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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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