpelly).</p> -</div></div> - -<p>As the mountains upon seacoasts are generally rising with -reference to the neighboring sea bottom, so the mountains which -hem in the deserts are generally growing upward with reference -to the inclosed desert floor. The marginal dislocations which -separate the two are often in evidence at the foot of the steep -slope (<a href="#f223">Fig. 223</a>), and these may even appear as visible earthquake<span class="pagenum"><a name="Page_213" id="Page_213">[213]</a></span> -faults to indicate that the uplift is more accelerated than -the deposition along the mountain front.</p> - -<div class="floatright"> - <img src="images/ill-264a.jpg" width="200" height="101" id="f224" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 224.</span>—Dry delta or alluvial fan at -the foot of a mountain range upon -the borders of a desert.</p> -</div></div> - -<p><b>The zone of the dwindling river.</b>—The rapid uplift so generally -characteristic of desert margins gives to the torrential streams -which develop after each cloudburst -such an unusual velocity -that when they emerge from the -mountain valleys on to the desert -floor, the current is suddenly -checked and the burden of sediment -in large part deposited at -the mouth of the valley so as to -form a coarse delta deposit which -is described as a <i>dry delta</i> (<a href="#f224">Fig. 224</a>). Dependent upon its steepness -of slope, this delta is variously referred to as an <i>alluvial fan</i> -or <i>apron</i>, or as an <i>alluvial cone</i>. Over the conical slopes of the -delta surface the stream is broken up into numerous distributaries -which divide and subdivide as do the roots of a tree. In the -Mohammedan countries described as <i>wadi</i>, these distributaries -upon dry deltas are on the Pacific coast of the United States -referred to as “washes” (<a href="#f225">Fig. 225</a>).</p> - -<div class="figcenter"> - <img src="images/ill-264b.jpg" width="400" height="228" id="f225" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 225.</span>—Map of the distributaries of neighboring streams which emerge at the -western base of the Sierra Nevadas in California (after W. D. Johnson).</p> -</div></div> - -<p>Fast losing their velocity after emerging from the mountains, -the various distributaries drop first of all the heavy bowlders,<span class="pagenum"><a name="Page_214" id="Page_214">[214]</a></span> -then the large pebbles and the sand, so that only the finer sand -and the silt are carried to the margin of the delta. As they -enlarge their boundaries, the neighboring deltas eventually -coalesce and so form an <i>alluvial bench</i> or “gravel piedmont” at -the foot of the range. Only the larger streams are able to entirely -cross this bench of parched deposits with its coarsely porous -structure, for the water is soon sucked up by the thirsty materials. -Encountering in its descent more clayey layers, this water -is conducted to the surface near the margin of the bench and -may there appear as a line of springs. At this level there develops, -therefore, a zone of vegetation, though there is no local rain.</p> - -<p>The alluvial bench grows upward by accretion of layers which -are thickest at the mountain end, so that the steepness of the -bench increases with time.</p> - - -<p><b>Erosion in and about the desert.</b>—The violent cloudburst that -is characteristic of the arid lands is a most potent agent in modeling -the surface of the ground wherever the rock materials are -not too firmly coherent. Under the dash of the rain a peculiar -type of “bad land” topography is developed (<a href="#p5b">plate 5 B</a> and <a href="#f226">Fig. 226</a>). -Such a rain-cut surface is a veritable maze -of alternating gully and ridge, a country -worthless for agricultural purposes and offering -the greatest difficulty in the way of penetrating -it. When composed of stiff clay with -scattered pebbles and bowlders, the effect of -the “rain erosion” is to fashion steep clay -pillars each capped by a pebble and described -as “demoiselles” (<a href="#f226">Fig. 226</a>).</p> - -<div class="floatleft"> - <img src="images/ill-265.jpg" width="180" height="272" id="f226" - alt="" - title="" /> - <div class="cf"><p class="ch180"><span class="smcap">Fig. 226.</span>—A group of -“demoiselles” in the -“bad lands” (after a -photograph by Fairbanks).</p> -</div></div> - -<p>Behind the mountain front the valleys out -of which the torrents are discharged are usually -short with steep side walls and a relatively -flat bottom, ending headward in an -amphitheater with precipitous walls (<a href="#f227">Fig. 227</a>). -In the western United States such -valleys are referred to as “box cañons”, but -in Mohammedan countries the name “wed” applies to the river -valley within the mountains and to the distributaries as well.</p> - - -<p><b>Characteristic features of the arid lands.</b>—It is characteristic -of erosion and deposition within humid regions that all outlines<span class="pagenum"><a name="Page_215" id="Page_215">[215]</a></span> -become softened into flowing curves, due to the protective mat -of vegetation. In arid lands those massive rocks which are -without structural planes of separation, partly as a consequence -of exfoliation, develop broad domes which are projected upon -the horizon as great semicircles, -broken in half it may be by -displacement. The same massive -rocks where intersected by vertical -joint planes yield, on the contrary, -sharp granite needles like those of -Harney Peak (plate 8 A). Similarly, -schistose or bedded rocks, when tilted -at a high angle, may yield forms -which are almost identical. Examples -of such needles are found in -the Garden of the Gods in Colorado.</p> - -<p>At lower levels, where the flying -sand becomes effective as an eroding -agent, flat bedded rocks become -etched into shelves and cornices, and -if intersected by joints, the shelves -and cornices are transformed into groups of castellated towers and -pinnacles of a high degree of ornamentation. These fantastic -erosion remnants are usually referred to as “chimneys” and may -be seen in numbers in the bad lands of Dakota, as they may in -Colorado either in Monument Park or in the new Monolithic -National Park (plate 8 B).</p> - -<div class="floatright"> - <img src="images/ill-266.jpg" width="200" height="253" id="f227" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 227.</span>—Amphitheater at the -head of the Wed Beni Sur (after -Walther).</p> -</div></div> - -<p>Where wind erosion plays a smaller rôle in the sculpture, but -where after an uplift a river has made its way, horizontally bedded -rocks are apt to be carved into broad <i>rock terraces</i>, nowhere shown -upon so grand a scale as about the Grand Cañon of the Colorado. -Each harder layer has here produced a floor or terrace which -ends in a vertical escarpment, and this is separated from the -next lower layer of more resistant rock by a slope of talus which -largely hides the softer intermediate beds. The great Desert of -Sahara is shaped in a series of rock terraces or steppes which -descend toward the interior of the basin.</p> - -<p>A single harder layer of resistant rock comes often to form -the flat capping of a plateau, and is then known as a <i>mesa</i>, or<span class="pagenum"><a name="Page_216" id="Page_216">[216]</a></span> -table mountain. Along its front, detached outliers usually stand -like sentinels before the larger mass, and according to their relative -proportions, these are referred to either as small mesas or -as the smaller <i>buttes</i> (<a href="#f228">Fig. 228</a>).</p> - -<div class="figcenter"> - <img src="images/ill-267a.jpg" width="400" height="145" id="f228" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 228.</span>—Mesa and outlying butte in the Leucite Hills of Wyoming (after Whitman -Cross, U. S. G. S.).</p> -</div></div> - -<p><b>The war of dune and oasis.</b>—In every desert the deposits -are arranged in consecutive belts or zones which are alternately -the work of wind and water. Surrounding the desert and upon -the flanks of the mountain wall there is found (1) the deposit of -loess derived from the dust that is carried out of the desert by -the wind. Immediately within the desert border at the base of -the mountains is (2) the zone of the dwindling river with its -sloping bench of coarse rubble and gravel.</p> - -<div class="figcenter"> - <img src="images/ill-267b.jpg" width="400" height="105" id="f229" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 229.</span>—Flat-bottomed basin separating dunes—<i>bajir</i> or <i>takyr</i> (after Ellsworth -Huntington).</p> -</div></div> - -<p>Next in order is -(3) the belt of the flying sand, a zone of dune ridges often separated -by narrow, flat-bottomed basins (<a href="#f229">Fig. 229</a>) into which the -strongest streams bring the finer sands and silt from the mountains. -Lastly, there is (4) the central sink or sinks, into which -all water not at once absorbed within the zone of alluviation or -in the zone of dunes is finally collected. Here are the true lacustrine -deposits of clay and separated salts (<a href="#f230">Fig. 230</a> and <a href="#f207">Fig. 207</a>, -<a href="#Page_201">p. 201</a>). The lake deposits fill in all the original irregularities -of the desert floor, out of which the tops of isolated ranges of -mountains now project like islands out of the surface of the sea. -The several zones of deposits -in their order from -the margin to the center -of the desert are given -schematically in <a href="#f231">Fig. 231</a>.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 8.</span></p> - -<div class="figcenter"> - <img src="images/ill-268a.jpg" width="400" height="246" id="p8a" - alt="" - title="" /> - <div class="caption"><p class="ch400"><i>A.</i> The granite needles of Harney Peak in the Black Hills of South Dakota (after -Darton).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-268b.jpg" width="400" height="299" id="p8b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Castellated erosion chimneys in El Cobra Cañon, New Mexico. -(<i>Photograph by E. C. Case.</i>)</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_217" id="Page_217">[217]</a></span></p> - -<div class="floatright"> - <img src="images/ill-270a.jpg" width="250" height="148" id="f230" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 230.</span>—Billowy surface of the salt crust on -the central sink in the Lop Desert of central -Asia (after Ellsworth Huntington).</p> -</div></div> - -<p>The zone of vegetation, -as already stated, lies near -the foot of the alluvial -bench, so that here are -found the oases about -which have clustered the -cities of the desert from -the earliest records of antiquity until now. Just without the line -of oases is the wall of dunes held back from further advance only -by the vegetation which in turn is dependent upon the rains in -the neighboring mountains. With every diminution in the water -supply, the dunes advance and encroach upon the oases (<a href="#p7b">plate 7 B</a>); -while with every considerable increase in this supply of moisture -the alluvial bench advances over the dunes and acquires a strip -of their territory. Thus with varying fortunes a war is continually -waged between the withering river and the flying sand, -and the alternations of climate are later recorded in the dovetailing<span class="pagenum"><a name="Page_218" id="Page_218">[218]</a></span> -together of the eolian and alluvial deposits at their common -junction (<a href="#f231">Fig. 231</a>).</p> - -<div class="figcenter"> - <img src="images/ill-270b.jpg" width="400" height="181" id="f231" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 231.</span>—Schematic diagram to show the zones of deposition in their order from -the margin to the center of a desert.</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-271a.jpg" width="250" height="199" id="f232" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 232.</span>—Mounds upon the site of the buried -city of Nippur (after the cast by Muret).</p> -</div></div> - -<p>In addition to the smaller periodic alternations of pluvial and -interpluvial climate—the -“pulse of Asia”—the -record of the Asiatic -deserts indicates a progressive -desiccation of -the entire region, which -has now given the victory -to the dune. The -ancient history of the -cities of the plains supplies -the records of many -that have been buried -in the dunes. To-day, -where once were prosperous -cities, nothing is -to be seen at the surface but a group of mounds (<a href="#f232">Fig. 232</a>). Exhumed -after much painstaking labor, the walls and palaces of -these ancient cities -have once more been -brought to the light -of day, and much -has thus been -learned of the civilization -of these -early times (<a href="#f233">Fig. 233</a>). -Quite recently -the mounds -which cover between -one and two -hundred buried villages -have been -found upon the borders -of the Tarim -basin of central Asia, where they were lost to history when -they were overwhelmed in the early centuries of the Christian -Era.</p> - -<div class="floatright"> - <img src="images/ill-271b.jpg" width="250" height="198" id="f233" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 233.</span>—Exhumed structures in the buried city of -Nippur (after Hilprecht).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_219" id="Page_219">[219]</a></span></p> - -<p><b>The origin of the high plains which front -the Rocky Mountains.</b>—To the eastward of -the great backbone of the North American -continent stretches a vast plain gently inclined -away from the range and generally -known as the High Plains region (plate 9). -The tourist who travels westward by train -ascends this slope so gradually that when he -has reached the mountain front it is difficult -to realize that he has climbed to an altitude -of five thousand feet above the level of the -sea. That he has also passed through several -climatic zones—a humid, a semiarid, and -an arid—and has now entered a semiarid -district, is more easily appreciated from study -of the vegetation (<a href="#f234">Fig. 234</a>). The surface of -the High Plains, where not cut into by rivers, -is remarkably even, so that it might be compared -to the quiet surface of a great lake.</p> - -<div class="figcenter"> - <img src="images/ill-272.jpg" width="450" height="86" id="f234" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 234.</span>—Section across the High Plains, showing the position of the terrace and the climatic zones above it (after W. D. Johnson).</p> -</div></div> - -<p>The materials which compose the surface -veneer of these plains are coarse conglomerates, -gravels, and sands, and the so-called -“mortar beds”, which are nothing but sands -cemented into sandstone by carbonate of lime. -The pebbles in all these deposits are far-traveled -and appear to have been derived -from erosion of those crystalline rocks which -compose the eastern front of the Rocky -Mountains. These different materials are -not arranged in strictly parallel beds, as are -the deposits of a lake or sea; but the beds -are made up of long threads of lenticular -cross section which are interlaced in the most -intricate fashion and which extend down the -slope, or outward from the mountain front -(<a href="#f235">Fig. 235</a>). It is thus shown that the High -Plains are a bench or plain of alluviation -formed at the front of the Rocky Mountains -during an earlier series of pluvial periods, and that subsequent<span class="pagenum"><a name="Page_220" id="Page_220">[220]</a></span> -uplift has produced the modern river valleys which are cut out of -the plain. The plexus of long threads of the coarser materials are -the courses of dwindling rivers which interlaced over the former -plain, and which in time were buried under other channel deposits -of the same nature but in different positions (<a href="#f236">Fig. 236</a>). The -pluvial periods in which this -bench was formed produced -in other latitudes the great -continental glaciers which -wrought such marvelous -changes in northern North -America and in northern -Europe.</p> - -<div class="figcenter"> - <img src="images/ill-273a.jpg" width="400" height="107" id="f235" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 235.</span>—Section across the great lenticular threads of alluvial deposits which -compose the veneer of the High Plains (after W. D. Johnson).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-273b.jpg" width="250" height="104" id="f236" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 236.</span>—Distributaries of the foothills -superimposed upon an earlier series (after -W. D. Johnson).</p> -</div></div> - -<p><b>Character profiles.</b>—In -contrast with the profiles in the landscapes of humid regions (see -<a href="#f187">Fig. 187</a>, <a href="#Page_177">p. 177</a>), those of arid lands are marked by straighter -elements (<a href="#f237">Fig. 237</a>). Almost the only exception of importance is -furnished by the domes of massive granite monoliths, which are -sometimes broken in half by great displacements. Below the -horizon the secondary lines in the landscape betray the same -straightness of the component elements by the gabled slopes -of talus which are many times repeated so as almost to reproduce -the lines in a house of cards, since the sloping lines are -maintained at the angle of repose of the materials (<a href="#f482">Fig. 482</a>, <a href="#Page_443">p. 443</a>). -Wherever the waves of desert lakes have made an attack upon the -rocks and have retired the projecting spurs, other gables characterized -by slightly different slopes are introduced into the landscape.</p> - -<div class="figcenter"> - <img src="images/ill-273c.jpg" width="400" height="213" id="f237" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 237.</span>—Character profiles in the landscapes of arid lands.</p> -</div></div> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 9.</span></p> - -<div class="figcenter"> - <img src="images/ill-274.jpg" width="400" height="631" id="p9" - alt="" - title="" /> -</div> - -</div> - -<p><span class="pagenum"><a name="Page_221" id="Page_221">[221]</a></span></p> - -<p class="prr"><span class="smcap">Reading References for Chapters XV and XVI</span></p> - -<p class="p1">General:—</p> - -<p class="pex"><span class="smcap">Johannes Walther.</span> Das Gesetz der Wüstenbildung in Gegenwart und -Vorzeit. Berlin, 1900, pp. 175, many plates. (This extremely valuable -work is now out of print, but both a revised edition and an English -translation are promised for 1912.)</p> - -<p class="pex"><span class="smcap">Siegfried Passarge.</span> Die Kalihari. Berlin, 1904, pp. 662.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> The Geographic Cycle in an Arid Climate, Jour. Geol., -vol. 13, 1905, pp. 381-407.</p> - -<p class="pex"><span class="smcap">Ellsworth Huntington.</span> The Pulse of Asia. New York and Boston, -1907, pp. 415.</p> - -<p class="pex"><span class="smcap">Sven Hedin.</span> Scientific Results of a Journey through Central Asia, 1899-1900. -Stockholm, 1904-1905, vols. 1 and 2, pp. 523 and 717, pls. -56 and 76.</p> - -<p class="pex"><span class="smcap">Joseph Barrell.</span> Relative Geological Importance of Continental, Littoral -and Marine Sedimentation, Jour. Geol., vol. 14, 1906, pp. 316-356, -429-457, 524-568.</p> - -<p class="pex"><span class="smcap">E. F. Gautier.</span> Études sahariennes, Ann. de Géogr., vol. 16, 1907, pp. -46-69, 117-138.</p> - -<p class="p1">The self-registering gauge of past climates:—</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Lake Bonneville, Mon. I, U. S. Geol. Surv., Chapter vi, -pp. 214-318.</p> - -<p class="pex"><span class="smcap">T. F. Jamieson.</span> The Inland Seas and Salt Lakes of the Glacial Period, -Geol. Mag. decade III, vol. 2, 1885, pp. 193-200.</p> - -<p class="pex"><span class="smcap">J. E. Talmage.</span> The Great Salt Lake, Present and Past. Salt Lake City, -1900, pp. 116, plates.</p> - -<p class="pex"><span class="smcap">E. Huntington.</span> Some Characteristics of the Glacial Period in Non-glaciated -Regions, Bull. Geol. Soc. Am., vol. 18, 1907, pp. 351-388, -pls. 31-39.</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin.</span> The Future Habitability of the Earth, Rept. -Smithson. Inst., 1910, pp. 371-389.</p> - -<p><span class="pagenum"><a name="Page_222" id="Page_222">[222]</a></span></p> - -<p class="p1">The red and brown desert varnish:—</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Subaërial Decay of Rocks and Origin of the Red Color -of Certain Formations. Bull. 52, U. S. Geol. Surv., 1889, pp. 65, pls. 5.</p> - -<p class="p1">Erosion in the desert:—</p> - -<p class="pex"><span class="smcap">J. A. Udden.</span> Erosion, Transportation, and Sedimentation performed by -the Atmosphere, Jour. Geol., vol. 2, 1894, pp. 318-331.</p> - -<p class="pex"><span class="smcap">S. Passarge.</span> Die pfannenförmigen Hohlformen der südafrikanischen -Steppen, Pet. Mitt., vol. 57, 1911, pp. 57-61, 130-135.</p> - -<p class="p1">The dust carried out of the desert:—</p> - -<p class="pex"><span class="smcap">F. von Richtofen.</span> China, Ergebnisse eigene Reisen und darauf gegründeten -Studien, Berlin, 1877, vol. 1, pp. 56-125.</p> - -<p><span class="smcap">E. Hilgard.</span> The Loess of the Mississippi Valley, Am. Jour. Sci., (3), -vol. 18, 1879, pp. 106-112.</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin</span> and <span class="smcap">R. D. Salisbury</span>. Preliminary Paper on the -Driftless Area of the Upper Mississippi Valley, 6th Ann. Rept. U. S. -Geol. Surv., 1885, pp. 278-307.</p> - -<p class="pex"><span class="smcap">E. E. Free.</span> The movement of soil material by the wind, with a bibliography -of eolian geology by S. C. Stuntz and E. E. Free, Bull. 68, -U. S. Bureau of Soils, 1911, pp. 272, pls. 5.</p> - -<p class="pex"><span class="smcap">M. Neumayr.</span> Erdgeschichte, vol. 1, pp. 510-514.</p> - -<p class="pex"><span class="smcap">E. de Martonne.</span> Géographie physique, pp. 663-668.</p> - -<p class="p1">Dunes:—</p> - -<p class="pex"><span class="smcap">Vaughan Cornish.</span> On the Formation of Sand-dunes, Geogr. Jour., -vol. 9, 1897, pp. 278-309 (a most important paper).</p> - -<p class="pex"><span class="smcap">F. Solger</span> and Others. Dünenbuch. Enke, Stuttgart, 1910, pp. 373.</p> - -<p class="p1">The zone of the dwindling river:—</p> - -<p class="pex"><span class="smcap">E. Huntington.</span> The Border Belts of the Tarim Basin, Bull. Am. Geogr. -Soc., vol. 38, 1906, pp. 91-96; The Pulse of Asia, pp. 210-222, 262-279.</p> - -<p class="p1">The war of dune and oasis:—</p> - -<p class="pex"><span class="smcap">R. Pumpelly.</span> Explorations in Turkestan, Expedition of 1904, etc., -Pub. 73, Carneg. Inst., Washington, vol. 1, pp. 1-13.</p> - -<p class="pex"><span class="smcap">E. Huntington.</span> The Oasis of Kharga, Bull. Am. Geogr. Soc., vol. 42. -1910, pp. 641-661.</p> - -<p class="pex"><span class="smcap">Th. H. Kearney.</span> The Country of the Ant Men, Nat. Geogr. Mag., vol. -22, 1911, pp. 367-382.</p> - -<p class="p1">Features of the arid lands:—</p> - -<p class="pex"><span class="smcap">C. E. Dutton.</span> Tertiary History of the Grand Cañon District, with -Atlas, Mon. II, U. S. Geol. Surv., 1882, pp. 264, pls. 42, maps 23.</p> - -<p class="pex"><span class="smcap">G. Sweinfurth.</span> Map Sheets of the Eastern Egyptian Desert. Berlin, -1901-1902, 8 sheets.</p> - -<p class="p1">The origin of the high plains:—</p> - -<p class="pex"><span class="smcap">W. D. Johnson.</span> The High Plains and their Utilization, 21st Ann. Rept. -U. S. Geol. Surv., Pt. iv, 1901, pp. 601-741.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_223" id="Page_223">[223]</a></span></p> - -<div class="chapter"> - -<h2 class="pc4">CHAPTER XVII</h2> - -<p class="pch">REPEATING PATTERNS IN THE EARTH RELIEF</p> - -<p><b>The weathering processes under control of the fracture system.</b>—In -an earlier chapter it was learned that the rocks which compose -the earth’s surface shell are intersected by a system of -joint fractures which in little-disturbed areas divide the surface -beds into nearly square perpendicular prisms (<a href="#f36">Fig. 36</a>, <a href="#Page_55">p. 55</a>), -more or less modified by additional diagonal joints, and often -also by more disorderly fractures. Throughout large areas these -fractures may maintain nearly constant directions, though either -one or more of the master series may be locally absent. This -distinctive architecture of the surface shell of the lithosphere has -exercised its influence upon the various weathering processes, as it -has also upon the activities of running water and of other less -common transporting agencies at the surface.</p> - -<p>Within high latitudes, where frost action is the dominant -weathering process, the water, by insinuating itself along the -joints and through repeated freezings, has broken down the rock -in the immediate neighborhood of these fractures, and so has -impressed upon the surface an image of the underlying pattern -of structure lines (<a href="#p10a">plate 10 A</a>).</p> - -<p>In much lower latitudes and in regions of insufficient rainfall, -the same structures are impressed upon the relief, but by other -weathering processes. In the case of the less coherent deposits -in these provinces, the initial forms of their erosional surface have -sometimes been determined by the dash of rain from the sudden -cloudburst. Thus the “bad lands” may have their initial gullies -directed and spaced in conformity with the underlying joint structures -(<a href="#f238">Fig. 238</a>).</p> - -<div class="floatleft"> - <img src="images/ill-279a.jpg" width="250" height="325" id="f238" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 238.</span>—Rain sculpturing under control -by joints. Coast of southern California -(after a photograph by Fairbanks).</p> -</div></div> - -<p>In such portions of the temperate regions as are favored by a -humid climate, the mat of vegetation holds down a layer of soil, -and mat and soil in coöperation are effective in preventing any<span class="pagenum"><a name="Page_224" id="Page_224">[224]</a></span> -such large measure of frostwork as is characteristic of the subpolar -regions or of high levels in the arid lands. In humid regions -the rocks become a prey especially -to the processes of solution -and accompanying chemical -decomposition, and these -processes, although guided by -the course of the percolating -ground water along the fracture -planes, do not afford such -striking examples of the control -of surface relief.</p> - -<p>Those limestones which -slowly pass into solution in -the percolating water do, however, -quite generally indicate -a localization of the solution -along the joint channels (<a href="#f239">Fig. 239</a> and <a href="#p6b">plate 6 B</a>). -Though in other rocks not so apparent, -yet solutions generally take -their courses along the same channels, and upon them is localized -the development of the newly formed hydrated and carbonate -minerals, as is well illustrated by -the phenomenon of spheroidal -weathering (<a href="#f155">Fig. 155</a>, <a href="#Page_150">p. 150</a>).</p> - -<div class="floatright"> - <img src="images/ill-279b.jpg" width="200" height="142" id="f239" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 239.</span>—Outcrop of flaggy limestone -which shows the effects of solution -along neighboring joints in a sagging -of the upper beds (after Gilbert, U. -S. G. S.).</p> -</div></div> - -<p><b>The fracture control of the drainage -lines.</b>—The etching out of -the earth’s architectural plan in -the surface relief, which we have -seen begun in the processes of -weathering, is continued after the -transporting agents have become -effective. It is often easy to see -that a river has taken its course -in rectangular zigzags like the -elbows of a jointed stove pipe, and that its walls are formed -of joint planes from which an occasional squared buttress projects -into the channel. This structure is rendered in the plan of<span class="pagenum"><a name="Page_225" id="Page_225">[225]</a></span> -the Abisko Cañon of northern Lapland (<a href="#f240">Fig. 240</a>). We are later -to learn that another great transporting agent, the water wave, -makes a selective attack upon the lithosphere -along the fractures of the joint -system (<a href="#f250">Fig. 250</a>, <a href="#Page_233">p. 233</a> and <a href="#f254">Fig. 254</a>, -<a href="#Page_235">p. 235</a>).</p> - -<div class="floatright"> - <img src="images/ill-280a.jpg" width="150" height="205" id="f240" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 240.</span>—Map of the -joint-controlled Abisko -Cañon in northern Lapland -(after Otto Sjögren).</p> -</div></div> - -<p>Where the scale of the example is large, -as in the cases which have been above -cited, the actual position and directions -of the joint wall are easily compared with -the near-by elements of the river’s course, -so that the connection of the drainage -lines with the underlying structure is at -once apparent. In many examples where -the scale is small, the evidence for the controlling -influence of the rock structure in -determining the courses of streams may -be found in the peculiar character of the drainage plan. To -illustrate: the course of the Zambesi River, within the gorge below -the famous Victoria Falls, not only makes repeated turnings at a -right angle, but its tributary streams, instead of making the usual -sharp angle where they join the -main stream, also affect the right -angle in their junctions (<a href="#f241">Fig. 241</a>).</p> - -<div class="floatleft"> - <img src="images/ill-280b.jpg" width="180" height="163" id="f241" - alt="" - title="" /> - <div class="cf"><p class="ch180"><span class="smcap">Fig. 241.</span>—Map of the gorge of the -Zambesi River below the Victoria -Falls (after Lamplugh).</p> -</div></div> - -<p><b>The repeating pattern in drainage -networks.</b>—It is a characteristic of -the joint system that the fractures -within each series are spaced with -approximation to uniformity. If -the plan of a drainage system has -been regulated in conformity with -the architecture of the underlying -rock basement, the same repeating -rectangles of the master joints may -be expected to appear in the lines of drainage—the so-called -drainage network.</p> - -<p>Such rectangular patterns do very generally appear in the -drainage network, though they are often masked upon modern -maps by what, to the geologist, seems impertinent intrusion of the<span class="pagenum"><a name="Page_226" id="Page_226">[226]</a></span> -black lines of overprinting which indicate railways, lines of highway, -and other culture elements. On river maps, which are -printed without culture, the pattern is much more easily recognized -(<a href="#f242">Figs. 242</a> and <a href="#f243">243</a>). Wherever the relief is strong, as is -the case in the Adirondack Mountain province of the State of -New York, individual hills may stand in relief between the bounding -streams which compose the rectangular network, like the -squared pedestals of monuments. Such a type of relief carved in -repeating patterns has been described as “checkerboard topography.”</p> - -<div class="floatleft"> - <img src="images/ill-281a.jpg" width="150" height="370" id="f242" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 242.</span>—Controlled -drainage network of -the Shepaug River -in Connecticut.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-281b.jpg" width="300" height="370" id="f243" - alt="" - title="" /> - <div class="cf"><p class="ch300"><span class="smcap">Fig. 243.</span>—A river network of repeating rectangular pattern. -Near Lake Temiskaming, Ontario (from the map -by the Dominion Government).</p> -</div></div> - -<p class="vh">————</p> - -<p><b>The dividing lines of the relief patterns—lineaments.</b>—The -repeating design outlined in the river network of the Temiskaming -district (<a href="#f243">Fig. 243</a>) would appear in greater perfection if we -could reproduce the relief without at the same time obscuring<span class="pagenum"><a name="Page_227" id="Page_227">[227]</a></span> -the lines of drainage; for where the pattern is not completely -closed by the course of the stream, there is generally found either -a dry valley or a ravine to complete the design. If these are -not present, a bit of straight coast line, a visible line of fracture, -a zone of fault breccia, or the boundary line separating -different formations may one or more of them fill in the gaps of -the parallel straight drainage lines which by their intersection -bring out the pattern. These significant lines of landscapes -which reveal the hidden architecture of the rock basement are -described as <i>lineaments</i> (<a href="#f82">Fig. 82</a>, <a href="#Page_87">p. 87</a>). They are the character -lines of the earth’s physiognomy.</p> - -<p>It is important to emphasize the essentially composite expression -of the lineament. At one locality it appears as a drainage -line, a little farther on it may be a line of coast; then, again, -it is a series of aligned waterfalls, a visible fault trace, or a rectilinear -boundary between formations; but in every case it is some -surface expression of a buried fracture. Hidden as they so generally -are, the fracture lines must be searched out by every means -at our disposal, if we are not to be misled in accounting for the -positions and the attitudes of disturbed rock masses.</p> - -<p>As we have learned, during earthquake shocks, as at no other -time, the surface of the earth is so sensitized as to betray the -position of its buried fractures. As the boundaries of orographic -blocks, certain of the fractures are at such times the seats of -especially heavy vibrations; they are the seismotectonic lines -of the earthquake province. Many lineaments are identical -with seismotectonic lines, and they therefore afford a means of -to some extent determining in advance the lines of greatest danger -from earthquake shock.</p> - - -<p><b>The composite repeating patterns of the higher orders.</b>—Not -only do the larger joint blocks become impressed upon the earth -relief as repeating diaper patterns, but larger and still larger composite -units of the same type may, in favorable districts, be found -to present the same characters. Attention has already been -more than once directed to the fact that the more perfect and -prominent fracture planes recur among the joints of any series at -more or less regular intervals (<a href="#f40">Fig. 40</a>, <a href="#Page_57">p. 57</a>, and <a href="#f41">Fig. 41</a>, <a href="#Page_58">p. 58</a>). -Nowhere, perhaps, is this larger order of the repeating pattern -more perfectly exemplified than in some recent deposits in the<span class="pagenum"><a name="Page_228" id="Page_228">[228]</a></span> -Syrian desert (<a href="#p10b">plate 10 B</a>). It is usually, however, in the older -sediments that such structures may be recognized; as, for example, -in the squared towers and buttresses of the Tyrolean -Dolomites (<a href="#f244">Fig. 244</a>). Here the larger blocks appear in the thick -bedded lower formation, the dolomite, divided into subordinate -sections of large dimensions; but in the overlying formations -in blocks of relatively small size, yet with similarly perfect subequal -spacing.</p> - -<div class="figcenter"> - <img src="images/ill-283.jpg" width="400" height="275" id="f244" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 244.</span>—Squared mountain masses which reveal a distribution of the joints in -block patterns of different orders of magnitude. The Pordoi range of the Sella -group of the Dolomites, seen from the Cima di Rossi (after Mojsisovics).</p> -</div></div> - -<p>The observing traveler who is privileged to make the journey -by steamer, threading its course in and out among the many islands -and skerries of the Norwegian coast, will hardly fail to be -struck by the remarkable profiles of most of the lower islands -(<a href="#f245">Fig. 245</a>). These profiles are generally convexly scalloped with a -noteworthy regularity, and not in one unit only, but in at least two -with one a multiple of the other (<a href="#f246">Fig. 246</a>). As the steamer passes -near to the islands, it is discovered that the smaller recognizable -units in the island profiles are separated by widely gaping joints -which do not, however, belong to the unit series, but to a larger -composite group (<a href="#f246">Fig. 246 <i>b</i></a>). Frostwork, which depends for its -action upon open spaces within the rocks, has here been the cause -of the excessive weathering above the more widely gaping joints.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 10.</span></p> - -<div class="figcenter"> - <img src="images/ill-284a.jpg" width="400" height="285" id="p10a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> View in Spitzbergen to illustrate the disintegration of rock under the control of -joints. -(<i>Photograph by O. Haldin.</i>)</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-284b.jpg" width="400" height="311" id="p10b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Composite pattern of the joint structures within recent alluvial deposits. -(<i>Photograph by Ellsworth Huntington.</i>)</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_229" id="Page_229">[229]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-286a.jpg" width="400" height="287" id="f245" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 245.</span>—Island groups of the Lofoten archipelago off the northwest coast of -Norway, which reveal repeating patterns of the relief in two orders of magnitude -(after a photograph by Knudsen).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-286b.jpg" width="250" height="88" id="f246" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 246.</span>—Diagrams to illustrate the composite profiles -of the islands on the Norwegian coast. <i>a</i>, distant view; -<i>b</i>, near view, showing the individual joints and the more -widely gaping fractures beneath each sag in the profile.</p> -</div></div> - -<p>High northern latitudes are thus especially favorable for revealing -all the details in the architectural pattern of the lithosphere -shell, and we need not be surprised that when the modern -maps of the Norwegian coast are examined, still larger repeating -patterns than any -that may be seen -in the field are to -be made out. The -Norwegian coast -was long ago shown -to be a complexly -faulted region, and -these larger divisions -of the relief -pattern, instead of being explained as a consequence solely of -selective weathering, must be regarded as due largely to fault -displacements of the type represented in our model (<a href="#p4c">plate 4 C</a>). -Yet whether due to displacements or to the more numerous -joints, all belong to the same composite system of fractures -expressed in the relief.</p> - -<p><span class="pagenum"><a name="Page_230" id="Page_230">[230]</a></span></p> - -<p class="prr"><span class="smcap">Reading References for Chapter XVII</span></p> - -<p class="pex p1"><span class="smcap">William H. Hobbs.</span> The River System of Connecticut, Jour. Geol., -vol. 9, 1901, pp. 469-485, pl. 1; Lineaments of the Atlantic Border -Region, Bull. Geol. Soc. Am., vol. 15, 1904, pp. 483-506, pls. 45-47; -The Correlation of Fracture Systems and the Evidences for Planetary -Dislocations within the Earth’s Crust, Trans. Wis. Acad. Sci., -etc., vol. 15, 1905, pp. 15-29; Repeating Patterns in the Relief and -in the Structure of the Land, Bull. Geol. Soc. Am., vol. 22, 1911, pp. -123-176, pls. 7-13.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_231" id="Page_231">[231]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XVIII</h2> - -<p class="pch">THE FORMS CARVED AND MOLDED BY WAVES</p> - -<p><b>The motion of a water wave.</b>—The motions within a wave -upon the surface of a body of water may be thought of in two -different ways. First of all, there is the motion of each particle -of water within an orbit of its own; and there is, further, the forward -motion of propagation of the wave considered as a whole.</p> - -<div class="floatright"> - <img src="images/ill-288.jpg" width="250" height="208" id="f247" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 247.</span>—Diagram to show the nature of the -motions within a free water wave.</p> -</div></div> - -<p>The water particle in a wave has a continued motion round and -round its orbit like that of a horse circling a race course, only that -here the track is in a -vertical plane, directed -along the line of propagation -of the wave (<a href="#f247">Fig. 247</a>). -Each particle of -water, through its friction -upon neighboring -particles, is able to -transmit its motion both -along the surface and -downward into the water -below. The force which -starts the water in motion -and develops the -wave, is the friction of -wind blowing over the -water surface, and the size of the orbit of the water particle at -any point is proportional to the wind’s force and to the stretch of -water over which it has blown. The wind’s effect is, therefore, -cumulative—the wave is proportional to the wind’s effect upon -all water particles in its rear, added to the local wind friction.</p> - -<p>The size or <i>height</i> of the wave is measured by the diameter of the -orbit of motion of the surface particle, and this is the difference -in height between trough and crest. The distance from crest -to crest, or from trough to trough, is called the <i>wave length</i>. -Though the wave motion is transmitted downward into the water<span class="pagenum"><a name="Page_232" id="Page_232">[232]</a></span> -there is a continued loss of energy which is here not compensated -by added wind friction, and so the orbital motion grows smaller and -smaller, until at the depth of about a wave length it has completely -died out. This level of no motion is called the <i>wave base</i>. In -quiet weather the level of no motion is practically at the water’s -surface, and inasmuch as the geological work of waves is in large -part accomplished during the great storms, the term “wave base” -refers to the lowest level of wave motion at the time of the heaviest -storms. Upon the ocean the highest waves that have been -measured have an amplitude of about fifty feet and a wave -length of about six hundred feet.</p> - - -<p><b>Free waves and breakers.</b>—So long as the depth of the water -is below wave base, there is obviously no possibility of interference -with the wave through friction upon the bottom. Under -these conditions waves are described as <i>free waves</i>, and their forms -are symmetrical except in so far as their crests are pulled over -and more or less dissipated in the spray of the “white caps” at -the time of high winds.</p> - -<div class="figcenter"> - <img src="images/ill-289.jpg" width="400" height="149" id="f248" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 248.</span>—Diagram to illustrate the transformation of a free wave into a breaker -as it approaches the shore.</p> -</div></div> - -<p>As a wave approaches a shore, which generally has a gentle -outward sloping surface, there is interposed in the way of a free -forward movement the friction upon the bottom. This friction -begins when the depth of water is less than wave base, and its -effect is to hold back the wave at the bottom. Carried slowly -upward in the water by the friction of particle upon particle, -the effect of this holding back is a piling up of the water, which increases -the wave height as it diminishes the wave length, and also -interferes with wave symmetry (<a href="#f248">Fig. 248</a>). Moving forward -at the top under its inertia of motion and held back at the bottom -by constantly increasing friction, a strong turning motion or -couple is started about a horizontal axis, the immediate effect -of which is to steepen the forward slope of the wave, and this continues -until it overhangs, -and, falling, “breaks” into -surf. Such a breaking -wave is called a “comber” -or “breaker” (<a href="#p11b">plate 11 B</a>).</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 11.</span></p> - -<div class="figcenter"> - <img src="images/ill-290a.jpg" width="400" height="255" id="p11a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Ripple markings within an ancient sandstone (courtesy of U. S. Grant).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-290b.jpg" width="400" height="263" id="p11b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> A wave breaking as it approaches the shore. -(<i>Photograph by Fairbanks.</i>)</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_233" id="Page_233">[233]</a></span></p> - -<div class="floatright"> - <img src="images/ill-292a.jpg" width="250" height="175" id="f249" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 249.</span>—Notched rock cliff cut by waves and -the fallen blocks derived from the cliff through -undermining. Profile Rock at Farwell’s -Point near Madison, Wisconsin.</p> -</div></div> - -<p><b>Effect of the breaking -wave upon a steep rocky -shore—the notched cliff.</b>—If -the shore rises abruptly -from deeper water, the top -of the breaking wave is -hurled against the cliff with -the force of a battering ram. -During storms the water of -shore waves is charged with sand, and each sand particle is driven -like a stone cutter’s tool under the stroke of his hammer. The effect -is thus both to chip and to batter away the rock of the shore to -the height reached by the wave, undermining it and notching -the rock at its base (<a href="#f249">Fig. 249</a>). When the notch has been cut -in this manner to a sufficient depth, the overhanging rock falls -by its own weight in blocks which -are bounded by the ever present -joints, leaving the upper cliff face -essentially vertical.</p> - -<div class="floatleft"> - <img src="images/ill-292b.jpg" width="250" height="274" id="f250" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 250.</span>—A wave-cut chasm under -control by joints, coast of Maine (after -Tarr).</p> -</div></div> - -<p><b>Coves, sea arches, and stacks.</b>—It -is the headland which is -most exposed to the work of the -waves, since with change of wind -direction it is exposed upon more -than a single face. The study of -headlands which have been cut -by waves shows that the joints -within the rock play a large rôle -in the shaping of shore features. -The attack of the waves under -the direction of these planes of<span class="pagenum"><a name="Page_234" id="Page_234">[234]</a></span> -ready separation opens out indentations of the shore (<a href="#f250">Fig. 250</a>) or -forms <i>sea caves</i> which, as they extend to the top of the cliff by the -process of sapping, yield the <i>coves</i> which are so common a feature -upon our rock-bound shores -(<a href="#f259">Fig. 259</a>, <a href="#Page_238">p. 238</a>). With continuation -of this process, the caves -formed on opposite sides of the -headland may be united to form -a <i>sea arch</i> (<a href="#f251">Fig. 251</a>).</p> - -<div class="floatleft"> - <img src="images/ill-293a.jpg" width="200" height="186" id="f251" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 251.</span>—The sea arch known as the -Grand Arch upon one of the Apostle -Islands in Lake Superior (after a photograph -by the Detroit Photographic -Company).</p> -</div></div> - -<p>A later stage in this selective -wave carving under the control -of joints is reached when the -bridge above the arch has -fallen in, leaving a detached -rock island with precipitous -walls. Such an offshore island -of rock with precipitous sides -is known as a <i>stack</i> (<a href="#f252">Fig. 252</a>), -or sometimes as a -“chimney”, though this latter -term is best restricted to other and similar forms which are the -product of selective weathering (<a href="#Page_300">p. 300</a>).</p> - -<div class="floatright"> - <img src="images/ill-293b.jpg" width="200" height="156" id="f252" - alt="" - title="" /> - <div class="cf"><p class="pc200"><span class="smcap">Fig. 252.</span>—Stack near the shore of Lake -Superior.</p> -</div></div> - -<p>Whenever the rock is less firmly consolidated, and so does not -stand upon such steep planes, -the stack is apt to have a -more conical form, and may -not be preceded in its formation -by the development of -the sea arch (<a href="#f260">Fig. 260</a>, <a href="#Page_239">p. 239</a>). -In the reverse case, or where -the rock possesses an unusual -tenacity, the stack may be -largely undermined and stand -supported like a table upon -thick legs or pillars of rock -(<a href="#f253">Fig. 253</a>). In <a href="#f254">Fig. 254</a> is -seen a group of stacks upon the coast of California, which show -with clearness the control of the joints in their formation, but -unlike the marble of the South American example the forms<span class="pagenum"><a name="Page_235" id="Page_235">[235]</a></span> -are not rounded, but retain -their sharp angles.</p> - -<div class="floatright"> - <img src="images/ill-294a.jpg" width="250" height="167" id="f253" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 253.</span>—The Marble Islands, stacks in -Lake Buenos Aires, southern Andes -(after F. P. Moreno).</p> -</div></div> - -<p><b>The cut rock terrace.</b>—When -waves first begin their -attack upon a steep, rocky -shore, the lower limit of the -action is near the wave base. -The action at this depth is, -however, less efficient, and as -the recession of the cliff is one -of the most rapid of erosional -processes, the rock floor outside the receding cliff comes to slope -gradually downward from the cliff to a maximum depth at the -edge of the terrace, approximately equal to wave base (<a href="#f255">Fig. 255</a>). -This cut terrace is extended seaward or lakeward, as the case may -be, in a <i>built terrace</i> constructed from a portion of the rock débris -acquired from the cliff.</p> - -<div class="figcenter"> - <img src="images/ill-294b.jpg" width="400" height="314" id="f254" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 254.</span>—Squared stacks which reveal the position of the joint planes which have -controlled in the process of carving by the waves. Pt. Buchon, California -(after a photograph by Fairbanks).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_236" id="Page_236">[236]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-295a.jpg" width="250" height="151" id="f255" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 255.</span>—Ideal section of a steep rocky -shore carved by waves into a notched cliff -and cut terrace, and extended by a built -terrace.</p> -</div></div> - -<p>The broken wave, after rising upon the terrace under the inertia -of its motion until all its energy has been dissipated, slides outward -by gravity, and though -checked and overridden by -succeeding breakers, it continues -its outward slide as -the “undertow” until it -reaches the end of the terrace. -Here it suddenly enters -deep water, and losing -its velocity, drops its burden -of rock, and builds the terrace -seaward after the manner -of construction of an -embankment. As we are to see, the larger portion of the wave-quarried -material is diverted to a different quarter.</p> - -<div class="floatright"> - <img src="images/ill-295b.jpg" width="250" height="201" id="f256" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 256.</span>—Map showing the outlines of the Island of -Heligoland at different stages in its recent history. The -peripheries given are in miles.</p> -</div></div> - -<p>To gain some conception of the importance of wave cutting -as an eroding process, we may consider the late history of Heligoland, -a sandstone island off the mouth of the Elbe in the North -Sea (<a href="#f256">Fig. 256</a>). From a periphery of 120 miles, which it possessed -in the ninth century -of the Christian -era, the -island has reduced -its outline to 45 -miles in the fourteenth -century, 8 -miles in the seventeenth, -and to -about 3 miles at -the beginning of -the twentieth century. -The German -government, which -recently acquired -this little remnant -from England, has -expended large -sums of money in an effort to save this last relic.</p> - -<p><span class="pagenum"><a name="Page_237" id="Page_237">[237]</a></span></p> - -<div class="floatright"> - <img src="images/ill-296a.jpg" width="250" height="96" id="f257" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 257.</span>—Cut and built terrace with bowlder pavement -shaped by waves on a steep shore formed of -loose materials.</p> -</div></div> - -<p><b>The cut and built terrace on a steep shore of loose materials.</b>—In -materials which lack the coherence of firm rock, no vertical -cliff can form; for as fast as undermined by the waves the loose -materials slide down -and assume a surface -of practically constant -slope—the “angle of -repose” of the materials -(<a href="#f257">Fig. 257</a>). The -terrace below this -sloping cliff will not -differ in shape from -that cut upon a rocky shore; but whenever the materials of the -shore include disseminated blocks too large for the waves to handle, -they collect upon the terrace near where they have been exhumed, -thus forming what has been called a “bowlder pavement” (<a href="#f258">Fig. 258</a>).</p> - -<div class="floatleft"> - <img src="images/ill-296b.jpg" width="250" height="235" id="f258" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 258.</span>—Sloping cliff and terrace with -bowlder pavement exposed at low tide -upon the shore at Scituate, Massachusetts.</p> -</div></div> - -<p>The edge of the cut and built terrace is, as already mentioned, -maintained at the depth of wave base. If one will study the submerged -contours of any of our -inland lakes, it will be found -that these basins are surrounded -by a gently sloping -marginal shelf,—the cut and -built terrace,—and that the -depth of this shelf at its outer -edge is proportioned to the -size of the lake. Upon Lake -Mendota at Madison, Wisconsin, -the large storm waves have -a length of about twenty feet, -which is the depth of the outer -edge of the shore terraces (<a href="#f267">Fig. 267</a>, <a href="#Page_242">p. 242</a>). The shelf surrounding -the continents has, -with few local exceptions, a uniform depth of 100 fathoms, or about -the wave base of the heaviest storm waves.</p> - - -<p><b>The work of the shore current.</b>—In describing the formation -of the built terrace, it was stated that the greater part of the rock<span class="pagenum"><a name="Page_238" id="Page_238">[238]</a></span> -material quarried upon headlands by the waves is diverted from -the offshore terrace. This diversion is the work of the shore current -produced by the wave.</p> - -<div class="figcenter"> - <img src="images/ill-297.jpg" width="400" height="288" id="f259" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 259.</span>—Map to show the nature of the shore current and the forms which are -molded by it.</p> -</div></div> - -<p>At but few places upon a shore will the storm waves beat perpendicularly, -and then for but short periods only. The broken -wave, as it crawls ever more slowly up the beach, carries the sand -with it in a sweeping curve, and by the time gravity has put a stop -to its forward movement, it is directed for a brief instant parallel -to the shore. Soon, however, the pull of gravity upon it has started -the backward journey in a more direct course down the slope of -the terrace; and here encountering the next succeeding breaker, -a portion of the water and the coarser sand particles with it are -again carried forward for a repetition of the zigzag journey. This -many times interrupted movement of the sand particles may be -observed during a high wind upon any sandy lee shore. The “set” -of the water along the shore as a result of its zigzag journeyings -is described as the <i>shore current</i> (<a href="#f259">Fig. 259</a>), and the effect upon -sand distribution is the same as though a steady current moved -parallel to the shore in the direction of the average trend of the -moving particles.</p> - -<p><b>The sand beach.</b>—The first effect of the shore current is to -deposit some portion of the sand within the first slight recess upon -the shore in the lee of the cliff. The earlier deposits near the cliff<span class="pagenum"><a name="Page_239" id="Page_239">[239]</a></span> -gradually force the shore current farther from the shore and -so lay down a sand selvage to the shore, which is shaped in the -form of an arc or crescent and known as a <i>beach</i> (<a href="#f259">Fig. 259</a> and -<a href="#f260">Fig. 260</a>).</p> - -<div class="figcenter"> - <img src="images/ill-298a.jpg" width="400" height="332" id="f260" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 260.</span>—Crescent-shaped beach formed in the lee of a headland. Santa -Catalina Island, California (after a photograph by Fairbanks).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-298b.jpg" width="150" height="48" id="f261" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 261.</span>—Cross section -of a beach pebble.</p> -</div></div> - -<p><b>The shingle beach.</b>—With heavy storms and an exceptional -reach of the waves, the shore currents are competent to move, not -the sand alone, but pebbles, the area of whose broader surface may -be as great as the palm of one’s hand. Such rock fragments are -shaped by the continued wear against their neighbors under the -restless breakers, until they have a lenticular -or watch-shaped form (<a href="#f261">Fig. 261</a>). -Such beach pebbles are described as <i>shingle</i>, -and they are usually built up into distinct -ridges upon the shore, which, under the -fury of the high breakers, may be piled several feet above the level -of quiet water (<a href="#f262">Fig. 262</a>). Such storm beaches have a gentle<span class="pagenum"><a name="Page_240" id="Page_240">[240]</a></span> -forward slope graded by the shore current, but a steep backward -slope on the angle of repose. Most storm beaches have -been largely shaped by the last great storm, such as comes only -at intervals of a number of years.</p> - -<div class="floatleft"> - <img src="images/ill-299a.jpg" width="230" height="148" id="f262" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 262.</span>—Storm beach of coarse -shingle about four feet in height at -the base of Burnt Bluff on the northeast -shore of Green Bay, Lake -Michigan.</p> -</div></div> - -<p><b>Bar, spit, and barrier.</b>—Wherever -the shore upon which -a beach is building makes a -sudden landward turn at the entrance -to a bay, the shore currents, -by virtue of their inertia -of motion, are unable longer to -follow the shore. The débris -which they carry is thus transported -into deeper water in a -direction corresponding to a continuation -of the shore just before -the point of turning (see <a href="#f259">Fig. 259</a>, <a href="#Page_238">p. 238</a>). The result is the -formation of a <i>bar</i>, which rises to near the water surface and is extended -across the entrance to the bay through continued growth -at its end, after the manner of constructing a railway embankment -across a valley.</p> - -<div class="floatright"> - <img src="images/ill-299b.jpg" width="250" height="165" id="f263" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 263.</span>—Spit of shingle on Au Train Island, -Lake Superior (after Gilbert).</p> -</div></div> - -<p>Over the deeper water near the bar the waves are at first not -generally halted and broken, as they are upon the shore, and so -the bar does not at once -build itself to the surface, -but remains an invisible -bar to navigation. From -its shoreward end, however, -the waves of even -moderate storms are -broken, and the bar is -there built above the water -surface, where it appears -as a narrow cape of sand -or shingle which gradually -thins in approaching its -apex. This feature is the well-known <i>spit</i> (<a href="#f263">Fig. 263</a>) which, as it -grows across the entrance to the bay, becomes a <i>barrier</i> or <i>barrier -beach</i> (<a href="#f264">Fig. 264</a>).</p> - -<p><span class="pagenum"><a name="Page_241" id="Page_241">[241]</a></span></p> - -<p>The continuation of the visible in the usually invisible bar, is -at the time of high winds made strikingly apparent, for the wave -base is below the crest of the bar, and at such times its crescentic -course beyond the spit can be followed by the eye in a white arc -of broken water.</p> - -<div class="floatright"> - <img src="images/ill-300.jpg" width="250" height="207" id="f264" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 264.</span>—Barrier beach in front of a lagoon -on Lake Mendota at Madison, Wisconsin. -The shallow lagoon behind the barrier is -filling up and is largely hidden in vegetation.</p> -</div></div> - -<p>The construction of a barrier across the entrance to a bay transforms -the latter into a separate body of water, a lagoon, within -which silting up and peat -formation usually lead to an -early extinction (<a href="#Page_429">p. 429</a>). The -formation of barriers thus -tends to straighten out the -irregularities of coast lines, -and opens the way to a -natural enlargement of the -land areas. While the coasts -of the United Kingdom of -Great Britain have been -losing some four thousand -acres through wave erosion, -there has been a gain by -growth in quiet lagoons which -amounts to nearly seven -times that amount. As evidence of the straightening of the shore -line which results from this process, the coast of the Carolinas or -of Nantucket (<a href="#f459">Fig. 459</a>) may serve for illustration.</p> - -<p><b>The land-tied island.</b>—We have seen that wave erosion operates -to separate small islands from the headlands, but the shore -currents counteract this loss to the continents by throwing out -barriers which join many separated islands to the mainland. Such -land-tied islands are a common feature on many rocky coasts, -and upon the New England coast they usually have been given the -name of “neck.” The long arc of Lynn Beach joins the former -island of Nahant, through its smaller neighbor Little Nahant, -to the coast of Massachusetts. A similar land-tied island is -Marblehead Neck. The Rock of Gibraltar, formerly an island, -is now joined to Spain by the low beach known as the “neutral -ground.” The Spanish name, <i>tombola</i>, has sometimes been employed -to describe an island thus connected to the shore.</p> - -<p><span class="pagenum"><a name="Page_242" id="Page_242">[242]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-301a.jpg" width="250" height="79" id="f265" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 265.</span>—Cross section of a barrier beach -with lagoon in its rear.</p> -</div></div> - -<p><b>A barrier series.</b>—The cross section of a barrier beach, like -that of a storm beach upon the shore, slopes gently upon the forward -side, and more steeply -at the angle, of repose upon -the rear or landward margin -(<a href="#f265">Fig. 265</a>). The thinning -wedge of shore deposits which -the barrier throws out to seaward -raises the level of the -lake bottom (<a href="#f266">Fig. 266</a>), and when coast irregularities are favorable -to it, new spits will develop upon the shore outside the -earlier one, and a new bar, and in its turn a barrier, will be found -outside the initial one, taking a course in a direction more or less -parallel to it (<a href="#f267">Fig. 267</a>).</p> - -<div class="figcenter"> - <img src="images/ill-301b.jpg" width="400" height="110" id="f266" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 266.</span>—Cross section of a series of barriers and an outer bar.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-301c.jpg" width="400" height="287" id="f267" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 267.</span>—Formation of barrier series and an outer bar in University Bay of -Lake Mendota, at Madison, Wisconsin. The water contour interval is five feet, -and the land contour interval ten feet (based on a map by the Wisconsin Geological -Survey).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_243" id="Page_243">[243]</a></span></p> - -<div class="floatright"> - <img src="images/ill-302a.jpg" width="200" height="179" id="f268" - alt="" - title="" /> - <div class="cf"><p class="pc200"><span class="smcap">Fig. 268.</span>—Series of barriers at the western end -of Lake Superior (after Gilbert).</p> -</div></div> - -<p>So soon as the first barrier is formed, processes are set in operation -which tend to transform -the newly formed lagoon -into land, and so with -a series of barriers, a zone -of water lilies between the -outer barrier and the bar, -a bog, and a land platform -may represent the successive -stages in this acquisition -of territory by the -lands. A noteworthy example -of barrier series -and extension of the land -behind them, is afforded by -the bay at the western end -of Lake Superior (<a href="#f268">Fig. 268</a>).</p> - -<div class="floatleft"> - <img src="images/ill-302b.jpg" width="200" height="150" id="f269" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 269.</span>—Character profiles resulting from wave -action upon shores.</p> -</div></div> - -<p><b>Character profiles.</b>—The character profiles yielded by the -work of waves are easy of recognition (<a href="#f269">Fig. 269</a>). The vertical -cliff with notch at its -base is varied by the -stack of sugar-loaf -form carved in softer -rocks, or the steeper -notched variety cut -from harder masses. -Sea caves and sea -arches yield variations -of a curve common -to the undercut -forms. Wherever the -materials of the shore -are loosely consolidated -only, the sloping -cliff is formed at the angle of repose of the materials. The -barrier beach, though projecting but a short distance above the -waves, shows an unsymmetrical curve of cross section with the -steeper slope toward the land.</p> - -<p><span class="pagenum"><a name="Page_244" id="Page_244">[244]</a></span></p> - -<p class="prr"><span class="smcap">Reading References for Chapter XVIII</span></p> - -<p class="pex p1"><span class="smcap">G. K. Gilbert.</span> The Topographic Features of Lake Shores, 5th Ann. -Rept. U. S. Geol. Surv., 1885, pp. 69-123, pls. 3-20; Lake Bonneville, -Mon. I, U. S. Geol. Surv., 1890, Chapters ii-iv, pp. 23-187.</p> - -<p class="pex"><span class="smcap">Vaughan Cornish.</span> On Sea Beaches and Sand Banks, Geogr. Jour., vol. -11, 1898, pp. 528-543, 628-658.</p> - -<p class="pex"><span class="smcap">F. P. Gulliver.</span> Shore Line Topography, Proc. Am. Acad. Arts and -Sci., vol. 34, 1899, pp. 149-258.</p> - -<p class="pex"><span class="smcap">N. S. Shaler.</span> The Geological History of Harbors, 13th Ann. Rept. U. S. -Geol. Surv., 1893, pp. 93-209.</p> - -<p class="pex"><span class="smcap">Sir A. Geikie.</span> The Scenery of Scotland, 1901, pp. 46-89.</p> - -<p><span class="smcap">W. H. Wheeler.</span> The Sea Coast. Longmans, London, 1902, pp. 1-78.</p> - -<p class="pex"><span class="smcap">G. W. von Zahn.</span> Die zerstörende Arbeit des Meeres an Steilküsten nach -Beobachtungen in der Bretagne und Normandie in den Jahren 1907 -und 1908, Mitt. d. Geogr. Ges. Hamb., vol. 24, 1910, pp. 193-284, -pls. 12-27.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_245" id="Page_245">[245]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XIX</h2> - -<p class="pch">COAST RECORDS OF THE RISE OR FALL OF THE -LAND</p> - -<p><b>The characters in which the record has been preserved.</b>—The -peculiar forms into which the sea has etched and molded its -shores have been considered in the last chapter. Of these the -more significant are the notched rock cliff, the cut rock terrace, -the sea cave, the sea arch, the stack, and the sloping cliff and terrace, -among the carved features; and the barrier beach and built -terrace, among the molded forms. It is important to remember -that the molded forms, by the very manner of their formation, -stand in a definite relationship to the carved features; so that -when either one has been in part effaced and made difficult of determination, -the discovery of the other in its correct natural position -may remove all doubt as to the origin of the relic.</p> - -<div class="floatleft"> - <img src="images/ill-305a.jpg" width="150" height="370" id="f270" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 270.</span>—The east -coast of Florida, with -shore line characteristic -of a raised -coast.</p> -</div></div> - -<p>In studies of the change of level of the land, it is customary to -refer all variations to the sea level as a zero plane of reference. -It is not on this account necessary to assume that the changes -measured from this arbitrary datum plane are the absolute upward -or downward oscillations which would be measured from the -earth’s center; for the sea, like the land, has been subject to its -changes of level. There need, however, be no apology for the -use of the sea surface as a plane of reference; for it is all that we -have available for the purpose, and the changes in level, even if -relative only, are of the greatest significance. It is probable that -in most cases where the coast line is rising from uplift, some portion -of the sea basin not far distant is becoming deepened, so that -the visible change of level is the algebraic sum of the two effects.</p> - -<p><b>Even coast line the mark of uplift.</b>—It was early pointed out -in this volume (<a href="#Page_158">p. 158</a>) that the floor of the sea in the neighborhood -of the land presents a relatively even surface. The carving by -waves, combined with the process of deposition of sediments, tends -to fill up the minor irregularities of surface and preserve only the<span class="pagenum"><a name="Page_246" id="Page_246">[246]</a></span> -features of larger scale, and these in much softened outlines. Upon -the continents, on the contrary, the running water, taking advantage -of every slight difference in elevation and -searching out the hidden structure planes -within the rock, soon etches out a surface of the -most intricate detail. The effect of elevation -of the sea floor into the light of day will therefore -be to produce an even shore line devoid of -harbors (<a href="#f270">Fig. 270</a>). If the coast has risen -along visible planes of faulting near the sea -margin, the coast line, in addition to being even, -will usually be made up of notably straight elements -joined to one another.</p> - -<div class="floatright"> - <img src="images/ill-305b.jpg" width="200" height="216" id="f271" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 271.</span>—Ragged coast line -of Alaska, the effect of subsidence.</p> -</div></div> - -<p><b>A ragged coast line the mark of subsidence.</b>—When -in place of uplift a subsidence -occurs upon the coast, -the intricately etched -surface, resulting from -erosion beneath the -sky, comes to be invaded -by the sea -along each trench and -furrow, so that a most -ragged outline is the result (<a href="#f271">Fig. 271</a>). -Such a coast -has many -harbors, -while the -uplifted coast is as remarkable for its -lack of them.</p> - -<p><b>Slow uplift of the coast—the -coastal plain and cuesta.</b>—A gradual -uplift of the coast is made apparent -in a progressive retirement of the sea -across a portion of its floor, thus exposing -this even surface of recent -sediments. The former shore land -will be easily recognized by it’s etched -surface, which will now come into<span class="pagenum"><a name="Page_247" id="Page_247">[247]</a></span> -sharp contrast with the new plain. It is therefore referred to as -the <i>oldland</i> and the newly exposed <i>coastal plain</i> as the <i>newland</i> -(<a href="#f272">Fig. 272</a>).</p> - -<div class="floatleft"> - <img src="images/ill-305c.jpg" width="200" height="208" id="f272" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 272.</span>—Portion of Atlantic -coastal plain and neighboring oldland -of the Appalachian Mountains.</p> -</div></div> - -<p>But the near-shore deposits upon the sea floor had an initial -dip or slope to seaward, and this inclination has been increased -in the process of uplift. The streams from the oldland have -trenched their way across these deposits while the shore was rising. -But the process being a slow one, deposits have formed -upon the seaward side of the plain after the landward portion was -above tide, and the coastal plain may come to have a “belted” -or zoned character. The streams tributary to those larger ones -which have trenched the plain may encounter in turn harder and -softer layers of the plain deposits, and at each hard layer will be -deflected along its margin so as to -enter the main streams more nearly -at right angles. They will also, as -time goes on, migrate laterally seaward -through undermining of the -harder layers, and thus will be -shaped alternating belts of lowland -separated by escarpments in the -harder rock from the residual higher slopes. Belts of upland of -this character upon a coastal plain are called <i>cuestas</i> (<a href="#f273">Fig. 273</a>).</p> - -<div class="floatright"> - <img src="images/ill-306.jpg" width="200" height="74" id="f273" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 273.</span>—Ideal form of cuestas -and intermediate lowlands carved -from a coastal plain (after Davis).</p> -</div></div> - -<p><b>The sudden uplifts of the coasts.</b>—Elevations of the coast -which yield the coastal plain must be accounted among the -slower earth movements that result in changes of level. Such -movements, instead of being accompanied by disastrous earthquakes, -were probably marked by frequent slight shocks only, -by subterranean rumblings, or, it may be, the land rose gradually -without manifestations of a sensible character.</p> - -<p>Upon those coasts which are often in the throes of seismic disturbance, -a quite different effect is to be observed. Here within -the rocks we will probably find the marks of recent faulting with -large displacements, and the movements have been upon such a -scale that shore features, little modified by subsequent weathering, -stand well above the present level of the seas. Above such coasts, -then, we recognize the characteristic marks of wave action, and -the evidence that they have been suddenly uplifted is beyond -question.</p> - -<p><span class="pagenum"><a name="Page_248" id="Page_248">[248]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-307a.jpg" width="400" height="307" id="f274" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 274.</span>—Uplifted sea cave, ten feet above the water upon the coast of California; -the monument to a former earthquake (after a photograph by Fairbanks).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-307b.jpg" width="400" height="471" id="f275" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 275.</span>—Double-notched cliff near Cape Tiro, Celebes (after a photograph by -Sarasin).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_249" id="Page_249">[249]</a></span></p> - -<p><b>The upraised cliff.</b>—Upon the coast of southern California -may be found all the features of wave-cut shores now in perfect -preservation, and in some cases as much as fifteen hundred feet -above the level of the sea. These features are monuments to the -grandest of earthquake disturbances which in recent time have -visited the region (<a href="#f274">Fig. 274</a>). Quite as striking an example of -similar movements is afforded by notched cliffs in hard limestone -upon the shore of the Island of Celebes (<a href="#f275">Fig. 275</a>). But the coast -of California furnishes the other characteristic coast features in the -high sea arch and the stack as additional monuments to the recent -uplift. Let one but imagine the stacks which now form the Seal -Rocks off the Cliff House at San Francisco to be suddenly raised -high above the sea, and the forms which they would then present -would differ but little from those which are shown in <a href="#f276">Fig. 276</a>.</p> - -<div class="figcenter"> - <img src="images/ill-308.jpg" width="400" height="266" id="f276" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 276.</span>—Jasper rock stacks uplifted on the coast of California (after a photograph -by Fairbanks).</p> -</div></div> - -<p><b>The uplifted barrier beach.</b>—Within the reëntrants of the -shore, the wave-cut cliff is, as we know, replaced by the barrier -beach, which takes its course across the entrance to a bay. After -an uplift, such a barrier composed of sand or shingle should be -connected with the headlands, often with a partially filled lagoon -behind it. Its cross section should be steep in the direction of -the lagoon, but quite gradual in front (<a href="#f277">Fig. 277</a>).</p> - -<p><span class="pagenum"><a name="Page_250" id="Page_250">[250]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-309a.jpg" width="400" height="219" id="f277" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 277.</span>—Uplifted shingle beach across the entrance to a former bay upon -the coast of southern California (after a photograph by Fairbanks).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-309b.jpg" width="250" height="94" id="f278" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 278.</span>—Raised beach terraces near Elie, -Fife, Scotland.</p> -</div></div> - -<p><b>Coast terraces.</b>—Upon those shores where to-day high mountains -front the sea, the coast may generally be seen to rise in a series -of terraces (<a href="#f278">Fig. 278</a>). This -is notably true of those coasts -which are to-day racked by -earthquakes, such as is the -eastern margin of the Pacific -from Alaska to Patagonia. -The traveler by steamer along -the coast from San Francisco -to Chili has for weeks almost constantly in sight these giant steps -on which the mountains have been uplifted from the sea. In -Alaska we are fortunate in having the history of the later stages in -this uplift (<a href="#f279">Fig. 279</a>). As described in a former chapter, portions -of this shore rose in the month of September of the year 1899 in<span class="pagenum"><a name="Page_251" id="Page_251">[251]</a></span> -some places as high as forty-seven feet, to the accompaniment of -a terrific earthquake and sea wave. Above the terrace which -marks the beach line of 1899 there is a higher terrace of similar -form now overgrown with trees, but none the less clearly to be recognized -as a shore line of the past century -which preceded in the long sequence the -uplift of 1899.</p> - -<div class="figcenter"> - <img src="images/ill-309c.jpg" width="400" height="186" id="f279" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 279.</span>—Uplifted sea cliffs and terraces on the coast -of Russell Fjord, Alaska (after Tarr and Martin).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-310a.jpg" width="400" height="168" id="f280" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 280.</span>—Diagrams to show how excessive sinking -upon the sea floor will cause the shore to migrate landward as it is -uplifted.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-310b.jpg" width="150" height="344" id="f281" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 281.</span>—A drowned river -mouth, or estuary upon a -coastal plain.</p> -</div></div> - -<p>As was noted in our study of earthquakes, -the recent instrumental records of -distant earthquakes tell us that the movements -upon the sea floor are many times -larger than those upon the continents, and -that while the mountainous coasts are generally -rising, the deeps of the sea are sinking. -The effect of this over-balance of -sinking, or resultant shrinking of the earth’s -shell, may be to compress the mountain -district and so cause the shore line to move -landward at the same time that it moves -upward (<a href="#f280">Fig. 280</a>).</p> - - -<p><b>The sunk or embayed coast.</b>—When -now, upon the other hand, a section of the -coast line sinks with reference to the sea, -the water invades all the near-shore valleys, -thus “drowning” them and yielding -the drowned river mouth or <i>estuary</i>. -If the relief of the shore was slight, as it generally is upon a -coastal plain, slight depression only will produce broad estuaries,<span class="pagenum"><a name="Page_252" id="Page_252">[252]</a></span> -such as Chesapeake Bay at the -drowned mouth of the Susquehanna -(<a href="#f281">Fig. 281</a>).</p> - -<p>If, on the other hand, the relief -of the shore is strong and the subsidence -is large, the entire coast -line will be transformed into an -archipelago of steep-walled rocky -islets which rise abruptly from the -sea (<a href="#f282">Figs. 282</a> and <a href="#f284">284</a>). A plateau -which is intersected by deep and -steep-walled valleys of <span class="font">U</span>-section -(<a href="#Page_341">p. 341</a>) under large submergence -yields the <i>fjords</i> so characteristic -of Scandinavia or Alaska. A ragged -coast line, fringed with islands -as a result of submergence, is described -as an <i>embayed coast</i>.</p> - -<div class="figcenter"> - <img src="images/ill-311a.jpg" width="400" height="265" id="f282" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 282.</span>—Archipelago of steep rocky islets due to large submergence of a coast -having strong relief. Entrance to Esquimalt Harbor, Vancouver Island (after -a photograph by Fairbanks).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-311b.jpg" width="200" height="340" id="f283" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 283.</span>—The submerged Hudsonian -channel which continues the -Hudson River across the continental -shelf.</p> -</div></div> - -<p><b>Submerged river channels.</b>—The -sinking of a coast of small -relief be sufficient to completely -submerge river valleys, -whose channels then begin to fill<span class="pagenum"><a name="Page_253" id="Page_253">[253]</a></span> -with sediment and whose courses can only be followed in soundings. -One of the most interesting of such channels is that which -continues the Hudson River across the continental shelf into the -deeper sea (<a href="#f283">Fig. 283</a>).</p> - - -<p><b>Records of an oscillation of movement.</b>—Because a coast -is deeply embayed is no ground for assuming that a subsidence -is now in progress, -or is, in -fact, the latest -movement recorded -upon the -coast. In many -cases it is easy -to see that such -is not the case. -The coast of -Maine is perhaps -as typical -of an embayed -shore line as -any that might -be cited, but a -study of the -river valleys in -the neighborhood -shows clearly that the present submergence of their mouths -is a fraction only of an earlier one which has left a record of its -existence in beds of marine clay which outline the earlier and far -deeper indentations (<a href="#f284">Fig. 284</a>).</p> - -<div class="floatright"> - <img src="images/ill-312.jpg" width="250" height="211" id="f284" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 284.</span>—Marine clay deposits near the mouths of the rivers -of Maine which preserve a record of earlier subsidence (after -Stone).</p> -</div></div> - -<p>If now we give a closer examination to the coast, it is found -that there are marks of recent uplift in an abandoned shore line -now far above the reach of the waves. There is here, then, the -record, first of subsidence and consequent embayment, and, later, -of an uplift which has reduced the raggedness of the coast outline -exposed the clay deposits, and raised the strands of the period of -deep subsidence to their present position.</p> - -<p>In countries which possess a more ancient civilization than our -own, the record of such oscillations in the level of the ground has -sometimes been entered upon human monuments, so that it is<span class="pagenum"><a name="Page_254" id="Page_254">[254]</a></span> -possible to date more or less definitely the periods of subsidence -or elevation. At the little town of Pozzuoli, upon the shore of -the Bay of Naples, is found one of the mos instructive of these -records.</p> - -<div class="floatleft"> - <img src="images/ill-313.jpg" width="230" height="177" id="f285" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 285.</span>—View of the three standing -columns of the temple of Jupiter Serapis -at Pozzuoli, showing the dark -and rough band nine feet in width -affected by the rock-boring mollusks -which now live in the Bay of Naples.</p> -</div></div> - -<p>In the ruins of the ancient temple of Jupiter Serapis are three -marble monoliths 40 feet in height, curiously marked by a -roughened surface between the -heights of 12 and 21 feet above -their pedestals (<a href="#f285">Fig. 285</a>). Closer -inspection shows that this roughened -surface has been produced -by a marine, rock-boring mollusk, -the <i>lithodomus</i>, which lives in the -waters of the Bay of Naples, and -the shells of this animal are still -to be found within the cavities -upon the surface of the columns. -Without recounting details which -have been many times recited -since these interesting monuments -were first geologically explored -by Babbage and Lyell, it may be stated that a record is -here preserved, first of subsidence amounting to some 40 feet, and -of subsequent elevation, of the low coast land on which stood the -temple in the old Roman city of Puteoli (<a href="#f286">Fig. 286</a>).</p> - -<p>At the time of deepest submergence the top of the lithodomus -zone upon the column stood at the level of the water in the Bay of -Naples, the smoother lower zone being buried at the time in the -sand at the bottom, and thus made inaccessible for the lithodomi. -It is to be added that studies made in the environs of Pozzuoli -have fully confirmed the changes of level revealed by the columns, -through the discovery of now elevated shore lines which are referable -to the period of deep submergence.</p> - - -<p><b>Simultaneous contrary movements upon a coast.</b>—In our -study of the changes in the level of the ground that take place -during earthquakes, it was learned that neighboring sections of -the earth’s crust may be moved at different rates or even in opposite -directions, notwithstanding the fact that the general movement -of the province is one of uplift. Thus during the Alaskan -earthquake of 1899, although portions of the coast line were elevated -by as much as forty-seven feet, neighboring sections were raised by -smaller amounts, and some small sections were sunk and so far -submerged that the salt water and the beach sand were washed -about the roots of forest trees.</p> - -<p><span class="pagenum"><a name="Page_255" id="Page_255">[255]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-314.jpg" width="350" height="652" id="f286" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 286.</span>—Pozzuoli in the 3rd, 9th, and 20th Centuries.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_256" id="Page_256">[256]</a></span></p> - -<p>A region racked by heavy earthquakes, where the present configuration -of the ground speaks strongly for a movement of somewhat -similar nature, but with average movement of elevation much -greater to the northward than in the opposite direction, is the extended -coast line of Chili. This country is characterized by a -great central north and south valley which separates the coast -range from the high chain of the Cordilleras to the eastward. To -the southward the floor of this valley descends, and has its continuance -in the Gulf of Corcovado behind the island of Chiloe and -the Chonos archipelago. The known recent uplift of the coast of -Chili, particularly in the northern sections and during the earthquakes -of the eighteenth, nineteenth, and twentieth centuries, -lends great interest to this topographic peculiarity. Indications -are not lacking that, during the earthquake of Concepcion in -1835, and of Valparaiso in 1907, the measure of uplift was greater -to the north than it was to the south.</p> - -<div class="figcenter"> - <img src="images/ill-315.jpg" width="400" height="143" id="f287" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 287.</span>—Map of San Clemente Island, California, showing the characteristic -topography of recent uplift (after U. S. Coast and Geodetic Survey).</p> -</div></div> - -<p><b>The contrasted islands of San Clemente and Santa Catalina.</b>—Perhaps -the most striking example of simultaneous opposite movements -observable in neighboring portions of the earth’s crust -is furnished by the coast of southern California. The coast itself -at San Pedro and the island of San Clemente, some fifty miles off -this point, in common with most portions of the neighboring coast -land, have been rising in interrupted movements from the sea, and -offer in rare perfection the characteristic coast terraces (<a href="#f287">Fig. 287</a> -and <a href="#f278">Fig. 278</a>, <a href="#Page_250">p. 250</a>). Midway between these two rising sections -of the crust, and less than twenty-five miles distant from either, is -the island of Santa Catalina, which has been sinking beneath the -waves, and apparently at a similarly rapid rate (<a href="#f288">Fig. 288</a>). The -topography of the island shows the intricate detail of a maturely -eroded surface, while that of the neighboring San Clemente shows -only the widely spaced, deep cañons of the infantile stage of erosion -(<a href="#f165">Fig. 165</a> and <a href="#p12a">pl. 12 A</a>). While Santa Catalina has been sinking, -San Pedro Hill has risen 1240 feet, and San Clemente, 1500 feet. -It is characteristic of a sinking coast line that the cliff recession is -abnormally rapid, and evidence for this is furnished by the shores -of Santa Catalina, upon which the waves are cutting the cliffs -back into the beds of cañons, and so causing small falls to develop -at the cañon mouths.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 12.</span></p> - -<div class="figcenter"> - <img src="images/ill-316a.jpg" width="400" height="253" id="p12a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> <span class="font">V</span>-shaped cañon cut in an upland recently elevated from the sea, San Clemente -Island, California (after W. S. Tangier-Smith).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-316b.jpg" width="400" height="286" id="p12b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> A “hogback” at the base of the Bighorn Mountains, Wyoming (after Darton).</p> -</div></div> - -</div> - - -<p><span class="pagenum"><a name="Page_257" id="Page_257">[257]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-318.jpg" width="400" height="174" id="f288" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 288.</span>—Map of Santa Catalina Island, California, showing the -characteristic surface of an area which has long been above the waves, -and the entire absence of coast terraces (after U. S. C. and G. S.).</p> -</div></div> - -<p><b>The Blue Grotto of Capri.</b>—We may now return to the Bay -of Naples for additional evidence that oscillations of level in -neighboring portions of the same coast are not necessarily synchronous, -and that near-lying sections may even move in opposite -directions at the same time, as has already been shown for the islands -off the California coast. For the Pozzuoli shore of the bay -it was learned that within the Christian Era a complete cycle of -downward, followed by later upward, movement has been largely -accomplished. Across the bay, and less than 20 miles distant, is -the Blue Grotto of Capri, a sea cave cut in limestone above an -earlier cave of the same nature which is now deep below the water -surface. It is the refracted sunlight which enters the cave through<span class="pagenum"><a name="Page_258" id="Page_258">[258]</a></span> -this lower submerged opening and has been robbed on the way of -all but its blue rays which gives to the famous grotto its special -charm (<a href="#f289">Fig. 289</a>).</p> - -<div class="floatleft"> - <img src="images/ill-319.jpg" width="230" height="151" id="f289" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 289.</span>—Cross section of the Blue Grotto on the Island of Capri, -showing the submerged sea cave through which most of the light enters -the grotto, and the higher artificial window now widened by wave action -(after von Knebel).</p> -</div></div> - -<p>It is known that the former, and now submerged, sea cave was -in use by Roman patricians as a -cool retreat from the oppressive -hot wind known as the sirocco, -and that an artificial entrance or -window was cut where is now the -only accessible entrance to the -grotto. In the ancient writings, -no mention is made, however, of -the remarkable blue illumination -for which it is now famous, and -the conditions at the time, as we -may see, were not such as to make -this possible. Later subsidence -of the coast has brought the -ancient window to the sea level, where it has been considerably -enlarged by the waves. The earlier grotto, abandoned as its -entrance was closed, was rediscovered in 1826 by the painter and -poet, August Kopisch.</p> - -<p>A grotto with green illumination (the Grotto Verde) is situated -upon the opposite side of the island, and a blue grotto, having its -origin in similar conditions to those of the famous Blue Grotto, -is found upon the island of Busi off the Dalmatian coast.</p> - -<p><b>Character profiles.