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They lift their materials miles above the level where they were originally laid, probably from a zone which is rarely if ever exposed to view, placing them near the surface, where the erosive agents can readily find access to them. Of the three agents which serve to export earth materials from its depths, volcanoes are doubtless the most important.
It is as if these striking forms were inherent in the rocks, waiting for the erosive forces to liberate them. The stratified rocks out of which they are carved were not laid down in forms that appeal to us, but layer upon layer, like the leaves of a book; neither has the crumpling and deformation of the earth's crust piled them up and folded them in a manner artistic and suggestive.
When, during a recent trip through the great Southwest, I saw the earth laid open by erosion as I had never before dreamed of, especially when I visited those halls of the gods, the Grand Canon and Yosemite Valley, I found my capacity to believe in the erosive power of water and the weather quite overtaxed.
We have already learned something about the way in which the ridges and hollows of the earth's surface are made. We have learned of the wrinkling of the crust, of the formation of fissures, and of the erosive work of running water. The interesting features of the Great Basin are mainly the result of two causes: the sinking of a portion of the earth's surface, and the lack of rainfall.
"Where they have been produced by volcanic action. There the walls are of volcanic rock." "Why is it that these underground channels are formed in this way?" "They are formed by the erosive action of the water wearing out the softer portions of the rock beneath a harder roof or wall.
In order that the cylinder, A, may be given a vertical motion, cords, M M, fixed to a piece, S, loose on the hub, D, wind round the drum of a windlass, T, after passing over the pulleys, p p. The rapid gyratory motion of the cylinder, along with the erosive action of the metallic agglomerate, rapidly wears away the rock, and causes the descent of the perforator.
But where the united river leaves Utah and passes into Arizona, it traverses a dry plateau country with little rain, where its waters have cut their way down through mountain limestone to a depth of 6000 feet. The strata are horizontal, and the whole series has been cleared away by the continued erosive power of water, aided by gravel and boulders.
He continues: "As a consequence of this uplift the erosive power of the streams was rejuvenated, the Cretaceous surface of gentle outline was dissected, and the rivers began to cut back behind the old divide, carrying their heads nearly to the present crest line that separates the slope of the Sierra from the depression of Lake Tahoe."
The steeper slope rising at the back is the remnant of the cliff against which the waves used to beat. If we are fortunate, we shall find at its base some water-worn pebbles and possibly a few fragments of sea-shells. The crumbling of the rocks and the erosive action of the rills are fast destroying the old cliffs, so that in many places they have entirely disappeared.
She crawled ahead slowly, scarcely conscious of the danger attending her mission. Large blocks of débris, tent-shaped erosive hillocks, impeded her progress; they crowded along the foot of the cliffs like protecting bulwarks, and the trail wound around them on a higher plane. But this trail she dared not follow, there was not enough darkness on it.
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