586 C. K. KBYES RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



tectonics. Basin-range structure has thus come to stand for a distinct 

 type of orogeny. It is characterized by simple faulting on a gigantic 

 scale, accompanied by a tilting of the fault blocks. This idea of the 

 desert ranges is, as Professor Davis observes, a very primitive theory of 

 mountain making and now finds little support from the studies of the 

 principal ranges of the globe. 



In ascribing to erosion an important role in the shaping of the Basin 

 ranges, as some of the more recent writers on the subject have been in- 

 clined to do, stream work alone has been mainly discussed. In fact, the 

 principles of water corrasion in a normally wet climate have been, without 

 material modification, directly applied to the arid lands. By such proce- 

 dure we arrive at an erosion scheme under conditions of aridity that are 

 essentially the same as that of the moist climate, with this difference 

 only, that there is somewhat less water involved. While this conception 

 is not an unnatural deduction, when based on experience in moist coun- 

 tries, it does not appear to be at all congruous with the idea of actual 

 desert conditions. 



Contrary to general opinion, the most effective water action in the 

 desert is not during the cooler portions of the year, as might be expected, 

 but during mid-summer. July and August are the months denominated 

 the "rainTJ- season" by the dwellers of the arid States. Moreover, the 

 more pronounced erosional effects of water are confined chiefly to the 

 slopes of the loftier mountain ranges. 



The intermont plains, which constitute four-fifths of the desert area, 

 are little affected by stream corrasion. Their surfaces are rarely trenched 

 by distinct drainageways. Their remarkable smoothness amply attests 

 the absence of stream influence. 



In general, in the desert region, where there is annually less than 10 

 inches of precipitation, the greater part of which sinks at once into the 

 spongy soil as soon as it has fallen, the effects of stream corrasion are 

 reduced to a minimum. In the mountains stream action is still an ero- 

 sive factor to be reckoned with; on the plains, most of which have high 

 gradients, it is an entirely negligible quantity. 



Since in proportion as the annual precipitation diminishes the po- 

 tency of the wind as an erosive agent increases, a point is finally reached, 

 as has already been shown, under conditions of aridity, when it becomes 

 the chief denuding influence. The full force of this observation appears 

 to be in large measure overlooked in most considerations of the origin of 

 the desert features. Mainly to eolian action must, therefore, be ascribed 

 the boldness in outline of the desert ranges, the bareness of the rock floors 

 of the intermont plains, the sharpness of meeting of mountain and plain. 