</b>—In the landscape of a coast which has been -slowly uplifted the characteristic line is the profile of the cuesta, -with short perpendicular element joined to a gently sloping and -longer section and continued in the horizontal portion corresponding -to the lowland (<a href="#f290">Fig. 290</a>). Rapidly uplifted coasts offer in -contrast the lines characteristic of wave erosion and deposition, -but at higher levels and in repeated sections. Most prominent -of all is the staircase constructed of coast terraces, with either -vertical or sloping risers and with outwardly inclining and gently -graded treads. Near the steep riser in the staircase may sometimes -be seen the sugar-loaf outline of the stack cut in softer material, -or the obelisk-like pillar undercut at its base, which is carved -in firmer rock masses. With excessively rapid uplift, the double-notched<span class="pagenum"><a name="Page_259" id="Page_259">[259]</a></span> -cliff or the double sea arch may appear in the landscape. -Upon a submerged coast the most significant lines in the view -are those of the rock islet and the steep-walled fjord.</p> - -<div class="figcenter"> - <img src="images/ill-320.jpg" width="400" height="258" id="f290" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 290.</span>—Character profiles in coast landscapes where -there has been either elevation or depression.</p> -</div></div> - -<p class="pch"><span class="smcap">Reading References for Chapter XIX</span></p> - -<p class="p1">General:—</p> - -<p class="pex"><span class="smcap">Sir Ch. Lyell.</span> Principles of Geology, vol. 2, pp. 180-197.</p> - -<p class="pex"><span class="smcap">Ed. Suess.</span> The Face of the Earth, Clarendon Press, Oxford, 1906, vol. -2, Chapters i and xiv, pp. 1-29, 535-556.</p> - -<p class="pex"><span class="smcap">Robert Sieger.</span> Seenschwankungen und Strandverschiebungen in Scandinavien, -Zeit. d. Gesell. f. Erdk., Berlin, vol. 28, 1893, pp. 1-106, -393-688, pl. 7.</p> - -<p class="p1">Elevated shore lines:—</p> - -<p class="pex"><span class="smcap">F. B. Taylor.</span> The Highest Old Shore Line of Mackinac Island, Am. -Jour. Sci., vol. 43, 1892, pp. 210-218.</p> - -<p class="pex"><span class="smcap">Thomas L. Watson.</span> Evidences of Recent Elevation of the Southern -Coast of Baffins Land, Jour. Geol., vol. 5, 1897, pp. 17-33.</p> - -<p class="pex"><span class="smcap">J. W. Goldthwait.</span> The Abandoned Shore Lines of Eastern Wisconsin. -Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 1-37.</p> - -<p class="p1">Evidences of depression:—</p> - -<p class="pex"><span class="smcap">W. B. Scott.</span> Introduction to Geology, New York, 1907, pp. 33-36.</p> - -<p class="pex"><span class="smcap">W. J. McGee.</span> The Gulf of Mexico as a Measure of Isostacy, Am. -Jour. Sci. (3), vol. 44, 1892, pp. 177-192.</p> - -<p><span class="pagenum"><a name="Page_260" id="Page_260">[260]</a></span></p> - -<p class="pex"><span class="smcap">A. Lindenkohl.</span> Notes on the Submarine Channel of the Hudson -River, etc., Am. Jour. Sci. (3), vol. 41, 1891, pp. 489-499, pl. 18.</p> - -<p class="pex"><span class="smcap">J. W. Spencer.</span> The Submarine Great Cañon of the Hudson River, -<i>ibid.</i> (4), vol. 19, 1905, pp. 1-15; Submarine Valleys off the American -Coast and in the North Atlantic, Bull. Geol. Soc. Am., vol. 14, 1903, -pp. 207-226, pls. 19-20.</p> - -<p class="pex"><span class="smcap">F. Nansen.</span> The Bathymetrical Features of the North Polar Sea, with a -Discussion of the Continental Shelves and Previous Oscillations of -Shore Line, Norwegian North Polar Expedition, vol. 4, 1904, pp. 99-231, -pl. 1.</p> - -<p class="pex"><span class="smcap">W. v. Knebel.</span> Höhlenkunde, etc., Braunschweig, 1906, pp. 175-177 -(the blue grotto of Capri).</p> - -<p class="p1">Oscillation of movement:—</p> - -<p class="pex"><span class="smcap">C. Lyell.</span> Principles of Geology, vol. 2, pp. 164-176 (Temple of Jupiter -Serapis).</p> - -<p class="pex"><span class="smcap">E. Ray Lankester.</span> Extinct Animals, New York, 1905, pp. 31-42.</p> - -<p class="pex"><span class="smcap">H. W. Fairbanks.</span> Oscillations of the Coast of California during the -Pliocene and Pleistocene, Am. Geol., vol. 20, 1897, pp. 213-245.</p> - -<p class="pex"><span class="smcap">G. H. Stone.</span> Mon. 34, U. S. Geol. Surv., 1899, pp. 56-58, pl. 2.</p> - -<p class="pex"><span class="smcap">Bailey Willis.</span> Ames Knob, North Haven, Maine. Bull. Geol. Soc. -Am., vol. 14, 1903, pp. 201-206, pls. 17-18.</p> - -<p class="p1">Simultaneous contrary movements on a coast:—</p> - -<p class="pex"><span class="smcap">A. C. Lawson.</span> The Post-Pliocene Diastrophism of the Coast of Southern -California, Bull. Univ. Calif. Dept. Geol., vol. 1, 1893, pp. 115-160, -pls. 8-9.</p> - -<p class="pex"><span class="smcap">W. S. Tangier-Smith.</span> A Geological Sketch of San Clemente Island, -18th Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 459-496, pls. 84-96.</p> - -<p class="pex"><span class="smcap">R. S. Tarr</span> and <span class="smcap">L. Martin</span>. Recent Changes of Level in the Yakutat -Bay Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, -pls. 12-23.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_261" id="Page_261">[261]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XX</h2> - -<p class="pch">THE GLACIERS OF MOUNTAIN AND CONTINENT</p> - -<p><b>Conditions essential to glaciation.</b>—Wherever for a sufficiently -protracted period the annual snowfall of a district is in -excess of the snow that is melted, a residue must remain from -each season to be added to that of succeeding ones. Eventually -so much snow will have accumulated that under its own weight -and in obedience to its peculiar properties, a movement will begin -within the mass tending to spread it and so to reduce the slope -of its upper surface (<a href="#p1">Frontispiece plate</a>). The conditions favorable -to glaciation are, therefore, heavy precipitation and low annual -temperature. If the precipitation is scanty, the small snowfall -is soon melted; and if the temperature be too high, the moisture -is precipitated not in the form of snow but as rain. It is important -here to keep in mind that snow is a poor heat conductor -and itself protects its deeper layers from melting.</p> - - -<p><b>The snow-line.</b>—Because of the low temperatures glaciers -should be most abundant or most extensive in high latitudes and -in high altitudes. The largest are found in polar and subpolar -regions, and they are elsewhere encountered only at considerable -elevations. The largest glaciers are the vast sheets of ice which -inwrap the continents of Greenland and Antarctica, but glaciers -of large size are to be found upon other large land masses of the -Arctic, as well as in Alaska, in the southern Andes, and in New -Zealand. Much smaller glaciers are characteristic of certain -highlands within temperate and tropical regions, but because -of specially favorable conditions both of altitude and precipitation -the Himalayas, although in relatively low latitudes, nourish -glaciers of large proportions. In general, it may be said that -the nourishing grounds of glaciers are largely restricted to those -areas where snow covers the ground throughout the year. The -lower margin of such areas is designated the <i>snow line</i>, and varies -but little from the line on which the average summer temperature -is at the freezing point of water—the so-called <i>summer<span class="pagenum"><a name="Page_262" id="Page_262">[262]</a></span> -isotherm of 32° Fahrenheit</i>. Within the tropics this line may -rise as high as 18,000 feet above the sea, whereas in polar latitudes -it descends to sea level.</p> - -<p><b>Importance of mountain barriers in initiating glaciers.</b>—The -precipitation within any district depends, however, not alone -upon the amount of moisture which is brought to it in the clouds, -but upon the amount which is abstracted before the clouds have -passed over it. The capacity of space to hold moisture increases -with its temperature, and hence any lowering of this temperature -will reduce the capacity. If lowered sufficiently, the point of -complete saturation will be reached and further cooling must -result in precipitation. Hence, anything which forces an air -current to rise into more rarefied zones above, will lower the pressure -upon it and so bring about a cooling effect in which no heat -is abstracted. This so-called <i>adiabatic refrigeration</i> of a gas -may be illustrated by the cool current which issues in a jet from -a warm expanded rubber tire after the cock has been opened; or -even better, by the instant solidification at extreme low temperatures -of such normal gases as carbonic acid when they are allowed -to issue under heavy pressure from a small orifice.</p> - -<p>As applied to moisture-laden and near-surface winds, the -effective agents of adiabatic cooling are the upland areas upon -the continents, and especially the ranges of mountains. These -barriers force the moving clouds to rise, cool, and deposit their -moisture. It is, therefore, the highland barriers which face the -oncoming, moisture-laden winds that receive the heaviest precipitation. -Within temperate regions, because of the prevalence -of westerly winds, those barriers which face the western shores -receive the heaviest fall. Within the tropics, on the other hand, -it is the barriers facing the eastern shores which, because of the -easterly “trades”, are most favorable to precipitation.</p> - -<p>Thus it is in the Sierra Nevadas of California, and not in the -Rockies or the Appalachians, that the glaciers of the United States -are found. The highland of the Swiss Alps lying likewise athwart -the “westerlies” of the temperate zone acquires the moisture -for nourishment of its glaciers from the western ocean—here -the Atlantic (<a href="#f291">Fig. 291</a>). Within the tropics the conditions are -reversed, and it is in general the ranges which lie nearer the eastern -coasts that are the more favored. If no barrier is found upon<span class="pagenum"><a name="Page_263" id="Page_263">[263]</a></span> -this coast, the clouds may travel over vast stretches of country -before being arrested by mountains and robbed of their moisture. -Thus in tropical Brazil the glaciers are found in the Andes upon -the Pacific coast though nourished by clouds from the Atlantic.</p> - -<div class="figcenter"> - <img src="images/ill-324.jpg" width="400" height="207" id="f291" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 291.</span>—Map showing the distribution of existing glaciers, and the two important -wind poles of the earth.</p> -</div></div> - -<p><b>Sensitiveness of glaciers to temperature changes.</b>—How -sensitive is the adjustment between snow precipitation and temperature -may be strikingly illustrated by the statement on excellent -authority that if the average annual temperature of the air -within the Scottish Highlands should be lowered by only three -degrees Fahrenheit, small glaciers would be the result; and a -moderate temperature fall within the region surrounding the -Laurentian lakes of North America would bring on glaciation, -otherwise expressed as a depression of the snow line of the region.</p> - - -<p><b>The cycle of glaciation.</b>—Though to-day buried beneath its -ice mantle, it is known that Greenland had more than once in earlier -geological ages a notably mild climate, and in some future age -it may revert to this condition. In other regions, also, we have -evidence that such a rotation of climatic changes has been successively -accomplished, the climate having steadily increased in -severity towards a culminating point, and been followed by a -reverse series of changes. Such a complete period may be called -a <i>cycle of glaciation</i>. While the climate is steadily becoming -more rigorous, we have to do with an <i>advancing hemicycle</i> of<span class="pagenum"><a name="Page_264" id="Page_264">[264]</a></span> -glaciation, but after the culminating point has been reached, the -period of amelioration of climate is the <i>receding hemicycle</i>.</p> - -<p><b>The advancing hemicycle.</b>—There is little reason to doubt -that whatever be the cause of the climatic changes which bring -on glacial conditions, these changes come on by insensible gradations. -The first visible evidence of the increased severity of -the climate is the longer persistence of the winter snows, at first -within the more elevated districts. In such positions drifts must -eventually continue throughout the warm season and so contribute -to the snow accumulations of the succeeding winter. This -point once reached, small glaciers are inevitable, even should the -average temperature fall no further, for the snow left over in -each season must steadily increase the depth of the deposits until -the weight brings about an internal motion of the mass from higher -to lower levels.</p> - -<div class="figcenter"> - <img src="images/ill-325.jpg" width="400" height="341" id="f292" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 292.</span>—An Alaskan glacier spreading out at the foot of the range which -nourishes it.</p> -</div></div> - -<p>The inherited depressions of the upland—the gentle hollows -at the heads of rivers—will first be filled, and so the valleys<span class="pagenum"><a name="Page_265" id="Page_265">[265]</a></span> -below become the natural channels for the outflow of the early -glaciers. With a continued lowering of the annual temperature -and consequent increased snowfall, the early glaciers become -more and more amply nourished. Snow and ice will, therefore, -cover larger areas of the upland, and the glaciers will push their -fronts farther down the valleys before they are wasted in the -warm air of the lower levels. As the valleys become thus more -completely invested by the glacier they are likewise filled to greater -and greater depths, and they may thus submerge portions of the -walls that separate adjacent valleys. Reaching at last the front -of the upland area, the glaciers may now be so well nourished at -their heads that they push out upon the flatter foreland and without -restraint from retaining walls spread broadly upon it (<a href="#f292">Fig. 292</a>).</p> - -<div class="figcenter"> - <img src="images/ill-326.jpg" width="400" height="253" id="f293" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 293.</span>—Surface of a glacier whose upper layers spread with slight restraint -from retaining walls. Surface of the Folgefond, an ice cap of southern Norway.</p> -</div></div> - -<p>The culmination of the progressive climatic change may ere -this have been reached and milder conditions have ensued. If, -however, the severity of the climate should be still further increased, -the expanded fronts of neighboring glaciers will coalesce -to form a common ice fan or apron along the foot of the upland -(<a href="#p18b">Plate 18 B</a>). This could hardly take place without a still further -deepening of the ice within the valleys above, and, probably, a -progressive submergence of the lower crests in the valley walls.<span class="pagenum"><a name="Page_266" id="Page_266">[266]</a></span> -This may even continue until all parts of the upland area have -been buried. The snow and ice now take the form of a covering -cap or carapace, and the upper portions being no longer restrained -at the sides, now spread into a broad dome, as would a viscous -liquid like thick molasses when poured out upon the floor (<a href="#f293">Fig. 293</a>). -The lower zones of the mass and the thinner marginal -portions still have their motion to a greater or less extent controlled -by the irregularity of the rock floor against which they rest.</p> - -<p>The reverse series of changes in the glacier is inaugurated by an -amelioration of the climate, and here, therefore, the advancing -hemicycle becomes merged in the receding hemicycle of glaciation.</p> - - -<p><b>Continental and mountain glaciers contrasted.</b>—The time -when the rock surface becomes submerged beneath the glacier -is, as regards both the surface forms and the erosive work, a critical -point of much significance; for the ice cap and larger continental -glacier obviously protect the rock surface from the action -of those chemical and mechanical processes in which the atmosphere -enters as chief agent, and which are collectively known as weathering -processes. Until submergence is accomplished, larger or -smaller portions of the rock surface project either through or -between the ice masses and are, therefore, exposed to direct -attack by the weather (see below, <a href="#Page_370">p. 370</a>).</p> - -<div class="figcenter"> - <img src="images/ill-328a.jpg" width="400" height="121" id="f294" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 294.</span>—Section through a mountain glacier (in solid black), showing how its -surface is determined by the irregularities in the rock basement (after Hess).</p> -</div></div> - -<p>Snow which falls in the mountains is not allowed to remain -long where it falls. By the first high wind it is swept off the -more elevated and exposed surfaces and collected under eddies in -any existing hollows, but especially those upon the lee slopes of -the range. We are to learn that glaciers carve the mountains by -enlarging the hollows which they find and producing great basins -for the collection of their snows; but with the initiation of glaciation -the inherited hollows are in most cases the unimportant -depressions at the heads of streams. Whatever they may be -and however formed, the snow first fills those hollows which are -sheltered from the wind, and as it accumulates and becomes -distributed as ice, assumes a surface of its own that is dependent -upon the form and the position of the basin which it occupies -(see <a href="#f294">Fig. 294</a>).</p> - -<div class="figcenter"> - <img src="images/ill-328b.jpg" width="450" height="82" id="f295" - alt="" - title="" /> - <div class="caption"><p class="ch450"><span class="smcap">Fig. 295.</span>—Profile across the largest of the Icelandic ice caps, with the vertical -scale greatly exaggerated (after Thoroddsen and Spethmann).</p> -</div></div> - -<p>When the quantity of accumulated snow is so great that all -hollows of the rock surface are filled, its own surface is no longer<span class="pagenum"><a name="Page_267" id="Page_267">[267]</a></span> -controlled by retaining rock walls, and it now assumes a form -largely independent of the irregularities in the upland. Experience -shows that this surface is approximately that of a flat dome -or shield, and as it covers all the upland, save where the ice thins -upon its margins, this type of glacier is called an <i>ice cap</i> (<a href="#f295">Fig. 295</a>). -All types of glacier in which rock projects above the -highest levels of the ice and snow are known as <i>mountain glaciers</i>.</p> - -<div class="figcenter"> - <img src="images/ill-328c.jpg" width="400" height="46" id="f296" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 296.</span>—Ideal section across a continental glacier, with the vertical scale and -the projecting rock masses of the marginal zone greatly magnified.</p> -</div></div> - -<p>The flat domes of ice which mantle the continents of Greenland -and Antarctica, though resembling in form the smaller ice -cap, are yet because of their vast size so distinct from them, particularly -in the manner of their nourishment, that they belong in -a separate class described as <i>inland ice</i> or <i>continental glaciers</i>. -Though they have some affinities with ice caps, they are most -sharply differentiated from all types of mountain glaciers. Of them -it is true that the lithosphere projects through them only in the -neighborhood of their margins (<a href="#f296">Fig. 296</a>), whereas in the case of<span class="pagenum"><a name="Page_268" id="Page_268">[268]</a></span> -mountain glaciers rock may project at any level but <i>always above -the highest snow surface</i>. Ice caps may be regarded as intermediate -between the two main classes of mountain and continental -glaciers (<a href="#f297">Fig. 297</a>). Because of the large rôle which continental -glaciers have played in geological history, it is thought best to consider -them first, leaving for later discussion the no less interesting -but less important mountain glaciers.</p> - -<div class="figcenter"> - <img src="images/ill-329.jpg" width="400" height="89" id="f297" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 297.</span>—View of the Eyriks-Jökull, an ice-cap of Iceland (after Grossman).</p> -</div></div> - -<p><b>The nourishment of glaciers.</b>—The life of a glacier is dependent -upon the continued deposition of snow in aggregate amount -in excess of that which is lost by melting or by other depleting -processes. Whenever, on the other hand, the waste exceeds the -precipitation, the glacier is in a receding condition and must -eventually disappear, if such conditions are sufficiently long continued. -The source of the snow is the water of the ocean evaporated -into the atmosphere and transported over the land in the -form of clouds. We are to learn that the changes which this -moisture undergoes before its delivery to the glacier are notably -different for the classes of continental and mountain glacier.</p> - - -<p><b>The upper and lower cloud zones of the atmosphere.</b>—Before -we can comprehend the nature of the processes by which glaciers -are nourished, it will be necessary to review the results of -recent studies made upon the earth’s atmospheric envelope. It -must be kept in mind that the sun’s rays are chiefly effective in -warming the atmosphere through being first absorbed by some -solid body such as rock or water and their heat then communicated -by contact to the immediately adjacent air layers. The layers thus -warmed being now lighter than before, they rise and are replaced -by colder air, which in its turn is warmed and likewise set in upward -motion. Such currents developed in the air by contact -with warmer solid bodies constitute the process known as convection.</p> - -<p><span class="pagenum"><a name="Page_269" id="Page_269">[269]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-330.jpg" width="400" height="356" id="f298" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 298.</span>—The zones of the lower atmosphere as revealed by recent kite and -balloon explorations.</p> -</div></div> - -<p>To a relatively small degree the atmosphere is heated by the -direct absorption of the sun’s rays which pass through it. Since -air has weight, it compresses the lower layers near the earth, and -hence as we ascend from the earth’s surface the air becomes continually -lighter. Convection currents must, therefore, adjust -themselves by the air expanding as it rises. But expansion of a -gas always results in its cooling, as every one must have observed -who has placed his finger in the air current which escapes from -the open valve of a warm rubber tire. Dry air is cooled a degree -Fahrenheit for every six hundred feet of ascent in the atmosphere. -At a height of about seven miles above the earth’s surface -all rising air currents have cooled to about 68° below the -zero of the Fahrenheit scale, and exploration with balloons has -shown that the currents rise no farther. At this level they<span class="pagenum"><a name="Page_270" id="Page_270">[270]</a></span> -move horizontally, just as rising vapor spreads out in a room beneath -the ceiling. Above this level, as far as exploration has gone, -or to a height of more than twelve miles, the temperature remains -nearly constant, and this upper zone is, therefore, called the <i>isothermal</i> -or the <i>advective zone</i>—the uniform temperature zone -of the lower atmosphere. Beneath the convective ceiling the -process of convection is characteristic, and this zone is therefore -described as the <i>convective zone</i> (<a href="#f298">Fig. 298</a>).</p> - -<p>A large part of the moisture which rises from the ocean’s surface -is condensed to vapor before it has ascended three miles, and in -this form it makes its transit over land as fleecy or stratiform -clouds—the so-called cumulus and stratus clouds and their many -intermediate varieties (see Frontispiece). This lower layer within -the convective zone is, therefore, a moist one overlaid by a relatively -drier middle layer of the convective zone. That moisture -which rises above the lower cloud layer is congealed by adiabatic -cooling to fine ice needles visible as the so-called cirrus -clouds which float as feathery fronds beneath the convective -ceiling (see frontispiece at right upper corner of picture). Thus -we have within the convective zone an upper layer more or less -charged with water in the form of ice needles. It is the clouds -of the lower zone whose moisture in the form of vapor supplies -the nourishment of mountain glaciers, and the high cirrus clouds -whose congealed moisture, after interesting transformations, is -responsible for the continued existence of continental glaciers.</p> - -<p>As we are to see, there are other noteworthy differences between -continental and mountain glaciers, in the manner of their -sculpture of the lithosphere, so that long after they have disappeared -the characters of each are easily identified in their handiwork. -How the lower clouds are forced upward and so compelled -to give up their moisture to feed the mountain glaciers, and how -the upper clouds are pulled downward to nourish the glaciers of -continents, can be best understood after the characteristics of -each glacier class have been studied.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_271" id="Page_271">[271]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXI</h2> - -<p class="pch">THE CONTINENTAL GLACIERS OF POLAR REGIONS</p> - -<div class="floatright"> - <img src="images/ill-332.jpg" width="250" height="410" id="f299" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 299.</span>—Map of Greenland showing the -area of inland-ice and the routes of different -explorers.</p> -</div></div> - -<p><b>The inland ice of Greenland.</b>—In Greenland and in Antarctica -the land is almost or quite buried under a cover of snow and -ice—the so-called “inland ice”—which -always assumes the -surface of a very flat dome or -shield. In Greenland there is -found a marginal ribbon of -land generally from five to -twenty-five miles in width -(<a href="#f299">Fig. 299</a>), but in Antarctica -all the land, with the exception -of a few mountain peaks, -is inwrapped in a mantle of -ice which is also extended upon -the sea in a broad shelf of snow -and ice. Neither of these vast -glaciers has been explored except -in its marginal portion, -yet such is the symmetry of -the profiles along the routes -traversed, and such the flatness -and monotony of the snow -surface within the margins, -that there is little reason to -doubt that the profile made -along Nansen’s route in southern -Greenland would, save only -for magnitude, fairly represent a section across the middle of the -continent (<a href="#f300">Fig. 300</a>).</p> - -<p><b>The mountain rampart and its portals.</b>—As soon as we examine -the coastal belt we observe that the “Great Ice” of<span class="pagenum"><a name="Page_272" id="Page_272">[272]</a></span> -Greenland is held in by a wall of mountains and so prevented from -spreading out to its natural surface in the marginal portions. -Through portals of the inclosing mountain ranges—the <i>outlets</i>—it -sends out <i>tongues</i> of ice which in many respects resemble -certain types of mountain glaciers.</p> - -<div class="figcenter"> - <img src="images/ill-333.jpg" width="450" height="110" id="f300" - alt="" - title="" /> - <div class="caption"><p class="ch450"><span class="smcap">Fig. 300.</span>—Profile in natural proportions across the southern end of the continental -glacier of Greenland, constructed upon an arc of the earth’s surface and based -upon Nansen’s profile corrected by Hess. The marginal portions of the profile -are represented below upon a magnified scale in order to bring out the characters -of the marginal slopes.</p> -</div></div> - -<p>Such measurements as have been made upon the inland ice -of Greenland at points back from, but yet comparatively near to, -the outlets, show that it has here a surface rate of motion amounting -to less than an inch per day, and it is highly probable that at -moderate distances from the margin this amount diminishes to -zero. Upon the outlets, on the contrary, surface rates as high as -59 feet per day have been measured, and even 100 feet per day has -been reported. We are thus justified in saying that glacier flow -within the outlets is from 700 to 1000 times as great as it is upon -the near-by inland ice, and that the glacier is in a measure drained -through the portals of the inclosing ranges. Back from these -outlet streams of ice, or tongues, the surface of the inland ice is -depressed to form a dimple or “basin of exudation” as is the surface -of a reservoir above the raceway when the water is being rapidly -drawn away (<a href="#f301">Fig. 301</a>).</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 13.</span></p> - -<div class="figcenter"> - <img src="images/ill-334a.jpg" width="400" height="335" id="p13a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Precipitous front of the Bryant glacier outlet of the Greenland inland-ice (after -Chamberlin).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-334b.jpg" width="400" height="319" id="p13b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Lateral stream beside the Benedict glacier outlet, Greenland (after R. E. Peary).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_273" id="Page_273">[273]</a></span></p> - -<div class="floatright"> - <img src="images/ill-336.jpg" width="200" height="508" id="f301" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 301.</span>—Map of a glacier tongue, with -dimple showing above and due to indraught -of the ice. Umanakfjord, Greenland -(after von Drygalski).</p> -</div></div> - -<p>Fissures in the ice, the so-called crevasses, are the recognized -marks of ice movement, and these are always concentrated at the -steep slopes of the ice surface in the neighborhood of its margins. -Upon the Greenland ice, crevasses are restricted in their distribution -to a zone which extends from seven to twenty-five miles -within the ice border.</p> - -<p><b>The marginal rock islands.</b>—From its margin the ice surface -rises so steeply as to be climbed only with difficulty, but this -gradient steadily diminishes until at a distance of between seventy-five -and a hundred miles its slope is less than two degrees. Where -crossed by Nansen near latitude 64° N. the broad central area of -ice was so nearly level as to -appear to be a plain.</p> - -<p>As we pass across the irregular -ice margin in the direction -of the interior, larger and larger -proportions of the land’s surface -are submerged, until only -projecting peaks rise above -the ice as islands which are -described as <i>nunataks</i> (<a href="#f302">Fig. 302</a>).</p> - -<p>Though not a universal observation, -it has been often -noted that the absorption of -the sun’s rays by rock masses -projecting through the snow -results in a radiation of the -heat and a lowering by melting -of the surrounding snow and -ice. For this reason nunataks -are often surrounded by a deep -trench due to a melting of the -snow. Such a depression in -the ice surface about the margin -of a nunatak, from its resemblance -to a trench about -an ancient castle, has been -designated a <i>moat</i> (<a href="#f303">Fig. 303</a>). -For the same reason, the outlet -tongues of ice which descend -in deep fjords between walls of -rock are melted away from -the walls and a lateral stream -of water is sometimes found -to flow between ice and rock -(<a href="#p13b">pl. 13 B</a>).</p> - -<p><span class="pagenum"><a name="Page_274" id="Page_274">[274]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-337a.jpg" width="250" height="156" id="f302" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 302.</span>—Edge of the Greenland inland ice, -showing the nunataks diminishing in size toward -the interior. The lines upon the ice are medial -moraines starting from nunataks (after Libbey).</p> -</div></div> - -<p><b>Rock fragments which travel with the ice.</b>—Rock surfaces -which are exposed to the atmosphere are in high latitudes broken -down through the freezing -of water within their -crevices. The fragments -resulting from -this rending process fall -upon the glacier surface -and are carried forward -as passengers in the direction -of the ice margin. They are either -visible as long and narrow -ridges or trains following -the directions of -the steepest slope (<a href="#f302">Fig. 302</a>), -or they become buried under fresh falls of snow and only -again become visible where summer melting has lowered the glacier -surface in the vicinity of its margin. These longitudinal trains of -rock fragments upon the glacier surface always have their starting -point at the lower margin of one of the nunataks, and are known -as <i>medial moraines</i> (<a href="#f301">Fig. 301</a>, <a href="#Page_273">p. 273</a>, and <a href="#f302">Fig. 302</a>). Inside -the zone of nunataks the glacier surface is, however, clear of rock -débris except where dust has -been blown on by the wind, -and this extends for a few -miles only. The material of -the medial moraines is a collection -of angular blocks whose -surfaces are the result of frost -rending, for in their travel -above the ice they are subjected -to no abrading processes.</p> - -<div class="floatright"> - <img src="images/ill-337b.jpg" width="250" height="208" id="f303" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 303.</span>—Moat surrounding a nunatak in -Victoria Land (after Scott).</p> -</div></div> - -<p>A contrasted type of surface -moraines upon the Greenland -glacier, instead of being parallel -to the direction of ice movement, is directed transversely or -parallel to the margins. The materials of these moraines are<span class="pagenum"><a name="Page_275" id="Page_275">[275]</a></span> -more rounded fragments of rock which have come up from the -bottom layers, and we shall again refer to the origin of such -moraines after the subglacial conditions have been considered.</p> - - -<p><b>The grinding mill beneath the ice.</b>—If, now, we examine the -front of a glacier tongue which goes out from the inland ice, we -find that while the upper portion is white and mainly free from rock -débris (<a href="#p13a">plate 13 A</a>), the lower zone is of a dark color and crowded -with layers of pebbles and bowlders which have been planed, -polished, and scratched in a quite remarkable manner. The ice -front is itself subject to forward and retrograde migrations of short -period, but it is easily seen that in the main its larger movement -has been a retrograde one. The ground from which it has lately -withdrawn is generally a hard rock floor unweathered, but smooth, -polished, and scratched in the same manner as the bowlders which -are imbedded within the ice. It is perfectly apparent that the -latter have been derived from some portion of the rock basement -upon which the glacier still rests, and that floor and bowlders have -alike been ground smooth by mutual contact under pressure.</p> - -<p>This erosion beneath the ice is accomplished by two processes; -namely, <i>plucking</i> and <i>abrasion</i>. Wherever the rock over which -the glacier moves has stood up in projecting masses and is riven -by fissure planes of any kind, the ice has found it easy to remove -it in larger or smaller fragments by a quarrying process described -as plucking. The rock may be said to be torn away in blocks which -are largely bounded by the preëxisting fissure planes. Over relatively -even surfaces plucking has little importance, but where -there are noteworthy inequalities of surface upon the glacier bed, -those sides which are away from the oncoming ice (<i>lee</i> side) are -degraded by plucking in such a manner as sometimes to leave -steep and ragged fracture surfaces. The tools of the ice thus acquired -in the process of plucking are quickly frozen into the lowest -ice layers, and being now dragged along the floor they abrade in -the same manner as does a rasp or file. These tools of the ice are -themselves worn away in the process and are thus given their -characteristic shapes. Just as the lapidary grinds the surface of -a jewel into facets by imbedding the gem in a matrix, first in one -and then in another position, each time wearing down the projecting -irregularities through contact with the abrading surface; -so in like manner the rock fragment is held fast at the bottom of<span class="pagenum"><a name="Page_276" id="Page_276">[276]</a></span> -the glacier until “soled” or “shod”, first upon one side and then -upon another. Accidental contact with some obstruction upon -the floor may suffice to turn the fragment and so expose a new surface -to wear upon the abrading floor. Minor obstructions coming -in contact with one side of the fragment only, may turn it in -its own plane without overturning. Evidence of such interruptions -can be later read in the different directions of striæ upon -the same facet (<a href="#p17a">plate 17 A</a>).</p> - -<div class="floatleft"> - <img src="images/ill-339.jpg" width="250" height="179" id="f304" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 304.</span>—A glacier pavement of Permo-Carboniferous -age in South Africa. The striæ -running in the direction of the observer are -prominent and a noteworthy gouging of the -surface is to be noted to the right in the -middle distance (after Davis).</p> -</div></div> - -<p>The floor beneath the glacier is reduced by the abrading process -to a more or less smooth and generally flattened or rounded surface—the -so-called <i>glacier pavement</i> (<a href="#f304">Fig. 304</a>). To accomplish -this all former mantle rock -due to weathering processes -must first be cleared away, -and the firm unaltered rock -beneath is wherever susceptible -of it given a smooth -polish although locally -scored and scratched by the -grinding bowlders. The -earlier projections of the -surface of the floor, if not -entirely planed away, are -at least transformed into -rounded shoulders of rock, -which from their resemblance -to closely crowded backs in a flock of sheep have been -called “sheep backs” or “<i>roches moutonnées</i>.” Thus the effect -of the combined action of the processes of plucking and abrasion -is to reduce the accent of the relief and to mold the contours of -the rock in smoothly flowing curves, generally of large radius.</p> - -<p><b>The lifting of the grinding tools and their incorporation -within the ice.</b>—Wherever the ice is locally held in check by the -projecting nunataks, relief is found between such obstructions, -and there the flow of the ice has a correspondingly increased velocity -(<a href="#f305">Fig. 305 <i>b</i></a>). If the obstructions are not of large dimensions, -the ice which flows around the outer edges is soon joined to that -which passes between the obstructions and so normal conditions -of flow are restored below the nunataks. The locally rapid flow<span class="pagenum"><a name="Page_277" id="Page_277">[277]</a></span> -of the ice is, therefore, restricted to a relatively short distance, the -passageway between the nunataks, and the conditions are thus -to be likened to the fall of water at a raceway due to the sudden -descent of its surface from the level of the reservoir to the level of -the stream in the outlet. As is well known, there is under these -conditions a prodigious scour upon the bottom which tends to dig -a pit just above and below the dam—a <i>scape colk</i>—and carry -the materials up to the surface below the pit. Such a tendency -was well illustrated by the behavior of the water at the opening -of the Neu Haufen dam below the city of Vienna (<a href="#f305">Fig. 305 <i>a</i></a>). In -the case of ice, material from the bottom may by the upward current -be brought up to the surface of the glacier at the lower edge -of the colk and thus produce a type of local surface moraine of -horseshoe form with its direction generally transverse to the direction -of ice movement (<a href="#f305">Fig. 305 <i>b</i></a>).</p> - -<div class="figcenter"> - <img src="images/ill-340.jpg" width="400" height="212" id="f305" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 305.</span>—<i>a</i>, Map showing pit excavated by the current below the opening in a -dam. <i>b</i>, Nunataks and surface moraines on the Greenland ice. Dalager’s -Nunataks (after Suess).</p> -</div></div> - -<p>Any obstruction upon the pavement of the glacier apparently -exerts a larger or smaller tendency to elevate the bowlders and -pebbles and incorporate them within the ice. Rock débris thus -incorporated is described as <i>englacial</i> drift. In the case of Greenland -glaciers this material seems at the ice front to be largely restricted -to the lower 100 feet (<a href="#p13a">plate 13 A</a>).</p> - -<p>Near the front of the inland ice the increased slope of the upper -surface greatly increases the flow of the upper ice layers in comparison<span class="pagenum"><a name="Page_278" id="Page_278">[278]</a></span> -with those nearer the bottom, so that the upper layers -override the lower as they would an obstruction. The englacial -drift is either for this reason or because of rock obstructions -brought to the surface, where it yields parallel ridges corresponding -in direction to the glacier margin. Such transverse surface moraines -are thus in many respects analogous to those which appear -about the lower margins of scape colks. In contrast to the -longitudinal or medial surface moraines the materials of the transverse -moraines are more faceted and rounded—they have been -abraded upon the glacier pavement.</p> - - -<p><b>Melting upon the glacier margins in Greenland.</b>—During the -short but warm summer season, the margins of the Greenland ice -are subject to considerable losses through surface melting. When -the uppermost ice layer has attained a temperature of 32° Fahrenheit, -melting begins and moves rapidly inward from the glacier -margin. In late spring the surface of the outer marginal zone is -saturated with water, and this zone of slush advances inward with -the season, but apparently never transgresses the inner border of -what we have generally referred to as the marginal zone of the ice -characterized by relatively steep slopes, crevasses, and nunataks. -Upon the ice within this zone are found streams large enough to be -designated as rivers and these are connected with pools, lakes, and -morasses. The dirt and rock fragments imbedded in the ice are -melted out in the lowering of the surface, so that late in the season -the ice presents a most dirty aspect. At the front of the great -mountain glaciers of Alaska, a more vigorous operation of the same -process has yielded a surface soil in which grow such rank forests -as entirely to mask the presence of the ice beneath.</p> - -<p>In addition to the visible streams upon the surface of the Greenland -ice, there are others which flow beneath and can be heard by -putting the ear to the surface. All surface streams eventually -encounter the marginal crevasses and plunge down in foaming -cascades, producing the well known “glacier wells” or “glacier -mills.” The progress of the water is now throughout in tunnels -within the ice until it again makes its appearance at the glacier -margin.</p> - -<div class="floatright"> - <img src="images/ill-342a.jpg" width="250" height="174" id="f306" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 306.</span>—Marginal moraine now forming at -the edge of Greenland inland ice, showing a -smooth rock pavement outside it. A small -lake with a partial covering of lake ice occupies -a hollow of this pavement (after von -Drygalski).</p> -</div></div> - -<p><b>The marginal moraines.</b>—Study of both the Greenland and -Antarctic glaciers has shown that if we disregard the smaller and -short-period migrations of the ice front, the general later movement<span class="pagenum"><a name="Page_279" id="Page_279">[279]</a></span> -has been a retrograde one—we live in a receding hemicycle -of glaciation. The earlier Greenland glacier has now receded so -as to expose large areas of -the former glacier pavement. -In places this -pavement is largely bare, -indicating a relatively rapid -retirement of the ice front, -but at all points at which -the ice margin was halted -there is now found a ridge -of unassorted rock materials -which were dropped -by the ice as it melted (<a href="#f306">Fig. 306</a>). -Such ridges, composed -of the unassorted -materials described as <i>till</i>, -come to have a festooned arrangement largely concentric to the ice -margin, and are the <i>marginal</i> or <i>terminal moraines</i> (see <a href="#f336">Fig. 336</a>, -<a href="#Page_312">p. 312</a>). Marginal moraines, if of large dimensions, usually have a -hummocky surface, and are apt to be composed of rock fragments -of a wide range of size from rock flour (clay) to large bowlders -(<a href="#p17a">plate 17 A</a>), which may represent many types since they have -been plucked by the glacier -or gathered in at its surface -from many widely separated -localities.</p> - -<div class="floatleft"> - <img src="images/ill-342b.jpg" width="250" height="178" id="f307" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 307.</span>—Small lake impounded between -the ice front and a moraine which it has -recently built. Greenland (after von Drygalski).</p> -</div></div> - -<p>As the glacier front retires -from the moraine which it -has built up, the water which -emerges from beneath the -ice is impounded behind the -new dam so as to form a -lake of crescentic outline -(<a href="#f307">Fig. 307</a>). Such lakes are -particularly short-lived, for -the reason that the water -finds an outlet over the lowest point in the crest of the moraine -and easily cuts a gorge through the loose materials, thus draining<span class="pagenum"><a name="Page_280" id="Page_280">[280]</a></span> -the lake (<a href="#f308">Fig. 308</a>). Thereafter, the escaping water flows in a -braided stream across the late lake bottom and thence at the -bottom of the gorge through the moraine.</p> - -<div class="figcenter"> - <img src="images/ill-343.jpg" width="400" height="263" id="f308" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 308.</span>—View of a drained lake bottom between the moraine-covered ice front -in the foreground and an abandoned marginal moraine in the middle distance. The -water flows from the ice front in a braided stream and passes out through the moraine -in a narrow gorge. Variegated glacier, Alaska (after Lawrence Martin).</p> -</div></div> - -<p><b>The outwash plain or apron.</b>—The water which descends from -the glacier surface in the glacier wells or mills, eventually arrives -at the bottom, where it follows a sinuous course within a tunnel -melted out in the ice. Much of this water may issue at the ice -front beneath the coarse rock materials which are found there, -and so be discovered with the ear rather than by the eye. The -water within the tunnels not flowing with a free surface but being -confined as though it were in a pipe, may, however, reach the -glacier margin under a hydrostatic pressure sufficient to carry it -up rising grades. Inasmuch as it is heavily charged with rock -débris and is suddenly checked upon arriving at the front it deposits -its burden about the ice margin so as to build up plains of -assorted sands and gravels, and over this surface it flows in ever -shifting serpentine channels of braided type (<a href="#f308">Fig. 308</a>). Such -plains of glacier outwash are described as <i>outwash plains</i> or <i>outwash -aprons</i>.</p> - -<p>Rising as it does under hydrostatic pressure the water issuing -at the glacier front may find its way upward in some of the crevasses<span class="pagenum"><a name="Page_281" id="Page_281">[281]</a></span> -and so emerge at a level considerably above the glacial -floor. It may thus come about that the outwash plain is built -up about the nose of the glacier so as partially to bury it from -sight. When now the ice front begins a rapid retirement, a depression -or <i>fosse</i> (<a href="#f309">Fig. 309</a> and <a href="#f339">Fig. 339</a>, <a href="#Page_314">p. 314</a>) is left behind the -outwash plain and in front of the moraine which is built up at the -next halting place.</p> - -<div class="figcenter"> - <img src="images/ill-344.jpg" width="400" height="189" id="f309" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 309.</span>—Diagrams to show the manner of formation and the structure of an -outwash plain, and the position of the fosse between this and the moraine.</p> -</div></div> - -<p><b>The continental glacier of Antarctica.</b>—In Victoria Land, upon -the continent of Antarctica, so far as exploration has yet gone, -the continental glacier is held back by a rampart of mountains, -as has been shown to be true of the inland ice of Greenland. The -same flat dome or shield has likewise been found to characterize -its upper surface (<a href="#f310">Fig. 310</a>).</p> - -<p>The most noteworthy differences between the inland ice masses -of Greenland and Antarctica are to be ascribed to the greater -severity of the Antarctic climate and to the more ample nourishment -of the southern glacier measured by the land area which it -has submerged. There is here no marginal land ribbon as in Greenland, -but the glacier covers all the land and is, moreover, extended -upon the sea as a broad floating terrace—the shelf ice (<a href="#f311">Fig. 311</a>). -This barrier at its margin puts a bar to all further navigation, -rising as it does in some cases 280 feet above the sea and descending -to even greater depths below (<a href="#p15b">plate 15 B</a>).</p> - -<p><span class="pagenum"><a name="Page_282" id="Page_282">[282]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-345.jpg" width="400" height="475" id="f310" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 310.</span>—Map showing the inland ice of Victoria Land bordered by the shelf -ice of the Great Ross Barrier. The arrows show the direction of the prevailing -winds (based on maps by Scott and Shackleton).</p> -</div></div> - -<p>In that portion of Antarctica which was explored by the German -expedition, the inland ice is not as in Victoria Land restrained -within walls of rock, but is spread out upon the continent so as to -assume its natural ice slopes, which are therefore much flatter -than those examined in Greenland and Victoria Land. Here in -Kaiser Wilhelm Land the ice rises at its sea margin in a cliff which -is from 130 to 165 feet in height, then upon a fairly steeply curving -slope to an elevation of perhaps a thousand feet. Here the grades -have become relatively level, and on ever flatter slopes the surface -appears to continue into the distant interior (<a href="#p14">plate 14</a>). Near -the ice margin numerous fissures betray a motion within the mass -which exact measurements indicate to be but one foot per day, and -at a distance of a mile and a quarter from the margin even this -slight value has diminished by fully one eighth. It can hardly -be doubted that at moderate distances only within the ice margin, -the glacier is practically without motion.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 14.</span></p> - -<div class="figcenter"> - <img src="images/ill-346.jpg" width="450" height="177" id="p14" - alt="" - title="" /> - <div class="caption"><p class="pc400">View of the margin of the Antarctic continental glacier in Kaiser Wilhelm Land (after E. v. Drygalski).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_283" id="Page_283">[283]</a></span></p> - -<p>Rain or general melting conditions being unknown in Antarctica, -a striking contrast is offered to the marginal zone of the Greenland -continent. This is to a large extent explained by the existence -upon the northern land mass of a coastland ribbon which becomes -quickly heated in the sun’s rays, and both by warming the air and -by radiating heat to the ice it causes melting and produces local -air temperatures which in summer may even be described as hot. -About Independence Bay in latitude 82° N. and near the northernmost -extremity of Greenland, Peary descended from the inland -ice into a little valley within which musk oxen were lazily -grazing and where bees buzzed from blossom to blossom over a -gorgeous carpet of flowers.</p> - -<div class="figcenter"> - <img src="images/ill-348.jpg" width="400" height="245" id="f311" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 311.</span>—Sections across the inland ice of Victoria Land, Antarctica, with the -shelf ice in front (after Shackleton).</p> -</div></div> - - -<p><b>Nourishment of continental glaciers.</b>—Explorations upon and -about the glaciers of Greenland and Antarctica have shown that -the circulation of air above these vast ice shields conforms to a -quite simple and symmetrical model subject to spasmodic pulsations<span class="pagenum"><a name="Page_284" id="Page_284">[284]</a></span> -of a very pronounced type. Each great ice mass with its -atmospheric cover constitutes a sort of refrigerating air engine -and plays an important part in the wind system of the globe. -(See <a href="#f291">Fig. 291</a>, <a href="#Page_263">p. 263</a>). Both the domed surface and the low temperature -of the glacier are essential to the continuation of this -pulsating movement within the atmosphere (<a href="#f312">Fig. 312</a>). The air -layer in contact with the ice is during a period of calm cooled, contracted, -and rendered heavier, so that it begins to slide downward -and outward upon the domed surface in all directions. The extreme -flatness of the greater portion of the glacier surface—a -fraction only of one degree—makes the engine extremely slow -in starting, but like all bodies which slide upon inclined planes, -the velocity of its movement is rapidly accelerated, until a blizzard -is developed whose vigor is unsurpassed by any elsewhere experienced.</p> - -<div class="figcenter"> - <img src="images/ill-349.jpg" width="400" height="105" id="f312" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 312.</span>—Diagram to show the nature of the fixed glacial anticyclone above -continental glaciers and the process by which their surface is shaped.</p> -</div></div> - -<p>The effect of such centrifugal air currents above the glacier is -to suck down the air of the upper currents in order to supply the -void which soon tends to develop over the central portion of the -glacier dome. This downward vortex, fed as it is by inward-blowing, -high-level currents, and drained by outwardly directed surface -currents, is what is known as an <i>anticyclone</i>, here fixed in -position by the central embossment of the dome.</p> - -<p>The air which descends in the central column is warmed by -compression, or adiabatically, just as air is warmed which is forced -into a rubber tire by the use of a pump. The moisture congealed -in the cirrus clouds floating in the uppermost layer of the convective -zone, is carried down in this vortex and first melted and in -turn evaporated, due to the adiabatic effect. This fusion and -evaporation of the ice by its transformation of latent, to sensible, -heat, in a measure counteracts, and so retards, the adiabatic elevation<span class="pagenum"><a name="Page_285" id="Page_285">[285]</a></span> -of temperature within the column. Eventually the warm -air now charged with water vapor reaches the ice surface, is at -once chilled, and its burden of moisture precipitated in the form of -fine snow needles, the so-called “frost snow”, which in accompaniment -to the sudden elevation of temperature is precipitated at the -termination of a blizzard.</p> - -<p>The warming of the air has, however, had the effect of damping -as it were, the engine stroke, and, as the process is continued, to -start a reverse or upward current within the chimney of the anticyclone. -The blizzard is thus suddenly ended in a precipitation -of the snow, which by changing the latent heat of condensation -to sensible heat tends to increase this counter current.</p> - -<div class="figcenter"> - <img src="images/ill-350.jpg" width="400" height="299" id="f313" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 313.</span>—Snow deltas about the margins of the Fan glacier outlet of Greenland -(after Chamberlin).</p> -</div></div> - -<p><b>The glacier broom.</b>—During the calm which succeeds to the -blizzard, heat is once more abstracted from the surface air layer, -and a new outwardly directed engine stroke is begun. The tempest -which later develops acts as a gigantic centrifugal broom which -sweeps out to the margins of the glacier all portions of the latest -snowfall which have not become firmly attached to the ice surface. -The sweepings piled up about the margin of continental glaciers -have been described as fringing glaciers, or the glacial fringe. The -northern coast of Greenland and Grant Land are bordered by a -fringe of this nature (<a href="#p14">plate 14</a>, and <a href="#f315">Fig. 315</a>, <a href="#Page_288">p. 288</a>). It is by the<span class="pagenum"><a name="Page_286" id="Page_286">[286]</a></span> -operation of the glacier broom that the inland ice is given its characteristic -shield-like shape (<a href="#f312">Fig. 312</a>). The granular nature of the -snow carried by the wind is well brought out by the little snow -deltas about the margins of Greenland ice tongues (<a href="#f313">Fig. 313</a>). -Obviously because of the presence of the vigorous anticyclone, no -snows such as nourish mountain glaciers can be precipitated upon -continental glaciers except within a narrow marginal zone, and, -as shown by Nansen rock dust from the coastland ribbon and -from the nunataks -of Greenland, is carried -by a few miles -inside the western -margin, and not -at all within the -eastern.</p> - -<div class="floatleft"> - <img src="images/ill-351.jpg" width="250" height="147" id="f314" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 314.</span>—Sea ice of the Arctic region in lat. 80° 5´ N. -and long. 2° 52´ E. (after Duc d’Orleans).</p> -</div></div> - -<p><b>Field and pack -ice.</b>—Within polar -regions the surface -of the sea freezes -during the long -winter season, the -product being known as <i>sea-ice</i> or <i>field-ice</i> (<a href="#f314">Fig. 314</a>). This ice -cover may reach a thickness by direct freezing of eight or more -feet, and by breaking up and being crowded above and below -neighboring fragments may increase to a considerably greater -thickness. Ice thus crowded together and more or less crushed is -described as <i>pack ice</i> or <i>the pack</i>.</p> - -<p>The pack does not remain stationary but is continually drifting -with the wind and tide, first in one direction and then in another, -but with a general drift in the direction of the prevailing winds. -Because of the vast dimensions of the pack, the winds over widely -separated parts may be contrary in direction, and hence when currents -blow toward each other or when the ice is forced against a -land area, it is locally crushed under mighty pressures and forced -up into lines of <i>hummocks</i>—the so-called <i>pressure ridges</i>. At -other times, when the winds of widely separated areas blow away -from each other, the pack is parted, with the formation of lanes or -<i>leads</i> of open water.</p> - -<p>If seen in bird’s-eye view the lines of hummocks would according<span class="pagenum"><a name="Page_287" id="Page_287">[287]</a></span> -to Nansen be arranged like the meshes of a net having roughly -squared angles and reaching to heights of 15 to 25, rarely 30, feet -above the general surface of the pack. The ice within each mesh of -the network is a <i>floe</i>, which at the times of pressure is ground against -its neighbors and variously shifted in position. At the margin of -the pack these floes become separated and float toward lower latitudes -until they are melted.</p> - - -<p><b>The drift of the pack.</b>—The discovery of the drift in the Arctic -pack is a romantic chapter in the history of polar exploration, and -has furnished an example of faith in scientific reasoning and judgment -which may well be compared with that of Columbus. The -great figure in this later discovery is the Norwegian explorer -Fridtjof Nansen, and to the final achievement the ill-fated <i>Jeannette</i> -expedition contributed an important part.</p> - -<p>The <i>Jeannette</i> carrying the American exploring expedition -was in 1879 caught in the pack to the northward of Wrangel Island -(<a href="#f315">Fig. 315</a>), and two years later was crushed by the ice and sunk to -the northward of the New Siberian Islands. In 1884 various -articles, including a list of stores in the handwriting of the commander -of the <i>Jeannette</i>, were picked up at Julianehaab near the -southern extremity of Greenland but upon the western side of -Cape Farewell. Nansen, having carefully verified the facts, -concluded that the recovered articles could have found their way -to Julianehaab only by drifting in the pack across the polar sea, -and that at the longest only five years had been consumed in the -transit. After being separated from the pack the articles must -have floated in the current which makes southward along the east -coast of Greenland and after doubling Cape Farewell flows northward -upon the west coast. It was clear that if they had come -through Smith Sound they would inevitably have been found -upon the other shore of Baffin Bay. In confirmation of this view -there was found at Godthaab, a short distance to the northward -of Julianehaab (<a href="#f315">Fig. 315</a>), an ornamented Alaskan “throwing -stick” which probably came by the same route. Moreover, -large quantities of driftwood reach the shores of Greenland which -have clearly come from the Siberian coast, since the Siberian -larch has furnished the larger quantity.</p> - -<p><span class="pagenum"><a name="Page_288" id="Page_288">[288]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-353.jpg" width="400" height="616" id="f315" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 315.</span>—Map of the north polar regions, showing the area of drift ice and the -tracks of the <i>Jeannette</i> and the <i>Fram</i> (compiled from various maps).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_289" id="Page_289">[289]</a></span></p> - -<p>Pinning his faith to these indubitable facts, Nansen built the -<i>Fram</i> in such a manner as to resist and elude the enormous pressures -of the ice pack, stocked her with provisions sufficient for -five years, and by allowing the vessel to be frozen into the pack -north of the New Siberian Islands, he consigned himself and -his companions to the mercy of the elements. The world knows -the result as one of the most remarkable achievements in -the long history of polar exploration. The track of the <i>Fram</i>, -charted in <a href="#f315">Fig. 315</a>, considered in connection with that of the -<i>Jeannette</i>, shows that the Arctic pack drifts from Bering Sea westward -until near the northeastern coast of Greenland.</p> - -<p>Special casks were for experimental purposes fastened in the -ice to the north of Behring Strait by Melville and Bryant, and two -of these were afterwards recovered, the one near the North Cape -in northern Norway, and the other in northeastern Iceland (see -map, <a href="#f315">Fig. 315</a>). Peary’s trips northward in 1906 and 1909 from -the vicinity of Smith Sound have indicated that between the Pole -and the shores of Greenland and Grant Land the drift is throughout -to the eastward, corresponding to the westerly wind. Upon -this border the great area of Arctic drift ice is in contact with -great continental glaciers bordered by a glacier fringe. Admiral -Peary has shown that instead of consisting of frozen sea ice, the -pack is here made up of great floes from 20 to 100 feet in thickness -and that these have been derived from the glacier fringe.</p> - -<p>Whenever the blizzards blow off the inland ice from the south, -leads are opened at the margin of the fringe and may carry strips -from the latter northward across the lead. With favorable conditions -these leads may be closed by thick sea ice so that with -the occurrence of counter winds from the north they do not entirely -return to their original position. A continuance of this process -may have resulted in the heavy floe ice to the northward of Greenland, -which, acting as an obstruction, may have forced the thinner -drift ice to keep on the European side of the Arctic pack.</p> - -<div class="figcenter"> - <img src="images/ill-355a.jpg" width="400" height="221" id="f316" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 316.</span>—The shelf ice of Coats Land with the surrounding pack ice showing -in the foreground (after Bruce).</p> -</div></div> - -<p>About the Antarctic continent there is a broad girdle of pack -ice which, while more indolent in its movements than the Arctic -pack, has been shown by the expeditions of the <i>Belgica</i> and the -<i>Pourquoi-Pas</i> to possess the same kind of shifting movements. -In the southern spring this pack floats northward and is to a large -extent broken up and melted on reaching lower latitudes.</p> - -<div class="floatleft"> - <img src="images/ill-355b.jpg" width="250" height="129" id="f317" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 317.</span>—Tidewater cliff at the front of a glacier -tongue from which icebergs are born.</p> -</div></div> - -<p><b>The Antarctic shelf ice.</b>—It has been already pointed out -that the inland ice of Antarctica is in part at least surrounded by<span class="pagenum"><a name="Page_290" id="Page_290">[290]</a></span> -a thick snow and ice terrace floating upon the sea and rising to -heights of more than 150 feet above it (<a href="#p15b">plate 15 B</a> and <a href="#f316">Fig. 316</a>). -The visible portions of this shelf-ice are of stratified compact -snow, and the areas which have thus far been studied are found -in bays from which dislodgment is less easily effected. The origin -of the shelf ice is believed to be a sea-ice which because not easily -detached at the time of the spring “break-up” is thickened in -succeeding seasons chiefly by the deposition of precipitated and -drifted snow upon its -surface, so that it is -bowed down under -the weight and sunk -to greater and greater -depths in the water. -To some extent, also, -it is fed upon its inner -margin by overflow -of glacier ice from -the inland ice masses.</p> - -<p><b>Icebergs and snowbergs and the manner of their birth.</b>—Greenland -reveals in the character of its valleys the marks of a large -subsidence of the continent—the serpentine inlets or fjords by -which its coast is so deeply indented. Into the heads of these fjords -the tongues from the inland ice descend generally to the sea level -and below. The glacier ice is thus directly attacked by the waves -as well as melted in the water, so that it terminates in the fjords -in great cliffs of ice (<a href="#f317">Fig. 317</a>). It is also believed to extend -beneath the water surface as -a long toe resting upon the -bottom (<a href="#f319">Fig. 319</a>).</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 15.</span></p> - -<div class="figcenter"> - <img src="images/ill-356a.jpg" width="400" height="188" id="p15a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> An Antarctic ice foot with boat party landing (after R. F. Scott).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-356b.jpg" width="400" height="320" id="p15b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> A near view of the front of the Great Ross Barrier, Antarctica (after R. F. Scott).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_291" id="Page_291">[291]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-358a.jpg" width="200" height="89" id="f318" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 318.</span>—A Greenlandic iceberg after a -long journey in warm latitudes.</p> -</div></div> - -<p>The exposed cliff is notched -and undercut by the waves in -the same manner as a rock cliff, -and the upper portions override -the lower so that at frequent intervals -small masses of ice from -this front separate on crevasses, and toppling over, fall into the -water with picturesque splashes. Such small bergs, whose birth -may be often seen at the cliff front of both the Greenland and -Alaskan glaciers, have little in common with those great floating -islands of ice that are drifted by the winds until, wasted to a fraction -only of their former proportions, they reach the lanes of transatlantic -travel and become a serious menace to navigation (<a href="#f318">Fig. 318</a>).</p> - -<div class="figcenter"> - <img src="images/ill-358b.jpg" width="400" height="123" id="f319" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 319.</span>—Diagram showing one way in which northern icebergs may be born -from the glacier tongue (after Russell).</p> -</div></div> - -<p>Northern icebergs of large dimensions are born either by the lifting -of a separated portion of the extended glacier toe lying upon the -bottom of the fjord, or else they separate bodily from the cliff -itself, apparently where it reaches water sufficiently deep to float it. -In either case the buoyancy of the sea water plays a large rôle in its -separation.</p> - -<p>If derived from the submerged glacier toe (<a href="#f319">Fig. 319</a>), a loud noise -is heard before any change is visible, and an instant later the great<span class="pagenum"><a name="Page_292" id="Page_292">[292]</a></span> -mass of ice rises out of the water some distance away from the -cliff, lifting as it does so a great volume of water which pours off on -all sides in thundering cascades and exposes at last a berg of the -deepest sapphire blue. The commotion produced in the fjord is -prodigious, and a vessel in close proximity is placed in jeopardy.</p> - -<p>Even larger bergs are sometimes seen to separate from the ice -cliff, in this case an instant before or simultaneously, with a loud -report, but such bergs float away with comparatively little commotion -in the water.</p> - -<div class="figcenter"> - <img src="images/ill-359.jpg" width="400" height="246" id="f320" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 320.</span>—A northern iceberg surrounded by sea ice.</p> -</div></div> - -<p>The icebergs of the south polar region are usually built upon a -far grander scale than those of the Arctic regions, and are, further, -both distinctly tabular in form and bounded by rectangular outlines -(<a href="#f321">Fig. 321</a>). Whereas the large bergs of Greenlandic origin -are of ice and blue in color, the tabular bergs of Antarctica might -better be described as <i>snowbergs</i>, since they are of a blinding whiteness -and their visible portions are either compacted snow or alternating -thick layers of compact snow and thin ribbons of blue ice, -the latter thicker and more abundant toward the base. All such -bergs have been derived from the shelf ice and not from the inland -ice itself. Blue icebergs which have been derived from the inland -ice have been described from the one Antarctic land that has been -explored in which that ice descends directly to the sea.</p> - -<p><span class="pagenum"><a name="Page_293" id="Page_293">[293]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-360.jpg" width="400" height="216" id="f321" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 321.</span>—Tabular Antarctic iceberg separating from the -shelf ice (after Shackleton).</p> -</div></div> - -<p>In both the northern and southern hemispheres those bergs -which have floated into lower latitudes have suffered profound -transformations. Their exposed surfaces have been melted in the -sun, washed by the rain, and battered by the waves, so that they -lose their relatively simple forms but acquire rounded surfaces in -place of the early angular ones (<a href="#f318">Fig. 318</a>, <a href="#Page_291">p.291</a>). Sir John Murray, -who had such extended opportunities of studying the southern icebergs -from the deck of the <i>Challenger</i>, has thus described their -beauties:</p> - -<p>“Waves dash, against the vertical faces of the floating ice island as -against a rocky shore, so that at the sea level they are first cut into ledges -and gullies, and then into caves and caverns of the most heavenly blue, -from out of which there comes the resounding roar of the ocean, and into -which the snow-white and other petrels may be seen to wing their way -through guards of soldier-like penguins stationed at the entrances. As -these ice islands are slowly drifted by wind and current to the north, they -tilt, turn and sometimes capsize, and then submerged prongs and spits are -thrown high into the air, producing irregular pinnacled bergs higher, possibly, -than the original table-shaped mass.”</p> - -<p class="prr"><span class="smcap">Reading References for Chapters XX and XXI</span></p> - -<p class="p1">General:—</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Glaciers of North America. Ginn, Boston, 1897, pp. -210, pls. 22.</p> - -<p class="pex"><span class="smcap">Chamberlin</span> and <span class="smcap">Salisbury</span>. Geology, vol. 1, pp. 232-308.</p> - -<p><span class="pagenum"><a name="Page_294" id="Page_294">[294]</a></span></p> - -<p class="pex"><span class="smcap">H. Hess.</span> Die Gletscher, Braunschweig, 1904, pp. 426 (illustrated).</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Characteristics of Existing Glaciers. Macmillan, -1911, pp. 301, pls. 34.</p> - -<p class="p1">Special districts of mountain glaciers:—</p> - -<p class="pex"><span class="smcap">James D. Forbes.</span> Travels Through the Alps of Savoy and other Parts -of the Pennine Chain with Observations on the Phenomena of Glaciers. -Edinburgh, 1845, pp. 456, pls. 9, maps 2.</p> - -<p class="pex"><span class="smcap">A. Penck</span>, <span class="smcap">E. Brückner</span>, et <span class="smcap">L. du Pasquier</span>. Le système glaciare des -alpes, etc., Bull. Soc. Sc. Nat. Neuchâtel, vol. 22, 1894, pp. 86.</p> - -<p class="pex"><span class="smcap">E. Richter.</span> Die Gletscher der Ostalpen. Stuttgart, 1888, pp. 306, -7 maps.</p> - -<p class="pex"><span class="smcap">James D. Forbes.</span> Norway and Its Glaciers, etc. Edinburgh, 1853, pp. -349, pls. 10, map.</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Existing Glaciers of the United States, 5th Ann. Rept. -U. S. Geol. Surv., 1885, pp. 307-355, pls. 32-55; Glaciers of Mt. -Ranier, 18th Ann. Rept. U. S. Geol. Surv., 1898, pp. 349-423, pls. -65-82.</p> - -<p class="pex"><span class="smcap">W. H. Sherzer.</span> Glaciers of the Canadian Rockies and Selkirks, Smith. -Cont. to Knowl. No. 1692, Washington, 1907, pp. 135, pls. 42.</p> - -<p class="pex"><span class="smcap">H. F. Reid.</span> Studies of Muir Glacier, Alaska, Nat. Geogr. Mag., vol. 4, -1892, pp. 19-84, pls. 1-16.</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Malaspina Glacier, Jour. Geol., vol. 1, 1893, pp. 219-245.</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Harriman Alaska Expedition, vol. 3, Glaciers, 1904, -pp. 231, pls. 37.</p> - -<p class="pex"><span class="smcap">W. M. Conway.</span> Climbing and Exploration in the Karakoram Himalayas, -Maps and Scientific Reports, 1894, map sheets I-II.</p> - -<p class="pex"><span class="smcap">Fanny Bullock Workman</span> and <span class="smcap">William Hunter Workman</span>. The Hispar -Glacier, Geogr. Jour., vol. 35, 1910, pp. 105-132, 7 pls. and map.</p> - -<p class="p1">The cycle of glaciation:—</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Cycle of Mountain Glaciation, Geogr. Jour., -vol. 36, 1910, pp. 146-163, 268-284.</p> - -<p class="p1">Upper and lower cloud zones of the atmosphere:—</p> - -<p class="pex"><span class="smcap">R. Assmann</span>, <span class="smcap">A. Berson</span>, and <span class="smcap">H. Gross</span>. Wissenschaftliche Luftfahrten -ausgeführt vom deutschen Verein zur Förderung der Luftschiffahrt -in Berlin, 1899-1900, 3 vols.</p> - -<p class="pex"><span class="smcap">E. Gold</span> and <span class="smcap">W. A. Harwood</span>. The Present State of our Knowledge of -the Upper Atmosphere as Obtained by the Use of Kites, Balloons, -and Pilot-balloons, Rept. Brit. Assoc. Adv. Sci., 1909, pp. 1-55.</p> - -<p class="pex"><span class="smcap">W. H. Moore.</span> Descriptive Meteorology, Appleton, New York, 1910, -pp. 95-136.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Pleistocene Glaciation of North America -Viewed in the Light of our Knowledge of Existing Continental -Glaciers, Bull. Am. Geogr. Soc., vol. 42, 1911, pp. 647-650.</p> - -<p><span class="pagenum"><a name="Page_295" id="Page_295">[295]</a></span></p> - -<p class="p1">The continental glacier of Greenland:—</p> - -<p class="pex"><span class="smcap">F. Nansen.</span> The First Crossing of Greenland, 2 vols, Longmans, London, -1890 (the scientific results are contained in an appendix to -volume 2, pp. 443-497).</p> - -<p class="pex"><span class="smcap">R. E. Peary.</span> A Reconnaissance of the Greenland Inland Ice, Jour. Am. -Geogr. Soc., vol. 19, 1887, pp. 261-289; Journeys in North Greenland, -Geogr. Jour., vol. 11, 1898, pp. 213-240.</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin.</span> Glacier Studies in Greenland, Jour. Geol., vol. 2, -1894, pp. 649-668, 768-788, vol. 3, pp. 61-69, 198-218, 469-480, 565-582, -668-681, 833-843, vol. 4, pp. 582-592, 769-810, vol. 5, pp. 229-245; -Recent glacial studies in Greenland (Presidential address), -Bull. Geol. Soc. Am., vol. 6, 1895, pp. 199-220, pls. 3-10.</p> - -<p class="pex"><span class="smcap">R. S. Tarr.</span> The Margin of the Cornell Glacier, Am. Geol., vol. 20, 1897, -pp. 139-156, pls. 6-12.</p> - -<p class="pex"><span class="smcap">R. D. Salisbury.</span> The Greenland Expedition of 1895, Jour. Geol., vol. 3, -1895, pp. 875-902.</p> - -<p class="pex"><span class="smcap">E. v. Drygalski.</span> Grönland Expedition der Gesellschaft für Erdkunde zu -Berlin 1891-1893, Berlin, 1897, 2 vols., pp. 551 and 571, pls. 53, -maps 10.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Characteristics of the Inland Ice of the Arctic -Regions, Proc. Am. Phil. Soc., vol. 49, 1910, pp. 57-129, pls. 26-30.</p> - -<p class="p1">The Antarctic continental glacier:—</p> - -<p class="pex"><span class="smcap">R. F. Scott.</span> The Voyage of the <i>Discovery</i>. London, 2 vols., 1905.</p> - -<p class="pex"><span class="smcap">E. H. Shackleton.</span> The Heart of the Antarctic. London, 2 vols., 1910.</p> - -<p class="pex"><span class="smcap">E. von Drygalski.</span> Zum Kontinent des eisigen Südens, Deutsche Südpolar-Expedition, -Fahrten und Forschungen des “Gauss”, 1901-1903, -Berlin, 1904, pp. 668, pls. 21.</p> - -<p class="pex"><span class="smcap">Otto Nordenskiöld</span> and <span class="smcap">J. S. Andersson</span>. Antarctica or Two Years -Amongst the Ice of the South Pole. London, 1905, pp. 608, illustrated.</p> - -<p class="pex"><span class="smcap">E. Philippi.</span> Ueber die fünf Landeis-Expeditionen, etc., Zeit. f. Gletscherk., -vol. 2, 1907, pp. 1-21.</p> - -<p class="p1">Nourishment of continental glaciers:—</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Characteristics of the Inland Ice of the Arctic -Regions, Proc. Am. Phil. Soc., vol. 49, 1910, pp. 96-110; The Ice -Masses on and about the Antarctic Continent, Zeit. f. Gletscherk., -vol. 5, 1910, pp. 107-120; Characteristics of Existing Glaciers. New -York, 1911, pp. 143-161, 261-289. Pleistocene Glaciation of North -America Viewed in the Light of our Knowledge of Existing Continental -Glaciers, Bull. Am. Geogr. Soc., vol. 43, 1911, pp. 641-659.</p> - -<p class="p1">Field and pack ice:—</p> - -<p class="pex"><span class="smcap">Emma de Long.</span> The Voyage of the <i>Jeannette</i>, the ship and ice journals -of George W. de Long, etc. Berlin, 1884, 2 vols., chart in back of -vol. 1.</p> - -<p><span class="pagenum"><a name="Page_296" id="Page_296">[296]</a></span></p> - -<p class="pex"><span class="smcap">Robert E. Peary.</span> The Discovery of the North Pole (for further references -on both sea and pack ice and Antarctic shelf ice, consult Hobbs’s -Characteristics of Existing Glaciers, pp. 210-213, 242-244).</p> - -<p class="p1">Icebergs:—</p> - -<p class="pex"><span class="smcap">Wyville Thomson.</span> Challenger Report, Narrative, vol. 1, 1865, Pt. i, -pp. 431-432, pls. B-D.</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> An Expedition to Mt. St. Elias, Nat. Geogr. Mag., vol. 3, -1891, pp. 101-102, fig. 1.</p> - -<p class="pex"><span class="smcap">H. F. Reid.</span> Studies of Muir Glacier, Alaska, <i>ibid.</i>, vol. 4, 1892, pp. 47-48.</p> - -<p class="pex"><span class="smcap">E. von Drygalski.</span> Grönland-Expedition, etc., vol. 1, pp. 367-404.</p> - -<p class="pex"><span class="smcap">M. C. Engell.</span> Ueber die Entstehung der Eisberge, Zeit. f. Gletscherk., -vol. 5, 1910, pp. 112-132.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_297" id="Page_297">[297]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXII</h2> - -<p class="pch">THE CONTINENTAL GLACIERS OF THE “ICE AGE”</p> - -<p><b>Earlier cycles of glaciation.</b>—Our study of the rocks composing -the outermost shell of the lithosphere tells us that in at least -three widely separated periods of its history the earth has passed -through cycles of glaciation during which considerable portions -of its surface have been submerged beneath continental glaciers. -The latest of these occurred in the yesterday of geology and has -often been referred to as the “ice age”, because until quite recently -it was supposed to be the only one of which a record was -preserved.</p> - -<div class="figcenter"> - <img src="images/ill-364.jpg" width="400" height="208" id="f322" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 322.</span>—Map of the globe showing the areas which were covered by the continental -glaciers of the so-called “ice-age” of the Pleistocene period. The arrows -show the directions of the centrifugal air currents in the fixed anticyclones above the -glaciers.</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-365a.jpg" width="230" height="119" id="f323" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 323.</span>—Glaciated granite bowlder -which has weathered out of a moraine -of Permo-Carboniferous age upon which -it rests. South Australia (after Howchin).</p> -</div></div> - -<p>This latest ice age represents four complete cycles of glaciation, -for it is believed that the continental ice developed and then -completely disappeared during a period of mild climate before the -next glacier had formed in its place, and that this alternation of -climates was no less than three times repeated, making four cycles -in all. At nearly or quite the same time ice masses developed in<span class="pagenum"><a name="Page_298" id="Page_298">[298]</a></span> -northern North America and -in northern Europe, the embossments -of the ice domes -being located in Canada and -in Scandinavia respectively -(<a href="#f322">Fig. 322</a>). There appears to -have been at this time no extensive -glaciation of the southern -hemisphere, though in the -next earlier of the known great -periods of glaciation—the so-called -Permo-Carboniferous—it was the southern hemisphere, and -not the northern, that was affected (<a href="#f323">Fig. 323</a> and <a href="#f304">Fig. 304</a>, <a href="#Page_276">p.276</a>). -From the still earlier glacial period our data are naturally much -more meager, but it seems probable that it was characterized by -glaciated areas within both the northern and the southern hemispheres.</p> - -<div class="figcenter"> - <img src="images/ill-365b.jpg" width="350" height="460" id="f324" - alt="" - title="" /> - <div class="caption"><p class="pc350"><span class="smcap">Fig. 324.</span>—Map to show the glaciated and nonglaciated regions of North America -(after Salisbury and Atwood).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_299" id="Page_299">[299]</a></span></p> - - -<p><b>Contrast of the glaciated and nonglaciated regions.</b>—Since -we have now studied in brief outline the characteristics of the existing -continental glaciers, we are in a position to review the evidences -of former glaciers, the records of which exist in their carvings, their -gravings, and their deposits.</p> - -<div class="figcenter"> - <img src="images/ill-366.jpg" width="400" height="296" id="f325" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 325.</span>—Map of the glaciated and nonglaciated areas of northern Europe. The -strongly marked morainal belts respectively south and north of the Baltic depression -represent halting places in the retreat of the latest continental glacier (compiled -from maps by Penck and Leverett).</p> -</div></div> - -<p>An observant person familiar with the aspects of Nature in both -the northern and southern portions of the central and eastern -United States must have noticed that the general courses of the -Ohio and Missouri rivers define a somewhat marked common border -of areas which in most respects are sharply contrasted (<a href="#f324">Fig. 324</a>). -Hardly less striking is the contrast between the glaciated and the -nonglaciated regions upon the continent of Europe (<a href="#f325">Fig. 325</a>).</p> - -<p>It is the northern of the two areas which in each case reveals the -characteristic evidences of glaciation, while there is entire absence<span class="pagenum"><a name="Page_300" id="Page_300">[300]</a></span> -of such marks to the southward of the common border. Within -the American glaciated region there is, however, an area surrounded -like an island, and within this district (<a href="#f324">Fig. 324</a>) none of the marks -characteristic of glaciation are to be found. This area is usually -referred to as the “driftless area”, and occupies portions of the -states of Wisconsin, Illinois, Minnesota, and Iowa. Even better -than the area to the southward of the Ohio and Missouri rivers, it -permits of a comparison of the nonglaciated with the drift-covered -region.</p> - -<div class="floatleft"> - <img src="images/ill-367.jpg" width="250" height="291" id="f326" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 326.</span>—“Stand Rock” near the “Dells” of the -Wisconsin river, an unstable erosion remnant characteristic -of the driftless area of North America -(after Salisbury and Atwood).</p> -</div></div> - -<p><b>The “driftless area.”</b>—Within this district, then, we have -preserved for our study a landscape which remains largely as it was -before the several ice -invasions had so profoundly -transformed the -general surface of the -surrounding country. -Speaking broadly, we -may say that it represents -an uplifted and -in part dissected plain, -which to the south and -east particularly reveals -the character of nearly -mature river erosion -(<a href="#f177">Fig. 177</a>, <a href="#Page_170">p. 170</a>). The -rock surface is here -everywhere mantled by -decomposed and disintegrated -rock residues -of local origin. The -soluble constituents of -the rock, such as the -carbonates, have been -removed by the process of leaching, so that the clays no longer -effervesce when treated with dilute mineral acid.</p> - -<p>Wherever favored by joints and by an alternation of harder -and softer rock layers, picturesque unstable erosion remnants or -“chimneys” may stand out in relief (<a href="#f326">Fig. 326</a>). Furthermore, the -driftless area is throughout perfectly drained—it is without lakes -or swamps—since all valleys are characterized throughout by -forward grades. The side valleys enter the main valleys as do the -branches a tree trunk; in other words, the drainage is described as -arborescent. In so far as any portions of a plane surface now remain -in the landscape, they are found at the highest levels (<a href="#p16a">plate 16 A</a>). -The topography is thus the result of a partial removal by erosion -of an upland and may be described as <i>incised topography</i>. Nowhere -within the area are there found rock masses foreign to the region, -but all mantle rock is the weathered product of the underlying -ledges.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 16.</span></p> - -<div class="figcenter"> - <img src="images/ill-368a.jpg" width="400" height="309" id="p16a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Incised topography within the “driftless area” (U. S. Geol. Survey).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-368b.jpg" width="400" height="308" id="p16b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Built-up topography within glaciated region (U. S. Geol. Survey).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_301" id="Page_301">[301]</a></span></p> - -<p><b>Characteristics of the glaciated regions.</b>—The topography of -the driftless area has been described as <i>incised</i>, because due to the -partial destruction of an uplifted plain; and this surface is, moreover, -perfectly drained. The -characteristic topography of the -“drift” areas is by contrast <i>built -up</i>; that is to say, the features of -the region instead of being <i>carved</i> -out of a plain are the result of -<i>molding</i> by the process of deposition -(plate 16 B). In so far as a -plane is recognizable, it is to be -found not at the highest, but at -the lowest level—a surface represented largely by swamps and -lakes—and above this plain rise the characteristic rounded hills -of various types which have been <i>built up</i> through deposition. The -process by which this has been accomplished is one easy to comprehend. -As it invaded the region, the glacier planed away beneath -its marginal zone all weathered mantle rock and deposited the -planings within the hollows of the surface (<a href="#f327">Fig. 327</a>). The -effect has been to flatten out the preëxisting irregularities of the -surface, and to yield at first a gently undulating plain upon which -are many undrained areas and a haphazard system of drainage -(<a href="#f328">Fig. 328</a>). All unstable erosion remnants, such as now are to be -found within the driftless area, were the first to be toppled over by -the invading glacier, and in their place there is left at best only -rounded and polished “shoulders” of hard and unweathered rock—the -well-known <i>roches moutonnées</i>.</p> - -<div class="floatleft"> - <img src="images/ill-370.jpg" width="200" height="86" id="f327" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 327.</span>—Diagram showing the manner -in which a continental glacier obliterates -existing valleys (after Tarr).</p> -</div></div> - -<p><b>The glacier gravings.</b>—The tools with which the glacier works<span class="pagenum"><a name="Page_302" id="Page_302">[302]</a></span> -are never quite evenly edged, and instead of an in all respects -perfect polish upon the rock pavement, there are left furrowings, -gougings, and scratches. Of whatever sort, these scorings indicate -the lines of ice movement and are thus indubitable records -graven upon the rock floor. When mapped over wide areas, a -most interesting picture is presented to our view, and one which -supplements in an important way the studies of existing continental -glaciers (<a href="#f334">Fig. 334</a>, <a href="#Page_308">p.308</a>, and <a href="#f336">Fig. 336</a>, <a href="#Page_312">p. 312</a>).</p> - -<div class="figcenter"> - <img src="images/ill-371.jpg" width="400" height="253" id="f328" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 328.</span>—Lake and marsh district in northern Wisconsin, the effect of glacial -deposition in former valleys (after Fairbanks).</p> -</div></div> - -<p>It has been customary to think of the glacier as everywhere -eroding its bed, although the only warrant for assuming degradation -by flow of the ice is restricted to the marginal zone, since -here only is there an appreciable surface grade likely to induce -flow. Both upon the advance and again during the retreat of a -glacier, all parts of the area overridden must be subjected to this -action. Heretofore pictured in the imagination as enlarged -models of Alpine glaciers, the vast ice mantles were conceived to -have spread out over the country as the result of a kind of viscous -flow like that of molasses poured upon a flat surface in cold -weather. The maximum thickness of the latest American glacier -of the ice age has been assumed to have been perhaps 10,000 feet -near the summit of its dome in central Labrador. From this<span class="pagenum"><a name="Page_303" id="Page_303">[303]</a></span> -point it was assumed that the ice traveled southward up the -northern slope of the Laurentian divide in Canada, and thence -to the Ohio river, a distance of over 1300 miles. If such a mantle -of ice be represented in its natural proportions in vertical section, -to cover the distance from center to margin we may use a line -six inches in length, and only <sup>1</sup>/<sub>100</sub> of an inch thick. Upon a reduced -scale these proportions are given in <a href="#f329">Fig. 329</a>. Obviously the -force of gravity acting within a viscous mass of such proportions -would be incompetent to effect a transfer of material from the -center to the periphery, even though the thickness should be -doubled or trebled. Yet until the fixed glacial anticyclone above -the glacier had been proven and its efficiency as a broom recognized, -no other hypothesis than that of viscous flow had been -offered in explanation. The inherited conception of a universal -plucking and abrasion on the bed of the glacier is thus made untenable -and can be accepted for the marginal portion only.