EECENCY 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 whethe.r 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 copiously — 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 past — 

 in Glacial and Post-glacial time — 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 ab- 

 sorbed 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 conclu- 

 sion 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 



