KELATIONS OF THE MOUNTAIN RANGES TO FLEXURES 559 



conspicuously folded structures usually observed in the Desert ranges be- 

 long to an ancient system of tectonics and have no genetic relations with 

 the rearing of the mountain ranges themselves. 



Relations of the Mountain Ranges to Flexures 



From the records of the observations recently carried on in all four of 

 the grand divisions of the American desert region the accumulated evi- 

 dences seem to be conclusive that the main flexing of the strata is very 

 much older than the present mountains possibly can be. The ancient 

 tectonics does not appear to be obliterated, or even notably disguised, by 

 any of the post-Triassic movements. As indicated by all the known 

 geologic cross-sections of the mountain ranges, the form of the latter can 

 not be regarded as having any direct genetic relationship whatever to the 

 flexed structures of the rock-masses. Arches, troughs, monoclines, and 

 flat-lying masses are truncated alike. Mainly to differential erosion 

 must the present surface inequalities be considered as owing their origin, 

 and the erosion is regarded as deflative in character. 



Geologic Evolution of Desert Ranges 



The anomalous climatic conditions of the desert region and the gen- 

 eral activities of desert-leveling or deflation have been already discussed 

 in another place.^^ 'No specific application of the principles to the 

 moulding of the Basin ranges, or rather Desert ranges, has yet been made. 

 The idea of desert-leveling, as now understood and applied to the Basin 

 ranges, does away with many of the obstacles which the fault theory has 

 long encountered. It does not necessitate the postulation of a former 

 humid climate and the development of wide branching drainage systems, 

 as recently urged by Spurr.^^ It makes it possible to formulate a con- 

 nected system of the salient geographic features of the region through 

 mere means of general wind denudation. The general relief effects are 

 not very unlike what they are in more humid lands, as the Appalachians, 

 differing chiefly only in the character of the principal erosive agency. 



According to the principles of general desert-leveling by deflation it is 

 not necessary to fancy, for the beginning of an arid cycle, an even surface 

 like a peneplain. Whatever the nature of the relief when such a cycle 

 began, the most active erosive forces would commence work just as they 

 found it. They would soon fashion all features after their own way. 



18 Bulletin of the Geological Society of America, vol. 19, 1908, p. 63. 

 " Bulletin of the Geological Society of America, vol. 12, 1901, p. 266. 