</p> - -<div class="figcenter"> - <img src="images/ill-372.jpg" width="450" height="20" id="f329" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 329.</span>—Cross section in approximate natural proportions of the latest North -American continental glacier of Pleistocene age from its center to its margin.</p> -</div></div> - -<p>Not only do the rock scorings show the lines of ice movement, -but the directions as well may often be read upon the rock. Wherever -there are pronounced irregularities of surface still existing on -the pavement, these are generally found to have gradual slopes -upon the side from which the ice came, and relatively steep falls -upon the lee or “pluck” side. If, however, we consider the irregularities -of smaller size, the unsymmetrical slopes of these protruding -portions of the floor are found to be reversed—it is the steep slope -which faces the oncoming ice and the flatter slope which is upon the -lee side. Such minor projections upon the floor usually have their -origin in some harder nodule which deflects the abrading tools and -causes them to pass, some on the one side and some upon the other. -By this process a staple-shaped groove comes to surround the -nodule, leaving an unsymmetrical elevated ridge within, which is -steep upon the stoss side and slopes gently away to leeward.</p> - -<div class="floatleft"> - <img src="images/ill-373.jpg" width="250" height="218" id="f330" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 330.</span>—Limestone surface at Sibley, Michigan.</p> -</div></div> - -<p><b>Younger records over older—the glacier palimpsest.</b>—Many -important historical facts have been recovered from the largely -effaced writing upon ancient palimpsests, or parchments upon -which an earlier record has been intentionally erased to make room<span class="pagenum"><a name="Page_304" id="Page_304">[304]</a></span> -for another. In the gravings upon the glacier pavement, earlier -records have been likewise in large part effaced by later, though in -favorable localities the two may be read together. Thus, as an -example, at the great limestone quarries of Sibley, in southeastern -Michigan, the glaciated rock surface wherever stripped of -its drift cover is a smoothly polished and relatively level floor -with striæ which are directed west-northwest. Beneath this general -surface there are, however, a number of elliptical depressions -which have their longer axes directed south-southwest, one -being from twenty-five to thirty feet long and some ten feet in -depth (<a href="#f330">Fig. 330</a>). These boat-shaped depressions are clearly the -remnants of an earlier -more undulating surface -which the latest -glacier has in large -part planed away, -since the bottoms of -the depressions are no -less perfectly glaciated -but have their striæ -directed in general -near the longer axis of -the troughs. Palimpsest-like -there are -here also the records -of more than one -graving.</p> - -<div class="floatright"> - <img src="images/ill-374.jpg" width="250" height="231" id="f331" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 331.</span>—Map to show the outcroppings of peculiar rock -types in the region of the Great Lakes, and some of the -localities where “float copper” has been collected (float -copper localities after Salisbury).</p> -</div></div> - -<p><b>The dispersion of the drift.</b>—Long before the “ice age” had -been conceived in the minds of Agassiz and his contemporaries, -it had been remarked that scattered over the North German plain -were rounded fragments of rock which could not possibly have been -derived from their own neighborhood but which could be matched -with the great masses of red granite in Sweden well known as the -“Swedish granite.” Buckland, an English geologist, had in 1815 -accounted for such “erratic” blocks of his own country, here of -Scotch granite, by calling in the deluge of Noah; but in the late -thirties of the nineteenth century, Sir Charles Lyell, with the results -of English Arctic explorers in mind, claimed that such traveled -blocks had been transported by icebergs emanating from the polar<span class="pagenum"><a name="Page_305" id="Page_305">[305]</a></span> -regions. A relic of Buckland’s earlier view we have in the word -“diluvium” still occasionally used in Germany for glacier transported -materials; while the term “drift” still remains in common -use to recall Lyell’s iceberg hypothesis, even though the original -meaning of the term has been abandoned. Drift is now a generic -term and refers to all deposits directly or indirectly referable to the -continental glaciers.</p> - -<p>In general the place of derivation of the glacial drift may be said -to be some point more distant from and within the former ice margin -at the time -when it was deposited; -in other -words, the dispersion -of the -drift was centrifugal -with reference -to the -glacier.</p> - -<div class="floatleft"> - <img src="images/ill-375.jpg" width="200" height="447" id="f332" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 332.</span>—Map of the “bowlder -train” from Iron Hill, R. I. -(based upon Shaler’s map, but -with the directions of glacial -striæ added).</p> -</div></div> - -<p>Wherever -rocks of unusual -and therefore -easily recognizable -character -can be shown to -occur in place -and with but limited -areas, the -dispersion of -such material is -easy to trace. -The areas of red Swedish and Scotch granite have been used to -follow out in a broad way the dispersion of drift over northern -Europe. Within the region of the Great Lakes of North America -are areas of limited size which are occupied by well marked rock -types, so that the journeyings of their fragments with the continental -glacier can be mapped with some care. Upon the northern -shore of Georgian Bay occurs the beautiful jasper conglomerate, -whose bright red pebbles in their white quartz field attract such -general notice. At Ishpeming in the northern peninsula of Michigan<span class="pagenum"><a name="Page_306" id="Page_306">[306]</a></span> -is found the equally beautiful jaspilite composed of puckered -alternating layers of black hematite and red jasper. On Keweenaw -Peninsula, which protrudes into Lake Superior from its southern -shore, is found that remarkable occurrence of native copper within -a series of igneous rocks of varied types and colors. Fragments -of this copper, some weighing several -hundreds of pounds each and masked -in a coat of green malachite, have under -the name of “drift” or “float” copper -been collected at many localities within -a broad “fan” of dispersal extending -almost to the very limits of glaciation -(<a href="#f331">Fig. 331</a>).</p> - -<p>Some miles to the north of Providence -in Rhode Island there is a hill -known as Iron Hill composed in large -part of black magnetite rock, the so-called -Cumberlandite. From this hill -as an apex there has been dispersed a -great quantity of the rock distributed -as a well marked “bowlder train” -within which the size and the frequency -of the dispersed bowlders is in -inverse ratio to the distance from the -parent ledge (<a href="#f332">Fig. 332</a>). Similar -though less perfect trains of bowlders -are found on the lee side of most projecting -masses of resistant rocks within -the area of the drift.</p> - -<p>Large bowlders when left upon a -ledge of notably different appearance -easily attract attention, and have been -described as “perched bowlders.” Resting as they sometimes do -upon a relatively small area, they may be nicely balanced and -thus easily given a pendular or rocking motion. Such “rocking -stones” are common enough, especially among the New England -hills (<a href="#p17b">plate 17 B</a>). Many such bowlders have made somewhat -remarkable peregrinations with many interruptions, having been -carried first in one direction by an earlier glacier to be later transported -in wholly different directions at the time of new ice invasions.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 17.</span></p> - -<div class="figcenter"> - <img src="images/ill-376a.jpg" width="400" height="174" id="p17a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Soled glacial bowlders which show differently directed striæ upon the same facet.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-376b.jpg" width="400" height="287" id="p17b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Perched bowlder upon a striated ledge of different rock type, Bronx Park, New -York (after Lungstedt).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-376c.jpg" width="400" height="147" id="p17c" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>C.</i> Characteristic knob and basin surface of a moraine.</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_307" id="Page_307">[307]</a></span></p> - -<p><b>The diamonds of the drift.</b>—Of considerable popular, even if -not economic, interest are the diamonds which have been sown -in the drift after long and interrupted journeyings with the ice -from some unknown home far to the northward in the wilderness -of Canada. The first stone to be discovered was taken by workmen -from a well opening near the little town of Eagle in Wisconsin -in the year 1876. Its nature not being known, it remained where -it was found as a curiosity only, and it was not until 1883 that it -was taken to Milwaukee and sold to a jeweler equally ignorant -of its value, and for the merely nominal sum of one dollar. Later -recognized as a diamond of the unusual weight of sixteen carats, -it was sold to the Tiffanys and became the cause of a long litigation -which did not end until the Supreme Court of Wisconsin had -decided that the Milwaukee jeweler, and not the finder, was entitled -to the price of the stone, since he had been ignorant of its -value at the time of purchase.</p> - -<div class="figcenter"> - <img src="images/ill-378.jpg" width="400" height="106" id="f333" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 333.</span>—Shapes and approximate natural sizes of some of the more important -diamonds from the Great Lakes region of the United States. In order from left -to right these figures represent the Eagle diamond of sixteen carats, the Saukville -diamond of six and one half carats, the Milford diamond of six carats, the Oregon -diamond of four carats, and the Burlington diamond of a little over two carats.</p> -</div></div> - -<p>An even larger diamond, of twenty-one carats weight, was found -at Kohlsville, and smaller ones at Oregon, Saukville, Burlington, -and Plum Creek in the state of Wisconsin; at Dowagiac in Michigan; -at Milford in Ohio, and in Morgan and Brown counties in -Indiana. The appearance of some of the larger stones in their -natural size and shape may be seen in <a href="#f333">Fig. 333</a>.</p> - -<p>While the number of the diamonds sown in the drift is undoubtedly -large, their dispersion is such that it is little likely they -can be profitably recovered. The distribution of the localities at -which stones have thus far been found is set forth upon <a href="#f334">Fig. 334</a>. -Obviously those that have been found are the ones of larger size, -since these only attract attention. In 1893, when the finding of -the Oregon stone drew attention to these denizens of the drift, -the writer prophesied that other stones would occasionally be discovered -under essentially the same conditions, and such discoveries -are certain to continue in the future.</p> - -<p><span class="pagenum"><a name="Page_308" id="Page_308">[308]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-379.jpg" width="400" height="608" id="f334" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 334.</span>—Glacial map of a portion of the Great Lakes region, showing the unglaciated -area and the areas of older and newer drift. The driftless area, the moraines -of the later ice invasion, and the distribution of diamond localities upon -the latter are also shown. With the aid of the directions of striæ some attempt -has been made to indicate the probable tracks of more important diamonds, which -tracks converge in the direction of the Labrador peninsula.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_309" id="Page_309">[309]</a></span></p> - -<p><b>Tabulated comparison of the glaciated and nonglaciated regions.</b>—It -will now be profitable to sum up in parallel columns -the contrasted peculiarities of the glaciated and the unglaciated -regions.</p> - -<table id="t07" summary="t07"> - - <tr> - <td class="tdc"><span class="smcap">Unglaciated Region</span></td> - <td class="tdc"><span class="smcap">Glaciated Region</span></td> - </tr> - - <tr> - <td class="tdc" colspan="2">TOPOGRAPHY</td> -</tr> - - <tr> - <td class="tdt5w">The topography is <i>destructional</i>; -the remnants of a plain are found -at the highest levels or upon the -hill tops; hills are <i>carved</i> -of a high plain; unstable erosion -remnants are characteristic.</td> - <td class="tdt5">The topography is <i>constructional</i>; -the remnants of a plain are found -at the lowest levels in lakes and -swamps; hills are <i>molded</i> above a -plain in characteristic forms; no -unstable erosion remnants, but only -rounded shoulders of rock.</td> - </tr> - - <tr> - <td class="tdc" colspan="2">DRAINAGE</td> -</tr> - - <tr> - <td class="tdt5">The area is completely drained, -and the drainage network is -<i>arborescent</i>.</td> - <td class="tdt5">The area includes undrained -areas,—lakes and swamps,—and -the drainage system is <i>haphazard</i>.</td> - </tr> - - <tr> - <td class="tdc" colspan="2">ROCK MANTLE</td> -</tr> - - <tr> - <td class="tdt5">The exposed rock is decomposed -and disintegrated to a -considerable depth; it is all of -local derivation and hence of few -types—<i>homogeneous</i>; the fragments -are angular; soils are leached and -hence do not contain carbonates.</td> - <td class="tdt5">No decomposed or disintegrated -rock is “in place”, but only -hard, fresh surface; loose rock -material is all foreign and of many -izes and types—<i>heterogeneous</i>; -rock bowlders and pebbles are -faceted and polished as well as -striated, usually in several -directions upon each facet; soils -are rock flour—the grist of the -glacial mill.</td> - </tr> - - <tr> - <td class="tdc" colspan="2">ROCK SURFACE</td> -</tr> - - <tr> - <td class="tdt5">Rock surface is rough and -irregular.</td> - <td class="tdt5">Rock surface is planed or grooved, -and polished. Shows glacial striæ.</td> - </tr> - -</table> - -<p><b>Unassorted and assorted drift.</b>—The drift is of two distinct -types; namely, that deposited directly by the glacier, which is<span class="pagenum"><a name="Page_310" id="Page_310">[310]</a></span> -without stratification, or unassorted; and that deposited by water -flowing either beneath or from the ice, and this like most fluid deposited -material is assorted or stratified. The unassorted material -is described as <i>till</i>, or sometimes as “bowlder clay”; the assorted -is sand or gravel, sometimes with small included bowlders, -and is described as <i>kame gravel</i>. To recall the parts which both -the glacier and the streams have played in its deposition, all water-deposited -materials in connection with glaciers are called <i>fluvio-glacial</i>.</p> - -<div class="floatleft"> - <img src="images/ill-381.jpg" width="250" height="223" id="f335" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 335.</span>—Section in coarse till. Note the -range in size of the materials, the lack of -stratification, and the “soled” form of the -bowlders.</p> -</div></div> - -<p>Till is, then, characterized by a noteworthy lack of homogeneity, -both as regards the size and the composition of its constituent -parts. As many as twenty -different rock types of varied -textures and colors may sometimes -be found in a single -exposure of this material, and -the entire gamut is run from -the finest rock flour upon the -one hand to bowlders whose -diameter may be measured -in feet (<a href="#f335">Fig. 335</a>).</p> - -<p>In contrast with those derived -by ordinary stream -action, the pebbles and -bowlders of the till are faceted -or “soled”, and usually -show striations upon their -faces. If a number of pebbles are examined, some at least are sure -to be found with striations in more than one direction upon a -single facet. As a criterion for the discrimination of the material -this may be an important mark to be made use of to distinguish -in special cases from rock fragments derived by brecciation and -slickensiding and distributed by the torrents of arid and semiarid -regions.</p> - -<p>Inasmuch as the capacity of ice for handling large masses is -greater than that of water, assorted drift is in general less coarse, -and, as its name implies, it is also stratified. From ordinary -stream gravels, the kame gravels are distinguished by the form of -their pebbles, which are generally faceted and in some cases<span class="pagenum"><a name="Page_311" id="Page_311">[311]</a></span> -striated. In proportion, however, as the materials are much -worked over by the water, the angles between pebble faces become -rounded and the original shapes considerably masked.</p> - - -<p><b>Features into which the drift is molded.</b>—Though the preëxisting -valleys were first filled in by drift materials, thus reducing -the accent of the relief, a continuation of the same process resulted -in the superimposition of features of characteristic shapes upon -the imperfectly evened surface of the earlier stages. These -features belong to several different types, according as they were -built up outside of, at and upon, or within the glacier margin. -The extra-marginal deposits are described as <i>outwash plains</i> or -<i>aprons</i>, or sometimes as <i>valley trains</i>; the marginal are either -<i>moraines</i> or <i>kames</i>; while within the border were formed the <i>till -plain</i> or <i>ground moraine</i>, and, locally also, the <i>drumlin</i> and the -<i>esker</i> or <i>os</i>. These characteristic features are with few exceptions -to be found only within the area covered by the latest of the ice -invasions. For the earlier ones, so much time has now elapsed -that the effect of weathering, wash, and stream erosion has been -such that few of the features are recognizable.</p> - -<p>Marginal and extra-marginal features are extended in the direction -of the margin or, in other words, perpendicular to the local -ice movement; while the intra-marginal deposits are as noteworthy -for being perpendicular to the margin, or in correspondence -with the direction of local ice movement. Each of these features -possesses characteristic marks in its form, its size, proportions, -surface molding and orientation, as well as in its constituent -materials. It should perhaps be pointed out that the existing -continental glaciers, being in high latitudes, work upon rock materials -which have been subjected to different weathering processes -from those characteristic of temperate latitudes. Moreover, the -melting of the Pleistocene glaciers having taken place in relatively -low latitudes, larger quantities of rock débris were probably released -from the ice during the time of definite climatic changes, and hence -heavier drift accumulations have for both of these reasons resulted.</p> - -<p><b>Marginal or “kettle” moraines.</b>—Wherever for a protracted -period the margin of the glacier was halted, considerable deposits -of drift were built up at the ice margin. These accumulations -form, however, not only about the margin, but upon the ice surface -as well; in part due to materials collected from melting down<span class="pagenum"><a name="Page_312" id="Page_312">[312]</a></span> -of the surface, and in part by the upturning of ice layers near the -margin (see <i>ante</i>, <a href="#Page_277">p. 277</a>).</p> - -<div class="floatleft"> - <img src="images/ill-383.jpg" width="200" height="293" id="f336" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 336.</span>—Sketch map of portions -of Michigan, Ohio, and Indiana, -showing the festooned outlines of -the moraines about the former ice -lobes, and the directions of ice -movement as determined by the -striæ upon the rock pavement -(after Leverett).</p> -</div></div> - -<p>An important rôle is played by the thaw water which emerges -at the ice margin, especially within the reëntrants or recesses of -the outline. The materials of moraines are, therefore, till with -large local deposits of kame gravel, and these form in a series of -ridges corresponding to the temporary positions of the ice front. -Their width may range from a few rods to a few miles, their height -may reach a hundred feet or more, -and they stretch across the country -for distances of hundreds or even -thousands of miles, looped in arcs -or scallops which are always convex -outward and which meet in sharp -cusps that in a general way point -toward the embossment of the -former glacier (<a href="#f334">Fig. 334</a>, <a href="#Page_308">p. 308</a>, and -<a href="#f336">Fig. 336</a>). These festoons of the -moraines outline the ice lobes of -the latest ice invasion, which in -North America were centered over -the depressions now occupied by the -Laurentian lakes. There was, thus, -a Lake Superior lobe, a Lake Michigan -lobe, etc. With the aid of -these moraine maps we may thus -in imagination picture in broad lines -the frontal contours of the earlier -glaciers. At specially favorable localities -where the ice front has -crossed a deep valley at the edge of -the Driftless Area, we may, even in a rough way, measure the slope -of the ice face. Thus near Devils Lake in southern Wisconsin the -terminal moraine crosses the former valley of the Wisconsin River, -and in so doing has dropped a distance of about four hundred feet -within the distance of a half mile or thereabouts (<a href="#f337">Fig. 337</a>).</p> - -<p>The characteristic surface of the marginal moraine is responsible -for the name “kettle” moraine so generally applied to it. The -“kettles” are roughly circular, undrained basins which lie among -hummocks or knobs, so that the surface has often been referred -to as “knob and basin” topography (<a href="#p17c">plate 17 C</a>).</p> - -<p><span class="pagenum"><a name="Page_313" id="Page_313">[313]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-384a.jpg" width="400" height="349" id="f337" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 337.</span>—Map of the vicinity of Devils Lake, Wisconsin, located within a reëntrant -of the “kettle” moraine upon the margin of the Driftless Area. The lake -lies within an earlier channel of the Wisconsin River which has been blocked at -both ends, first by the glacier and later by its moraine. The stippled area upon -the heights and next the moraine represents the clay deposits of a former lake -(based on map by Salisbury and Atwood).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-384b.jpg" width="400" height="179" id="f338" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 338.</span>—Moraine with outwash apron in front, the latter in part eroded by a -river. Westergötland, Sweden (after H. Munthe).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_314" id="Page_314">[314]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-385.jpg" width="250" height="170" id="f339" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 339.</span>—Fosse between an outwash plain -(in the foreground) and the moraine, -which rises to the left in the middle distance. -Ann Arbor, Michigan.</p> -</div></div> - -<p><b>Kames.</b>—Within reëntrants or recesses of the ice margin the -drift deposits were especially heavy, so that high hills of hummocky -surface have been built up, which are described as <i>kames</i>. Most -of the higher drift hills have this origin. They rarely have any -principal extension along a single direction, but are composed in -large part of assorted materials. In contrast with other portions -of the morainal ridges they lack the prominent basins known as -kettles. Other <i>kames</i> are high hills of assorted materials not in -direct association with moraines -and believed to have -been built up beneath glacier -wells or mills (<a href="#Page_278">p. 278</a>).</p> - -<p><b>Outwash plains.</b>—Upon the -outer margin of the moraine -is generally to be found a plain -of glacial “outwash” composed -of sand or gravel deposited -by the braided streams -(<a href="#f308">Fig. 308</a>, <a href="#Page_280">p. 280</a>) flowing from -the glacier margin. Such -plains, while notably flat (<a href="#f338">Fig. 338</a>), -slope gently away from the moraine. Between the outwash -plain and the moraine there is sometimes found a pit, or <i>fosse</i> -(<a href="#f309">Fig. 309</a>, <a href="#Page_281">p. 281</a>), where a part of the ice front was in part buried -in its own outwash (<a href="#f339">Fig. 339</a>).</p> - -<div class="floatright"> - <img src="images/ill-386a.jpg" width="250" height="108" id="f340" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 340.</span>—View looking along an esker in southern -Maine (after Stone).</p> -</div></div> - -<p><b>Pitted plains and interlobate moraines.</b>—Where glacial outwash -is concentrated within a long and narrow reëntrant, separating -glacial lobes, strips of high plain are sometimes built up which -overtop the other glacial deposits of the district. The sand and -gravel which compose such plains have a surface which is pitted by -numerous deep and more or less circular lakes, so that the term -“pitted plain” has been applied to them. The surface of such a -plain steadily rises toward its highest point in the angle between -the ice lobes. Though consisting almost entirely of assorted -materials, and built up largely without the ice margins, such -gently sloping pitted platforms are described as <i>interlobate moraines</i>. -Upon a topographic map the course of such an interlobate<span class="pagenum"><a name="Page_315" id="Page_315">[315]</a></span> -moraine may often be followed by the belts of small pit -lakes (see <a href="#f336">Fig. 336</a>).</p> - -<div class="figcenter"> - <img src="images/ill-386b.jpg" width="400" height="367" id="f341" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 341.</span>—Outline map showing the eskers of Finland trending southeasterly toward -the festooned moraines at the margin of the ice. The characteristic lakes -of a glaciated region appear behind the moraines (after J. J. Sederholm).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-387.jpg" width="200" height="488" id="f342" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 342.</span>—Small sketch maps -showing the relationships in -size, proportions, and orientation -of drumlins and eskers in -southern Wisconsin. The eskers -are in solid black (after -Alden).</p> -</div></div> - -<p><b>Eskers.</b>—Intra-morainal features, or those developed beneath -the glacier but relatively near its margin, include the “serpentine -kame”, <i>esker</i>, or, -as it is called in -Scandinavia, the <i>os</i> -(plural <i>osar</i>) (<a href="#f340">Fig. 340</a>). -These diminutive -ridges -have a width seldom -exceeding a -few rods, and a -height a few tens -of feet at most, but with slightly sinuous undulations they may be -followed for tens or even hundreds of miles in the general direction -of the local ice movement (<a href="#f341">Fig. 341</a>). They are composed of<span class="pagenum"><a name="Page_316" id="Page_316">[316]</a></span> -poorly stratified, thick-bedded sands, gravels, and “worked over” -materials, and are believed to have been formed by subglacial -rivers which flowed in tunnels beneath the ice. Inasmuch as the -deposits were piled against the ice walls, the beds were disturbed -at the sides when these walls disappeared, -and the stratification, which -was somewhat arched in the beginning, -has been altered by sliding at both -margins. As already stated, eskers -have not a general distribution within -the glaciated area, but are often found -in great numbers at specially favored -localities. Formed as they are beneath -the ice, it is believed that many have -their materials redistributed so soon as -uncovered at the glacier margin, because -of the vigorous drainage there. -They are thus to be found only at those -favored localities where for some reason -border drainage is less active, or where -the ice ended in a body of water.</p> - -<p><b>Drumlins.</b>—A peculiar type of small -hill likewise found behind the marginal -moraine in certain favored districts has -the form of an inverted boat or canoe, -the long axis of which is parallel to -the direction of ice movement, as is -that of the esker (<a href="#f342">Fig. 342</a>). Unlike -the esker, this type of hill is composed -of till, and from being found in Ireland -it is called a <i>drumlin</i>, the Irish word -meaning a little hill (<a href="#f343">Fig. 343</a>). Drumlins -are usually found in groups more -or less radial and not far behind the -outermost moraine, to which their radiating axes are perpendicular. -The manner of their formation is involved in some uncertainty, -but it is clear that they have been formed beneath the margin of -the glacier, and have been given their shape by the last glacier -which occupied the district.</p> - -<p><span class="pagenum"><a name="Page_317" id="Page_317">[317]</a></span></p> - -<p>The mutual relationships of nearly all the molded features -resulting from continental glaciation may be read from <a href="#f344">Fig. 344</a>.</p> - -<div class="figcenter"> - <img src="images/ill-388a.jpg" width="400" height="77" id="f343" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 343.</span>—View of a drumlin, showing an opening in the till. Near Boston, Massachusetts -(after Shaler and Davis).</p> -</div></div> - -<p><b>The shelf ice of the ice age.</b>—Shelf ice, such as we have become -familiar with in Antarctica as a marginal snow-ice terrace floating -upon the sea, no doubt existed during the ice age above the Gulf -of Maine (see <a href="#f324">Fig. 324</a>, <a href="#Page_298">p. 298</a>), and perhaps also over the deep sea -to the westward of Scotland. Though the inland ice probably -covered the North Sea, and upon the American side of the Atlantic -the Long Island Sound, both these basins are so shallow that -the ice must have rested upon the bottom, for neither is of -sufficient depth to entirely submerge one of the higher European -cathedrals.</p> - -<div class="figcenter"> - <img src="images/ill-388b.jpg" width="400" height="252" id="f344" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 344.</span>—Outline map of the front of the Green Bay lobe of the latest continental -glacier of the United States. Drumlins in solid black, moraines with diagonal -hachure, outwash plains and the till plain or ground moraine in white (after -Alden).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_318" id="Page_318">[318]</a></span></p> - -<p><b>Character profiles.</b>—All surface features referable to continental -glaciers, whether carved in rock or molded from loose materials, -present gently flowing outlines which are convex upward (<a href="#f345">Fig. 345</a>). -The only definite features carved from rock are the <i>roches -moutonnées</i>, with their flattened shoulders, while the hillocks upon -moraines and kames, and the drumlins as well, approximate to -the same profile. The esker in its cross sections is much the same, -though its serpentine extension may offer some variety of curvature -when viewed from higher levels.</p> - -<div class="figcenter"> - <img src="images/ill-389.jpg" width="400" height="102" id="f345" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 345.</span>—Character profiles referable to continental glacier.</p> -</div></div> - -<p class="prr"><span class="smcap">Reading References for Chapter XXII</span></p> - -<p class="p1">General:—</p> - -<p class="pex"><span class="smcap">James Geikie.</span> The Great Ice Age. 3d ed. London, 1894, pp. 850, -maps 18.</p> - -<p class="pex"><span class="smcap">Chamberlin</span> and <span class="smcap">Salisbury</span>. Geology, vol. 3, 1906, pp. 327-516.</p> - -<p class="pex"><span class="smcap">Frank Leverett.</span> The Illinois Glacial Lobe, Mon. 38, U. S. Geol. Surv., -1899, pp. 817, pls. 34; Glacial formations and Drainage Features of -the Erie and Ohio Basins, Mon. 41, <i>ibid.</i>, 1902, pp. 802, pls. 25; -Comparison of North American and European Glacial Deposits, Zeit. -f. Gletscherk., vol. 4, 1910, pp. 241-315, pls. 1-5.</p> - -<p class="p1">Former glaciations previous to Ice Age:—</p> - -<p class="pex"><span class="smcap">A. Strahan.</span> The Glacial Phenomena of Paleozoic Age in the Varanger -Fjord, Quart. Jour. Geol. Soc., London, vol. 53, 1897, pp. 137-146, pls. -8-10.</p> - -<p class="pex"><span class="smcap">Bailey Willis</span> and <span class="smcap">Eliot Blackwelder</span>. Research in China, Pub. 54, -Carnegie Inst. Washington, vol. 1, 1907, pp. 267-269, pls. 37-38.</p> - -<p class="pex"><span class="smcap">A. P. Coleman.</span> A Lower Huronian Ice Age, Am. Jour. Sci. (4), vol. 23, -1907, pp. 187-192.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> Observations in South Africa, Bull. Geol. Soc. Am., vol. -17, 1906, pp. 377-450, pls. 47-54.</p> - -<p class="pex"><span class="smcap">David White.</span> Permo-Carboniferous Climatic Changes in South America, -Jour. Geol., vol. 15, 1907, pp. 615-633.</p> - -<p><span class="pagenum"><a name="Page_319" id="Page_319">[319]</a></span></p> - -<p class="p1">Driftless and drift areas:—</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin</span> and <span class="smcap">R. D. Salisbury</span>. Preliminary Paper on the -Driftless Areas of the Upper Mississippi Valley, 6th Ann. Rept. U. S. -Geol. Surv., 1885, pp. 199-322, pls. 23-29.</p> - -<p class="pex"><span class="smcap">R. D. Salisbury.</span> The Drift, its Characteristics and Relationships, -Jour. Geol., vol. 2, 1894, pp. 708-724, 837-851.</p> - -<p class="pex"><span class="smcap">R. H. Whitbeck.</span> Contrasts between the Glaciated and the Driftless -Portions of Wisconsin, Bull. Geogr. Soc., Philadelphia, vol. 9, 1911, -pp. 114-123.</p> - -<p class="p1">Glacier gravings:—</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin.</span> The Rock Scorings of the Great Ice Invasions, 7th -Ann. Rept. U. S. Geol. Surv., 1888, pp. 147-248, pl. 8.</p> - -<p class="p1">The dispersion of the drift:—</p> - -<p class="pex"><span class="smcap">R. D. Salisbury.</span> Notes on the Dispersion of Drift Copper, Trans. Wis. -Acad. Sci., etc., vol. 6, 1886, pp. 42-50, pl.</p> - -<p class="pex"><span class="smcap">N. S. Shaler.</span> The Conditions of Erosion beneath Deep Glaciers, -based upon a Study of the Bowlder Train from Iron Hill, Cumberland, -Rhode Island, Bull. Mus. Comp. Zoöl. Harv. Coll., vol. 16, No. 11, -1893, pp. 185-225, pls. 1-4 and map.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Diamond Field of the Great Lakes, Jour. -Geol., vol. 7, 1899, pp. 375-388, pls. 2 (also Rept. Smithson. Inst., -1901, pp. 359-366, pls. 1-3).</p> - -<p class="p1">Glacial features:—</p> - -<p class="pex"><span class="smcap">T. C. Chamberlin.</span> Preliminary Paper on the Terminal Moraine of the -Second Glacial Epoch, 3d Ann. Rept. U. S. Geol. Surv., 1883, pp. -291-402, pls. 26-35.</p> - -<p class="pex"><span class="smcap">G. H. Stone.</span> Glacial Gravels of Maine and their Associated Deposits, -Mon. 34, U. S. Geol. Surv., 1899, pp. 489, pls. 52.</p> - -<p class="pex"><span class="smcap">W. C. Alden.</span> The Delaven Lobe of the Lake Michigan Glacier of the -Wisconsin Stage of Glaciation and Associated Phenomena. Prof. Pap. -No. 34, U. S. Geol. Surv., 1904, pp. 106, pls. 15; The Drumlins of -Southeastern Wisconsin, Bull. 273, U. S. Geol. Surv., 1905, pp. 46, -pls. 9.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> Structure and Origin of Glacial Sand Plains, Bull. Geol. -Soc. Am., vol. 1, 1890, pp. 196-202, pl. 3; The Subglacial Origin -of Certain Eskers, Proc. Bost. Soc. Nat. Hist., vol. 35, 1892, pp. 477-499.</p> - -<p class="pex"><span class="smcap">F. P. Gulliver.</span> The Newtonville Sand Plain, Jour. Geol., vol. 1, 1893, -pp. 803-812.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_320" id="Page_320">[320]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXIII</h2> - -<p class="pch">GLACIAL LAKES WHICH MARKED THE DECLINE OF -THE LAST ICE AGE</p> - -<div class="floatleft"> - <img src="images/ill-391.jpg" width="250" height="127" id="f346" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 346.</span>—The Illinois River where it passes through -the outer moraine at Peoria, Illinois, showing the -flood plain of the ancient stream as an elevated -terrace into which the modern stream has cut its -gorge (after Goldthwait).</p> -</div></div> - -<p><b>Interference of glaciers with drainage.</b>—Every advance and -every retreat of a continental glacier has been marked by a complex -series of episodes in the history of every river whose territory -it has invaded. Whenever the valley was entered from the direction -of its divide, the -effect of the advancing -ice front has generally -been to swell -the waters of the river -into floods to which -the present streams -bear little resemblance -(<a href="#f346">Fig. 346</a>). Because -of the excessive melting, -this has been even -more true of the ice -retreat, but here <i>when -the ice front retired up the valley</i> toward the divide. A sufficiently -striking example is furnished by the Wabash, Kaskaskia, Illinois, -and other streams to the southward of the divide which surrounds -the basin of the Great Lakes (<a href="#f347">Fig. 347</a>).</p> - -<div class="floatright"> - <img src="images/ill-392a.jpg" width="250" height="279" id="f347" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 347.</span>—Broadly terraced valleys outside the -divide of the St. Lawrence basin, which remain to -mark the floods that issued from the latest continental -glacier during its retreat (after Leverett).</p> -</div></div> - -<p>Wherever the relief was small there occurred in the immediate -vicinity of the ice front a temporary diversion of the streams by the -parallel moraines, so that the currents tended to parallel the ice -front. This temporary diversion known as “border drainage” -was brought to a close when the partially impounded waters had, -by cutting their way through the moraines, established more permanent -valleys (<a href="#f348">Fig. 348</a>).</p> - -<p><span class="pagenum"><a name="Page_321" id="Page_321">[321]</a></span></p> - - -<p><b>Temporary lakes due to ice blocking.</b>—Whenever, on the contrary, -the advancing ice front entered a valley from the direction -of its mouth, or a <i>retreating -ice front retired -down the valley</i>, quite -different results followed, -since the waters -were now impounded -by the ice front serving -as a dam. Though the -histories of such blocking -of rivers are often -quite complex, the principles -which underlie -them are in reality simple -enough. Of the -lakes formed during advancing -hemicycles of -glaciation, and of all -save the latest receding -hemicycle, no satisfactory -records are preserved, -for the reason -that the lake beaches and the lake deposits were later disturbed -and buried by the overriding ice sheets. We have, however, every -reason to suppose that the histories of each of these hemicycles -were in every way as complex and interesting as that of the one -which we are permitted to study.</p> - -<p><span class="pagenum"><a name="Page_322" id="Page_322">[322]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-392b.jpg" width="400" height="126" id="f348" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 348.</span>—Border drainage about the retreating ice front south of Lake Erie. -The stippled areas are the morainal ridges and the hachured bands the valleys -of border drainage (after Leverett).</p> -</div></div> - -<p>As an introduction to the study of the ice-blocked lakes of North -America, and to set forth as clearly as may be the fundamental -principles upon which such lakes are dependent, we shall consider -in some detail the late glacial history of certain of the Scottish -glens, since their area is so small -and the relief so strong that relationships -are more easily seen; it -is, so to speak, a pocket edition -of the history of the more extended -glacial lakes.</p> - -<div class="floatleft"> - <img src="images/ill-393a.jpg" width="230" height="129" id="f349" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 349.</span>—The “parallel roads” of -Glen Roy in the southern highlands -of Scotland (after Jamieson).</p> -</div></div> - -<p><b>The “parallel roads” of the -Scottish glens.</b>—In a number -of neighboring glens within the -southern highlands of Scotland -there are found faint terraces upon the glen walls which under the -name of the “parallel roads” (<a href="#f349">Fig. 349</a>) have offered a vexed -problem to scientists. Of the many scientists who long attempted -to explain them, though in vain, was Charles Darwin, the father -of modern evolution. He offered it as his view that the “roads” -were beaches formed at a time when the sea entered the glens -and stood at these levels. When, however, Jamieson’s studies -had discovered their true history, Darwin, with a frankness characteristic -of some of the greatest scientists, admitted how far astray<span class="pagenum"><a name="Page_323" id="Page_323">[323]</a></span> -he had been in his reasoning. Let us, then, first examine the facts, -and later their interpretation. The map of <a href="#f350">Fig. 350</a> will suffice -to set forth with sufficient clearness the course of the several -“roads.” These “roads” are found in a number of glens tributary -to Loch Lochy, and of the three neighboring valleys, Glen -Roy has three, Glen Glaster two, and Glen Spean one “road.” -The facts of greatest significance in arriving at their interpretation -relate to their elevations with reference to the passes at the valley -heads, their abrupt terminations down-valleyward, and the morainic -accumulations which are found where they terminate. The -single “road” of Glen Spean is found at an elevation of 898 -feet, a height which corresponds to that of the pass or col at the -head of its valley and to the lowest of the “roads” in both Glens -Glaster and Roy. Similarly the upper of the two “roads” in -Glen Glaster is at the height of the pass at its head (1075 feet) -and corresponds in elevation to the middle one of the three “roads” -in Glen Roy. Lastly, the highest of the “roads” in Glen Roy is -found at an elevation of 1151 feet, the height of the col at the head -of the Glen. In the neighboring Glen Gloy is a still higher “road” -corresponding likewise in elevation to that of the pass through -which it connects with Glen Roy.</p> - -<div class="figcenter"> - <img src="images/ill-393b.jpg" width="400" height="203" id="f350" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 350.</span>—Map of Glen Roy and neighboring valleys of the Scottish highlands with -the so-called “roads” entered in heavy lines. Glens Roy, Glaster, and Spean -have three “roads”, two “roads”, and one “road”, respectively (after Jamieson).</p> -</div></div> - -<p>To come now to the explanation of the “roads”, it may be said -at the outset that they are, as Darwin supposed, beach terraces -cut by waves, not as he believed of the ocean, but of lakes which -once filled portions of the glens when glaciers proceeding from -Ben Nevis to the southwestward were blocking their lower portions. -The several episodes of this lake history will be clear from -a study of the three successive idealistic diagrams in <a href="#f351">Fig. 351</a>.</p> - -<p><span class="pagenum"><a name="Page_324" id="Page_324">[324]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-395.jpg" width="400" height="694" id="f351" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 351.</span>—Three successive diagrams to set forth in order the late glacial lake -history of the Scottish glens.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_325" id="Page_325">[325]</a></span></p> - -<p>To derive the principles underlying this history, it is at once -seen that <i>all changes are initiated by the retirement of the ice front -to such a point that it unblocks for the waters of a lake an outlet that -is lower than the one in service at the time</i>. This is the principle -which explains nearly all episodes of glacial lake history. Thus, -when the ice front had retired so as to open direct connections -between Glen Roy and Glen Glaster, the col at the head of Glen -Roy was abandoned as an outlet, and the waters fell to the level -fixed for Glen Glaster. A still further retirement at last opened -direct connection between Glen Glaster and Glen Spean, so that -the lake common to Glens Glaster and Roy fell to the level of the -col which was the outlet of the Spean valley at the time. This -stage continued until the ice front had retired so far that the waters -drained naturally down the river Spean to Loch Lochy and thence -to the ocean.</p> - -<div class="figcenter"> - <img src="images/ill-396a.jpg" width="400" height="131" id="f352" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 352.</span>—Harvesting time on the fertile floor of the glacial Lake Agassiz (after -Howell).</p> -</div></div> - -<p>Only in their far grander scale and in the lesser relief of the land -over which they formed, do the complex histories of the great -ice-blocked lakes of North America differ from these little valley -lakes whose beaches may be visited and the relationships worked -out, thanks to Jamieson, in a single day’s strolling.</p> - -<div class="floatright"> - <img src="images/ill-396b.jpg" width="250" height="225" id="f353" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 353.</span>—Map of Lake Agassiz (after -Upham).</p> -</div></div> - -<p><b>The glacial Lake Agassiz.</b>—The grandest of the temporary lakes -referable to blocking by the continental glaciers of the ice age -must be looked for in the largest -valleys that lay within the territory -invaded and <i>which normally -drain toward the retiring ice front</i>. -In North America these rivers are -the Red River of the North in -Minnesota, the Dakotas, and Manitoba; -and the St. Lawrence River -system. To the ice dam which lay -across the Red River valley we -owe the fertility of that vast plain -of lake deposits where is to-day the -most intensive wheat farming of -the northwest (<a href="#f352">Fig. 352</a>). Lakes Winnipeg, Winnipegoosis, and -Manitoba, and the Lake of the Woods, are all that now remain of -this greatest of the glacial lakes, which in honor of the distinguished -founder of the glacial theory has been called Lake Agassiz (<a href="#f353">Fig. 353</a>). -With their natural outlet blocked by the ice in northern<span class="pagenum"><a name="Page_326" id="Page_326">[326]</a></span> -Manitoba and Keewatin, the waters of the Red were swollen by -melting from the retiring glacier and spread over a vast area before -finding a southern outlet along the course of the present Lake -Traverse and the valley of the Minnesota River. Along this route -there flowed a mighty flood which carved out a broad valley many -times too large for the Minnesota, its present occupant, and this -giant prehistoric river has been called the Warren River (<a href="#f354">Fig. 354</a>).</p> - -<div class="figcenter"> - <img src="images/ill-397.jpg" width="400" height="558" id="f354" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 354.</span>—Map of the southern end of the Lake Agassiz basin, showing -the position of some of the beaches and the outlet through the former -Warren River (after Upham).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_327" id="Page_327">[327]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-398a.jpg" width="400" height="118" id="f355" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 355.</span>—Narrows of the Warren River below Big Stone Lake, where it passed -between jaws of hard granite and gneiss (after Upham).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-398b.jpg" width="250" height="250" id="f356" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 356.</span>—Map of the valley of the Warren -River in the vicinity of Minneapolis, with -the young valley of the Mississippi entering -it at Fort Snelling (after Sardeson).</p> -</div></div> - -<p>It is interesting to follow this ancient waterway and to discover -that, like our normal, present-day streams, it was held up in narrows -wherever outcroppings of harder rock had constricted its channel -(<a href="#f355">Fig. 355</a>). The upper end of the Warren River valley is now -occupied by the long and relatively narrow Lakes Traverse and -Big Stone, each the result of blocking by delta deposits where a -tributary stream has emerged into the valley, but this gigantic -channel continues down to and beyond Minneapolis, occupied as -far as Fort Snelling by the -Minnesota River—a mere -pygmy compared to its predecessor. -To the earnest student -of glacial geology there can be -few sights more impressive than -are obtained by standing at -Fort Snelling, just above the -confluence of the Minnesota -and the Mississippi rivers, and -surveying first the steep and -narrow valley of the Mississippi -above the junction,—a -stream fitted to its valley for -the simple reason that it has -carved it,—and then gazing -up and down that broad valley -in which the great Warren River once flowed majestically to the -sea, now the bed of the Minnesota above the Fort and of the Mississippi -below it (<a href="#f356">Fig. 356</a>).</p> - -<p><span class="pagenum"><a name="Page_328" id="Page_328">[328]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-399.jpg" width="400" height="332" id="f357" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 357.</span>—Portion of the Herman quadrangle of Minnesota, showing the position -of the Herman beach on the shore of the former Lake Agassiz. The lake basin is -to the left, and the pitted morainal deposits appear to the right (U. S. G. S.).</p> -</div></div> - -<p>Just as the “parallel roads” of Glen Roy, roads in name only, -are the beaches of earlier glacial lake stages, so in Lake Agassiz -we have parallel beaches of the barrier type which are often roads -in fact as well as in name, and which mark the stages of successive -lakes within this vast basin. The Herman beach, corresponding -to the highest level of the lake, is thus a sharp topographic boundary -between lake deposits and morainal accumulations, and is -further itself a well-marked topographic feature composed of wave-washed -and hence well-drained materials (<a href="#f357">Fig. 357</a>). Farmers of -the district have been quick to realize that these level and slightly -elevated ridges lack the clay which would render them muddy in -the wet seasons, and are thus ideally adapted for roads. They -have in many sections been thus used over long stretches and are -known as the “ridge roads.”</p> - -<p><span class="pagenum"><a name="Page_329" id="Page_329">[329]</a></span></p> - -<p><b>Episodes of the glacial lake history within the St. Lawrence -valley.</b>—Within this great drainage basin it has apparently -been possible to read the records of each stage in the latest lake -history—complex as this has been. We have only to recall the -lake stages cited from the Scottish glens and remember that each -new stage was begun in a retirement of the glacier front which unblocked -an outlet of lower level than the last. This sequence -might, however, have been varied by a temporary readvance of the -ice, as indeed once occurred in the Huron-Erie lobe of the great -North American glacier.</p> - -<div class="figcenter"> - <img src="images/ill-400.jpg" width="400" height="323" id="f358" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 358.</span>—The continental glacier of North America in an early stage of its recession, -when it covered the entire St. Lawrence drainage basin. The dashed line -is the approximate position of the divide (based on a map by Goldthwait).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-401a.jpg" width="200" height="192" id="f359" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 359.</span>—Outline map of the -early Lake Maumee, with the -bordering moraine and the -water-laid moraine remaining -on the site of the former ice cliff.</p> -</div></div> - -<p><b>The crescentic lakes of the earlier stages.</b>—So long as the -glacier covered the entire drainage basin of the St. Lawrence -River system, all water was freely drained away by streams which -flowed <i>away from</i> the ice front (<a href="#f358">Fig. 358</a>). So soon, however, -as at any point the front had retired behind the divide, impounding -of the waters must locally have occurred. Lakes of this type -are to-day to be seen in Greenland and in the southern Andes; -and though upon a diminutive scale, some idea of their aspect may -be obtained from the appearance of the Märjelen Lake of Switzerland, -here blocked by a mountain glacier (<a href="#f446">Fig. 446</a>, <a href="#Page_411">p. 411</a>).<span class="pagenum"><a name="Page_330" id="Page_330">[330]</a></span> -Within all areas of small relief, such as -the prairie country surrounding the -present Laurentian lakes, the earlier -and smaller stages of such ice-blocked -lakes are generally crescentic in outline. -This is because a moraine in -most cases forms the land margin of -the lake, and because the ice cliff -upon the opposite border, although -somewhat straightened, as a consequence -of wave-cutting and iceberg -formation, still retains the convex -outlines characteristic of ice lobes -(<a href="#f359">Fig. 359</a>).</p> - -<div class="figcenter"> - <img src="images/ill-401b.jpg" width="400" height="382" id="f360" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 360.</span>—Map to show the first stages of the ice-dammed lakes within the -St. Lawrence basin (after Leverett and Taylor).</p> -</div></div> - -<p>Within each of the Great Lake basins a crescentic lake early appeared -at that end of the depression which was first uncovered<span class="pagenum"><a name="Page_331" id="Page_331">[331]</a></span> -by the glacier: Lake Duluth in the Superior basin, Lake Chicago -in the Michigan basin, and Lake Maumee in the Huron-Erie -basin (<a href="#f360">Fig. 360</a>).</p> - -<p>We may now, with profit, trace the successive episodes of the -glacial lake history, considering for the earlier stages those changes -which occurred within the Huron-Erie basin, since, these are in -essential respects like those of the Michigan and Superior basins, -although worked out in greater detail. Lake Chicago must, -however, be brought into consideration, since in all save the earliest -and the later stages, the waters from the Huron-Erie depression -were discharged through the Grand River into this lake and -thence by the so-called “Chicago outlet” into the Mississippi -(<a href="#p20">plate 20 A</a>).</p> - - -<p><b>The early Lake Maumee.</b>—The area, outline, and outlet of -this lake are indicated upon <a href="#f360">Fig. 360</a>. Its ancient beaches have -been traced, as well as the water-laid moraine beneath its former -ice cliff; and no observant traveler who should take his way -down the ancient outlet from Fort Wayne, Indiana, past the town -of Huntington, could fail to be impressed by its size, suggesting -as it does the great volume of water which must once have flowed -along it. Now a channel a mile or more in width, its bed for the -twenty-five miles between Fort Wayne and Huntington may be -seen from the tracks of the Wabash Railway as a series of swamps -merely, while at Huntington the Wabash river enters by a young -<span class="font">V</span>-shaped valley at the side, much as the Mississippi emerges into -the old channel of the Warren River at Fort Snelling, Minnesota -(see <a href="#Page_327">p. 327</a>).</p> - -<p>The Huron River of southern Michigan, which now discharges -into Lake Erie, then found its lower course blocked by the glacier -and was thus compelled to find a southerly directed channel now -easily followed to the northern horn of the crescent of Lake -Maumee.</p> - - -<p><b>The later Lake Maumee.</b>—When the ice lobe had retired its -front sufficiently, an outlet lower than that at Fort Wayne was -uncovered past the city of Imlay, Michigan, into the Grand -River, and thence through Lake Chicago and its outlet into the -Mississippi. This old outlet south of Chicago follows the course -of the present Drainage Canal and the line of the Chicago & -Alton Railway. The traveler journeying southward by train from<span class="pagenum"><a name="Page_332" id="Page_332">[332]</a></span> -Chicago has thus the opportunity of observing first the beaches -of the former lake, and then the several channels which were -joined in the main outlet at the station of Sag (<a href="#p20">plate 20 A</a>).</p> - -<div class="figcenter"> - <img src="images/ill-403.jpg" width="400" height="293" id="f361" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 361.</span>—Outline map of the later Lake Maumee and of its “Imlay outlet” to -Lake Chicago (after Leverett).</p> -</div></div> - -<p>In this stage of our history Lake Maumee pushed a shrunk -arm up past the site of Ypsilanti in Michigan (<a href="#f361">Fig. 361</a>), the well-marked -beach being found on Summit Street opposite the State -Normal College. The Huron River, which in the first lake stage -had followed the valley now occupied by the Raisin River southward -into Indiana, now discharged directly into a bay upon this -arm of Lake Maumee, and so formed a delta at Ann Arbor.</p> - -<div class="figcenter"> - <img src="images/ill-404a.jpg" width="400" height="211" id="f362" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 362.</span>—Outline map of Lakes Whittlesey and Saginaw (after Leverett).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-404b.jpg" width="400" height="206" id="f363" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 363.</span>—Map of the glacial Lake Warren, the last of the lakes in the Huron-Erie -basin, which discharged through the “Grand River outlet” into the Mississippi -(after Leverett).</p> -</div></div> - -<p><b>Lakes Arkona and Whittlesey.</b>—The ice front in the Huron-Erie -basin now retired so far that the impounded waters, instead -of following the more direct “Imlay outlet” to the Grand, passed -at a lower level completely around “the thumb” of Michigan -into the Saginaw basin. Meanwhile a crescent-shaped lake had -developed in that basin, so that now the waters of the Maumee -basin were joined to those in the Saginaw basin as a common -lake, just as the lowering of the waters in Glen Roy caused a -union with those of Glen Glaster in the example cited for illustration.<span class="pagenum"><a name="Page_333" id="Page_333">[333]</a></span> -Our records of this third North American lake stage, -referred to as Lake Arkona, are however most imperfect, for the -reason that it was followed by a readvance of the ice front which -closed the passage around “the thumb” and raised the level of -the waters until an outlet was found past the town of Ubly at a -lower level than the “Imlay outlet.” When the waters of a -lake are thus rising, strong beach formations result, and those of -this stage, which is known as the Lake Whittlesey stage, are much -the strongest that are found within the Huron-Erie basin. Traced<span class="pagenum"><a name="Page_334" id="Page_334">[334]</a></span> -for some three hundred miles entirely around the southern and -western margins of Lake Erie, this beach is for much of the distance -the famous “ridge road” (<a href="#f362">Fig. 362</a>).</p> - - -<p><b>Lake Warren.</b>—As the ice advance which had produced Lake -Whittlesey came to an end, the normal recession was resumed -and a lake once more formed as a body common to the Saginaw -and Erie basins. This lake, known as Lake Warren, extended -a shrunk arm far eastward along the ice front into western New -York, though it was still blocked from entering the great Mohawk -valley (<a href="#f363">Fig. 363</a>).</p> - -<div class="figcenter"> - <img src="images/ill-405.jpg" width="400" height="313" id="f364" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 364.</span>—Map of the Glacial Lake Algonquin (after Leverett).</p> -</div></div> - -<p><b>Lakes Iroquois and Algonquin.</b>—It must be evident that -toward the close of the Lake Warren stage a profound change was -imminent—a transfer of the glacial waters from their course to -the Mississippi and the Gulf to the trench which crosses New -York State and enters the Atlantic. So soon as the ice front had -retired sufficiently to lay bare the bed of the Mohawk, an outlet -was found by this route and its continuation down the Hudson -valley to the sea. The Lake Ontario basin now became occupied -by a considerably larger water body known as Lake Iroquois, and<span class="pagenum"><a name="Page_335" id="Page_335">[335]</a></span> -the three upper lakes, then joined as Lake Algonquin, discharged -their combined waters into Lake Iroquois at first through a great -channel now strongly marked across Ontario in the course of the -Trent River and Lake Simcoe, the so-called “Trent outlet.” -At this time a smaller Lake Erie probably occupied the basin of -that lake, and later the Trent outlet was abandoned for the Port -Huron outlet (<a href="#f364">Fig. 364</a>).</p> - -<div class="figcenter"> - <img src="images/ill-406.jpg" width="400" height="310" id="f365" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig.</span> 365.—Outline map of the Nipissing Great Lakes with their outlet past North -Bay into the Champlain Sea.</p> -</div></div> - -<p><b>The Nipissing Great Lakes.</b>—We have now followed the ice -front step by step in its retreat across the valley of the St. Lawrence -system. The successive unblocking of outlets offers but -one further possibility—the opening of the French River-Nipissing -Lake-Ottawa River, or “North Bay outlet.” Though not -so to-day, the bed of this ancient channel was then much lower -than that of the “Mohawk outlet”, and so soon as the glacier -had in its retreat uncovered this northern channel, the waters of -the upper lakes discharged through it past the site of Ottawa -and into an arm of the sea which then occupied the lower St. -Lawrence valley and has been called the Champlain Gulf or Sea<span class="pagenum"><a name="Page_336" id="Page_336">[336]</a></span> -(<a href="#f365">Fig. 365</a>). The level of the waters was lowered and the area -of the lakes correspondingly reduced.</p> - -<p>The reader who has had no opportunity to observe these ancient -channels which carried the swollen waters of the former -glacier lakes, will find it interesting to consider that every one of -them has been fixed upon by engineers for improvement as artificial -waterways. Thus we have the Illinois Drainage Canal -and projected ship canal along the “Chicago outlet”, the projected -Mississippi-Lake Erie Canal along the “Fort Wayne outlet”, -the Grand River canal project to connect Lake Michigan and -Saginaw Bay along the course of the “Grand River outlet”, the -Trent Canal along the “Trent outlet”, the Erie Canal along the -“Mohawk outlet”, and, lastly, the proposed Georgian Bay ship -canal to the ocean along the “North Bay” or “Nipissing outlet.”</p> - -<p><b>Summary of lake stages.</b>—We have omitted in this summary -of late lake history in the Laurentian basin all the less -important lake stages, including some of a transitional nature -which were represented by beaches and outlets easily traced to-day. -This is because it is an outline only which it seems best to -present, and the episodes of this abridged history may be tabulated -as follows:</p> - -<p class="pch">EPISODES OF GLACIAL LAKE HISTORY</p> - -<p class="pc"><span class="smcap">Mississippi Drainage</span></p> - -<p class="pi6 p1">Lake Maumee (early), Fort Wayne outlet.</p> -<p class="pi6">Lake Maumee (late), Imlay City outlet.</p> -<p class="pi6">Lake Arkona, “thumb” outlet.</p> -<p class="pi6">Lake Whittlesey (with readvance of glacier), Ubly outlet.</p> -<p class="pi6">Lake Warren, “thumb” outlet.</p> - -<p class="pc p1"><span class="smcap">Atlantic Drainage</span></p> - -<p class="pi6 p1">Lakes Iroquois and Algonquin (early), Trent and Mohawk outlets.</p> -<p class="pi6">Lakes Iroquois and Algonquin (late), Port Huron and Mohawk outlets.</p> -<p class="pi6">Nipissing Great Lakes, North Bay outlet.</p> - -<p class="p1"><b>Permanent changes of drainage affected by the glacier.</b>—While -the lake history which we have sketched is made up of episodes -which endured only while the ice front lay between certain stations -upon its retreat, there were none the less brought about the<span class="pagenum"><a name="Page_337" id="Page_337">[337]</a></span> -profoundest of permanent modifications in the drainage of the -region. It is possible to restore upon maps in part only the preglacial -drainage of the north central states, but we know at least -that it was as different as may be from that which we find to-day. -The Missouri and the Ohio take their courses to-day along the -margin of the glaciated area as an inheritance from the border -drainage of the ice age. Within the glaciated regions rivers -have in many cases been compelled by morainal obstructions to -enter upon new courses, or even to travel -in the opposite direction along their -former channels. In districts of considerable -relief these diversions have -sometimes caused the streams to plunge -over the walls of deep valleys, and it -may truthfully be said that we owe -much of our most beautiful scenery in -part to the carving and molding of -glaciers, but especially to the cascades -and waterfalls directly due to their interference -with drainage.</p> - -<div class="floatright"> - <img src="images/ill-408.jpg" width="200" height="367" id="f366" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 366.</span>—Probable preglacial -drainage of the upper Ohio -region (after Chamberlin and -Leverett).</p> -</div></div> - -<p>Many diversions or reversals of former -drainage lines, through the influence of -the continental glacier, are at once suggested -by the abnormal stream courses, -which appear upon our maps, and the -correctness of these suggestions may -often be confirmed by very simple observations -made upon the ground. -The map of <a href="#f366">Fig. 366</a> shows how different -was the preglacial drainage of the upper Ohio region from -that of to-day.</p> - -<p>An interesting additional example is furnished by the Still -River which in Connecticut is tributary to the Farmington, and -is no less remarkable for its abnormal northerly course and sluggish -current perpetuated in its name, than for the way in which it is joined -to the Farmington system (<a href="#f367">Fig. 367 <i>A</i></a>). A careful study of the -district has shown that the Still River was once a part of the -Naugatuck and flowed southward toward Long Island Sound like -other rivers of the district (<a href="#f367">Fig. 367 <i>B</i></a>). It possessed, however,<span class="pagenum"><a name="Page_338" id="Page_338">[338]</a></span> -an advantage in a narrow belt of softer rock along its course, and -because of this advantage it captured a portion of one of the tributaries -to the Farmington (<a href="#f367">Fig. 367 <i>C</i></a>). The continental glacier -later covered the region, and on its retreat laid down morainal -obstructions directly across this river and also at the head of the -severed arm of the Farmington tributary (<a href="#f367">Fig. 367 <i>D</i></a>). The now -impounded waters found their lowest outlet near Sandy Brook, -and in waterfalls and cascades the now reversed river falls one -hundred feet to the bed of that stream. With the aid of the -excellent topographic maps which are now supplied by a generous -government at a merely nominal price, such bits of recent history -may be read at many places within the glaciated region.</p> - -<div class="figcenter"> - <img src="images/ill-409.jpg" width="400" height="189" id="f367" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 367.</span>—Diagrams to illustrate the episodes in the recent history of the Still -River tributary to the Farmington in Connecticut. <i>A</i>, present drainage; <i>B</i>, early -stage; <i>C</i>, after capture of a tributary to the Farmington; <i>D</i>, after blocking by -morainal obstructions of the ice age.</p> -</div></div> - -<p><b>Glacial Lake Ojibway in the Hudson Bay drainage basin.</b>—When -by passing over the “height of land” in northern Ontario -the greatly reduced continental glacier had vacated the basin -of St. Lawrence drainage, it was in a position to impound those -waters which normally drained to Hudson Bay. The lake which -then came into existence has been called Lake Ojibway and was the -latest of the entire series. Though of but recent discovery in -a country till lately a trackless wilderness, its extension seems to -have been that of the clay beds suited for farming. The beaches -and outlets remain to be mapped when the country has been -made more easily accessible.</p> - -<p><span class="pagenum"><a name="Page_339" id="Page_339">[339]</a></span></p> - -<p class="prr"><span class="smcap">Reading References for Chapter XXIII</span></p> - -<p class="p1">Parallel roads of Glen Roy:—</p> - -<p class="pex"><span class="smcap">Charles Darwin.</span> Observations on the Parallel Roads of Glen Roy -and of Other Parts of Lochaber in Scotland, with an attempt to prove -that they are of Marine Origin, Phil. Trans., vol. 8, 1839, pp. 39-82.</p> - -<p class="pex"><span class="smcap">Louis Agassiz.</span> Geological Sketches, Boston, 1876, vol. 2, pp. 32-76.</p> - -<p class="pex"><span class="smcap">T. T. Jamieson.</span> On the Parallel Roads of Glen Roy and their Place in -the History of the Glacial Period, Quart. Jour. Geol. Soc. Lond., -vol. 19, 1863, pp. 235-259.</p> - -<p class="p1">Glacial Lake Agassiz:—</p> - -<p class="pex"><span class="smcap">Warren Upham.</span> The Glacial Lake Agassiz. Mon. 25, U. S. Geol. Surv., -pp. 658, pls. 38.</p> - -<p class="pex"><span class="smcap">F. W. Sardeson.</span> Beginning and Recession of St. Anthony’s Falls, -Bull. Geol. Soc. Am., vol. 19, 1908, pp. 29-36.</p> - -<p class="p1">Glacial lakes in the St. Lawrence valley:—</p> - -<p class="pex"><span class="smcap">Chamberlin and Salisbury.</span> Geology, vol. 3, pp. 394-405.</p> - -<p class="pex"><span class="smcap">Frank Leverett.</span> Outline of the History of the Great Lakes (Presidential -Address), 12th Rept. Mich. Acad. Sci., 1910, pp. 19-42. The -Pleistocene Features and Deposits of the Chicago Area. Chicago, -1897, pp. 86, pls. 8 (Chicago Outlet).</p> - -<p class="pex"><span class="smcap">H. L. Fairchild.</span> Glacial Lakes in Western New York, Bull. Geol. Soc. -Am., vol. 6, 1895, pp. 353-374, pls. 18-23; Glacial Waters in Central -New York. Bull. 127, N. Y. State Mus., 1909, pp. 66, pls. 42, and -maps in cover.</p> - -<p class="p1">Early lakes in the Erie basin:—</p> - -<p class="pex"><span class="smcap">Frank Leverett.</span> On the Correlation of Moraines with Raised Beaches -of Lake Erie, Am. Jour. Sci. (3), vol. 43, 1892, pp. 281-301.</p> - -<p class="pex"><span class="smcap">F. B. Taylor.</span> The Great Ice Dams of Lakes Maumee, Whittlesey, and -Warren, Am. Geol., vol. 24, 1899, pp. 6-38, pls. 2-3; Relation of -Lake Whittlesey to the Arkona Beaches, 7th Rept. Mich. Acad. Sci., -1905, pp. 30-36.</p> - -<p class="pex"><span class="smcap">Frank Leverett.</span> The Ann Arbor Folio, Folio No. 155, U. S. Geol. Surv., -1908, pp. 10-12.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_340" id="Page_340">[340]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXIV</h2> - -<p class="pch">THE UPTILT OF THE LAND AT THE CLOSE OF THE -ICE AGE</p> - -<p><b>The response of the earth’s shell to its ice mantle.</b>—There -is now good reason to believe that the earth’s outer shell makes -a response by oscillations of level due to the loading by ice, on the -one hand, and to the removal of this burden upon the other. We -know, at least, that both in northern Europe and in North America -areas which have undergone depression during and elevation after -the ice age, correspond closely to the regions which were ice covered. -Wherever in these regions there was high relief before the -advent of the ice, river valleys were drowned at the land margins -and were also gouged out into troughs through erosion by the -outlet tongues upon the margin of the ice sheet. Such furrowed -and half-submerged valleys have a characteristic <span class="font">U</span>-shaped section, -so that their walls rise precipitously from the sea. From -their typical occurrence in Scandinavian countries the name <i>fjord</i> -has been applied to them.</p> - -<p>It is now no less clear that the removal of the ice blanket brought -from the earth a relatively quick response in uplift, which began -before the ice front had retired across the present international -boundary of the United States, and that this uplift continued -until the final disappearance of the ice. A far slower elevation of -a somewhat different nature has continued, even to the present -day.</p> - -<p>It is obvious that at the time of their formation all shore lines -referable to the work of waves must have been horizontal, and -hence any variations from a perfect level which they reveal to-day -must indicate that a tilting movement of the ground has occurred -since the waters departed from their basins. We have thus -provided for us in the positions of these ancient water planes, -particularly because of their wide extent, a complete record the -refinement of which is not easily overstated. Interpreting this<span class="pagenum"><a name="Page_341" id="Page_341">[341]</a></span> -record, we find that it was the uptilt of the land to the northward -which brought the glacial lake history to an end and inaugurated -the present system of St. Lawrence drainage. The outlet of the -Nipissing Great Lakes is to-day more than a hundred feet above -the level of the outlet at Port Huron, where the upper lakes are -now discharging their waters, and this difference in level can -only be ascribed to an upward tilting of the land since the latest -of the glacial lake stages.</p> - - -<p><b>The abandoned strands as they appear to-day.</b>—The traveler -by steamer upon the upper lakes, as he comes within view of -each rocky headland, may note -how the profile against the horizon -is notched by a series of -steps or terraces (<a href="#f368">Fig. 368</a>), -and if he has followed the discussion -in previous chapters, -he will suspect that these terraces -mark the now abandoned -shore lines which have come -to their present position through a series of uplifts of the ground -accompanied by earthquake shocks. As his steamer skirts the -shore he may chance to note a cave within the rock cliff which -represents the now elevated sea-arch of an ancient shore.</p> - -<div class="floatright"> - <img src="images/ill-412.jpg" width="250" height="114" id="f368" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 368.</span>—The notched rock headland -of Boyer Bluff between Green Bay and -Lake Michigan (after Goldthwait).</p> -</div></div> - -<p>Disembarking from the steamer and traveling inland at any -point where the shores are high, the traveler is certain to come -upon still more convincing proofs of the ancient strands; perhaps -in a storm beach of the unmistakable “shingle”, half buried though -it may be under dunes of newly drifted sand, or possibly at higher -levels the highway has been cut through a shingle barrier as -fresh and unmistakable as though formed upon the present shore. -Sometimes it is the rock cliff and terrace, at other times barrier -ridges of shingle, or, again, it is the sloping cliff and terrace cut -in the drift deposits; but of whatever sort, if studied with proper -regard to the topography of the district, the evidence is clear -and unmistakable.</p> - - -<p><b>The records of uplift about Mackinac Island.</b>—Nowhere are -the records of the recent uplift of the lake region more easily read -than about Mackinac Island in the straits connecting Lake Michigan -with Lake Huron. Approaching the island by steamer from<span class="pagenum"><a name="Page_342" id="Page_342">[342]</a></span> -St. Ignace, its profile upon the horizon is worthy of remark (<a href="#f369">Fig. 369</a>). -From a central crest broken by minor irregularities and -bounded on all sides by a cliff, the island profile slopes gently -away to a still lower cliff, below which is another terrace.</p> - -<div class="figcenter"> - <img src="images/ill-413a.jpg" width="400" height="172" id="f369" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 369.</span>—View of Mackinac Island from the direction of St. Ignace. The irregular -central portion is the only part of the island that was not submerged in -Lake Algonquin. The terrace at its base is the old shore line of Lake Algonquin, -and the lower terrace the strand of Lake Nipissing (after a photograph by -Taylor).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-413b.jpg" width="250" height="153" id="f370" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 370.</span>—The “Sugar Loaf”, a stack -near the shore of Lake Algonquin, as -it is seen from the observatory upon -Mackinac Island (after a photograph -by Taylor).</p> -</div></div> - -<p>When we have reached the island and have climbed to the -summit, we there find the surface which is characteristic of erosion -by running water, whereas at lower levels are found the forms -carved or molded by the action of waves. This central “island”, -superimposed upon the larger island, is all that rose above Lake -Algonquin, the earliest of the glacial lakes in this northern district; -and as we look out from the observatory upon the summit, -it is easy to call up a picture of -the country when the lake stood -at the base of this highest cliff. -To the northward one sees the -“Sugar Loaf” rise out of a sea -of foliage, as it formerly did -from the waters of Lake Algonquin -(<a href="#f370">Fig. 370</a>). It is a huge -stack near the former island -shore. If we turn now to the -southward and direct our gaze -toward the Fort, we encounter -a veritable succession of beach ridges formed of shingle and ranged -like a series of waves within the cleared space of the “Short -Target Range” (<a href="#f371">Fig. 371</a>). These ridges mark each a stage within<span class="pagenum"><a name="Page_343" id="Page_343">[343]</a></span> -a series of successive uplifts which have brought the island to -its present height.</p> - -<div class="figcenter"> - <img src="images/ill-414a.jpg" width="400" height="307" id="f371" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 371.</span>—View from the observatory upon Mackinac Island across the “Short -Target Range” toward the Fort. Beach ridges appear in succession within the -cleared space (after a photograph by Rossiter).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-414b.jpg" width="400" height="308" id="f372" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 372.</span>—Notched stack of the Nipissing Great Lakes at St. Ignace -(after a photograph by Taylor).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_344" id="Page_344">[344]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-415.jpg" width="200" height="392" id="f373" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 373.</span>—Series of diagrams to -illustrate the evolution of ideas -concerning the uplift of the lake -region since the ice age. <i>A</i>, -simple northerly up-canting -(Gilbert): <i>B</i>, northerly acceleration -of the up-canting (Spencer -and Upham); <i>C</i>, northerly -“feathering out” of beaches -(Spencer and Upham); <i>D</i>, hinge, -line of up-canting found within -the lake region (Leverett); <i>E</i>, -multiple and northwardly migrating -hinge lines of up-canting -(Hobbs).</p> -</div></div> - -<p>If now we descend from our position and visit the “battlefield”, -we find there a great ridge of level crest, behind which -the British force was stationed in its defense of the island in -1812. Near by in the woods is Pulpit Rock, a strikingly perfect -stack of the Nipissing Lake. Across the straits at St. Ignace is an -even finer example of the notched -stack (<a href="#f372">Fig. 372</a>). Other less prominent -beaches, but all later than the -Nipissing Lakes, intervene between -this level and the present shore to -mark the stages in the continued uplift -of the land.</p> - -<p><b>The present inclinations of the uplifted -strands.</b>—It is not enough that -we should have recognized the marks -of former shores now at considerable -elevations above the existing lakes; -if we are to know the nature of the -uplift, we must prepare accurate maps -based upon measurements by precise -leveling at many localities. Such -methods are, however, of comparatively -recent application in this field; -and, as in the investigation of so many -other problems, the earlier observations -were largely of the nature of -reconnaissances with the elevation of -beaches estimated by comparatively -crude methods only. The evolution -of ideas concerning the uptilt has, -therefore, been a gradual one.</p> - -<div class="floatright"> - <img src="images/ill-416.jpg" width="200" height="159" id="f374" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 374.</span>—Map of the Great Lakes region to show isobases -and hinge lines of uptilt. <i>a</i>, isobase of the Chicago -outlet; <i>b</i>, main hinge line of the Lake Whittlesey beach -(Leverett); <i>b<sup>1</sup></i>, hinge line of the Lake Warren beach (Taylor); -<i>c</i>, isobase of the Port Huron outlet; <i>d</i>, main hinge -line of highest Algonquin beach (Goldthwait); <i>e</i>, <i>f</i>, <i>g</i>, <i>h</i>, -additional hinge lines of Algonquin beaches in Door County -peninsula (Hobbs); <i>l</i>, isobase of the Lake Superior outlet -for the Algonquin beaches (Leverett): <i>m</i>, isobase of the -same outlet for the Nipissing beaches (Leverett).</p> -</div></div> - -<p>It was early observed that the -beaches corresponding to a given lake -stage were higher to the northward -and northeastward, and the natural -conclusion from this was that the -earth’s crust had here been canted -like a trap door (<a href="#f373">Fig. 373, <i>A</i></a>). As we are to see, this but half-correct -assumption has led to a striking prophecy relating to future<span class="pagenum"><a name="Page_345" id="Page_345">[345]</a></span> -changes within the lake region which we now know to be without -warrant in the facts. Later it was learned that the uptilt -of the lake beaches is much accelerated to the northward (<a href="#f373">Fig. 373,<i> B</i></a>), -and that new beaches make their appearance from beneath -others as -we proceed in -this direction—there -is a “feathering -out” of -beaches to the -northward (<a href="#f373">Fig. 373, <i>C</i></a>).</p> - -<p><b>The hinge -lines of uptilt.</b>—Still -later in -the study of the -region, it was -learned that the -axis or fulcrum -about which the -region has been -uptilted, instead -of lying to the -southward of the -lake district, as -had been assumed -by Gilbert, lay within the region and about halfway up -the basin of Lake Michigan (<a href="#f373">Fig. 373, <i>D</i></a>, and <a href="#f374">Fig. 374</a>). Similarly, -in the uptilt which followed the ice retreat in northern -Europe a definite hinge line of movement has been discovered.</p> - -<p>Lastly, it has been shown, as a result of the use of precise leveling -methods, that not one but several hinge lines of movement -lie within the region, and that the separate sections into which -they divide the area are each in turn characterized by increased -up-cant as we proceed to the northward (<a href="#f373">Fig. 373, <i>E</i></a>. and <a href="#f374">Fig. 374</a>).</p> - -<p><span class="pagenum"><a name="Page_346" id="Page_346">[346]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-417.jpg" width="400" height="455" id="f375" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 375.</span>—Series of idealistic diagrams to indicate the nature of the quick recovery -of the crust by uplift in blocks unloaded of the ice in succession. A further and -slower uptilt, added after the completion of the first movement, is brought out in -the last diagram (<i>b</i>´).</p> -</div></div> - -<p>The beaches of Lake Maumee, the earliest of the series of lakes -within the Huron-Erie lobe and within the extreme southern -portion of the Great Lakes area, show only the slightest possible -northerly uptilt, and the well-marked hinge line disclosed in the -Whittlesey beach is evidence that the elastic recoil, as it were, -from the weight of the mantling glacier did not begin until after -the draining of Lake Whittlesey. The determination by Taylor -that there is a similar initial hinge line in the Warren beach—that -this strand begins its uptilt some fifteen miles farther northeast -than does the Whittlesey beach—is one of the greatest importance -in obtaining a correct idea of the recent uplift; for it -shows that the draining of Lake Whittlesey was followed by -a period of quick uplift and seismic activity, that the stage of<span class="pagenum"><a name="Page_347" id="Page_347">[347]</a></span> -Lake Warren was one of comparative stability of the land, -and, lastly, that the draining of Lake Warren was followed by a -second period of rapid uplift and earthquake disturbance. -The strongly marked hinge lines, additional to the initial one -indicated for the Algonquin beaches in the profiles by Goldthwait -from the west shore of Lake Michigan, when considered in -the light of this northeasterly migration of the still earlier hinge -line in the southern district, are best explained through the assumption -of a succession of quick recoveries of the crust by uplift, -separated by periods of relative stability, and brought on by -the removal in turn of the ice burden from successive blocks of -the shell which are separated by the several hinge lines (<a href="#f375">Fig. 375</a>).</p> - -<p>The elaborate study of erosion in the outlet of Lake Agassiz -had indicated identical interruptions in the up-canting process -for that basin.</p> - - -<p><b>Future consequences of the continued uptilt within the lake -region.</b>—One of the most distinguished of American geologists, -Dr. G. K. Gilbert, in order to determine whether the uptilt revealed -by canted beach lines is still in progress, carried out an elaborate -study upon the gauge records preserved at the various gauging -stations about the Great Lakes. Upon the basis of these studies, -he concluded that the uplift continues, that the axes of equal -uplift (isobases) take their course about fifteen degrees north of -west, so that the lines of greatest uptilt should be perpendicular to -this direction, or fifteen degrees east of north. He further believed -that the basin was undergoing an up-cant in the simple manner of -a trap door, the hinge of which lay to the southward of Chicago, -and the study of the gauge records led him to believe that “the -rate of change is such that the two ends of a line one hundred miles -long and lying in a south-southwest direction are relatively displaced -four tenths of a foot in one hundred years.”</p> - - -<p><b>Gilbert’s prophecy of a future outlet of the Great Lakes to -the Mississippi.</b>—The <i>natural</i> rock sill, over which the waters -of Lake Chicago once flowed to the Mississippi, is to-day but -eight feet above the common mean level of Lakes Michigan and -Huron, and if the tilting of the lake region were to continue upon -Gilbert’s assumption of a canting plane with the hinge of the -movement to the south of Chicago, a time must come when the -“Chicago outlet” will again come into use and the lakes once<span class="pagenum"><a name="Page_348" id="Page_348">[348]</a></span> -more drain to the Mississippi and the Gulf. Upon the basis of -his measurements, Gilbert ventured the prophecy that the first -high-water discharge into the Mississippi should occur in from -five hundred to six hundred years, and for continuous discharge -in fifteen hundred years. In twenty-five hundred years Niagara -Falls should at low water stages be dry from this cause, and in -thirty-five hundred years it should have become extinct.</p> - -<p>This prophecy, emanating from a high scientific authority and -relating to changes of such profound economic and commercial -importance, has been often quoted and has taken a firm hold upon -the popular imagination. Obviously, it depends upon the now -exploded theory that the lake basin has been canted <i>as a plane</i> -and that the axis of uptilt lies somewhere to the southward of -the lake region, or, in any event, to the southward of the present -Port Huron outlet. We know to-day that instead of being uniformly -distributed over the entire lake region, the uptilting goes -on at a much higher rate within the northern areas, and that -since the early stage of Lake Whittlesey the hinge line of uplift -has been steadily migrating northward with the retreat of the -ice and is now well to the northward of the present outlet. There -is, therefore, no known uptilt of the district which separates -the present from the former Chicago outlet, and there is no apparent -natural cause which should result in the reoccupation of -the old outlet to the Mississippi. The prophecy must be regarded -as one that has been outgrown with the progress of science.</p> - - -<p><b>Geological evidences of continued uplift.</b>—It has recently -been claimed, on the basis of a reëxamination of Gilbert’s study -of the lake gauge records, that his methods are open to serious -criticism and that in reality the figures afford no evidence of continued -uplift of the region. However this may be, there are not -lacking geological evidences which do not admit of doubt, and -these are in a striking way confirmatory of the latest conclusions -upon the manner of the recent uplift.</p> - -<p>If our conclusions have been correct, the several lake basins -should now be behaving in different ways as regards the changes -upon their shores. If it is true that the lines of greatest uptilt -run north-northeasterly, there should be, speaking broadly, a -“spilling over” of waters upon the south-southwesterly shores -and a laying bare of the north-northeasterly shore terraces of the<span class="pagenum"><a name="Page_349" id="Page_349">[349]</a></span> -basins. This should, however, be true only of basins whose -outlets are to the northeastward of the existing main hinge line -of uptilt. Lake Huron, having its outlet at the southern margin -of its basin, should not have its waters encroaching upon the -southern shore, for the simple reason that any continued uptilt -of the basin can only have the effect of pouring more water through -the outlet. Lake Michigan and Saginaw Bay, which are arms of -the Huron basin, ought, however, to become flooded upon their -southern shores, <i>were it not that the hinge line of uptilt to-day lies -to the northward of the outlet at Port Huron, and, further, that the -two connecting channels still have their beds lower than the sill of -the outlet channel</i>. Now the evidence goes to show that no encroachment -of waters is occurring upon the Chicago shore of Lake -Michigan, and although the shores of Saginaw Bay are so excessively -flat as to reveal slight changes of level by large migrations -of the strand, yet the ancient meander posts fixed by the early -surveys are still found near the water’s edge.</p> - - -<p><b>Drowning of southwestern shores of Lakes Superior and Erie.</b>—Within -the basins occupied by Lakes Superior and Erie, a wholly -different condition is found. In each case the outlet is found -to the northeastward (<a href="#f374">Fig. 374</a>, <a href="#Page_345">p. 345</a>), and the northwesterly trend -of the isobases from these outlets is responsible for a continued -elevation from uptilt of the outlets with reference to the western -and southern shores. In consequence, the waters are encroaching -upon these shores, and rivers which there enter the lake are -drowned at their mouths, with the formation of estuaries. Upon -Lake Superior these changes are very marked near Duluth and particularly -in the St. Louis River, within which, since the early treaty -with the Indians, certain rapids have disappeared and submerged -trunks of trees are now found in the channel of the river. As -far east as Ontonagon essentially the same conditions are found.</p> - -<p>Upon the shores within the Porcupine Mountain district, the -waters are clearly rising. Here old cedar trees may be seen, in -some cases dead but still upright and standing in from six to eight -inches of water a number of feet out from the present shore, -while others near the shore, but upon the land and still living, are -washed by the waves, and losing their lower bark in consequence. -An old road along the shore has had to be abandoned because of -the encroaching water.