590 C. R. KEYES RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



The presence of lofty mountain ranges in true desert regions ^'ery nat- 

 urally gives rise, at first thought, to the impression that arid climate has 

 come upon the region since they were upraised and their present aspects 

 molded. Such a region as the Great Basin province, which is sometimes 

 cited as a type of arid youthfulness, is a good illustration. That the 

 highlands themselves may simulate the physiographic stage mentioned 

 has been noted already as possible, since the mountains have their sur- 

 faces largely sculptured by normal water action, while the greater part of 

 the region is still under strictly desert climate conditions. 



With regional youthfulness premised, a number of incongruous conclu- 

 sions necessarily follow. In the first place, the present highlands, as 

 mere topographic features, must be regarded as mainly structural up- 

 risings due wholly to frequent and profound differential displacement. 

 In support of this view, the most recent investigations of a critical nature 

 do not furnish as many facts as could be desired. A very large mass of 

 the evidence is strongly against the acceptance of this hypothesis, which 

 in its essence must be recognized as "practically a phase of one of the 

 more primitive theories of mountain building that now finds few advo- 

 cates in any part of the world." In the desert region the evidences 

 against its validity are, perhaps, more obvious than anywhere else. A 

 critical point that should alone invalidate such a theory is the fact that 

 the mountains are composed of hard rocks and the valleys or intermont 

 plains occupy areas of weak rocks and are in the main as truly rock- 

 floored as the mountains themselves, rather than extensively aggraded 

 areas. 



Secondly, if the present expression of the desert ranges be considered, 

 as urged by Spurr, as mainly resulting from extended dissection at times 

 when the region possessed a moister climate than at present, and was, in 

 fact, well watered, and that there was, not so very long ago, abrupt 

 changes from humid to a dry climate, it necessitates the postulation of 

 water action as the sole, or at best chief, erosive agency. It further 

 postulates extensive dissection of the mountains and extensive and uni- 

 versal aggradation of the plains, which, as lately pointed out, does not 

 appear always to be the case. Moreover, if there were water sufficient to 

 accomplish results of this kind, it is, indeed, singular that so few of the 

 bolsons have points in the rims low enough to permit the ready flow of 

 waters from one basin to another. 



Thirdly, if the lowlands, which in the American arid regions occupy 

 four-fifths of the desert areas, be assumed to be tracts of aggradation, it 

 is remarkable that the recent observations show conclusively that many, 

 if not the majority, of these intermont plains have in large part at least 



