William Herbert Hobbs

“ 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 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. ”

“ 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. ”

“ 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) . ”

“ 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 [292] 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. ”

“ 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 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.” ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The dust carried out of the desert.—When, standing upon the mountain wall that surrounds a desert, the traveler gazes out to [207] 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”