</p> - -<p><span class="pagenum"><a name="Page_350" id="Page_350">[350]</a></span></p> - -<p>Upon the opposite or northeastern shore of the lake, on the -other hand, the land is everywhere rising out of the water, and -the waves are now building storm beaches well out upon the wave-cut -terrace. Here the streams, instead of forming estuaries by -drowning, drop down -in rapids to the level -of the lake.</p> - -<div class="floatleft"> - <img src="images/ill-421.jpg" width="250" height="192" id="f376" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 376.</span>—Portion of the Inner Sandusky Bay, to -afford a comparison of the shore line of 1820 with -that of to-day (after Moseley).</p> -</div></div> - -<p>At the southwestern -margin of Lake -Erie there is everywhere -evidence of a -rapid encroachment -by the water. In the -caves of South Bass -Island stalactites, -which must obviously -have formed above -the lake level, are -now permanently submerged. -It is, however, about Sandusky Bay upon the southwest -shore that the most striking observations have been made. -Moseley has collected historical records of the killing of forest -trees through a submergence which was the result of an advance -of the water upon the shores. It seems to be proven from his -studies that the water is now rising in Sandusky Bay at a rate of -about 2.14 feet per century. In <a href="#f376">Fig. 376</a> there is a comparison -of the shores of the inner bay separated by an interval of about -ninety years.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter XXIV</span></p> - -<p class="p1">Uptilt in basin of Lake Agassiz:—</p> - -<p class="pex"><span class="smcap">Warren Upham.</span> The Glacial Lake Agassiz, Mon. 25, U. S. Geol. Surv., -pp. 474-522.</p> - -<p class="p1">Uptilt in Laurentian Basin:—</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Recent Earth Movement in the Great Lakes Region, -18th Ann. Rept. U. S. Geol. Surv., 1898, Pt. ii, pp. 595-647.</p> - -<p class="pex"><span class="smcap">J. W. Spencer.</span> Deformation of the Algonquin Beach, etc., Am. Jour. -Sci. (3), vol. 41, 1891, pp. 14-16.</p> - -<p class="pex"><span class="smcap">F. B. Taylor.</span> The Highest Old Shore Line of Mackinac Island, Am. -Jour. Sci. (3), vol. 43, 1892, pp. 210-218.</p> - -<p><span class="pagenum"><a name="Page_351" id="Page_351">[351]</a></span></p> - -<p class="pex"><span class="smcap">A. C. Lawson.</span> Sketch of the Coastal Topography of the North Side of -Lake Superior, with reference to the abandoned strands, etc., 20th -Ann. Rept. Geol. and Nat. Hist. Surv. Minn., 1893, pp. 181-289, -pls. 7-12.</p> - -<p class="pex"><span class="smcap">J. B. Woodworth.</span> Ancient Water Levels of the Champlain and Hudson -Valleys, Bull. 84, N.Y. State Mus., 1905, pp. 265, pls. 28.</p> - -<p class="pex"><span class="smcap">E. L. Moseley.</span> Formation of Sandusky Bay and Cedar Point, Proc. -Ohio State Acad. Sci., vol. 4, 1905, Pt. v, pp. 179-238.</p> - -<p class="pex"><span class="smcap">F. E. Wright.</span> Rept. Geol. Surv. Mich. for 1903, 1905, p. 37.</p> - -<p class="pex"><span class="smcap">J. W. Goldthwait.</span> The Abandoned Shore Lines of Eastern Wisconsin, -Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 37; A -Reconstruction of Water Planes of the Extinct Glacial Lakes in the -Lake Michigan Basin, Jour. Geol., vol. 16, 1908, pp. 459-476; Isobases -of the Algonquin and Iroquois Beaches and their Significance, -Bull. Geol. Soc. Am., vol. 21, 1910, pp. 227-248, pl. 5; An Instrumental -Survey of the Shore Lines of the Extinct Lakes Algonquin and -Nipissing in Southwestern Ontario, Mem. 10, Dept. of Mines, Canada, -1910, pp. 57, pls. 4.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Late Glacial and Post-glacial Uplift of the -Michigan Basin, Pub. 5, Mich. Geol. and Biol. Surv., 1911, pp. 68, -pls. 2.</p> - -<p class="pex"><span class="smcap">Lawrence Martin.</span> [Post-glacial Modifications in and Around the Great -Lakes], Mon. 52, U. S. Geol. Surv., 1911, pp. 455-459.</p> - -<p class="p1">Uptilt in northern Europe:—</p> - -<p class="pex"><span class="smcap">G. de Geer.</span> Quaternary Changes of Level in Scandinavia, Bull. Geol. -Soc. Am., vol. 3, 1892, pp. 65-68, pl. 2.</p> - -<p class="pex"><span class="smcap">H. Munthe.</span> Studies in the Late Quaternary History of Southern -Sweden, paper No. 25, Livret Guide, Cong. Géol. Intern., 1910, pp. -96, many plates and maps.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_352" id="Page_352">[352]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXV</h2> - -<p class="pch">NIAGARA FALLS A CLOCK OF RECENT GEOLOGICAL -TIME</p> - -<p><b>Features in and about the Niagara gorge.</b>—A striking example -of those permanent alterations of drainage which have -resulted from the presence of the late continental glacier in North -America is to be found in the Niagara gorge between Lakes Erie -and Ontario. With the aid of borings many of the now buried -channels of the region have been followed out, and in a later paragraph -we shall refer to some of the stronger lines of the earlier -drainage system. Before undertaking the study of Niagara history, -it is essential that one become somewhat familiar with the -present topography in and about the Niagara gorge.</p> - -<p>Below the present cataract the river flows through a deep gorge -for about seven miles before issuing at the Lewiston Escarpment -(<a href="#f381">Fig. 381</a>, <a href="#Page_355">p. 355</a>). This gorge has been cut in beds of rock sediments -which dip at a gentle angle southward toward Lake Erie. -The capping of the rock series is a compact and relatively resistant -limestone which is known as the Niagara limestone, beneath -which there are alternating beds of shale with thinner limestone -and sandstone. The plain formed by the upper surface of the -limestone capping terminates in the Lewiston Escarpment, which -is transverse to the direction of the gorge and seven miles distant -below the Falls. The depth of the gorge varies markedly, the -above-water portion being represented at the upper end by the -height of the cataract, one hundred and sixty-five feet, while at -its lower end near Lewiston it is twice that amount. Halfway -down the gorge a sharp turn is made at an angle of more than -ninety degrees, and the upstream arm is extended to form a -basin which contains the famous whirlpool. This visible extension -of the upper gorge is continued in a buried channel, the St. -Davids Gorge, which extends to the escarpment, broadening as -it does so in the form of a trumpet. The materials which fill -this earlier channel are notably coarse glacial deposits (<a href="#f389">Fig. 389</a>).</p> - -<p><span class="pagenum"><a name="Page_353" id="Page_353">[353]</a></span></p> - -<div class="floatright"> - <img src="images/ill-424a.jpg" width="200" height="172" id="f377" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 377.</span>—Ideal cross section -of the Niagara gorge to show -the marginal terrace.</p> -</div></div> - -<p>Directly above the whirlpool the Niagara gorge is first contracted, -but almost immediately swells out into the form of a -sausage, which under the name of the Eddy Basin extends to the -constricted channel occupied by the Whirlpool Rapids. This Gorge -of the Whirlpool Rapids extends to and a little above the railroad -bridges, where it again suddenly widens and deepens and with -surprisingly uniform cross section now continues as far as the cataract. -This uppermost section is known as the Upper Great -Gorge. About a mile below the whirlpool -is that remarkable projection into -the gorge from the Canadian wall which -is known as Wintergreen Flats, below -which and nearer the river are Fosters -Flats. Almost throughout its entire -length the Niagara gorge is bordered -on either side by a narrow and gently -incurving terrace eroded below the general -level of the plain and meeting the -gorge in a sharp angle (<a href="#f377">Fig. 377</a>).</p> - -<p>The features immediately about the cataract show that the Falls -are to-day in a condition which, so far as we know, has occurred -but once before in their entire history—the waters of the river -are divided unequally by an island, and for this reason, as we shall -see, the cataract enters over the <i>side wall</i> of the gorge instead of -at its <i>end</i> (<a href="#f381">Fig. 381</a>), although the turning of the channel from this -cause is combined with a bend of the river.</p> - -<div class="floatleft"> - <img src="images/ill-424b.jpg" width="250" height="138" id="f378" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 378.</span>—View of the bed of the Niagara -River above the cataract, where water -has been drained off in installing a power -plant. Some separated blocks of limestone -are still in place (after J. W. -Spencer).</p> -</div></div> - -<p><b>The drilling of the gorge.</b>—There appear to be two important -processes which are responsible -for the recession of the Falls, -the rate of which is determined -largely by the resistance of the -limestone capping and the tenacity -of the looser shale beneath -it. One of the eroding processes -operates from below and undermines -the cap until the unsupported -cornice falls in blocks -to the bottom of the gorge; -the other makes its attack directly<span class="pagenum"><a name="Page_354" id="Page_354">[354]</a></span> -from above, selecting for the purpose the lines of jointing -of the rock which it widens by solution and corrasion until the -included blocks are in so far separated that they are torn out and -go over the brink of the Falls (<a href="#f378">Fig. 378</a>). This process of overhead -attack in the powerful currents just above a cataract is even -better illustrated by the Falls of St. Anthony near Minneapolis, -which have had a similar history of recession to that of the Niagara -Falls (<a href="#f379">Fig. 379</a>).</p> - -<div class="figcenter"> - <img src="images/ill-425.jpg" width="400" height="263" id="f379" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 379.</span>—Falls of St. Anthony, looking westward from Hennepin Island in 1851 -(after N. H. Winchell, daguerreotype by Hessler of Chicago).</p> -</div></div> - -<p>The blocks of the capping limestone at Niagara Falls are to -some extent fixed in size by the joint planes present in them, and -as they fall to the bottom of the gorge, they promote or retard the -further recession of the Falls according as they can or cannot be -moved about by the churning currents beneath the cataract. Of -the retarding effect there is an illustration in the accumulation of -the blocks below the American and the intermediate Luna Falls -(<a href="#p23a">plate 23 A</a>), which the weaker currents upon the American side -find too heavy to handle.</p> - -<div class="floatleft"> - <img src="images/ill-426a.jpg" width="230" height="247" id="f380" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 380.</span>—Ideal section to show -the nature of the drilling process -beneath the cataract.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-426b.jpg" width="230" height="527" id="f381" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 381.</span>—Plan and section of the Niagara -gorge, showing how in each section the -depth is proportional to the width, except -in the lowest section where subsequent river -action of the normal type has modified the -bed of the channel (plan after Taylor and -section after Gilbert).</p> -</div></div> - -<p class="vh">————</p> - -<p>The Canadian Fall, with its much greater -power, is an example of the promotion of recession through the -churning about of the blocks at the base of the cataract. We have -here to do with a churn drill which bores its way into the bottom<span class="pagenum"><a name="Page_355" id="Page_355">[355]</a></span> -of the gorge with increasing radius of rotary motion with each increase -in volume of the falling water. Under this rotary churning -the soft shales are torn out near the bottom and in succession -the harder layers -above until the capping is -reached (<a href="#f380">Fig. 380</a>). The conditions -appear now to be such -that the effective work is -largely concentrated, as it -usually has been, near the -middle of the channel; and -so the gorge recedes with a -margin of the earlier river -bed remaining as a terrace on -either side and extending to -the former river bank (<a href="#f377">Fig. 377</a>).</p> - -<p>As must have been noted, -one peculiarity of the operation -of the churn drill beneath -the cataract is that the depth -of the gorge will bear a direct -proportion to its width, and<span class="pagenum"><a name="Page_356" id="Page_356">[356]</a></span> -if the volume of water has varied during the process of recession, -these changes in volume will be registered in the width and also -in the depth of that section of the gorge which was drilled at the -time—the cross section of the gorge at any place is proportional -to the volume of the water falling in the cataract which produced -it, modified, however, by the competency to handle the joint blocks -of definite size (<a href="#f381">Fig. 381</a>).</p> - -<div class="figcenter"> - <img src="images/ill-427.jpg" width="300" height="352" id="f382" - alt="" - title="" /> - <div class="cf"><p class="ch300"><span class="smcap">Fig. 382.</span>—Comparison of a sketch of the Canadian Fall made with the aid of a -camera lucida in 1827 with a photograph taken from the same view point in 1895 -(after Gilbert).</p> -</div></div> - -<p><b>The present rate of recession.</b>—There are various sketches, -more or less accurate, made in the early part of the nineteenth -century, and from the later period there are daguerreotypes, photographs, -and maps, which refer especially to the Canadian Fall; and -which, taken together, render possible a comparison of the earlier -with the later brinks. By comparing the earliest with the recent, -views it is seen at a glance that the Falls are receding, and at a -quite appreciable rate (<a href="#f382">Fig. 382</a>). A careful comparison of the<span class="pagenum"><a name="Page_357" id="Page_357">[357]</a></span> -maps made in 1842, 1875, 1886, 1890, and 1905 of the brink of -the Canadian Fall (<a href="#f383">Fig. 383</a>) indicates that for the period covered -the rate of recession has been about five feet per year, and similar -studies made of the -American Fall show that -it has been receding at -the rate of only three -inches per year, or one -twentieth the rate of the -recession of the Canadian -Fall.</p> - -<div class="floatright"> - <img src="images/ill-428.jpg" width="250" height="357" id="f383" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 383.</span>—Map to show the recession of the brink -of the Canadian Fall, based upon maps of different -dates (after Gilbert).</p> -</div></div> - -<p><b>Future extinction of the -American Fall.</b>—It is -because of this many -times more rapid recession -of the Canadian -Fall that the Niagara -cataract, instead of lying -athwart the gorge, enters -it from its side. The -Canadian Fall is thus in -reality swinging about -the American, and the -time can already be -roughly estimated when -this more effective drilling -tool will have brought -about a capture, so to speak, of the American Fall through the -cutting off of its water supply. It will then be drained and left -literally “high and dry”, an enduring witness to the geological -effect of an island in making an unequal division of the waters for -the work of two cataracts.</p> - -<p>As already pointed out, the inefficiency of the American Fall -as an eroding agent is amply attested by the wall of blocks -already appearing above the water below it. The tourist who a -thousand years hence pays a visit to the Niagara cataract, provided -the water flow is allowed to remain as it has been, will find -above this rampart of blocks a bare cliff in part undermined, and -surmounted by a nearly flat table surface which is cut off from the<span class="pagenum"><a name="Page_358" id="Page_358">[358]</a></span> -existing cataract by a higher section of the gorge (<a href="#f384">Fig. 384</a>). It -is quite likely that this table will furnish the most satisfactory -viewpoint of the future cataract of that date.</p> - -<div class="figcenter"> - <img src="images/ill-429a.jpg" width="400" height="167" id="f384" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 384.</span>—Comparison of the present with the future falls.</p> -</div></div> - -<p><b>The captured Canadian Fall at Wintergreen Flats.</b>—What we -have predicted for the future of the present American Fall will -be the better understood from the study of a monument to earlier -capture made long before the upper section of the gorge had -been cut or the whirlpool had come into existence. The tables -were then turned, for it was a fall upon the Canadian side of the -gorge that was captured by one upon the American. The locality -is known as Wintergreen Flats, or sometimes as Fosters Flats; -though the first name properly applies to a higher surface near the<span class="pagenum"><a name="Page_359" id="Page_359">[359]</a></span> -brink of the gorge, and Fosters Flats to a lower plain near the level -of the river (see <a href="#f381">Fig. 381</a>, <a href="#Page_355">p. 355</a>). The peculiar topographic features -at this locality are well brought out in Gilbert’s bird’s-eye -view of the locality (<a href="#f385">Fig. 385</a>); in fact, in some respects better -than they appear to the tourist upon the ground, for the reason -that the abandoned channel and the Flats on the site of the since -undermined island are both heavily forested and so not easy to -include in a single view. For one who has studied the existing -cataract this early monument is full of meaning. Standing, as -one may, upon the very brink of the former cataract, it is easy -to call up in imagination the grandeur of the earlier surroundings -and to hear the thunder of the falling water. A particularly vivid -touch is added when, in digging over the sand about the great -blocks of fallen limestone underneath the brink, one comes upon -the shells of an animal still living in the Niagara River, though only -in the continual spray beneath the cataract.</p> - -<div class="figcenter"> - <img src="images/ill-429b.jpg" width="400" height="268" id="f385" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 385.</span>—Bird’s-eye view of the captured Canadian Fall at Wintergreen Flats, -showing the section of the river bed above the cliff and the blocks of fallen Niagara -limestone strewn over the abandoned channel below (after Gilbert).</p> -</div></div> - -<p><b>The Whirlpool Basin excavated from the St. Davids Gorge.</b>—It -has already been pointed out that a rock channel now filled with -glacial deposits extends from the Whirlpool Basin to the edge of -the escarpment at St. Davids (<a href="#f389">Fig. 389</a>, <a href="#Page_363">p. 363</a>). In plan this -buried gorge has a trumpet form, being more than two miles wide -at its mouth and narrowing to the width of the upper gorge before -it has reached the Whirlpool. Near the Whirlpool it has been in -part excavated by Bowman Creek, thus revealing walls that are -well glaciated. Different opinions have been expressed concerning -the origin of this channel, one being that it is the course either of -a preglacial river or one incised between consecutive glacial invasions; -and another that it is a cataract gorge drilled out between -glacial invasions after the manner of the later Niagara gorge. In -either case its contours have been much modified by the later -glacier or glaciers, whose work of planing, polishing, and widening -is revealed in the exposed surfaces; and it is not improbable that -a cataract has receded along the course of an earlier river valley.</p> - -<p>As we shall see, there are facts which point rather clearly to an -earlier cataract which ended its life immediately above the present -Whirlpool. When the later Niagara cataract had receded to near -the upper end of the Cove section, or near the present Whirlpool, -the falling water must have been separated from this older channel -and its filling of till deposits by only a thin wall of rock, and this<span class="pagenum"><a name="Page_360" id="Page_360">[360]</a></span> -must have been constantly weakened as its thickness was further -reduced.</p> - -<p>When this weakened dam at last gave way, it must have produced -a debacle grand in the extreme. It is hardly to be conceived -that the “washout” of the ancient channel to form the Whirlpool -Basin could have occupied more than a small fraction of a -day, though it is highly probable that the broken rock partition -below the Whirlpool was not immediately removed entire. The -mandible-like termination of the Eddy Basin immediately above -the Whirlpool has led Taylor to believe that the cataract quickly -reëstablished itself at this point upon the last site of the extinct -St. Davids cataract. If reduced in power for a short interval, as a -result of the obstructions still remaining in the lately broken dam -below the Whirlpool, the remarkable narrowing of the gorge at -this point would be sufficiently accounted for.</p> - -<p>Being compelled to turn through more than a right angle after -it enters the Whirlpool Basin, the swift current of the Niagara -River is forced to double upon itself against the opposite bank -and dive below the incoming current before emerging into the -Cove section below the Whirlpool (<a href="#f386">Fig. 386</a>).</p> - -<div class="floatleft"> - <img src="images/ill-431.jpg" width="250" height="209" id="f386" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 386.</span>—Map of the Whirlpool Basin, -showing the rock side walls like those of -the Niagara Gorge, and the drift bank -which forms the northwest wall (after -Gilbert).</p> -</div></div> - -<p>In tearing out the loose deposits which had filled this part of -the buried St. Davids Gorge, -many bowlders of great size -were left which slid down the -slope and in time produced an -armor about the looser deposits -beneath, so as to protect them -and prevent continued excavation. -Thus it is found that the -submerged northwestern wall -of the basin is sheathed with -bowlders large enough to retain -their positions and so stop a -natural process of placer outwashing -upon a gigantic scale -(<a href="#f386">Fig. 386</a>).</p> - - -<p><b>The shaping of the Lewiston Escarpment.</b>—To understand -the formation of the Lewiston Escarpment cut in the hard Niagara -limestone, it is necessary to consider the geology of a much larger<span class="pagenum"><a name="Page_361" id="Page_361">[361]</a></span> -area—that of the Great Lakes region as a whole. To the north -of the Lakes in Canada is found a most ancient continent which -was in existence when all the area to the southward lay below the -waters of the ocean. In a period still very many times as long -ago as the events we have under discussion, there were laid down -off the shore of this oldland a series of unconsolidated deposits -which, hardened in the course of time, and elevated, are now represented -by the shales, sandstone, and limestone which we find, one -above the other, in the Niagara gorge in the order in which they -were laid down upon the ocean floor. The formations represented -in the gorge are but a part of the entire series, for other higher members -are represented by rocks about Lake Erie and even farther -to the southward. These strata, having been formed upon an outward -sloping sea floor, had a small initial dip to the southward, -and this has been probably increased by subsequent uptilt, including -the latest which we have so recently had under discussion. At -the present time the beds dip southward by an angle of less than -four degrees, or about thirty-five feet in each mile.</p> - -<div class="figcenter"> - <img src="images/ill-432.jpg" width="400" height="311" id="f387" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 387.</span>—Map to show the cuestas which have played so important a part in -fixing the boundaries of the Lake basins, and also the principal preglacial rivers -by which they have been trenched (based upon a map by Grabau).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_362" id="Page_362">[362]</a></span></p> - -<p>When the elevation of the land in the vicinity of this shore had -caused a recession of the waters, there was formed a coastal plain -on the borders of the oldland like that which is now found upon -our Atlantic border between the Appalachians and the sea (<a href="#f272">Fig. 272</a>, -<a href="#Page_246">p. 246</a>). The rivers from the oldland cut their way in narrow -trenches across the newland, and because of the harder limestone -formations, their tributaries gradually became diverted from their -earlier courses until they entered the trunk stream nearly at right -angles and produced the type of drainage -network which is called “trellis drainage.” -It is characteristic of this drainage that -few tributaries of the second order will -flow up the natural slope of the beds, but -on the contrary these natural slopes are -followed in the softer rock nearly at right -angles again to the tributaries of the first -order of magnitude (<a href="#f387">Fig. 387</a>). Thus are -produced a series of more or less parallel -escarpments formed in the harder rock and -having at their base a lowland which rises -gradually in the direction of the oldland -until a new escarpment is reached in the -next lower of the hard formations. Such -flat-topped uplands in series with intermediate -lowlands and separated by sharp -escarpments are known as <i>cuestas</i> (see <a href="#Page_246">p. 246</a>), -and the Lewiston Escarpment limits that formed in Niagara -limestone (<a href="#f38">Figs. 387</a> and <a href="#f388">388</a>).</p> - -<div class="floatleft"> - <img src="images/ill-433.jpg" width="200" height="326" id="f388" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 388.</span>—Bird’s-eye view -of the cuestas south of -Lakes Ontario and Erie -(after Gilbert).</p> -</div></div> - -<p><b>Episodes of Niagara’s history and their correlation with those -of the Glacial Lakes.</b>—Of the early episodes of Niagara’s history, -our knowledge is not as perfect as we could desire, but the later -events are fully and trustworthily recorded. The birth of the -Falls is to be dated at the time when the ice front had here first -retired into what is now Canadian territory, thus for the first time -allowing the waters from the Erie basin to discharge over the Lewiston -Escarpment into the basin of the newly formed Lake Iroquois -(<a href="#f364">Fig. 364</a>, <a href="#Page_334">p. 334</a>). Since the level of Lake Iroquois was far above -that of the present Lake Ontario, the new-born cataract was not -the equivalent in height of the escarpment to-day. The Iroquois<span class="pagenum"><a name="Page_363" id="Page_363">[363]</a></span> -waters then bathed all the lower portion of the escarpment, so -that the foot of the Fall was upon the borders of the Lake.</p> - -<p>In order to interpret the history of the Niagara gorge, we must -remember that the effective drilling of this gorge was in each stage -dependent mainly upon -the volume of water discharged -from Lake Erie, -a large discharge being -recorded by a channel -drilled both wide and -deep, while that produced -by the discharge -of a smaller volume was -correspondingly narrow -and shallow. To-day -the gorges of large cross -section have, moreover, -a relatively placid surface, -whereas through the -constricted sections the -water of the river is unable -to pass without first -raising its level at the -upper end and under the -head thus produced rushing -through under an increased -velocity. The -best illustration of such a constricted section is the Gorge of the -Whirlpool Rapids.</p> - -<div class="floatright"> - <img src="images/ill-434.jpg" width="250" height="326" id="f389" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 389.</span>—Sketch map of the greater portion of -the Niagara Gorge to show the changes in cross -section in their relations to Niagara history -(based upon a map by Taylor).</p> -</div></div> - -<p>Our reading of the history should begin at the site of the present -cataract, since the records of later events are so much the more -complete and legible, and it should ever be our plan to proceed -from the clearly written pages to those half effaced and illegible.</p> - -<p>As we have learned, the most abrupt change in the cross section -of the gorge is found a little above the railroad bridges, where the -Upper Great Gorge is joined to the Gorge of the Whirlpool -Rapids (<a href="#f389">Fig. 389</a>). In view of the remarkably uniform cross -section of the Upper Great Gorge, there is no reason to doubt that -it has been drilled throughout under essentially the same volume<span class="pagenum"><a name="Page_364" id="Page_364">[364]</a></span> -of water, and that its lower limit marks the position of the former -cataract when the waters from the upper lakes were transferred -from the “North Bay Outlet” into the present or “Port Huron -Outlet” and Lake Erie. As the upper limit of the Gorge of the -Whirlpool Rapids thus corresponds to the closing of the “North -Bay Outlet” and the extinction of the Nipissing Great Lakes, -so its lower limit doubtless corresponds to the opening of that outlet -and the termination of the preceding Algonquin stage; for in the -stage of the Nipissing lakes the water of the upper lakes, as we -have learned, reached the ocean through the northern outlet.</p> - -<p>Mr. Frank Taylor, who has given much study to the problem -of Niagaran history, believes that the Middle Great Gorge, comprising -the Eddy Basin and the Cove section, represents the gorge -drilling which occurred during the later stage of Lake Algonquin -after the “Trent Outlet” had been closed and the waters of the -upper lakes had been turned into the Erie Basin.</p> - -<p>Summarizing, then, the episodes of the lake and the gorge history -are to be correlated as follows:—</p> - -<table id="t08" summary="t08"> - - <tr> - <td class="tdc"><span class="smcap">Glacial Lake</span></td> - <td class="tdc"><span class="smcap">Niagara Gorge</span></td> - </tr> - - <tr> - <td class="tdt5w">Early Lakes Iroquois and Algonquin.</td> - <td class="tdt5">Drilling of the gorge from the -Lewiston Escarpment to the Cove -section above the Wintergreen -Flats.</td> - </tr> - - <tr> - <td class="tdt5">Later Lakes Iroquois and Algonquin -with upper lakes discharging -into Erie basin.</td> - <td class="tdt5">Drilling of Middle Great Gorge.</td> - </tr> - - <tr> - <td class="tdt5">Nipissing Great Lakes with the -upper lake waters diverted from -Lake Erie.</td> - <td class="tdt5">Drilling of the narrow Gorge of -the Whirlpool Rapids.</td> - </tr> - - <tr> - <td class="tdt5">Recent St. Lawrence drainage -since the waters of the upper lakes -were discharged into Lake Erie -through occupation of the Port -Huron Outlet.</td> - <td class="tdt5">Drilling of Upper Great Gorge to -the present cataract.</td> - </tr> - -</table> - -<p class="p1"><b>Time measures of the Niagara clock.</b>—In primitive civilizations -time has sometimes been measured by the lapse necessary -to accomplish a certain task, such, for example, as walking the -distance between two points; and the natural clock of Niagara -has been of this type. But men possess differences in strength -and speed, and the same man is at some times more vigorous than<span class="pagenum"><a name="Page_365" id="Page_365">[365]</a></span> -at others, and so does not work at a uniform rate. The cataract -of Niagara, charged with the pent-up energy of the waters of all -the Great Lakes, can rush its work as it is clearly unable to do at -times when the greater part of this energy has been diverted. -Units of distance measured along the gorge are therefore too unreliable -for our use, with the unique exception of the stretch from -the railroad bridges to the site of the present cataract, within -which stretch the gorge cross sections are so nearly uniform as to -indicate an approximation to continued application of uniform -energy. This energy we may actually measure in the existing -cataract, and so fix upon a unit of time that can be translated into -years.</p> - -<p>In order to secure the normal rate of recession of this Upper -Great Gorge, we should add to the volume of water in the Canadian -Fall that now passing over the American; and for the reason that -the blocks which fall from the cataract cornice and are the tools -of the drilling instrument approximate to a definite size fixed by -their joint planes, the effect of this added energy it is not easy -to estimate. We may be sure, however, that the drilling action -would be somewhat increased by the junction of the two Falls, -and thus are assured that the average rate of recession within the -Upper Great Gorge has been somewhat in excess of the five feet -per year determined by Gilbert for the present Canadian Fall. -The Upper Great Gorge is about two miles in length, and its beginning -may thus be dated near the dawning of the Christian Era. -The Whirlpool Gorge was cut when the ice vacated the North Bay -Outlet in Canada, and still lay as a broad mantle over all northeastern -Canada. For the earlier gorge and lake stages, the time -estimates are hardly more than guesses, and we need not now concern -ourselves with them.</p> - -<p><b>The horologe of late glacial time in Scandinavia.</b>—A glacial -timepiece of somewhat different construction and of greater refinement -has been made use of in Scandinavia to derive the “geochronology -of the last 12,000 years.” Instead of retreating over -the land and impounding the drainage as it did so, the latest continental -glacier of Scandinavia ended below sea level, and as it -retired, its great subglacial river laid down a giant esker known as -the Stockholm Os, which was bordered by a delta and fringed on -either side by water-laid moraines of the block type. These recessional<span class="pagenum"><a name="Page_366" id="Page_366">[366]</a></span> -moraines are upon the average less than 1000 feet apart, -and are believed to have each been formed in a single season. The -delta deposits which surround the esker are of thin-banded clay, -and as an additional uppermost band is found outside every moraine, -these bands are also believed to represent each the delta -deposit of a single year. In studies extending over many years, -Baron de Geer, with the aid of a large body of student helpers, -has succeeded in completing a count of moraines and clay layers, -and so in determining the time to be 12,000 years since the ice -front of the latest continental glacier lay across southern Sweden. -The fertility of conception and the thoroughness of execution of -this epoch-making investigation recommend its conclusion to the -scientific reader.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter XXV</span></p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Niagara Falls and their History, Nat. Geogr. Soc. -Mon., vol. 1, No. 7, 1895, pp. 203-236.</p> - -<p class="pex"><span class="smcap">F. B. Taylor.</span> Origin of the Gorge of the Whirlpool Rapids at Niagara, -Bull. Geol. Soc. Am., vol. 9, 1898, pp. 59-84.</p> - -<p class="pex"><span class="smcap">A. W. Grabau.</span> Guide to the Geology and Paleontology of Niagara -Falls and Vicinity, Bull. N. Y. State Mus., vol. 9, No. 45, 1901, pp. -1-85, pls. 1-11.</p> - -<p class="pex"><span class="smcap">J. W. Spencer.</span> The Falls of Niagara, etc. Dept. of Mines, Geol. Surv. -Branch, Canada, 1907, pp. 490, pls. 43.</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Rate of Recession of Niagara Falls, etc. Bull. 306, U. S. -Geol. Surv., 1907, pp. 31, pls. 11.</p> - -<p class="pex"><span class="smcap">G. de Geer.</span> Quaternary Sea Bottoms of Western Sweden. Paper 23, -Livret Guide Cong. Géol. Intern., 1910, pp. 57, pls. 3.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_367" id="Page_367">[367]</a></span></p> - -<div class="chapter"> - -<h2>CHAPTER XXVI</h2> - -<p class="pch">LAND SCULPTURE BY MOUNTAIN GLACIERS</p> - -<p><b>Contrasted sculpturing of continental and mountain glaciers.</b>—In -discussing in a previous chapter the rock pavement lately uncovered -by the Greenland glacier, we learned that this surface had -been lowered by the processes of plucking and abrasion, the combined -effect of which is always to reduce the irregularities of the -surface, soften its outlines, and from sharply projecting masses to -develop rounded shoulders of rock—<i>roches moutonnées</i>.</p> - -<p>Though the same processes act in much the same manner beneath -mountain glaciers, though here upon all parts of the bed, they are, -in the earlier stages at least, subordinated to a third process more -important than the two acting together. Sculpture by mountain -glaciers, instead of reducing surface irregularities and softening -outlines, increases the accent of the relief and produces the most -sharply rugged topography that is known. In nearly all places -where Alpinists resort for difficult rock climbing, mountain glaciers -are to be seen, or the evidence for their former presence may -be read in unmistakable characters.</p> - - -<p><b>Wind distribution of the snow which falls in mountains.</b>—Until -quite recently students of glaciation have concerned themselves -but little with the work of the wind in lifting and redistributing -the snow after it has fallen. We have already seen that, -for the continental glaciers, wind appears to be the chief transporting -agent, if we except the marginal lobes where glacier flow -assumes large importance. In the case of mountain glaciers, also, -we are to find that for the earlier stages particularly wind is of the -first importance as a redistributing agent. In the higher levels -snow is swept up from the ground by all high winds, and does not -find a resting place until it is dropped beneath an eddy in some -irregularity of the surface; and if the inherited surface be relatively<span class="pagenum"><a name="Page_368" id="Page_368">[368]</a></span> -smooth, this will be found in most cases upon the lee of the -mountain crest.</p> - -<p>In normal cases at least the inherited irregularities of the higher -zones of mountain upland are the gentle depressions which develop -at the heads of streams. These become, then, the sites of snowdrifts -that are augmented in size from year to year, though at -first they melt away in the late summer.</p> - -<div class="figcenter"> - <img src="images/ill-439.jpg" width="400" height="200" id="f390" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 390.</span>—Snowdrift hollowing its bed by nivation and building a delta (at the -left). Quadrant Mountain, Yellowstone National Park.</p> -</div></div> - -<p><b>The niches which form on snowdrift sites.</b>—Wherever a drift -is formed, a process is set in operation, the effect of which is to -hollow out and lower the ground beneath it, a process which has -been called <i>nivation</i>. The drift shown in <a href="#f390">Fig. 390</a> was photographed -in late summer at an elevation of some 9000 feet in the -Yellowstone National Park. The very gently sloping surface -surrounding the drift is covered with grass, but within a zone a -few feet in width on the borders of the drift no grass is growing, -and in its place is found a fine brown soil which is fast becoming -the prey of the moving water derived by melting of the drift. -This is explained by the water permeating the crevices of the rock -and being rent by the nightly freezing. Farther from the drift -the ground is dry, and no such action is possible. With each succeeding -spring the augmented drift as it melts carries all finely -comminuted rock material down slopes beneath the snow to emerge -at the lowest margin and be there deposited in the form of a delta. -By the operation of this process of nivation the higher parts of the<span class="pagenum"><a name="Page_369" id="Page_369">[369]</a></span> -drift site are lowered as deposition goes on upon the lower. The -combined effect is thus to produce a <i>niche</i> or faintly etched amphitheater -upon the slope of the mountain (<a href="#f391">Fig. 391</a>).</p> - -<div class="figcenter"> - <img src="images/ill-440.jpg" width="400" height="311" id="f391" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 391.</span>—Amphitheater formed on a drift site in northern Lapland (after a -photograph by G. von Zahn).</p> -</div></div> - -<p><b>The augmented snowdrift moves down the valley—birth of -the glacier.</b>—In still lower air temperatures the drifts enlarge with -each succeeding year until they endure throughout the summer -season. From this stage on, an increment of snow is left from each -succeeding season. No longer entirely wasted by melting, the -time soon comes when the upper snow layers will by their weight -compress the lower into ice, and the mass will begin to creep down -the slope along the course of the inherited valley. The enlarged -snowdrift which feeds this ice stream is called the <i>névé</i> or <i>firn</i>.</p> - -<p>Against the sloping cliff which had been shaped by nivation -at the upper margin of the snowdrift, that snow which is not of -sufficient depth to begin a movement towards the valley separates -from the moving portion, opening as it does so a cleft or crevasse<span class="pagenum"><a name="Page_370" id="Page_370">[370]</a></span> -parallel to the wall. This crack in the snow is called by its German -name <i>Bergschrund</i> or <i>Randspalte</i>, and may perhaps be referred -to as the marginal crevasse -(<a href="#f392">Fig. 392</a>).</p> - -<div class="floatleft"> - <img src="images/ill-441.jpg" width="200" height="291" id="f392" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 392.</span>—The marginal crevasse or -Bergschrund on the highest margin -of a glacier (after Gilbert).</p> -</div></div> - -<p><b>The excavation of the glacial -amphitheater or cirque.</b>—It has -been found that the marginal crevasse -plays a most important rôle -in the sculpture of mountains by -glaciers, for the great amphitheater -which is everywhere the collecting -basin for the nourishment of mountain -glaciers is not an inherited -feature, but the handiwork of the -ice itself. This was the discovery -of Mr. W. D. Johnson, an American -topographer and geologist, who, in -order to solve the problem of the -amphitheater allowed himself to be -lowered into such a crevasse upon -the Mount Lyell glacier of the -Sierra Nevadas in California.</p> - -<div class="floatright"> - <img src="images/ill-442a.jpg" width="200" height="157" id="f393" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 393.</span>—Niches and cirques in the same -vicinity in the Bighorn Mountains of -Wyoming. <i>A, A</i>, unmodified valleys; -<i>B, B</i>, niches on drift sites; <i>C, C</i>, cirques -on small glacier sites (after map by -F. E. Mathes, U. S. G. S.).</p> -</div></div> - -<p>Let down a distance of a hundred and fifty feet, he reached the -bottom of the crack, and in a drizzling rain of thaw water stood -upon a floor composed of rock masses in part dislodged from a wall -which extended some twenty feet upwards upon the cliff side of the -crevasse. It was evident that the warm air of the day produced -the thaw water which was constantly dripping and which filled -every crack and cranny of the rock surface. With the sinking of -the sun below the peaks the sudden chill, so characteristic of the -end of the day in high mountains, causes this water to freeze and -thus rend the rock along its planes of jointing. Broad and thin -plates of ice, loosened by melting at the walls, could be extracted -from the crevices of the rock as mute witnesses to the powerful -stresses developed by this most vigorous of weathering processes.</p> - -<div class="floatleft"> - <img src="images/ill-442b.jpg" width="230" height="272" id="f394" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 394.</span>—Subordinate small cirques -in the amphitheater on the -west face of the Wannehorn -above the Great Aletsch Glacier -of Switzerland.</p> -</div></div> - -<p>In short, the rock wall above the glacier, which in its initial -stage was the upper wall of the niche hollowed beneath the snowdrift, -is first steepened and later continually both recessed and -deepened by an intensive frost rending which is in operation at<span class="pagenum"><a name="Page_371" id="Page_371">[371]</a></span> -the base of the marginal crevasse. The same process does not go -on as rapidly above the surface of the névé for the reason that <i>the -necessary wetting of the rock surface does not there so generally result -from the daily summer thaw</i>. -At the bottom of the marginal -crevasse alone is this condition -fully realized. Intensive frost -action <i>where the rock is wet with -thaw water daily</i> is thus a -fundamental cause, both of the -hollowing of the early drift site -to form the niche, and of the -later enlargement of this niche -into an amphitheater or cirque -when the drift has been transformed -into the névé of a -glacier. Inasmuch as the crevasse -forms where the snow and -ice pull away from the rock -toward the middle of the depression, the cirque wall in its early -stage has the outline of a semicircle. In the Bighorn Mountains -of Wyoming, all stages, from the unmodified valley heads to the -full-formed cirque, may be seen near -one another (<a href="#f393">Fig. 393</a>). It will be -noted that wherever a glacier has -formed, as indicated by the cirque, -there is a series of lakes which have -developed in the valley below (see -<a href="#Page_412">p. 412</a>).</p> - -<div class="figcenter"> - <img src="images/ill-443a.jpg" width="400" height="174" id="f395" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 395.</span>—“Biscuit cutting” effect of glacial sculpture in the Uinta Mountains of -Wyoming (after Atwood).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-443b.jpg" width="250" height="360" id="f396" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 396.</span>—Two intersecting inverted -cones representing glacial cirques of different -sizes, to show that their intersection -is the arc of a hyperbola, the curve -to which the col approximates.</p> -</div></div> - -<p><b>Life history of the cirque.</b>—In its -earliest stage the cirque is more or -less uniformly supplied with snow -from all sides, and so it enlarges by -recession in a manner to retain its -early semicircular outline. In a later -stage a larger proportion of the snow -reaches the cirque at its sides so that -its further enlargement causes it to -broaden and to flatten somewhat that<span class="pagenum"><a name="Page_372" id="Page_372">[372]</a></span> -part of its outline which represents the head of the valley (<a href="#f389">Fig. 389</a>, -<a href="#Page_364">p. 364</a>). As the territory of the upland is still further invested -by the cirques, their nourishment -becomes still more irregular, -and the circular outline -gives place to a scalloped -border, as the amphitheater -becomes differentiated into -subordinate smaller cirques, -each of which corresponds to a -scallop of the outline (<a href="#f398">Fig. 398</a> -and <a href="#f394">Fig. 394</a>).</p> - -<p><b>Grooved and fretted uplands.</b>—The -partial investment -by cirques of a mountain -upland yields a type of topography -quite unlike that produced -by any other geological -process. The irregularly connected -remnants of the inherited -upland resemble nothing -so much as a layer of dough -from which biscuits have been -cut (<a href="#f395">Fig. 395</a>). The surface as -a whole, furrowed as it is below -the cirques, may be described as a <i>grooved upland</i> (<a href="#p19a">plate 19 A</a>). -A further continuation of the process removes all traces of the -earlier upland, for the cirques intersect from opposite sides and -thus yield palisades of sharp rock pinnacles which rise on precipitous -walls from a terraced floor. This ultimate product of -cirque sculpture by glaciers is called a <i>fretted upland</i> (<a href="#p18a">plate 18 A</a> and <a href="#p19b">19 B</a>).</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 18.</span></p> - -<div class="figcenter"> - <img src="images/ill-444a.jpg" width="400" height="214" id="p18a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Fretted upland of the Alps seen from the summit of Mount Blanc.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-444b.jpg" width="400" height="333" id="p18b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Model of the Malaspina Glacier and the fretted upland above it (after model by -L. Martin).</p> -</div></div> - -</div> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 19.</span></p> - -<div class="figcenter"> - <img src="images/ill-446a.jpg" width="400" height="278" id="p19a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Contour map of a grooved upland, Bighorn Mountains, Wyoming -(U. S. Geol. Survey).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-446b.jpg" width="400" height="280" id="p19b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Contour map of a fretted upland, Philipsburg Quadrangle, Montana -(U. S. Geol. Survey).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_373" id="Page_373">[373]</a></span></p> - -<p><b>The features carved above the glacier.</b>—The ranges of pinnacles -carved out by mountain glaciers have become known by -various names of foreign derivation, such as <i>arête</i>, <i>grat</i>, <i>aiguille</i> -mountains, “files of <i>gendarmes</i>”, etc. They may, perhaps, be -best referred to as <i>comb ridges</i>, and according to their position they -are differentiated into main and lateral comb ridges, as will be -clear from the second map of <a href="#p19b">plate 19</a>.</p> - -<div class="figcenter"> - <img src="images/ill-448.jpg" width="400" height="333" id="f397" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 397.</span>—A col shaped like a hyperbola between Mount Sir Donald and Yogo -Peak in the Selkirks (after a plate by the Keystone Plate Co.).</p> -</div></div> - -<p>With the gradual invasion of the upland upon which the cirques -have made their attack, the area from which winds may gather<span class="pagenum"><a name="Page_374" id="Page_374">[374]</a></span> -up the snow is steadily diminished, and hence cirque recession is -correspondingly retarded. Cirques which have approached each -other from opposite sides of the ridge until they have become tangent -at one point may, however, still receive nourishment at the -sides and so continue to cut down the intervening rock wall to -form a pass or <i>col</i>. The theoretical curve which results from -this intersection is that -known as the hyperbola, -of which an illustration -is afforded by <a href="#f396">Fig. 396</a>. -An approximation to this -form is clearly furnished -by most of the mountain -passes in glaciated mountain -districts, and a particularly -good illustration -is furnished from the -vicinity of Glacier on the -line of the Canadian Pacific -Railway (<a href="#f397">Fig. 397</a>).</p> - -<div class="floatleft"> - <img src="images/ill-449.jpg" width="250" height="215" id="f398" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 398.</span>—Diagrams to illustrate the progressive -investment of an upland by cirques with -the formation of comb ridges, cols, and horns. -I, early stage, youth; II, intermediate stage; -III, late stage, maturity.</p> -</div></div> - -<p>Upon either side of the -col the land mass is left -in high relief, rising from -a more or less triangular -base (<a href="#f398">Fig. 398</a>, III) into a sharp horn or tooth. An illustration -of such a <i>horn</i> is furnished by the Matterhorn in the Swiss Alps, -or by Mount Sir Donald in the Selkirks, though less noteworthy -examples may be found in every maturely glaciated mountain -district.</p> - -<p><b>The features shaped beneath the glacier.</b>—Those features -which are carved above the glacier—the comb ridge, the col, -and the horn—are all shaped as a result of intensive weathering -upon the cirque wall. The shaping at lower levels is accomplished -by processes in operation below the glacier surface, where weathering -is excluded and where plucking and abrasion work together -to tear away and grind off the rock surface. By their joint action -the valley is both deepened and widened, directly to the height of -the glacier surface, and indirectly through undermining as far up -as rock extends. Thus the valley is transformed into one of broad<span class="pagenum"><a name="Page_375" id="Page_375">[375]</a></span> -and flat bed and precipitous side walls—the <span class="font">U</span>-shaped section -illustrated by valleys of the Swiss Alps and in fact in all districts -which have been strongly glaciated by mountain glaciers (<a href="#f399">Fig. 399</a>).</p> - -<div class="floatright"> - <img src="images/ill-450a.jpg" width="200" height="136" id="f399" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 399.</span>—The <span class="font">U</span>-shaped Kern valley -in the Sierra Nevadas of California -(after W. B. Scott).</p> -</div></div> - -<p>As high up in the valley as it has been occupied by the glacier, -the bed is rounded, smoothed, and polished, and marked by the -characteristic glacial scorings or -striæ which point down the valley. -Above the level of the glacier’s -upper surface, on the other -hand, erosion is accomplished -through undermining or sapping, -a process which always leaves -precipitous slopes of ragged surface -made up of the joint planes -on which the fallen blocks have -separated from the cliff. Thus -there is found a sharp line which separates the smoothly rounded<span class="pagenum"><a name="Page_376" id="Page_376">[376]</a></span> -rock surface below from the jagged and precipitous one above -(<a href="#f400">Fig. 400</a>). Inasmuch as this boundary usually separates the -scalable from the inaccessible slopes above, snow is apt to lodge -at this level and make it strikingly apparent.</p> - -<div class="figcenter"> - <img src="images/ill-450b.jpg" width="400" height="292" id="f400" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 400.</span>—Glaciated valley wall in the Sierra Nevadas of California, showing the -sharp line which separates the abraded from the undermined rock surface (after -a photograph by Fairbanks).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-451.jpg" width="250" height="179" id="f401" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 401.</span>—View of the Vale of Chamonix from the -séracs of the Glacier des Bossons. The alb of the -opposite side is well brought out.</p> -</div></div> - -<p>If uplift of the land occurs while glaciers occupy the valleys of -mountains, an increased capacity for deepening the valley is imparted -to these ice -streams, and we find, -as a result, a deep -central valley of <span class="font">U</span> -cross section excavated -within a relatively -broad trough -visible above the -shoulder on either side -of the later furrow. -Save only for its -characteristic curves, -such a valley bears -close resemblance to -a mature stream valley -which has been rejuvenated (see <a href="#Page_173">p. 173</a>). The remnants of the -earlier glacier-carved valley are, as already stated, gently curving -high terraces so common in Switzerland, where they are known as -<i>albs</i> or high mountain meadows. These albs may be seen to special -advantage on the sides of the Chamonix valley (<a href="#f401">Fig. 401</a>), the -Lauterbrunnen valley, or in fact almost any of the larger Alpine -valleys.</p> - - -<p><b>The cascade stairway in glacier-carved valleys.</b>—If now, instead -of giving our attention to the cross section, we follow the course -of the valley that has been occupied by a glacier, we find that it -descends by a series of steps or terraces having many backwardly -directed treads (<a href="#p19a">plate 19</a>), whereas a normal and well-established -river valley has only forward grades. Because of these backward -grades the stream waters are impounded, and so lakes -are found strung along the valley in chains as the larger beads -are found in a rosary, and these are the characteristic <i>rock basin -lakes</i> sometimes referred to as “Paternoster Lakes” (see <a href="#Page_412">p. 412</a> -and <a href="#f402">Fig. 402</a>).</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 20.</span></p> - -<div class="figcenter"> - <img src="images/ill-452.jpg" width="400" height="514" id="p20" - alt="" - title="" /> - <div class="caption"><p class="pc400">Map of the surface modeled by mountain glaciers in the Sierra Nevadas of California -(after I. C. Russell).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_377" id="Page_377">[377]</a></span></p> - -<p>When the backward grades upon the valley floor are especially -steep, the rock step becomes a <i>rock bar</i>, or <i>Riegel</i>, of which nearly -every Alpine valley has its examples. In a walk from the Grimsel -to Meiringen many such bars are passed. Carrying in suspension -the sharp rock sand from the glacier deposits along its bed, the -stream which succeeds to the glacier as it vacates its valley saws -its way through these obstructions with a rapidity that is amazing, -thus producing narrow defiles, of which the Gorge of the Aar near -Meiringen and that of the Gorner near Zermatt are such well-known -examples (<a href="#f403">Fig. 403</a>).</p> - -<div class="figcenter"> - <img src="images/ill-454.jpg" width="400" height="283" id="f402" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 402.</span>—Map of an area near the continental divide in Colorado, showing an -unglaciated surface to the west of the divide, where the westerly winds have cleared -the ground of snow, and the glacier-carved country to the eastward. Note the -regular forms of the youthful cirque, the glacier stairway, and the rock basin lakes -(U. S. G. S.).</p> -</div></div> - -<div class="floatleft"> - <img src="images/ill-455a.jpg" width="280" height="400" id="f403" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 403.</span>—Gorge of the Albula River near -Berkum in the Engadine, cut through a rock -bar by the river which has succeeded to the -earlier glacier.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-455b.jpg" width="280" height="148" id="f404" - alt="" - title="" /> - <div class="cf"><p class="ch280"><span class="smcap">Fig. 404.</span>—Idealistic sketch showing both glaciated -and nonglaciated side valleys tributary to a glaciated -main valley (after Davis).</p> -</div></div> - -<p>It is characteristic of rivers that the tributaries cut their valleys -more rapidly than does the main stream within the neighboring -section, though they cannot cut lower than their outlets—the -side streams enter <i>accordantly</i>. This is easily explained because -the grades of the tributary streams are the steeper, and, as -we well know, the corrosion of a valley is augmented at a most<span class="pagenum"><a name="Page_378" id="Page_378">[378]</a></span> -amazing rate for each increase of its grade. No such law controls -the processes of plucking and abrasion by which the glacier lowers -its floor, for these processes -appear to depend for their -efficiency upon the depth of -the ice, and the supply of -cutting tools, quite as much -as upon the grade of the -bed. To apply a homely -illustration, the hollowing -of flagstones upon our walks -is dependent more upon the -number of persons that pass -over them, and upon their -size and the number of protruding -nails in their boot -heels, than upon the grades -upon which they are placed. -At all events we find that -the main glacier valleys are -cut deeper than the side -valleys, so that the latter -become <i>hanging valleys</i>—they -enter the main valley, -not upon its bed, but some -distance above it (<a href="#f404">Fig. 404</a>).</p> - -<p>The <span class="font">U</span>-shaped hanging valleys, like the main valley, are much -too large for the -streams which now fill -them, and these diminutive -side streams -plunge over the steep -wall of the main valley -in ribbon-like falls so -thin that the wind -turns them aside and -disperses the water in -the spray of a “bridal -veil.” Such falls are<span class="pagenum"><a name="Page_379" id="Page_379">[379]</a></span> -found by the hundred in every glaciated mountain district, imparting -to it one of the greatest of its scenic charms.</p> - -<div class="figcenter"> - <img src="images/ill-456a.jpg" width="400" height="197" id="f405" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 405.</span>—Character profiles in landscapes sculptured by mountain glaciers.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-456b.jpg" width="400" height="233" id="f406" - alt="" - title="" /> - <div class="caption"><p class="pc"><span class="smcap">Fig. 406.</span>—Flat dome shaped under the margin of a Norwegian ice cap with projecting -rock knobs and moraines in foreground.</p> -</div></div> - -<p><b>The character profiles which result from sculpture by mountain -glaciers.</b>—The lines which are repeated in landscapes carved by -mountain glaciers are easy to recognize (<a href="#f405">Fig. 405</a>). The highest -horizon lines are the outlines of horns which are separated by cols. -Minaret-like palisades, or “files of <i>gendarmes</i>”, often run for long -distances as the characteristic comb ridges. Lower down and<span class="pagenum"><a name="Page_380" id="Page_380">[380]</a></span> -lacking the lighter background of the sky, we make out with less -distinctness the <span class="font">U</span>-valley, either with or without the albs to show -that the sculpturing process has been interrupted by uplift.</p> - -<div class="figcenter"> - <img src="images/ill-457.jpg" width="400" height="487" id="f407" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 407.</span>—Two views illustrating successive stages in the shaping of tinds -or “beehive” mountains.</p> -</div></div> - -<p><b>The sculpture accomplished by ice caps.</b>—In the case of ice -caps, the only rock exposed is found in the neighborhood of the -margin—the projecting islands known as nunataks. It is essential -for the existence of the ice cap that the rock base should<span class="pagenum"><a name="Page_381" id="Page_381">[381]</a></span> -have relatively slight irregularities compared to the dimensions of -the cap itself. Except in very high latitudes this base must be -somewhat elevated, for like mountain glaciers ice caps are nourished -by the surface air currents, and their snows are deposited -above the snow line.</p> - -<p><b>The Norwegian tind or beehive mountain.</b>—Within temperate -or tropical climes the snow line lies so high that only the loftier -mountains are able to support glaciers. It follows that those -which are formed flow upon relatively high grades with correspondingly -high rate of movement and increased cutting power. -Within high latitudes the snow is found nearer the sea level, and -glaciers are for the most part correspondingly sluggish in their -movements as well as less active denuding agents.</p> - -<p>To this condition characteristic of high latitude glaciers, there -is added in Norway another in the peculiar shape of the basement -beneath the recent and the still existing glaciers. The plateau of -Norway is intersected by a network of deep and steep walled fjords, -and the glaciers have developed as small ice caps perched upon -veritable pedestals of rock, over the margins of which their outlet -tongues of ice descend on steep slopes into the fjord. The tops -of the pedestals thus come to be shaped by the plucking and abrading -processes into flat domes (<a href="#f406">Fig. 406</a>), while the knobs of rock, -which as nunataks reach above the surface of the ice, divide the -outflowing ice tongues at the margin of the pedestal. These -tongues being much more active denuding agents, because of their -steep gradients, continually lower their beds, thus transforming -the earlier knobs of rock into high and steep mountains of more or -less circular base. Such “beehive” mountains upon the margins -of the fjords are the characteristic Norwegian <i>tinds</i> (<a href="#f407">Fig. 407</a>).</p> - -<p class="prr"><span class="smcap">Reading References for Chapter XXVI</span></p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Quaternary History of Mono Valley, California, 8th -Ann. Rept. U. S. Geol. Surv., 1889, pp. 329-371, pls. 27-37.</p> - -<p class="pex"><span class="smcap">F. E. Matthes.</span> Glacial Sculpture of the Bighorn Mountains, Wyoming, -21st Ann. Rept. U. S. Geol. Surv., 1900, Pt. ii, pp. 179-185, -pl. 23.</p> - -<p class="pex"><span class="smcap">W. D. Johnson.</span> Maturity in Alpine Glacial Erosion, Jour. Geol., vol. 12, -1904, pp. 569-578.</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Systematic Asymmetry of Crest Lines in the High -Sierras of California, <i>ibid.</i>, pp. 579-588.</p> - -<p><span class="pagenum"><a name="Page_382" id="Page_382">[382]</a></span></p> - -<p class="pex"><span class="smcap">Emm. de Martonne.</span> Sur la Formation des Cirques, Ann. de Géogr., -vol. 10, 1901, pp. 10-16.</p> - -<p class="pex"><span class="smcap">W. M. Davis.</span> Glacial Erosion in North Wales, Quart. Jour. Geol. Soc. -Lond., vol. 65, 1909, pp. 281-350, pl. 14.</p> - -<p class="pex"><span class="smcap">Ed. Brückner.</span> Die Glazialen Züge im Antlitz der Alpen, Naturw. -Wochenschr., N. F., vol. 8, 1909.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Characteristics of Existing Glaciers, pp. 1-96.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_383" id="Page_383">[383]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXVII</h2> - -<p class="pch">SUCCESSIVE GLACIER TYPES OF A WANING -GLACIATION</p> - -<p><b>Transition from the ice cap to the mountain glacier.</b>—A study -of existing glaciers leads inevitably to the conclusion that although -subject to short period advances and retreats, yet, broadly speaking, -glaciers are now gradually wasting away, surrounded by wide -areas upon which are the evidences of their recent occupation. -We are thus living in a receding hemicycle of glaciation.</p> - -<div class="figcenter"> - <img src="images/ill-460.jpg" width="400" height="388" id="f408" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 408.</span>—Schematic diagram to show the relationships of glacier types formed -in succession during a receding hemicycle of glaciation.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_384" id="Page_384">[384]</a></span></p> - -<p>Many mountain districts which now support small glaciers only, -or none at all, were once nearly or quite submerged beneath snow -and ice. If once covered by an ice carapace or cap, our present -interest in them begins at that stage of the receding hemicycle -<i>when the rock surface has made its reappearance above the surface -of the snow-ice mass</i>. At this stage intensive frostwork, the characteristic -high level weathering, begins, and cirques develop above -the scars of those earlier amphitheaters formed in the advancing -hemicycle.</p> - - -<p><b>The piedmont glacier.</b>—In this early stage of transition from -the ice cap to the mountain glacier, the ice flows outward to the -mountain front in ill-defined streams divided by the projecting -ridges, and upon reaching the mountain front these streams deploy -upon it so as to coalesce in a great stagnant ice apron whose upper -surface slopes gently forward at an angle of a few degrees at the -most (<a href="#f408">Fig. 408</a>, stage I). This is the <i>piedmont glacier</i>, a type -found to-day in the high latitudes of Alaska and in the southern -Andes (<a href="#f409">Fig. 409</a> and <a href="#p18b">pl. 18 B</a>).</p> - -<div class="figcenter"> - <img src="images/ill-461.jpg" width="400" height="290" id="f409" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 409.</span>—Map of the Malaspina glacier of Alaska, the best known of existing -piedmont glaciers (after Russell).</p> -</div></div> - -<p>During this stage the cirques may be but poorly defined, and -ice flows in both directions from rock divides so that the streams -transect the range, and later, after the glaciers have disappeared, -may expose a pass smoothed and polished upon its floor and with<span class="pagenum"><a name="Page_385" id="Page_385">[385]</a></span> -striæ directed in opposite directions from the highest point. The -pass of the Grimsel in Switzerland furnishes an excellent illustration -of such earlier transection of the range.</p> - -<div class="floatright"> - <img src="images/ill-462a.jpg" width="250" height="136" id="f410" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 410.</span>—Map of the Baltoro glacier of the -Himalayas, a typical glacier of the dendritic -type.</p> -</div></div> - -<p><b>The expanded-foot glacier.</b>—As air temperatures continue to become -milder, the glacier streams within the mountains are less deep -and hence more clearly -defined, and instead of -coalescing upon the mountain -foreland, they now -issue from the mountains -to form individual aprons -and are described as <i>expanded-foot -glaciers</i> (<a href="#f408">Fig. 408</a>, -stage II, and <a href="#f292">Fig. 292</a>, -<a href="#Page_264">p. 264</a>).</p> - -<div class="floatleft"> - <img src="images/ill-462b.jpg" width="230" height="369" id="f411" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 411.</span>—The Triest glacier, a -hanging glacieret separated from -the Great Aletsch glacier to -which it was lately a tributary.</p> -</div></div> - -<p><b>The dendritic glacier.</b>—Still -later in the hemicycle nourishment of the glaciers is diminished -as depletion from melting increases, so that the glacier -streams no longer reach to the mountain front. Branches continue -to enter the main valley from -the several side valleys like the short -branches of a tall tree, and because of -this arrangement such a glacier may -be described as a <i>dendritic glacier</i> -(<a href="#f408">Fig. 408</a>, stage III, and <a href="#f410">Fig. 410</a>).</p> - -<p>Inasmuch as the depletion from -melting increases at a rapid rate in -descending to lower levels, the tributary -glacier valleys “hanging” above -the main valley in the lower stretches -become separated, and may continue -to exist as series of hanging glacierets -upon either side of the main valley below -the glacier front (<a href="#f408">Fig. 408</a>, stage -III, and <a href="#f411">Fig. 411</a>). It must be clear -from this that any attempt to name -each separated ice stream without -regard to its relationship must lead -to endless confusion, for glacier size<span class="pagenum"><a name="Page_386" id="Page_386">[386]</a></span> -is in such sensitive adjustment to air temperature that a fall or rise -of a few degrees only in the average annual temperature of the district -may prove sufficient to fuse many glaciers into one or separate -one ice mass into many smaller ones.</p> - -<p>When in high latitudes a dendritic glacier descends in fjords -to below the level of the sea, it is attacked by the water in the same -manner as are the outlets of Greenland glaciers, and is then known -as a “tidewater glacier”, -which may thus be a -subtype or variety of the -dendritic glacier (<a href="#f412">Fig. 412</a>).</p> - -<div class="floatleft"> - <img src="images/ill-463a.jpg" width="250" height="128" id="f412" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 412.</span>—The Harriman fjord glacier of Alaska, -a tidewater variety of dendritic glacier (after a -map by Gannett).</p> -</div></div> - -<p><b>The radiating (Alpine) -glacier.</b>—In the progressive -wastings of -dendritic glaciers, there -comes a time when their -dendritic outlines give -place to radiating ones. Attention has already been called to the -division of the cirque into subordinate basins separated by small -rock arêtes and yielding a markedly scalloped border (<a href="#f394">Fig. 394</a>, -<a href="#Page_371">p. 371</a>). When the ice front retires from the main valley into one -of these mature cirques, the now wasted ice stream is broken up -into subordinate glacierets, each of which occupies one of the -basins within the larger cirque, and these ice streams -flow together to produce a glacier whose component -elements radiate like the sticks within a lady’s -fan (<a href="#f408">Fig. 408</a>, stage IV, and <a href="#f413">Fig. 413</a>).</p> - -<div class="floatright"> - <img src="images/ill-463b.jpg" width="150" height="219" id="f413" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 413.</span>—Map -of the Rotmoos -glacier, a radiating -glacier -of Switzerland -(after Sonklar).</p> -</div></div> - -<p><b>The horseshoe glacier.</b>—As the glacier draws -near to its final extinction, it is crowded hard -against the wall of the amphitheater in which it -has so long been nourished. Up to this stage it -has offered a swelling front outwardly convex as a -direct consequence of the laws controlling its flow. -No longer amply nourished, for the first time its -front is hollowed, and it awaits its final dissolution -curled up against the cirque wall (<a href="#f408">Fig. 408</a>, -stage V, and <a href="#f414">Fig. 414</a>). Practically all the glaciers of the United -States and southern Canada are of this type.</p> - -<p><span class="pagenum"><a name="Page_387" id="Page_387">[387]</a></span></p> - -<p>The above classification is one depending directly upon glacier -nourishment, and hence also upon size, and upon the stage of the -glacial hemicycle. In order to determine the type of any glacier -it is necessary to know the outlines of the mountain valley—its -divide—and those of the glacier or glaciers within it. It -is likely that the types of the advancing hemicycle of glaciation -would be much the same, save only for the <i>new-born</i> or <i>nivation -glacier</i>, which would be as different as possible from the horseshoe -type, to which in size it corresponds. Upon the continent -of Antarctica, where the absence of any general melting of the ice, -even in the summer season and near the sea level, introduces special -conditions, some additional glacier types are found, which, however, -it is not necessary that we consider here.</p> - -<div class="figcenter"> - <img src="images/ill-464.jpg" width="400" height="314" id="f414" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 414.</span>—Outline map of the Asulkan glacier in the Selkirks, a typical horseshoe -glacier.</p> -</div></div> - -<p><b>The inherited-basin glacier.</b>—It may be, however, that glaciers -have developed, not upon mountains shaped in a cycle of -river erosion, nor yet in succession to an ice cap, as in the normal -cases which we have considered. On the contrary, glaciers<span class="pagenum"><a name="Page_388" id="Page_388">[388]</a></span> -may develop where basins of one sort or another have been -inherited from the preceding period. In such cases inherited depressions -may become more important than the auto-sculpture of -the glacier. Glaciers which develop under such conditions may -be described as <i>inherited-basin glaciers</i>.</p> - -<div class="figcenter"> - <img src="images/ill-465.jpg" width="400" height="478" id="f415" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 415.</span>—Outline map of the Illecillewaet glacier, an inherited-basin glacier in -the Selkirks.</p> -</div></div> - -<p>A partly closed basin between ridges may supply a collecting -ground for snows carried from neighboring slopes by the wind,<span class="pagenum"><a name="Page_389" id="Page_389">[389]</a></span> -and so may yield a broad névé, approaching in size a small ice cap, -yet without developing definite ice streams except upon its border. -Such a glacier is the Illecillewaet glacier of the Selkirks (<a href="#f415">Fig. 415</a>).</p> - -<p>Again in low latitudes the high and pointed volcanic peaks -may push up beyond the snow line into the upper atmosphere, -and so become snow-capped. Definite cirques do not develop well -under these circumstances, and the loose materials of which such -peaks are always composed are attacked in somewhat irregular -fashion from the different sides. This is the case of Mount Rainier -and similar peaks of the Cascade range of North America.</p> - - -<p><b>Summary of types of mountain glacier.</b>—In tabular form the -various types of mountain glacier may be arranged as follows:—</p> - -<p class="prr">MOUNTAIN GLACIERS</p> - -<p><i>Piedmont glacier.</i> Mountain valleys entirely occupied and largely -submerged, with overflow upon the foreland to form a common ice apron -through coalescence of neighboring streams.</p> - -<p><i>Expanded-foot glacier.</i> Valley entirely occupied and an overflow upon -the foreland sufficient to produce individual ice apron.</p> - -<p><i>Dendritic glacier.</i> Valley not completely occupied but with tributary -ice streams ranged along the sides of the main stream, and with hanging -glacierets separated near the glacier foot.</p> - -<p><i>Radiating glacier.</i> Glacier largely included in a cirque with subordinate -glacierets converging below like the sticks in a lady’s fan.</p> - -<p><i>Horseshoe glacier.</i> Small glacier remnants hugging the cirque wall -and having an incurving front.</p> - -<p><i>Inherited-basin glacier.</i> Of form dependent on a basin inherited and -not shaped by the glacier itself.</p> - -<p class="prr"><span class="smcap">Reading Reference for Chapter XXVII</span></p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> The Cycle of Mountain Glaciation, Geogr. Jour., -vol. 37, 1910, pp. 268-284.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_390" id="Page_390">[390]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXVIII</h2> - -<p class="pch">THE GLACIER’S SURFACE FEATURES AND THE -DEPOSITS UPON ITS BED</p> - -<p><b>The glacier flow.</b>—The downward flow of the ice within a -mountain glacier has been the subject of many investigations and -the topic of many heated discussions since the time when Louis -Agassiz and his companions set a line of stakes across the Aar -glacier and numbered the surface bowlders in -preparation for repeated observations. Their -first observation was that the line of stakes, -which had run straight across the glacier, was -distorted into a curve which was convex downstream -(<a href="#f416">Fig. 416</a>, A´), thus showing that the -surface layers have more rapid motion in proportion -as they are distant from the side margins. -Summarizing these and later studies, it -may be stated that the glacier increases its rate -of motion from its side margin towards its center -line, from its bed upwards towards its surface, -and below the névé the velocity is greatest -where the fall is greatest and also wherever the -cross section diminishes. In all these particulars, -then, the ice of the glacier behaves like a -stream of water. The average rate of flow of -Alpine glaciers varies from a few inches to a few -feet per day, and is greater during the warm -summer season. The Muir glacier of Alaska -has been shown to move at the rate of about -seven feet per day.</p> - -<div class="floatleft"> - <img src="images/ill-467.jpg" width="200" height="377" id="f416" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 416.</span>—Diagram -to illustrate the migrations -of lines of -stakes crossing a -glacier, due to its -surface movement, -<i>A</i>, original position -of lines; <i>A´</i>, later -positions; <i>a</i> and <i>a´</i>, -original and distorted -forms of a -square section of -the glacier surface -near its margin; <i>r</i>, -<i>r´</i>, diagonal crevasses.</p> -</div></div> - -<p>In traveling from the névé downward to the -glacier foot, the snow not only changes into -ice, but it undergoes a granulating process with continued increase -in the size of the nodules until at the foot of the glacier these may<span class="pagenum"><a name="Page_391" id="Page_391">[391]</a></span> -be picked out of the partially melted ice as articulating balls the -size of the fist or larger. Glacier ice has therefore a structure -quite different from that of lake ice, since the latter is developed -in parallel needles perpendicular to the freezing surface.</p> - - -<p><b>Crevasses and séracs.</b>—Prominent surface indications of glacier -movement are found in the open cracks or <i>crevasses</i>, which -are the marks of its yielding to tensional stresses. Crevasses -are apt to run either directly across the glacier, wherever there is -a steep descent upon its bed, or diagonally, running in from the -margin and directed up-glacier (<i>r</i>, <i>r</i>, <i>r</i>, of <a href="#f416">Fig. 416</a>), though they -occasionally run longitudinally with the glacier when there is -a rock terrace at the side of the valley beneath the ice. The -diagonal crevasses at the glacier margin are due to the more -sluggish movement where the ice is held back by friction upon the -walls of the valley, as will be clear from <a href="#f416">Fig. 416</a>. The square <i>a</i> -has by this movement been distorted into the lozenge <i>a´</i>, so that -the line <i>xy</i> has been extended into <i>x´y´</i>, with the obvious tendency -to open cracks in the direction <i>ss</i>.</p> - -<p>Every glacier surface below its névé is marked by steps or -terraces, which are well understood to overlie corresponding steps -of the cascade stairway to be seen in all vacated glacier valleys -(<a href="#p19a">plate 19</a>). The steep risers of these steps are usually marked -by parallel crevasses which cross the glacier. Under the rays -of the sun, which strike them more from one side than from -the other, the slices into which the ice -is divided are transformed into sharpened -blades and needles which are -known as <i>séracs</i> (<a href="#f401">Fig. 401</a>, <a href="#Page_376">p. 376</a>, and -<a href="#f417">Fig. 417</a>).</p> - -<div class="floatright"> - <img src="images/ill-468.jpg" width="200" height="106" id="f417" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 417.</span>—Transverse crevasses -at the fall below a glacier step -transformed by unsymmetrical -melting into séracs.</p> -</div></div> - -<p>The numerous crevasses tell us that -the ice is many times wrenched apart -during its journey down the glacier. -This has been illustrated by somewhat -grewsome incidents connected with accidents to Alpinists, -but as they illustrate in some measure both the mode and the rate -of motion of Swiss glaciers, they are worthy of our consideration.</p> - - -<p><b>Bodies given up by the Glacier <i>des Bossons</i>.</b>—In the year -1820, during one of the earlier ascents of Mont Blanc, three guides -were buried beneath an avalanche near the <i>Rochers Rouges</i> in<span class="pagenum"><a name="Page_392" id="Page_392">[392]</a></span> -the névé of the Glacier des Bossons (<a href="#f418">Fig. 418</a>). In 1858 Dr. -Forbes, who had measured the rate of flow of a number of Alpine -glaciers, predicted that the bodies of the victims of this accident -would be given up by the glacier after being entombed from thirty-five -to forty years. In the year 1861, or forty-one years after the -disaster, the heads of the three guides, separated from their bodies, -with some hands and fragments of clothing, appeared at the foot -of the Glacier des Bossons, and in such a state of preservation that -they were easily recognized by a guide who had known them in -life. Inasmuch as these fragments of the bodies had required -forty-one years to travel in the ice the three thousand meters -which separate the place of the accident from the foot of the -glacier, the rate of movement was twenty centimeters, or eight -inches, per day.</p> - -<div class="figcenter"> - <img src="images/ill-469.jpg" width="400" height="257" id="f418" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 418.</span>—View of the <i>Glacier des Bossons</i> upon the slopes of Mont Blanc showing -the position of accidents to Alpinists and the place of reappearance of their -bodies.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-470a.jpg" width="250" height="103" id="f419" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 419.</span>—Lines of flow upon the surface of the -Hintereisferner glacier in the Alps (after Hess).</p> -</div></div> - -<p>Various separated parts of the body of Captain Arkwright, who -had been lost in 1866 upon the névé of the same glacier, reappeared -at its foot after entombment in the ice for a period of thirty-one -years. To-day the time of reappearance of portions of the -bodies of persons lost upon Mont Blanc is rather accurately predicted, -so that friends repair to Chamonix to await the giving up -of its victims by the Glacier des Bossons.</p> - -<p><span class="pagenum"><a name="Page_393" id="Page_393">[393]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-470b.jpg" width="200" height="279" id="f420" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 420.</span>—Lateral and medial -moraines of the <i>Mer de glace</i> -and its tributary ice streams.</p> -</div></div> - -<p><b>The moraines.</b>—The horns and comb ridges which rise above -the glacier surface are continually subject to frost weathering, -and from time to time drop their separated fragments upon the -glacier. Falling as these do from considerable heights, they reach -the ice under a high velocity, and rebounding, sometimes travel -well out upon its surface before coming to a temporary rest. Upon -a fresh snow surface of the névé their tracks may sometimes be -followed with the eye for considerable distances, and their fall -is a constant menace to Alpine climbers. Below the névé the -larger number of such fragments -remain near the -cliff, and the lines of flow -of the ice within the glacier -surface are such that -blocks which reach points -farther out upon the glacier -are later gathered in -beneath the cliff at the side (<a href="#f419">Fig. 419</a>). The ridge of angular rock -débris which thus forms at the side of the glacier is called a -<i>lateral moraine</i> (see <a href="#f411">Fig. 411</a>, <a href="#Page_385">p. 385</a>, and <a href="#f420">Fig. 420</a>).</p> - -<p>At the junction of two glacier -streams, the lateral moraines are joined, -and there move out upon the ice surface -of the resultant glacier as a <i>medial -moraine</i>. Thus from the number of -medial moraines upon a glacier surface -it is possible to say that the important -tributary glaciers number one -more (<a href="#f420">Fig. 420</a>).</p> - -<div class="figcenter"> - <img src="images/ill-471a.jpg" width="400" height="211" id="f421" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 421.</span>—Ideal cross-section of a mountain glacier to show the position of -moraines and other peculiarities characteristic of the surface of the bed.</p> -</div></div> - -<p>The plucking and abrading processes -in operation beneath the glacier, quarry -the rock upon its bed, and after shaping -and smoothing the separated rock -fragments, these are incorporated within -the lower layers of the ice as <i>englacial</i> -rock débris. In spaces favorable -for its accumulation, a portion of this material, together with much -finer débris and rock flour, is left behind as a ground moraine -upon the bed of the glacier (see <a href="#f421">Fig. 421</a>).</p> - -<p><span class="pagenum"><a name="Page_394" id="Page_394">[394]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-471b.jpg" width="400" height="240" id="f422" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 422.</span>—Fragments of rock of different sizes, to bring out their different -effects upon the melting of the glacier surface.</p> -</div></div> - -<p>At the foot of the glacier the relatively angular rock débris, -which has been carried upon the surface, and the soled and polished -englacial material from near the bottom, are alike deposited in a -common marginal ridge known as the <i>terminal</i> or <i>end moraine</i> -(<a href="#p21b">plate 21 B</a>).</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 21.</span></p> - -<div class="figcenter"> - <img src="images/ill-472a.jpg" width="400" height="234" id="p21a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> View of the Harvard Glacier, Alaska, showing the -characteristic terraces (after U. S. Grant).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-472b.jpg" width="400" height="260" id="p21b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> The terminal moraine at the foot of a mountain glacier (after George Kinney).</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_395" id="Page_395">[395]</a></span></p> - -<p><b>Selective melting upon the glacier surface.</b>—The white surface -of the glacier generally reflects a large proportion of the sun’s -rays which reach it, and its more rapid melting is largely accomplished -through the agency of rock fragments spread upon its -surface. Such fragments, however, promote or retard the melting -process in inverse proportion to their size up to a certain limit, -and above that size their action is always to protect the glacier -from the sun. This nice adjustment to the size of the rock fragments -will be clear from examination of <a href="#f422">Fig. 422</a>, for rock is a -poor conductor of heat, and in even the longest summer day a -thin outer layer only is appreciably -warmed. Large rock blocks, -grouped in the medial and lateral -moraines, hold back the process of -lowering the glacier surface during -the summer, so that late in the -season these moraines stand fifty -feet or more above the glacier as -armored ice ridges.</p> - -<div class="floatright"> - <img src="images/ill-474.jpg" width="200" height="204" id="f423" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 423.</span>—Small glacier table upon -the surface of the Great Aletsch -glacier in 1908.</p> -</div></div> - -<p>Isolated and large rock slabs, as -the season advances, may come to -form the capping of an ice pedestal -which they overhang and are known -as <i>glacier tables</i> (<a href="#f423">Fig. 423</a>). Such -tables the sun attacks more upon one side than upon the other, -so that the slab inclines more and more to the south and may -eventually slip down until its edges rest against the glacier surface. -Rounded bowlders, which less frequently become perched -upon ice pedestals, may, from a similar process, slide down upon -the southern side and leave a pyramid of ice furrowed upon this -side and known as an <i>ice pyramid</i>.</p> - -<p>Fine dirt when scattered over the glacier surface is, on the other -hand, most effective in lowering its level by melting. Use was -made of this knowledge to lower the great drifts of snow which -had to be removed each season during the construction of the -new Bergen railway of southern Norway. Each dirt particle, -being warmed throughout by the sun’s rays, melts its way rapidly -into the glacier surface until the <i>dust well</i> which it has formed is -so deep that the slanting rays of the sun no longer reach it. When -the dirt particles are near together, the thin walls which separate -the dust wells are attacked from the sides in the warm air of summer -days, thus producing from a patch of dirt upon the glacier -surface a <i>bath tub</i> (<a href="#f424">Fig. 424 <i>d</i></a>). At night the water which fills these -basins is frozen to form a lining of ice needles projecting inward -from the wall, and this, repeated in succeeding nights, may<span class="pagenum"><a name="Page_396" id="Page_396">[396]</a></span> -entirely close the basin with water ice and produce the familiar -<i>glacier star</i> (<a href="#f424">Fig. 424 <i>c</i></a>).</p> - -<div class="figcenter"> - <img src="images/ill-475a.jpg" width="400" height="241" id="f424" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 424.</span>—Effects of differential melting and subsequent refreezing upon the -glacier surface. <i>a</i>, dust wells; <i>b</i>, glacier <i>tub</i> produced by melting about a group of -scattered dust particles; <i>c</i>, glacier star produced when the inclosed water of the -glacier well has frozen in successive nights; <i>d</i>, “bath tub.”</p> -</div></div> - -<p>If the dirt upon the glacier surface, instead of being scattered, -is so disposed as to make a patch completely covering the ice to -the thickness of an inch or more, the effect is altogether different. -Protecting as it now does the ice below, a local ice hillock rises -upon its site as the surrounding surface is lowered, and as this -grows in height its declivities increase and a portion of the dirt -slides down the side. The final product of this shaping is an -almost perfectly conical ice hill encased in dirt and known as a<span class="pagenum"><a name="Page_397" id="Page_397">[397]</a></span> -<i>débris</i>, <i>sand</i>, or <i>dirt cone</i> (<a href="#f425">Fig. 425</a>). The novice in glacier study -is apt to assume that these black cones contain only dirt, but is -rudely awakened to the reality when he attempts to kick them to -pieces. Both glacier tubs and débris cones may assume large -dimensions; as, for example, in Alaska, where they may be properly -described as lakes and hills.</p> - -<div class="figcenter"> - <img src="images/ill-475b.jpg" width="400" height="147" id="f425" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 425.</span>—Dirt cone and one with its casing in part removed. Victoria glacier -(after Sherzer).