34 T11IJEE GEOLOGICAL PKOVINCES. 



the conditions for great erosion now found in the Basin Ranges have existed 

 but for a short period,-*-, e., the blocks were certainly not upheaved ante- 

 cedent to Cenozoic time ; and it would seem probable that it must have been 

 in late Tertiary. 



It seems proper to add here a remark concerning certain conditions of 

 erosion, though I have elsewhere discussed the subject more fully. 



The lesser or greater rapidity of erosion depends chiefly upon three 

 conditions: first, elevation above the base level of erosion; second, the 

 induration of the rocks ; and, third, the amount of rain fall. But erosion 

 does not increase in ratio with the increase of the precipitation of moisture, 

 for increasing moisture serves to increase the protection derived from 

 vegetation. 



Nor does induration greatly preserve rocks from erosion, for on most 

 exposures the action of the elements in disintegrating the rocks is in excess 

 of the power of the streams to carry the material away. The exceptional 

 exposures are found on steep slopes ; yet the difference in the induration of 

 beds has an effect as seen in the minor or concomitant forms of all mountain 

 regions. The principal factor in maximum erosion is elevation above the 

 base level, and the power of erosion increases in geometric ratio with the 

 elevation. The power of the streams to transport the material of erosion 

 is increased in geometric ratio and the power of the water in corrasion is in 

 like manner increased ; and the corrasion of deep channels by rapid streams 

 filled with sands, gravels and bowlders produces another condition of surface 

 favorable to general degradation that is, the walls of these deep channels 

 are broken down by gravity, which is further increased by an undermining 

 process where harder and softer beds alternate. With these facts in view, 

 we need not enter into a consideration of the difference of texture or indu- 

 ration of the rocks of the Plateau and Park Provinces and those of the 

 Basin Province ; but we may remark that of the 30,000 feet eroded from 

 the Uinta uplift, more than 16,000 feet were of beds of Paleozoic Age, and 

 with a texture as firm as the rocks of the Basin Ranges. 



It is manifest that the result of all these movements of displacement in 

 the three provinces was general upheaval. But this upheaval in the three 

 provinces was unequal ; it was great in the Basin Province, greater in the 



