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Show EEOENCY OF FAULTS. 41 die of the Pliocene we shall have dealt generously with any one who may be disposed to push them back to the remotest possible epoch. But it may be asked if erosion may not after all have proceeded slowly in this region on account of the arid climate, and whether there may not have been long intervals when its rate was insignificant. I think the answer must be decidedly in the negative so far as the time is concerned which lies on this side of the epoch of displacement. The High Plateaus are not arid, but are watered copiouslyâ€"less, indeed, than the regions east of the Mississippi, but far more abundantly than the deserts which lie to the east and to the west of them. It must be remembered that their altitude is great, and that their length and breadth is far greater than most of the Rocky Ranges. They are the most prominent topographical barrier which the westerly winds strike after leaving the Sierra Nevada, and though the plains and even the ragged ridges of the Great Basin are parched and dry, yet the High Plateaus wring from the air notable quantities of moisture. The rainfall is not known, but 30 inches per annum is a small estimate of the probable precipitation on the Plateau summits. In the valley plains of the Great Basin the rainfall seldom exceeds 8 inches, and in the painted desert to the east of the High Plateaus it could not reasonably be expected to amount to so much as 4 inches. But there is evidence that in the pastâ€" in Glacial and Post-glacial timeâ€"the rainfall was far more abundant than now. The drainage of three-fourths of the district was gathered in those periods into the grand expanse of Lake Bonneville, of which Great Salt Lake and Sevier Lake are the remnants. At present this drainage is absorbed and finally evaporated in Sevier Lake alone. Very abundant must have been the rainfall and moist the atmosphere which, with such a relatively .moderate water-shed, could have kept such a lake as Bonneville brimming. Nor is there at present any evidence that the erosion was materially affected either in degree or kind by the presence of ice during the Glacial epoch. On the contrary, the evidence is strongly in favor of the conclusion that in that period the climate was not glacial in this district. The ravines and valleys are conspicuously water-carved and conspicuously not ice-carved. As if to furnish proof that the absence of all indications of ice action in the valleys and plateau flanks should be construed as |