</p> -</div></div> - -<p>A patch of hard and dense snow which is less easily melted -than that upon which it rests may lead to the formation of snow -cones upon the glacier surface similar in size and shape to the -better known débris cones. Such cones of snow have, with -doubtful propriety, been designated “penitents”, for it is pretty -clear that the interesting bowed snow figures, which really resemble -penitents and which were first described from the southern -Andes under the name of <i>nieves penitentes</i>, are of somewhat different -character.</p> - -<div class="floatright"> - <img src="images/ill-476.jpg" width="150" height="201" id="f426" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 426.</span>—Schematic -diagram to show the -manner of formation -of glacier cornices.</p> -</div></div> - -<p>One further ice feature shaped by differential melting around -rock particles remains to be mentioned. Wherever the seasonal -snowfalls of the névé are exposed in crevasses, they are generally -found to be separated by layers of dirt, and lines of pebbles similarly -separate those ice layers which are revealed at the foot -of the glacier. In either case, if the sun’s rays can reach these -layers in an opened crevasse, the half-buried -rock fragments are warmed by the sun upon -their exposed surfaces and slowly melt their -way down the ice surface, thus removing from -it a thin layer of snow or ice and causing that -part above the pebble layer to project like -a cornice. This process will go on until the -overhanging cornice protects the pebbles from -any further warming by the sun, but each -lower pebble layer that is reached by the sun -will produce an additional cornice, so that -the original surface may at the bottom have -been retired by the process a number of inches. These features -are described as <i>glacier cornices</i> (<a href="#f426">Fig. 426</a>).</p> - -<div class="floatleft"> - <img src="images/ill-477.jpg" width="250" height="177" id="f427" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 427.</span>—Superglacial stream upon the Great -Aletsch glacier.</p> -</div></div> - -<p><b>Glacier drainage.</b>—Already in the early morning of every -warm summer day, active melting has begun upon the surface of -the Swiss glaciers. Rills of icy water soon make their way along -depressions upon the surface, and are joined to one another so that<span class="pagenum"><a name="Page_398" id="Page_398">[398]</a></span> -they sometimes form brooks of considerable size (<a href="#f427">Fig. 427</a>). Such -streams continue their serpentine courses until these are intersected -by a crevasse down which the waters plunge in a whirling -vortex which soon develops a vertical shaft of circular section -within the ice. Such shafts with their descending columns of -whirling water are the -well-known <i>moulins</i>, -or “<i>mills</i>”, which -may be detected from -a distance by their -gurgling sounds. The -first plunge of the -water may not reach -to the bottom of the -glacier, in which case -the stream finds a -passageway below the -surface but above the -floor until another -crevasse is encountered and a new plunge made, here perhaps to -the bottom. Once upon the valley floor the stream is joined by -others, and pursues its course within an ice tunnel of its own -making (<a href="#f421">Fig. 421</a>, <a href="#Page_394">p. 394</a>) until it issues at the glacier front.</p> -<p>The coarser of the rock débris which was gathered up by the -stream upon the glacier surface is deposited within the tunnel in -imperfect assortment (gravel and sand), while all finer material -and that lifted from the floor (rock flour) is retained in suspension -and gives to the escaping stream its opaque white appearance. -This <i>glacier milk</i> may generally be traced far down the valleys or -out upon the foreland, and is often the traveler’s first indication -that a range which he is approaching supports glaciers.</p> - -<div class="figcenter"> - <img src="images/ill-478a.jpg" width="400" height="90" id="f428" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 428.</span>—Ideal form of the surface left on the site of the apron of a piedmont -glacier. <i>M</i>, moraine; <i>T</i>, outwash; <i>C</i>, basin usually occupied by a lake; <i>D</i>, drumlins -(after Penck).</p> -</div></div> - -<p><b>Deposits within the vacated valley.</b>—For every excavation -of the higher portions of the upland through glacial sculpture, -there is a corresponding deposit of the excavated materials in -lower levels. So far as these materials are deposited directly by -the ice, they form the lateral, medial, ground, and terminal moraines -already described. A considerable proportion of them are, however, -deposited by the water outside the terminal moraine; but -as with the shrinking glacier the ice front retires in halting movements<span class="pagenum"><a name="Page_399" id="Page_399">[399]</a></span> -over the area earlier ice-covered, the terminal moraines are -ranged along the vacated valley as <i>recessional moraines</i>, each with -a <i>valley train</i> of outwash below. About the apron of the piedmont -glacier, such deposits are particularly heavy (<a href="#f428">Fig. 428</a>). During -the “ice age” the Swiss glaciers extended down the valleys below -the existing ice remnants and spread upon the Swiss foreland as -great piedmont glaciers such as may now be seen in Alaska. To-day -we find there moraines and glacial outwash, a lake in the -middle of the apron site, and sometimes a group of radiating drumlins -like those found within the ice lobes of the continental glacier -in southern Wisconsin (<a href="#f429">Fig. 429</a>, and <a href="#f344">Fig. 344</a>, <a href="#Page_317">p. 317</a>).</p> - -<div class="figcenter"> - <img src="images/ill-478b.jpg" width="400" height="268" id="f429" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 429.</span>—Moraines and drumlins about Lake Constance upon the site of the -earlier piedmont glacier of the Upper Rhine. The white area outside the outermost -moraine is buried in glacial outwash (after Penck and Brückner).</p> -</div></div> - -<p><span class="pagenum"><a name="Page_400" id="Page_400">[400]</a></span></p> - -<p>Behind the recessional moraines within the glaciated valley are -found the valley moraine lakes (<a href="#f448">Fig. 448</a>, <a href="#Page_413">p. 413</a>), in association -with the rock basin lakes due to glacial sculpture (<a href="#f447">Fig. 447</a>, <a href="#Page_412">p. 412</a>). -After the glacier has vacated its valley, the precipitous side walls -become the prey of frostwork and are the scenes of disastrous -avalanches or landslides. Within the cirques, drifts of snow are -nourished long after the ice has disappeared, and as a consequence -the amphitheater walls succumb to the process of solifluxion -(<a href="#Page_153">p. 153</a>).</p> - -<p>Diversions and reversals of drainage, which are so characteristic -of the work of continental glaciers, are hardly less common to -glaciated mountain districts. Many of our most beautiful waterfalls -have resulted from either the temporary or permanent obstruction -of earlier valleys above the falls. The famous Yosemite -Falls offers an interesting illustration of the shifting of an earlier -waterfall, itself no doubt due to ice blocking in a still earlier glaciation -(<a href="#p22b">plate 22 B</a>).</p> - -<p><b>Marks of the earlier occupation of mountains by glaciers.</b>—It -is well that we should now bring together within a small compass -those evidences by which the existence of earlier mountain glaciers -may be proven in any district. These marks are so deeply stamped -upon the landscape that no one need err in their interpretation.</p> - -<p class="prr">MARKS OF MOUNTAIN GLACIERS</p> - -<p><i>High-level sculpture.</i> The grooved upland with its cirques, or the fretted -upland with its cirques, cols, horns, and comb ridges.</p> - -<p><i>Low-level sculpture.</i> The <span class="font">U</span>-shaped main valley, the hanging side -valleys with their ribbon falls, the glacier staircase with its rock bars and -gorges, the rounded, polished, and striated rock floor.</p> - -<p><i>Deposits.</i> The recessional moraines of till and the valley trains of -sand and gravel, the soled erratic blocks derived always from higher -levels of the valley.</p> - -<p><i>Lakes.</i> The valley moraine lakes and the chains of rock basin lakes.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter XXVIII</span></p> - -<p>Glacier movement:—</p> - -<p class="pex"><span class="smcap">L. Agassiz.</span> Nouvelles Études et Expériences sur les Glaciers Actuels, -etc., Paris, 1847, pp. 435-539.</p> - -<p class="pex"><span class="smcap">H. Hess.</span> Die Gletscher, Braunschweig, 1904, pp. 115-150.</p> - -<p class="pex"><span class="smcap">H. F. Reid.</span> The Mechanics of Glaciers, Jour. Geol., vol. 4, 1896, pp. 912-928; -Glacier Bay and Its Glaciers, 16th Ann. Rept. U. S Geol. -Surv., Pt. i, 1898, pp. 445-448.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 22.</span></p> - -<div class="figcenter"> - <img src="images/ill-480a.jpg" width="400" height="461" id="p22a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Model of the vicinity of Chicago, showing the position of the ancient -beaches and the outlet of the former Lake Chicago.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-480b.jpg" width="400" height="485" id="p22b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Map of Yosemite Falls and its earlier site near Eagle Peak (after -F. E. Matthes).</p> -</div></div> - -</div> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_401" id="Page_401">[401]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXIX</h2> - -<p class="pch">A STUDY OF LAKE BASINS</p> - -<p><b>Freshwater and saline lakes.</b>—Lakes require for their existence -a basin within which water may be impounded, and a supply -of water more than sufficient to meet the losses from seepage and -evaporation. If there is a surplus beyond what is needed to meet -these losses, lakes have outlets and remain fresh; their content -of mineral matter is then too slight to be detected by the palate. -If, on the other hand, supply is insufficient for overflow, continued -evaporation results in a concentration of the mineral content of -the water, subject as it is to continual augmentation from the inflowing -streams.</p> - -<p>As we have seen, there are in areas of small rainfall special -weathering processes which tend to bring out the salts from the -interior of rock masses, these concentrated salts generally first -appearing as a surface efflorescence which is ultimately transferred -through the agency of wind and cloudburst to the characteristically -saline desert lakes.</p> - -<p>Lake basins may be formed in many ways. Depressions of -the land surface may result from tectonic movements of the crust; -they may be formed by excavating processes; but in by far the -greater number of instances they result from the obstruction in -some manner of valleys which were before characterized by uniformly -forward grades. In relatively few cases loose materials -are heaped up in such a manner as to produce fairly symmetrical -basins.</p> - -<div class="figcenter"> - <img src="images/ill-483a.jpg" width="400" height="274" id="f430" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 430.</span>—Map and diagram to bring out the characteristics of newland lakes.</p> -</div></div> - -<p><b>Newland lakes.</b>—On land recently elevated from the sea, -basins of lakes may be merely the inherited slight irregularities -of the earlier sea floor, in which case they may be assumed to be -largely the result of an irregular distribution of deposits derived -from the land. Lakes of this type are especially well exhibited -in Florida, and are known as newland lakes (<a href="#f430">Fig. 430</a>). Such -lakes are exceptionally shallow, and are apt to have irregular outlines<span class="pagenum"><a name="Page_402" id="Page_402">[402]</a></span> -and extremely low banks. Under these circumstances, they -are soon filled with a rank growth of vegetation, so that it is sometimes -difficult to properly distinguish lake and marsh.</p> - -<div class="figcenter"> - <img src="images/ill-483b.jpg" width="400" height="207" id="f431" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 431.</span>—View of the Warner Lakes, Oregon (after Russell).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-484a.jpg" width="250" height="205" id="f432" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 432.</span>—Schematic diagrams to illustrate the -characteristics of basin-range lakes.</p> -</div></div> - -<p><b>Basin-range lakes.</b>—Newland lakes may be said to have their -origin in an uplift of the land and sea floor near their common -margin. A lake type dependent upon movements of the earth’s crust -but within interior areas has been described as the basin-range -type and is exemplified by the Warner Lakes of Oregon. In this<span class="pagenum"><a name="Page_403" id="Page_403">[403]</a></span> -district great rectangular blocks of the earth’s crust, which in their -upper portions at least are composed of basaltic lavas, have undergone -vertical adjustments in level and have been tilted so that -the corresponding corners of neighboring blocks have been given -a similar degree of down-tilt -(<a href="#f431">Fig. 431</a>). Lakes -formed in this way are -of triangular outline, are -bounded on the two -shorter sides by cliffs, -but have extremely flat -shores on their longest -side. From this shore the -water increases gradually -in depth and attains a -maximum depth at or -near the opposite angle. -Such lakes naturally betray -a tendency to appear -in series (<a href="#f432">Fig. 432</a>), and are unfortunately much too often illustrated -on a small scale after a shower by the tilted blocks of -imperfectly made cement sidewalks.</p> - -<div class="figcenter"> - <img src="images/ill-484b.jpg" width="400" height="140" id="f433" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 433.</span>—Schematic diagrams of rift-valley lakes, and the rift valley of the Jordan -with the Dead Sea and the Sea of Galilee as remnants of a larger lake in which -their basins were included.</p> -</div></div> - -<p><b>Rift-valley lakes.</b>—Another type of lake basin which has its -origin in faulted block movements is known as the rift-valley lake, -and is best exemplified by the great lakes of east Central Africa. -In this type a strip of crust, many times as long as it is wide, has -been relatively sunk between the blocks on either side so as to -produce a deep rift, or what in Germany is known as a <i>Graben</i><span class="pagenum"><a name="Page_404" id="Page_404">[404]</a></span> -(trench). Such a basin when occupied by water yields a lake which -is long, straight, deep, and narrow, and is in addition bounded on -the sides by steep rock cliffs. At the ends the -shores are generally by contrast decidedly low. -If the hard rock at the bottom of the lake -could be examined, it would be found to be of -the same type as that exposed near the top of -the side cliffs. The valley of the Jordan in -Palestine is a rift of this character and was at -one time occupied by a long and narrow lake -of which the Dead Sea and the Sea of Galilee -are the existing remnants (<a href="#f433">Fig. 433</a>).</p> - -<div class="floatleft"> - <img src="images/ill-485a.jpg" width="150" height="410" id="f434" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 434.</span>—Map showing -the rift valley -lakes of east Central -Africa.</p> -</div></div> - -<p>One of the most striking examples of a rift -valley lake is Lake Tanganyika, while Albert -Nyanza, Nyassa, and -Rudolf in the same -region are similar -(<a href="#f434">Fig. 434</a>).</p> - -<div class="floatright"> - <img src="images/ill-485b.jpg" width="200" height="394" id="f435" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 435.</span>—Earthquake -lakes which were formed -in the flood plain of the -lower Mississippi during -the earthquake of 1811 -(after Humphreys).</p> -</div></div> - -<p><b>Earthquake lakes.</b>—The -complex adjustments -in level of the -surface of the ground -at the time of sensible -earthquakes are many -of them made apparent in no other way -than by the derangements of the surface -water. This is at such times impounded -either in pools or in broad lakes, which -inasmuch as they date from known earthquakes -have been called “earthquake -lakes”, even though in a strict sense any -lake which has originated in earth movements -might properly be regarded as an -earthquake lake. To avoid unnecessary -confusion, the term must, however, be restricted -to those lakes which are known to -have been formed at the time of definite earthquakes (<a href="#f435">Fig. 435</a>). -Reelfoot Lake in Tennessee, which in late years has acquired -undesirable notoriety because of the feuds between the fishermen<span class="pagenum"><a name="Page_405" id="Page_405">[405]</a></span> -of the district and the constituted authorities, is a lake more than -twenty miles across and came into existence during the great -earthquake of the lower Mississippi valley in 1811.</p> - - -<p><b>Crater lakes.</b>—The craters of volcanic mountains are natural -basins in which surface waters are certain to be collected, provided -only the supply is sufficient and seepage into the loose materials is -not excessive. Some craters, still visibly more or less active, are -occupied by lakes (<a href="#f436">Fig. 436</a>).</p> - -<div class="figcenter"> - <img src="images/ill-486.jpg" width="400" height="344" id="f436" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 436.</span>—View of lake in Poas Crater in Costa Rica, a volcanic crater more -than half a mile across and with walls 800 feet deep. At intervals there is an -ejection of steam mixed with mud and ash after the manner of a geyser (after -H. Pittier).</p> -</div></div> - -<p>In the larger number of cases in which craters become occupied -by lakes, the evidence of continued activity is lacking, and it would -appear in such cases that the lava of the chimney had consolidated -into a volcanic plug, closing the bottom of the crater. Notable -groups of crater lakes are the <i>Caldera</i> of the Roman Campagna -(<a href="#f437">Fig. 437</a>) and the so-called <i>maare</i> of the Eifel about the Lower -Rhine. Crater lakes are easy to recognize by their circular plan,<span class="pagenum"><a name="Page_406" id="Page_406">[406]</a></span> -their steep walls of volcanic materials, and their considerable -depth with a maximum near the center.</p> - -<p>One of the most remarkable of these water-filled basins is Crater -Lake in Oregon, which has a diameter of about six miles and is -believed to have resulted from the incaving of a great volcanic -cone in the latest stage of its activity. This remarkable feature -has now been made a national park and will soon be conveniently -reached by tourists and counted one of the greatest nature wonders -of the Pacific slope.</p> - -<div class="figcenter"> - <img src="images/ill-487a.jpg" width="400" height="175" id="f437" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 437.</span>—Diagrams to illustrate the characteristics of crater lakes. The Roman -Campagna is a plain formed of volcanic ash, with the crater lakes of Bracciano, -Vico, and Bolseno arranged on a line traversing it.</p> -</div></div> - -<p><b>Coulée lakes.</b>—Far more important as lakes are those volcanic -basins which arise from the flow of a stream of lava across the valley -of a river so as to impound its -waters (<a href="#f438">Fig. 438</a>).</p> - -<p>At the time of the great eruption -under Skaptár Jökull in 1783 -the river Skaptár and many of -its tributaries were blocked by -the flow of lava, which it is estimated -exceeded in bulk the mass -of Mont Blanc.</p> - -<div class="floatleft"> - <img src="images/ill-487b.jpg" width="250" height="187" id="f438" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 438.</span>—View of Snag Lake, a <i>coulée</i> -lake with lava dam shown in middle -distance (after Fairbanks).</p> -</div></div> - -<p><b>Morainal lakes.</b>—As we have -learned, the obstruction of drainage, -due to the distribution of -rock débris by continental glaciers, -has yielded lakes in almost countless numbers. Probably ninety -per cent or more of the known lakes have had this origin, and the<span class="pagenum"><a name="Page_407" id="Page_407">[407]</a></span> -type is so common within the once glaciated regions that it forms -perhaps the best distinguishing mark of former glaciation. The -hummocky surface of morainal deposits is so characteristic that -the lakes of this type are never very large and are correspondingly -irregular in outline. They have often numerous islands, and their -banks are formed of the combination of rock flour and ice-worn materials -known as till (<a href="#f439">Fig. 439</a>). The smallest of the morainal -lakes are mere kettles on the marginal moraine, and these rapidly -become replaced by peat bogs. In contrast with pit lakes, morainal -lakes lack the steep surrounding slopes and the encircling -plain.</p> - -<div class="figcenter"> - <img src="images/ill-488.jpg" width="400" height="191" id="f439" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 439.</span>—Diagrams to illustrate the characteristics of morainal lakes, and a -sample map of such lakes from the glaciated region of North America.</p> -</div></div> - -<p><b>Pit lakes.</b>—The so-called pit lakes have their origin in continental -glaciation, and are found in groups within broad plains -of glacial outwash (mainly sand and gravel), which are for this -reason described as “pitted plains” (see <a href="#Page_314">p. 314</a>). Those areas -which lay between neighboring lobes of the ice sheet were subject -to particularly heavy deposits of outwash material, and are, in -consequence, particularly likely to be occupied by pit lakes. As -has been pointed out in an earlier section, the water derived from -surface melting within the marginal portions of a continental -glacier descends to the bottom in the crevasses and thereafter -flows in an ice tunnel under the same conditions as water flowing -in a pipe. Having in most cases a considerable head at the outer -margin of the ice, this water may rise and issue well above the lower -ice layers and so cover a portion of the ice margin beneath sand<span class="pagenum"><a name="Page_408" id="Page_408">[408]</a></span> -and gravel (<a href="#f440">Fig. 440</a>). Separated blocks, often of massive proportions, -are thus buried beneath nonconducting materials by -which they are long protected from further melting. Eventually, -however, with the approach of still milder climates they disappear, -thus causing the overlying sand and gravel to descend and form a -pit of steep walls similar to the sawdust pits over melted ice blocks -within our storehouses.</p> - -<div class="figcenter"> - <img src="images/ill-489a.jpg" width="400" height="132" id="f440" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 440.</span>—Diagram to show the manner of formation -of pit lakes.</p> -</div></div> - -<p>Pit lakes are thus easily recognized by their occurrence usually -in groups within a plain of glacial outwash and by their characteristic -banks inclined at the angle of repose of such materials -(<a href="#f441">Fig. 441</a>).</p> - -<div class="figcenter"> - <img src="images/ill-489b.jpg" width="400" height="220" id="f441" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 441.</span>—Diagrams to illustrate the characteristics of pit lakes and a sample -map from the glaciated region of North America.</p> -</div></div> - -<p><b>Glint or colk lakes.</b>—It has been found to be true of existing -continental glaciers that where their mass has been held back by a -mountain wall, their current at the portals within this rampart -becomes greatly accelerated. Though the upper layers of the<span class="pagenum"><a name="Page_409" id="Page_409">[409]</a></span> -glacier in the vicinity may move forward with a velocity of but an -inch per day, the current within the outlet may be as much as -seven hundred or a thousand times as great. In many respects -these conditions are similar to those about the raceway of a reservoir -where the near-by surface of the water is lowered by the indraught -of the outlet and the current in the raceway is so accelerated -that, unless protected, the bottom of the race is carried away -and a basin excavated which extends a short distance both above -and below the position of the dam. In Holland such basins hollowed -out beneath breaks in the dykes are known as colks. Basins -which were excavated beneath the glacier outlets by a similar process -would not be open to our -inspection until after the ice had -disappeared from the region; -but it is most significant that in -Scandinavia, where the Pleistocene -continental glacier, advancing -westward from the Baltic, -was held in check by the escarpment -at the Norwegian boundary -(the <i>glint</i>), lake basins have -been excavated in hard rock -whose walls show the abrading -and polishing which are characteristic -of glacial sculpture, and -whose positions are such that -they lie beneath the former outlets -partly above and in part -below the line of the escarpment. Their position in reference to -the rampart and to the former outlets is brought out in <a href="#f442">Fig. 442</a>. -The largest of the glint lakes of this series is Torneträsk in -northern Lapland (see <a href="#Page_277">p. 277</a> and <a href="#f443">Fig. 443</a>).</p> - -<div class="figcenter"> - <img src="images/ill-490a.jpg" width="400" height="66" id="f442" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 442.</span>—Diagram to show the manner of formation of glint or outlet lakes where -the continental glacier of Scandinavia issued from the Baltic depression through -portals in its mountain rampart.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-490b.jpg" width="250" height="262" id="f443" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 443.</span>—Map showing a series of -glint lakes which lie across the international -boundary of Sweden and -Norway.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_410" id="Page_410">[410]</a></span></p> - -<p><b>Ice-dam lakes.</b>—Whenever a continental glacier, either in advancing -its front or in retiring, lies across the lines of drainage upon -their downstream side, water is impounded along the ice front -so as to form ice-dam lakes. Such lakes -are found to-day in Greenland and in -the southern Andes, and similar bodies -of water of far greater size and importance -came into existence in Pleistocene -times each time that the continental -glaciers of northern North America -and Europe advanced upon or retired -from suitably directed river systems. -Thus above the Baltic depression, when -the ice front lay to the eastward of the -main watershed, each easterly sloping -valley was obstructed by the ice and -occupied by an ice-dam lake (<a href="#f444">Fig. 444</a>), -the beaches of which may all be traced -to-day (<a href="#f445">Fig. 445</a>).</p> - -<div class="floatleft"> - <img src="images/ill-491a.jpg" width="200" height="219" id="f444" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 444.</span>—Ice-dam lakes (in -black) between the front of -the late Pleistocene glacier -of northern Europe and the -divide near the Norwegian -boundary (after G. de Geer).</p> -</div></div> - -<p>One side of each ice-dam lake is formed by an ice cliff at the -glacier front, and if the region is relatively flat, the remaining -shores are likely to be formed by a marginal moraine which the -glacier has abandoned in its retreat. In their smaller stages, -therefore, ice-dam lakes on prairie country have the form of a -crescent, which is the more pronounced because the waves by their -attack upon the ice front flatten the curvature of its outline (see -<a href="#f360">Fig. 360</a>, <a href="#Page_330">p. 330</a>).</p> - -<p><span class="pagenum"><a name="Page_411" id="Page_411">[411]</a></span></p> - -<p>The life of an ice-dam lake is begun and ended in important -changes of glacier outline, and after the draining of lakes by this -process the land shores may be traced in beaches, and the ice margin -by a water-laid moraine of low relief (<a href="#f359">Fig. 359</a>, <a href="#Page_330">p. 330</a>).</p> - -<p>A much smaller but in many respects similar ice-dam lake is -to-day to be seen at the side of the Great Aletsch glacier, a mountain -glacier of Switzerland. The traveler who makes the easy -ascent of the Eggishorn may look directly down upon this crescent-shaped -lake with its ice cliff on the glacier side (see <a href="#f446">Fig. 446</a>).</p> - -<div class="figcenter"> - <img src="images/ill-491b.jpg" width="400" height="165" id="f445" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 445.</span>—Wave-cut terrace at an elevation of 177.5 meters above sea on the -southern slope of the northern Dala valley north of Baggedalen in Sweden. To -the right in the foreground is a peat bog (after Munthe).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-492.jpg" width="400" height="215" id="f446" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 446.</span>—View of the Márjelen Lake at the side of the Great Aletsch glacier, -seen looking directly down from the summit of the Eggishorn (after a photograph -by I. D. Scott).</p> -</div></div> - -<p><b>Glacier lobe lakes.</b>—Upon the sites of the former lobes of the -Pleistocene glacier of North America are found the basins of the -Laurentian River system, the largest freshwater lakes in the world. -There has been much controversy concerning the manner of formation -of these lakes, but the view which has seemed to have the -largest following is that they were excavated by the eroding action -of the continental glacier over the drainage basins of former -rivers. It is but one phase of the long controversy between opposing -schools, which have advocated on the one hand the efficiency -of glacier ice as an eroding agent, and upon the other its supposed -protection from the weathering processes. The positions and the -outlines of the several lakes of the series sufficiently proclaim their -connection with the former glacial lobes, and the name which we -have adopted leaves the exact manner of their formation a still<span class="pagenum"><a name="Page_412" id="Page_412">[412]</a></span> -open question. The recognition of the importance of the glacial -anticyclone, in giving shape to the glacier surface and in effecting -a transfer of snow from the central to the marginal portions, has -had the effect of emphasizing the relative importance of erosion -under the marginal and lobate portions. Thus the importance of ice -lobes has been greatly accentuated, though this applies only to -the shaping of the basins and not in any important way to the impounding -of the present waters. The present Laurentian Lakes -owe their existence to the elevation by successive uplifts of the -country to the northward and eastward, since the glacier retired -from the lake region. When the ice front lay to the northward of -the Ottawa River, the discharge of the upper lakes was by a channel -through Nipissing River and Lake and thence down the Ottawa -River to a gulf in the lower St. Lawrence. The uplift of the land -has had the effect of raising a barrier where the former outlet -existed, and diverting the waters to a roundabout channel by way -of Detroit and Lake Erie (see <a href="#f365">Fig. 365</a>, <a href="#Page_335">p. 335</a>).</p> - -<div class="figcenter"> - <img src="images/ill-493.jpg" width="400" height="156" id="f447" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 447.</span>—Diagrams to illustrate the arrangement and the characters of rock-basin -lakes, together with a map of such lakes from the Bighorn Mountains in -Wyoming.</p> -</div></div> - -<p><b>Rock-basin lakes.</b>—The reversed grades which develop in a -valley deepened by mountain glaciers—the back-tilted treads of -the cascade stairway (see <a href="#Page_376">p. 376</a>)—furnish a series of basins -hollowed in rock which are strung along the course of the valley -like pearls upon a thread, or, far better, like the larger beads in a -rosary (<a href="#f447">Fig. 447</a>). This characteristic arrangement accounts for -the name “Paternoster Lakes” which has sometimes been applied -to them in Europe. Their positions in series within <span class="font">U</span>-shaped -mountain valleys, and their rock shores with characteristically<span class="pagenum"><a name="Page_413" id="Page_413">[413]</a></span> -smoothed and striated surfaces, make them easy of determination. -In the higher portions of the valley, where the treads of the -cascade stairway are relatively narrow, such lakes are often approximately -circular in outline, but in the lower levels and upon -wider treads they may be ribbon-like, though lakes of this type -are to a large extent replaced in the lower levels by the valley -moraine type or a combination of the two.</p> - -<div class="figcenter"> - <img src="images/ill-494.jpg" width="400" height="302" id="f448" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 448.</span>—Convict Lake, a lake behind a moraine dam within a glaciated valley -of the Sierra Nevadas, California (after a photograph by Fairbanks).</p> -</div></div> - -<p><b>Valley moraine lakes.</b>—The recessional moraines which mark -the halting stations of mountain glaciers, while retiring up their -valleys, form dams in the later river and so produce a type of lake -which is in contrast with the morainal lakes which result from -continental glaciation. They may, therefore, be distinguished -by the name <i>valley moraine lakes</i>. Their positions on the bed of a -<span class="font">U</span>-shaped mountain valley, and the glacial materials which compose -the dams, are sufficient for their identification (<a href="#f448">Fig. 448</a>). -Moraine Lake and Lake Louise in the Canadian Rockies are typical -examples. Rock basin and valley moraine lakes may occur -in alternation or combined in mountain valleys.</p> - -<p><span class="pagenum"><a name="Page_414" id="Page_414">[414]</a></span></p> - -<div class="floatleft"> - <img src="images/ill-495a.jpg" width="250" height="57" id="f449" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 449.</span>—Lake basins produced by successive slides -from the steep walls of a glaciated mountain valley -(after Russell).</p> -</div></div> - -<p><b>Landslide lakes.</b>—The sheer-walled valleys which are carved -by mountain glaciers are too steep to long retain their perpendicularity -when the support of the glacier has been removed. Aided -by the ever present joint planes, which admit water to the rock, -they succumb to frost action, and further give way in avalanches -whenever the rock -is of sufficiently -porous material to -become saturated -with water. Landslides -sometimes -occur successively -until the original valley wall has been replaced by a terraced slope. -The treads of the steps in this terrace have generally a backward-sloping -grade, so that basins are formed to be filled by relatively -long and narrow lakes or by successions of small pools (<a href="#f449">Fig. 449</a> -and <a href="#p23b">plate 23 B</a>).</p> - -<div class="floatright"> - <img src="images/ill-495b.jpg" width="200" height="252" id="f450" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 450.</span>—Lake Garda, a -border lake upon the site of a -piedmont apron at the margin -of the Alpine highland -(after Penck and Brückner).</p> -</div></div> - -<p>When the avalanched material is so disposed as to dam the valley, -much larger lakes of this type come into existence. During -an earthquake which occurred on January 25, 1348, there was a -landslide within the valley of the Gail, -Carinthia, which destroyed seventeen -villages and produced a lake which -even to-day is represented by a great -marsh.</p> - -<p><b>Border lakes.</b>—Whenever mountain -glaciers push out their fronts -beyond the borders of the mountain -range by which they are nourished, -they spread upon the foreland in -broad aprons about which morainic -accumulations are particularly heavy. -This elevation of morainal walls about -the margins of the aprons yields natural -basins that are occupied by lakes so -soon as the glacier retires its front -within the valley. Because such lakes -are found at the borders of upland districts they have been called -<i>border lakes</i>. The beautiful Lakes Constance, Lucerne, Maggiore, -Lugano, Como, and Garda (<a href="#f450">Fig. 450</a>), on the borders of the Alpine -highland, are all of this type.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 23.</span></p> - -<div class="figcenter"> - <img src="images/ill-496a.jpg" width="400" height="212" id="p23a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> View of the American Fall at Niagara, showing the accumulation of rocks beneath -(after Grabau).</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-496b.jpg" width="400" height="261" id="p23b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> Crystal Lake, a landslide lake in Colorado.<br /> -(<i>Photograph by Howland Bancroft.</i>)</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_415" id="Page_415">[415]</a></span></p> - -<p><b>Ox-bow lakes.</b>—The cutting off of a meander within the flood -plain of a river yields a lake which is of horseshoe (ox-bow) outline -and lies generally with low banks within a plain composed of -river silt. Before separating from the parent stream the meander -had begun to silt up, especially at the ends. Ox-bow lakes are, -however, relatively deep near the convex shore and correspondingly -shallow toward the concave margin (<a href="#f451">Fig. 451</a>).</p> - -<div class="figcenter"> - <img src="images/ill-498a.jpg" width="400" height="162" id="f451" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 451.</span>—Diagrams to bring out the characteristics of ox-bow lakes, together -with a map of such lakes from the flood plain of the Arkansas River.</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-498b.jpg" width="200" height="67" id="f452" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 452.</span>—Diagrammatic section to -illustrate the formation of saucer-like -basins between the levees of -streams flowing in a flood plain.</p> -</div></div> - -<p><b>Saucer lakes.</b>—As we have learned, a river meandering in its -flood plain has banks which are higher than the average level of -the plain, for the reason that at flood time the main current of the -stream still persists in the channel, thus allowing the burden of -sediment to be dropped in the -relatively slack water upon its -margin. Because of these natural -embankments or levees, tributary -streams are often compelled to -flow long distances in nearly parallel -direction before effecting a -junction. Between the trunk -stream and its tributaries, likewise bounded by levees, and between -streams and the valley walls, there thus exist low basins -which are more or less saucer-shaped (<a href="#f452">Fig. 452</a>). At flood time, -when the levees are overflowed or crevassed, water enters these -depressions, and an additional supply may be derived from the -walls of the valley. Good illustrations of such lakes are furnished<span class="pagenum"><a name="Page_416" id="Page_416">[416]</a></span> -by the flood plain of the former river Warren near the banks of -the present Minnesota River (<a href="#f453">Fig. 453</a>).</p> - -<div class="figcenter"> - <img src="images/ill-499.jpg" width="400" height="177" id="f453" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 453.</span>—Saucer lakes upon the bed of the former river Warren (from the -Minneapolis sheet, U. S. G. S.).</p> -</div></div> - -<p><b>Crescentic levee lakes.</b>—As we approach the delta of a river, -the size and importance of the levee increases, and here a new type -of levee lake may develop in series (<a href="#f454">Fig. 454</a>). At flood time the -levee is breached near the point of sharpest curvature on the convex -side (<a href="#f454">Fig. 454</a> <i>a</i>). When the waters are subsiding, the current -is kept away from the old channel by the rising grade of the -levee as well as by the inertia of the current, and an entrance to -the old channel is first found below the next change in curvature -of the meander, since here scour becomes effective in cutting -through the levee. The new channel is thus established in the -form of a loop inclosing the old one, and the process of levee building -now erects a wall about the territory newly acquired by the -meander. This territory has the form of a crescent, and when -occupied by water produces a crescentic levee lake often joined -to its neighbors in series. The abandoned channel now closed -at both ends by levees may be occupied by water to produce a -subordinate ribbon type of curving trench (<a href="#f454">Fig. 454</a> <i>b</i>, <i>c</i>).</p> - -<p>The importance of levees in obstructing drainage to form lakes -is only beginning to be appreciated. It has quite recently been -shown that when trunk streams are greatly swollen and burdened -with sediment while flowing from a receding continental glacier, -they may build such high levees as to aggrade their tributary -streams above the junctions, even producing reversed grades -and so impounding the waters to form extensive lakes. During -the “ice age” lakes of this type were formed in Illinois and Kentucky<span class="pagenum"><a name="Page_417" id="Page_417">[417]</a></span> -rivers just above their junctions with the Ohio. The old -lake floor with its eastern shore line and its protruding islands is -easily made out upon the new topographic maps of Kentucky.</p> - -<div class="figcenter"> - <img src="images/ill-500.jpg" width="400" height="501" id="f454" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 454.</span>—Levee lakes developed concentrically in series within meanders of a -stream tributary to the Mississippi and flowing upon its delta plain. <i>b</i> and <i>c</i> are -examples of the ribbon type of levee lake due to occupation of the abandoned -river channel. The larger number of lakes, of which Sip Lake and Texas Lake -are examples, have the form of crescents and lie between abandoned levees (from -recent map of U. S. G. S.).</p> -</div></div> - -<p><b>Raft lakes.</b>—Within humid regions the flood plains of our larger -rivers are generally forested, and as the river swings from side to<span class="pagenum"><a name="Page_418" id="Page_418">[418]</a></span> -side in its perpetual meanderings, the timber which grows upon -the convex side of each meander is progressively undermined by -the river and felled upon its bank. The prostrate trees remain -upon the banks during the low water of the summer season, to be -gathered up at the time of flood in the next spring season. It -is log jams thus acquired which so generally block the main channel -of a river and turn the current across the neck of the meander -when cut-offs occur with the formation of ox-bow lakes. When -the mass of timber thus gathered up by the river is excessive, as, -for example, within the flood plain of the Red River of Arkansas -and Louisiana, huge log rafts are produced -which dam up the river so effectively -as to produce temporary lakes. -The impounded waters soon find an -outlet over the levee at some point -higher up the river, and the waters -flowing off through the timbered -bottom lands, other logs are caught -by the standing timber as in a weir. -A second dam is thus formed which is -separated from the initial one by open -water, and in this way the driftwood -dam acquires enormous proportions -as it gradually moves up the river. -After a period of perhaps a century -or more, the lower sections of the -jam become decayed and dislodged so as to float down the river.</p> - -<div class="floatleft"> - <img src="images/ill-501.jpg" width="200" height="215" id="f455" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 455.</span>—Raft lakes along -the banks of the Red River in -Arkansas and Louisiana at -their fullest recorded development -(after A. C. Veatch, -U. S. G. S.).</p> -</div></div> - -<p>In the lower Red River a great raft of alternating jams and -open water reached a length of about one hundred and sixty miles -and moved up the river at the average rate of something less -than a mile per year. Within the limits of the dam all tributary -streams were blocked, so that secondary lakes were formed in a -double fringe about the main river (<a href="#f455">Fig. 455</a>). The great raft -which formed here in the latter part of the fifteenth century -has now at the beginning of the twentieth been largely removed -and measures have been adopted to prevent its re-formation.</p> - -<div class="floatright"> - <img src="images/ill-502a.jpg" width="200" height="138" id="f456" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 456.</span>—The Swiss lakes Thun and Brienz, -formed by deltas at the junction of streams -tributary to a steep-walled valley.</p> -</div></div> - -<p><b>Side-delta lakes.</b>—It is characteristic of river drainage that -the tributary streams enter the main valley on steeper gradients -than the trunk stream at the point of junction. Wherever the<span class="pagenum"><a name="Page_419" id="Page_419">[419]</a></span> -difference in velocity of the two streams at the junction is large, -and the side stream is charged with sediment, a delta will be -formed at the mouth of -the tributary stream. -Such deltas push out -from the shore and may -eventually block the main -channel so as to form a -more or less sausage-shaped -expansion of the -river—a side-delta lake. -Traverse and Big Stone -Lakes in the valley of the -Warren River in Minnesota -have been formed in -this way (<a href="#f354">Fig. 354</a>, <a href="#Page_326">p. 326</a>). Lakes Thun and Brienz in the Swiss -Alps are of similar origin, the beautiful city of Interlaken being -built upon the delta plain over the valley of the earlier river -(<a href="#f456">Fig. 456</a>). The Mississippi has similarly been expanded to form -Lake Pepin above the delta at the mouth of the Chippewa River.</p> - -<div class="floatleft"> - <img src="images/ill-502b.jpg" width="230" height="228" id="f457" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 457.</span>—Delta lakes formed at -the mouth of the Mississippi -through the junction of the levees -of radiating distributaries with -the shore of the estuary (after -Berghaus).</p> -</div></div> - -<p><b>Delta lakes.</b>—A somewhat different -type of delta lake has been -formed in Louisiana, where the -“father of waters” discharges into -the gulf. Here the various distributaries -radiate from the main -channel to produce the “bird-foot” -delta type and the toes in this foot -by their junction with the banks -which outline the ancient estuary, -have separated in succession a series -of basins that before were in direct -connection with the sea (<a href="#f457">Fig. 457</a>). -Lake Pontchartrain is the largest -of this series, while the so-called -Lake Borgne is in process of -separation.</p> - -<p>Where large deltas push out from the shore into the open sea, -the levees which border the individual distributaries are attacked<span class="pagenum"><a name="Page_420" id="Page_420">[420]</a></span> -by the waves and their materials are transported by the shore -currents and built into barriers. These barriers cut off the re-entrants -between neighboring distributaries so as to produce -lagoons or lakes (<a href="#f458">Fig. 458</a>).</p> - -<div class="floatleft"> - <img src="images/ill-503a.jpg" width="200" height="155" id="f458" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 458.</span>—A type of delta lakes -formed by levees in part destroyed -and built into barriers -on the margin of the delta of the -Nile (after Supan).</p> -</div></div> - -<p>A type of delta lake, which more resembles the side-delta lake -above described, has formed at the mouth of the Colorado River, -where it enters the Gulf of Lower -California. The Imperial Valley -lying to the north of this delta is -the desiccated floor of the earlier -Gulf of Lower California which has -been captured from the sea by the -delta of the Colorado. The rampart -of mountains, by which this valley -is surrounded, has cut it off from -any water supply derived from -clouds, and its waters being no -longer renewed from the sea, the -region has passed through a period -of desiccation which has left the -Salton Sink as the only existing remnant of the earlier lagoon. It -will be remembered that careless operations in diverting distributaries -of the Colorado recently reversed this process so that the -waters rose in the valley, and expensive emergency operations -were necessary in order to again turn the waters of the Colorado -into their accustomed channels.</p> - -<div class="figcenter"> - <img src="images/ill-503b.jpg" width="400" height="160" id="f459" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 459.</span>—Diagrams to illustrate the characteristics of barrier lakes, with an -example from the southern coast of the Island of Nantucket.</p> -</div></div> - -<p><b>Barrier lakes.</b>—The Salton Sink illustrates a type of lake -which is formed at the border of the sea through the erection of<span class="pagenum"><a name="Page_421" id="Page_421">[421]</a></span> -some kind of barrier which captures a small area of the ocean’s -surface. Though such lakes may be properly described as strand -lakes, it is usually at the mouth of a river that the process becomes -effective. The common type of <i>barrier lakes</i> is found -developed on most ragged coast lines where the -shore currents have formed first bars and later -barriers at the mouths of the estuaries (<a href="#f459">Fig. 459</a>). -Such embankments are usually gently curving -or crescent shaped and are composed of sand or -shingle which presents a steep landward and a -gradual seaward slope.</p> - -<div class="floatright"> - <img src="images/ill-504a.jpg" width="150" height="318" id="f460" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 460.</span>—Dune -lakes on the coast -of France (after -Berghaus).</p> -</div></div> - -<p><b>Dune lakes.</b>—Within the narrow strips of -shore in which all the fine soil that could be -available for plant life has been washed away by -the waves, beach sand is exposed to the direct -action of the winds. In time of storm the sand -is picked up and after drifting in the wind is -collected in long ridges parallel to the shore. -Constantly traveling along shore, these dunes block the mouths -of rivers and thus produce a series of lakes such as are indicated -in <a href="#f460">Fig. 460</a>.</p> - -<p><b>Sink lakes.</b>—Another class of lakes are due either directly -or indirectly to the work of underground waters. In districts -which are underlain by limestone, the surface water descending<span class="pagenum"><a name="Page_422" id="Page_422">[422]</a></span> -along the joints of the limestone may widen these passageways -through solution of the rock and at lower levels flow on the floors -of caverns eaten out by the same process on bedding planes of the -formation. At the intersections of joints, more or less circular -shafts known as “swallow-holes” go down to the caves from the -surface. Locally, also the cavern roofs give way so as to choke -the galleries with rubble and leave a basin at the surface which -has an irregular but generally a more or less oval outline. If -sufficiently clogged at the bottom by finer rock débris, these basins -become occupied by small lakes which are known as sinks, and -constitute one of the best surface indications of a limestone -country.</p> - -<div class="figcenter"> - <img src="images/ill-504b.jpg" width="400" height="236" id="f461" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 461.</span>—Sink lakes in Florida, with a schematic diagram to illustrate the -manner of their formation (map from U. S. G. S.).</p> -</div></div> - -<p><b>Karst lakes—poljen.</b>—In the limestone country to the north -and east of the Adriatic Sea—the so-called Karst region—there -are many interesting features which are directly traceable to the -solution of the country rock. Here all the surface water descends -in certain districts along the widened joint planes so that the -drainage is largely subterranean. The so-called <i>dolines</i> or sinks of -very regular and symmetrical forms resembling deep bowls cover -a large part of the surface.</p> - -<p>The entire country is, moreover, faulted in the most intricate -fashion into many rift valleys. The drainage being so largely -subterranean, these downthrown blocks of crust, the so-called -<i>poljen</i>, become flooded at certain seasons of the year when the -subterranean passages become choked or are too small to carry -away all the water. A seasonal lake of this character is the -Zirknitz Lake (<a href="#Page_189">p. 189</a>).</p> - - -<p><b>Playa lakes.</b>—It is the law of the desert that the arid region -be walled in by mountains. This encircling rampart forces the -clouds to rise, and by robbing them of their moisture leaves the -desert dry and barren. Those waters which fall upon the inner -margin of the ranges drain toward the interior of this pan-like -depression and are not returned to the sea—the desert is without -an outlet. Infrequent though they be, the desert rains are of -the cloudburst type and in the hills develop torrents whose waters, -emerging upon the desert floor, develop lakes in the space of a -few minutes or at most hours. In the hot and dry atmosphere -the waters of these shallow basins may be sucked up in the space -of a few hours but reappear in the same basins at the time of the<span class="pagenum"><a name="Page_423" id="Page_423">[423]</a></span> -next succeeding cloudburst. Such ephemeral lakes are known -as playas.</p> - - -<p><b>Salines.</b>—Desert lakes more favored in their supply of water -may be relatively long lived and persist for periods measured in -years or centuries. Such lakes are, however, extremely sensitive -to climatic changes (see <a href="#Page_198">p. 198</a>).</p> - -<p>For the reason that they have no outlet the waters of desert -lakes become salt through continued evaporation. They are, -therefore, spoken of as <i>salines</i>. Lake Bonneville, so long as it -discharged its waters over the sill of the Red Rock Pass, must -have remained fresh; but when the level of its waters had fallen -below this outlet, its waters became salt and the content increased -as the volume diminished.</p> - -<p>The shallow basins upon the floors of desert lakes may have -come into existence in various ways; but it would appear that -the irregular removal of the soil by the winds, modified as this is by -differences in composition of the rock materials and by vegetable -growth, and the deposition of sand by the same agent, are by far -the most important. Many of the types of tectonic and volcanic -lakes which have been described are characteristic of humid and -arid regions alike.</p> - - -<p><b>Alluvial-dam lakes.</b>—Within the mountains upon the desert -borders, the alluvial fans which form at the mouths of valleys, -because of the characteristic cloudburst, sometimes obstruct a -main valley at the junction with its tributaries. By this process -the waters of the main river are impounded in essentially the -same manner as are the rivers of humid regions by the deltas -of their tributaries.</p> - - -<p><b>Résumé.</b>—The types of lakes which we have now considered -are arranged below in tabular form so as to show their relationship -to important geological processes. While not complete, -the list includes the more important classes, as well as others -which, while not of common occurrence, are yet of interest in giving -further illustration to the processes which have been treated -in earlier chapters.</p> - -<p>By giving careful attention to criteria which have been above -suggested, it should be possible in the greater number of instances -at least to determine whether any lake which is visited has had its -origin in one or another of the processes described.</p> - -<p><span class="pagenum"><a name="Page_424" id="Page_424">[424]</a></span></p> - - -<p class="prr">CLASSIFICATION OF LAKES</p> - -<table id="t09" summary="t09"> - - <tr> - <td class="tdc"><i>Tectonic Lakes</i></td> - <td class="tdc"><i>Volcanic Lakes</i></td> - </tr> - - <tr> - <td class="tdt6w">Newland lakes<br /> -Basin-range lakes<br /> -Rift-valley lakes<br /> -Earthquake lakes</td> - <td class="tdt6">Crater lakes<br /> -Coulée lakes</td> - </tr> - - <tr> - <td class="tdc"><i>Continental Glaciation Lakes</i></td> - <td class="tdc"><i>Mountain Glaciation Lakes</i></td> - </tr> - - <tr> - <td class="tdt6">Morainal lakes<br /> -Pit lakes<br /> -Glint or colk lakes<br /> -Ice-dam lakes<br /> -Glacier-lobe lakes</td> - <td class="tdt6">Rock-basin lakes<br /> -Valley moraine lakes<br /> -Landslide lakes<br /> -Border lakes</td> - </tr> - - <tr> - <td class="tdc"><i>River Lakes</i></td> - <td class="tdc"><i>Strand Lakes</i></td> - </tr> - - <tr> - <td class="tdt6">Ox-bow lakes<br /> -Saucer lakes<br /> -Crescentic levee lakes<br /> -Raft lakes<br /> -Side-delta lakes<br /> -Delta lakes</td> - <td class="tdt6">Barrier lakes<br /> -Dune lakes</td> - </tr> - - <tr> - <td class="tdc"><i>Ground Water Lakes</i></td> - <td class="tdc"><i>Desert Lakes</i></td> - </tr> - - <tr> - <td class="tdt6">Sink lakes<br /> -Karst lakes—<i>poljen</i></td> - <td class="tdt6">Playa lakes<br /> -Salines<br /> -Alluvial dam lakes.</td> - </tr> - -</table> - -<p class="prr"><span class="smcap">Reading References for Chapter XXIX</span></p> - -<p class="p1">General:—</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> Lakes of North America. Boston, 1895, pp. 125, pls. 23.</p> - -<p class="pex"><span class="smcap">A. P. Brigham.</span> Lakes, A Study for Teachers, Jour. Sch. Geogr., vol. 1, -1897, pp. 65-72.</p> - -<p class="pex"><span class="smcap">N. M. Fenneman.</span> The Lakes of Southeastern Wisconsin, Bul. 8, Wis. -Geol. and Nat. Hist. Surv., 1902 (Rev. Ed., 1910), pp. 188, pls. 37.</p> - -<p class="pex"><span class="smcap">A. Delebecque.</span> Les Lacs Français (with Atlas). Paris, 1898. (Work -crowned by the Society of Geology of Paris.)</p> - -<p class="pex"><span class="smcap">H. R. Mill.</span> Bathymetrical Survey of the English Lakes, Geogr. Jour., -vol. 6, 1895, pp. 46-73, 135-166.</p> - -<p class="pex"><span class="smcap">A. Supan.</span> Grundzüge der Physischen Erdkunde. Leipzig, 1896, pp. -531-548.</p> - -<p class="pex"><span class="smcap">H. Berghaus.</span> Atlas der Hydrographie. Gotha, 1891, pl. 3.</p> - -<p class="pex"><span class="smcap">R. D. Salisbury.</span> Physiography. 1907, pp. 292-327.</p> - -<p class="pex"><span class="smcap">Charles Rabot.</span> Revue de limnologie, La Géographie, Vol. 4, 1901, -pp. 110-119, 172, 189.</p> - -<p><span class="pagenum"><a name="Page_425" id="Page_425">[425]</a></span></p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> A Geological Reconnaissance in Southern Oregon, 4th -Ann. Rept. U. S. Geol. Surv., 1884, pp. 442-447. (Basin range -lakes.)</p> - -<p class="pex"><span class="smcap">Ed. Suess.</span> The Face of the Earth, vol. 4, 1909, pp. 268-286. (Rift valley -lakes.)</p> - -<p class="pex"><span class="smcap">J. S. Diller.</span> Crater Lake, Nat. Geogr. Mag., vol. 8, 1897, pp. 33-48, -pl. 1; Geology of Lassen Peak Quadrangle, California, Geol. Fol. 15, -U. S. Geol. Surv., 1895. (Coulée lakes.)</p> - -<p class="pex"><span class="smcap">N. M. Fenneman.</span> Lakes of Southeastern Wisconsin, <i>l.c.</i>, pp. 4-6. (Pit -lakes.)</p> - -<p class="pex"><span class="smcap">Ed. Suess.</span> The Face of the Earth, vol. 2, 1906, pp. 340-346, pl. 7. (Glint -lakes.)</p> - -<p class="pex"><span class="smcap">I. C. Russell.</span> A Preliminary Paper on the Geology of the Cascade -Mountains in Northern Washington, 20th Ann. Rept. U. S. Geol. Surv. -Pt. ii, 1900, pl. 14. (View of a rock-basin lake.)</p> - -<p class="pex"><span class="smcap">E. W. Shaw.</span> Preliminary Statement concerning a New System of -Quaternary Lakes in the Mississippi Basin, Jour. Geol., 1911, pp. 481-491. -(New type of levee lakes.)</p> - -<p class="pex"><span class="smcap">A. C. Veatch.</span> Formation and Destruction of the Lakes of the Red -River Valley, Prof. Pap. No. 46, U. S. Geol. Surv., pp. 60-62, pls. 29-33. -(Raft lakes.)</p> - -<p class="pex"><span class="smcap">M. Neumeyer.</span> Erdgeschichte, vol. 1, pp. 595-596. (Poljen.)</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_426" id="Page_426">[426]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXX</h2> - -<p class="pch">THE EPHEMERAL EXISTENCE OF LAKES</p> - -<p><b>Lakes as settling basins.</b>—Of all the processes which conspire -to blot out the lakes with which our northern landscapes are -dotted, the one of greatest importance is in most cases a process -of filling by the sediments brought in by tributary streams. The -carrying of sediment in suspension depends, as we know, upon the -velocity of the current, and as this is checked where it reaches -the lake margin, all coarser material is at once deposited to form -a delta, while the finer sediments are held longer in suspension and -finally settle in thin layers over the entire bottom of the lake. -Clay deposits surrounded by coarser sediments are thus characteristic -of filled lake basins.</p> - -<div class="figcenter"> - <img src="images/ill-509.jpg" width="400" height="171" id="f462" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 462.</span>—Map of the Arve and the upper Rhone to show the importance of -Lake Geneva as a settling basin of the larger stream.</p> -</div></div> - -<p>How waters are clarified by their passage through a lake is -indicated by a comparison of a river system such as the St. Lawrence, -with a river like the Missouri and Mississippi. Not only -are the lower stretches of the St. Lawrence in striking contrast -with the muddy floods of the Missouri and Mississippi; but the -delta, which is so remarkable a feature in the Mississippi, has -no counterpart in the northern river.</p> - -<p><span class="pagenum"><a name="Page_427" id="Page_427">[427]</a></span></p> - -<p>The most noteworthy examples of settling are, however, furnished -by the lakes of Switzerland, for the reason that <span class="smcap">Swiss</span> -rivers are heavily charged with rock flour produced beneath the -numerous glaciers at the valley heads, and, further, because these -rivers descend with turbulent currents to near the borders of the -larger lakes. To look out upon the murky waters of the upper -Rhone, where they enter Lake Geneva near Villeneuve, and then -to watch the flood of crystal water which issues from the lake -and passes under the bridge at Geneva, is an object lesson which -no traveling student should miss (<a href="#f462">Fig. 462</a>). Yet even more instructive -is a visit to the <i>Bois de la Bâtie</i> at the junction of this -clear stream with the Arve, a half hour’s walk only below Geneva. -The waters of the Arve have come on a steep descent directly -from the glaciers of the Mont Blanc district, and as they meet -the cleared waters of the Rhone, they flow beside them down -the common valley without mingling. Dull and opaque, the -Arve waters can be discerned for a long distance as a white belt -against the left bank of the river, sharply defined against the blue -reflecting surface of the Rhone waters (<a href="#f463">Fig. 463</a>). Upon the -banks of the Arve, just above its junction, a cement manufactory -has been established to utilize the clays which are here deposited.</p> - -<div class="figcenter"> - <img src="images/ill-510.jpg" width="400" height="248" id="f463" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 463.</span>—View looking upstream across the opaque waters of the Arve to the -clear reflecting surface of the Rhone. To the right across the Arve is seen the -cement works for recovering the Arve sediments.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_428" id="Page_428">[428]</a></span></p> - -<p>Wherever lakes are contained in long and narrow valleys, the -greater part of the tributary drainage enters at the upper end, -and the delta which there forms -extends from bank to bank. As -it continues to advance into the -lake, the earlier water basin is -gradually transformed into a level -plain of delta deposit, a feature -so common as to be deserving of a -special name. The Scottish lochs, -which are lakes of this type, are -each extended in a longer or shorter -delta plain described as a <i>strath</i>, -and this local term may well be -given a general application (frontispiece). -The city of Ithaca, the -seat of Cornell University, is built -upon a strath at the head of Lake Cayuga, and numberless Scottish -and Swiss hamlets have been located upon such fertile plains -(<a href="#f464">Fig. 464</a>).</p> - -<div class="floatleft"> - <img src="images/ill-511.jpg" width="250" height="241" id="f464" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 464.</span>—The village of Poschiavo -in eastern Switzerland, built upon -a strath at the head of Lake Poschiavo.</p> -</div></div> - -<p><b>Drawing off of water by erosion of outlet.</b>—Next in importance -to the filling up of lake basins as a factor in their early -extinction is the cutting down of their channels of outflow. -Whenever the walls of the outlet are cut in rock, this draining -process is apt to be slow, for the reason that the outlet -stream is of filtered water and so lacks the necessary cutting -tools. By far the larger number of lakes are, however, held -back by dams of loose drift deposits laid down by the earlier -continental glaciers; and so the very clarity of the water promotes -the erosion of the outlet by allowing the stream’s full -burden of sediment to be lifted and then removed from the -channel.</p> - - -<p><b>The pulling in of headlands and the cutting off of bays.</b>—The -removal of projecting headlands by wave action, though it increases -the area of the lake, yet it decreases directly the volume -of lake water through formation of the built terrace, and indirectly -in far larger measure through the transformation of bays -into quiet lagoons within which the extinguishing process of peat -growth is set in operation.</p> - -<p><span class="pagenum"><a name="Page_429" id="Page_429">[429]</a></span></p> - - -<p><b>Lake extinction by peat growth.</b>—The first condition for the -growth of lake vegetation is quiet water. Within small lakes, -such as the kettle basins upon moraines, aquatic vegetation develops -rapidly, and bogs of peat might almost be included among -the most important distinguishing marks of a glaciated country. -Within larger lakes it is only after barrier beaches have been thrown -across the mouths of the bays to form natural breakwaters for -the waves that this process of lake extinction by peat growth -can become effective.</p> - -<div class="floatright"> - <img src="images/ill-512.jpg" width="250" height="177" id="f465" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 465.</span>—View of the floating bog and surrounding -zones of vegetation in a small glacial lake of the Yellowstone -National Park (after a photograph by Fairbanks).</p> -</div></div> - -<p>Many erroneous notions are still held concerning the prime -importance of sphagnum in peat formation, owing to the peculiar -local conditions -under which the -early studies were -made. Within the -glaciated districts of -the United States, -the formation of -peat involves the -successive growths -of a number of -zones of vegetation -and the formation -of a floating bog -which advances into -the lake from the -shores, followed in -turn by belts of low shrubs, tamaracks, and lastly deciduous -trees (<a href="#f465">Fig. 465</a>).</p> - -<p>In most cases the first plants to develop in a quiet lake are the -water lilies, though these are sometimes preceded by chara and -floating bladderwort. Next behind the water lilies come the -sedges, which form a mat of floating bog by their grasslike stems -sinking down in the water and being there interwoven with the -rhizomes below. This mat of sedge is often so firm that cattle -may advance upon it to the water’s edge, but it is separated -by a layer of water from the bed of growing peat at the bottom -of the lake (<a href="#f466">Fig. 466</a>). This bed of peat appears to grow upward -toward the surface and become joined to the shore end of the<span class="pagenum"><a name="Page_430" id="Page_430">[430]</a></span> -floating bog by decaying vegetation which is dropped from the -bottom of the mat above.</p> - -<div class="figcenter"> - <img src="images/ill-513.jpg" width="400" height="105" id="f466" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 466.</span>—Diagram to show how small lakes are transformed into peat bogs -(after C. A. Davis).</p> -</div></div> - -<p>In order behind the floating bog come the advanced plants -of the conifer group, with sphagnum and low shrub here upon a -peat base extending to the lake bottom. Behind the belt of -shrubs arise the tamaracks and spruces, and lastly, toward the -shore, come the deciduous trees and especially poplars, maples, -and marginal willows. Upon the margin of the basin there is -usually a low trench, or “fosse”, filled with water during wet seasons, -as a result, no doubt, of seasonal inwash that does not reach -the residual lake toward the center of the basin.</p> - -<p><b>Extinction of lakes in desert regions.</b>—In arid regions there -are special causes of lake extinction. Thus the blowing in of -sand and dust carried for long distances in the air, a by no -means negligible factor even in humid regions, here assumes -large importance. The now exposed basins of extinct desert -lakes afford the evidence, however, of an even greater factor -of extinction, in climatic change. The clouds, which at one -time found their way into the drainage basin of a lake, may -later through the rise of a mountain barrier be cut off, and -so with reduced water supply a period of lake desiccation -is begun. When, in this process of drying up, the lake level -has fallen below that of the outlet, the saline content of the -waters begins to increase, and later a stage is reached, as in -Great Salt Lake, when the sodium salts are precipitated. When -the lake has become extinct, these deposits remain as a witness -to the changed climatic condition.</p> - - -<p><b>The rôle of lakes in the economy of nature.</b>—It is natural, -in considering the extinction of lakes, to give some attention to -the rôle which they play in the economy of nature. That lakes<span class="pagenum"><a name="Page_431" id="Page_431">[431]</a></span> -filter the water of rivers, and prevent the formation of important -delta deposits, has already been noticed. A curious exception -to this general rule is furnished by the great delta at the head of -Lake St. Clair, just below the outlet of Lake Huron. This anomaly -is, however, explained by the peculiar currents of Lake Huron, -which are so directed as to sweep the beach sand into the swift -current of the outlet, to be deposited in the quiet -waters of Lake St. Clair (<a href="#f467">Fig. 467</a>).</p> - -<div class="floatright"> - <img src="images/ill-514.jpg" width="150" height="276" id="f467" - alt="" - title="" /> - <div class="cf"><p class="ch150"><span class="smcap">Fig. 467.</span>—Map -to show anomalous -position -of the delta in -Lake St. Clair, -due to the peculiar -currents -in Lake Huron -(after maps by -Cole).</p> -</div></div> - -<p>As regulators of the flow of rivers, lakes perform -an important function. Such disastrous floods as -are characteristic of the spring season within the -basin of the lower Mississippi could not occur in -the lower St. Lawrence, for the reason that the -great basins of the lakes serve as distributing reservoirs. -The annual floods, upon which the agriculture -of Egypt depends, are explained by the -flood waters from the high mountains of Abyssinia -entering the Nile <i>below</i> the lakes of its upper basin.</p> - -<p>In one further respect large inland bodies of -water have an important function as regulators. -It is the property of water to respond but slowly -to the variations in the quantity of heat which -reaches the earth’s surface from the sun. A larger -quantity of heat must be added to or abstracted -from a body of water, in order to change its temperature -by one degree, than would be required for a like change -in the same bulk of earth or rock. Thus bodies of water by more -slowly acquiring the summer’s heat retard the coming spring, and -by storing up this energy and carrying it over into the autumn -the warm season is prolonged and early frosts prevented. The -fruit belts about the lower Great Lakes are thus dependent upon -this regulating property of the lake waters. The discomfort of -the long spring of raw weather is thus compensated by an unusually -salubrious harvest season.</p> - - -<p><b>Ice ramparts on lake shores.</b>—Small ridges known as ice ramparts -are formed upon lake shores by the action of lake ice, though -subject to so many qualifying conditions that the range of their -occurrence is somewhat limited. Within districts where a winter -ice cover of some thickness is formed, the shores of lakes are apt<span class="pagenum"><a name="Page_432" id="Page_432">[432]</a></span> -to present ridges of bowlders parallel to and near the water’s -edge, and such lakes have sometimes become known as “wall -lakes” (<a href="#f468">Fig. 468</a>).</p> - -<div class="floatleft"> - <img src="images/ill-515.jpg" width="250" height="173" id="f468" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 468.</span>—A bowlder wall upon the shore -of a small lake in the Adirondacks of New -York.</p> -</div></div> - -<p>In many cases these small ridges have been formed at the time -of the spring “break up” of the ice; for the ice cover, when once -loosened, is drifted in great -rafts first against one shore, -and later, with a change of -wind direction, against another. -Under the impact of -such heavy rafts, the half-submerged -bowlders near the -shore are forced up the beach -until they lie in a ridge or -bowlder wall.</p> - -<p>At other times such bowlder -walls, and far more interesting -ridges as well, result from a kind of ice shove independent of the -wind, but caused by expansion within the ice itself during a sudden -rise of temperature of the surrounding air. Such ice ramparts -require for their explanation a consideration of the sequence of -events from the time the ice cover closes the lakes.</p> - -<div class="figcenter"> - <img src="images/ill-516a.jpg" width="400" height="135" id="f469" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 469.</span>—Diagrams to show the effect of ice shove in producing ice ramparts -upon the shores of lakes (after Buckley with a slight modification).</p> -</div></div> - -<p>The first lake ice of early winter forms in most cases with air -temperatures a few degrees only below the freezing point of the -water. When later a severe “cold wave” arrives, the ice cover -is contracted and becomes too small for the lake surface. To this -contraction it yields and opens cracks up which the water rises, -and in the prevailing low temperature this water is quickly frozen -and the lake cover again made complete. Skaters are familiar -with the opening of these cracks and the loud “roaring” which -accompanies it on cold mornings, the sharp skate runners sometimes -starting a crack in the strained ice, as does a light scratch -upon glass that is in a similar strained condition.</p> - -<div class="fl1"> - <img src="images/ill-516b.jpg" width="230" height="183" id="f470a" - alt="" - title="" /> -</div> - -<div class="fr1"> - <img src="images/ill-516c.jpg" width="230" height="183" - alt="" - title="" /> -</div> - -<div class="fr2"> - <img src="images/ill-516d.jpg" width="230" height="155" - alt="" - title="" /> - <div class="cf"><p class="ch230"><span class="smcap">Fig. 470.</span>—Various forms of ice -ramparts (after Buckley).</p> -</div></div> - -<p class="vh">———</p> - -<p>The original ice cover of the lake, which was formed at near-freezing -temperatures, has now received a number of inserted -wedges of new ice at a time when its contracted volume has made -this possible. If now a “warm wave” succeeds to the “cold -wave” in the air, the ice cover expands at a rate corresponding -to its rate of contraction, so that a strong pressure is exerted<span class="pagenum"><a name="Page_433" id="Page_433">[433]</a></span> -against the shore (<a href="#f469">Fig. 469</a>). Sliding up the sloping surface of -the cut and built terrace, the force of this shove may be deflected -upward against the cliff, and if this is of loose materials, the effect -may be to ram bowlders into the bank, to push up ramparts or -ridges, to overturn trees, etc. (<a href="#f470a">Fig. 470</a>). In marsh land the -frozen surface layer may slide over -its unfrozen base and be forced up -into broken folds (lower diagram -of <a href="#f469">Figs. 469</a> and <a href="#f470a">470</a>).</p> - -<div class="floatleft"> - <img src="images/ill-517.jpg" width="250" height="162" id="f471" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 471.</span>—Map of Lake Mendota at Madison, Wisconsin, -showing the position of the ridge which forms -from ice expansion, and the ice ramparts about the -shores of the bays (based on Buckley’s map).</p> -</div></div> - -<p>In order that ice ramparts may -be formed, it is necessary that the -winter climate of the district be -severe and characterized by alternating -cold and warm waves, involving -considerable range of air temperature below the freezing -point. If the lake is small, the push of the ice will be through so -small a distance as not to yield appreciable ramparts. If, on the -other hand, the lake is too large, the ice cover is not rigid enough -to transmit the push to the distant shore, but, like a long beam<span class="pagenum"><a name="Page_434" id="Page_434">[434]</a></span> -employed in the same manner to transmit a compressive stress, -it is bent out of a straight line and later broken. Thus in a broad -lake, with the coming of a “warm wave”, the ice cover opens in -a crack from shore -to shore and finds -relief from the stress -by pushing up a ridge -above the crack. On -such lakes ice ramparts -are found only -about the shores of -bays whose expanse -does not greatly exceed -a mile (<a href="#f471">Fig. 471</a>).</p> - -<p>When there is -heavy snowfall, ice -ramparts either do -not form or are of -smaller dimensions, probably in part because the ice is blanketed -by the snow and so prevented from sudden elevation of temperature -during the “warm wave”, but even more because the ice -cover is sensibly bowed down under its load and so rendered -incompetent to transmit the developed stresses to the shores.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter XXX</span></p> - -<p class="p1">Lake extinction by peat growth:</p> - -<p class="pex"><span class="smcap">C. A. Davis.</span> Peat, Essays on its Origin, Uses, and Distribution in Michigan, -Ann. Rept. Mich. Geol. Surv. for 1906, 1907, pp. 105-182; -Peat Deposits as Geological Records, 10th Rept. Mich. Acad. Sci., -1908, pp. 107-112.</p> - -<p class="pex"><span class="smcap">G. P. Burns.</span> Bog Studies. Ann Arbor, 1906, pp. 13.</p> - -<p class="p1">Ice ramparts:</p> - -<p class="pex"><span class="smcap">C. H. Hitchcock.</span> Shore Ramparts in Vermont, Proc. Am. Assoc. Adv. -Sci., vol. 13, 1869, pp. 335-337.</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Lake Bonneville, Mon. 1, U. S. Geol. Surv., 1890, pp. 71-72.</p> - -<p class="pex"><span class="smcap">E. R. Buckley.</span> Ice Ramparts, Trans. Wis. Acad. Sci., etc., vol. 13, 1900, -pp. 141-162, pls. 1-18.</p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Requisite Conditions for the Formation of Ice -Ramparts, Jour. Geol., vol. 19, 1911, pp. 157-160.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_435" id="Page_435">[435]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">CHAPTER XXXI</h2> - -<p class="pch">THE ORIGIN AND THE FORMS OF MOUNTAINS</p> - -<p><b>A mountain defined.</b>—As ordinarily understood, mountains -are elevations upon the earth’s surface which rise above the -general level of the country. Their summits need not be at great -heights above the sea, but it is essential that they project above -the average level of the surrounding country by at least a quarter -of a mile. Lower elevations are described as hills. On the other -hand, the elevation of a plateau like the “High Plains” of the -western United States may be as much as a mile, but the vast -expanse of nearly level surface precludes the use of the term -“mountain.” The word is thus applied to a feature of the earth -and not merely to an elevated tract.</p> - -<p>In a collective sense, though more often in the plural form, -the term is properly applied to groups of similar features which -have a common origin in local uplift of the land. The origin of -mountains used in this sense of mountain complexes is thus -connected with some essentially local uplift of the earth’s surface. -This may take place by the processes of folding and superincumbent -fault displacement, by volcanic extravasations or ejections, -or by a deeper seated and essentially hydrostatic elevation -of rock beds over molten rock material.</p> - -<p>The existing <i>forms</i> of mountains, as we are to see, are largely -shaped by the erosional processes which are set in operation -by the uplift itself, though often completed long subsequent -to it.</p> - - -<p><b>The festoons of mountain arcs.</b>—From our earliest studies -of school geographies, we have become familiar with the arrangement -of the more important mountains in long chains or systems. -Comparatively few persons have given any further attention to -the arrangement of the chains, though over large areas of the -earth’s surface the distribution of mountain ranges is deeply significant. -The map of Asia in particular presents a series of great -sweeping arcs or crescents which are grouped as though hung<span class="pagenum"><a name="Page_436" id="Page_436">[436]</a></span> -upon the map in festoons with knots or vertexes to separate -neighboring groups (<a href="#f474">Fig. 474</a>, <a href="#Page_438">p. 438</a>, and <a href="#f472">Fig. 472</a>).</p> - -<div class="floatleft"> - <img src="images/ill-519.jpg" width="250" height="223" id="f472" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 472.</span>—The great multiple mountain arc of -Sewestan, British India (after de Saint Martin -and Schrader).</p> -</div></div> - -<p>The significance of these mountain groupings in the evolution -of the earth’s surface -has been pointed out -by the great Viennese -geologist Suess, to whom -we are indebted for focusing -upon the plan of -the earth an amount of -attention which before -had been largely given -to the preparation of -hypothetical sections -of strata which were -largely buried from sight -beneath the earth’s surface. -Broadly speaking, -the mountain arcs may -be said to be grouped -about those shields of older rock which geological studies have -shown to be the oldest land masses upon the globe. Within the -northern hemisphere these original continents are represented by -the areas of crystalline rock centered over Hudson Bay, the Baltic -Sea, and an area in northeastern Siberia known to geologists as -Angara Land. In our study of the figure of the earth (Chapter II) -it was found that these shields represent the truncated angles of -the rounded tetrahedral form toward which the planet is tending -(<a href="#f3">Fig. 3</a>, <a href="#Page_12">p. 12</a>).</p> - - -<p><b>Theories of origin of the mountain arcs.</b>—The mountain -arcs, when studied in detail, are found to be composed of closely -folded rock strata, the flexures of which are generally so overturned -that their axial planes dip toward the center of the arc (<a href="#f473">Fig. 473</a>). -It was the view of Suess that these arcs are to be explained -by a pushing outward of the rock strata from the center of the -arc toward its periphery, thus causing a wrinkling of the surface -strata and an overriding of the surrounding formations, which -upon this hypothesis opposed a greater resistance to the sliding -movement. The folding together of the strata due to the sliding<span class="pagenum"><a name="Page_437" id="Page_437">[437]</a></span> -naturally involves a very considerable diminution of the surface -area presented by the strata (<a href="#f22">Fig. 22</a>, <a href="#Page_42">p. 42</a>). In the case of the -Alpine chains it has been estimated that a flat land area, four -hundred to eight hundred miles across, has by the folding process -been reduced to a width of only about one hundred miles, or from -a fourth to an eighth of its former width.</p> - -<div class="floatright"> - <img src="images/ill-520.jpg" width="200" height="277" id="f473" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 473.</span>—<i>a</i>, diagram to illustrate -the Suess’ theory of the -origin of mountain arcs; <i>b</i>, the -author’s modification of this -view.</p> -</div></div> - -<p>The weakness of Professor Suess’ theory lies in the fact that -such compression as it implies is assumed to be due to an -<i>outward</i> movement of the relatively -small area of the earth’s outer shell -which is included <i>within</i> the arc. It -must be obvious that such a movement, -being from a center toward three -sides at once, would for this circumscribed -area involve enormous proportionate -reduction in superficial area -of the strata and could only result in -a hiatus near the center of the arc. -No such gap is to be found, and one -would, moreover, be difficult to account -for upon any plausible hypothesis. On -the other hand, the general contraction -of the planet as a whole, involving -as it does reduction of surface over -large areas, is a well-recognized fact; -and if it be true that the shields -formed by the older continents are less subject to contraction than -the remaining portions of the surface, it is easy to understand why -the earth’s outer skin should be wrinkled by <i>underfolding</i> and -thrusting about these continental margins. The contrast of this -view with that of Professor Suess is expressed in the diagrams of -<a href="#f473">Fig. 473</a>.</p> - -<div class="floatleft"> - <img src="images/ill-521a.jpg" width="250" height="208" id="f474" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 474.</span>—Festoons of mountain arcs about the borders -of the Pacific Ocean—Pacific type of coast (based upon -Suess).</p> -</div></div> - -<p>We may illustrate this conception by a stretched sheet of rubber -cloth such as is in common use by dentists, upon which a -thin layer of hot Canada balsam has been spread. This substance -congeals upon cooling to near-normal temperatures, and if a small -local area of the balsam layer be chilled and the tension upon the -rubber then released, the viscous balsam of the unchilled portion -of the layer is thrown into wrinkles about the cooled and more<span class="pagenum"><a name="Page_438" id="Page_438">[438]</a></span> -resistant areas. These more resistant portions of the stratum -may thus represent the ancient continental shields of our planet.</p> - -<p><b>The Atlantic and -Pacific coasts contrasted.</b>—In -his -studies of mountain -arcs in their -relation to the -plan of the earth, -Professor Suess -has shown how -the arrangements -of the mountain -chains about the -two larger oceans -represent two -strongly contrasted -types. -Whereas about -the Pacific margin -the mountain arcs are, as it were, strung in festoons which trend -parallel to and are convex toward the coast, or else lie in fringing -garlands of islands in the same attitude (<a href="#f474">Fig. 474</a>); the mountain -chains about the Atlantic become sharply truncated as they reach -the coast, and thus indicate -that the basin of this ocean -has been produced by an inthrow -or depression between -great marginal displacements -in some period subsequent -to the formation of -the mountains.</p> - -<div class="floatright"> - <img src="images/ill-521b.jpg" width="250" height="209" id="f475" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 475.</span>—The interrupted system of the -Armorican Mountains common to western -Europe and eastern North America (after -Arldt).</p> -</div></div> - -<p>Thus the mountain folds -of the Appalachian system -are in Newfoundland cut -off abruptly at the coast -line, and the same beds, -similarly truncated, are encountered -again across the<span class="pagenum"><a name="Page_439" id="Page_439">[439]</a></span> -expanse of ocean in the folds at the coast of western Europe (<a href="#f475">Fig. 475</a>). -In discontinuous remnants this ancient mountain chain may -be traced in an east and west direction across western and central -Europe. We have thus here to do with a single mountain -system which extends from central Europe to northern Alabama, -out of which a great link has been taken by the subsequent -sinking in of the basin of the Atlantic Ocean.</p> - -<div class="floatright"> - <img src="images/ill-522a.jpg" width="200" height="108" id="f476" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 476.</span>—Schematic representation -of a “zone of diverse displacement” -in the Great Basin of -the western United States (after -Powell).</p> -</div></div> - -<p><b>The block type of mountain.</b>—The inclusion of most elevations -in mountain chains and arcs is one of the most obvious -facts to any one who has examined -world atlases with this subject in -mind. Such chains are almost invariably -composed of folded rocks, -thus indicating that erosion has -removed great superincumbent -masses of strata since the crustal -compression produced the folds at -considerable depths below the then -surface.</p> - -<p>There are, however, large elevated tracts upon the earth’s surface -which are intersected by deep valleys, but where no arrangement -of the elevated portions within chains or ranges is to be -detected. In such cases the distribution of mountain and valley -may bear a resemblance to a mosaic of disturbed parts which -stand at different levels -(<a href="#f476">Fig. 476</a>).</p> - -<div class="floatleft"> - <img src="images/ill-522b.jpg" width="250" height="184" id="f477" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 477.</span>—Section of an East African block -mountain (after J. W. Gregory).</p> -</div></div> - -<p>Such block mountain districts -are to be found in -many parts of the earth’s -surface, but notably within -the Great Basin of the -western United States, and -in the land area which -borders the Indian Ocean -upon the west and northwest. -In contrast with the -mountain arcs, so strikingly -exemplified by the continent of Asia as a whole, its extreme southwestern -portion is made up of an alternation of plateau and rift<span class="pagenum"><a name="Page_440" id="Page_440">[440]</a></span> -valley separated from each other by great displacements. Though -modified to some extent by erosion, the elevations seem generally -to represent the displaced crust blocks which in mutual adjustments -have been left at the highest levels (<a href="#f477">Fig. 477</a>). The valley -of the Jordan, with the mountains of Lebanon rising above it, is -near the northern extremity of this faulted mountain region (<a href="#f434">Fig. 434</a>, -<a href="#Page_404">p. 404</a>), while the Great Rift valley, crossing east Central -Africa, and the many neighboring rifts to the east and west, are -graven in lines so deep that an observer upon a neighboring planet -might perhaps detect them.</p> - -<p>It is not necessary in all cases to assume that the block mountains -of a faulted district represent the blocks which in the adjustments -were left the highest. Erosion in the course of time -accomplishes marvels of transformation, and it may result that -heavy masses of more resistant rock eventually project the highest, -even though they may represent the downthrown blocks in -the fault mosaic (<a href="#f43">Fig. 43</a>, <a href="#Page_60">p. 60</a>).</p> - -<div class="figcenter"> - <img src="images/ill-523.jpg" width="400" height="121" id="f478" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 478.</span>—Tilted crust blocks in the Queantoweap valley.</p> -</div></div> - -<p>Where in addition to undergoing changes of level the earth -blocks have been tilted, the features long since described from our -western interior basin as “Basin Range structure” are developed. -Here the upper surface of the disturbed earth blocks betrays the -evidence of a definite tilt in some one direction (<a href="#f478">Fig. 478</a>, and <a href="#f431">Fig. 431</a>, -<a href="#Page_402">p. 402</a>).</p> - -<p><b>Mountains of outflow or upheap.