RECAPlTULlATIOlSr 



563 



are so impressive on every hand; while the recognized absence of an abun- 

 dance of running waters makes it appear that the progress of erosion 

 must be extremely slow. The latter is, of course, the only possible alter- 

 native when water is assumed to be the sole erosive agent. 



Eeoapitulation" 



From the foregoing consideration of the most obvious geologic struc- 

 tures presented by the Desert ranges of western United States, it appears 

 that : 



1. The folded structures, such as are displayed in some of the Basin 

 ranges and by many of the Desert ranges outside of the confines of the 

 Great Basin, belong chiefly to an ancient system of tectonics (Early 

 Mesozoic) . 



2. These flexed structures have usually no direct genetic relationships 

 with the rearing of the present mountains. 



3. The major faulting is also mainly old and long since completely 

 mastered by erosion, and has small direct connection with the rearing of 

 the present ranges. 



4. The entire flexed and faulted region was planed off to the condition 

 of a peneplain before it was upraised and presented to the sculpturing 

 agencies through which the present mountains took form. 



5. The tectonics of the region have merely an accidental role rather 

 than a direct genetic influence in the shaping of the Desert ranges. The 

 present facial expression of the arid region and its evolution into abrupt, 

 narrow, and lofty mountain ranges and broad, even plains are due directly 

 to the differential effects of general desert erosion on alternating belts of 

 very resistant and very weak rock-masses. 



6. The present profiles of the desert ranges are thus largely inde- 

 pendent of faulting movements, and the erosional shapes assumed only 

 emphasize the gently flexed bedding structures. 



7. The general desert-leveling and lowering of the country goes on 

 much in the same way as planation proceeds in humid regions, except 

 that there is no downward limitation at sealevel. 



