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EARTH FEATURES AND THEIR MEANING
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 The series of readings co
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PREFACE
PREFACE
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 so
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EXPLANATORY LIST OF ABBREVIATIONS FOR JOURNAL NAMES IN READING REFERENCES
EXPLANATORY LIST OF ABBREVIATIONS FOR JOURNAL NAMES IN READING REFERENCES
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.
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CHAPTER I
CHAPTER I
Tabular recapitulation. —In a slightly different arrangement from the above order of mention, the subdivisions of geology are as follows:— Subdivisions of Geology 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 t
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CHAPTER II
CHAPTER II
The oblateness of the earth. —Every schoolboy is to-day familiar with the fact that the earth departs from a perfect spherical figure by being flattened at the ends of its axis of rotation. The polar diameter is usually given as 1 / 299 shorter than the equatorial one. This oblateness of the spheroid was proven by geodesists when they came to compare the lengths of measured degrees of arc upon meridians in high and in low latitudes. Fig. 1. —Diagrams to afford a correct impression of the measure
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CHAPTER III
CHAPTER III
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
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CHAPTER IV
CHAPTER IV
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
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CHAPTER V
CHAPTER V
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
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CHAPTER VI
CHAPTER VI
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,
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CHAPTER VII
CHAPTER VII
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 populat
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CHAPTER VIII
CHAPTER VIII
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
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CHAPTER IX
CHAPTER IX
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 th
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CHAPTER X
CHAPTER X
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
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CHAPTER XI
CHAPTER XI
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 intersec
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CHAPTER XII
CHAPTER XII
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
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CHAPTER XIII
CHAPTER XIII
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 di
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CHAPTER XIV
CHAPTER XIV
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 ). 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 unco
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CHAPTER XV
CHAPTER XV
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 1
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CHAPTER XVI
CHAPTER XVI
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
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CHAPTER XVII
CHAPTER XVII
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 gro
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CHAPTER XVIII
CHAPTER XVIII
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. Fig. 248. —Diagram to illustrate the transformation of a free wave into a breaker as it approa
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CHAPTER XIX
CHAPTER XIX
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
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CHAPTER XX
CHAPTER XX
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
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CHAPTER XXI
CHAPTER XXI
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 h
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CHAPTER XXII
CHAPTER XXII
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 a
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CHAPTER XXIII
CHAPTER XXIII
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 fro
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CHAPTER XXIV
CHAPTER XXIV
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
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CHAPTER XXV
CHAPTER XXV
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
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CHAPTER XXVI
CHAPTER XXVI
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 gr
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CHAPTER XXVII
CHAPTER XXVII
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 for
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CHAPTER XXVIII
CHAPTER XXVIII
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 bene
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CHAPTER XXIX
CHAPTER XXIX
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 circumsta
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CHAPTER XXX
CHAPTER XXX
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 la
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CHAPTER XXXI
CHAPTER XXXI
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
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I. The Minerals of Economic Importance
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 eas
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II. The Minerals important as Rock Makers
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.
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2. Extrusive Rocks
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 cry
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3. Sedimentary Rocks of Mechanical Origin
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 surfa
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4. Sedimentary Rocks of Chemical Precipitation
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 concen
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5. Sedimentary Rocks of Organic Origin
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
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6. Metamorphic Rocks
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
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APPENDIX C
APPENDIX C
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
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APPENDIX D
APPENDIX D
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 ( Cha
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APPENDIX E
APPENDIX E
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 h
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