</b>—An important type of mountain, -generally described as volcanic, may be due either to the outflow -of lava at the earth’s surface, or to accumulations of separated -fragments of lava, first thrown into the air, and then deposited -by gravity or admixed with water as volcanic mud. Such mountains, -both before and after modification by erosion, assume the -strikingly characteristic forms which have been fully discussed in -Chapters IX and X. The dominant types are the lava dome and<span class="pagenum"><a name="Page_441" id="Page_441">[441]</a></span> -the puy, the cinder cone, and the more complex composite cone. -Excepting only the surface produced by the few great fissure eruptions -and the semivolcanic mesa type, the individual mountains -of volcanic origin develop features with notably circular bases.</p> - -<div class="figcenter"> - <img src="images/ill-524a.jpg" width="450" height="85" id="f479" - alt="" - title="" /> - <div class="caption"><p class="ch450"><span class="smcap">Fig. 479.</span>—Pen drawing of the laccolite of the Carriso Mountain by W. H. -Holmes, which shows the jagged surface of the igneous rock core and the sloping -tables which still remain of the roof of sedimentary rocks (after Cross).</p> -</div></div> - -<div class="floatright"> - <img src="images/ill-524b.jpg" width="250" height="257" id="f480" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 480.</span>—Map of laccolitic mountains. A portion of the -Judith Mountains, Montana. The intrusive igneous rock is -shown in black (after Weed).</p> -</div></div> - -<p><b>Domed mountains of uplift—laccolites.</b>—At a considerable -number of widely separated localities upon the earth’s surface, -mountainous regions are encountered, the central areas or cores -of which are composed of intrusive igneous rock such as granite, -and about this core the sediments dip away in all directions as -though they -had once -formed a continuous -roof -above it and -had been -forced into -this dome by -hydrostatic -pressure of -the once viscous -material -beneath (<a href="#f152">Fig. 152</a>, -<a href="#Page_143">p. 143</a>, -and <a href="#f479">Figs. 479</a> -and <a href="#f480">480</a>). Examples -of such -domed mountains -of uplift -were first described -by -Gilbert from<span class="pagenum"><a name="Page_442" id="Page_442">[442]</a></span> -the Henry Mountains of Utah, but instances are furnished by -many elevated tracts, especially within the western United States. -Such mountains are known as <i>laccolites</i>, -but when one margin at least -of the igneous core corresponds to -a displacement, the mountain is described -as a <i>bysmalite</i> (<a href="#f481">Fig. 481</a>).</p> - -<div class="floatleft"> - <img src="images/ill-525.jpg" width="200" height="149" id="f481" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 481.</span>—Ideal sections of laccolite -and bysmalite.</p> -</div></div> - -<p>When subjected to long-continued -erosion, the generally fissured granitic -core of the laccolite weathers in a -wholly different manner from the -bedded sediments which surround -and still in part mount over it. The former usually presents a -more or less jagged surface which contrasts sharply with the gently -sloping tables of the latter (<a href="#f479">Fig. 479</a>). About the high granite core -of the mountain, the several strata of the uptilted formations present -each a steep slope toward this higher land, and a gentler slope -in the opposite direction. Such unsymmetrical ridges which surround -the mountain area are often referred to as “hog backs” -(<a href="#p12b">plate 12 B</a>). The arrangement of the strata in the hog backs thus -presents an overlapping series like the shingles upon a roof, except -that the overlapping is here from the bottom instead of the -top, and the exposed ends thus face toward the crest. Unlike a -shingle roof the hog backs do not shed the water which descends to -them from the higher levels, but, on the contrary, they cause it to -flow in troughs parallel to the base of the slope except where outlets -are found through them.</p> - - -<p><b>Mountains carved from plateaus.</b>—In the mountain types -thus far discussed, the local uplifting of the land has itself developed -features which in the aggregate may be referred to as mountains, -even though the characters of the original surface are soon destroyed -by erosive processes of one sort or the other. Erosive -processes are, however, quite competent to produce mountain -forms from a featureless plateau, and particularly through the -incision by streams of running water, the best studied process of -mountain sculpture (see <a href="#Page_149">Chapters XI-XIII</a>). This process of -throwing valleys about an elevated section of the earth’s surface, -and so carving out mountains, is sometimes described as <i>circumvallation</i>; -and if the term “mountain” be applied in its ordinary<span class="pagenum"><a name="Page_443" id="Page_443">[443]</a></span> -sense to describe an individual feature, it is clear that most mountains -have been formed in this way.</p> - -<p>To discuss the characteristic shapes of such mountains would -be largely to review the contents of this book, and especially those -portions which discuss the character profiles resulting from the -action of each sculpturing or molding agent. The work of frost -and other weathering agencies, of running water, of mountain and -of continental glacier, would all have to be considered in order to -evolve the history of each mountain.</p> - -<p>In addition to discovering the agents which were chiefly responsible -for the shaping of the mountain, we may, further, in -many cases determine at what stage the work of one agent has -been succeeded by that of another, and at least at what stage -of its complete cycle of activity the latest agent is now at work.</p> - -<div class="figcenter"> - <img src="images/ill-526.jpg" width="400" height="147" id="f482" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 482.</span>—The gabled façade so largely developed in desert landscapes and -sharply contrasted with the recurring curves in the landscapes of humid districts -(from a painting of the Grand Cañon of the Colorado by Moran).</p> -</div></div> - -<p><b>The climatic conditions of the mountain sculpture.</b>—Since -the different geological agencies operate either in a different manner -or with differences in vigor according to the varying climatic -conditions, the mountains of arid regions may in most cases be -readily differentiated from those of the more habitable humid sections -of country. In broad lines these differences may be summed -up in the greater prevalence of the curving line within the landscapes -of humid districts. This may be largely ascribed to the -influence of the mat of vegetation, which protects the rock surface -from more rapid mechanical degeneration, and arrests the -sliding movements within the already loosened rock débris. In -place of the reversed curves of the lines of beauty, so generally -observed in the landscapes of well-watered regions, the desert -lands present ever a repetition of the vertical cliff alternating with<span class="pagenum"><a name="Page_444" id="Page_444">[444]</a></span> -a sort of many gabled façade which is occasionally due to truncation -of mountain spurs by the waves of former lakes, but far more -often the outlines of débris cones built up beneath each prominent -joint of the cliff walls (<a href="#f482">Fig. 482</a>).</p> - - -<p><b>The effect of the resistant stratum.</b>—In a striking manner -mountain landscapes may disclose the influence of the diversified -rock materials and of the rock structures as well. After prolonged -erosion there is likely to be little correspondence between the positions -of the anticlinal folds and the crests of the higher mountains. -Such mountains are, in fact, much more likely to rise over synclines -than upon the site of anticlines. The traveler who enters -the Alps by any of the several railways, or who journeys by steamer -over the beautiful lake of Lucerne, has a most favorable opportunity -to study the position of the rock folds in the mountain -sections that are unrolled in succession before him. Rarely indeed -will he find a definite anticline in correspondence with a mountain -peak, for the layers which are most resistant have developed -the peaks, and it is because the outer layers of the anticlines open -by local tension (see <a href="#f26">Fig. 26</a>, <a href="#Page_45">p. 45</a>) that they were first cut away -by erosion, so that the hard -layers within the synclines -are likely to constitute the -peaks within the existing surface.</p> - -<div class="floatleft"> - <img src="images/ill-527.jpg" width="250" height="103" id="f483" - alt="" - title="" /> - <div class="cf"><p class="ch250"><span class="smcap">Fig. 483.</span>—The Mythen, composed of Jurassic -and Cretaceous sediments, and resting -upon softer Tertiary formations. View -from a balloon (after a photograph by C. -Schmidt).</p> -</div></div> - -<p>When, as sometimes happens, -an older and likewise -more resistant bed has been -folded back upon younger and -softer formations, an isolated -remnant may be found “unrooted” to its base, upon which it appears -as though floating within a billowy sea of the softer formations -(<a href="#f483">Fig. 483</a>).</p> - - -<p><b>The mark of the rift in the eroded mountains.</b>—Applying -the term “mountain” in its collective sense for a circumscribed -area of uplifted crust, whether represented to-day by a folded or -a faulted complex, a lava mass, or a granite dome; the period of -uplift has marked the beginning of the activity of sculpturing -agencies. By these the mass is pared down as it is shaped into -a more or less intricate design of component and essentially<span class="pagenum"><a name="Page_445" id="Page_445">[445]</a></span> -repeating units. In the vernacular the word “mountain” is -applied to these units into which the larger mountain mass is -subdivided.</p> - -<div class="floatright"> - <img src="images/ill-528a.jpg" width="200" height="148" id="f484" - alt="" - title="" /> - <div class="cf"><p class="ch200"><span class="smcap">Fig. 484.</span>—The battlement type of -erosion mountains. Die Drei Zinnen -(Three Battlements) in the -Dolomites (after Marr).</p> -</div></div> - -<p>It has been one of the main objects of this work to point out -that the peculiar shapes of these elementary mountains are each -characteristic of the erosive agents which produced them, and that -each surface has marks which may be recognized in those lines of -profile which recur within the landscape—the -character profiles. In -the subdivision of the larger mass—the -<i>genetical</i> mountain—to form -the numerous smaller masses—the -<i>erosional</i> or <i>circumvallational</i> mountains—there -is disclosed a pattern -of fractures which has guided the -erosional agents in their incisional -operations (see Chapter XVII). In -high altitudes, where the action of -frost is so potent in prying at the -wider fractures, this subdivision of the mass may be revealed by -the sculpturing of squared towers or battlements (<a href="#f484">Fig. 484</a>).</p> - -<div class="figcenter"> - <img src="images/ill-528b.jpg" width="400" height="157" id="f485" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 485.</span>—Symmetrically formed low islands repeated in ranks upon Temagami -Lake, Ontario.</p> -</div></div> - -<p>For other examples in which the sculptured surface is largely -the handiwork of a single erosional agent, as over vast areas in the -Canadian wilderness, the revelation of the fracture design is no -less apparent. Here a series of crystalline rocks underlie broad -expanses of territory and are without noteworthy variations of -hardness and almost bare of surface débris. Sculptured beneath a -mantling ice sheet, excavation has naturally been concentrated<span class="pagenum"><a name="Page_446" id="Page_446">[446]</a></span> -above the more widely gaping fissures of the joint-fault system, -doubtless already marked out in the river network which the -glacier overrode. The result has been a division of the surface -into a series of low, oval ridges or hummocks, which over vast areas -are repeated with monotonous regularity. Wherever the lower -levels have been flooded, symmetrical low islands of nearly uniform -elevation rise from the expanse of water and may be counted -by thousands. Though the smaller islands have notably regular -shore lines, the larger ones disclose their composition from smaller -units by the breaking of their shores into similar bays spaced with -regular intervals (<a href="#f485">Fig. 485</a>, and <a href="#f243">Figs. 243</a> and <a href="#f245">245</a>, <a href="#Page_229">p. 229</a>).</p> - -<p>The ever repeating fracture design of the earth’s crust is not -restricted to the mountain masses which it has broken up, and the -unity of which it has done so much to conceal. It extends far -outside the margin of these masses, and is in fact common to whole -continents and perhaps even to the planet as a whole. The part -played by this design of fractures in the control of the sculpture -of landscapes it would be hard to overestimate. Through its -influence the striking features molded by one agent have been -merged in the contrasted shapes developed by another. It is the -great outline blender in the creation of nature’s masterpieces of -form and color. Thus the lines of this mysterious fracture network, -though stamped in indelible characters upon our landscapes, -are generally lost in the ensemble effect and may long remain undiscovered. -Like a moss-grown inscription upon a slab of marble, -though veiled, it may yet be deciphered; and if the veil be withdrawn, -the runic characters are disclosed, and one of nature’s laws -lies open before us.</p> - -<p class="prr"><span class="smcap">Reading References for Chapter XXXI</span></p> - -<p class="p1">Mountain arcs or festoons:—</p> - -<p class="pex"><span class="smcap">Ed. Suess.</span> The Face of the Earth, vol. 2, 1906, pp. 201-207; vol. 4, 1909, -pp. 498-542.</p> - -<p class="p1">Block mountains:—</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Surveys West of the 100th Meridian (Wheeler), vol. 3, -Geology, Washington, 1875, Pt. 1, pp. 19 <i>et seq.</i>, 48.</p> - -<p class="pex"><span class="smcap">J. W. Powell.</span> Report on the Geology of the Eastern Portion of the -Uinta Mountains and a Region of Country Adjacent thereto, U. S. -Geol. and Geogr. Surv. Ter., II Div. Washington, 1876, pp. 218.</p> - -<p class="pex"><span class="smcap">John W. Gregory.</span> The Great Rift Valley. London, 1896, pp. 422.</p> - -<p><span class="pagenum"><a name="Page_447" id="Page_447">[447]</a></span></p> - -<p class="p1">Laccolites and bysmalites:—</p> - -<p class="pex"><span class="smcap">G. K. Gilbert.</span> Report on the Geology of the Henry Mountains, U. S. -Geol. and Geogr. Surv. Ter., 1877, pp. 18-98.</p> - -<p class="pex"><span class="smcap">Whitman Cross.</span> The Laccolitic Mountain Groups of Colorado, Utah, -and Arizona, 14th Ann. Rept. U. S. Geol. Surv., 1895, pp. 157-241, -pls. 7-16.</p> - -<p class="pex"><span class="smcap">W. H. Weed</span> and <span class="smcap">L. V. Pirsson</span>. Geology and Mineral Resources of the -Judith Mountains of Montana, 18th Ann. Rept. U. S. Geol. Surv., -Pt. iii, 1898, pp. 485-556, pl. 75.</p> - -<p class="pex"><span class="smcap">W. H. Weed.</span> Geology of the Little Belt Mountains, Montana, etc., -20th Ann. Rept. U. S. Geol. Surv., Pt. iii, 1900, pp. 387-400.</p> - -<p><span class="smcap">Vera de Derwies.</span> Recherches géologiques et pétrographiques sur les -loccolithes des environs de Piatigorsk (Caucase du Nord). Geneva, -1905, pp. 84, pls. 3.</p> - -<p class="pex"><span class="smcap">R. A. Daly.</span> The Mechanics of Igneous Intrusion, Am. Jour. Sci. (4), vol. -15, 1903, pp. 269-278; vol. 16, 1903, pp. 107-126.</p> - -<p class="pex"><span class="smcap">Joseph Barrell.</span> Geology of the Marysville Mining District, Montana. -A study of Igneous Intrusion and Contact Metamorphism. Prof. -Pap. 57, U. S. Geol. Surv., 1907, pp. 151-178.</p> - -<p class="p1">Climatic condition in relation to land sculpture:—</p> - -<p class="pex"><span class="smcap">C. E. Dutton.</span> Tertiary History of the Grand Canyon District, Mon. 2, -U. S. Geol. Surv., 1882, pp. 264, pls. 42.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_448" id="Page_448">[448]</a><br /><a name="Page_449" id="Page_449">[449]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">APPENDIX A</h2> - -<p class="pch">THE QUICK DETERMINATION OF THE COMMON MINERALS</p> - -<p>Before one may gain a knowledge of rocks or the architecture of their -arrangement within the earth’s crust, it is quite essential that some familiarity -should be acquired with the appearance and properties of the -commonest minerals, and particularly those which enter as essential -constituents into the more abundant rocks. To be a competent mineralogist, -one must have a rather extended knowledge both of inorganic chemistry -and of the science of crystallography, which, fascinating as it is to -study, involves some technical knowledge of mathematics and much -laboratory experience. Though necessary to any one who contemplates -making a career as a geologist, this special study is not essential to a -cultural course like the present one. The attempt will here be made to -bring together a body of fact, from the study of which the student may -quickly learn to recognize the commonest minerals in their usual varieties. -The tests he is to apply are mainly physical, and in place of an -elaborate discussion of crystal symmetry, pictures only can be supplied.</p> - -<p>To the beginner the usual textbook of mineralogy is difficult to read -intelligently, for the reason that for each mineral species it sets before him -a catalogue of each physical property in its turn, with little indication of -those data which in the individual case have special diagnostic value. -None the less, however, the student is advised to consider the several -properties of each mineral in a definite order, and the following may serve -as well as any: crystal or other form, cleavage, fracture, luster, color, -streak, transparency, tenacity, hardness, magnetism, and specific gravity. -In endeavoring to connect the specific values of these properties with -individual mineral species, the chemical composition and the manner of -occurrence are not to be forgotten. It is well for the student to be -supplied with a small pocket lens and with a pocket knife the blade of -which has been magnetized.</p> - -<p><b>Crystal form.</b>—Some mineral species generally occur in more or less -definite crystals—are bounded by definite plane surfaces developed when -the mineral was formed; others in groups of interfering crystals or aggregates, -in which case the mineral is said to be crystalline; while still others -are rarely found crystallized at all. Thus in a given case crystal form -may, or may not, be important for the diagnosis of the substance. If<span class="pagenum"><a name="Page_450" id="Page_450">[450]</a></span> -a mineral species is usually to be found in crystals, the student should -be aware of the fact, and if possible should have a mental picture of the -common crystal shape or shapes. Without an extended knowledge of -crystallography, this must be supplied him by drawings. Since crystals -of most species are apt to be distorted, owing to the fact that some -planes within the same group appear upon the crystal with a larger development -than others, it is convenient to remember that markings, such -as lines or etchings upon the crystal faces, are the same throughout the -same group of planes, and in the text figures such groups of planes are -indicated by the use of a common letter. For crystalline aggregates -such terms as fibrous, radiating, massive, or granular have their usual -meanings.</p> - -<p><b>Cleavage.</b>—It is characteristic of most crystals that they break or -<i>cleave</i> along certain directions so as to leave plane or nearly plane surfaces, -and the luster of the cleaved surface measures the perfection of the cleavage -property. It is important always to note how many such directions -of cleavage are present, and, roughly at least, at what angles they intersect—whether -they are perpendicular to each other or inclined at some -other angle. Further, it should be noted whether a given cleavage is -<i>perfect</i>, that is, easy, which will be indicated by the thinness of the plates -which can be secured. An extremely perfect cleavage is possessed by -the mineral mica, whose plates are thinner than the thinnest paper. In -the case of imperfect or interrupted cleavage, the fracture surfaces are -not plane throughout, but interrupted, the surface “jumping” from one -plane to a neighboring parallel one. It is especially important to note -whether, in the case of several cleavages possessed by a crystal, all have -the same degree of perfection, or whether they exhibit differences.</p> - -<p><b>Fracture.</b>—In minerals with poorly developed cleavage, the fracture -surface is described as <i>fracture</i>. Fracture is thus perfect in proportion -as cleavage is imperfect. The fracture is described as conchoidal -when it shows waving spherical surfaces like broken glass. For -fine aggregates the fracture is described as even, uneven, earthy, etc., -names which are generally intelligible.</p> - -<p><b>Luster.</b>—This term is applied especially to the manner in which light -is reflected from mineral surfaces. The most important distinction -is made between those minerals which have a <i>metallic</i> luster and those -which have not, the former being always opaque. Other characteristic -lusters are adamantine (like oiled glass), vitreous (glassy), resinous, -waxy, etc.</p> - -<p><b>Color.</b>—For minerals which possess metallic luster the color is always -practically the same, and hence it becomes a valuable diagnostic property. -Of minerals which have nonmetallic luster, the color may be always<span class="pagenum"><a name="Page_451" id="Page_451">[451]</a></span> -the same and hence characteristic, but in the case of many minerals it -ranges between wide limits and sometimes runs almost the entire gamut -of hues, yet without appreciable changes in the chemical composition of -the mineral.</p> - - -<p><b>Streak.</b>—This term is applied to the color of the mineral powder, -and is usually fairly constant, even when the surface color of different -specimens may vary within wide limits. In the case of fairly soft minerals -the streak is best examined by making a mark on a piece of unglazed -porcelain (streak stone).</p> - - -<p><b>Transparency</b> (<b>diaphaneity</b>).—The terms “transparent”, “translucent”, -“subtranslucent”, and “opaque” are used to describe decreasing grades -of permeability by light rays. Through transparent bodies print may -be read, while translucent bodies allow the light to be transmitted in -considerable quantity through them, though without rendering the image -of objects.</p> - - -<p><b>Tenacity.</b>—This comprehensive term includes such properties as -brittleness, flexibility, elasticity, malleability, etc.</p> - - -<p><b>Hardness.</b>—Quite erroneous notions are held concerning the meaning -of this very common word, which properly implies a resistance offered -to abrasion. It is one of the most valuable properties for the quick determination -of minerals, since minerals range from diamond upon the one -hand—the hardest of substances—to talc and graphite, which are so -soft as to be deeply scratched by the thumb nail. For practical purposes -it is sufficient to make use of a rough scale of hardness made up -from common or well-known minerals. If we exclude the gem minerals, -this scale need include but seven numbers, which are: talc, 1; gypsum, 2; -calcite, 3; fluor spar, 4; apatite, 5; feldspar, 6; and quartz, 7. A given -mineral is softer than a mineral in the scale when it can be visibly scratched -by a scale mineral, but will not leave a scratch when the conditions are -reversed. If each will scratch the other with equal readiness, the two -minerals have the same hardness.</p> - -<p>Since it may often be desirable to test mineral hardness when no scale -is at hand, the following substitutes may be made use of: 1, greasy feel -and easily scratched by the thumb nail; 2, takes a scratch from the thumb -nail, but much less readily; 3, scratched by a copper coin and very -easily by a pocket knife; 4, scratched without difficulty by a knife; -5, scratched with difficulty by a knife, but easily by window glass; -6, scratched by window glass; 7, scratches window glass with readiness, -but a grain of sand may be substituted to represent quartz in the scale.</p> - - -<p><b>Magnetism.</b>—Though nearly all minerals which contain important -quantities of the elements iron, cobalt, or nickel may be attracted to a -strong electromagnet, there are but two common minerals, and these<span class="pagenum"><a name="Page_452" id="Page_452">[452]</a></span> -of widely different appearance, whose powder is lifted by a common -magnet. Others are, however, lifted after strong heating in the air -(<i>ignition</i>), and this is a valuable test.</p> - -<p><b>Specific gravity.</b>—Rough tests of relative weight, or specific gravity, -may be made by lifting fair-sized specimens in the hand. Better determinations -require the use of a spring balance.</p> - -<p><b>Treatment with acid.</b>—The carbonate minerals react with warm -and dilute mineral acid so as to give a boiling effect (effervescence), -since carbonic acid gas escapes into the air in the process.</p> - -<p class="pch">PROPERTIES OF THE COMMON MINERALS</p> - -<p>The more important common minerals fall into two classes according -as they have large economic importance as ores, or enter in an important -way into the composition of rocks.</p> - -<h3>I. The Minerals of Economic Importance</h3> - -<p><b>Hematite.</b>—The sesquioxide of iron, Fe<sub>2</sub>O<sub>3</sub>, and by far the most important -ore of iron. Rarely in good crystals, but sometimes in thin opaque -scales bearing some resemblance to mica and known as micaceous or -specular iron ore. At other times in nodules built up from radial needles -(needle ore); in hard masses mixed with fine quartz grains (hard hematite); -or in soft reddish brown earth (soft hematite). Color, black to -cherry red. The powdered mineral always cherry red or reddish brown, -and easily lifted by the magnet after ignition. Hardness 5.5-6.5; -specific gravity 5.</p> - -<p><b>Magnetite.</b>—The magnetic oxide of iron, Fe<sub>3</sub>O<sub>4</sub>, often in crystals like -<a href="#f486">Fig. 486</a>, <sup>1-2</sup>. Black and opaque with a metallic luster. Streak black. -Lifted by a magnet and sometimes itself capable of lifting filings of -soft iron (lodestone). Hardness 5.5-6.5. Specific gravity 5.</p> - -<p><b>Limonite.</b>—The most abundant and most valuable of the hydrated -iron ores, 2 Fe<sub>2</sub>O<sub>3</sub>. 3 H<sub>2</sub>O. Chemical composition the same as iron rust, -with which in the earthy form it is identical. Never in crystals, but often -in mammillary or rounded pendant forms resembling icicles, or sometimes -clusters of grapes. Its yellow (rust) streak is its best diagnostic -property. Ignited it gives off water and becomes magnetic. The streak -and its notably lower specific gravity distinguish it from certain forms of -hematite which it outwardly resembles. Hardness 5-5.5. Specific -gravity 3.6-4.</p> - -<p><b>Pyrite, iron pyrites, or “fool’s gold.”</b>—The sulphide of iron, FeS<sub>2</sub>. -The most widely distributed sulphide mineral and now a chief source of<span class="pagenum"><a name="Page_453" id="Page_453">[453]</a></span> -the great chemical reagent, sulphuric acid or vitriol. Often, but not always, -in crystals (<a href="#f486">Fig. 486</a>, <sup>3-5</sup>) which have peculiar striæ upon their -faces. At other times the mineral is found massive or in radiated needles. -Bright metallic luster with the color of new brass, though often tarnished -or altered upon the surface to limonite. Hard and brittle, and so distinguished -from gold, which is soft and malleable and of the color of the -paler old brass (which contained a larger percentage of zinc). Gold is, -further, about four times as heavy as pyrite. Hardness 6-6.5. Specific -gravity 5.</p> - -<p><b>Chalcopyrite, copper pyrites.</b>—A mixed sulphide of copper and iron. -If in crystals, like <a href="#f486">Fig. 486</a>, <sup>6</sup>; otherwise massive or compact. Luster metallic. -Color orange-yellow, often with local blue and green iridescence -like a pigeon’s throat. Distinguished from pyrite by the deeper color -and lower hardness, and from gold, particularly, by its brittleness and -lower specific gravity. Hardness 3.5-4. Specific gravity 4.</p> - -<p><b>Galenite, galena.</b>—Sulphide of lead, PbS. The chief ore of lead, and, -from admixture of a silver mineral, of silver as well. Usually found in -crystals (<a href="#f486">Fig. 486</a>, <sup>7</sup>). Always cleaves into blocks bounded by six very -perfect rectangular faces which, when freshly broken, show a bright silvery -luster and quickly tarnish to a peculiarly “leaden” surface. Very -heavy. Color and streak lead-gray. Hardness 2.5. Specific gravity 7.5.</p> - -<p><b>Sphalerite, zinc blende.</b>—Sulphide of zinc, ZnS, usually with considerable -admixture of sulphide of iron. The great ore of zinc. Not infrequently -in crystals (<a href="#f486">Fig. 486</a>, <sup>8-9</sup>), but more often in cleavable crystalline -aggregates. The cleavage in fine aggregates is sometimes difficult to -make out, but in coarse-grained masses it is seen to be equally and highly -perfect in six different directions, so that a symmetrical twelve-faced form -may sometimes be broken out (dodecahedron). Luster like that of rosin -(rosin jack), though when with large iron admixture the color may approach -black (black jack). The lighter colored varieties are translucent. Hardness -3.5-4. Specific gravity 4.</p> - -<p><b>Malachite.</b>—Hydrated (basic) copper carbonate. The green copper -ore and the common surface alteration product of other copper minerals. -Usually has a microscopic structure made up of fine needle-like crystals, -but generally massive in various imitative shapes not unlike those of the -iron ores. Sometimes earthy. Its color is bright green, and it is usually -found in association with other characteristic copper ores, such as chalcopyrite -and azurite. When relatively pure and in large masses, it is -a beautiful ornamental stone. Effervesces with acid. Hardness 3.5-4. -Specific gravity 4.</p> - -<p><span class="pagenum"><a name="Page_454" id="Page_454">[454]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-537.jpg" width="400" height="639" id="f486" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 486.</span>—Forms of Crystals: 1-2, magnetite; 3-5, pyrite; 6, chalcopyrite; -7, galenite; 8-9, sphalerite; 10-13, calcite.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_455" id="Page_455">[455]</a></span></p> - -<p><b>Azurite.</b>—Hydrated (basic) copper carbonate, less hydrated than -malachite, and known as the blue carbonate of copper. Generally in -very minute and quite complex crystals, but also in imitative shapes -similar to those of malachite, and at other times earthy. Slightly lighter -in weight than malachite, from which it is easily distinguished, as from -most other minerals, by its bright azure blue color and its somewhat -lighter blue streak. Effervesces with nitric acid. Hardness 3.5-4. -Specific gravity 3.7-3.8.</p> - -<p><b>Calcite.</b>—Calcium carbonate, CaCO<sub>3</sub>. Almost always in crystals (<a href="#f486">Fig. 486</a>, -<sup>10-13</sup>), or in confused crystal aggregates, though rarely fibrous or -dull and earthy. Some of the forms of the crystals are described as -“dog-tooth spar”, others as “nail-head spar”, while still others are modified -hexagonal prisms. There is a beautifully perfect cleavage of the -mineral along three directions which make angles of about 105° with each -other, so that under the hammer the substance breaks into blocks which -are shaped like the crystal of <a href="#f486">Fig. 486</a>, <sup>10</sup>. Usually white or gray, but -occasionally faintly tinted. Streak white. Effervesces with cold and -dilute mineral acids. An associate of many ores and the chief mineral -of limestone. A similar mineral—dolomite—contains in addition magnesium -carbonate, has simpler crystals (like the drawing of <a href="#f486">Fig. 486</a>, <sup>10</sup>, -but often with rounded faces), and effervesces only when the acid is warmed. -Hardness 3. Specific gravity 2.7.</p> - -<p><b>Gypsum.</b>—Hydrated calcium sulphate, CaSO<sub>4</sub>.2 H<sub>2</sub>O, and the source -of plaster of Paris. Often in simple crystals (<a href="#f487">Fig. 487</a>, <sup>1</sup>) or else “swallow -tail”, like <a href="#f487">Fig. 487</a>, <sup>2</sup>; in which case the mineral is generally either -transparent or translucent and is described as selenite. Such crystals -show a cleavage approaching in perfection that of the micas, but, unlike -the mica laminæ, those produced by cleavage in gypsum though flexible -are not elastic. There are also fibrous forms of gypsum (satin spar), -a fine-grained form (alabaster), and the impure earthy form (rock gypsum). -Very soft, light in weight, and difficultly fusible. Color usually -white, gray, or pale yellow. Hardness 2. Specific gravity 2.3.</p> - -<p><b>Copper glance.</b>—A sulphide of copper, Cu<sub>2</sub>S. Not usually well crystallized, -but generally massive and associated or variously admixed with -other copper ores such as chalcopyrite, malachite, etc. Fracture conchoidal, -luster metallic, color and streak blackish lead-gray, though often -tarnished blue or green from surface alterations to the copper carbonates. -Softer and heavier than chalcopyrite. Blowpipe or chemical tests are necessary -for its identification. Hardness 2.5-3. Specific gravity 5.5-5.8.</p> - -<p><b>Cerussite.</b>—The white or carbonate lead ore, PbCO<sub>3</sub>, and an important -ore of silver as well. Often in crystals of considerable complexity, though -<a href="#f487">Fig. 487</a>, <sup>3-4</sup>, shows some common shapes. Often granular, massive, or -earthy (gray carbonate ore). Very brittle and with conchoidal fracture. -The luster is adamantine or like that of oiled glass. Color generally<span class="pagenum"><a name="Page_456" id="Page_456">[456]</a></span> -white or gray. Very heavy, the heaviest of light colored and nonmetallic -minerals. Dissolves in nitric acid with effervescence. Hardness 3-3.5. -Specific gravity 6.5.</p> - -<p><b>Siderite.</b>—The carbonate or “spathic” ore of iron, FeCO<sub>3</sub>. Either -in crystals resembling in form <a href="#f486">Fig. 486</a>, <sup>10</sup>, but with rounded faces, or -cleavable massive to finely granular and earthy. The crystalline varieties -cleave easily into smaller blocks of the same form as those of calcite. Color -usually gray or brown and streak white. On strongly igniting, the white -powder becomes black and magnetic. Lighter in both color and weight -than the other iron ores, and unlike them siderite effervesces with acid. -Distinguished from calcite by its higher specific gravity and its change -upon being ignited. Hardness 3.5-4. Specific gravity 3.9.</p> - -<p><b>Smithsonite.</b>—Carbonate of zinc, ZnCO<sub>3</sub>, and an important ore of -that metal. Seldom found in crystals except as a replacement of calcite -crystals, in which case it shows the forms characteristic of the latter mineral. -Usually kidney-shaped, stalactitic, or else in incrustations upon -other minerals. Sometimes granular or earthy. Brittle. Luster vitreous, -color white or greenish gray, though often stained yellow with iron -rust. Streak white except when the mineral is stained with iron. Effervesces -with warm acid. Hardness 5. Specific gravity 4.4.</p> - -<p><b>Pyrolusite.</b>—Black oxide of manganese, MnO<sub>2</sub>, though generally impure -from admixture with other manganese oxides. Usually in intricate -aggregates which may be columnar, fibrous, mammillary, earthy, etc. -Opaque, with color and streak both black. Soft and easily soils the fingers. -With hydrochloric acid gives off the choking fumes of chlorine. Hardness -2-2.5. Specific gravity 4.8.</p> - -<h3>II. The Minerals important as Rock Makers</h3> - -<p>These minerals are in most cases complex silicates of one or more of a -certain number of metals such as aluminium, calcium, magnesium, iron, -sodium, potassium, or hydroxyl (OH). For their identification an examination -of the physical properties is usually sufficient, whereas of the -typical ore minerals already considered, additional chemical tests may be -necessary.</p> - -<p><b>Feldspars.</b>—A group of similar alumino-silicates of potassium, sodium, -and calcium. The most important of all rock-making minerals. Although -with wide variation in chemical composition, the feldspars are yet broadly -divided into two classes; the one striated, and the other an unstriated -potash or orthoclase variety. The pocket lens is usually necessary in order -to make out the striations upon the crystal or cleavage surfaces. When -formed in veins, feldspar appears in crystals (<a href="#f487">Fig. 487</a>, <sup>5-6</sup>), but as a rock -constituent the mutual interference of crystals prevents the development -of bounding faces. Two cleavage directions, nearly or quite perpendicular -to each other, are notably different in their perfection. Hard enough -to scratch glass, but easily scratched by sand. Color pink (usually orthoclase -or microline), white (often albite) to gray. Sometimes with beautiful -“pigeon’s throat” effect of iridescence (labradorite). Low specific -gravity. Hardness 6. Specific gravity 2.5-2.8.</p> - -<p><span class="pagenum"><a name="Page_457" id="Page_457">[457]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-540.jpg" width="400" height="618" id="f487" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 487.</span>—Forms of Crystals: 1-2, gypsum; 3-4, cerussite; 5-6, feldspar; 7, -quartz; 8, pyroxene (cross section); 9, hornblende (cross section); 10, garnet; -11, nephelite; 12-14, staurolite; 15-16, tourmaline (cross sections); 17, olivine.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_458" id="Page_458">[458]</a></span></p> - -<p><b>Quartz.</b>—Oxide of silicon or silica, SiO<sub>2</sub>. Both an important vein -mineral associated with the ores and a rock maker. In the former case -particularly, often in crystals of notably simple forms (<a href="#f487">Fig. 487</a>, <sup>7</sup>). Few -minerals which are not gems are so hard. Remarkable freedom from -cleavage so that the mineral breaks much like window glass—conchoidal -fracture. Wide range in both transparency and color. Transparent and -colorless crystalline variety (rock crystal), brown translucent (smoky -quartz), turbid white (milky quartz), and various colored varieties (carnelian, -jasper, jet, etc.). Insoluble in acids and infusible. Hardness 7. -Specific gravity 2.6.</p> - -<p><b>Micas.</b>—Like the feldspars a group of complex silicates, but here -chiefly of potassium, magnesium, iron, and hydroxyl. Abundant as rock -makers, the micas are all characterized by the thinnest and toughest -of elastic cleavage plates, such as are generally known as isinglass. When -a needle is driven sharply through a thin scale of mica, a six-rayed puncture -star forms about the needle point. The darker common variety of -mica is rich in iron and magnesium and is called biotite, and the lighter -colored alkaline variety, muscovite. Hardness 2.5-3.1. Specific gravity -2.7-3.1.</p> - -<p><b>Chlorite.</b>—Generally an intricate mixture of more or less similar -microscopic crystals having varying and rather complex chemical compositions -and related to the micas, but all characterized by a peculiar leaf -green color. These minerals are a common product of hydration weathering -in rocks which are rich in magnesium and iron—especially those that -contain biotite, pyroxene, or hornblende (see below). Hardness 1-2.5. -Specific gravity 2.5-3.</p> - -<p><b>Pyroxenes.</b>—An important group of related rock-making minerals all -of which are silicates of the bases magnesium, calcium, aluminium, iron, -and manganese. Quite generally developed either in columnar or needle-like -crystals which are uniformly shaped in cross section like <a href="#f487">Fig. 487</a>, <sup>8</sup>. -Two rather imperfect cleavages are directed parallel to the longer axis -of the crystal and nearly at right angles to each other. The colors of all -but the lime varieties are dark and generally green, dark brown, bronze, -or black. The lime varieties are white, gray, or pale green. A dark -colored and common iron variety is known as augite. Streak generally -either white or lightly tinted. Hardness 5-6. Specific gravity 3.2-3.6.</p> - -<p><span class="pagenum"><a name="Page_459" id="Page_459">[459]</a></span></p> - -<p><b>Amphiboles.</b>—A group of minerals of the same chemical composition -as the pyroxenes, with which also in most physical properties they agree. -The principal distinction is found in the shape of the cross section and in -the cleavage (<a href="#f487">Fig. 487</a>, <sup>9</sup>). Whereas the cross sections of pyroxenes are -generally eight sided, those of the amphiboles have six sides, and whereas -the cleavage directions of pyroxenes are nearly at right angles to each -other (87°), the similar but much more perfect cleavage directions of -the amphiboles are inclined at an obtuse angle (124½°). Owing to the -obliquity of the amphibole cleavage, fractured surfaces of the mineral -appear splintery, which is not in the same measure true of the pyroxenes. -A fibrous variety of amphibole, and occasionally other varieties of the -mineral, is a not uncommon product of weathering of pyroxenes. Other -physical properties of the amphiboles are in the main almost identical with -those of the pyroxenes.</p> - -<p><b>Garnet.</b>—Complex alumino-silicates or ferro-silicates of calcium, -magnesium, iron, or manganese, or several of these combined. Nearly -always in crystals, and usually found in mica schist (see below). The -crystals usually have twelve similar faces, each a lozenge (dodecahedron), -or else twenty-four similar faces, or the two forms combined (<a href="#f487">Fig. 487</a>, -<sup>10</sup>). Brittle. From any but the gem minerals garnet is easily -distinguished by its hardness, which in different varieties ranges from -somewhat below to somewhat above that of quartz. The luster is vitreous, -and the color runs the gamut of reds, browns, and greens, but with -the common hue dark red to black. Streak white. Hardness 6.5-7.5. -Specific gravity 3.1-4.3.</p> - -<p><b>Nephelite</b> (<b>nephelene</b>).—An alumino-silicate of sodium and potassium. -In certain special provinces this mineral is developed in abundance as an -essential constituent of igneous rocks, but elsewhere practically unknown. -The rare crystals are hexagonal prisms (<a href="#f487">Fig. 487</a>, <sup>11</sup>), but the mineral is most -easily determined by its general resemblance to feldspar, but with the differences -of cleavage, luster, and reaction with acid. Whereas the feldspars -have two cleavages, either nearly or quite perpendicular to each other -and of different degrees of perfection, nephelite has three equal cleavages -inclined 60° and 120° to each other and of less perfection than either -feldspar cleavage. The luster of nephelite is perhaps the best clew -to its identity, since this is greasy and simulated by but few minerals. -The fine powder of the mineral treated for some time with strong hydrochloric -acid forms a perfect jelly of silicic acid, whereas the feldspars -do not. Though itself gray or white and unobtrusive, nephelite -is usually associated with brightly colored minerals, which are often the -first clew to its presence in a rock. Hardness 5.5-6. Specific gravity -2.5-2.6.</p> - -<p><span class="pagenum"><a name="Page_460" id="Page_460">[460]</a></span></p> - -<p><b>Talc</b> (<b>soapstone</b>).—A silicate of magnesium and hydroxyl which is -an important alteration product through weathering of certain pyroxene -rocks especially. Usually a foliated mass, this product is occasionally -fibrous or even granular. Talc is one of the softest of minerals, having a -greasy feel and being easily scratched with the thumb nail. The luster -of the foliated varieties is apt to be pearly, and the color apple-green to -white, though sometimes stained brown from oxide of iron. The streak -of the mineral is white except when stained by iron. Although the -rocks which are composed mainly of talc (soapstone) are exceedingly -soft, they are very tough and remarkably resistant. Hardness 1-1.5. -Specific gravity 2.7-2.8.</p> - -<p><b>Serpentine.</b>—Like talc, serpentine is a silicate of magnesium and -hydroxyl, and an important product of the breaking down of magnesium -minerals in the process of weathering. The mineral is usually found as a -fine web of microscopic needle-like fibers, and is best roughly diagnosed -by its color and its associated minerals. Like talc it is usually developed -within those igneous rocks from which feldspar is lacking, but where either -pyroxene or olivine is found in abundance or was previous to alteration. -The characteristic color of serpentine is leek-green. The rock largely -composed of serpentine is called by the same name, and being exceedingly -tough and unchanging is, in spite of its softness, a valuable building and -ornamental stone. A red magnesium garnet is apt to be associated with -such serpentine masses. Hardness 2.5-4, because of impurities. Specific -gravity 2.5-2.6.</p> - -<p><b>Staurolite.</b>—A silicate of aluminium, iron, and hydroxyl. Found in -metamorphic rocks usually in association with garnet. Always in crystals -bounded by simple forms generally crossed, as shown in <a href="#f487">Fig. 487</a>, <sup>12-14</sup>. -The color is dark reddish brown, and the streak is colorless to grayish. -The hardness is exceptional and higher than that of quartz. Hardness -7-7.5. Specific gravity 3.6-3.7.</p> - -<p><b>Tourmaline.</b>—An exceptionally complex silicate of boron and aluminium -as well as iron, magnesium, and the alkalies. Found in metamorphic -rocks and always crystallized. The crystals are columns or needles -whose cross section is the best guide to their identity, since this is a modified -triangle unlike that of any other mineral (<a href="#f487">Fig. 487</a>, <sup>15-16</sup>). Additional -diagnostic properties are the characteristic striations which run lengthwise -of the crystals upon prism faces, and the lack of any cleavage (difference -from hornblende). The hardness is also a valuable property, since this -is greater than that of quartz. The mineral is brittle and the fracture -subconchoidal. The range in color is as great as, or greater than, that of -garnet, though the common forms are jet black. Streak uncolored. -Hardness 7-7.5. Specific gravity 3-3.2.</p> - -<p><span class="pagenum"><a name="Page_461" id="Page_461">[461]</a></span></p> - -<p><b>Olivine.</b>—A silicate of magnesium and iron and a rock-making mineral -found only in those igneous rocks which have little or no feldspar. -It easily suffers alteration by weathering and passes into serpentine, and -in fact is seldom found except when at least partially altered to the fibrous -webs of that mineral. The form of the unaltered crystals within the -rocks is shown in <a href="#f487">Fig. 487</a>, <sup>17</sup>, and, cut in sections, the mineral appears -in more or less elongated hexagons. The hardness of the unaltered mineral -is about that of quartz. It has rather imperfect cleavages in two -rectangular directions, and is usually translucent, with a vitreous luster -and a color which is olive-green when not stained brown by oxide of -iron. Streak uncolored. Hardness 6.5-7. Specific gravity 3.2-3.3.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_462" id="Page_462">[462]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">APPENDIX B</h2> - -<p class="pch">SHORT DESCRIPTIONS OF SOME COMMON ROCKS</p> - -<p>In Chapter IV the classification and the structure of rocks have been -briefly discussed. Below are added brief descriptions of the more important -common rocks. For rocks as for minerals it is, however, essential -that a collection of well-chosen specimens be studied for purposes of -comparison. A small pocket lens is a valuable aid in making out the -component minerals and the textures of the finer grained rocks.</p> - -<h3>1. Intrusive Rocks</h3> - -<p><b>Granite.</b>—Of granitic texture, though sometimes porphyritic as well. -The most abundant mineral constituent is a pink or white feldspar, usually -without visible striations, with which there is usually in subordinate -quantity a white striated feldspar. Next in importance to the feldspar -is quartz, which because of its lack of cleavage shows a peculiar gray -surface resembling wet sugar. In addition to feldspar and quartz there is -generally, though not universally, a dark colored mineral, either mica or -hornblende. The mica is usually biotite, though often associated with -muscovite.</p> - -<p><b>Syenite.</b>—Like granite, but without quartz, with more striated feldspar, -and generally also the rock has a darker average tint. While biotite -is the commonest dark colored constituent of granite, hornblende is more -apt to take its place in syenite. Less common than granite, to which it is -closely related in origin and in composition.</p> - -<p><b>Gabbro.</b>—A dark colored rock of granitic texture composed of striated -feldspar with broad cleavage surfaces and usually an abundance of pyroxene. -In contrast to the feldspars of granite, those of gabbroes are often -dull and colored grayish yellow or greenish. The pyroxene is often in -part changed to fibrous amphibole. Magnetite may be an abundant -accessory mineral.</p> - -<p><b>Diabase.</b>—In color dark like gabbro, and of similar constitution. In -diabase, however, the feldspar crystals, instead of being broad and of -irregularly interrupted outline, are relatively long (“lath-shaped”), and -the pyroxene acts as a filler of the residual space between them.</p> - -<p><b>Peridotite.</b>—A heavy and dark colored rock of granitic texture which -is nearly or quite devoid of feldspar but contains olivine. When altered,<span class="pagenum"><a name="Page_463" id="Page_463">[463]</a></span> -as it generally is, it is largely a mass of serpentine, talc, and chlorite, surrounding -cores, it may be, of still unaltered pyroxene and olivine. Magnetite -is an abundant constituent, and a red garnet is apt to be present.</p> - -<h3>2. Extrusive Rocks</h3> - -<p><b>Obsidian.</b>—A rock glass rich in silica. It is usually black and breaks -with a perfect conchoidal fracture. It often passes over through insensible -gradations into pumice, which differs only in its vesicular structure. -As regards chemical composition, obsidian and pumice are not notably -different from rhyolite (below).</p> - -<p><b>Rhyolite.</b>—A light colored rock of porphyritic texture, often also with -fluxion or spherulitic textures, or both combined. The porphyritic appearance -is given the rock by large crystals of a glassy, unstriated feldspar -and crystals of quartz. Rhyolite is a very siliceous lava containing rather -more silica than granite, to which of the intrusive rocks it is most closely -related, and from which it differs in its texture and in the manner of its -occurrence in nature. Whereas granite is found in great batholites, -laccolites, and bysmalites, and consolidated in most cases beneath the -earth’s surface, rhyolite generally occurs in sheets, flows, or dikes, and -consolidated either above or in fissures near to the surface.</p> - -<p><b>Trachyte.</b>—Similar to rhyolite, but usually with a peculiar gray aspect -from the greater abundance of feldspar crystals. The rock is less siliceous -than rhyolite, contains no quartz crystals, and approaches a feldspar -in its average composition.</p> - -<p><b>Andesite.</b>—Similar to rhyolite in appearance and in origin, but more -basic and correspondingly dark in color. The porphyritic crystals are of -lath-shaped, striated feldspar, with which are associated crystals of either -biotite or hornblende or both. A fluxion texture is particularly characteristic -of this type of extrusive rock.</p> - -<p><b>Basalt.</b>—A dark colored or black basic rock of porphyritic texture -which differs but little from diabase. It may show under the lens fine -lath-shaped crystals of striated feldspar associated with crystals of augite, -but more frequently the rock is dense and without visible mineral constituents. -It is particularly likely to occur divided up into columns six -inches to a foot in diameter and known as basaltic columns. Especially -fine examples are known from the Giant’s Causeway and other localities -in the western British Isles.</p> - -<h3>3. Sedimentary Rocks of Mechanical Origin</h3> - -<p><b>Conglomerate</b> (“<b>pudding stone</b>”).—A rock made up from pebbles -which are cemented together with sand and finer materials. The pebbles -are usually worn by work of the waves upon a shore, and may vary in<span class="pagenum"><a name="Page_464" id="Page_464">[464]</a></span> -size from a pea to large bowlders. They may consist of almost any hard -mineral or rock, though the sand about them is largely quartz.</p> - -<p><b>Sandstone.</b>—A rock composed of sand cemented together either by -calcareous, siliceous, or ferruginous materials. Sandstones are described -as friable when their surface grains are easily rubbed off, or as compact -when they are more firmly cemented. Sandstones are often distinctly -banded and are sometimes variously stained with oxide of iron. Those -sandstones which have been formed upon a seacoast are known as marine -sandstones, while those derived from accumulations collected by the wind -in deserts are distinguished as continental deposits. Sandstones form -much thicker formations than conglomerates, the latter usually constituting -a basal layer only of the sandstone formation (basal conglomerate).</p> - -<p><b>Shale.</b>—A consolidated mud stone which is probably the most abundant -rock formation. In large part clay admixed in varying proportions -with extremely fine sandy grains.</p> - -<h3>4. Sedimentary Rocks of Chemical Precipitation</h3> - -<p><b>Calcareous tufa</b> (<b>travertine</b>).—Not to be confused with tuff, which -is a fragmental extrusive or volcanic rock. Calcareous tufa is formed -when waters which contain carbonic acid gas and lime carbonate in solution, -give off the gas and with it the power to hold the lime in solution. -Such a liberation of the gas may occur when the stream is dashed into -spray above a cascade, and the lime is then deposited about the site of the -falls. Travertine is generally porous and formed of more or less concentric -layers or incrustations. A remarkable illustration is furnished by the -travertine deposits of Tivoli and other localities near Rome, since here -the material supplies a valuable building stone.</p> - -<p><b>Oölitic limestone</b> (<b>oolite</b>).—This rock is made up of spherical nodules -and so has the appearance of fish roe. Broken apart, each grain reveals -in its center a core of siliceous sand about which carbonate of lime has been -deposited in concentric layers. It is thought that waters charged with -carbonate of lime, in issuing from a river near a sea beach, coat the sand -grains of the latter with successive thin films of lime carbonate due to the -rhythmic ebb and flow of the tides, evaporation of the adhering water -taking place when the sands are exposed at low tide.</p> - -<h3>5. Sedimentary Rocks of Organic Origin</h3> - -<p><b>Limestone.</b>—A generally white or gray rock composed of carbonate -of lime with varying proportions of clay, silica, and other impurities. The -lime carbonate is usually derived from the hard parts of marine organisms, -and the argillaceous and siliceous impurities from the finer land-derived -sediments which descend with them to the bottom.</p> - -<p><span class="pagenum"><a name="Page_465" id="Page_465">[465]</a></span></p> - -<p><b>Dolomite</b> (<b>dolomitic or magnesium limestone</b>).—Differs from limestone -in containing varying proportions of the mineral dolomite (<i>ante</i>, -<a href="#Page_455">p. 455</a>), which is made up of equal parts of calcium and magnesium carbonates. -Difficult to distinguish from limestone unless a chemical test -is made for magnesium, though it may be said in general that dolomite -is less soluble in cold mineral acids.</p> - -<p><b>Peat.</b>—An accumulation of decomposed vegetable matter within -small lakes and in lagoons separated from larger ones (<i>ante</i>, <a href="#Page_429">p. 429</a>). -Peat represents the first stage in the formation of coal from vegetable matter, -and differs from the coals by its larger proportion of contained water. -Because of this water its fuel value is correspondingly small. It is usually -dark brown or black and reveals something of the structure of the -plants out of which it was formed.</p> - -<h3>6. Metamorphic Rocks</h3> - -<p><b>Gneiss.</b>—A generally more or less banded (gneissic) metamorphic -rock with a mineral constitution similar to granite, and often developed -by metamorphic processes from that rock. It may at other times, by processes -not essentially different, be derived from sedimentary formations. -It usually contains as important constituents unstriated feldspar and -quartz, but in addition it may include a striated feldspar, biotite, muscovite, -or hornblende, or several of these combined. In proportion as -mica or hornblende is abundant, it has a marked banded texture, but it -differs from mica schist (see below) not only in the presence of its feldspar, -but in the smaller proportion of mica. Biotite gneiss, hornblende gneiss, -etc., are terms used to designate varieties in which one or the other of the -dark colored constituents predominate.</p> - -<p><b>Mica schist.</b>—A metamorphic rock without feldspar and mainly -composed of quartz and light colored mica (muscovite). The abundant -mica lends to the rock its characteristic schistose texture, which differs -from the usual gneissic texture. In some cases the mica is wrapped about -the grains of quartz, but at other times it forms a series of almost continuous -membranes separating layers of quartz.</p> - -<p><b>Sericite schist.</b>—A variety of schist which is characterized by an -abundance of a peculiar silvery mica rich in the element group hydroxyl. -The mica scales are often microscopic and wrought into an intricate -web with the quartz constituent.</p> - -<p><b>Talc schist.</b>—A schist made up largely of talc, but with varying -proportions of quartz, magnetite, etc. From the abundance of the talc -it is usually pale green or white.</p> - -<p><b>Chlorite schist.</b>—A greenish, fine-grained metamorphic rock in which -chlorite is the principal mineral, but in which magnetite is a quite characteristic -accessory constituent.</p> - -<p><span class="pagenum"><a name="Page_466" id="Page_466">[466]</a></span></p> - -<p><b>Staurolitic garnetiferous mica schist.</b>—A mica schist in which garnet -and staurolite are so abundant as to be essential constituents.</p> - -<p><b>Clay slate.</b>—A metamorphosed mud stone or shale. In the process -of metamorphism the rock has been hardened, given a slaty cleavage, -and innumerable minute scales of mica have developed to produce a -silky luster upon the cleavage faces. The color may be gray, green, -purple, or black.</p> - -<p><b>Quartzite.</b>—A metamorphosed sandstone in which the sand grains -have become enlarged by accretion of silica. Whereas a sandstone fractures -about its constituent grains, a break in quartzite is continued through -the grains and the cement alike. In contrast to sandstones, the quartzites -derived from them are usually lighter in color and often nearly white.</p> - -<p><b>Marble</b> (<b>crystalline limestone</b>).—The result of metamorphism upon -limestones. Usually white in color but sometimes gray, blue gray, or -yellow, and sometimes variously broken or brecciated and stained with -iron oxide. Effervesces with cold dilute acid.</p> - -<p><b>Coals.</b>—Under the head of peat the first stage in the formation of -coals from vegetable matter has been briefly described. Lignite, or -brown coal, represents a further stage and one in which the vegetable -structure is still recognizable. It is usually brownish black or black -in color and contains a considerable proportion of water. With increased -pressure or dynamic metamorphism, further percentages of the volatile -constituents are eliminated, and when from seventy-five to ninety -per cent of carbon remains, the material burns with a yellow flame and -is known as bituminous coal. This is the great fuel for the production -of steam. A continuation of the metamorphic processes carries off a -further proportion of the volatile matter and leaves a dense, hard, black -substance with sometimes as much as ninety-five per cent of carbon. -This is the so-called “hard coal” or anthracite generally used for fuel -in our houses, for which purpose it is so well adapted because it burns -with a production of much heat and almost without smoke.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_467" id="Page_467">[467]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">APPENDIX C</h2> - -<p class="pch">THE PREPARATION OF TOPOGRAPHICAL MAPS</p> - -<p><b>Topographical maps a library of physiography.</b>—For the satisfactory -working out in detail of the geology of any region of complex structure, -an accurate topographical map is prerequisite. This is so much the more -true because nearly all complexly folded or faulted rock masses are to be -found in mountainous, or at least in hilly regions. The making of the -topographical map must, therefore, precede that of the geological map, -and in modern usage the latter is a topographical and a geological map -combined in one.</p> - -<p>Within certain narrow limits, predictions concerning the geological -history of a province may often be made by an expert geologist from -examination of an accurate topographical map. Just as in forecasting -the weather upon the basis of the usual weather maps, such predictions -can sometimes be made with entire confidence in their accuracy, while -at other times a guess only may be hazarded. The great value of the -modern topographical map is becoming, however, universally acknowledged, -and every highly civilized nation has either completed or has in -preparation sectional topographical maps of its domain on such a scale -as is warranted by its financial condition and its state of development. -Thus there is now being accumulated a vast library of geographical and -to some extent geological information, of which the student of geology -must be prepared to make use.</p> - -<p><b>The nature of a contour map.</b>—More and more the contour map is -replacing the earlier and less scientific methods of representing topography -on the large scale sectional maps, and hence this type only need -here be considered. In the contour map, the relief of the land is represented -by a series of curving lines, each the intersection of a particular -horizontal plane with the land surface, and the several planes separated -by uniform differences of elevation. This altitude interval is known as -the contour interval. Its choice is a matter of considerable importance, -for though regions of relatively simple topography may be adequately -represented upon a map of large contour interval, say one hundred feet, -another district may require an interval as short as five feet. A contour -map with this interval may be conceived to have been made by flooding<span class="pagenum"><a name="Page_468" id="Page_468">[468]</a></span> -the region which it represents and preparing maps of the shore lines for -each rise of five feet of the water surface, and superimposing the several -maps thus derived with accurate registration one above the other. Wherever -the land slopes are steep, the shore lines of the several maps will be -crowded closely together and give the effect of a relatively dark local -shade; where, upon the other hand, the surface is relatively flat, the -several shores will be widely spaced and the effect will be to produce a -white area upon the map. Thus in contour maps dark tones indicate -steep gradients and pale tones a flatness of surface.</p> - -<p><b>The selection of scale and contour interval.</b>—With the use of the -small scale in the contour map, the tones of the map will be correspondingly -dark, though the relative differences in tone will remain the same. -With the use of a closer contour interval the tones will deepen throughout. -The adjustment of scale and contour interval to any given region is a -matter requiring experience in topographical mapping, and in addition -a knowledge of the geological significance of topographic features. Unfortunately, -the element of expense and the special commercial objects -held in view, conspire to select scales and contour intervals which are -often little adapted to the districts surveyed.</p> - -<p><b>The method of preparing a topographical map.</b>—Having fixed upon -the scale and the contour interval which is to be employed, the task of -the topographical surveyor is next to fix accurately the positions and the -elevations of a sufficient number of points to <i>control</i> the map, and then -to hang, as it were, upon these points as attachments the design represented -by the relief. Were the surface of the ground to be represented -by a flexible fabric, the map maker might raise from a flat base a series of -stout posts of the heights and in the positions which he has determined, -and upon these supports arrange the slopes of the fabric much as drapery -is adjusted. The determination of the exact positions and the elevations -of his control stations is, therefore, a process coldly precise and formal; -whereas in the shaping of the surfaces his attention should be fixed -more upon correctly reproducing the shapes than upon fixing accurately -the position of every point. As a matter of fact, the position of the -average point will be most accurately fixed when the shapes of the features -are most clearly comprehended. To some extent, therefore, the -topographer should be familiar with the geological significance of the -earth features which he is representing.</p> - -<p><b>Laboratory exercises in the preparation of topographical maps.</b>—The -principles which underlie the surveyor’s method for preparing a topographical -map may be learned in the laboratory by the use of models and -the simple device shown in plate 24 A and B. To represent the section -of country to be mapped a model in plaster of Paris is substituted, and this -is placed within a rectangular tank to which locating carriages and altitude -gauges are attached that allow the student to fix the position and the -elevation of any point upon the surface of the model.</p> - -<div class="bord p2"> - -<p class="pr5"><span class="smcap">Plate 24.</span></p> - -<div class="figcenter"> - <img src="images/ill-552a.jpg" width="400" height="325" id="p24a" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>A.</i> Apparatus for exercise in the preparation of topographic maps.</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-552b.jpg" width="400" height="265" id="p24b" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>B.</i> The same apparatus in use for testing the contours of a map.)</p> -</div></div> - -<div class="figcenter"> - <img src="images/ill-552c.jpg" width="400" height="169" id="p24c" - alt="" - title="" /> - <div class="caption"><p class="pc400"><i>C.</i> Modeling apparatus in use.</p> -</div></div> - -</div> - -<p><span class="pagenum"><a name="Page_469" id="Page_469">[469]</a></span></p> - -<p>Upon each model the student “locates”, or fixes, the position of a -sufficient number of points for the control of his map, entering upon an -appropriate map base for each position the altitude which was read from -the gauges. Now <i>with the map always before him</i> he “sketches in” the -forms of the surface by means of contour lines. For this purpose it -is often desirable to fix roughly the direction of the steepest slope at a -number of places, and noting the differences in elevation between control -stations, divide up the distance in accordance with the curves of slope -and start the contours at right angles to the slope. Afterwards such -sections are connected by sketching in with the model always in view -for control (<a href="#f488">Fig. 488</a>).</p> - -<div class="figcenter"> - <img src="images/ill-554.jpg" width="400" height="398" id="f488" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 488.</span>—A student’s map prepared from a model by the use of the contour -apparatus represented in <a href="#p24a">plate 24 A</a>.</p> -</div></div> - -<p><b>The verification of the map.</b>—The map prepared, its accuracy may -be tested by a simple method which is denied the topographer who has -to do with the actual surface of the ground. The locating carriages -and altitude gauges are removed from the tank, which is next filled with<span class="pagenum"><a name="Page_470" id="Page_470">[470]</a></span> -water and leveled by means of guide marks upon the interior. A few -drops of milk or of ordinary clothes blueing are added to the water to -render it opaque, and it is then drawn off at the faucet in successive installments, -so that the surface drops by layers corresponding in thickness -to the contour interval of the map, plate 24 B. As each layer is withdrawn, -that contour of the map to which the shore line should correspond -is carefully examined and corrected. By such corrections the nature of -the first errors made is soon appreciated, and the method of procedure -is thus more easily acquired. At the same time the significance of the -design of the map is more quickly learned than by a mere examination -of the standard government maps.</p> - -<p>The work above outlined calls for waterproofed models of suitable -form and size, and a series, each of which sets forth some typical feature -or series of features, has been designed by Mr. Irving D. Scott.<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a></p> - -<p><b>The preparation of physiographic models.</b>—The apparatus used to -prepare the topographic map is adapted also for preparing a physiographic -model from a standard topographical map. For this purpose -the method is essentially reversed, though the tank is replaced to advantage -by a light metal frame elevated upon one side so as to permit a free -use of the hands in modeling the clay.</p> - -<p>The material used in preparing the model is artists’ modeling clay<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> -which has a base of beef suet, and hence does not dry out and crack as -does ordinary clay. Its form is, therefore, retained indefinitely, and it -may be used again and again. Most maps must be enlarged in modeling, -and the simplest way is often to photographically or by pantograph -enlarge the map to the scale of the model. The map prepared, -it is covered by a thin celluloid plate which has cut upon it a series of -crossed lines spaced in inches and larger subdivisions to correspond to -those of the locating carriages (<a href="#p24c">plate 24 C</a>).</p> - -<p>The enlargement of the map is not essential to experienced workers, -and the standard map may be covered in similar manner by a transparent -plate with “checkerboard” design, the squares of which bear some -simple relation in size to the larger divisions of the locating carriages -(<a href="#p24c">Plate 24 C</a>, rear).</p> - -<p>The method of preparing the model is comparatively simple. Beginning -at any point upon the map, the intersection of a heavy contour -line with one of the guide lines of the celluloid “position plate” is carefully -noted. Both the position and the elevation of this point are fixed by -the point of the altitude gauge of the modeling frame, and the clay built -<span class="pagenum"><a name="Page_471" id="Page_471">[471]</a></span> -up beneath it to that height. With the fingers the clay is now roughly -shaped in various directions from this point, the altitude gauge is advanced -by the locating carriage so as to correspond in position to the -intersection of the next heavy contour line with the same guide line of -the position plate, and the elevation for this point similarly adjusted -upon the model. As before, the surface of the clay is roughly shaped in -advance and upon the sides so as to conform to the indications of the -map; and this process is repeated until the work is finished. Corrections -for intermediate positions may be carried to any desired degree of -refinement which the scale and the accuracy of the map permit. Models -which are larger than the area of the modeling frame are prepared by -making a square foot at a time by the above described process, and then -moving the frame forward and adjusting in a new position by means -of the sharp pins in the legs of the apparatus.</p> - -<p class="prr"><span class="smcap">Reading References</span></p> - -<p class="pex"><span class="smcap">William H. Hobbs</span>, New Laboratory Methods for Instruction in Geography, -Journal of Geography, vol. 7, 1909, pp. 97-104. Also Scot. -Geogr. Mag., vol. 24, 1908, pp. 643-652. The Modeling of Physiographic -Forms in the Laboratory, <i>ibid.</i>, vol. 8, 1910, pp. 225-228.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_472" id="Page_472">[472]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">APPENDIX D</h2> - -<p class="pch">LABORATORY MODELS FOR STUDY IN THE INTERPRETATION -OF GEOLOGICAL MAPS</p> - -<div class="floatleft"> - <img src="images/ill-557a.jpg" width="250" height="80" id="f489" - alt="" - title="" /> - <div class="cf"><p class="pc250"><span class="smcap">Fig. 489.</span>—Models to represent outcrops of rock.</p> -</div></div> - -<p>The laboratory models which have been described on page 63, and are -used to represent outcrops in the study of geological maps, are shown -in <a href="#f489">Fig. 489</a>. The drum-shaped blocks serve to represent massive rocks -which occur in irregularly -shaped masses such as batholites -and flows. The long, -narrow strips are for intrusive -rocks in the form of -dikes, while the larger blocks -provided with a swivel joint -are used for outcrops of sedimentary rocks, and after adjustment they -give the dip and strike of the exposure. The wing bolts used in their -construction should be of bronze, because of the effect of iron upon the -compass. For the same reason tables should not be placed near iron -beams or columns. All these blocks can be made by an ordinary carpenter, -and should be available in sufficient numbers to arrange problems -like those of <a href="#f47">Figs. 47</a>, <a href="#f48">48</a>, and <a href="#f490">490</a>. With a view to supplying suggestions -for other problems of the same general nature, the three additional field -maps of <a href="#f491">Fig. 491</a> have been introduced.</p> - -<div class="figcenter"> - <img src="images/ill-557b.jpg" width="400" height="217" id="f490" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 490.</span>—Special laboratory table set with a problem in geological mapping -which is solved in <a href="#f47">Figs. 47</a> and <a href="#f48">48</a>.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_473" id="Page_473">[473]</a></span></p> - -<div class="figcenter"> - <img src="images/ill-558.jpg" width="400" height="641" id="f491" - alt="" - title="" /> - <div class="caption"><p class="ch400"><span class="smcap">Fig. 491.</span>—Three field maps to be used as suggestions in arranging laboratory -tables for problems in the preparation of areal geological maps.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_474" id="Page_474">[474]</a></span></p> - -<p>The list of questions given below is intended to indicate the nature of -some of the problems which the student should be asked to solve in the -preparation of each map. The numbers in parentheses refer to pages in -this book where further information is given:—</p> - -<p class="prr"><span class="smcap">Stratigraphical</span></p> - -<p>1. Of the formations represented what ones are sedimentary and what -igneous (<a href="#Page_30">Chap. IV</a>, <a href="#Page_462">App. B</a>)?</p> - -<p>2. Which formations, if any, are separated by unconformities (<a href="#Page_51">51-53</a>)?</p> - -<p>3. What is the order of age of the sedimentary formations (<a href="#Page_65">65</a>)?</p> - -<p>4. What are the <i>exposed</i> thicknesses of each of these formations (<a href="#Page_48">48-49</a>)?</p> - -<p>5. Do any of these values represent full thickness of the formation, -and if so, which ones?</p> - -<p>6. What is the age in terms of the sedimentary formations of each of -the igneous rock masses (<a href="#Page_65">65</a>)?</p> - -<p>7. Which igneous rocks, if any, occur in batholites (<a href="#Page_143">143</a>, <a href="#Page_441">441</a>)? Which, -if any, in dikes (<a href="#Page_140">140</a>)?</p> - -<p class="prr"><span class="smcap">Structural</span></p> - -<p>8. What formations, if any, have monoclinal dip (<a href="#Page_42">42</a>)?</p> - -<p>9. Indicate upon the map by dashed lines the crests of all anticlines -and the trough lines of synclines.</p> - -<p>10. Indicate by arrows the direction of pitch of all plunging anticlines -and synclines wherever disclosed by changes of dip and strike (<a href="#Page_43">43</a>).</p> - -<p>11. Indicate the approximate position of all faults whose position is -disclosed (<a href="#Page_58">58-61</a>), and, if possible, state which limb is the one downthrown.</p> - -<p>12. Prepare suitable geological sections.</p> - -<p class="prr"><span class="smcap">Reading Reference</span></p> - -<p class="pex"><span class="smcap">William H. Hobbs.</span> Apparatus for Instruction in Geography and Structural -Geology. III. The Interpretation of Geologic Maps. School -Science and Mathematics, vol. 9, 1909, pp. 644-653.</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_475" id="Page_475">[475]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">APPENDIX E</h2> - -<p class="pch">SUGGESTED ITINERARIES FOR PILGRIMAGES TO STUDY -EARTH FEATURES</p> - -<p>The chief value of the laboratory studies discussed in the preceding -appendices is as a preparation for observations made in the field—the -laboratory <i>par excellence</i> of the geologist. The pilgrimages whose itineraries -are here suggested have been planned especially for impressing by -observation the lessons of this book. Such journeys are best interrupted -at a relatively small number of localities which, because already studied -in some detail, are specially adapted to serve as centers for local excursions. -These localities will in most cases be the great scenic places to which -tourists resort, or the seats of universities near which specially detailed -explorations have been often made.</p> - -<p>Within the United States a few local geological guides have been published, -and the Geologic Folios published by the United States Geological -Survey are already available for a number of such centers. For one long -geological pilgrimage we are fortunate in having a carefully prepared -guide, namely, from New York to the Yellowstone National Park and -back, with a side trip to the Grand Cañon of the Colorado. Except for -the side trip this route, in large measure, corresponds with one here chosen, -and for the return journey especially the student is referred to it for information -(Geological Guide Book of the Rocky Mountain Excursion, -edited by Samuel Franklin Emmons. Comte Rendu de la Congrés -Géologique Internationale, 5me Session, Washington, 1891, 1893, pp. 253-487, -map and plates 13, figs. 32).</p> - -<p>Our journey is begun at New York City, which is built about the deeply -submerged channels of an estuary choked with glacial deposits, though -the channel may be followed as a deep cañon across the continental shelf -to its margin (<a href="#Page_252">252</a>,<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a> <a href="#p17b">pl. 17 B</a>). New York City is also upon the margin -of the glaciated area, the outer terminal moraine of which is well represented -on Long Island (<a href="#Page_298">298</a>). Across the Hudson in New Jersey is the -great Coastal Plain which meets the oldland in a well-defined margin (<a href="#Page_159">159</a>, -<a href="#Page_246">246</a>, <a href="#Page_247">247</a>). A local geological guide of the vicinity of the metropolis has -been written by Gratacap (Geology of the City of New York, Greater New -York. Brentanos, New York, 1904, pp. 119, pls. and map).</p> - -<p><span class="pagenum"><a name="Page_476" id="Page_476">[476]</a></span></p> - -<p>Traveling by the New York Central Railway, we follow up the Mohawk -outlet of the glacial lakes Iroquois and Algonquin (<a href="#Page_334">334</a>), first skirting upon -the east the great sills of intrusive basalt known as the Palisades, with -their markedly columnar jointing and intersections by numerous faults. -Above Peekskill we enter the picturesque narrows of the river (<a href="#Page_174">174</a>), cut -in the hard crystalline rocks of the Highlands. Entering the Mohawk -Valley, we pass Syracuse with limestone caverns and well-oriented joints -widened by solution through the agency of the descending ground water -(<a href="#Page_181">181</a>, <a href="#p6b">pl. 6 B</a>). A branch line to the southwest reaches the vicinity of -Cayuga Lake and Ithaca, where are well-oriented joints which have controlled -the drainage directions, and there is also a typical strath (<a href="#Page_55">55</a>, <a href="#Page_87">87</a>, -<a href="#Page_428">428</a>).</p> - -<p>To Niagara Falls at least a day should be allotted for the “gorge ride” -by trolley car, thus making the complete circuit of the brink of the gorge -with interruptions and local studies at all important points (<a href="#Page_352">352-366</a>, -<a href="#p23a">pl. 23 A</a>). From Niagara Falls over the Michigan Central Railway we reach -Detroit on the present outlet of the upper Great Lakes as well as of the -later Lake Algonquin (<a href="#Page_334">334</a>). From this city as a center a trip is made by -electric railway to Ypsilanti and Ann Arbor, across the bottoms of the -early glacial lakes from the first Maumee to Warren (<a href="#Page_330">330-333</a>). The -strong Whittlesey beach is encountered at the little station of Ridge -Road, and one of the Maumee beaches on Summer Street in Ypsilanti. -The city of Ypsilanti is built upon a terrace (<a href="#Page_165">165</a>) of the Huron River, -and another terrace in the same series is crossed by the electric line. In -an excursion of a few miles down the river, passing meanders (<a href="#Page_164">164-165</a>) -and ox-bow lakes (<a href="#Page_165">165</a>, <a href="#Page_415">415</a>), is found an interesting case of stream capture -near the little village of Rawsonville (<a href="#Page_175">175</a>. See Isaiah Bowman, Jour. -Geol., Vol. 12, 1904, pp. 326-334).</p> - -<p>Continuing our journey from Ypsilanti over a high moraine (<a href="#Page_312">312</a>), Ann -Arbor is reached, built upon the level plain of outwash with fosses sometimes -separating it from the moraine (<a href="#Page_281">281</a>, <a href="#Page_314">314</a>). Upon the campus of -the university are great bowlders of jasper conglomerate and jaspilite, -which were transported from the north by the continental glacier (<a href="#Page_305">305</a>). -Across the river from the Michigan Central station and behind the little -church is a delta formed in one of the glacial lakes Maumee and here -opened in section (<a href="#Page_168">168</a>). West of the city is a great valley which was the -former course of the Huron River when thus diverted by the continental -glacier lying to the eastward of Ann Arbor—border drainage (see Ann -Arbor folio by the U. S. G. S., and, further, R. C. Allen and I. D. Scott, -An Aid to Geological Field Studies in the Vicinity of Ann Arbor, George -Wahr, publisher, Ann Arbor).</p> - -<p>Returning to Detroit (M. C. Ry.), the great Sibley quarries in limestone<span class="pagenum"><a name="Page_477" id="Page_477">[477]</a></span> -near Trenton may be visited. They display perfect jointing, numerous -fossils, and especially well-glaciated surfaces interrupted by deep troughs -and showing striæ of several glaciations (<a href="#Page_304">304</a>). From Detroit the journey -is continued by steamer to Mackinac Island in the strait connecting Lakes -Michigan and Huron, passing on the way through the peculiar delta of -the St. Clair River (<a href="#Page_431">431</a>), and coming in view of the notched headlands, -which are a monument to the post-glacial uplift of the glaciated area (<a href="#Page_250">250</a>, -<a href="#Page_341">341</a>). A day is spent at Mackinac Island and St. Ignace in order to study -with some care these uplifted strands of the late glacial lakes (<a href="#Page_341">341-344</a>). -Chicago may now be reached either by steamer or by rail, and in its vicinity -we may see the elevated beaches and the ancient outlet of Lake Chicago -(<a href="#Page_331">331-332</a>, <a href="#Page_347">347</a>, <a href="#p22a">pl. 22 A</a>. See Chicago Folio, U. S. G. S.). By the -Chicago and Northwestern Railway the area of recessional moraines and -intermediate outwash plains, and later that of the drumlins, are crossed in -journeying to Madison, Wisconsin. By examination of the maps on -pages 308 and 317 in connection with the larger scale atlas sheets of the -United States Geological Survey (Janesville, Evansville, and Madison -sheets), this car journey can be made most instructive in gaining familiarity -with the characteristic glacial features, and this study is continued to -special advantage in excursions about Madison as a center (<a href="#Page_316">316-317</a>, <a href="#Page_407">407</a>). -This is the more true since at numerous localities in the vicinity of Madison -the well-striated glacier pavement is exposed for comparison of the -striæ as regards direction with the axes of the several types of glacial -features.</p> - -<p>An especially instructive excursion may be made by carriage in a single -day to the “driftless area” some twelve miles west of the city. Before -reaching it we cross in alternation a series of recessional terminal moraines -(<a href="#p17c">pl. 17 C</a>) and outwash plains, and near Cross Plains encounter the partially -dissected upland with its arborescent drainage and even sky line (<a href="#Page_298">298</a>, -<a href="#Page_300">300-301</a>, <a href="#Page_312">312-313</a>, <a href="#p16a">pl. 16 A</a> and <a href="#p16b">B</a>). Typical shore formations (<a href="#Page_233">233</a>, <a href="#Page_241">241</a>, -<a href="#Page_242">242</a>) are studied to advantage about Lake Mendota in a walking trip to -and beyond Picnic Point, where are found the best ice ramparts (<a href="#Page_431">431-434</a>. -See Buckley, Trans. Wis. Acad. Sci., Vol. 13, pp. 141-162, pls. 18).</p> - -<p>Our journey is now continued over the Chicago and Northwestern -Railway to Devils Lake near Baraboo, where we cross a salient of the -driftless area, within which lies Devils Lake, imprisoned in a former valley -of the Wisconsin River, since diverted to another course as a result of the -glacial invasion (<a href="#Page_312">312-313</a>). The valley here is a former narrows in hard -quartzite (<a href="#Page_466">466</a>), which towers above the lake in unstable chimneys (<a href="#Page_300">300</a>), -such as the Devils Tower, but such remnants are not found on the other -side of the moraine, being there replaced by rounded rock shoulders. Just -north of the lake the marginal moraine which blocks the valley is so<span class="pagenum"><a name="Page_478" id="Page_478">[478]</a></span> -characteristic as to merit special study (<a href="#p17c">pl. 17 C</a>). Only a few miles northward -along the railway from Devils Lake is Ableman, where, exposed in -a high cliff, the hard purple quartzite with beautiful ripple marks to reveal -its plane of sedimentation (<a href="#p11a">pl. 11 A</a>) dips vertically, and is overlain by -horizontally bedded yellow sandstone. The marked angular unconformity -which is thus displayed is further made evident by a basal layer of conglomerate -(<a href="#Page_463">463</a>) in the sandstone (<a href="#Page_51">51-53</a>). Here also are deposits of loess -along the river, which display their vertical joint surfaces (<a href="#Page_207">207</a>). An -excellent geological guide to this interesting district and that of the neighboring -“Dalles” of the Wisconsin River has been written by Salisbury -and Atwood (The Geography of the Region about Devils Lake and the -Dalles of the Wisconsin, etc., Bull. 5, Wis. Geol. and Nat. Hist. Surv., 1900, -pp. 151, pls. 38, figs. 47).</p> - -<p>If we have taken a conveyance at Devils Lake for Ableman, we may -continue in the same manner to Kilbourn, where begin the picturesque -Dalles of the Wisconsin River—here a young gorge cut in sandstone, -because the Wisconsin was diverted from its old valley to border drainage -at the edge of the driftless area (<a href="#Page_300">300</a>, <a href="#Page_321">321</a>). The side cañons of the river, -through their abrupt zigzags, reveal the control of their courses by the joint -system (<a href="#Page_224">224</a>). In the journey up the rapids by steamer to inspect the -Dalles, we observe many beautiful examples of cross bedding in the sandstone -(<a href="#Page_37">37</a>).</p> - -<p>From Kilbourn we continue our journey to Minneapolis over the Chicago, -Milwaukee, and St. Paul Railway, and near Camp Douglas are over a peneplain, -out of which rise prominent monadnocks (<a href="#Page_171">171</a>). At La Crosse the -Mississippi River is reached, flowing beneath bluffs of sandstone which are -capped by loess (<a href="#Page_207">207</a>). The meanderings and the numerous cut-offs of -the Mississippi may be observed to the left (<a href="#Page_415">415</a>). Lake Pepin is a side-delta -lake blocked by the deposits of the Chippewa River (<a href="#Page_419">419</a>).</p> - -<p>From Minneapolis an excursion is made to Fort Snelling to view the -young gorge of the Mississippi, cut by the Falls of St. Anthony for a distance -of about eight miles in manner similar to that of the seven miles of Niagara -gorge (<a href="#Page_354">354</a>), and to compare this narrow gorge with the broad valley of the -Warren River which drained Lake Agassiz (<a href="#Page_327">327</a>). Somewhat farther up -the Warren River are examples of saucer lakes (<a href="#Page_416">416</a>).</p> - -<p>From Minneapolis the journey may be continued by the Great Northern -Railway to Livingston, Montana, thus crossing between the stations of -Muscoda and Buffalo the bed of Lake Agassiz and its marginal beaches -(<a href="#Page_325">325-328</a>. For local geology of Minnesota consult C. W. Hall, Geology -of Minnesota, Vol. 1, Minneapolis, 1903).</p> - -<p>The Yellowstone Park is entered from Livingston (Livingston Geological -Folio, U. S. G. S.) and departure from it made at the relatively new<span class="pagenum"><a name="Page_479" id="Page_479">[479]</a></span> -Union Pacific terminal at the southwest margin. The regular trip -through the Park includes visits to the several geyser basins (<a href="#Page_191">191-194</a>), -Obsidian Cliff (<a href="#Page_33">33</a>, <a href="#Page_463">463</a>), the Cañon of the Yellowstone, etc. Good climbers -can make a side trip from near the Mammoth Hot Springs to the top of -Quadrant Mountain, the remnant of a “biscuit cut” upland (<a href="#Page_372">372</a>), and -there study the nivation process (<a href="#Page_368">368</a>, Yellowstone National Park Folio, -U. S. G. S.).</p> - -<p>The trip from the Park to Salt Lake City, over the Union Pacific Railway, -passes through the Red Rock Pass, the former outlet of Lake Bonneville -(<a href="#Page_423">423</a>), into the desert of the Great Basin ( <a href="#Page_197">Chaps. XV</a> and <a href="#Page_209">XVI</a>). -Great Salt Lake is a saline lake or sink with an interesting record of climatic -changes (<a href="#Page_198">198</a>, <a href="#Page_401">401</a>). The front of the Wasatch Range, in view and -easily reached from Salt Lake City, is deeply scored by the horizontal -shore terraces of Lake Bonneville (<a href="#Page_198">198</a>, <a href="#Page_199">199</a>), and these terraces are extended -at every reëntrant by barrier beaches of great perfection. In the -Pleistocene period mountain glaciers in part occupied the valleys of this -range, though they did not always extend as far as the mountain front. -Big Cottonwood Cañon, which realizes this condition, and the neighboring -Little Cottonwood Cañon, from whose front its glacier spread into an -expanded foot (<a href="#Page_264">264</a>), thus show for comparison in a single view the <span class="font">V</span> -and the low <span class="font">U</span> sections respectively (<a href="#Page_172">172</a>, <a href="#Page_376">376</a>). Here are also alluvial -fans (<a href="#Page_213">213</a>) and recent faults which intersect them.</p> - -<p>From Salt Lake City the return to New York may be made by the -Denver and Rio Grande Railway across deserts and through the Royal -Gorge, the cañon of the Arkansas River. A full itinerary of the points -of geological interest along this route, and continued to Chicago, Washington, -and New York, is supplied in much detail in the guide of the geological -excursion to the Rocky Mountains above cited. This the traveling geologist -should not fail to study. Some references to points along this -journey will be found on preceding pages of this book (<a href="#Page_219">219-220</a>, High -Plains; <a href="#Page_170">170</a>, Allegheny Plateau in West Virginia; <a href="#Page_176">176</a>, water gap of -Harper’s Ferry; <a href="#Page_176">176-177</a>, <a href="#Page_184">184-186</a>, side trip up the Shenandoah Valley -to Luray Caverns and Snickers Gap; <a href="#Page_251">251</a>, Chesapeake Bay).</p> - -<p>Instead of returning directly from Salt Lake City, the traveler, if he -has sufficient time at his disposal, may extend his journey southwestward -across the Great Basin to Los Angeles. A branch line from this route -leaves the Vegas Valley and passes within reach of the famous Death -Valley (<a href="#Page_201">201</a>) to Tonopah (<a href="#Page_79">79</a>) and the Owens Valley (<a href="#Page_77">77-78</a>, <a href="#Page_92">92</a>), where are -many surface faults dating from the earthquake of 1872 and other less -recent disturbances. Returning to the junction point, the route continues -across the Colorado and Mohave deserts to Los Angeles. From Los Angeles -as a center the exceptionally interesting terraces, caves, and stacks of an<span class="pagenum"><a name="Page_480" id="Page_480">[480]</a></span> -uplifted coast are to be seen to best advantage near Pt. Harford -(<a href="#Page_245">Chap. XIX</a>). The islands of San Clemente and Santa Catalina may also be -reached from Los Angeles (<a href="#Page_239">239</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>, <a href="#Page_250">250</a>, <a href="#Page_256">256</a>, <a href="#Page_257">257</a>, <a href="#p5b">pls. 5 B</a>, <a href="#p7a">7 A</a>, -<a href="#p12a">12 A</a>). The return to the East, if made by the Santa Fe Railway, permits -of a visit to the Grand Cañon (<a href="#Page_174">174</a>, <a href="#Page_443">443</a>) from the station of Williams. -From that point eastward the geology of the route is fully covered in Emmons’ -Guide to the Rocky Mountain Excursion already cited.</p> - -<hr class="tb" /> - -<p>For the benefit of those who are privileged to travel in Europe, and the -number increases yearly, a pilgrimage is suggested which may easily be -made to correspond with plans laid out on the basis of historical, artistic, -and scenic points of interest. The only popular guide of a general nature -written for geologists traveling abroad appears to be a brief but valuable -little paper by Professor Lane (The Geological Tourist in Europe, Popular -Science Monthly, Vol. 33, 1888, pp. 216-229). The publishing house of -Gebrüder Bornträger in Berlin is now publishing a quite valuable series -of geological guides dealing with special districts and written by well-known -authorities (Sammlung Geologischer Führer). Of this series some -thirteen numbers have already been issued. Many other valuable local -guides of a geological nature are the Livrets Guides of the International -Geological and Geographical Congresses, and the similar pamphlets supplied -in connection with annual meetings of national or provincial geological -societies.</p> - -<p>Passengers on steamships sailing from the harbor of New York pass out -over a deeply submerged cañon (<a href="#Page_252">252</a>) largely filled with glacial deposits, -through the Narrows (<a href="#Page_174">174</a>), and in sight of Sandy Hook, a modified spit -(<a href="#Page_238">238</a>, <a href="#Page_240">240</a>). To the left are seen the great morainic accumulations at the -border of the glaciated area on Long Island (<a href="#Page_298">298</a>). In the course of the -trans-Atlantic voyage a much-rounded iceberg may be encountered (<a href="#Page_291">291</a>), -though this is much more apt to occur upon the northern routes from -Quebec, and late in the season. Upon entering the English Channel the -land on both coasts rises in steep cliffs, where are found all the common shore -features well developed (<a href="#Page_231">Chap. XVIII</a>). The German steamships pass -in sight of Heligoland, that last remnant of wave erosion (<a href="#Page_236">236</a>).</p> - -<p>While traveling in Europe, the student should consult a map of the -glaciated area (<a href="#Page_299">299</a>), and so learn to recognize its peculiarities, and carefully -mark its marginal moraine (<a href="#Page_311">311</a>) and other strongly marked features.</p> - -<p>If the British Isles are visited and the more rugged areas are selected, -one may study the cirques and other characteristic features due to the -presence of mountain glaciers about Snowdon (<a href="#Page_367">Chap. XXVI</a>). More -mature stages of the same processes are to be found in the Scottish Highlands<span class="pagenum"><a name="Page_481" id="Page_481">[481]</a></span> -and the Inner Hebrides, but especially upon the Island of Skye (<a href="#f492">Fig. 492</a>). -A very valuable aid to excursions in this district is Baddeley’s -Scotland (part I, Dulau, London) and Sir Archibald Geikie’s Explanatory -Notes to accompany Bartholomew’s Geological Map of Scotland -(map and notes in cover, Edinburgh, 1892, pp. 23).</p> - -<div class="figcenter"> - <img src="images/ill-566.jpg" width="400" height="374" id="f492" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 492.</span>—Sketch map of Western Scotland and the Inner Hebrides to show -location of some points of special geological interest.</p> -</div></div> - -<p>It is from Oban, the “Charing Cross of the Highlands”, that one should -start out upon the summer steamers in order to reach both Skye and -Staffa, the latter with fine basaltic columns (<a href="#Page_463">463</a>), and Fingal’s Cave. In -sailing to Skye one passes upon either shore of the narrow fjords many -relics left in the dissection of volcanoes (<a href="#Page_139">139-143</a> and Sir A. Geikie, Ancient -Volcanoes of Great Britain, Vol. II); also rocky islands and skerries -marking submergence (<a href="#Page_252">252</a>), and the coast terraces which register a later -uplift (<a href="#Page_250">250</a>). Skye is a complex of many intrusive and volcanic rocks of<span class="pagenum"><a name="Page_482" id="Page_482">[482]</a></span> -such markedly different colors as to appear as tints in the landscape. -In the Cuchillin Hills of dark green rises the massive gabbro (<a href="#Page_462">462</a>) cut by -cirques into the jagged pinnacles of horns and comb ridges (<a href="#Page_373">373</a>); while -lower down and to the east are rounded domes of rhyolite (<a href="#Page_463">463</a>) abraded -beneath the glaciers and of a delicate salmon tint. Still lower and to the -westward are flat mesas composed of horizontal layers of black basalt -under a rich carpeting of the brightest verdure. Eastward across the -channel are seen the purplish walls of an ancient sandstone. The jagged -gabbro core of the island thus represents a fretted upland (<a href="#Page_372">372</a>) and is -now the training ground of the Alpinist (Abraham, Rock Climbing in -Skye, Longmans, London, 1908), while nestled in one of the bottoms of a -<span class="font">U</span>-valley is Loch Coruisk, a typical rock-basin lake (<a href="#Page_412">412</a>), its shores of hard -rock planed and scored.</p> - -<p>From Skye we may go to study the remarkable thrusts (<a href="#Page_45">45</a>) on the north -shore of Loch Maree, a marked lineament, and one directed at right angles -to that on the course of the Caledonian Canal connecting Loch Linne with -Loch Ness. This northeast wall of Loch Maree is a strikingly rectilinear -fault represented by an escarpment, up which we climb to find at the top -the crushed and fluted thrust planes of movement dipping southeastward -at a flat angle. Here also are beautiful rock-basin lakes, lying in hollows -molded beneath the continental glacier. On our way from Skye we have -passed up Loch Carron, a sea loch or fjord (<a href="#Page_252">252</a>), and along the strath at its -head known as Strathcarron (<a href="#Page_428">428</a>).</p> - -<p>Returning now to Oban, it is but a short trip by steamer up Loch Linne -to Fort William along the striking lineament (<a href="#Page_226">226</a>) which continues to -Loch Ness and beyond (<a href="#f492">Fig. 492</a>), and thence by rail to Glen Roy and the -neighboring glens of Lochaber (<a href="#Page_322">322-325</a>).</p> - -<p>From Paris as a starting point, we may visit in a most picturesque region -the beautifully preserved craters of extinct volcanoes in the Auvergne of -Central France (<a href="#Page_105">105</a>, <a href="#Page_124">124</a>, <a href="#Page_145">145</a>), which district is entered from Clermont-Ferrand. -Here are found the characteristic puys, steep lava domes of -viscous lava (<a href="#Page_105">105</a>), which figured largely in the early controversies of geologists -concerning the origin of rocks.</p> - -<div class="figcenter"> - <img src="images/ill-568.jpg" width="400" height="486" id="f493" - alt="" - title="" /> - <div class="caption"><p class="pc400"><span class="smcap">Fig. 493.</span>—Outline map of a geological pilgrimage across the continent of Europe.</p> -</div></div> - -<p>The rest of our pilgrimage will be so planned as to enter the noble river -Rhine at its mouth (<a href="#f493">Fig. 493</a>), ascend its course to its birthplace in the -snows of Switzerland, and after further exploration of the features of this -fretted upland, traverse northern and central Italy so as to make our -departure for America by the southern route. Entering then upon this -course in the Low Countries, we have first the opportunity of observing -the characteristics of a great delta with natural levees artificially -strengthened as dikes (<a href="#Page_165">165-168</a>). Here also are found dunes of beach -material which has been raised by the wind into a great rampart near the<span class="pagenum"><a name="Page_483" id="Page_483">[483]</a></span> -shore (<a href="#Page_209">209-211</a>). Such a wall of dune sand is well displayed at the bathing -resort at Scheveningen near the Hague (<a href="#Page_421">421</a>). The flood plain of the -Rhine (<a href="#Page_162">162-165</a>) may be studied in a journey up the river to the university -town of Bonn, from whence a day’s excursion should be devoted -to the relics of volcanoes known as the Seven Mountains (H. von Dechen, -Geognostischer Führer in das Siebengebirge, Bonn, 1861). As a preparation -for this trip and others in the volcanic Eifel higher up the river, a visit<span class="pagenum"><a name="Page_484" id="Page_484">[484]</a></span> -should be made to the mineral and rock collections of the Poppelsdorfer -Schloss at the University. In the volcanic Eifel are found some of the -most interesting of crater lakes (<a href="#Page_405">405</a>), the largest being Lake Laach with -its somewhat peculiar volcanic ejectamenta and its picturesque abbey (see -von Dechen, Geognostischer Führer zu der Vulkanreihe der Vorder-Eifel, -etc., Bonn, 1886. Consult also Lane, A Geological Tourist in Europe, <i>l.c.</i>).</p> - -<p>Continuing our course up the river from Bonn, we soon enter the gorge -of the Rhine cut in an uplifted peneplain (<a href="#Page_169">169</a>, <a href="#Page_171">171</a>, <a href="#Page_174">174</a>). From Coblenz, -where the Moselle enters the Rhine, a side trip may be made up this tributary -river past Zell with its entrenched meanders (<a href="#Page_173">173</a>) to the ancient -Roman city of Treves. Above Bingen on the Rhine we leave behind us -the narrow gorge and rapid current of the river and continue over the broad -floor at the bottom of a rift valley (<a href="#Page_403">403</a>), lying between the forest of Odin -and the Black Forest on the east and the “Blue Alsatian Mountains” -far away to the west. At the margins of this plain are beds of loess with -their characteristic joint structures and inclusions (<a href="#Page_207">207</a>), and in the higher -hills on either hand a wealth of intrusive igneous rocks.</p> - -<p>At the entrance of the Neckar River to this broad plain is nestled the -picturesque castle and university town of Heidelberg, a convenient center -for excursions (Julius Ruska, Geologische Streifzüge in Heidelbergs -Umgebung, etc., Nägele, Leipzig, 1908, pp. 208, map). At Strassburg -(Schwarzwaldstrasse 12) is located the German Chief Station for Earthquake -Study, with a particularly large set of modern seismographs. In -the university cabinet is also one of the largest and most representative -mineral collections in Europe. For excursions in the neighborhood consult -Benecke, Sammlung Geognostische Führer, Vol. 5, Elsass, 1900.</p> - -<p>From Strassburg we may go by the Black Forest Railway to the Hegau -with its volcanic plugs (<a href="#Page_140">140</a>), each surmounted by a picturesque castle. -We enter next the broadly extended piedmont apron site, above which -Lake Constance still remains as a border lake (<a href="#Page_399">399</a>). Outwash aprons -(<a href="#Page_314">314</a>), moraines (<a href="#Page_311">311</a>), and drumlins (<a href="#Page_317">317</a>) are each in turn encountered. -Still continuing our course up the Rhine from Bregenz, we enter the fretted -upland (<a href="#Page_372">372</a>) of the Alps, mountains composed of great folds and thrusts -about a core of intrusive rock (Rothpletz, Sammlung Geologische Führer, -Vol. 10, 1902, Thrusts in the Alps between Lake Constance and the -Engadine). Some fourteen miles above Chur we pass the terrace produced -by successive landslides (<a href="#Page_414">414</a>), known far and wide as the Flimser -Bergstürz. The further assent of the cascade stairway of this glacier-carved -valley brings us to the Furka Pass, from which point magnificent -views of the fretted upland are obtained. At the Känzli, a mile from -the hotel, one may view the névé of the Rhone Glacier, which may also -be easily visited.</p> - -<p><span class="pagenum"><a name="Page_485" id="Page_485">[485]</a></span></p> - -<p>We have now followed a great river from its mouth in the sands of -Holland to its source in the snows of the higher Alps. Passing over the -divide and descending to Gletsch, we may observe the lower end, or foot, -of the Rhone glacier and the crevasses and séracs (<a href="#Page_391">391</a>) on the steep descent -of this radiating glacier (<a href="#Page_383">383</a>, <a href="#Page_386">386</a>). The response which glaciers make to -climatic changes is here well illustrated by the recession of the glacier -front from near the hotel (its position in the ’50s of the nineteenth century) -to its present position about a mile farther up the valley.</p> - -<p>The characteristics of a glaciated mountain valley may be further -illustrated by climbing to the Grimsel Pass, which is scratched and striated -(<a href="#Page_377">377</a>, <a href="#Page_385">385</a>), and then descending the valley of the Aar to Meyringen (<a href="#Page_377">377</a>). -Near the Grimsel Hospice are the characteristic rock basin lakes (<a href="#Page_412">412</a>), -and upon the Aar Glacier to our left were carried out the epoch-making -researches of Louis Agassiz, the founder of the glacial theory for explaining -the drift. We encounter some thirteen rock bars (<a href="#Page_377">377</a>). Just before -reaching Meyringen we pass the last of these, the Gorge of the Aar, cut -by the stream through limestone.</p> - -<p>Interlaken (<a href="#Page_419">419</a>) may be made the center for additional excursions up -the Lauterbrunnen Valley, with its prominent albs (<a href="#Page_376">376</a>) and its ribbon -fall of the Staubbach (<a href="#Page_378">378</a>). By the Jungfrau Mountain railway we may -now ascend partly in tunnels of the rock to the Ewigeismeer, and look -down upon the névé and bergschrunds of the Great Aletsch Glacier (<a href="#Page_370">370</a>, -see Baltzer, Sammlung Geologische Führer, Vol. 10, Bernese Oberland, -1906). Returning to Interlaken by way of Grindelwald, one may study -the foot of a radiating glacier, the Untergrindelwald glacier, with its tunnel -and its milky and braided stream.</p> - -<p>Crossing now the Alpine foreland to Villeneuve at the upper end of Lake -Geneva and upon a well-developed strath (<a href="#Page_426">426</a>, <a href="#Page_428">428</a>), we may look out -upon the turbid waters extending far from the shore of the lake. Journeying -to Geneva by steamer we note the gradual clearing of the water until -at the outlet of the lake it is as clear as crystal. A walking trip from -Geneva takes us to the Bois de la Bâtie, where the Arve with turbid waters -meets this clear stream (<a href="#Page_427">427</a>).</p> - -<p>The railroad to Chamonix ascends another cascade stairway (<a href="#Page_376">376</a>), -affords views of complexly folded sedimentary rocks (<a href="#Page_43">43</a>), and at Chamonix -itself the mer de glace supplies opportunities for the study of moraines -(<a href="#Page_386">386</a>, <a href="#Page_393">393</a>) and glacial movement (<a href="#Page_390">390-392</a>). To experienced Alpinists -the summit of Mount Blanc offers a remarkably extended outlook over the -fretted upland of the Alps (<a href="#p18a">pl. 18 A</a>). From the station of LeFayet below -Chamonix, one may ascend to the Désert de la Platé, where are Schratten -in limestone due to solution (<a href="#Page_188">188</a>).</p> - -<p>Crossing by one of the passes to the valley of the Rhone at Martigny<span class="pagenum"><a name="Page_486" id="Page_486">[486]</a></span> -we may reach Zermatt, to-day the climbing center of the Alps. From the -subordinate cirques surrounding this village descend the Gorner, Findelen, -St. Theodul, and other components of this radiating glacier. A black -tooth of rock, the Matterhorn, towers above the other peaks and shows to -greatest advantage this feature of glacial sculpture (<a href="#Page_374">374</a>), while the Gorge -of the Gorner is a severed rock bar like that of the Aar (<a href="#Page_377">377</a>). Either on -foot or over the mountain railway we may ascend to the Gorner Grat, -a subordinate comb ridge (<a href="#Page_373">373</a>) which affords one of the most magnificent -and instructive views of radiating glaciers.</p> - -<p>From Brig, farther up the Rhone Valley, an excursion is made to the -Eggishorn Hotel, a center for study on and about the Great Aletsch -Glacier (<a href="#Page_329">329</a>, <a href="#Page_371">371</a>, <a href="#Page_385">385</a>, <a href="#Page_388">388</a>, <a href="#Page_395">395</a>, <a href="#Page_410">410</a>). The easy ascent of the Eggishorn -is rewarded by a view almost directly downward upon the ice-dammed -Márjelen Lake (<a href="#Page_329">329</a>, <a href="#Page_411">411</a>).</p> - -<p>From Brig one may make his entry into Italy, either over the picturesque -Simplon route afoot or by diligence, or else beneath it through the -railway tunnel. By an alternation of short steamboat and rail trips the -journey is continued in a direction transverse to the longer axes of the -border lakes Maggiore, Lugano, and Como, and later southward to Milan. -In leaving the village of Como we pass over heavy morainic deposits on -the apron borders of the expanded-foot glacier (<a href="#Page_383">383</a>, <a href="#Page_385">385</a>) which once -occupied the valley above. On the journey from Milan to Venice, over -the fertile plains of Lombardy, the similar accumulations about Lake -Garda (<a href="#Page_414">414</a>) are first encountered at the little station of Lonato and left -behind at Somma Campagna (Tornquist, Sammlung Geologische Führer, -Vol. 9, Northern Italy, 1902).</p> - -<p>The city of Venice is built upon pile foundations in the lagoon behind -the barrier beach known as the Lido (<a href="#Page_242">242</a>, <a href="#Page_428">428-429</a>). From here we may -reach the Karst country by way of Trieste, some of the more interesting -and typical features being found near Divača (<a href="#Page_187">187-189</a>, <a href="#Page_422">422</a>, <a href="#p6a">pl. 6 A</a>). In -a different direction from Venice by way of Belluno we enter the Dolomites -with their patterned relief and battlemented towers (<a href="#Page_228">228</a>, <a href="#Page_445">445</a>).</p> - -<p>Additional centers for geological excursions on the route to our point of -departure from Italy are Rome and Naples. At the Italian capitol and -in its neighborhood we may study the volcanic Campagna with its beds -of tuff (<a href="#Page_105">105</a>) and its crater lakes (405. See Sir A. Geikie, The Roman Campagna, -Landscape in History and other Essays, Macmillan, 1905, pp. 308-352; -also Deecke, Sammlung Geologische Führer, Vol. 8, Campagna, 1901). -From Rome it is an easy journey to the cataract of Tivoli with its deposits -of travertine (<a href="#Page_184">184</a>). In the opposite direction from Rome across the -Campagna rise the Alban Hills, ruins of a composite cone with several -crater lakes on the sites of former vents. On the summit of the encircling<span class="pagenum"><a name="Page_487" id="Page_487">[487]</a></span> -crater rim, like the Monte Somma of the Vesuvian Mountain now a crescent -only, is located the chief Italian station for earthquake study.</p> - -<p>From Naples we may reach in short excursions and study with some care -still active volcanic mountains. To the east is Mount Vesuvius (<a href="#Page_94">94</a>, <a href="#Page_97">97</a>, -<a href="#Page_122">122</a>, <a href="#Page_124">124</a>, <a href="#Page_127">127-137</a>), which was in grand eruption in April, 1906. Westward -from Naples are the Campi Phlegraeii, or burning fields, with many craters. -Of these Astroni offers a fine example of a large-cratered cinder cone (<a href="#Page_105">105</a>). -In the same vicinity are Monte Nuovo (<a href="#Page_96">96</a>) and the Solfatara (<a href="#Page_97">97</a>), the -latter a type of volcano which no longer erupts lava, but in its place emits -carbon dioxide and other gaseous emanations (Grotto del Cane). The -starting point for excursions in the Phlegræan fields is Pozzuoli with its -Temple of Jupiter Serapis (<a href="#Page_254">254-255</a>), reached from Naples by an electric -line which pierces the wall of an immense crater (Posilippo) composed of -fine yellow volcanic ash known as Pozzuolan.</p> - -<p>From Naples steamers make short excursions to Sorrento with its deep -ash deposits, and to Capri with its blue grotto (<a href="#Page_257">257-258</a>). Herculaneum -(<a href="#Page_139">139</a>) and Pompeii (<a href="#Page_122">122</a>), buried during the eruption of 79 <span class="smcap">A.D.</span>, are on the -line of the Circum-Vesuvian Railway.</p> - -<p>Steamships to New York from Naples call at Gibraltar, the land-tied -island <i>par excellence</i> (<a href="#Page_241">241</a>). Most steamships of the southern route pass -through or near the volcanic islands of the Azores, and certain boats touch -at Algiers, from which a line of railway gives access to Biskra on the -borders of the Desert of Sahara.</p> - -<p>Throughout these pilgrimages the traveler should be on the alert to -note not only the agent responsible for the features which come under his -observation, but, especially where this is the common sculpturing agent -of running water, he should not fail to notice the stage of the erosion -cycle which is represented (<a href="#Page_169">Chapter XIII</a>).</p> - -<hr class="chap" /> - -</div> - -<p><span class="pagenum"><a name="Page_488" id="Page_488">[488]</a><br /><a name="Page_489" id="Page_489">[489]</a></span></p> - -<div class="chapter"> - -<h2 class="p4">INDEX</h2> - -<p class="pn"><span class="pl">A</span>brasion, beneath glaciers, <a href="#Page_275">275</a>.</p> - -<p class="pni">Abyssinia, fissure eruptions in, <a href="#Page_101">101</a>.</p> - -<p class="pni">Accordance, of tributary valleys, <a href="#Page_162">162</a>.</p> - -<p class="pni">Adiabatic refrigeration, in relation to glaciers, <a href="#Page_262">262</a>.</p> - -<p class="pni">Adolescence, in cycle of erosion, <a href="#Page_169">169</a>.</p> - -<p class="pni">Advancing hemicycle of glaciation, <a href="#Page_263">263-266</a>.</p> - -<p class="pni">Advective zone, of atmosphere, <a href="#Page_270">270</a>.</p> - -<p class="pni">Aftershocks, of earthquakes, <a href="#Page_83">83</a>.</p> - -<p class="pni">Agassiz, glacial lake, <a href="#Page_325">325-328</a>.</p> - -<p class="pni">Agassiz, Louis, cited, <a href="#Page_339">339</a>, <a href="#Page_400">400</a>.</p> - -<p class="pni">Age, of strata, <a href="#Page_38">38</a>, <a href="#Page_52">52</a>.</p> - -<p class="pni">Aggradation, <a href="#Page_162">162</a>.</p> - -<p class="pni">Aktian deposits, <a href="#Page_36">36</a>.</p> - -<p class="pni">Alaskan coast, map of, <a href="#Page_79">79</a>.</p> - -<p class="pni">Albs, <a href="#Page_376">376</a>.</p> - -<p class="pni">Alden, W. C., cited, <a href="#Page_316">316</a>, <a href="#Page_318">318</a>, <a href="#Page_319">319</a>.</p> - -<p class="pni">Algæ, growth of, in hot springs, <a href="#Page_194">194</a>.</p> - -<p class="pni">“Alkali” in deserts, <a href="#Page_201">201</a>.</p> - -<p class="pni">Alluvial bench, <a href="#Page_214">214</a>.</p> - -<p class="pni">Alluvial cone, <a href="#Page_213">213</a>.</p> - -<p class="pni">Alluvial-dam lakes, <a href="#Page_423">423</a>.</p> - -<p class="pni">Alluvial fan, <a href="#Page_213">213</a>.</p> - -<p class="pni">Alpine glaciers, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>.</p> - -<p class="pni">Alterations of minerals, <a href="#Page_27">27</a>.</p> - -<p class="pni">Altitude, of different parts of lithosphere, <a href="#Page_18">18</a>.</p> - -<p class="pni">American Falls, future extinction of, <a href="#Page_357">357</a>.</p> - -<p class="pni">Amphiboles, <a href="#Page_459">459</a>.</p> - -<p class="pni">Amphitheaters, formed on drift sites, <a href="#Page_369">369</a>.</p> - -<p class="pni">Amundsen, R., cited, <a href="#Page_23">23</a>.</p> - -<p class="pni">Analysis, of folds, <a href="#Page_54">54</a>.</p> - -<p class="pni">Anderson, Tempest, cited, <a href="#Page_146">146</a>, <a href="#Page_147">147</a>.</p> - -<p class="pni">Andersson, J. G., cited, <a href="#Page_157">157</a>, <a href="#Page_295">295</a>.</p> - -<p class="pni">Andesite, <a href="#Page_463">463</a>.</p> - -<p class="pni">Angular unconformity, <a href="#Page_53">53</a>.</p> - -<p class="pni">Antarctica, <a href="#Page_154">154</a>, <a href="#Page_281">281</a>.</p> - -<p class="pni">Antarctic protuberance, <a href="#Page_17">17</a>.</p> - -<p class="pni">Antarctic shelf ice, <a href="#Page_289">289</a>, <a href="#Page_290">290</a>.</p> - -<p class="pni">Anticlinal folds, <a href="#Page_42">42</a>.</p> - -<p class="pni">Anticlines, <a href="#Page_42">42</a>;</p> -<p class="pnii">tension in, <a href="#Page_45">45</a>.</p> - -<p class="pni">Anticyclone, glacial, <a href="#Page_284">284</a>.</p> - -<p class="pni">Ants, factor in rock decomposition, <a href="#Page_156">156</a>.</p> - -<p class="pni">Apron, alluvial, <a href="#Page_213">213</a>.</p> - -<p class="pni">Aprons, outwash, <a href="#Page_280">280</a>, <a href="#Page_281">281</a>.</p> - -<p class="pni">Arbenz, P., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">Arches, of folded strata, <a href="#Page_42">42</a>;</p> -<p class="pnii">sea, <a href="#Page_233">233</a>, <a href="#Page_234">234</a>.</p> - -<p class="pni">Architecture, of fractured earth superstructure, <a href="#Page_55">55</a>.</p> - -<p class="pni">Arctic depression, <a href="#Page_17">17</a>.</p> - -<p class="pni">Areal geological map, <a href="#Page_62">62</a>.</p> - -<p class="pni">Arêtes, <a href="#Page_373">373</a>.</p> - -<p class="pni">Arldt, Theodore, cited, <a href="#Page_11">11</a>, <a href="#Page_19">19</a>, <a href="#Page_438">438</a>.</p> - -<p class="pni">Arnold, Ralph, cited, <a href="#Page_157">157</a>.</p> - -<p class="pni">Arrangement of oceans and continents, <a href="#Page_10">10</a>.</p> - -<p class="pni">Artesian wells, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_196">196</a>.</p> - -<p class="pni">Ash, volcanic, <a href="#Page_122">122</a>.</p> - -<p class="pni">Askja, eruption of, in 1875, <a href="#Page_101">101</a>.</p> - -<p class="pni">Assmann, R., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Astronomical <i>vs.</i> geodetic observations, <a href="#Page_12">12</a>.</p> - -<p class="pni">Atlantis, North, <a href="#Page_16">16</a>.</p> - -<p class="pni">Atmosphere, compressibility of, <a href="#Page_8">8</a>.</p> - -<p class="pni">Attack, of the weather, <a href="#Page_149">149</a>.</p> - -<p class="pni">Atwood, W. W., cited, <a href="#Page_7">7</a>, <a href="#Page_160">160</a>, <a href="#Page_298">298</a>, <a href="#Page_300">300</a>, <a href="#Page_313">313</a>, <a href="#Page_372">372</a>.</p> - -<p class="pni">Axial plane, of folds, <a href="#Page_42">42</a>.</p> - -<p class="pni">Axis, of folds, <a href="#Page_42">42</a>.</p> - -<p class="pni">Azurite, <a href="#Page_453">453</a>.</p> - -<p class="pn"><span class="pl">B</span>acteria, part taken in weathering, <a href="#Page_156">156</a>.</p> - -<p class="pni">“Bad Lands”, control of relief in, <a href="#Page_223">223</a>, <a href="#Page_224">224</a>.</p> - -<p class="pni">“Bad Land” topography, <a href="#Page_214">214</a>.</p> - -<p class="pni"><i>Bajir</i>, <a href="#Page_216">216</a>.</p> - -<p class="pni">Balance, between degradation and aggradation, <a href="#Page_161">161</a>.</p> - -<p class="pni">Bandai-san, dissection of, <a href="#Page_141">141</a>.</p> - -<p class="pni">Barchans, <a href="#Page_211">211</a>.</p> - -<p class="pni">Barrancoes, <a href="#Page_139">139</a>.</p> - -<p class="pni">Barrell, J., cited, <a href="#Page_221">221</a>, <a href="#Page_447">447</a>.</p> - -<p class="pni">Barrier beaches, <a href="#Page_240">240</a>;</p> -<p class="pnii">sections of, <a href="#Page_242">242</a>;</p> -<p class="pnii">uplifted, <a href="#Page_249">249</a>, <a href="#Page_250">250</a>.</p> - -<p class="pni">Barrier lakes, <a href="#Page_420">420</a>.</p> - -<p class="pni">Barriers, <a href="#Page_240">240</a>;</p> -<p class="pnii">mountain, in relation to glaciers, <a href="#Page_262">262</a>.</p> - -<p class="pni">Bars, <a href="#Page_240">240</a>.</p> - -<p class="pni">Basal conglomerate, <a href="#Page_37">37</a>, <a href="#Page_53">53</a>.</p> - -<p class="pni">Basalt, <a href="#Page_463">463</a>;</p> -<p class="pnii">faulted blocks of, <a href="#Page_58">58</a>;</p> -<p class="pnii">of Hawaii, <a href="#Page_105">105</a>.</p> - -<p class="pni">Base level, <a href="#Page_159">159</a>.</p> - -<p class="pni">Basin-range lakes, <a href="#Page_402">402</a>, <a href="#Page_403">403</a>.</p> - -<p class="pni">Basin Range structure, <a href="#Page_440">440</a>.</p> - -<p class="pni">Basins, flat bottomed, separating dunes, <a href="#Page_216">216</a>;</p> -<p class="pnii">of exudation, <a href="#Page_272">272</a>;</p> -<p><span class="pagenum"><a name="Page_490" id="Page_490">[490]</a></span></p><p class="pnii">of sedimentation, earlier, <a href="#Page_38">38</a>.</p> - -<p class="pni">Bastin, E. S., cited, <a href="#Page_210">210</a>.</p> - -<p class="pni">Batholites, <a href="#Page_143">143</a>.</p> - -<p class="pni">“Bath tubs”, <a href="#Page_395">395</a>.</p> - -<p class="pni">Beach pebbles, <a href="#Page_239">239</a>.</p> - -<p class="pni">Beach sand, <a href="#Page_206">206</a>, <a href="#Page_238">238</a>.</p> - -<p class="pni">Beaches, remaining from ice-dam lakes, <a href="#Page_410">410</a>;</p> -<p class="pnii">shingle, <a href="#Page_239">239</a>;</p> -<p class="pnii">storm, <a href="#Page_240">240</a>;</p> -<p class="pnii">uplifted, “feathering out” of, <a href="#Page_344">344</a>.</p> - -<p class="pni">Bedded structure of rocks, <a href="#Page_31">31</a>.</p> - -<p class="pni">Beede, J. W., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">“Bee-hive” mountains, <a href="#Page_380">380</a>, <a href="#Page_381">381</a>.</p> - -<p class="pni"><i>Belgica</i> expedition, <a href="#Page_289">289</a>.</p> - -<p class="pni">Belt of sea which divides land masses, <a href="#Page_11">11</a>.</p> - -<p class="pni">Berghaus, H., cited, <a href="#Page_424">424</a>.</p> - -<p class="pni">Bergschrund, <a href="#Page_370">370</a>.</p> - -<p class="pni">Berson, A., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Berthaut, General, cited, <a href="#Page_7">7</a>.</p> - -<p class="pni">“Bird-foot” delta, <a href="#Page_167">167</a>.</p> - -<p class="pni">“Biscuit cutting” effect of glacial sculpture, <a href="#Page_372">372</a>.</p> - -<p class="pni">Blackwelder, E., cited, <a href="#Page_318">318</a>.</p> - -<p class="pni">Block mountains, <a href="#Page_446">446</a>.</p> - -<p class="pni">Blocks, orographic, <a href="#Page_58">58</a>.</p> - -<p class="pni"><i>Bocchi</i>, <a href="#Page_125">125</a>.</p> - -<p class="pni">Bog, floating, <a href="#Page_429">429</a>.</p> - -<p class="pni">Bogs, of peat, <a href="#Page_429">429</a>, <a href="#Page_430">430</a>.</p> - -<p class="pni">Bonney, T. G., cited, <a href="#Page_146">146</a>.</p> - -<p class="pni">Borax deposits, in deserts, <a href="#Page_201">201</a>.</p> - -<p class="pni">Border drainage, about glaciers, <a href="#Page_316">316</a>, <a href="#Page_320">320</a>, <a href="#Page_321">321</a>.</p> - -<p class="pni">Border lakes, <a href="#Page_399">399</a>, <a href="#Page_414">414</a>.</p> - -<p class="pni">Bosses, <a href="#Page_143">143</a>.</p> - -<p class="pni">“Bottoms”, from entrenched meanders, <a href="#Page_173">173</a>.</p> - -<p class="pni">“Bowlder clay”, <a href="#Page_310">310</a>.</p> - -<p class="pni">“Bowlder pavement”, <a href="#Page_237">237</a>.</p> - -<p class="pni">Bowlders, faceted, <a href="#Page_310">310</a>;</p> -<p class="pnii">glacial, <a href="#Page_298">298</a>;</p> -<p class="pnii">“soled”, <a href="#Page_276">276</a>, <a href="#Page_310">310</a>;</p> -<p class="pnii">thrown up during earthquakes, <a href="#Page_69">69</a>.</p> - -<p class="pni">Bowlder trains, <a href="#Page_306">306</a>.</p> - -<p class="pni">Bowman, Isaiah, cited, <a href="#Page_179">179</a>.</p> - -<p class="pni">Box cañons, <a href="#Page_214">214</a>.</p> - -<p class="pni">Braided streams, <a href="#Page_280">280</a>.</p> - -<p class="pni">Branner, J. C., cited, <a href="#Page_6">6</a>, <a href="#Page_91">91</a>.</p> - -<p class="pni">“Bread-crust” lava projectiles, <a href="#Page_119">119</a>.</p> - -<p class="pni">Breakers, <a href="#Page_232">232</a>.</p> - -<p class="pni">Breccia, fault, <a href="#Page_60">60</a>.</p> - -<p class="pni">Bridges, nature of damage to, during earthquakes, <a href="#Page_75">75</a>, <a href="#Page_76">76</a>.</p> - -<p class="pni">Brigham, A. P., cited, <a href="#Page_424">424</a>.</p> - -<p class="pni">Brögger, W. C., cited, <a href="#Page_66">66</a>.</p> - -<p class="pni">Bruce, W. S., cited, <a href="#Page_290">290</a>, <a href="#Page_382">382</a>, <a href="#Page_399">399</a>, <a href="#Page_414">414</a>.</p> - -<p class="pni">Bryant, H. G., cited, <a href="#Page_289">289</a>.</p> - -<p class="pni">Buckley, E. R., cited, <a href="#Page_433">433</a>, <a href="#Page_434">434</a>.</p> - -<p class="pni">Built terraces, <a href="#Page_235">235</a>.</p> - -<p class="pni">Bunsen, cited, <a href="#Page_192">192</a>.</p> - -<p class="pni">Burns, G. P., cited, <a href="#Page_434">434</a>.</p> - -<p class="pni">Burton, W. K., cited, <a href="#Page_92">92</a>.</p> - -<p class="pni">Buttes, <a href="#Page_216">216</a>.</p> - -<p class="pni">Bysmalite, <a href="#Page_442">442</a>, <a href="#Page_447">447</a>.</p> - -<p class="pn"><span class="pl">C</span>alcareous ooze, <a href="#Page_36">36</a>.</p> - -<p class="pni">Calcareous sinter, <a href="#Page_184">184</a>.</p> - -<p class="pni">Calcareous tufa, <a href="#Page_464">464</a>.</p> - -<p class="pni">Calcite, <a href="#Page_455">455</a>.</p> - -<p class="pni">Caldera, <a href="#Page_405">405</a>, of composite volcanic cones, <a href="#Page_126">126</a>.</p> - -<p class="pni">Camiguin volcano, birth of, <a href="#Page_96">96</a>, <a href="#Page_97">97</a>.</p> - -<p class="pni">Campbell, M. R., cited, <a href="#Page_178">178</a>.</p> - -<p class="pni">Cañons, <a href="#Page_160">160</a>;</p> -<p class="pnii">box, <a href="#Page_214">214</a>.</p> - -<p class="pni">Capri, blue grotto of, <a href="#Page_257">257</a>, <a href="#Page_258">258</a>.</p> - -<p class="pni">Capture, river, <a href="#Page_175">175</a>, <a href="#Page_176">176</a>, <a href="#Page_179">179</a>.</p> - -<p class="pni">Carbonization, <a href="#Page_151">151</a>.</p> - -<p class="pni">Cascade Mountains, fissure eruptions of, <a href="#Page_102">102</a>.</p> - -<p class="pni">Cascade stairway, <a href="#Page_376">376</a>.</p> - -<p class="pni">Caspian Depression, <a href="#Page_14">14</a>.</p> - -<p class="pni">Cauliflower cloud, <a href="#Page_130">130</a>.</p> - -<p class="pni">Caverns, galleries directed by joints, <a href="#Page_182">182</a>;</p> -<p class="pnii">of limestone, <a href="#Page_182">182</a>, <a href="#Page_195">195</a>;</p> -<p class="pnii">refuge of predatory animals, <a href="#Page_185">185</a>.</p> - -<p class="pni">Caves, sea, <a href="#Page_234">234</a>.</p> - -<p class="pni">Cellular structure, of lava domes, <a href="#Page_112">112</a>.</p> - -<p class="pni">Centers of dispersion, of North American Pleistocene glaciers, <a href="#Page_298">298</a>.</p> - -<p class="pni">Centrosphere, <a href="#Page_8">8</a>.</p> - -<p class="pni">Cerussite, <a href="#Page_455">455</a>.</p> - -<p class="pni">Chaix, A., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">Chaix, E., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">Chalcopyrite, <a href="#Page_453">453</a>.</p> - -<p class="pni">Challenger expedition, <a href="#Page_38">38</a>, <a href="#Page_96">96</a>, <a href="#Page_97">97</a>, <a href="#Page_293">293</a>.</p> - -<p class="pni">Chamberlin, T. C., cited, <a href="#Page_29">29</a>, <a href="#Page_156">156</a>, <a href="#Page_191">191</a>, <a href="#Page_196">196</a>, <a href="#Page_205">205</a>, <a href="#Page_221">221</a>, <a href="#Page_222">222</a>, <a href="#Page_293">293</a>, <a href="#Page_295">295</a>, <a href="#Page_318">318</a>, <a href="#Page_319">319</a>, <a href="#Page_337">337</a>, <a href="#Page_339">339</a>.</p> - -<p class="pni">Character profiles, coast, due to uplift or depression, <a href="#Page_259">259</a>;</p> -<p class="pnii">composite, <a href="#Page_229">229</a>;</p> -<p class="pnii">directly due to volcanic agencies, <a href="#Page_145">145</a>, <a href="#Page_146">146</a>;</p> -<p class="pnii">from stream erosion in humid climates, <a href="#Page_177">177</a>;</p> -<p class="pnii">of arid lands, <a href="#Page_220">220</a>;</p> -<p class="pnii">of shore features, <a href="#Page_243">243</a>;</p> -<p class="pnii">referable to continental glaciers, <a href="#Page_318">318</a>;</p> -<p class="pnii">referable to mountain glaciers, <a href="#Page_379">379</a>.</p> - -<p class="pni">“Checkerboard topography”, <a href="#Page_226">226</a>.</p> - -<p class="pni">Chemical sediments, <a href="#Page_34">34</a>.</p> - -<p class="pni">Chicago outlet, <a href="#Page_331">331</a>.</p> - -<p class="pni">Chimneys, in “driftless area”, <a href="#Page_300">300</a>.</p> - -<p class="pni">Chimneys, shore feature, <a href="#Page_234">234</a>.</p> - -<p class="pni">China, loess of, <a href="#Page_207">207</a>.</p> - -<p class="pni">Chlorite, <a href="#Page_458">458</a>.</p> - -<p class="pni">Chlorite schist, <a href="#Page_465">465</a>.</p> - -<p class="pni">Cicatrice, from dissection of volcanoes, <a href="#Page_142">142</a>.</p> - -<p class="pni">Cinder cones, <a href="#Page_105">105</a>;</p> -<p class="pnii">corrugations upon, <a href="#Page_138">138</a>;</p> -<p class="pnii">diameter of crater in relation to violence of explosions, <a href="#Page_123">123</a>;</p> -<p><span class="pagenum"><a name="Page_491" id="Page_491">[491]</a></span></p><p class="pnii">grander eruptions of, <a href="#Page_117">117</a>;</p> -<p class="pnii">profiles of, <a href="#Page_123">123</a>;</p> -<p class="pnii">secondary, <a href="#Page_111">111</a>.</p> - -<p class="pni">Cinder eruptions, artificially simulated, <a href="#Page_122">122</a>.</p> - -<p class="pni">Cirques, <a href="#Page_371">371</a>;</p> -<p class="pnii">life history of, <a href="#Page_371">371</a>;</p> -<p class="pnii">subordinate, <a href="#Page_371">371</a>.</p> - -<p class="pni">Cities, destruction of, by drifting sand, <a href="#Page_218">218</a>.</p> - -<p class="pni">Clastic rocks, <a href="#Page_30">30</a>.</p> - -<p class="pni">Clay slate, <a href="#Page_466">466</a>.</p> - -<p class="pni">Cleavage, mineral, <a href="#Page_27">27</a>, <a href="#Page_450">450</a>;</p> -<p class="pnii">rock, <a href="#Page_44">44</a>.</p> - -<p class="pni">Clefts, volcanic, in Iceland, <a href="#Page_99">99</a>.</p> - -<p class="pni">Cliffs, notched, <a href="#Page_233">233</a>.</p> - -<p class="pni">Climatic conditions, in relation to mountain sculpture, <a href="#Page_443">443</a>.</p> - -<p class="pni">Clinometer, <a href="#Page_48">48</a>.</p> - -<p class="pni">Cloudbursts, in deserts, <a href="#Page_201">201</a>, <a href="#Page_212">212</a>.</p> - -<p class="pni">Cloud zones, <a href="#Page_268">268</a>, <a href="#Page_269">269</a>, <a href="#Page_294">294</a>.</p> - -<p class="pni">Coals, <a href="#Page_466">466</a>.</p> - -<p class="pni">Coast, Dalmatian, grottoes of, <a href="#Page_258">258</a>.</p> - -<p class="pni">Coast, elevation of, during earthquakes, <a href="#Page_80">80</a>;</p> -<p class="pnii">submergences of, during earthquakes, <a href="#Page_80">80</a>.</p> - -<p class="pni">Coastal plains, <a href="#Page_246">246</a>;</p> -<p class="pnii">belted, <a href="#Page_247">247</a>.</p> - -<p class="pni">Coast lines, even, <a href="#Page_246">246</a>;</p> -<p class="pnii">indicative of uplift or submergence, <a href="#Page_245">245</a>, <a href="#Page_246">246</a>;</p> -<p class="pnii">ragged, <a href="#Page_246">246</a>.</p> - -<p class="pni">Coast records, <a href="#Page_245">245</a>.</p> - -<p class="pni">Coasts, Atlantic and Pacific contrasted, <a href="#Page_438">438</a>;</p> -<p class="pnii">embayed, <a href="#Page_251">251</a>.</p> - -<p class="pni">Coast terraces, <a href="#Page_80">80</a>, <a href="#Page_250">250</a>, <a href="#Page_241">241</a>;</p> -<p class="pnii">uplift, effect of, on sediments, <a href="#Page_38">38</a>.</p> - -<p class="pni">Coats Land, shelf ice of, <a href="#Page_290">290</a>.</p> - -<p class="pni">Cobalt, in meteorites, <a href="#Page_23">23</a>.</p> - -<p class="pni">Cobb, Collier, cited, <a href="#Page_179">179</a>.</p> - -<p class="pni">Coigns, of earth’s tetrahedral figure, <a href="#Page_15">15</a>.</p> - -<p class="pni">Coleman, A. P., cited, <a href="#Page_318">318</a>.</p> - -<p class="pni">Colk lakes, <a href="#Page_408">408</a>, <a href="#Page_409">409</a>.</p> - -<p class="pni">Colks, scape, <a href="#Page_277">277</a>.</p> - -<p class="pni">Collet, L. W., cited, <a href="#Page_39">39</a>.</p> - -<p class="pni">Colorado desert, <a href="#Page_74">74</a>.</p> - -<p class="pni">Color, of minerals, <a href="#Page_450">450</a>.</p> - -<p class="pni">Cols, <a href="#Page_374">374</a>;</p> -<p class="pnii">origin of in cirque intersection, <a href="#Page_372">372</a>.</p> - -<p class="pni">Comb ridges, <a href="#Page_373">373</a>.</p> - -<p class="pni">Compass, geologist’s, <a href="#Page_47">47</a>, <a href="#Page_48">48</a>.</p> - -<p class="pni">Competent layer, <a href="#Page_42">42</a>;</p> -<p class="pnii">in relation to lava reservoirs, <a href="#Page_144">144</a>.</p> - -<p class="pni">Composite cones, <i>caldera</i> of, <a href="#Page_126">126</a>, <a href="#Page_127">127</a>.</p> - -<p class="pni">Composite groups of joints, <a href="#Page_57">57</a>.</p> - -<p class="pni">Composite volcanic cones, <a href="#Page_105">105</a>.</p> - -<p class="pni">Composition of earth, <a href="#Page_29">29</a>.</p> - -<p class="pni">Composition of the earth’s core, <a href="#Page_21">21</a>.</p> - -<p class="pni">Compression of a district during earthquakes, <a href="#Page_76">76</a>.</p> - -<p class="pni">Cones, alluvial, <a href="#Page_213">213</a>;</p> -<p class="pnii">cinder, <a href="#Page_105">105</a>;</p> -<p class="pnii">composite volcanic, <a href="#Page_105">105</a>.</p> - -<p class="pni">Conformable series, <a href="#Page_51">51</a>.</p> - -<p class="pni">Conglomerate, <a href="#Page_34">34</a>, <a href="#Page_463">463</a>;</p> -<p class="pnii">basal, <a href="#Page_37">37</a>, <a href="#Page_53">53</a>.</p> - -<p class="pni">Constructional topography, <a href="#Page_309">309</a>.</p> - -<p class="pni">Construction of buildings, in earthquake regions, <a href="#Page_89">89-91</a>.</p> - -<p class="pni">Continental glacier, behind rampart, <a href="#Page_281">281</a>;</p> -<p class="pnii">in Victoria Land, <a href="#Page_280">280-285</a>;</p> -<p class="pnii">of Antarctica, literature of, <a href="#Page_295">295</a>;</p> -<p class="pnii">of Greenland, <a href="#Page_271">271</a>;</p> -<p class="pnii">of Greenland, melting on margin of, <a href="#Page_278">278</a>;</p> -<p class="pnii">of Greenland, literature, <a href="#Page_295">295</a>.</p> - -<p class="pni">Continental glaciers, contrasted with mountain glaciers, <a href="#Page_266">266-268</a>;</p> -<p class="pnii">defined, <a href="#Page_266">266-267</a>;</p> -<p class="pnii">of “ice age”, <a href="#Page_297">297</a>;</p> -<p class="pnii">of ice age, cross section of, <a href="#Page_302">302</a>;</p> -<p class="pnii">nourishment of, <a href="#Page_283">283</a>, <a href="#Page_286">286</a>, <a href="#Page_295">295</a>;</p> -<p class="pnii">profiles of, <a href="#Page_267">267</a>.</p> - -<p class="pni">Continental platform, <a href="#Page_19">19</a>.</p> - -<p class="pni">Continental shelves, <a href="#Page_18">18</a>, <a href="#Page_19">19</a>;</p> -<p class="pnii">origin, <a href="#Page_232">232</a>.</p> - -<p class="pni">Continents, arrangement of, <a href="#Page_10">10</a>;</p> -<p class="pnii">development of, <a href="#Page_14">14</a>;</p> -<p class="pnii">increase in area of, through wave action, <a href="#Page_241">241</a>;</p> -<p class="pnii">past history of, <a href="#Page_14">14</a>.</p> - -<p class="pni">Contortions of the strata, <a href="#Page_40">40</a>.</p> - -<p class="pni">Contours, of topographic maps, <a href="#Page_62">62</a>.</p> - -<p class="pni">Contraction of earth’s surface, during earthquakes, <a href="#Page_74">74</a>.</p> - -<p class="pni">Contrary movements upon coasts, <a href="#Page_254">254</a>, <a href="#Page_257">257</a>.</p> - -<p class="pni">Convective zone, of atmosphere, <a href="#Page_270">270</a>.</p> - -<p class="pni">Conway, W. M., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Copernicus, cited, <a href="#Page_10">10</a>.</p> - -<p class="pni">Copper glance, <a href="#Page_455">455</a>.</p> - -<p class="pni">Coquina, <a href="#Page_35">35</a>.</p> - -<p class="pni">Cornish, Vaughan, cited, <a href="#Page_211">211</a>, <a href="#Page_222">222</a>, <a href="#Page_244">244</a>.</p> - -<p class="pni">Corrasion, <a href="#Page_162">162</a>.</p> - -<p class="pni">Corrosion, of rocks, <a href="#Page_156">156</a>.</p> - -<p class="pni">Coulée lakes, <a href="#Page_406">406</a>.</p> - -<p class="pni">Coves, <a href="#Page_233">233</a>, <a href="#Page_234">234</a>.</p> - -<p class="pni">Cracks, earthquake, <a href="#Page_74">74</a>.</p> - -<p class="pni">Crater, evolution of form of, <a href="#Page_128">128</a>.</p> - -<p class="pni">Crater lakes, <a href="#Page_405">405</a>, <a href="#Page_406">406</a>.</p> - -<p class="pni">Craterlets, <a href="#Page_84">84</a>;</p> -<p class="pnii">sections of, <a href="#Page_85">85</a>.</p> - -<p class="pni">Craters, mechanics of explosions in, <a href="#Page_115">115</a>.</p> - -<p class="pni">Crater, volcanic, <a href="#Page_95">95</a>.</p> - -<p class="pni">Credner, G. R., cited, <a href="#Page_179">179</a>.</p> - -<p class="pni">Crescentic levee lakes, <a href="#Page_416">416</a>, <a href="#Page_417">417</a>.</p> - -<p class="pni">Crestline, of an anticline, <a href="#Page_42">42</a>.</p> - -<p class="pni">Crevasse, marginal, on mountain glaciers, <a href="#Page_370">370</a>.</p> - -<p class="pni">Crevasses, in connection with river cut-offs, <a href="#Page_164">164</a>;</p> -<p class="pnii">on glaciers, <a href="#Page_391">391</a>.</p> - -<p class="pni">Cross, Whitman, cited, <a href="#Page_216">216</a>, <a href="#Page_441">441</a>, <a href="#Page_447">447</a>.</p> - -<p class="pni">Cross-bedded structure, <a href="#Page_37">37</a>.</p> - -<p class="pni">“Crystal cellars”, <a href="#Page_27">27</a>.</p> - -<p class="pni">Crystal form, of minerals, <a href="#Page_449">449</a>.</p> - -<p class="pni">Crystals, behavior under special treatment, <a href="#Page_24">24</a>, <a href="#Page_25">25</a>;</p> -<p class="pnii">essential nature of, <a href="#Page_23">23</a>;</p> -<p class="pnii">forms of, <a href="#Page_454">454</a>, <a href="#Page_457">457</a>;</p> -<p><span class="pagenum"><a name="Page_492" id="Page_492">[492]</a></span></p><p class="pnii">individuality of, <a href="#Page_24">24</a>;</p> -<p class="pnii">mutilated, later growth of, <a href="#Page_26">26</a>;</p> -<p class="pnii">symmetry of form of, <a href="#Page_23">23</a>.</p> - -<p class="pni">Crustal shortening, <a href="#Page_42">42</a>.</p> - -<p class="pni">Cuestas, <a href="#Page_246">246</a>, <a href="#Page_247">247</a>;</p> -<p class="pnii">south of Lake Ontario, <a href="#Page_361">361</a>, <a href="#Page_362">362</a>.</p> - -<p class="pni">Cut and built terrace, on steep shore of loose materials, <a href="#Page_237">237</a>.</p> - -<p class="pni">Cut-offs, of meanders, <a href="#Page_164">164</a>.</p> - -<p class="pni">Cut rock terraces, <a href="#Page_235">235</a>.</p> - -<p class="pni">Cuvier, cited, <a href="#Page_199">199</a>.</p> - -<p class="pni">Cvijić, J., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">Cycle of glaciation, <a href="#Page_263">263</a>, <a href="#Page_294">294</a>.</p> - -<p class="pni">Cycles, of glaciation, Pleistocene, <a href="#Page_297">297</a>;</p> -<p class="pnii">of stream meanders, <a href="#Page_163">163</a>.</p> - - -<p class="pn"><span class="pl">D</span>ana, J. D., cited, <a href="#Page_6">6</a>, <a href="#Page_104">104</a>, <a href="#Page_106">106</a>, <a href="#Page_109">109</a>, <a href="#Page_111">111</a>, <a href="#Page_146">146</a>, <a href="#Page_147">147</a>.</p> - -<p class="pni">Dana, E. S., cited, <a href="#Page_29">29</a>.</p> - -<p class="pni">Daly, R. A., cited, <a href="#Page_447">447</a>.</p> - -<p class="pni">Dante, cited, <a href="#Page_9">9</a>.</p> - -<p class="pni">Darton, N. H., cited, <a href="#Page_179">179</a>.</p> - -<p class="pni">Darwin, Charles, cited, <a href="#Page_199">199</a>, <a href="#Page_322">322</a>, <a href="#Page_323">323</a>, <a href="#Page_339">339</a>.</p> - -<p class="pni">Daubrée, A., cited, <a href="#Page_54">54</a>.</p> - -<p class="pni">David, T. W. E., cited, <a href="#Page_23">23</a>.</p> - -<p class="pni">Davis, C. A., cited, <a href="#Page_434">434</a>.</p> - -<p class="pni">Davis, W. M., cited, <a href="#Page_7">7</a>, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>, <a href="#Page_221">221</a>, <a href="#Page_247">247</a>, <a href="#Page_276">276</a>, <a href="#Page_317">317-319</a>, <a href="#Page_378">378</a>, <a href="#Page_382">382</a>.</p> - -<p class="pni">Deceptive unconformity, <a href="#Page_53">53</a>.</p> - -<p class="pni">Decomposition, <a href="#Page_149">149</a>, <a href="#Page_156">156</a>;</p> -<p class="pnii">mechanical results of, <a href="#Page_150">150</a>.</p> - -<p class="pni">Débris cones, <a href="#Page_395">395</a>.</p> - -<p class="pni">Deep sea deposits, <a href="#Page_36">36</a>, <a href="#Page_38">38</a>.</p> - -<p class="pni">Deflation, <a href="#Page_204">204</a>.</p> - -<p class="pni">Deforestation, in relation to agriculture, <a href="#Page_156">156</a>;</p> -<p class="pnii">of Karst region, <a href="#Page_188">188</a>;</p> -<p class="pnii">relation to erosion, <a href="#Page_157">157</a>.</p> - -<p class="pni">Degeneration, <a href="#Page_149">149</a>.</p> - -<p class="pni">De Geer, G., cited, <a href="#Page_351">351</a>, <a href="#Page_366">366</a>, <a href="#Page_410">410</a>.</p> - -<p class="pni">Degradation, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>.</p> - -<p class="pni">Dekkan, fissure eruptions of, <a href="#Page_101">101</a>.</p> - -<p class="pni">Delebecque, A., cited, <a href="#Page_424">424</a>.</p> - -<p class="pni">De Lorenzo, cited, <a href="#Page_125">125</a>, <a href="#Page_132">132</a>.</p> - -<p class="pni">Delta, “Bird-foot”, <a href="#Page_167">167</a>;</p> -<p class="pnii">bottom-set beds, <a href="#Page_167">167</a>;</p> -<p class="pnii">dry, <a href="#Page_213">213</a>;</p> -<p class="pnii">of Mississippi River, rate of growth of, <a href="#Page_168">168</a>.</p> - -<p class="pni">Delta deposits, manner of growth of, <a href="#Page_167">167</a>.</p> - -<p class="pni">Delta lakes, <a href="#Page_419">419</a>, <a href="#Page_420">420</a>.</p> - -<p class="pni">Delta region, of a river, <a href="#Page_35">35</a>.</p> - -<p class="pni">Deltas, abnormal, below outlets of lakes, <a href="#Page_431">431</a>;</p> -<p class="pnii">in relation to agriculture, <a href="#Page_166">166</a>;</p> -<p class="pnii">in relation to population, <a href="#Page_166">166</a>;</p> -<p class="pnii">lake, <a href="#Page_428">428</a>;</p> -<p class="pnii">of rivers, <a href="#Page_165">165</a>, <a href="#Page_166">166</a>, <a href="#Page_179">179</a>;</p> -<p class="pnii">sections of, <a href="#Page_168">168</a>.</p> - -<p class="pni">Dendritic glaciers, <a href="#Page_383">383</a>, <a href="#Page_385">385</a>, <a href="#Page_386">386</a>.</p> - -<p class="pni">Deniston, cited, <a href="#Page_121">121</a>.</p> - -<p class="pni">Deposition, in zones about desert, <a href="#Page_216">216</a>, <a href="#Page_217">217</a>.</p> - -<p class="pni">Deposits, aktian, <a href="#Page_36">36</a>;</p> -<p class="pnii">chemical, <a href="#Page_34">34</a>;</p> -<p class="pnii">continental, <a href="#Page_37">37</a>;</p> -<p class="pnii">deep sea, <a href="#Page_36">36</a>, <a href="#Page_38">38</a>;</p> -<p class="pnii">delta, manner of growth of, <a href="#Page_167">167</a>;</p> -<p class="pnii">fluviatile, <a href="#Page_35">35</a>;</p> -<p class="pnii">fluvio-glacial, <a href="#Page_31">31</a>, <a href="#Page_310">310</a>;</p> -<p class="pnii">in valley vacated by glacier, <a href="#Page_398">398</a>;</p> -<p class="pnii">glacial, <a href="#Page_31">31</a>;</p> -<p class="pnii">lacustrine, <a href="#Page_35">35</a>, <a href="#Page_217">217</a>;</p> -<p class="pnii">littoral, <a href="#Page_36">36</a>;</p> -<p class="pnii">marine, <a href="#Page_35">35</a>;</p> -<p class="pnii">mechanical, <a href="#Page_34">34</a>;</p> -<p class="pnii">organic, <a href="#Page_34">34</a>;</p> -<p class="pnii">salt, <a href="#Page_217">217</a>;</p> -<p class="pnii">shoal water, <a href="#Page_26">26</a>;</p> -<p class="pnii">sinter, <a href="#Page_184">184</a>;</p> -<p class="pnii">terrigenous, <a href="#Page_36">36</a>.</p> - -<p class="pni">Derangement of water flow, during earthquakes, <a href="#Page_83">83</a>, <a href="#Page_84">84</a>.</p> - -<p class="pni">Derwies, V. de, cited, <a href="#Page_447">447</a>.</p> - -<p class="pni">Descent of ground water, <a href="#Page_180">180</a>.</p> - -<p class="pni">Desert, due to deforestation, <a href="#Page_156">156</a>;</p> -<p class="pnii">erosion in, <a href="#Page_214">214</a>, <a href="#Page_222">222</a>;</p> -<p class="pnii">law of, <a href="#Page_197">197</a>.</p> - -<p class="pni">Desert lakes, <a href="#Page_423">423</a>.</p> - -<p class="pni">Desert landscapes, features in, <a href="#Page_209">209</a>.</p> - -<p class="pni">Desert rains, <a href="#Page_212">212</a>.</p> - -<p class="pni">Desert rocks, red color of, <a href="#Page_222">222</a>.</p> - -<p class="pni">Desert varnish, <a href="#Page_201">201</a>, <a href="#Page_222">222</a>.</p> - -<p class="pni">Deserts, former shore lines in, <a href="#Page_198">198</a>;</p> -<p class="pnii">self-registering gauge of past climates, <a href="#Page_198">198</a>.</p> - -<p class="pni">Destructional topography, <a href="#Page_309">309</a>.</p> - -<p class="pni">Detection of plunging folds, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>.</p> - -<p class="pni">Detonations, during Vulcanian eruptions, <a href="#Page_131">131</a>.</p> - -<p class="pni">Device, to simulate building of cinder cones, <a href="#Page_122">122</a>.</p> - -<p class="pni">Diabase, <a href="#Page_462">462</a>.</p> - -<p class="pni">Diagram, to illustrate formation of lava reservoirs, <a href="#Page_143">143</a>.</p> - -<p class="pni">Diagrams for comparison of fold types, <a href="#Page_42">42</a>;</p> -<p class="pnii">to show the effect of spheroidal weathering, <a href="#Page_150">150</a>.</p> - -<p class="pni">Diamonds, in the drift, <a href="#Page_307">307</a>.</p> - -<p class="pni">Diffission, <a href="#Page_204">204</a>.</p> - -<p class="pni">Dikes, hollow, <a href="#Page_140">140</a>;</p> -<p class="pnii">in China, <a href="#Page_167">167</a>;</p> -<p class="pnii">in Holland, <a href="#Page_166">166</a>;</p> -<p class="pnii">from volcanic dissection, <a href="#Page_140">140</a>.</p> - -<p class="pni">Diller, J. S., cited, <a href="#Page_39">39</a>, <a href="#Page_425">425</a>.</p> - -<p class="pni">“Diluvium”, <a href="#Page_305">305</a>.</p> - -<p class="pni">Dimples, on margin of continental glaciers, <a href="#Page_272">272</a>.</p> - -<p class="pni">Dip, <a href="#Page_46">46</a>.</p> - -<p class="pni">Dirt cones, <a href="#Page_396">396</a>.</p> - -<p class="pni">Disintegration, <a href="#Page_156">156</a>;</p> -<p class="pnii">of rocks in deserts, <a href="#Page_202">202</a>;</p> -<p class="pnii">through root expansion, <a href="#Page_154">154</a>;</p> -<p class="pnii">through tree growth, <a href="#Page_154">154</a>, <a href="#Page_155">155</a>.</p> - -<p class="pni">Dislocations, marginal, about deserts, <a href="#Page_212">212</a>.</p> - -<p class="pni">Dispersion of the drift, <a href="#Page_304">304-309</a>, <a href="#Page_319">319</a>.</p> - -<p class="pni">Displacement, total, on faults, <a href="#Page_59">59</a>.</p> - -<p class="pni">Dissection of volcanoes, <a href="#Page_139">139</a>.</p> - -<p class="pni">Distributaries, on alluvial fans, <a href="#Page_213">213</a>, <a href="#Page_220">220</a>.</p> - -<p class="pni">Divides, <a href="#Page_170">170</a>;</p> -<p class="pnii">migration of, <a href="#Page_175">175</a>.</p> - -<p><span class="pagenum"><a name="Page_493" id="Page_493">[493]</a></span></p><p class="pni">Dolines, of Karst region, <a href="#Page_187">187</a>, <a href="#Page_422">422</a>.</p> - -<p class="pni">Dolomite, <a href="#Page_465">465</a>.</p> - -<p class="pni">Dolomites, <a href="#Page_203">203</a>, <a href="#Page_228">228</a>, <a href="#Page_445">445</a>.</p> - -<p class="pni">Domed mountains of uplift, <a href="#Page_441">441</a>.</p> - -<p class="pni">Dome structure, of granite masses, <a href="#Page_152">152</a>, <a href="#Page_157">157</a>.</p> - -<p class="pni">Domes, lava, <a href="#Page_105">105</a>.</p> - -<p class="pni">Dovetailing, of sea and land, <a href="#Page_11">11</a>, <a href="#Page_17">17</a>.</p> - -<p class="pni">Drainage, changes of, due to glaciation, <a href="#Page_336">336-338</a>;</p> -<p class="pnii">haphazard, of glaciated area, <a href="#Page_301">301</a>;</p> -<p class="pnii">interference of glaciers with, <a href="#Page_320">320</a>;</p> -<p class="pnii">of glaciers, <a href="#Page_397">397</a>;</p> -<p class="pnii">reversals of, due to glaciation, <a href="#Page_337">337</a>, <a href="#Page_338">338</a>;</p> -<p class="pnii">trellis, <a href="#Page_175">175</a>.</p> - -<p class="pni">Drainage lines, control of, by fractures, <a href="#Page_224">224</a>.</p> - -<p class="pni">Drainage networks, controlled by fractures, <a href="#Page_225">225</a>, <a href="#Page_226">226</a>;</p> -<p class="pnii">repeating pattern in, <a href="#Page_225">225</a>.</p> - -<p class="pni">Drake, Sir Francis, circumnavigation of the globe, <a href="#Page_10">10</a>.</p> - -<p class="pni"><i>Dreikanten</i>, <a href="#Page_205">205</a>.</p> - -<p class="pni">Driblet cones, <a href="#Page_104">104</a>, <a href="#Page_125">125</a>;</p> -<p class="pnii">of Kilauea, <a href="#Page_107">107</a>.</p> - -<p class="pni">“Drift”, <a href="#Page_305">305</a>.</p> - -<p class="pni">Drift, assorted, <a href="#Page_309">309</a>;</p> -<p class="pnii">dispersion of, <a href="#Page_304">304-309</a>;</p> -<p class="pnii">englacial, <a href="#Page_277">277</a>, <a href="#Page_278">278</a>;</p> -<p class="pnii">unassorted, <a href="#Page_309">309</a>.</p> - -<p class="pni">“Driftless area”, <a href="#Page_300">300</a>, <a href="#Page_313">313</a>, <a href="#Page_318">318</a>.</p> - -<p class="pni">Driftless area, map of, <a href="#Page_298">298</a>.</p> - -<p class="pni">Drift sites, <a href="#Page_368">368</a>, <a href="#Page_369">369</a>.</p> - -<p class="pni">Drowned rivers, <a href="#Page_251">251</a>.</p> - -<p class="pni">Drumlins, <a href="#Page_311">311</a>, <a href="#Page_316">316</a>, <a href="#Page_317">317</a>, <a href="#Page_399">399</a>.</p> - -<p class="pni">Dry deltas, <a href="#Page_213">213</a>.</p> - -<p class="pni">Drygalski, E. von, cited, <a href="#Page_273">273</a>, <a href="#Page_279">279</a>, <a href="#Page_295">295</a>, <a href="#Page_296">296</a>.</p> - -<p class="pni">Dry weathering, in deserts, <a href="#Page_201">201</a>.</p> - -<p class="pni">Dune, war with oasis, <a href="#Page_216">216</a>.</p> - -<p class="pni">Dune lakes, <a href="#Page_421">421</a>.</p> - -<p class="pni">Dunes, <a href="#Page_222">222</a>;</p> -<p class="pnii">forms of, <a href="#Page_210">210</a>, <a href="#Page_211">211</a>;</p> -<p class="pnii">in relation to obstructions, <a href="#Page_209">209</a>, <a href="#Page_210">210</a>;</p> -<p class="pnii">stopped by vegetation, <a href="#Page_211">211</a>;</p> -<p class="pnii">wandering, <a href="#Page_209">209</a>, <a href="#Page_211">211</a>.</p> - -<p class="pni">Dust, carried out of desert, <a href="#Page_206">206</a>, <a href="#Page_222">222</a>;</p> -<p class="pnii">volcanic, <a href="#Page_122">122</a>.</p> - -<p class="pni">Dust wells, <a href="#Page_395">395</a>.</p> - -<p class="pni">Dutton, C. E., cited, <a href="#Page_85">85</a>, <a href="#Page_92">92</a>, <a href="#Page_178">178</a>, <a href="#Page_200">200</a>, <a href="#Page_222">222</a>, <a href="#Page_447">447</a>.</p> - -<p class="pn"><span class="pl">E</span>arlier figures of the earth, <a href="#Page_14">14</a>.</p> - -<p class="pni">Earth, a magnet, <a href="#Page_23">23</a>;</p> -<p class="pnii">composition of, <a href="#Page_20">20</a>;</p> -<p class="pnii">oblateness of, <a href="#Page_10">10</a>;</p> -<p class="pnii">rigidity of, <a href="#Page_20">20</a>, <a href="#Page_21">21</a>, <a href="#Page_29">29</a>;</p> -<p class="pnii">scale of its elevations, <a href="#Page_10">10</a>, <a href="#Page_11">11</a>;</p> -<p class="pnii">theories of origin of, <a href="#Page_20">20</a>, <a href="#Page_29">29</a>;</p> -<p class="pnii">surface shell, chemical constitution of, <a href="#Page_23">23</a>;</p> -<p class="pnii">surface shell, response to load, <a href="#Page_340">340</a>.</p> - -<p class="pni">Earth features, shaped by running water, <a href="#Page_169">169</a>.</p> - -<p class="pni">Earth figure, evolution of ideas concerning, <a href="#Page_9">9</a>.</p> - -<p class="pni">Earthquake cracks, <a href="#Page_74">74</a>.</p> - -<p class="pni">Earthquake fountains, <a href="#Page_190">190</a>.</p> - -<p class="pni">Earthquake lakes, <a href="#Page_404">404</a>.</p> - -<p class="pni">Earthquake, of Alaska, 1899, <a href="#Page_72">72</a>, <a href="#Page_77">77</a>, <a href="#Page_79">79</a>, <a href="#Page_80">80</a>, <a href="#Page_81">81</a>;</p> -<p class="pnii">of Assam, 1897, <a href="#Page_72">72</a>, <a href="#Page_77">77</a>;</p> -<p class="pnii">of California, 1906, <a href="#Page_70">70</a>, <a href="#Page_72">72</a>, <a href="#Page_73">73</a>, <a href="#Page_74">74</a>, <a href="#Page_90">90</a>, <a href="#Page_91">91</a>;</p> -<p class="pnii">of Casamicciola, 1883, <a href="#Page_87">87</a>;</p> -<p class="pnii">of Costa Rica, 1910, <a href="#Page_68">68</a>;</p> -<p class="pnii">of India, 1819, <a href="#Page_84">84</a>;</p> -<p class="pnii">of Jamaica, 1692, <a href="#Page_80">80</a>;</p> -<p class="pnii">of Jamaica, 1907, <a href="#Page_80">80</a>;</p> -<p class="pnii">of Japan, 1891, <a href="#Page_72">72</a>, <a href="#Page_75">75</a>;</p> -<p class="pnii">of lower Mississippi Valley, 1811, <a href="#Page_83">83</a>;</p> -<p class="pnii">of Messina, 1908, <a href="#Page_68">68</a>;</p> -<p class="pnii">of Owens Valley, California, 1872, <a href="#Page_73">73</a>, <a href="#Page_77">77</a>, <a href="#Page_78">78</a>, <a href="#Page_79">79</a>;</p> -<p class="pnii">of Servia, 1904, <a href="#Page_84">84</a>;</p> -<p class="pnii">of South Carolina, 1886, <a href="#Page_85">85</a>.</p> - -<p class="pni">Earthquake shocks, heavy over loose foundations, <a href="#Page_88">88</a>.</p> - -<p class="pni">Earthquakes, aftershocks of, <a href="#Page_83">83</a>;</p> -<p class="pnii">associated with growing mountains, <a href="#Page_86">86</a>;</p> -<p class="pnii">changes in earth’s surface during, <a href="#Page_71">71</a>;</p> -<p class="pnii">connected with lines of fracture, <a href="#Page_86">86</a>;</p> -<p class="pnii">descriptive reports upon, <a href="#Page_92">92</a>;</p> -<p class="pnii">due to adjustments between blocks of shell, <a href="#Page_78">78</a>, <a href="#Page_79">79</a>;</p> -<p class="pnii">faults and fissures, <a href="#Page_71">71</a>;</p> -<p class="pnii">focused at fault intersections, <a href="#Page_87">87</a>;</p> -<p class="pnii">fountains during, <a href="#Page_83">83</a>, <a href="#Page_86">86</a>;</p> -<p class="pnii">localized at corners of earth blocks, <a href="#Page_87">87</a>;</p> -<p class="pnii">manifestations of changes in level, <a href="#Page_68">68</a>;</p> -<p class="pnii">nature of shocks, <a href="#Page_67">67</a>;</p> -<p class="pnii">of Ischia, localization of, <a href="#Page_87">87</a>;</p> -<p class="pnii">shown by coast terraces, <a href="#Page_250">250</a>;</p> -<p class="pnii">special lines of heavy shock, <a href="#Page_86">86</a>;</p> -<p class="pniii">in unstable areas of earth’s crust, <a href="#Page_86">86</a>;</p> -<p class="pnii">wave motions of, <a href="#Page_68">68</a>;</p> -<p class="pnii">zones in distribution of, <a href="#Page_86">86</a>.</p> - -<p class="pni">Earth relief, repeating patterns in, <a href="#Page_223">223</a>.</p> - -<p class="pni">Eckert, cited, <a href="#Page_188">188</a>.</p> - -<p class="pni">Effect of contraction upon a spherical body, <a href="#Page_13">13</a>.</p> - -<p class="pni">Egg-spinning demonstration of earth rigidity, <a href="#Page_20">20</a>.</p> - -<p class="pni">“Elevation-crater” theory of volcanoes, <a href="#Page_95">95</a>, <a href="#Page_139">139</a>.</p> - -<p class="pni">Embankments, shore, <a href="#Page_240">240</a>.</p> - -<p class="pni">Embayed coasts, <a href="#Page_251">251</a>.</p> - -<p class="pni">Emerson, B. K., cited, <a href="#Page_19">19</a>.</p> - -<p class="pni">End moraines, <a href="#Page_394">394</a>.</p> - -<p class="pni">Engell, M. C., cited, <a href="#Page_296">296</a>.</p> - -<p class="pni">Englacial débris, <a href="#Page_393">393</a>.</p> - -<p class="pni">Englacial drift, <a href="#Page_277">277</a>, <a href="#Page_278">278</a>.</p> - -<p class="pni"><i>Entonnoirs</i>, <a href="#Page_182">182</a>.</p> - -<p class="pni">Entrenchment of meanders, <a href="#Page_172">172</a>, <a href="#Page_173">173</a>, <a href="#Page_179">179</a>.</p> - -<p class="pni">Eolian sand, <a href="#Page_206">206</a>.</p> - -<p class="pni">Eolian sediments, <a href="#Page_30">30</a>.</p> - -<p class="pni">Erosional unconformity, <a href="#Page_53">53</a>.</p> - -<p class="pni">Erosion cycle, <a href="#Page_159">159</a>.</p> - -<p class="pni">Erosion, effect of, in adding curves to landscape, <a href="#Page_65">65</a>;</p> -<p><span class="pagenum"><a name="Page_494" id="Page_494">[494]</a></span></p><p class="pnii">glacial, in contrast with normal weathering, <a href="#Page_377">377</a>;</p> -<p class="pnii">in desert, <a href="#Page_214">214</a>;</p> -<p class="pnii">shadow, <a href="#Page_206">206</a>;</p> -<p class="pnii">stream, as modified by resistant rocks, <a href="#Page_174">174</a>.</p> - -<p class="pni">“Erratic blocks”, <a href="#Page_304">304</a>.</p> - -<p class="pni">Eruptions, Strombolian, <a href="#Page_117">117</a>;</p> -<p class="pnii">Vulcanian, <a href="#Page_117">117</a>, <a href="#Page_125">125</a>.</p> - -<p class="pni">Escarpments, from faults, <a href="#Page_59">59</a>.</p> - -<p class="pni">Eskers, <a href="#Page_311">311</a>, <a href="#Page_315">315</a>, <a href="#Page_316">316</a>, <a href="#Page_363">363</a>.</p> - -<p class="pni">Estes, L. A., cited, <a href="#Page_93">93</a>.</p> - -<p class="pni">Estuaries, <a href="#Page_251">251</a>.</p> - -<p class="pni">Etna, eruption of 1669, <a href="#Page_122">122</a>.</p> - -<p class="pni">Evolution, doctrine of, in connection with fossils, <a href="#Page_38">38</a>.</p> - -<p class="pni">Evolution of ideas concerning the earth’s figure, <a href="#Page_9">9</a>.</p> - -<p class="pni">Exfoliation, <a href="#Page_151">151</a>, <a href="#Page_203">203</a>.</p> - -<p class="pni">Expanded foot glaciers, <a href="#Page_383">383</a>, <a href="#Page_385">385</a>.</p> - -<p class="pni">Experiment, to illustrate relation of earthquake shocks to foundations, <a href="#Page_88">88</a>.</p> - -<p class="pni">Experiments, on fracture and flow, <a href="#Page_40">40</a>, <a href="#Page_41">41</a>;</p> -<p class="pnii">for demonstration of earthquakes, <a href="#Page_81">81</a>, <a href="#Page_82">82</a>.</p> - -<p class="pni">Exposures, rock, <a href="#Page_46">46</a>.</p> - -<p class="pni">Extrusive rocks, <a href="#Page_463">463</a>.</p> - -<p class="pn"><span class="pl">F</span>airbanks, H. W., cited, <a href="#Page_155">155</a>, <a href="#Page_170">170</a>, <a href="#Page_174">174</a>, <a href="#Page_201">201</a>, <a href="#Page_205">205</a>, <a href="#Page_214">214</a>, <a href="#Page_224">224</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>, <a href="#Page_250">250</a>, <a href="#Page_260">260</a>, <a href="#Page_302">302</a>, <a href="#Page_375">375</a>, <a href="#Page_406">406</a>, <a href="#Page_413">413</a>, <a href="#Page_429">429</a>.</p> - -<p class="pni">Fairchild, H. L., cited, <a href="#Page_339">339</a>.</p> - -<p class="pni">Falls, “Bridal veil”, <a href="#Page_378">378</a>.</p> - -<p class="pni">Falls, ribbon, <a href="#Page_378">378</a>.</p> - -<p class="pni">Fan, alluvial, <a href="#Page_213">213</a>.</p> - -<p class="pni">Farrington, O. C., cited, <a href="#Page_29">29</a>.</p> - -<p class="pni">Fault, drag upon, <a href="#Page_60">60</a>.</p> - -<p class="pni">Fault breccia, <a href="#Page_60">60</a>.</p> - -<p class="pni">Fault topography, <a href="#Page_65">65</a>.</p> - -<p class="pni">Faults, <a href="#Page_58">58</a>, <a href="#Page_440">440</a>;</p> -<p class="pnii">during earthquakes, <a href="#Page_71">71</a>;</p> -<p class="pnii">earthquake, change in throw upon, <a href="#Page_76">76</a>, <a href="#Page_77">77</a>, <a href="#Page_78">78</a>;</p> -<p class="pnii">earthquake, disappear in loose materials, <a href="#Page_73">73</a>;</p> -<p class="pnii">earthquake, of small displacements, <a href="#Page_74">74</a>;</p> -<p class="pnii">earthquake, plan of, <a href="#Page_76">76</a>, <a href="#Page_78">78</a>;</p> -<p class="pnii">illusory nature of, <a href="#Page_59">59</a>;</p> -<p class="pnii">methods of detecting, <a href="#Page_59">59</a>;</p> -<p class="pnii">post-glacial, <a href="#Page_74">74</a>;</p> -<p class="pnii">relation of escarpments to, <a href="#Page_60">60</a>;</p> -<p class="pnii">shown by changes in strike and dip, <a href="#Page_61">61</a>;</p> -<p class="pnii">shown by offsets, <a href="#Page_61">61</a>.</p> - -<p class="pni">Feldspars, <a href="#Page_456">456</a>.</p> - -<p class="pni">Fenneman, N. M., cited, <a href="#Page_424">424</a>, <a href="#Page_425">425</a>.</p> - -<p class="pni">Festoons of mountain arcs, <a href="#Page_435">435</a>, <a href="#Page_436">436</a>.</p> - -<p class="pni">Field ice, <a href="#Page_286">286</a>.</p> - -<p class="pni">Field map, geological, <a href="#Page_62">62</a>, <a href="#Page_63">63</a>.</p> - -<p class="pni">Figure of the earth, the, <a href="#Page_8">8</a>.</p> - -<p class="pni">Figures, earlier, of the earth, <a href="#Page_14">14</a>;</p> -<p class="pnii">earth, evolution of, <a href="#Page_15">15</a>.</p> - -<p class="pni">Figure toward which the earth is tending, <a href="#Page_12">12</a>.</p> - -<p class="pni">“Fire girdle” of the Pacific, <a href="#Page_98">98</a>.</p> - -<p class="pni">Firn, <a href="#Page_369">369</a>.</p> - -<p class="pni">Fissure eruptions, of volcanoes, <a href="#Page_101">101</a>.</p> - -<p class="pni">Fissures, during earthquakes, <a href="#Page_71">71</a>;</p> -<p class="pnii">earthquake, <a href="#Page_74">74</a>;</p> -<p class="pnii">in connection with volcanoes, <a href="#Page_99">99-101</a>.</p> - -<p class="pni">Fissure springs, <a href="#Page_61">61</a>, <a href="#Page_190">190</a>, <a href="#Page_195">195</a>.</p> - -<p class="pni">Fjords, <a href="#Page_290">290</a>, <a href="#Page_340">340</a>.</p> - -<p class="pni">“Float copper”, <a href="#Page_305">305</a>.</p> - -<p class="pni">Flooded portions of continents, <a href="#Page_18">18</a>.</p> - -<p class="pni">Flood plain, <a href="#Page_178">178</a>;</p> -<p class="pnii">manner of grading of, <a href="#Page_162">162</a>.</p> - -<p class="pni">Floors of hydrosphere and atmosphere, <a href="#Page_18">18</a>.</p> - -<p class="pni">Flow, experiments on, <a href="#Page_41">41</a>;</p> -<p class="pnii">zone of, <a href="#Page_40">40</a>.</p> - -<p class="pni">Flow texture, of extrusive rocks, <a href="#Page_33">33</a>.</p> - -<p class="pni">Fluviatile deposits, <a href="#Page_35">35</a>.</p> - -<p class="pni">Fluvio-glacial deposits, <a href="#Page_31">31</a>.</p> - -<p class="pni">Fluxion texture, of extrusive rocks, <a href="#Page_33">33</a>.</p> - -<p class="pni">Folds, analysis of, <a href="#Page_54">54</a>;</p> -<p class="pnii">comparison of shapes of, <a href="#Page_44">44</a>;</p> -<p class="pnii">mutilated, restoration of, <a href="#Page_45">45</a>;</p> -<p class="pnii">pitching, <a href="#Page_43">43</a>;</p> -<p class="pnii">secondary, <a href="#Page_44">44</a>;</p> -<p class="pnii">shapes of, <a href="#Page_43">43</a>.</p> - -<p class="pni">Fold topography, <a href="#Page_65">65</a>.</p> - -<p class="pni">Forbes, J. D., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Fore-set beds, <a href="#Page_167">167</a>.</p> - -<p class="pni">Forest, destruction of, in relation to agriculture, <a href="#Page_156">156</a>.</p> - -<p class="pni">Formation of lava reservoirs, <a href="#Page_143">143</a>.</p> - -<p class="pni">Formations, measurement of thickness of, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>.</p> - -<p class="pni">Fort Snelling, on Warren River, <a href="#Page_327">327</a>, <a href="#Page_331">331</a>.</p> - -<p class="pni">Fosses, glacial, <a href="#Page_281">281</a>, <a href="#Page_314">314</a>;</p> -<p class="pnii">in connection with peat bogs, <a href="#Page_430">430</a>.</p> - -<p class="pni">Fracture control, of drainage lines, <a href="#Page_224">224</a>.</p> - -<p class="pni">Fracture, experiments on, <a href="#Page_41">41</a>;</p> -<p class="pnii">of minerals, <a href="#Page_450">450</a>;</p> -<p class="pnii">zone of, <a href="#Page_40">40</a>, <a href="#Page_46">46</a>.</p> - -<p class="pni">Fractures, in rocks, shown by rectilinear lines on map, <a href="#Page_65">65</a>;</p> -<p class="pnii">system of, <a href="#Page_55">55</a>.</p> - -<p class="pni">Free, E. E., cited, <a href="#Page_222">222</a>.</p> - -<p class="pni">Free waves, <a href="#Page_232">232</a>.</p> - -<p class="pni">Fretted upland, <a href="#Page_372">372</a>, <a href="#Page_373">373</a>.</p> - -<p class="pni">Frost, prying work of, <a href="#Page_152">152</a>.</p> - -<p class="pni">Frost action, <a href="#Page_223">223</a>.</p> - -<p class="pni">Frost snow, <a href="#Page_285">285</a>.</p> - -<p class="pni">Fuller, M. L., cited, <a href="#Page_157">157</a>, <a href="#Page_195">195</a>.</p> - -<p class="pni">Fumeroles, <a href="#Page_97">97</a>.</p> - - -<p class="pn"><span class="pl">G</span>abbro, <a href="#Page_462">462</a>.</p> - -<p class="pni">Gabled façade, in desert landscapes, <a href="#Page_221">221</a>, <a href="#Page_443">443</a>.</p> - -<p class="pni">Galenite, <a href="#Page_453">453</a>.</p> - -<p class="pni">Gannett, Henry, cited, <a href="#Page_178">178</a>, <a href="#Page_386">386</a>.</p> - -<p class="pni">Gaps, water, <a href="#Page_176">176</a>;</p> -<p class="pnii">wind, <a href="#Page_176">176</a>.</p> - -<p class="pni">Garnet, <a href="#Page_459">459</a>.</p> - -<p class="pni">Gautier, E. F., cited, <a href="#Page_221">221</a>.</p> - -<p class="pni">Geikie, A., cited, <a href="#Page_6">6</a>, <a href="#Page_7">7</a>, <a href="#Page_148">148</a>, <a href="#Page_178">178</a>, <a href="#Page_244">244</a>, <a href="#Page_318">318</a>.</p> - -<p class="pni">Geikie, James, cited, <a href="#Page_6">6</a>, <a href="#Page_318">318</a>.</p> - -<p><span class="pagenum"><a name="Page_495" id="Page_495">[495]</a></span></p><p class="pni">Geoid, departure from spherical surface of, <a href="#Page_10">10</a>.</p> - -<p class="pni">Geological map, <a href="#Page_46">46</a>, <a href="#Page_54">54</a>;</p> -<p class="pnii">areal, <a href="#Page_62">62</a>, <a href="#Page_63">63</a>;</p> -<p class="pnii">base of, <a href="#Page_61">61</a>;</p> -<p class="pnii">field, <a href="#Page_62">62</a>, <a href="#Page_63">63</a>.</p> - -<p class="pni">Geological section, <a href="#Page_46">46</a>, <a href="#Page_47">47</a>.</p> - -<p class="pni">Geology, defined, <a href="#Page_1">1</a>.</p> - -<p class="pni">Geyserite, <a href="#Page_194">194</a>.</p> - -<p class="pni">Geysers, <a href="#Page_191">191-194</a>;</p> -<p class="pnii">effect of plugging with sod, <a href="#Page_193">193</a>;</p> -<p class="pnii">in relation to drainage lines, <a href="#Page_191">191</a>;</p> -<p class="pnii">soaping of, <a href="#Page_194">194</a>.</p> - -<p class="pni"><i>Geysir</i>, <a href="#Page_192">192</a>.</p> - -<p class="pni">Gilbert, G. K., cited, <a href="#Page_93">93</a>, <a href="#Page_148">148</a>, <a href="#Page_157">157</a>, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>, <a href="#Page_198">198</a>, <a href="#Page_221">221</a>, <a href="#Page_224">224</a>, <a href="#Page_240">240</a>, <a href="#Page_244">244</a>, <a href="#Page_294">294</a>, <a href="#Page_344">344</a>, <a href="#Page_345">345</a>, <a href="#Page_347">347</a>, <a href="#Page_350">350</a>, <a href="#Page_355">355</a>, <a href="#Page_356">356</a>, <a href="#Page_357">357</a>, <a href="#Page_358">358</a>, <a href="#Page_359">359</a>, <a href="#Page_362">362</a>, <a href="#Page_366">366</a>, <a href="#Page_370">370</a>, <a href="#Page_381">381</a>, <a href="#Page_434">434</a>, <a href="#Page_446">446</a>, <a href="#Page_447">447</a>.</p> - -<p class="pni"><i>Gjás</i>, volcano fissures in Iceland, <a href="#Page_99">99</a>.</p> - -<p class="pni">Glacial anticyclone, <a href="#Page_284">284</a>.</p> - -<p class="pni">Glacial deposits, <a href="#Page_30">30</a>, <a href="#Page_31">31</a>.</p> - -<p class="pni">Glacial fringe, of Grant Land, <a href="#Page_285">285</a>.</p> - -<p class="pni">Glacial Lake Agassiz, <a href="#Page_325">325-328</a>, <a href="#Page_339">339</a>.</p> - -<p class="pni">Glacial lakes, at close of ice age, <a href="#Page_320">320</a>;</p> -<p class="pnii">of St. Lawrence Valley, <a href="#Page_329">329</a>.</p> - -<p class="pni">Glaciated regions, aspects of, <a href="#Page_302">302</a>;</p> -<p class="pnii">characteristics of, <a href="#Page_301">301</a>;</p> -<p class="pnii">contrasted with nonglaciated, <a href="#Page_299">299</a>, <a href="#Page_309">309</a>.</p> - -<p class="pni">Glaciation, conditions essential to, <a href="#Page_261">261</a>;</p> -<p class="pnii">cycle of, <a href="#Page_263">263</a>;</p> -<p class="pnii">Permo-Carboniferous, <a href="#Page_298">298</a>.</p> - -<p class="pni">Glaciations, following changes in earth’s figure, <a href="#Page_15">15</a>;</p> -<p class="pnii">previous to “ice age”, literature of, <a href="#Page_318">318</a>.</p> - -<p class="pni">Glacier broom, over continental ice, <a href="#Page_285">285</a>.</p> - -<p class="pni">Glacier cornices, <a href="#Page_397">397</a>.</p> - -<p class="pni">Glacier deposits, upon its bed, <a href="#Page_390">390</a>.</p> - -<p class="pni">Glacier drainage, <a href="#Page_397">397</a>.</p> - -<p class="pni">Glacier flow, <a href="#Page_390">390</a>, <a href="#Page_400">400</a>;</p> -<p class="pnii">data from accidents to Alpinists, <a href="#Page_392">392</a>.</p> - -<p class="pni">Glacier gravings, <a href="#Page_301">301</a>, <a href="#Page_319">319</a>;</p> -<p class="pnii">multiple records, <a href="#Page_304">304</a>.</p> - -<p class="pni">Glacier lobe lakes, <a href="#Page_411">411</a>.</p> - -<p class="pni">Glacier milk, <a href="#Page_398">398</a>.</p> - -<p class="pni">Glacier mills, <a href="#Page_278">278</a>.</p> - -<p class="pni">Glacier pavement, <a href="#Page_276">276</a>.</p> - -<p class="pni">Glaciers, birth of, <a href="#Page_369">369</a>;</p> -<p class="pnii">crevasses on, <a href="#Page_391">391</a>;</p> -<p class="pnii">dendritic, <a href="#Page_383">383</a>, <a href="#Page_385">385</a>, <a href="#Page_386">386</a>;</p> -<p class="pnii">grinding tools of, <a href="#Page_276">276</a>;</p> -<p class="pnii">horseshoe, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>, <a href="#Page_387">387</a>;</p> -<p class="pnii">inherited basin, <a href="#Page_387">387-389</a>;</p> -<p class="pnii">initiation of, <a href="#Page_262">262</a>;</p> -<p class="pnii">in relation to wind direction, <a href="#Page_262">262</a>;</p> -<p class="pnii">main types of, <a href="#Page_266">266</a>;</p> -<p class="pnii">mountain, cross sections of, <a href="#Page_394">394</a>;</p> -<p class="pnii">mountain, expanded-foot type, <a href="#Page_264">264</a>;</p> -<p class="pnii">mountain, land sculpture by, <a href="#Page_367">367</a>;</p> -<p class="pnii">mountain, successive stages, <a href="#Page_383">383</a>;</p> -<p class="pnii">nivation, <a href="#Page_387">387</a>;</p> -<p class="pnii">nourishment of, <a href="#Page_268">268-270</a>;</p> -<p class="pnii">piedmont, <a href="#Page_383">383</a>, <a href="#Page_384">384</a>;</p> -<p class="pnii">radiating, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>;</p> -<p class="pnii">sensitiveness to temperature changes, <a href="#Page_263">263</a>;</p> -<p class="pnii">séracs, <a href="#Page_391">391</a>;</p> -<p class="pnii">surface features of, <a href="#Page_390">390</a>;</p> -<p class="pnii">tide water, <a href="#Page_290">290</a>, <a href="#Page_386">386</a>.</p> - -<p class="pni">Glacier stars, <a href="#Page_395">395</a>.</p> - -<p class="pni">Glacier tables, <a href="#Page_395">395</a>.</p> - -<p class="pni">Glacier types, successive, during waning glaciation, <a href="#Page_383">383</a>.</p> - -<p class="pni">Glacier wells, <a href="#Page_278">278</a>.</p> - -<p class="pni">Glassy texture, of extrusive rocks, <a href="#Page_32">32</a>.</p> - -<p class="pni">Glen Roy, <a href="#Page_322">322</a>, <a href="#Page_339">339</a>.</p> - -<p class="pni">Glint, <a href="#Page_409">409</a>.</p> - -<p class="pni">Glint lakes, <a href="#Page_408">408</a>, <a href="#Page_409">409</a>.</p> - -<p class="pni">Gneiss, <a href="#Page_465">465</a>.</p> - -<p class="pni">Gneiss banding, <a href="#Page_31">31</a>.</p> - -<p class="pni">Goethe, cited on volcano structure, <a href="#Page_139">139</a>.</p> - -<p class="pni">Gold, E., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Goldthwait, J. W., cited, <a href="#Page_259">259</a>, <a href="#Page_320">320</a>, <a href="#Page_341">341</a>, <a href="#Page_345">345</a>, <a href="#Page_351">351</a>.</p> - -<p class="pni">Gondwana Land, <a href="#Page_16">16</a>.</p> - -<p class="pni">Gorges, through rock bars, <a href="#Page_378">378</a>.</p> - -<p class="pni">Grabau, A. W., cited, <a href="#Page_361">361</a>, <a href="#Page_366">366</a>.</p> - -<p class="pni">Grading of flood plain, <a href="#Page_162">162</a>.</p> - -<p class="pni">Grand Cañon of the Colorado, <a href="#Page_146">146</a>, <a href="#Page_169">169</a>, <a href="#Page_174">174</a>, <a href="#Page_215">215</a>, <a href="#Page_443">443</a>.</p> - -<p class="pni">Grand River outlet, <a href="#Page_333">333</a>.</p> - -<p class="pni">Granite, <a href="#Page_462">462</a>;</p> -<p class="pnii">dome structure in, <a href="#Page_152">152</a>, <a href="#Page_157">157</a>.</p> - -<p class="pni">Granite domes, <a href="#Page_221">221</a>.</p> - -<p class="pni">Granitic texture, of igneous rocks, <a href="#Page_33">33</a>.</p> - -<p class="pni"><i>Grats</i>, <a href="#Page_373">373</a>.</p> - -<p class="pni">Gravel, kame, <a href="#Page_310">310</a>.</p> - -<p class="pni">“Gravel piedmont”, <a href="#Page_214">214</a>.</p> - -<p class="pni">Great Basin, <a href="#Page_190">190</a>, <a href="#Page_198">198</a>, <a href="#Page_439">439</a>.</p> - -<p class="pni">Great Lakes, probable future of, <a href="#Page_347">347</a>, <a href="#Page_348">348</a>;</p> -<p class="pnii">submergence of certain shores of, <a href="#Page_349">349</a>, <a href="#Page_350">350</a>.</p> - -<p class="pni">Great Ross Barrier, <a href="#Page_282">282</a>.</p> - -<p class="pni">Great Salt Lake, <a href="#Page_199">199</a>;</p> -<p class="pnii">fluctuations of level of, <a href="#Page_198">198</a>.</p> - -<p class="pni">Green, W. Lowthian, cited, <a href="#Page_19">19</a>.</p> - -<p class="pni">Gregory, J. W., cited, <a href="#Page_11">11</a>, <a href="#Page_19">19</a>, <a href="#Page_439">439</a>, <a href="#Page_446">446</a>.</p> - -<p class="pni">Grooved upland, <a href="#Page_372">372</a>, <a href="#Page_373">373</a>.</p> - -<p class="pni">Gross, H., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Grossman, cited, <a href="#Page_268">268</a>.</p> - -<p class="pni">Grottoes, sea, colors of, <a href="#Page_258">258</a>.</p> - -<p class="pni">Ground water, <a href="#Page_180">180</a>;</p> -<p class="pnii">descent of, in relation to joints, <a href="#Page_181">181</a>.</p> - -<p class="pni">Ground water lakes, <a href="#Page_424">424</a>.</p> - -<p class="pni">Grund, A., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">Gullies, early stages of, <a href="#Page_160">160</a>.</p> - -<p class="pni">Gulliver, F. P., cited, <a href="#Page_244">244</a>, <a href="#Page_319">319</a>.</p> - -<p class="pni">Gullying process, started by deforestation, <a href="#Page_156">156</a>.</p> - -<p class="pni">Gypsum, <a href="#Page_455">455</a>.</p> - -<p class="pn"><span class="pl">H</span>ade, on faults, <a href="#Page_59">59</a>.</p> - -<p class="pni">Hague, Arnold, cited, <a href="#Page_196">196</a>.</p> - -<p class="pni">Halemaumau, Kilauea, <a href="#Page_107">107</a>, <a href="#Page_108">108</a>.</p> - -<p class="pni">Hamilton, Sir William, cited, <a href="#Page_128">128</a>.</p> - -<p class="pni">Hanging valleys, <a href="#Page_378">378</a>.</p> - -<p class="pni">Hardness, of minerals, <a href="#Page_451">451</a>.</p> - -<p class="pni">Harwood, W. A., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Haug, E., cited, <a href="#Page_7">7</a>, <a href="#Page_133">133</a>, <a href="#Page_211">211</a>.</p> - -<p><span class="pagenum"><a name="Page_496" id="Page_496">[496]</a></span></p><p class="pni">Haughton, Samuel, cited, <a href="#Page_56">56</a>.</p> - -<p class="pni">Hawaii, lava domes of, <a href="#Page_105">105</a>;</p> -<p class="pnii">lava surfaces of, <a href="#Page_113">113</a>;</p> -<p class="pnii">map of, <a href="#Page_106">106</a>;</p> -<p class="pnii">section through, <a href="#Page_106">106</a>.</p> - -<p class="pni">Hayes, C. W., cited, <a href="#Page_156">156</a>.</p> - -<p class="pni">Headlands, notched, <a href="#Page_341">341</a>.</p> - -<p class="pni">Heave, of faults, <a href="#Page_59">59</a>.</p> - -<p class="pni">Hebrews, conception of the universe, <a href="#Page_9">9</a>.</p> - -<p class="pni">Hedin, Sven, cited, <a href="#Page_221">221</a>.</p> - -<p class="pni">Heilprin, A., cited, <a href="#Page_148">148</a>.</p> - -<p class="pni">Heim, A., cited, <a href="#Page_54">54</a>.</p> - -<p class="pni">Heligoland, <a href="#Page_236">236</a>.</p> - -<p class="pni">Helland, A., cited, <a href="#Page_99">99</a>.</p> - -<p class="pni">Hematite, <a href="#Page_452">452</a>.</p> - -<p class="pni">Hemicycles, of glaciation, <a href="#Page_263">263</a>, <a href="#Page_264">264</a>.</p> - -<p class="pni">Herculaneum, buried beneath mud flows, <a href="#Page_139">139</a>.</p> - -<p class="pni">Hess, H., cited, <a href="#Page_267">267</a>, <a href="#Page_272">272</a>, <a href="#Page_294">294</a>, <a href="#Page_393">393</a>, <a href="#Page_400">400</a>.</p> - -<p class="pni">High plains, <a href="#Page_435">435</a>;</p> -<p class="pnii">origin of, <a href="#Page_219">219</a>.</p> - -<p class="pni">Hilgard, E., cited, <a href="#Page_222">222</a>.</p> - -<p class="pni">Hinge lines, of uptilt, <a href="#Page_344">344-347</a>.</p> - -<p class="pni">Hitchcock, C. H., cited, <a href="#Page_106">106</a>, <a href="#Page_147">147</a>, <a href="#Page_434">434</a>.</p> - -<p class="pni">Hobson, B., cited, <a href="#Page_120">120</a>.</p> - -<p class="pni">Hogarth, William, cited, <a href="#Page_170">170</a>.</p> - -<p class="pni">Hogarthian line of beauty, in landscapes, <a href="#Page_170">170-171</a>.</p> - -<p class="pni">“Hog backs”, <a href="#Page_442">442</a>.</p> - -<p class="pni">Holmes, W. H., cited, <a href="#Page_441">441</a>.</p> - -<p class="pni">Horns, <a href="#Page_374">374</a>.</p> - -<p class="pni">Horseshoe glaciers, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>, <a href="#Page_387">387</a>.</p> - -<p class="pni">Hot springs, <a href="#Page_191">191</a>;</p> -<p class="pnii">colors in, due to algæ, <a href="#Page_194">194</a>.</p> - -<p class="pni">Hovey, E. O., cited, <a href="#Page_136">136</a>, <a href="#Page_137">137</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Hovey, H. C., cited, <a href="#Page_183">183</a>, <a href="#Page_195">195</a>.</p> - -<p class="pni">Howchin, W., cited, <a href="#Page_298">298</a>.</p> - -<p class="pni">Howe, E., cited, <a href="#Page_140">140</a>.</p> - -<p class="pni">Howell, cited, <a href="#Page_325">325</a>.</p> - -<p class="pni">Hudson River, narrows of, <a href="#Page_174">174</a>.</p> - -<p class="pni">Hudsonian channel, <a href="#Page_252">252</a>.</p> - -<p class="pni">Hummocks, on pack ice, <a href="#Page_286">286</a>.</p> - -<p class="pni">Humphrey, R. L., cited, <a href="#Page_90">90</a>, <a href="#Page_93">93</a>.</p> - -<p class="pni">Humphreys, cited, <a href="#Page_404">404</a>.</p> - -<p class="pni">Humus, in relation to weathering, <a href="#Page_156">156</a>.</p> - -<p class="pni">Huntington, Ellsworth, cited, <a href="#Page_216">216</a>, <a href="#Page_217">217</a>, <a href="#Page_221">221</a>, <a href="#Page_222">222</a>.</p> - -<p class="pni">Hus, H. T. A. de L., cited, <a href="#Page_183">183</a>.</p> - -<p class="pni">Hydration, <a href="#Page_151">151</a>.</p> - -<p class="pni">Hydrosphere, <a href="#Page_8">8</a>.</p> - -<p class="pni">Hypothesis, the value of, <a href="#Page_6">6</a>;</p> -<p class="pnii">Laplacian, of the universe, <a href="#Page_20">20</a>.</p> - -<p class="pn"><span class="pl">I</span>cebergs, <a href="#Page_296">296</a>;</p> -<p class="pnii">Antarctic, <a href="#Page_292">292</a>, <a href="#Page_293">293</a>;</p> -<p class="pnii">Antarctic, formation of, <a href="#Page_292">292</a>;</p> -<p class="pnii">blue, <a href="#Page_292">292</a>;</p> -<p class="pnii">manner of formation of, <a href="#Page_291">291</a>, <a href="#Page_292">292</a>;</p> -<p class="pnii">northern, <a href="#Page_291">291</a>.</p> - -<p class="pni">Ice caps, profiles of, <a href="#Page_267">267</a>, <a href="#Page_268">268</a>;</p> -<p class="pnii">sculpture, <a href="#Page_380">380</a>.</p> - -<p class="pni">Ice-dammed lakes, <a href="#Page_321">321</a>, <a href="#Page_323">323</a>, <a href="#Page_410">410</a>, <a href="#Page_411">411</a>;</p> -<p class="pnii">in St. Lawrence Valley, <a href="#Page_339">339</a>;</p> -<p class="pnii">of Scottish glens, <a href="#Page_322">322</a>.</p> - -<p class="pni">Ice floes, <a href="#Page_287">287</a>.</p> - -<p class="pni">Iceland, fissure eruptions of, <a href="#Page_102">102</a>.</p> - -<p class="pni">Ice pyramids, <a href="#Page_395">395</a>.</p> - -<p class="pni">Ice ramparts, <a href="#Page_431">431-434</a>;</p> -<p class="pnii">manner of formation of, <a href="#Page_433">433</a>.</p> - -<p class="pni">Igneous rocks, <a href="#Page_30">30</a>;</p> -<p class="pnii">textures of, <a href="#Page_32">32</a>.</p> - -<p class="pni">Imlay outlet, <a href="#Page_332">332</a>.</p> - -<p class="pni">Inbreak, of lava surface, <a href="#Page_107">107</a>.</p> - -<p class="pni">Incised topography, <a href="#Page_301">301</a>.</p> - -<p class="pni">Inherited basin glacier, <a href="#Page_387">387-389</a>.</p> - -<p class="pni">Interlobate moraines, <a href="#Page_314">314</a>.</p> - -<p class="pni">Inter-pluvial periods, <a href="#Page_198">198</a>.</p> - -<p class="pni">Intricate pattern of river etchings, <a href="#Page_158">158</a>.</p> - -<p class="pni">Intrusive rocks, <a href="#Page_32">32</a>, <a href="#Page_462">462</a>.</p> - -<p class="pni">Islands, land-tied, <a href="#Page_241">241</a>;</p> -<p class="pnii">steep rocky, due to submergence, <a href="#Page_252">252</a>.</p> - -<p class="pni">Isobases, <a href="#Page_347">347</a>.</p> - -<p class="pni">Isoclinal folds, <a href="#Page_42">42</a>.</p> - -<p class="pni">Isothermal zone of atmosphere, <a href="#Page_270">270</a>.</p> - -<p class="pn"><span class="pl">J</span>agger, T. A., Jr., cited, <a href="#Page_148">148</a>.</p> - -<p class="pni">Jamieson, T. F., cited, <a href="#Page_221">221</a>, <a href="#Page_322">322</a>, <a href="#Page_339">339</a>.</p> - -<p class="pni">Jeannette exploring expedition, <a href="#Page_287">287</a>, <a href="#Page_295">295</a>.</p> - -<p class="pni">Jensen, H. I., cited, <a href="#Page_110">110</a>, <a href="#Page_113">113</a>, <a href="#Page_147">147</a>.</p> - -<p class="pni">Johnson, D. W., cited, <a href="#Page_7">7</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Johnson, W. D., cited, <a href="#Page_77">77</a>, <a href="#Page_213">213</a>, <a href="#Page_219">219</a>, <a href="#Page_220">220</a>, <a href="#Page_222">222</a>, <a href="#Page_370">370</a>, <a href="#Page_381">381</a>.</p> - -<p class="pni">Johnston-Lavis, H. J., cited, <a href="#Page_87">87</a>, <a href="#Page_131">131</a>, <a href="#Page_132">132</a>, <a href="#Page_134">134</a>, <a href="#Page_138">138</a>, <a href="#Page_147">147</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Joint blocks, in Niagara limestone, <a href="#Page_353">353</a>.</p> - -<p class="pni">Joint plane, seat of frost action, <a href="#Page_370">370</a>.</p> - -<p class="pni">Joints, <a href="#Page_56">56</a>;</p> -<p class="pnii">effect on surface features, <a href="#Page_57">57</a>;</p> -<p class="pnii">closed during earthquakes, <a href="#Page_76">76</a>;</p> -<p class="pnii">composite nature of, <a href="#Page_58">58</a>;</p> -<p class="pnii">composite groups of, <a href="#Page_57">57</a>;</p> -<p class="pnii">disorderly, <a href="#Page_57">57</a>;</p> -<p class="pnii">displacements upon, <a href="#Page_58">58</a>;</p> -<p class="pnii">master, <a href="#Page_56">56</a>;</p> -<p class="pnii">space intervals of, <a href="#Page_58">58</a>;</p> -<p class="pnii">sets of, <a href="#Page_55">55</a>;</p> -<p class="pnii">system of, <a href="#Page_55">55</a>.</p> - -<p class="pni">Joint series, combinations of, <a href="#Page_56">56</a>.</p> - -<p class="pni">Joint systems, <a href="#Page_66">66</a>.</p> - -<p class="pni">Jorullo, birth of, <a href="#Page_96">96</a>.</p> - -<p class="pni">Judd, John W., cited, <a href="#Page_116">116</a>, <a href="#Page_118">118</a>, <a href="#Page_139">139</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Julien, A. A., <a href="#Page_156">156</a>.</p> - -<p class="pni">Jura Mountains, <a href="#Page_46">46</a>.</p> - -<p class="pn"><span class="pl">K</span>ame gravel, <a href="#Page_310">310</a>.</p> - -<p class="pni">Kames, <a href="#Page_311">311</a>, <a href="#Page_314">314</a>.</p> - -<p class="pni">Kammerbühl, <a href="#Page_139">139</a>.</p> - -<p class="pni"><i>Karrenfelder</i>, <a href="#Page_188">188</a>.</p> - -<p class="pni">Karst, characters of, <a href="#Page_186">186-187</a>;</p> -<p class="pnii">once forested, <a href="#Page_188">188</a>.</p> - -<p class="pni">Karst conditions, <a href="#Page_195">195</a>.</p> - -<p class="pni">Karst lakes, <a href="#Page_422">422</a>.</p> - -<p class="pni"><i>Katavothren</i>, <a href="#Page_188">188</a>.</p> - -<p class="pni">Katzer, F., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">Kearney, Th. H., cited, <a href="#Page_222">222</a>.</p> - -<p class="pni">Kelvin, Lord, cited, <a href="#Page_20">20</a>, <a href="#Page_29">29</a>.</p> - -<p class="pni">“Kettle moraines”, <a href="#Page_311">311-314</a>.</p> - -<p><span class="pagenum"><a name="Page_497" id="Page_497">[497]</a></span></p><p class="pni">“Kettles” on moraines, <a href="#Page_312">312</a>.</p> - -<p class="pni">Kikuchi, Y., cited, <a href="#Page_148">148</a>.</p> - -<p class="pni">Kilauea, <a href="#Page_101">101</a>, <a href="#Page_106">106</a>;</p> -<p class="pnii">draining of lava in crater of, <a href="#Page_108">108</a>;</p> -<p class="pnii">eruption of 1840, <a href="#Page_109">109</a>, <a href="#Page_111">111</a>, <a href="#Page_112">112</a>;</p> -<p class="pnii">lava movements in, <a href="#Page_106">106</a>, <a href="#Page_107">107</a>;</p> -<p class="pnii">moving platform in crater, <a href="#Page_107">107</a>;</p> -<p class="pnii">range in height of lava in, <a href="#Page_107">107</a>.</p> - -<p class="pni">King, F. H., cited, <a href="#Page_157">157</a>, <a href="#Page_195">195</a>.</p> - -<p class="pni">Knebel, W. von, cited, <a href="#Page_185">185</a>, <a href="#Page_195">195</a>, <a href="#Page_258">258</a>, <a href="#Page_260">260</a>.</p> - -<p class="pni">“Knob and basin” topography, <a href="#Page_314">314</a>.</p> - -<p class="pni">Knott, C. G., cited, <a href="#Page_92">92</a>.</p> - -<p class="pni">Kopisch, August, cited, <a href="#Page_258">258</a>.</p> - -<p class="pni">Kotô, B., cited, <a href="#Page_92">92</a>.</p> - -<p class="pni">Krakatoa, dissected by eruption, <a href="#Page_142">142</a>.</p> - -<p class="pni">Krakatoa, eruption of 1883, <a href="#Page_141">141</a>, <a href="#Page_142">142</a>.</p> - -<p class="pni"><i>Kuppen</i>, <a href="#Page_105">105</a>.</p> - -<p class="pni">Kurische Nehrung, wandering dunes of, <a href="#Page_210">210</a>.</p> - -<p class="pn"><span class="pl">L</span>aboratory apparatus, for simulation of cinder eruptions, <a href="#Page_122">122</a>.</p> - -<p class="pni">Laboratory models, for study of geological maps, <a href="#Page_63">63</a>.</p> - -<p class="pni">Laccolites, <a href="#Page_143">143</a>, <a href="#Page_441">441</a>, <a href="#Page_442">442</a>, <a href="#Page_447">447</a>.</p> - -<p class="pni">Lacroix, A., cited, <a href="#Page_148">148</a>.</p> - -<p class="pni">Lacustrine deposits, <a href="#Page_35">35</a>.</p> - -<p class="pni">Lake Agassiz, glacial, <a href="#Page_325">325-328</a>.</p> - -<p class="pni">Lake Algonquin, <a href="#Page_334">334</a>, <a href="#Page_342">342</a>.</p> - -<p class="pni">Lake Arkona, <a href="#Page_332">332</a>, <a href="#Page_333">333</a>.</p> - -<p class="pni">Lake basins, study of, <a href="#Page_401">401</a>.</p> - -<p class="pni">Lake Bonneville, <a href="#Page_199">199</a>.</p> - -<p class="pni">Lake Chicago, <a href="#Page_330">330</a>, <a href="#Page_332">332</a>, <a href="#Page_333">333</a>.</p> - -<p class="pni">Lake Eulalie, draining of, during earthquake, <a href="#Page_83">83</a>.</p> - -<p class="pni">Lake Iroquois, <a href="#Page_334">334</a>, <a href="#Page_335">335</a>.</p> - -<p class="pni">Lake Maumee, <a href="#Page_330">330</a>, <a href="#Page_331">331</a>, <a href="#Page_332">332</a>, <a href="#Page_345">345</a>.</p> - -<p class="pni">Lake Ojibway, glacial, <a href="#Page_338">338</a>.</p> - -<p class="pni">Lake stages, in St. Lawrence Valley, <a href="#Page_336">336</a>.</p> - -<p class="pni">Lake Warren, <a href="#Page_333">333</a>, <a href="#Page_334">334</a>.</p> - -<p class="pni">Lake Whittlesey, <a href="#Page_332">332</a>, <a href="#Page_333">333</a>.</p> - -<p class="pni">Lakes, alluvial dam, <a href="#Page_423">423</a>;</p> -<p class="pnii">as regulators of air temperature, <a href="#Page_431">431</a>;</p> -<p class="pnii">as regulators of river flow, <a href="#Page_431">431</a>;</p> -<p class="pnii">as settling basins, <a href="#Page_426">426-428</a>;</p> -<p class="pnii">barrier, <a href="#Page_420">420</a>;</p> -<p class="pnii">basin range, <a href="#Page_402">402</a>, <a href="#Page_403">403</a>;</p> -<p class="pnii">become extinct through wave action, <a href="#Page_428">428</a>;</p> -<p class="pnii">border, <a href="#Page_399">399</a>, <a href="#Page_414">414</a>;</p> -<p class="pnii">classification of, <a href="#Page_424">424</a>;</p> -<p class="pnii">colk, <a href="#Page_408">408</a>, <a href="#Page_409">409</a>;</p> -<p class="pnii">continental glaciation, <a href="#Page_424">424</a>;</p> -<p class="pnii">coulée, <a href="#Page_406">406</a>;</p> -<p class="pnii">crater, <a href="#Page_405">405</a>, <a href="#Page_406">406</a>;</p> -<p class="pnii">crescentic, <a href="#Page_329">329</a>, <a href="#Page_330">330</a>;</p> -<p class="pnii">crescentic levee, <a href="#Page_416">416</a>, <a href="#Page_417">417</a>;</p> -<p class="pnii">currents in, <a href="#Page_431">431</a>;</p> -<p class="pnii">delta, <a href="#Page_419">419</a>, <a href="#Page_420">420</a>;</p> -<p class="pnii">desert, <a href="#Page_424">424</a>;</p> -<p class="pnii">drained by cutting down of outlet, <a href="#Page_428">428</a>;</p> -<p class="pnii">dune, <a href="#Page_421">421</a>;</p> -<p class="pnii">drained during earthquakes, explanation of, <a href="#Page_83">83</a>;</p> -<p class="pnii">earthquake, <a href="#Page_404">404</a>;</p> -<p class="pnii">ephemeral existence of, <a href="#Page_426">426</a>;</p> -<p class="pnii">extinction by peat growth, <a href="#Page_429">429-430</a>;</p> -<p class="pnii">extinction of, in desert regions, <a href="#Page_430">430</a>;</p> -<p class="pnii">fresh water, <a href="#Page_401">401</a>;</p> -<p class="pnii">glacier lobe, <a href="#Page_411">411</a>;</p> -<p class="pnii">glint, <a href="#Page_408">408</a>, <a href="#Page_409">409</a>;</p> -<p class="pnii">ground water, <a href="#Page_424">424</a>;</p> -<p class="pnii">ice dam, <a href="#Page_410">410</a>, <a href="#Page_411">411</a>;</p> -<p class="pnii">intramorainal, about continental glaciers, <a href="#Page_279">279</a>, <a href="#Page_280">280</a>;</p> -<p class="pnii">karst, <a href="#Page_422">422</a>;</p> -<p class="pnii">landslide, <a href="#Page_414">414</a>;</p> -<p class="pnii">morainal, <a href="#Page_315">315</a>, <a href="#Page_406">406</a>, <a href="#Page_407">407</a>;</p> -<p class="pnii">mountain glaciation, <a href="#Page_424">424</a>;</p> -<p class="pnii">newland, <a href="#Page_401">401</a>, <a href="#Page_402">402</a>;</p> -<p class="pnii">ox-bow, <a href="#Page_165">165</a>, <a href="#Page_415">415</a>;</p> -<p class="pnii">pit, <a href="#Page_315">315</a>, <a href="#Page_407">407</a>, <a href="#Page_408">408</a>;</p> -<p class="pnii">playa, <a href="#Page_422">422</a>;</p> -<p class="pnii">raft, <a href="#Page_417">417</a>, <a href="#Page_418">418</a>;</p> -<p class="pnii">rift-valley, <a href="#Page_403">403</a>, <a href="#Page_404">404</a>;</p> -<p class="pnii">river, <a href="#Page_424">424</a>;</p> -<p class="pnii">rock basin, <a href="#Page_376">376</a>, <a href="#Page_377">377</a>, <a href="#Page_400">400</a>, <a href="#Page_412">412</a>;</p> -<p class="pnii">rock basin about continental glaciers, <a href="#Page_279">279</a>;</p> -<p class="pnii">rôle of, in economy of nature, <a href="#Page_430">430</a>;</p> -<p class="pnii">saline, <a href="#Page_401">401</a>;</p> -<p class="pnii">salines, <a href="#Page_423">423</a>;</p> -<p class="pnii">saucer, <a href="#Page_415">415</a>, <a href="#Page_416">416</a>;</p> -<p class="pnii">seasonal, <a href="#Page_189">189</a>, <a href="#Page_422">422</a>;</p> -<p class="pnii">side delta, <a href="#Page_326">326</a>, <a href="#Page_327">327</a>, <a href="#Page_418">418</a>, <a href="#Page_419">419</a>;</p> -<p class="pnii">sink, <a href="#Page_421">421</a>;</p> -<p class="pnii">strand, <a href="#Page_424">424</a>;</p> -<p class="pnii">tectonic, <a href="#Page_424">424</a>;</p> -<p class="pnii">valley moraine, <a href="#Page_400">400</a>, <a href="#Page_413">413</a>;</p> -<p class="pnii">volcanic, <a href="#Page_424">424</a>;</p> -<p class="pnii">“wall”, <a href="#Page_432">432</a>.</p> - -<p class="pni">Laki, eruption in 1783, <a href="#Page_99">99</a>.</p> - -<p class="pni">Laminated structure, of rocks, <a href="#Page_31">31</a>.</p> - -<p class="pni">Lamplugh, G. W., cited, <a href="#Page_225">225</a>.</p> - -<p class="pni">Land, growth of, from volcanic outflow, <a href="#Page_113">113</a>, <a href="#Page_114">114</a>;</p> -<p class="pnii">sliced during earthquake, <a href="#Page_80">80</a>;</p> -<p class="pnii">uptilt of, at close of ice age, <a href="#Page_340">340</a>.</p> - -<p class="pni">Land areas, concentration of, in northern hemisphere, <a href="#Page_11">11</a>.</p> - -<p class="pni">Land sculpture, by mountain glaciers, <a href="#Page_367">367</a>;</p> -<p class="pnii">in relation to climatic conditions, <a href="#Page_443">443</a>;</p> -<p class="pnii">referable to ice caps, <a href="#Page_380">380</a>.</p> - -<p class="pni">Land shields, <a href="#Page_15">15</a>.</p> - -<p class="pni">Landslide lakes, <a href="#Page_414">414</a>.</p> - -<p class="pni">Land-tied islands, <a href="#Page_241">241</a>.</p> - -<p class="pni">Lane, A. C., cited, <a href="#Page_148">148</a>.</p> - -<p class="pni">Lankester, E. Ray, cited, <a href="#Page_260">260</a>.</p> - -<p class="pni">La Noe, G. de, cited, <a href="#Page_7">7</a>.</p> - -<p class="pni"><i>Lapilli</i>, <a href="#Page_119">119</a>, <a href="#Page_122">122</a>.</p> - -<p class="pni">Laplacian hypothesis of the universe, <a href="#Page_20">20</a>.</p> - -<p class="pni">Lateral moraines, <a href="#Page_393">393</a>.</p> - -<p class="pni">Lateral movements, deep seated, during earthquakes, <a href="#Page_81">81</a>.</p> - -<p class="pni">Lava, <a href="#Page_32">32</a>;</p> -<p class="pnii">block, <a href="#Page_113">113</a>;</p> -<p class="pnii">composition and properties of, <a href="#Page_103">103</a>;</p> -<p class="pnii">discharging from tunnel, <a href="#Page_111">111</a>;</p> -<p class="pnii">fluidity of basic, <a href="#Page_103">103</a>;</p> -<p class="pnii">movements, in caldron of Kilauea, <a href="#Page_107">107</a>;</p> -<p class="pnii">probable origin from shale, <a href="#Page_144">144</a>;</p> -<p class="pnii">ropy, <a href="#Page_113">113</a>;</p> -<p class="pnii">viscosity of siliceous, <a href="#Page_103">103</a>.</p> - -<p class="pni">Lava domes, probable structure of walls of, <a href="#Page_112">112</a>;</p> -<p class="pnii">slopes of, <a href="#Page_103">103</a>, <a href="#Page_104">104</a>, <a href="#Page_105">105</a>.</p> - -<p class="pni">Lava projectiles, pear-shaped type, <a href="#Page_121">121</a>.</p> - -<p class="pni">Lava reservoirs, formation of, <a href="#Page_143">143</a>.</p> - -<p class="pni">Lava streams, appearance of, <a href="#Page_133">133</a>, <a href="#Page_134">134</a>.</p> - -<p class="pni">Lava surface, <a href="#Page_113">113</a>, <a href="#Page_124">124</a>.</p> - -<p class="pni">Law of the desert, <a href="#Page_197">197</a>.</p> - -<p class="pni">Lawson, A. C., cited, <a href="#Page_92">92</a>, <a href="#Page_260">260</a>, <a href="#Page_351">351</a>.</p> - -<p class="pni">Leads, in pack ice, <a href="#Page_286">286</a>.</p> - -<p class="pni">Le Conte, Joseph, cited, <a href="#Page_6">6</a>.</p> - -<p class="pni">Leffingwell crater, California, <a href="#Page_104">104</a>.</p> - -<p><span class="pagenum"><a name="Page_498" id="Page_498">[498]</a></span></p><p class="pni">Levees, <a href="#Page_166">166</a>.</p> - -<p class="pni">Leverett, Frank, cited, <a href="#Page_6">6</a>, <a href="#Page_104">104</a>, <a href="#Page_166">166</a>, <a href="#Page_312">312</a>, <a href="#Page_318">318</a>, <a href="#Page_321">321</a>, <a href="#Page_330">330</a>, <a href="#Page_332">332</a>, <a href="#Page_333">333</a>, <a href="#Page_334">334</a>, <a href="#Page_337">337</a>, <a href="#Page_339">339</a>, <a href="#Page_344">344</a>, <a href="#Page_345">345</a>.</p> - -<p class="pni">Lewiston escarpment, at Niagara, shaping of, <a href="#Page_360">360-362</a>.</p> - -<p class="pni">Libbey, W., cited, <a href="#Page_274">274</a>.</p> - -<p class="pni">Life histories, of rivers, <a href="#Page_158">158</a>.</p> - -<p class="pni">Light figure, from surface of crystal, <a href="#Page_25">25</a>.</p> - -<p class="pni">Lightning, in connection with volcanic eruptions, <a href="#Page_130">130</a>.</p> - -<p class="pni">Limbs of faults, <a href="#Page_59">59</a>;</p> -<p class="pnii">of folds, <a href="#Page_43">43</a>.</p> - -<p class="pni">Limestone, <a href="#Page_464">464</a>;</p> -<p class="pnii">origin of, <a href="#Page_36">36</a>;</p> -<p class="pnii">sinks, <a href="#Page_182">182</a>.</p> - -<p class="pni">Limestone, caverns of, <a href="#Page_182">182</a>.</p> - -<p class="pni">Limonite, <a href="#Page_452">452</a>.</p> - -<p class="pni">Linck, G., cited, <a href="#Page_122">122</a>.</p> - -<p class="pni">Lindenkohl, A., cited, <a href="#Page_260">260</a>.</p> - -<p class="pni">Lineaments, <a href="#Page_87">87</a>, <a href="#Page_226">226</a>, <a href="#Page_227">227</a>.</p> - -<p class="pni">Line of beauty, Hogarthian, in landscapes, <a href="#Page_170">170</a>, <a href="#Page_171">171</a>.</p> - -<p class="pni"><i>Lithodomus</i>, borings of, in records of oscillation, <a href="#Page_254">254</a>.</p> - -<p class="pni">Lithosphere, a complex of interlocking crystals, <a href="#Page_25">25</a>;</p> -<p class="pnii">and its envelopes, <a href="#Page_8">8</a>.</p> - -<p class="pni">Littoral deposits, <a href="#Page_36">36</a>.</p> - -<p class="pni">Loess, <a href="#Page_35">35</a>, <a href="#Page_207">207</a>;</p> -<p class="pnii">erosion of, <a href="#Page_208">208</a>.</p> - -<p class="pni">Loessmännchen, <a href="#Page_208">208</a>.</p> - -<p class="pni">Lubbock, Sir John, cited, <a href="#Page_7">7</a>.</p> - -<p class="pni">Luray caverns, Virginia, <a href="#Page_186">186</a>.</p> - -<p class="pni">Luster, of minerals, <a href="#Page_450">450</a>.</p> - -<p class="pni">Lyell, Sir Charles, cited, <a href="#Page_7">7</a>, <a href="#Page_96">96</a>, <a href="#Page_146">146</a>, <a href="#Page_199">199</a>, <a href="#Page_259">259</a>, <a href="#Page_260">260</a>, <a href="#Page_304">304</a>.</p> - - -<p class="pn"><i><span class="pl">M</span>aare</i>, <a href="#Page_405">405</a>.</p> - -<p class="pni">McGee, W. J., cited, <a href="#Page_157">157</a>, <a href="#Page_259">259</a>.</p> - -<p class="pni">Mackinac Island, records of uplift of, <a href="#Page_341">341-344</a>.</p> - -<p class="pni">Madison, Wisconsin, <a href="#Page_233">233</a>, <a href="#Page_237">237</a>, <a href="#Page_241">241</a>, <a href="#Page_317">317</a>, <a href="#Page_434">434</a>.</p> - -<p class="pni">Magellan, circumnavigation of globe, <a href="#Page_9">9</a>.</p> - -<p class="pni">Magma, defined, <a href="#Page_30">30</a>.</p> - -<p class="pni">Magnetism, of minerals, <a href="#Page_451">451</a>.</p> - -<p class="pni">Magnetite, <a href="#Page_452">452</a>.</p> - -<p class="pni">Malachite, <a href="#Page_453">453</a>.</p> - -<p class="pni"><i>Mamelons</i>, <a href="#Page_105">105</a>.</p> - -<p class="pni">Mammoth Cave, <a href="#Page_182">182</a>, <a href="#Page_183">183</a>.</p> - -<p class="pni">Mantle, rock, <a href="#Page_155">155</a>.</p> - -<p class="pni">Map, contour, nature of, <a href="#Page_467">467</a>;</p> -<p class="pnii">of Armorican mountains, <a href="#Page_438">438</a>;</p> -<p class="pnii">of barrier beaches, <a href="#Page_242">242-243</a>;</p> -<p class="pnii">of bowlder train from Iron Hill, <a href="#Page_306">306</a>;</p> -<p class="pnii">of cirques and niches, in Bighorn Mountains, <a href="#Page_371">371</a>;</p> -<p class="pnii">of coast lines, <a href="#Page_246">246</a>;</p> -<p class="pniii">geological, <a href="#Page_54">54</a>, <a href="#Page_61">61</a>;</p> -<p class="pniii">geological, method of preparing, <a href="#Page_46">46</a>, <a href="#Page_63">63</a>;</p> -<p class="pnii">of continental divide in Colorado, <a href="#Page_377">377</a>;</p> -<p class="pnii">of continental glacier in Victoria Land, <a href="#Page_282">282</a>;</p> -<p class="pnii">of Dalager’s nunataks, <a href="#Page_277">277</a>;</p> -<p class="pnii">of expanded foot glaciers, <a href="#Page_264">264</a>;</p> -<p class="pnii">of front of Green Bay lobe, <a href="#Page_317">317</a>;</p> -<p class="pnii">of glacial features, Southern Finland, <a href="#Page_315">315</a>;</p> -<p class="pnii">of glacial Lake Agassiz, <a href="#Page_325">325</a>, <a href="#Page_326">326</a>, <a href="#Page_328">328</a>;</p> -<p class="pnii">of glaciated area, Europe, <a href="#Page_299">299</a>;</p> -<p class="pnii">of glaciated area, North America, <a href="#Page_298">298</a>;</p> -<p class="pnii">of ice ramparts on Lake Mendota, <a href="#Page_434">434</a>;</p> -<p class="pnii">of inner Sandusky Bay, <a href="#Page_350">350</a>;</p> -<p class="pnii">of Kilauea and neighboring slopes, <a href="#Page_109">109</a>;</p> -<p class="pnii">of Lake Chicago and later Lake Maumee, <a href="#Page_332">332</a>;</p> -<p class="pnii">of Lake Maumee, <a href="#Page_330">330</a>;</p> -<p class="pnii">of Lakes Whittlesey and Saginaw, <a href="#Page_333">333</a>;</p> -<p class="pnii">of lava outflows on Vesuvius, 1906, <a href="#Page_131">131</a>;</p> -<p class="pnii">of lava streams on Mauna Loa, <a href="#Page_126">126</a>;</p> -<p class="pnii">of marginal moraines, <a href="#Page_312">312</a>;</p> -<p class="pnii">of mountain arcs of Eastern Asia, <a href="#Page_438">438</a>;</p> -<p class="pnii">of mountain arc of Sewestan, <a href="#Page_436">436</a>;</p> -<p class="pnii">of North Polar regions, <a href="#Page_288">288</a>;</p> -<p class="pnii">of part of “fire girdle” of the Pacific, <a href="#Page_98">98</a>;</p> -<p class="pnii">of Scottish glens, <a href="#Page_322">322-324</a>;</p> -<p class="pnii">of Volcano, <a href="#Page_118">118</a>;</p> -<p class="pnii">of volcano belts, <a href="#Page_98">98</a>;</p> -<p class="pnii">of Warren River, <a href="#Page_326">326</a>, <a href="#Page_327">327</a>;</p> -<p class="pnii">topographical, <a href="#Page_61">61</a>;</p> -<p class="pnii">topographical, preparation of, <a href="#Page_467">467</a>, <a href="#Page_468">468</a>;</p> -<p class="pnii">topographical, verification of, <a href="#Page_469">469</a>;</p> -<p class="pnii">to show dispersion of diamonds in Lake region, <a href="#Page_308">308</a>;</p> -<p class="pnii">to show dispersion of peculiar rocks, <a href="#Page_305">305</a>;</p> -<p class="pnii">to show distribution of existing glaciers, <a href="#Page_263">263</a>;</p> -<p class="pnii">to show formation of shore features, <a href="#Page_238">238</a>;</p> -<p class="pnii">to show glaciated areas of Pleistocene period, <a href="#Page_297">297</a>;</p> -<p class="pnii">to show reciprocal relation of land and sea, <a href="#Page_11">11</a>.</p> - -<p class="pni">Marble, <a href="#Page_466">466</a>.</p> - -<p class="pni">Margerie, Emm. de, cited, <a href="#Page_7">7</a>, <a href="#Page_54">54</a>.</p> - -<p class="pni">Marginal moraines, <a href="#Page_278">278-280</a>, <a href="#Page_311">311-314</a>.</p> - -<p class="pni">Marine clays, as marks of uplift, <a href="#Page_253">253</a>.</p> - -<p class="pni">Marine deposits, <a href="#Page_35">35</a>.</p> - -<p class="pni">Märjelen Lake, <a href="#Page_329">329</a>, <a href="#Page_411">411</a>.</p> - -<p class="pni">Marks, of origin of rocks, <a href="#Page_30">30</a>;</p> -<p class="pnii">of uplift, on coasts, <a href="#Page_245">245</a>.</p> - -<p class="pni">Marr, John E., cited, <a href="#Page_7">7</a>, <a href="#Page_445">445</a>.</p> - -<p class="pni">Martel, E. A., cited, <a href="#Page_181">181</a>, <a href="#Page_187">187</a>, <a href="#Page_195">195</a>.</p> - -<p class="pni">Martin, Lawrence, cited, <a href="#Page_77">77</a>, <a href="#Page_92">92</a>, <a href="#Page_260">260</a>, <a href="#Page_280">280</a>, <a href="#Page_351">351</a>.</p> - -<p class="pni">Martonne, E. de, cited, <a href="#Page_7">7</a>, <a href="#Page_195">195</a>, <a href="#Page_222">222</a>, <a href="#Page_382">382</a>.</p> - -<p class="pni">Massive structure, of rocks, <a href="#Page_31">31</a>.</p> - -<p class="pni">Master joints, <a href="#Page_56">56</a>.</p> - -<p class="pni">Matavanu, eruption in 1906, <a href="#Page_110">110</a>, <a href="#Page_113">113</a>, <a href="#Page_147">147</a>.</p> - -<p class="pni">Mat of vegetation, shield to lithosphere, <a href="#Page_155">155</a>.</p> - -<p class="pni">Matthes, F. E., cited, <a href="#Page_7">7</a>, <a href="#Page_371">371</a>, <a href="#Page_381">381</a>.</p> - -<p class="pni">Maturity, of upland, <a href="#Page_170">170</a>.</p> - -<p class="pni">Mauna Loa, <a href="#Page_106">106</a>;</p> -<p class="pnii">eruptions of, <a href="#Page_109">109</a>.</p> - -<p class="pni">Meander scars, <a href="#Page_165">165</a>.</p> - -<p class="pni">Meanders, entrenchment of, <a href="#Page_172">172</a>, <a href="#Page_173">173</a>, <a href="#Page_179">179</a>;</p> -<p class="pnii">stream, <a href="#Page_163">163</a>;</p> -<p><span class="pagenum"><a name="Page_499" id="Page_499">[499]</a></span></p><p class="pnii">stream, undermining by, <a href="#Page_164">164</a>.</p> - -<p class="pni">Measurement of thickness, of formations, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>.</p> - -<p class="pni">Mechanical sediments, <a href="#Page_34">34</a>.</p> - -<p class="pni">Medial moraines, <a href="#Page_393">393</a>;</p> -<p class="pnii">from nunataks, <a href="#Page_274">274</a>.</p> - -<p class="pni">Mediterranean seas, <a href="#Page_14">14</a>.</p> - -<p class="pni">Melting, selective, on glacier surface, <a href="#Page_394">394</a>.</p> - -<p class="pni">Melville, G. W., cited, <a href="#Page_289">289</a>.</p> - -<p class="pni">Mercalli, G., cited, <a href="#Page_89">89</a>, <a href="#Page_117">117</a>, <a href="#Page_119">119</a>, <a href="#Page_147">147</a>.</p> - -<p class="pni">Merrill, George P., cited, <a href="#Page_156">156</a>.</p> - -<p class="pni">Mesa, <a href="#Page_215">215</a>, <a href="#Page_216">216</a>;</p> -<p class="pnii">origin of, <a href="#Page_112">112</a>.</p> - -<p class="pni">Metamorphic rocks, <a href="#Page_30">30</a>, <a href="#Page_31">31</a>, <a href="#Page_465">465</a>.</p> - -<p class="pni">Meteorites, compared with earth, <a href="#Page_22">22</a>;</p> -<p class="pnii">composition of, <a href="#Page_21">21</a>, <a href="#Page_23">23</a>.</p> - -<p class="pni">Mica, <a href="#Page_458">458</a>.</p> - -<p class="pni">Mica schist, <a href="#Page_465">465</a>.</p> - -<p class="pni">Michailovitch, J., cited, <a href="#Page_84">84</a>.</p> - -<p class="pni">Microscopical petrography, <a href="#Page_27">27</a>.</p> - -<p class="pni">Migration, of divides, <a href="#Page_175">175</a>.</p> - -<p class="pni">Mill, H. R., cited, <a href="#Page_424">424</a>.</p> - -<p class="pni">Mills, glacier, <a href="#Page_398">398</a>.</p> - -<p class="pni">Milne, John, cited, <a href="#Page_75">75</a>, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>.</p> - -<p class="pni">Mineral fragments, possibility of growth of, <a href="#Page_24">24</a>.</p> - -<p class="pni">Minerals, alterations of, <a href="#Page_27">27</a>, <a href="#Page_28">28</a>;</p> -<p class="pnii">common, properties of, <a href="#Page_452">452-461</a>;</p> -<p class="pnii">of economic importance, <a href="#Page_452">452-456</a>;</p> -<p class="pnii">important as rock makers, <a href="#Page_456">456-461</a>;</p> -<p class="pnii">properties of, <a href="#Page_26">26</a>, <a href="#Page_27">27</a>;</p> -<p class="pnii">quick determination of, <a href="#Page_449">449</a>.</p> - -<p class="pni">Mississippi River, <a href="#Page_167">167</a>.</p> - -<p class="pni">Mitchell, G. E., cited, <a href="#Page_157">157</a>.</p> - -<p class="pni">Moats, about nunataks, <a href="#Page_273">273</a>, <a href="#Page_274">274</a>.</p> - -<p class="pni">Models, laboratory, for study of geological maps, <a href="#Page_63">63</a>.</p> - -<p class="pni">Mojsisovics von Mojsvár, E., cited, <a href="#Page_228">228</a>.</p> - -<p class="pni">Mokuaweoweo, crater of, <a href="#Page_106">106</a>.</p> - -<p class="pni">“Mole-hill” effect, after earthquakes, <a href="#Page_73">73</a>.</p> - -<p class="pni">Molten rock, rise to earth’s surface, <a href="#Page_94">94</a>.</p> - -<p class="pni">Monadnocks, <a href="#Page_172">172</a>.</p> - -<p class="pni">Monte Nuovo, <a href="#Page_96">96</a>.</p> - -<p class="pni">Monte Somma, <i>caldera</i> of, <a href="#Page_127">127</a>.</p> - -<p class="pni">Montessus de Ballore, de F., cited, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>.</p> - -<p class="pni">Monti Rossi, crystal rain from, <a href="#Page_122">122</a>;</p> -<p class="pnii">parasitic cones of, <a href="#Page_125">125</a>.</p> - -<p class="pni">Mont Pelé, post-eruption stage of, <a href="#Page_135">135-138</a>;</p> -<p class="pnii">spine of, <a href="#Page_136">136</a>, <a href="#Page_137">137</a>, <a href="#Page_138">138</a>.</p> - -<p class="pni">Moore, W. H., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Morainal lakes, <a href="#Page_315">315</a>, <a href="#Page_406">406</a>, <a href="#Page_407">407</a>.</p> - -<p class="pni">Moraines, interlobate, <a href="#Page_314">314</a>;</p> -<p class="pnii">lateral, <a href="#Page_393">393</a>;</p> -<p class="pnii">marginal, <a href="#Page_278">278-280</a>;</p> -<p class="pnii">medial, <a href="#Page_393">393</a>;</p> -<p class="pnii">medial, from nunataks, <a href="#Page_274">274</a>;</p> -<p class="pnii">of mountain glaciers, <a href="#Page_393">393</a>, <a href="#Page_394">394</a>;</p> -<p class="pnii">recessional, <a href="#Page_399">399</a>;</p> -<p class="pnii">surface, <a href="#Page_277">277</a>;</p> -<p class="pnii">terminal, <a href="#Page_311">311-314</a>, <a href="#Page_394">394</a>;</p> -<p class="pnii">water-laid, <a href="#Page_330">330</a>.</p> - -<p class="pni">Moreno, F. P., cited, <a href="#Page_235">235</a>.</p> - -<p class="pni">Moseley, E. L., cited, <a href="#Page_350">350</a>, <a href="#Page_351">351</a>.</p> - -<p class="pni">Moselle River, with entrenched meanders, <a href="#Page_173">173</a>.</p> - -<p class="pni">Motive power, of rivers, <a href="#Page_158">158</a>.</p> - -<p class="pni">Moulins, <a href="#Page_398">398</a>.</p> - -<p class="pni">Mountain arcs, festoons of, <a href="#Page_435">435</a>, <a href="#Page_436">436</a>;</p> -<p class="pnii">theories of origin of, <a href="#Page_436">436</a>, <a href="#Page_437">437</a>.</p> - -<p class="pni">Mountain glaciation lakes, <a href="#Page_424">424</a>.</p> - -<p class="pni">Mountain glaciers, contrasted with continental glaciers, <a href="#Page_266">266-268</a>;</p> -<p class="pnii">defined, <a href="#Page_266">266-268</a>;</p> -<p class="pnii">dendritic, <a href="#Page_383">383</a>, <a href="#Page_385">385</a>, <a href="#Page_386">386</a>;</p> -<p class="pnii">expanded-foot type, <a href="#Page_264">264</a>;</p> -<p class="pnii">horseshoe, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>, <a href="#Page_387">387</a>;</p> -<p class="pnii">land sculpture by, <a href="#Page_367">367</a>;</p> -<p class="pnii">marks of, <a href="#Page_400">400</a>;</p> -<p class="pnii">piedmont, <a href="#Page_383">383</a>, <a href="#Page_384">384</a>;</p> -<p class="pnii">profiles of, <a href="#Page_267">267</a>;</p> -<p class="pnii">radiating, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>;</p> -<p class="pnii">studies of special districts, <a href="#Page_294">294</a>;</p> -<p class="pnii">summary of types of, <a href="#Page_389">389</a>.</p> - -<p class="pni">Mountain ramparts, about continental glaciers, <a href="#Page_271">271</a>.</p> - -<p class="pni">Mountains, battlement type, <a href="#Page_228">228</a>, <a href="#Page_445">445</a>;</p> -<p class="pnii">block type, <a href="#Page_439">439</a>;</p> -<p class="pnii">carved from plateaux, <a href="#Page_442">442</a>;</p> -<p class="pniii">of circumvallation, <a href="#Page_442">442</a>, <a href="#Page_445">445</a>;</p> -<p class="pnii">defined, <a href="#Page_435">435</a>;</p> -<p class="pnii">domed, of uplift, <a href="#Page_441">441</a>;</p> -<p class="pnii">erosional, <a href="#Page_445">445</a>;</p> -<p class="pnii">evidence for occupation by mountain glaciers, <a href="#Page_400">400</a>;</p> -<p class="pnii">genetical, <a href="#Page_445">445</a>;</p> -<p class="pnii">largely shaped by erosion, <a href="#Page_435">435</a>;</p> -<p class="pnii">of outflow and upheap, <a href="#Page_440">440</a>;</p> -<p class="pnii">origin and forms of, <a href="#Page_435">435</a>;</p> -<p class="pnii">truncated at coast lines, <a href="#Page_438">438</a>.</p> - -<p class="pni">Mt. Etna, <a href="#Page_125">125</a>, <a href="#Page_126">126</a>.</p> - -<p class="pni">Mt. Vesuvius, <a href="#Page_94">94</a>;</p> -<p class="pnii">appearance of, from Naples at night, <a href="#Page_129">129</a>;</p> -<p class="pnii">ash curtain, during eruption, <a href="#Page_132">132</a>;</p> -<p class="pnii">ash-fall over, 1906, <a href="#Page_133">133</a>;</p> -<p class="pnii">“cauliflower” cloud over, <a href="#Page_133">133</a>;</p> -<p class="pnii">changed appearance after eruption of 1906, <a href="#Page_132">132</a>;</p> -<p class="pnii">eruption of 79 <span class="smcap">A.D.</span>, <a href="#Page_97">97</a>;</p> -<p class="pnii">eruption of 1872, <a href="#Page_124">124</a>;</p> -<p class="pnii">eruption of 1906, <a href="#Page_127">127-137</a>;</p> -<p class="pnii">history of, <a href="#Page_97">97</a>;</p> -<p class="pnii">lavas of, <a href="#Page_32">32</a>.</p> - -<p class="pni">Mud cones, <a href="#Page_84">84</a>;</p> -<p class="pnii">aligned upon a fissure, <a href="#Page_84">84</a>.</p> - -<p class="pni">Mud-crack structure, <a href="#Page_37">37</a>.</p> - -<p class="pni">Mud, flocculent calcareous, of Florida, <a href="#Page_36">36</a>.</p> - -<p class="pni">Mud flows, which destroyed Herculaneum, <a href="#Page_139">139</a>.</p> - -<p class="pni">Mud veneer, from eruption of Taal, <a href="#Page_121">121</a>.</p> - -<p class="pni">Muir, John, cited, <a href="#Page_7">7</a>.</p> - -<p class="pni">Munthe, H., cited, <a href="#Page_313">313</a>, <a href="#Page_351">351</a>, <a href="#Page_410">410</a>.</p> - -<p class="pni">Murray, Sir John, cited, <a href="#Page_39">39</a>, <a href="#Page_293">293</a>.</p> - -<p class="pni">“Mushroom rocks”, <a href="#Page_205">205</a>.</p> - -<p class="pn"><span class="pl">N</span>ansen, F., cited, <a href="#Page_17">17</a>, <a href="#Page_260">260</a>, <a href="#Page_271">271</a>, <a href="#Page_272">272</a>, <a href="#Page_287">287</a>, <a href="#Page_295">295</a>.</p> - -<p class="pni">Narrows, river, <a href="#Page_174">174</a>, <a href="#Page_327">327</a>.</p> - -<p><span class="pagenum"><a name="Page_500" id="Page_500">[500]</a></span></p><p class="pni">Natural Bridge, near Lexington, Virginia, <a href="#Page_184">184</a>.</p> - -<p class="pni">Natural bridges, <a href="#Page_184">184</a>.</p> - -<p class="pni">Natural sand blast, <a href="#Page_204">204</a>.</p> - -<p class="pni">Nature of materials in the lithosphere, <a href="#Page_20">20</a>.</p> - -<p class="pni">Necks, volcanic, <a href="#Page_140">140</a>.</p> - -<p class="pni">Nephelite, <a href="#Page_459">459</a>.</p> - -<p class="pni">Neumayr, Melchior, cited, <a href="#Page_7">7</a>, <a href="#Page_146">146</a>, <a href="#Page_195">195</a>, <a href="#Page_196">196</a>, <a href="#Page_222">222</a>, <a href="#Page_425">425</a>.</p> - -<p class="pni">Névé, <a href="#Page_369">369</a>.</p> - -<p class="pni">Newborn glacier, <a href="#Page_387">387</a>.</p> - -<p class="pni">Newland, <a href="#Page_159">159</a>, <a href="#Page_247">247</a>.</p> - -<p class="pni">Newland lakes, <a href="#Page_401">401</a>, <a href="#Page_402">402</a>.</p> - -<p class="pni">New Madrid earthquake, <a href="#Page_83">83</a>.</p> - -<p class="pni">New River, of Cumberland plateau, <a href="#Page_173">173</a>.</p> - -<p class="pni">Niagara Falls, <a href="#Page_352">352-366</a>;</p> -<p class="pnii">episodes in history of, <a href="#Page_362">362-365</a>;</p> -<p class="pnii">the clock of recent geological time, <a href="#Page_364">364</a>.</p> - -<p class="pni">Niagara gorge, <a href="#Page_352">352-366</a>;</p> -<p class="pnii">drilling of, <a href="#Page_353">353</a>, <a href="#Page_355">355</a>;</p> -<p class="pnii">episodes in history of, in connection with glacial lakes, <a href="#Page_364">364</a>;</p> -<p class="pnii">plan and section of, <a href="#Page_355">355</a>;</p> -<p class="pnii">rate of recession of, <a href="#Page_356">356</a>.</p> - -<p class="pni">Niches, <a href="#Page_371">371</a>;</p> -<p class="pnii">beneath snowdrift sites, <a href="#Page_368">368</a>, <a href="#Page_369">369</a>.</p> - -<p class="pni">Nickel, in meteorites, <a href="#Page_23">23</a>.</p> - -<p class="pni"><i>Nieves penitentes</i>, <a href="#Page_397">397</a>.</p> - -<p class="pni">Nipissing Great Lakes, <a href="#Page_335">335</a>, <a href="#Page_342">342</a>.</p> - -<p class="pni">Nipissing outlet, <a href="#Page_335">335</a>, <a href="#Page_336">336</a>.</p> - -<p class="pni">Nippur, sand mounds over, <a href="#Page_218">218</a>.</p> - -<p class="pni">Nivation, <a href="#Page_368">368</a>.</p> - -<p class="pni">Nivation glacier, <a href="#Page_387">387</a>.</p> - -<p class="pni">Noble, F. H., cited, <a href="#Page_147">147</a>.</p> - -<p class="pni">Nordenskiöld, Otto, cited, <a href="#Page_154">154</a>, <a href="#Page_157">157</a>, <a href="#Page_295">295</a>.</p> - -<p class="pni">North Atlantis, <a href="#Page_16">16</a>.</p> - -<p class="pni">North Bay outlet, <a href="#Page_335">335</a>.</p> - -<p class="pni">Northwest Highlands of Scotland, thrusts of, <a href="#Page_45">45</a>.</p> - -<p class="pni">Norway, repeating patterns of, <a href="#Page_229">229</a>.</p> - -<p class="pni">Notched cliffs, <a href="#Page_233">233</a>;</p> -<p class="pnii">elevated, <a href="#Page_248">248</a>.</p> - -<p class="pni">Nourishment of continental glaciers, <a href="#Page_295">295</a>.</p> - -<p class="pni">Nunataks, <a href="#Page_272">272</a>, <a href="#Page_274">274</a>, <a href="#Page_277">277</a>.</p> - -<p class="pni">Nussbaum, F., cited, <a href="#Page_161">161</a>.</p> - -<p class="pn"><span class="pl">O</span>asis, <a href="#Page_216">216</a>.</p> - -<p class="pni">Oblateness, of the earth, <a href="#Page_10">10</a>.</p> - -<p class="pni">Observational geology <i>vs.</i> speculative philosophy, <a href="#Page_5">5</a>.</p> - -<p class="pni">Obsidian, <a href="#Page_463">463</a>.</p> - -<p class="pni">Obsidian Cliff, <a href="#Page_33">33</a>.</p> - -<p class="pni">Ocean of Tethys, <a href="#Page_16">16</a>.</p> - -<p class="pni">Oceanic platform, <a href="#Page_19">19</a>.</p> - -<p class="pni">Oceans, arrangement of, <a href="#Page_10">10</a>.</p> - -<p class="pni">Oldham, R. D., cited, <a href="#Page_72">72</a>, <a href="#Page_76">76</a>, <a href="#Page_92">92</a>.</p> - -<p class="pni">Oldland, <a href="#Page_159">159</a>, <a href="#Page_247">247</a>.</p> - -<p class="pni">Olivine, <a href="#Page_461">461</a>.</p> - -<p class="pni">Omori, F., cited, <a href="#Page_147">147</a>.</p> - -<p class="pni">Oölite, <a href="#Page_464">464</a>.</p> - -<p class="pni">Oölitic limestone, <a href="#Page_464">464</a>.</p> - -<p class="pni">Ooze, calcareous, <a href="#Page_36">36</a>;</p> -<p class="pnii">composition of, <a href="#Page_39">39</a>.</p> - -<p class="pni">Optical mineralogy, <a href="#Page_27">27</a>.</p> - -<p class="pni">Order of deposition, during marine transgression, <a href="#Page_37">37</a>.</p> - -<p class="pni">Order of superposition, of strata, <a href="#Page_52">52</a>.</p> - -<p class="pni">Organic sediments, <a href="#Page_34">34</a>.</p> - -<p class="pni"><i>Orgeln</i>, <a href="#Page_182">182</a>.</p> - -<p class="pni">Orleans, Duc d’, cited, <a href="#Page_286">286</a>.</p> - -<p class="pni">Orographic blocks, <a href="#Page_58">58</a>.</p> - -<p class="pni">Osar, <a href="#Page_311">311</a>, <a href="#Page_315">315</a>, <a href="#Page_316">316</a>.</p> - -<p class="pni">Oscillations of movement, on coasts, <a href="#Page_253">253</a>.</p> - -<p class="pni">Outcrop blocks, for study of maps, <a href="#Page_63">63</a>.</p> - -<p class="pni">Outcroppings, <a href="#Page_46">46</a>.</p> - -<p class="pni">Outlets, from continental glaciers, <a href="#Page_271">271</a>;</p> -<p class="pnii">of glacial lakes, <a href="#Page_326">326</a>, <a href="#Page_327">327</a>.</p> - -<p class="pni">Outwash plains, <a href="#Page_280">280</a>, <a href="#Page_281">281</a>, <a href="#Page_311">311</a>, <a href="#Page_313">313</a>, <a href="#Page_314">314</a>, <a href="#Page_399">399</a>, <a href="#Page_408">408</a>.</p> - -<p class="pni">Overthrust, <a href="#Page_45">45</a>.</p> - -<p class="pni">Owens Valley, California, map of earthquake faults in, <a href="#Page_78">78</a>.</p> - -<p class="pni">“Ox-bow”, of river, <a href="#Page_165">165</a>.</p> - -<p class="pni">Ox-bow lakes, <a href="#Page_165">165</a>, <a href="#Page_415">415</a>.</p> - - -<p class="pn"><span class="pl">P</span>ack, drift of, <a href="#Page_287">287</a>;</p> -<p class="pnii">the, <a href="#Page_286">286</a>.</p> - -<p class="pni">Pack ice, <a href="#Page_286">286</a>.</p> - -<p class="pni">Pagination, of the earth record, <a href="#Page_38">38</a>.</p> - -<p class="pni"><i>Pahoehoe</i> type of lava surface, <a href="#Page_113">113</a>.</p> - -<p class="pni">Pan form of deserts, <a href="#Page_197">197</a>.</p> - -<p class="pni">Panum crater, <i>caldera</i> of, <a href="#Page_126">126</a>.</p> - -<p class="pni">“Parallel roads”, of Scottish glens, <a href="#Page_322">322-325</a>, <a href="#Page_328">328</a>, <a href="#Page_339">339</a>.</p> - -<p class="pni">Partially dissected upland, <a href="#Page_160">160</a>.</p> - -<p class="pni">Passarge, S., cited, <a href="#Page_221">221</a>, <a href="#Page_222">222</a>.</p> - -<p class="pni">“Paternoster lakes”, <a href="#Page_376">376</a>.</p> - -<p class="pni">Pattern, of river etchings, <a href="#Page_158">158</a>.</p> - -<p class="pni">Patterns, repeating, <a href="#Page_223">223</a>.</p> - -<p class="pni">Pavement, bowlder, <a href="#Page_237">237</a>;</p> -<p class="pnii">glacier, <a href="#Page_276">276</a>;</p> -<p class="pnii">tessellated from soil flow, <a href="#Page_154">154</a>.</p> - -<p class="pni">Pavlow, A. P., cited, <a href="#Page_108">108</a>.</p> - -<p class="pni">Peale, A. C., cited, <a href="#Page_195">195</a>, <a href="#Page_196">196</a>.</p> - -<p class="pni">Peary, R. E., cited, <a href="#Page_17">17</a>, <a href="#Page_283">283</a>, <a href="#Page_289">289</a>, <a href="#Page_295">295</a>, <a href="#Page_296">296</a>.</p> - -<p class="pni">Peat, <a href="#Page_465">465</a>;</p> -<p class="pnii">formation of, <a href="#Page_429">429</a>, <a href="#Page_430">430</a>.</p> - -<p class="pni">Peat bogs, <a href="#Page_429">429</a>.</p> - -<p class="pni">“Pelé’s Hair”, <a href="#Page_107">107</a>.</p> - -<p class="pni">Pelé, spine of, <a href="#Page_148">148</a>.</p> - -<p class="pni">Penck, A., cited, <a href="#Page_294">294</a>, <a href="#Page_399">399</a>, <a href="#Page_414">414</a>.</p> - -<p class="pni">Peneplain, <a href="#Page_171">171</a>, <a href="#Page_179">179</a>.</p> - -<p class="pni">“Penitents”, <a href="#Page_397">397</a>.</p> - -<p class="pni">“Perched bowlders”, <a href="#Page_306">306</a>.</p> - -<p class="pni">Peridotite, <a href="#Page_462">462</a>.</p> - -<p class="pni">Periods, interpluvial, <a href="#Page_198">198</a>;</p> -<p class="pnii">pluvial, <a href="#Page_198">198</a>.</p> - -<p class="pni">Peripheral granulation, <a href="#Page_31">31</a>.</p> - -<p class="pni">Perret, F. A., cited, <a href="#Page_148">148</a>.</p> - -<p class="pni">Philippi, E., cited, <a href="#Page_295">295</a>.</p> - -<p class="pni">Phillips, John, cited, <a href="#Page_56">56</a>.</p> - -<p class="pni">Physiographic models, preparation, of, <a href="#Page_470">470</a>.</p> - -<p class="pni">Piedmont glaciers, <a href="#Page_383">383</a>, <a href="#Page_384">384</a>.</p> - -<p><span class="pagenum"><a name="Page_501" id="Page_501">[501]</a></span></p><p class="pni"><i>Pino</i>, <a href="#Page_119">119</a>, <a href="#Page_130">130</a>.</p> - -<p class="pni">Pipes, volcanic, <a href="#Page_140">140</a>.</p> - -<p class="pni">Piracy, river, <a href="#Page_175">175</a>, <a href="#Page_176">176</a>.</p> - -<p class="pni">Pirsson, L. V., cited, <a href="#Page_39">39</a>, <a href="#Page_447">447</a>.</p> - -<p class="pni">Pitch, <a href="#Page_43">43</a>.</p> - -<p class="pni">Pitching folds, <a href="#Page_43">43</a>.</p> - -<p class="pni">Pit lakes, <a href="#Page_315">315</a>, <a href="#Page_407">407</a>, <a href="#Page_408">408</a>.</p> - -<p class="pni">Pitted plains, <a href="#Page_314">314</a>, <a href="#Page_407">407</a>, <a href="#Page_408">408</a>.</p> - -<p class="pni">Pittier, H., cited, <a href="#Page_405">405</a>.</p> - -<p class="pni">Plains, flood, <a href="#Page_178">178</a>;</p> -<p class="pnii">coastal, <a href="#Page_246">246</a>;</p> -<p class="pnii">outwash, <a href="#Page_280">280</a>, <a href="#Page_281">281</a>;</p> -<p class="pnii">pitted, <a href="#Page_314">314</a>, <a href="#Page_407">407</a>, <a href="#Page_408">408</a>.</p> - -<p class="pni">Platform, continental, <a href="#Page_18">18</a>, <a href="#Page_19">19</a>;</p> -<p class="pnii">oceanic, <a href="#Page_18">18</a>, <a href="#Page_19">19</a>.</p> - -<p class="pni">Playa lakes, <a href="#Page_422">422</a>.</p> - -<p class="pni">Playfair, Sir John, cited, <a href="#Page_178">178</a>.</p> - -<p class="pni">Plucking, beneath glaciers, <a href="#Page_275">275</a>.</p> - -<p class="pni">Plugs, volcanic, <a href="#Page_140">140</a>.</p> - -<p class="pni">Plunge and flow structure, <a href="#Page_37">37</a>.</p> - -<p class="pni">Plunging folds, <a href="#Page_43">43</a>;</p> -<p class="pnii">detection of, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>.</p> - -<p class="pni">Pluvial periods, <a href="#Page_198">198</a>.</p> - -<p class="pni">Pocket rocks, in desert, <a href="#Page_200">200</a>, <a href="#Page_201">201</a>, <a href="#Page_202">202</a>.</p> - -<p class="pni">Poles, wind, of the earth, <a href="#Page_263">263</a>;</p> -<p class="pnii">earlier, <a href="#Page_297">297</a>.</p> - -<p class="pni"><i>Poljen</i>, <a href="#Page_189">189</a>, <a href="#Page_422">422</a>.</p> - -<p class="pni">Pompeii, destruction of, <a href="#Page_97">97</a>;</p> -<p class="pnii">volcanic materials over, <a href="#Page_122">122</a>.</p> - -<p class="pni"><i>Ponores</i>, <a href="#Page_188">188</a>.</p> - -<p class="pni">Porphyritic texture, of certain igneous rocks, <a href="#Page_32">32</a>.</p> - -<p class="pni">Portals, in mountain rampart, surrounding continental glaciers, <a href="#Page_271">271</a>.</p> - -<p class="pni">Potato shape, of earth, <a href="#Page_7">7</a>.</p> - -<p class="pni"><i>Pourquoi-Pas</i> expedition, <a href="#Page_289">289</a>.</p> - -<p class="pni">Powell, J. W., cited, <a href="#Page_178">178</a>, <a href="#Page_439">439</a>, <a href="#Page_446">446</a>.</p> - -<p class="pni">Pratt, W. E., cited, <a href="#Page_147">147</a>.</p> - -<p class="pni">Precipitation, in relation to glaciation, <a href="#Page_261">261</a>.</p> - -<p class="pni">Pressure ridges, on pack ice, <a href="#Page_286">286</a>.</p> - -<p class="pni">Prinz, cited, <a href="#Page_14">14</a>, <a href="#Page_19">19</a>, <a href="#Page_54">54</a>, <a href="#Page_133">133</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Processes by which rocks are formed, <a href="#Page_30">30</a>.</p> - -<p class="pni">Profile, cut by waves on steep rocky shore, <a href="#Page_236">236</a>.</p> - -<p class="pni">Profiles, character, <a href="#Page_177">177</a>, <a href="#Page_318">318</a>;</p> -<p class="pnii">character, directly due to volcanic agencies, <a href="#Page_145">145</a>, <a href="#Page_146">146</a>;</p> -<p class="pnii">character, coast, due to uplift or depression, <a href="#Page_259">259</a>;</p> -<p class="pnii">character, of arid lands, <a href="#Page_220">220</a>;</p> -<p class="pnii">character, of shore features, <a href="#Page_243">243</a>;</p> -<p class="pnii">character, referable to mountain glaciers, <a href="#Page_379">379</a>;</p> -<p class="pnii">of cinder cones, <a href="#Page_123">123</a>.</p> - -<p class="pni">Projectiles, lava, “bread-crust” type, <a href="#Page_119">119</a>;</p> -<p class="pnii">volcanic, <a href="#Page_121">121</a>.</p> - -<p class="pni">Prying work of frost, <a href="#Page_152">152</a>.</p> - -<p class="pni">“Pudding stone”, <a href="#Page_463">463</a>.</p> - -<p class="pni">Pumiceous texture, of extrusive rocks, <a href="#Page_32">32</a>.</p> - -<p class="pni">Pumpelly, Raphael, cited, <a href="#Page_222">222</a>.</p> - -<p class="pni">Pumpelly, R. W., cited, <a href="#Page_212">212</a>.</p> - -<p class="pni"><i>Puys</i>, <a href="#Page_105">105</a>.</p> - -<p class="pni"><i>Puys</i> of Auvergne, <a href="#Page_124">124</a>.</p> - -<p class="pni">Pyrite, <a href="#Page_452">452</a>.</p> - -<p class="pni">Pyrolusite, <a href="#Page_456">456</a>.</p> - -<p class="pni">Pyroxenes, <a href="#Page_458">458</a>.</p> - -<p class="pn"><span class="pl">Q</span>uartz, <a href="#Page_458">458</a>.</p> - -<p class="pni">Quartzite, <a href="#Page_466">466</a>.</p> - -<p class="pni"><i>Quebradas</i>, <a href="#Page_75">75</a>.</p> - -<p class="pn"><span class="pl">R</span>abot, C., cited, <a href="#Page_424">424</a>.</p> - -<p class="pni">Radiating glaciers, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>.</p> - -<p class="pni">Raft lakes, <a href="#Page_417">417</a>, <a href="#Page_418">418</a>.</p> - -<p class="pni">Rafts, log, in Red River, <a href="#Page_418">418</a>.</p> - -<p class="pni">Railway tracks, buckled, during earthquakes, <a href="#Page_75">75</a>.</p> - -<p class="pni">Rain erosion, <a href="#Page_214">214</a>.</p> - -<p class="pni">Rainfall, infrequent in deserts, <a href="#Page_197">197</a>.</p> - -<p class="pni">Raised beaches, <a href="#Page_326">326</a>, <a href="#Page_328">328</a>.</p> - -<p class="pni">Ramparts, ice, <a href="#Page_431">431-434</a>.</p> - -<p class="pni"><i>Randspalte</i>, <a href="#Page_370">370</a>.</p> - -<p class="pni">Rapids, in Rhine gorge, <a href="#Page_169">169</a>.</p> - -<p class="pni"><i>Rapilli</i>, <a href="#Page_122">122</a>.</p> - -<p class="pni">Rath, G. vom, cited, <a href="#Page_147">147</a>.</p> - -<p class="pni">Reaction rims, about minerals, <a href="#Page_28">28</a>.</p> - -<p class="pni">Receding hemicycle of glaciation, <a href="#Page_264">264</a>.</p> - -<p class="pni">Recessional moraines, <a href="#Page_399">399</a>.</p> - -<p class="pni">Reciprocal relation, of land and sea, map to show, <a href="#Page_11">11</a>.</p> - -<p class="pni">Réclus, E., cited, <a href="#Page_147">147</a>.</p> - -<p class="pni">Records, of rise or fall of land, <a href="#Page_245">245</a>.</p> - -<p class="pni">Red clay, of the deep sea, <a href="#Page_39">39</a>.</p> - -<p class="pni">Red color, of desert rocks, <a href="#Page_202">202</a>.</p> - -<p class="pni">Reid, H. F., cited, <a href="#Page_294">294</a>, <a href="#Page_296">296</a>, <a href="#Page_400">400</a>.</p> - -<p class="pni">Rejuvenated rivers, <a href="#Page_173">173</a>, <a href="#Page_174">174</a>.</p> - -<p class="pni">Relief forms, carved by waves, <a href="#Page_213">213</a>.</p> - -<p class="pni">Relief patterns, dividing lines of, <a href="#Page_226">226</a>.</p> - -<p class="pni">Repeating patterns, in earth relief, <a href="#Page_223">223</a>;</p> -<p class="pnii">composite, <a href="#Page_227">227</a>.</p> - -<p class="pni">Reservoirs, of lava, local, <a href="#Page_95">95</a>.</p> - -<p class="pni">Residual rocks, <a href="#Page_30">30</a>.</p> - -<p class="pni">Resistant rocks, in relation to erosion, <a href="#Page_174">174</a>.</p> - -<p class="pni">Rhine, gorge of, <a href="#Page_169">169</a>.</p> - -<p class="pni">Rhyolite, <a href="#Page_463">463</a>.</p> - -<p class="pni">Ribbon falls, <a href="#Page_378">378</a>.</p> - -<p class="pni">Richter, E., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Richtofen, Freiherr von, cited, <a href="#Page_207">207</a>, <a href="#Page_222">222</a>.</p> - -<p class="pni">“Ridge roads”, <a href="#Page_328">328</a>.</p> - -<p class="pni"><i>Riegel</i>, <a href="#Page_377">377</a>.</p> - -<p class="pni">Rifting, in eroded mountains, <a href="#Page_444">444</a>.</p> - -<p class="pni">Rift-valley lakes, <a href="#Page_403">403</a>, <a href="#Page_404">404</a>.</p> - -<p class="pni">Rift valleys, <a href="#Page_440">440</a>.</p> - -<p class="pni">Rigidity of the earth, <a href="#Page_20">20</a>, <a href="#Page_29">29</a>.</p> - -<p class="pni">Ripple markings, <a href="#Page_36">36</a>.</p> - -<p class="pni">River, zone of the dwindling, <a href="#Page_213">213</a>.</p> - -<p class="pni">River capture, <a href="#Page_175">175</a>.</p> - -<p class="pni">River deltas, <a href="#Page_179">179</a>.</p> - -<p class="pni">River etchings, intricate pattern of, <a href="#Page_158">158</a>.</p> - -<p><span class="pagenum"><a name="Page_502" id="Page_502">[502]</a></span></p><p class="pni">River lakes, <a href="#Page_424">424</a>.</p> - -<p class="pni">River narrows, <a href="#Page_174">174</a>, <a href="#Page_327">327</a>.</p> - -<p class="pni">River networks, in relation to precipitation, <a href="#Page_161">161</a>;</p> -<p class="pnii">in relation to rock architecture, <a href="#Page_161">161</a>;</p> -<p class="pnii">meshes of, <a href="#Page_161">161</a>.</p> - -<p class="pni">Rivers, braided, <a href="#Page_280">280</a>;</p> -<p class="pnii">cross sections of, in successive stages, <a href="#Page_172">172</a>;</p> -<p class="pnii">drowned, <a href="#Page_251">251</a>, <a href="#Page_340">340</a>;</p> -<p class="pnii">early aspects of, <a href="#Page_159">159</a>;</p> -<p class="pnii">life begun in uplift, <a href="#Page_159">159</a>;</p> -<p class="pnii">life histories of, <a href="#Page_158">158</a>;</p> -<p class="pnii">motive power of, <a href="#Page_158">158</a>;</p> -<p class="pnii">rejuvenated, <a href="#Page_173">173</a>, <a href="#Page_174">174</a>;</p> -<p class="pnii">submerged channels of, <a href="#Page_252">252</a>;</p> -<p class="pnii">swollen during melting of continental glaciers, <a href="#Page_320">320</a>;</p> -<p class="pnii">tributary, accordant, <a href="#Page_377">377</a>;</p> -<p class="pnii">young, <a href="#Page_159">159</a>, <a href="#Page_160">160</a>.</p> - -<p class="pni">River terraces, <a href="#Page_165">165</a>, <a href="#Page_178">178</a>.</p> - -<p class="pni">River valley, longitudinal section of, <a href="#Page_161">161</a>.</p> - -<p class="pni"><i>Roches moutonnées</i>, <a href="#Page_276">276</a>, <a href="#Page_301">301</a>, <a href="#Page_367">367</a>.</p> - -<p class="pni">Rock bars, <a href="#Page_377">377</a>;</p> -<p class="pnii">cut through by gorges, <a href="#Page_378">378</a>.</p> - -<p class="pni">Rock basin lakes, <a href="#Page_376">376</a>, <a href="#Page_377">377</a>, <a href="#Page_400">400</a>, <a href="#Page_412">412</a>.</p> - -<p class="pni">Rock cleavage, <a href="#Page_44">44</a>.</p> - -<p class="pni">“Rock glaciers”, <a href="#Page_153">153</a>.</p> - -<p class="pni">“Rocking stones”, <a href="#Page_306">306</a>.</p> - -<p class="pni">Rock mantle, <a href="#Page_155">155</a>;</p> -<p class="pnii">relation to topography, <a href="#Page_156">156</a>.</p> - -<p class="pni">Rock pedestals, <a href="#Page_381">381</a>.</p> - -<p class="pni">Rock terraces, <a href="#Page_215">215</a>.</p> - -<p class="pni">Rocks, clastic, <a href="#Page_30">30</a>;</p> -<p class="pnii">corrosion of, <a href="#Page_156">156</a>;</p> -<p class="pnii">description of some common, <a href="#Page_462">462-466</a>;</p> -<p class="pnii">extrusive, <a href="#Page_32">32</a>, <a href="#Page_463">463</a>;</p> -<p class="pnii">igneous, <a href="#Page_30">30</a>;</p> -<p class="pnii">igneous, textures of, <a href="#Page_32">32</a>;</p> -<p class="pnii">igneous, massive structure of, <a href="#Page_31">31</a>;</p> -<p class="pnii">intrusive, <a href="#Page_32">32</a>, <a href="#Page_462">462</a>, <a href="#Page_463">463</a>;</p> -<p class="pnii">laminated structure of, <a href="#Page_31">31</a>;</p> -<p class="pnii">marks of origin of, <a href="#Page_30">30</a>;</p> -<p class="pnii">metamorphic, <a href="#Page_30">30</a>, <a href="#Page_31">31</a>, <a href="#Page_465">465</a>;</p> -<p class="pnii">residual, <a href="#Page_30">30</a>;</p> -<p class="pnii">sedimentary, <a href="#Page_30">30</a>;</p> -<p class="pnii">sedimentary, of chemical precipitation, <a href="#Page_464">464</a>;</p> -<p class="pnii">sedimentary, of mechanical origin, <a href="#Page_463">463</a>;</p> -<p class="pnii">sedimentary, of organic origin, <a href="#Page_464">464</a>;</p> -<p class="pnii">sedimentary, rounded grains of, <a href="#Page_31">31</a>;</p> -<p class="pnii">volcanic, <a href="#Page_32">32</a>.</p> - -<p class="pni">Ross Barrier, <a href="#Page_282">282</a>.</p> - -<p class="pni">Rudolph, E., cited, <a href="#Page_92">92</a>.</p> - -<p class="pni">Rudski, M. P., cited, <a href="#Page_19">19</a>.</p> - -<p class="pni">Russell, I. C., cited, <a href="#Page_126">126</a>, <a href="#Page_147">147</a>, <a href="#Page_148">148</a>, <a href="#Page_175">175</a>, <a href="#Page_178">178</a>, <a href="#Page_222">222</a>, <a href="#Page_293">293</a>, <a href="#Page_294">294</a>, <a href="#Page_296">296</a>, <a href="#Page_381">381</a>, <a href="#Page_384">384</a>, <a href="#Page_414">414</a>, <a href="#Page_424">424</a>, <a href="#Page_425">425</a>.</p> - -<p class="pn"><span class="pl">S</span>t. Anthony Falls, recession of, <a href="#Page_327">327</a>, <a href="#Page_354">354</a>.</p> - -<p class="pni">St. David’s gorge, near Niagara, <a href="#Page_352">352</a>, <a href="#Page_359">359</a>, <a href="#Page_360">360</a>, <a href="#Page_363">363</a>.</p> - -<p class="pni">St. Goars, on Rhine, <a href="#Page_169">169</a>.</p> - -<p class="pni">Saint Martin, cited, <a href="#Page_436">436</a>.</p> - -<p class="pni">St. Paul’s rocks, a dissected volcano, <a href="#Page_141">141</a>.</p> - -<p class="pni">Salients, of newly incised upland, <a href="#Page_169">169</a>.</p> - -<p class="pni">Salines, <a href="#Page_423">423</a>.</p> - -<p class="pni">Salisbury, R. D., cited, <a href="#Page_156">156</a>, <a href="#Page_160">160</a>, <a href="#Page_205">205</a>, <a href="#Page_222">222</a>, <a href="#Page_293">293</a>, <a href="#Page_295">295</a>, <a href="#Page_298">298</a>, <a href="#Page_300">300</a>, <a href="#Page_305">305</a>, <a href="#Page_313">313</a>, <a href="#Page_318">318</a>, <a href="#Page_319">319</a>, <a href="#Page_339">339</a>, <a href="#Page_424">424</a>.</p> - -<p class="pni">Salton sink, <a href="#Page_420">420</a>.</p> - -<p class="pni">Sand, beach, <a href="#Page_206">206</a>;</p> -<p class="pnii">eolian, <a href="#Page_206">206</a>;</p> -<p class="pnii">volcanic, <a href="#Page_122">122</a>.</p> - -<p class="pni">Sand blast, natural, <a href="#Page_204">204</a>.</p> - -<p class="pni">Sand cones, <a href="#Page_84">84</a>.</p> - -<p class="pni">“Sand devils”, <a href="#Page_209">209</a>.</p> - -<p class="pni">Sandstone, <a href="#Page_464">464</a>.</p> - -<p class="pni">Sand storms, <a href="#Page_209">209</a>.</p> - -<p class="pni">Santa Catalina, <a href="#Page_239">239</a>, <a href="#Page_257">257</a>.</p> - -<p class="pni">Sapper, K., cited, <a href="#Page_111">111</a>, <a href="#Page_147">147</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Sarasin, P. and F., cited, <a href="#Page_248">248</a>.</p> - -<p class="pni">Sardeson, F. W., cited, <a href="#Page_327">327</a>, <a href="#Page_339">339</a>.</p> - -<p class="pni">Saucer lakes, <a href="#Page_415">415</a>, <a href="#Page_416">416</a>.</p> - -<p class="pni">Sawa Lake, of Persian desert, <a href="#Page_199">199</a>.</p> - -<p class="pni">Scaling, <a href="#Page_151">151</a>.</p> - -<p class="pni">Scape colks, <a href="#Page_277">277</a>.</p> - -<p class="pni">Scars, from dissection of volcanoes, <a href="#Page_142">142</a>;</p> -<p class="pnii">meander, <a href="#Page_165">165</a>.</p> - -<p class="pni">Schist, chlorite, <a href="#Page_465">465</a>;</p> -<p class="pnii">mica, <a href="#Page_465">465</a>;</p> -<p class="pnii">sericite, <a href="#Page_465">465</a>;</p> -<p class="pnii">talc, <a href="#Page_465">465</a>.</p> - -<p class="pni">Schistosity, <a href="#Page_31">31</a>.</p> - -<p class="pni">Schrader, cited, <a href="#Page_436">436</a>.</p> - -<p class="pni"><i>Schratten</i>, <a href="#Page_188">188</a>.</p> - -<p class="pni">Scidmore, E. R., cited, <a href="#Page_70">70</a>.</p> - -<p class="pni">Scoriaceous texture, of extrusive rocks, <a href="#Page_32">32</a>.</p> - -<p class="pni">Scott, I. D., cited, <a href="#Page_411">411</a>, <a href="#Page_470">470</a>.</p> - -<p class="pni">Scott, R. F., cited, <a href="#Page_282">282</a>, <a href="#Page_295">295</a>.</p> - -<p class="pni">Scott, W. B., cited, <a href="#Page_6">6</a>, <a href="#Page_60">60</a>, <a href="#Page_72">72</a>, <a href="#Page_259">259</a>, <a href="#Page_274">274</a>, <a href="#Page_375">375</a>.</p> - -<p class="pni">“Scree”, <a href="#Page_152">152</a>.</p> - -<p class="pni">Scrope, P., cited, <a href="#Page_96">96</a>, <a href="#Page_124">124</a>, <a href="#Page_146">146</a>.</p> - -<p class="pni">Sea caves, <a href="#Page_234">234</a>;</p> -<p class="pnii">elevated, <a href="#Page_248">248</a>.</p> - -<p class="pni">Sea coves, <a href="#Page_233">233</a>.</p> - -<p class="pni">Sea ice, <a href="#Page_286">286</a>, <a href="#Page_292">292</a>.</p> - -<p class="pni">Seaquakes, <a href="#Page_69">69</a>;</p> -<p class="pnii">distribution of, <a href="#Page_70">70</a>;</p> -<p class="pnii">downward movement of sea floor during, <a href="#Page_81">81</a>;</p> -<p class="pnii">number and magnitude of, <a href="#Page_81">81</a>.</p> - -<p class="pni">Seasonal lakes, <a href="#Page_189">189</a>, <a href="#Page_422">422</a>.</p> - -<p class="pni">Section, geological, <a href="#Page_46">46</a>, <a href="#Page_47">47</a>;</p> -<p class="pnii">across mountain wall about desert, <a href="#Page_212">212</a>.</p> - -<p class="pni">Sederholm, J. J., cited, <a href="#Page_315">315</a>.</p> - -<p class="pni">Sedimentary rocks, <a href="#Page_30">30</a>;</p> -<p class="pnii">of chemical precipitation, <a href="#Page_464">464</a>;</p> -<p class="pnii">of mechanical origin, <a href="#Page_463">463</a>;</p> -<p class="pnii">of organic origin, <a href="#Page_464">464</a>.</p> - -<p class="pni">Seismic sea wave, <a href="#Page_69">69</a>;</p> -<p class="pnii">Japan, 1896, <a href="#Page_70">70</a>.</p> - -<p class="pni">Seismotectonic lines, <a href="#Page_87">87</a>.</p> - -<p class="pni">Sekiya, S., cited, <a href="#Page_141">141</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Séracs, <a href="#Page_391">391</a>.</p> - -<p class="pni">Serapeum, at Pozzuoli, <a href="#Page_254">254</a>.</p> - -<p class="pni">Sericite schist, <a href="#Page_465">465</a>.</p> - -<p class="pni">Series, conformable, <a href="#Page_51">51</a>;</p> -<p class="pnii">unconformable, <a href="#Page_51">51</a>.</p> - -<p class="pni">Serpentine, <a href="#Page_460">460</a>.</p> - -<p class="pni">Shackleton, Sir Ernest, cited, <a href="#Page_17">17</a>, <a href="#Page_282">282</a>, <a href="#Page_283">283</a>, <a href="#Page_292">292</a>, <a href="#Page_295">295</a>.</p> - -<p class="pni">Shadow erosion, <a href="#Page_206">206</a>.</p> - -<p><span class="pagenum"><a name="Page_503" id="Page_503">[503]</a></span></p><p class="pni">Shadow weathering, <a href="#Page_203">203</a>.</p> - -<p class="pni">Shale, <a href="#Page_464">464</a>.</p> - -<p class="pni">Shaler, N. S., cited, <a href="#Page_7">7</a>, <a href="#Page_157">157</a>, <a href="#Page_244">244</a>, <a href="#Page_306">306</a>, <a href="#Page_317">317</a>, <a href="#Page_319">319</a>.</p> - -<p class="pni">Shapes of rock folds, <a href="#Page_43">43</a>.</p> - -<p class="pni">Shaw, E. W., cited, <a href="#Page_425">425</a>.</p> - -<p class="pni">Shearing, in folds, <a href="#Page_45">45</a>.</p> - -<p class="pni">“Sheep backs”, <a href="#Page_276">276</a>.</p> - -<p class="pni">Shelf, continental, <a href="#Page_18">18</a>, <a href="#Page_19">19</a>.</p> - -<p class="pni">Shelf ice, <a href="#Page_281">281</a>, <a href="#Page_282">282</a>, <a href="#Page_283">283</a>;</p> -<p class="pnii">Antarctic, <a href="#Page_289">289</a>, <a href="#Page_290">290</a>;</p> -<p class="pnii">of ice age, <a href="#Page_317">317</a>.</p> - -<p class="pni">Sherzer, W. H., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Shields, of lithosphere, <a href="#Page_436">436</a>.</p> - -<p class="pni">Shingle, <a href="#Page_239">239</a>.</p> - -<p class="pni">Shoal water deposits, <a href="#Page_36">36</a>.</p> - -<p class="pni">Shore current, work of, <a href="#Page_237">237</a>, <a href="#Page_238">238</a>.</p> - -<p class="pni">Shore lines, elevated, <a href="#Page_340">340</a>;</p> -<p class="pnii">migration of landward with uplift, <a href="#Page_251">251</a>.</p> - -<p class="pni">Side delta lakes, <a href="#Page_418">418</a>, <a href="#Page_419">419</a>.</p> - -<p class="pni">Siderite, <a href="#Page_456">456</a>.</p> - -<p class="pni">Sieberg, A., cited, <a href="#Page_92">92</a>.</p> - -<p class="pni">Sieger, R., cited, <a href="#Page_259">259</a>.</p> - -<p class="pni">Siliceous lava, viscous, <a href="#Page_103">103</a>.</p> - -<p class="pni">Siliceous sinter, <a href="#Page_194">194</a>.</p> - -<p class="pni">Sills, <a href="#Page_142">142</a>.</p> - -<p class="pni">Sinclair, W. J., cited, <a href="#Page_152">152</a>.</p> - -<p class="pni">Sink lakes, <a href="#Page_421">421</a>.</p> - -<p class="pni">Sinks, in limestone, <a href="#Page_182">182</a>.</p> - -<p class="pni">Sinter, calcareous, <a href="#Page_184">184</a>;</p> -<p class="pnii">siliceous, <a href="#Page_194">194</a>.</p> - -<p class="pni">Sinter columns, formation of, <a href="#Page_185">185</a>.</p> - -<p class="pni">Sinter deposits, <a href="#Page_184">184</a>.</p> - -<p class="pni">Sjögren, Otto, cited, <a href="#Page_225">225</a>.</p> - -<p class="pni">Skaptár fissure in Iceland, <a href="#Page_99">99</a>.</p> - -<p class="pni">Skyline, straight, of mature upland, <a href="#Page_170">170</a>.</p> - -<p class="pni">Slate, clay, <a href="#Page_466">466</a>.</p> - -<p class="pni">Slichter, C. S., cited, <a href="#Page_195">195</a>.</p> - -<p class="pni">Slickensides, on fault, <a href="#Page_60">60</a>.</p> - -<p class="pni">Smith, George Otis, cited, <a href="#Page_173">173</a>.</p> - -<p class="pni">Smithsonite, <a href="#Page_456">456</a>.</p> - -<p class="pni">“Smoke” of volcanoes, nature of, <a href="#Page_128">128</a>.</p> - -<p class="pni">Smyth, C. H., Jr., cited, <a href="#Page_157">157</a>.</p> - -<p class="pni">Snake river, Idaho, lava plains of, <a href="#Page_102">102</a>.</p> - -<p class="pni">Snickers Gap, <a href="#Page_177">177</a>.</p> - -<p class="pni">Snow, B. W., cited, <a href="#Page_193">193</a>.</p> - -<p class="pni">Snowbergs, <a href="#Page_292">292</a>, <a href="#Page_293">293</a>.</p> - -<p class="pni">Snowdrift sites, <a href="#Page_368">368</a>.</p> - -<p class="pni">Snow line, <a href="#Page_261">261</a>.</p> - -<p class="pni">Soil flow, <a href="#Page_153">153</a>, <a href="#Page_157">157</a>.</p> - -<p class="pni">Soil striping, <a href="#Page_154">154</a>.</p> - -<p class="pni">Solfatara condition of volcanoes, <a href="#Page_97">97</a>.</p> - -<p class="pni">Solger, F., cited, <a href="#Page_222">222</a>.</p> - -<p class="pni">Solifluxion, <a href="#Page_153">153</a>, <a href="#Page_157">157</a>.</p> - -<p class="pni">Sonklar, cited, <a href="#Page_386">386</a>.</p> - -<p class="pni">Spallanzani, cited, <a href="#Page_115">115</a>.</p> - -<p class="pni">Spatter cones, <a href="#Page_104">104</a>.</p> - -<p class="pni">Speculative philosophy <i>vs.</i> observational geology, <a href="#Page_5">5</a>.</p> - -<p class="pni">Spencer, J. W., cited, <a href="#Page_260">260</a>, <a href="#Page_344">344</a>, <a href="#Page_350">350</a>, <a href="#Page_353">353</a>, <a href="#Page_366">366</a>.</p> - -<p class="pni">Spethmann, H., cited, <a href="#Page_267">267</a>.</p> - -<p class="pni">Sphalerite, <a href="#Page_453">453</a>.</p> - -<p class="pni">Spherulites, <a href="#Page_33">33</a>.</p> - -<p class="pni">Spherulitic texture, of igneous rocks, <a href="#Page_33">33</a>.</p> - -<p class="pni">Sphinx, erosion by natural sand blast, <a href="#Page_205">205</a>.</p> - -<p class="pni">Spits, <a href="#Page_240">240</a>.</p> - -<p class="pni">Spitzbergen, <a href="#Page_154">154</a>.</p> - -<p class="pni">Springs, fissure, <a href="#Page_190">190</a>, <a href="#Page_195">195</a>;</p> -<p class="pnii">surface, <a href="#Page_181">181</a>;</p> -<p class="pnii">thermal, <a href="#Page_190">190</a>.</p> - -<p class="pni">Stability, not the order of nature, <a href="#Page_4">4</a>.</p> - -<p class="pni">Stacks, <a href="#Page_233">233</a>;</p> -<p class="pnii">elevated, <a href="#Page_249">249</a>, <a href="#Page_343">343</a>.</p> - -<p class="pni">Stage of adolescence, <a href="#Page_169">169</a>, <a href="#Page_170">170</a>.</p> - -<p class="pni">Stairway, cascade, <a href="#Page_376">376</a>.</p> - -<p class="pni">Stalactites, growth of, <a href="#Page_184">184</a>.</p> - -<p class="pni">Stalagmites, formation of, <a href="#Page_185">185</a>.</p> - -<p class="pni">Staurolite, <a href="#Page_460">460</a>.</p> - -<p class="pni">Steppes, <a href="#Page_215">215</a>.</p> - -<p class="pni">Still river, of Connecticut, history of, <a href="#Page_338">338</a>.</p> - -<p class="pni">Stone, G. H., cited, <a href="#Page_253">253</a>, <a href="#Page_260">260</a>, <a href="#Page_315">315</a>, <a href="#Page_319">319</a>.</p> - -<p class="pni">“Stone ginger”, <a href="#Page_208">208</a>.</p> - -<p class="pni">“Stone lattice”, <a href="#Page_205">205</a>, <a href="#Page_206">206</a>.</p> - -<p class="pni">“Stone rivers”, <a href="#Page_153">153</a>.</p> - -<p class="pni">Strahan, A., cited, <a href="#Page_318">318</a>.</p> - -<p class="pni">Strand lakes, <a href="#Page_424">424</a>.</p> - -<p class="pni">Strata, conformable, <a href="#Page_51">51</a>;</p> -<p class="pnii">contortions of, <a href="#Page_40">40</a>.</p> - -<p class="pni">Straths, <a href="#Page_428">428</a>.</p> - -<p class="pni">Streak, of minerals, <a href="#Page_451">451</a>.</p> - -<p class="pni">Stream capture, <a href="#Page_179">179</a>.</p> - -<p class="pni">Stream, meandering, cross section of, <a href="#Page_163">163</a>;</p> -<p class="pnii">braided, <a href="#Page_280">280</a>;</p> -<p class="pnii">intermittent, <a href="#Page_180">180</a>.</p> - -<p class="pni">Stream velocity, determined by gradient, <a href="#Page_158">158</a>.</p> - -<p class="pni">Strike, <a href="#Page_46">46</a>.</p> - -<p class="pni">Striped ground, <a href="#Page_154">154</a>.</p> - -<p class="pni"><i>Strokr</i>, <a href="#Page_193">193</a>.</p> - -<p class="pni">Strombolian eruptions, <a href="#Page_117">117</a>.</p> - -<p class="pni">Stromboli, cinder cone of, <a href="#Page_115">115</a>;</p> -<p class="pnii">excentric crater of, <a href="#Page_115">115</a>;</p> -<p class="pnii">explanation of eruptions in, <a href="#Page_116">116</a>, <a href="#Page_117">117</a>.</p> - -<p class="pni">Structure, cross-bedded, <a href="#Page_37">37</a>.</p> - -<p class="pni">Submerged channels, of rivers, <a href="#Page_252">252</a>.</p> - -<p class="pni">Submergence of land, during earthquakes, <a href="#Page_80">80</a>.</p> - -<p class="pni">Suess, E., cited, <a href="#Page_19">19</a>, <a href="#Page_142">142</a>, <a href="#Page_259">259</a>, <a href="#Page_277">277</a>, <a href="#Page_425">425</a>, <a href="#Page_436">436</a>, <a href="#Page_437">437</a>, <a href="#Page_438">438</a>, <a href="#Page_446">446</a>.</p> - -<p class="pni"><i>Suffioni</i>, arrangement on faults, <a href="#Page_87">87</a>.</p> - -<p class="pni">Supan, A., <a href="#Page_420">420</a>, <a href="#Page_424">424</a>.</p> - -<p class="pni">Surface moraines, <a href="#Page_277">277</a>.</p> - -<p class="pni">Surface springs, <a href="#Page_181">181</a>.</p> - -<p><span class="pagenum"><a name="Page_504" id="Page_504">[504]</a></span></p><p class="pni">“Swallow holes”, <a href="#Page_182">182</a>, <a href="#Page_422">422</a>.</p> - -<p class="pni">Swamp lands, drained during earthquakes, <a href="#Page_83">83</a>.</p> - -<p class="pni">Sweinfurth, G., cited, <a href="#Page_222">222</a>.</p> - -<p class="pni">Syenite, <a href="#Page_462">462</a>.</p> - -<p class="pni">Symbols, T., to express strike and dip, <a href="#Page_48">48</a>.</p> - -<p class="pni">Synclinal folds, <a href="#Page_42">42</a>.</p> - -<p class="pni">Synclines, <a href="#Page_42">42</a>.</p> - -<p class="pni">System of fractures, <a href="#Page_55">55</a>.</p> - -<p class="pn"><span class="pl">T</span>aal volcano, double explosive eruption of 1911, <a href="#Page_120">120</a>, <a href="#Page_121">121</a>.</p> - -<p class="pni">Table mountains, origin of, <a href="#Page_112">112</a>.</p> - -<p class="pni"><i>Takyr</i>, <a href="#Page_216">216</a>.</p> - -<p class="pni">Talc, <a href="#Page_460">460</a>.</p> - -<p class="pni">Talc schist, <a href="#Page_465">465</a>.</p> - -<p class="pni">Talmage, J. E., cited, <a href="#Page_221">221</a>.</p> - -<p class="pni">Talus, <a href="#Page_152">152</a>, <a href="#Page_153">153</a>, <a href="#Page_215">215</a>.</p> - -<p class="pni">Tangier-Smith, W. S., cited, <a href="#Page_260">260</a>.</p> - -<p class="pni">Tarr, R. S., cited, <a href="#Page_77">77</a>, <a href="#Page_92">92</a>, <a href="#Page_233">233</a>, <a href="#Page_260">260</a>, <a href="#Page_295">295</a>, <a href="#Page_301">301</a>.</p> - -<p class="pni">Taylor, F. B., cited, <a href="#Page_259">259</a>, <a href="#Page_330">330</a>, <a href="#Page_339">339</a>, <a href="#Page_342">342</a>, <a href="#Page_343">343</a>, <a href="#Page_346">346</a>, <a href="#Page_350">350</a>, <a href="#Page_355">355</a>, <a href="#Page_366">366</a>.</p> - -<p class="pni">Tectonic lakes, <a href="#Page_424">424</a>.</p> - -<p class="pni">Temperature, diurnal changes of, in deserts, <a href="#Page_202">202</a>.</p> - -<p class="pni">Temple of Jupiter Serapis, oscillations of level of, <a href="#Page_254">254</a>, <a href="#Page_255">255</a>.</p> - -<p class="pni">Terminal moraine, of Pleistocene glaciations, <a href="#Page_298">298</a>, <a href="#Page_299">299</a>.</p> - -<p class="pni">Terminal moraines, of mountain glaciers, <a href="#Page_394">394</a>.</p> - -<p class="pni">Terraced valleys, <a href="#Page_320">320</a>, <a href="#Page_321">321</a>.</p> - -<p class="pni">Terraces, built, <a href="#Page_235">235</a>;</p> -<p class="pnii">coast, <a href="#Page_80">80</a>, <a href="#Page_235">235</a>, <a href="#Page_341">341</a>;</p> -<p class="pnii">river, <a href="#Page_165">165</a>, <a href="#Page_178">178</a>, <a href="#Page_320">320</a>, <a href="#Page_321">321</a>;</p> -<p class="pnii">rock, <a href="#Page_215">215</a>.</p> - -<p class="pni">Terra Rossa, of Karst region, <a href="#Page_188">188</a>.</p> - -<p class="pni">Tessellated pavement, from soil flow, <a href="#Page_154">154</a>.</p> - -<p class="pni">Tethys, ocean of, <a href="#Page_16">16</a>.</p> - -<p class="pni">Tetrahedron, reciprocal relations of antipodal parts, <a href="#Page_13">13</a>;</p> -<p class="pnii">truncated, toward which earth is tending, <a href="#Page_12">12</a>.</p> - -<p class="pni">Tetrahedrons, twin, <a href="#Page_16">16</a>.</p> - -<p class="pni">Thaw water, soil flow in presence of, <a href="#Page_153">153</a>.</p> - -<p class="pni">Theory, evolved from working hypothesis, <a href="#Page_6">6</a>;</p> -<p class="pnii">mixture with observation, on maps, <a href="#Page_63">63</a>.</p> - -<p class="pni">Thermal springs, <a href="#Page_190">190</a>.</p> - -<p class="pni">Thickness of formations, <a href="#Page_65">65</a>.</p> - -<p class="pni">Thompson, Bertha, cited, <a href="#Page_155">155</a>.</p> - -<p class="pni">Thomson and Tait, cited, <a href="#Page_29">29</a>.</p> - -<p class="pni">Thomson, Wyville, cited, <a href="#Page_296">296</a>.</p> - -<p class="pni">Thoroddsen, Th., cited, <a href="#Page_103">103</a>, <a href="#Page_123">123</a>, <a href="#Page_147">147</a>, <a href="#Page_267">267</a>.</p> - -<p class="pni">Throw, on faults, <a href="#Page_59">59</a>.</p> - -<p class="pni">Thrusts, <a href="#Page_45">45</a>.</p> - -<p class="pni">“Tidal waves”, <a href="#Page_70">70</a>.</p> - -<p class="pni">Tides, effect on a fluid earth, <a href="#Page_20">20</a>.</p> - -<p class="pni">Tidewater glaciers, <a href="#Page_290">290</a>, <a href="#Page_386">386</a>.</p> - -<p class="pni">Till, <a href="#Page_31">31</a>, <a href="#Page_310">310</a>.</p> - -<p class="pni">Tillite, <a href="#Page_31">31</a>.</p> - -<p class="pni">Till plains, <a href="#Page_311">311</a>.</p> - -<p class="pni">Tinds, <a href="#Page_380">380</a>, <a href="#Page_381">381</a>.</p> - -<p class="pni">Tivoli, travertine of, <a href="#Page_184">184</a>.</p> - -<p class="pni">Tombolas, <a href="#Page_241">241</a>.</p> - -<p class="pni">Tongues, ice, on margin of continental glaciers, <a href="#Page_272">272</a>.</p> - -<p class="pni">Topographic maps, <a href="#Page_61">61</a>;</p> -<p class="pnii">preparation of, <a href="#Page_467">467</a>.</p> - -<p class="pni">Topography, built up, <a href="#Page_301">301</a>;</p> -<p class="pnii">constructional, <a href="#Page_309">309</a>;</p> -<p class="pnii">destructional, <a href="#Page_309">309</a>;</p> -<p class="pnii">fault, <a href="#Page_65">65</a>;</p> -<p class="pnii">fold, <a href="#Page_65">65</a>;</p> -<p class="pnii">incised, <a href="#Page_301">301</a>;</p> -<p class="pnii">knob and basin, <a href="#Page_314">314</a>.</p> - -<p class="pni">Top-set beds, <a href="#Page_167">167</a>.</p> - -<p class="pni">Tourmaline, <a href="#Page_460">460</a>.</p> - -<p class="pni">Tower, W. S., cited, <a href="#Page_178">178</a>.</p> - -<p class="pni">Trachyte, <a href="#Page_463">463</a>.</p> - -<p class="pni">Transgression, of the sea, <a href="#Page_37">37</a>.</p> - -<p class="pni">Transparency, of minerals, <a href="#Page_451">451</a>.</p> - -<p class="pni">Travertine, <a href="#Page_184">184</a>, <a href="#Page_464">464</a>.</p> - -<p class="pni">Trees, how affected by advancing lava, <a href="#Page_133">133</a>;</p> -<p class="pnii">undermined on stream meanders, <a href="#Page_164">164</a>.</p> - -<p class="pni">“Trellis drainage”, <a href="#Page_175">175</a>.</p> - -<p class="pni">Troughline, of a syncline, <a href="#Page_42">42</a>.</p> - -<p class="pni">Trunk channels of descending water, <a href="#Page_181">181</a>.</p> - -<p class="pni">Tsunamis, <a href="#Page_70">70</a>.</p> - -<p class="pni">T symbols, to express strike and dip, <a href="#Page_48">48</a>.</p> - -<p class="pni">Tufa, calcareous, <a href="#Page_464">464</a>.</p> - -<p class="pni">Tunnels, lava, <a href="#Page_111">111</a>, <a href="#Page_112">112</a>, <a href="#Page_125">125</a>.</p> - -<p class="pni">Twin tetrahedrons, <a href="#Page_16">16</a>.</p> - -<p class="pni">Tyndall, John, cited, <a href="#Page_192">192</a>, <a href="#Page_196">196</a>.</p> - -<p class="pn"><span class="pl">U</span>dden, J. A., cited, <a href="#Page_222">222</a>.</p> - -<p class="pni">Unconformable series, <a href="#Page_51">51</a>.</p> - -<p class="pni">Unconformity, <a href="#Page_65">65</a>;</p> -<p class="pnii">episodes in history of, <a href="#Page_52">52</a>;</p> -<p class="pnii">meaning of, <a href="#Page_51">51</a>.</p> - -<p class="pni">Underfolding, of earth’s shell, <a href="#Page_437">437</a>.</p> - -<p class="pni">Underground water, <a href="#Page_180">180</a>.</p> - -<p class="pni">Undertow, <a href="#Page_236">236</a>.</p> - -<p class="pni">Unstable erosion remnants, in “driftless area”, <a href="#Page_300">300</a>.</p> - -<p class="pni">Upham, Warren, cited, <a href="#Page_325">325</a>, <a href="#Page_327">327</a>, <a href="#Page_339">339</a>, <a href="#Page_344">344</a>, <a href="#Page_350">350</a>.</p> - -<p class="pni">Upland, fretted, <a href="#Page_372">372</a>, <a href="#Page_373">373</a>;</p> -<p class="pnii">grooved, <a href="#Page_372">372</a>, <a href="#Page_373">373</a>;</p> -<p class="pnii">maturely dissected, <a href="#Page_170">170</a>;</p> -<p class="pnii">mature, unfavorable to commercial development, <a href="#Page_171">171</a>;</p> -<p class="pnii">newly incised, <a href="#Page_169">169</a>;</p> -<p class="pnii">partially dissected, <a href="#Page_160">160</a>;</p> -<p class="pnii">progressive investment of, by cirques, <a href="#Page_374">374</a>.</p> - -<p class="pni">Uplift, marks of, on coasts, <a href="#Page_245">245</a>;</p> -<p class="pnii">sudden, of coasts, <a href="#Page_247">247</a>.</p> - -<p><span class="pagenum"><a name="Page_505" id="Page_505">[505]</a></span></p><p class="pni">Upraised cliffs, <a href="#Page_249">249</a>.</p> - -<p class="pni">Uptilt, in basin of Lake Agassiz, <a href="#Page_350">350</a>;</p> -<p class="pnii">of glaciated area, evidence that it continues, <a href="#Page_348">348-350</a>;</p> -<p class="pnii">of glaciated area, supposed nature of, <a href="#Page_344">344-347</a>.</p> - -<p class="pni">U-shaped valleys, <a href="#Page_374">374</a>.</p> - -<p class="pni">Usu-san (New Mountain), birth of, <a href="#Page_96">96</a>.</p> - -<p class="pn"><span class="pl">V</span>alley moraine lakes, <a href="#Page_400">400</a>, <a href="#Page_413">413</a>.</p> - -<p class="pni">Valleys, hanging, <a href="#Page_378">378</a>;</p> -<p class="pnii">of V-form, <a href="#Page_172">172</a>;</p> -<p class="pnii">U-shaped, <a href="#Page_374">374</a>.</p> - -<p class="pni">Valley trains, <a href="#Page_311">311</a>, <a href="#Page_399">399</a>.</p> - -<p class="pni">Van Hise, C. R., cited, <a href="#Page_54">54</a>.</p> - -<p class="pni">Varnish, desert, <a href="#Page_201">201</a>.</p> - -<p class="pni">Veatch, A. C., cited, <a href="#Page_418">418</a>, <a href="#Page_425">425</a>.</p> - -<p class="pni">Verbeek, R. D. M., cited, <a href="#Page_100">100</a>, <a href="#Page_142">142</a>, <a href="#Page_147">147</a>, <a href="#Page_148">148</a>.</p> - -<p class="pni">Vesicular texture, of extrusive rocks, <a href="#Page_32">32</a>.</p> - -<p class="pni">Victoria Falls, <a href="#Page_225">225</a>.</p> - -<p class="pni">Vincentius of Beauvais, cited, <a href="#Page_9">9</a>.</p> - -<p class="pni">Volcanic ash, <a href="#Page_122">122</a>.</p> - -<p class="pni">“Volcanic bombs”, <a href="#Page_121">121</a>.</p> - -<p class="pni">Volcanic dust, <a href="#Page_122">122</a>.</p> - -<p class="pni">Volcanic eruptions, during changes in earth’s figure, <a href="#Page_15">15</a>.</p> - -<p class="pni">Volcanic lakes, <a href="#Page_424">424</a>.</p> - -<p class="pni">Volcanic mountains, of ejected materials, <a href="#Page_115">115</a>;</p> -<p class="pnii">of exudation, <a href="#Page_94">94</a>.</p> - -<p class="pni">Volcanic necks, <a href="#Page_140">140</a>.</p> - -<p class="pni">Volcanic pipes, <a href="#Page_140">140</a>.</p> - -<p class="pni">Volcanic plugs, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>.</p> - -<p class="pni">Volcanic projectiles, <a href="#Page_121">121</a>.</p> - -<p class="pni">Volcanic rocks, <a href="#Page_32">32</a>.</p> - -<p class="pni">Volcanic sand, <a href="#Page_122">122</a>.</p> - -<p class="pni">Volcano belts, of the earth, <a href="#Page_98">98</a>.</p> - -<p class="pni">Volcano, definition of, <a href="#Page_95">95</a>.</p> - -<p class="pni">Volcano, eruption in 1888, <a href="#Page_118">118</a>, <a href="#Page_120">120</a>, <a href="#Page_147">147</a>;</p> -<p class="pnii">history of, <a href="#Page_118">118</a>, <a href="#Page_119">119</a>.</p> - -<p class="pni">Volcanoes, active, <a href="#Page_97">97</a>;</p> -<p class="pnii">arrangement over fissures, <a href="#Page_99">99</a>;</p> -<p class="pnii">birth of, <a href="#Page_96">96</a>;</p> -<p class="pnii">cone-producing period of, <a href="#Page_127">127</a>;</p> -<p class="pnii">convulsive eruptions of, <a href="#Page_105">105</a>;</p> -<p class="pnii">crater-producing period of, <a href="#Page_128">128</a>;</p> -<p class="pnii">dissection of, <a href="#Page_139">139</a>, <a href="#Page_148">148</a>;</p> -<p class="pnii">dormant, <a href="#Page_97">97</a>;</p> -<p class="pnii">early views concerning, <a href="#Page_95">95</a>;</p> -<p class="pnii">“elevation-crater” theory of, <a href="#Page_95">95</a>;</p> -<p class="pnii">explosive eruptions of, <a href="#Page_105">105</a>;</p> -<p class="pnii">extinct, <a href="#Page_97">97</a>;</p> -<p class="pnii">fissure eruptions of, <a href="#Page_101">101</a>;</p> -<p class="pnii">location at fissure intersections, <a href="#Page_100">100</a>;</p> -<p class="pnii">map of, in Java, <a href="#Page_100">100</a>;</p> -<p class="pnii">migration of vent along fissure, <a href="#Page_101">101</a>, <a href="#Page_124">124</a>;</p> -<p class="pnii">misconceptions concerning, <a href="#Page_94">94</a>;</p> -<p class="pnii">mud flows after eruptions, <a href="#Page_138">138</a>;</p> -<p class="pnii">of Gulf of Guinea, <a href="#Page_101">101</a>;</p> -<p class="pnii">regarded as retaining walls, <a href="#Page_124">124</a>, <a href="#Page_125">125</a>;</p> -<p class="pnii">relation to mountain ranges, <a href="#Page_144">144</a>;</p> -<p class="pnii">sequence of events within chimney of, during eruption, <a href="#Page_134">134</a>, <a href="#Page_135">135</a>;</p> -<p class="pnii">solfataric activity of, <a href="#Page_97">97</a>;</p> -<p class="pnii">three types of, <a href="#Page_105">105</a>.</p> - -<p class="pni">V-shaped valley, <a href="#Page_172">172</a>.</p> - -<p class="pni">Vulcanello, <a href="#Page_119">119</a>.</p> - -<p class="pni">Vulcanian eruptions, <a href="#Page_117">117</a>, <a href="#Page_125">125</a>.</p> - -<p class="pn"><span class="pl">W</span>altershausen, S. von, cited, <a href="#Page_148">148</a>.</p> - -<p class="pni">Walther, Johannes, cited, <a href="#Page_201">201</a>, <a href="#Page_202">202</a>, <a href="#Page_203">203</a>, <a href="#Page_204">204</a>, <a href="#Page_205">205</a>, <a href="#Page_206">206</a>, <a href="#Page_211">211</a>, <a href="#Page_215">215</a>, <a href="#Page_221">221</a>.</p> - -<p class="pni">Wandering dunes, <a href="#Page_209">209</a>.</p> - -<p class="pni">Warren river, <a href="#Page_416">416</a>.</p> - -<p class="pni">“Washes”, <a href="#Page_213">213</a>.</p> - -<p class="pni">Water, derangement of flow during earthquakes, <a href="#Page_83">83</a>;</p> -<p class="pnii">ground, <a href="#Page_180">180</a>;</p> -<p class="pnii">percolating, rôle of, <a href="#Page_149">149</a>;</p> -<p class="pnii">running, earth features shaped by, <a href="#Page_169">169</a>;</p> -<p class="pnii">shot up in sheets during earthquake, <a href="#Page_83">83</a>;</p> -<p class="pnii">thaw, soil flow in presence of, <a href="#Page_153">153</a>.</p> - -<p class="pni">Water gaps, <a href="#Page_176">176</a>.</p> - -<p class="pni">Water pipes, buckled in ground, during earthquakes, <a href="#Page_75">75</a>.</p> - -<p class="pni">Water table, <a href="#Page_180">180</a>;</p> -<p class="pnii">extreme depth of, <a href="#Page_201">201</a>, <a href="#Page_203">203</a>.</p> - -<p class="pni">Water wave, effect of breaking on shore, <a href="#Page_233">233</a>;</p> -<p class="pnii">free, <a href="#Page_232">232</a>;</p> -<p class="pnii">motion of, <a href="#Page_231">231</a>.</p> - -<p class="pni">Watson, T. L., cited, <a href="#Page_259">259</a>.</p> - -<p class="pni">Wave, water, the motion of, <a href="#Page_231">231</a>.</p> - -<p class="pni">Wave base, <a href="#Page_232">232</a>.</p> - -<p class="pni">Wave length, <a href="#Page_231">231</a>.</p> - -<p class="pni">Weathering, carbonization, <a href="#Page_151">151</a>;</p> -<p class="pnii">chemical, <a href="#Page_149">149</a>;</p> -<p class="pnii">chemical agents of, <a href="#Page_149">149</a>;</p> -<p class="pnii">dry, <a href="#Page_201">201</a>;</p> -<p class="pnii">exfoliation, <a href="#Page_151">151</a>;</p> -<p class="pnii">frost action, <a href="#Page_152">152</a>;</p> -<p class="pnii">hydration, <a href="#Page_151">151</a>;</p> -<p class="pnii">in relation to climate, <a href="#Page_150">150</a>;</p> -<p class="pnii">internal, in deserts, <a href="#Page_201">201</a>;</p> -<p class="pnii">mechanical, <a href="#Page_149">149</a>;</p> -<p class="pnii">of lithosphere surface, <a href="#Page_29">29</a>;</p> -<p class="pnii">shadow, <a href="#Page_203">203</a>;</p> -<p class="pnii">spheroidal, <a href="#Page_150">150</a>, <a href="#Page_151">151</a>;</p> -<p class="pnii">two contrasted processes of, <a href="#Page_149">149</a>.</p> - -<p class="pni"><i>Wed</i> (<i>Wadi</i>), <a href="#Page_212">212</a>, <a href="#Page_213">213</a>, <a href="#Page_214">214</a>.</p> - -<p class="pni">Weed, W. H., cited, <a href="#Page_196">196</a>, <a href="#Page_441">441</a>, <a href="#Page_447">447</a>.</p> - -<p class="pni">West Indies, seismotectonic lines of, <a href="#Page_88">88</a>.</p> - -<p class="pni">Wheeler, W. H., cited, <a href="#Page_244">244</a>.</p> - -<p class="pni">Whirlpool basin, at Niagara, <a href="#Page_359">359</a>;</p> -<p class="pnii">excavation of, <a href="#Page_360">360</a>.</p> - -<p class="pni">Whitbeck, R. H., cited, <a href="#Page_319">319</a>.</p> - -<p class="pni">White, David, cited, <a href="#Page_318">318</a>.</p> - -<p class="pni">Willis, Bailey, cited, <a href="#Page_45">45</a>, <a href="#Page_54">54</a>, <a href="#Page_157">157</a>, <a href="#Page_260">260</a>, <a href="#Page_318">318</a>.</p> - -<p class="pni">Winchell, N. H., cited, <a href="#Page_354">354</a>.</p> - -<p class="pni">Wind, in relation to location of glaciers, <a href="#Page_377">377</a>;</p> -<p class="pnii">in relation to mountain glaciers, <a href="#Page_367">367</a>.</p> - -<p class="pni">Wind distribution of snow, <a href="#Page_367">367</a>.</p> - -<p class="pni">Wind gaps, <a href="#Page_176">176</a>.</p> - -<p class="pni"><i>Windkanten</i>, <a href="#Page_205">205</a>.</p> - -<p class="pni">Wind poles, of the earth, <a href="#Page_263">263</a>;</p> -<p class="pnii">of earth, earlier, <a href="#Page_297">297</a>.</p> - -<p class="pni">Wintergreen Flats, site of captured fall, <a href="#Page_358">358</a>.</p> - -<p><span class="pagenum"><a name="Page_506" id="Page_506">[506]</a></span></p><p class="pni">Wisconsin diamonds, <a href="#Page_307">307</a>, <a href="#Page_308">308</a>.</p> - -<p class="pni">Woodworth, J. B., cited, <a href="#Page_74">74</a>, <a href="#Page_351">351</a>.</p> - -<p class="pni">Worcester, Dean C., cited, <a href="#Page_96">96</a>.</p> - -<p class="pni">Working hypothesis, <a href="#Page_6">6</a>.</p> - -<p class="pni">Workman, Fanny Bullock, cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Workman, W. H., cited, <a href="#Page_294">294</a>.</p> - -<p class="pni">Wright, F. E., cited, <a href="#Page_351">351</a>.</p> - -<p class="pn"><span class="pl">Y</span>ellowstone National Park, <a href="#Page_33">33</a>, <a href="#Page_191">191</a>, <a href="#Page_193">193</a>, <a href="#Page_194">194</a>.</p> - -<p class="pni">Yosemite Valley, <a href="#Page_59">59</a>, <a href="#Page_152">152</a>.</p> - -<p class="pni">Young rivers, <a href="#Page_159">159</a>, <a href="#Page_160">160</a>.</p> - -<p class="pn"><span class="pl">Z</span>ahn, G. W. von, cited, <a href="#Page_244">244</a>.</p> - -<p class="pni">Zigzag ranges, due to plunging folds, <a href="#Page_51">51</a>.</p> - -<p class="pni">Zittel, K. v., cited, <a href="#Page_19">19</a>.</p> - -<p class="pni">Zone of diverse displacement, <a href="#Page_439">439</a>.</p> - -<p class="pni">Zone of flow, <a href="#Page_40">40</a>, <a href="#Page_143">143</a>.</p> - -<p class="pni">Zone of fracture, <a href="#Page_40">40</a>, <a href="#Page_46">46</a>.</p> - -<p class="pni">Zones, of deposition, surrounding desert, <a href="#Page_216">216</a>, <a href="#Page_217">217</a>;</p> -<p class="pnii">upper and lower cloud, <a href="#Page_268">268</a>, <a href="#Page_269">269</a>.</p> - -<hr class="tb" /> - -<p class="pc small">Printed in the United States of America.</p> - -</div> - -<div class="chapter"> - -<h2 class="p4">FOOTNOTES:</h2> - -<div class="footnotes"> - -<p class="pfn4"><span class="ln1"><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a></span> -Italian for mouth; plural <i>bocchi</i>.</p> - -<p class="pfn4"><span class="ln1"><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a></span> -These models and the contouring apparatus are now manufactured for the use -of schools and colleges by Eberbach and Son, Ann Arbor, Mich.</p> - -<p class="pfn4"><span class="ln1"><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a></span> -This clay is manufactured by the A. H. Abbott Company, art dealers, Wabash -Avenue, Chicago.</p> - -<p class="pfn4"><span class="ln1"><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a></span> -Numbers in parenthesis refer to pages in this book, where further information is -to be found.</p> - -</div> -<p> </p> -<p> </p> -<hr class="tb" /> -<p> </p> -<p> </p> - -</div> -<div class="transnote"> -<p class="pc large">TRANSCRIBER’S NOTE:</p> -<p class="ptn">—Obvious typographical errors have been silently corrected. -All other variations in spelling punctuation and accents have been left unchanged.</p> -<p class="ptn">—A border has been added to plates.</p> -</div> -</div> - -<p> </p> -<p> </p> -<hr class="full" /> -<p>***END OF THE PROJECT GUTENBERG EBOOK EARTH FEATURES AND THEIR MEANING***</p> -<p>******* This file should be named 50671-h.htm or 50671-h.zip *******</p> -<p>This and all associated files of various formats will be found in:<br /> -<a href="http://www.gutenberg.org/dirs/5/0/6/7/50671">http://www.gutenberg.org/5/0/6/7/50671</a></p> -<p> -Updated editions will replace the previous one--the old editions will -be renamed.</p> - -<p>Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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