COMPARRISON OF DEFLATION AND CORRASION 587 



the complete isolation of the different mountain ranges, and the forma- 

 tion of the remarkable plateau plains which are so characteristic of the 

 dry country. 



Eecent descriptions of desert regions by Bronhardt, Passarge, Penck, 

 Walther, and others emphasize the wind as a potent erosive agent. Ac- 

 cordingly may not the great arid tracts of southwestern United States 

 and northern Mexico be advantageously regarded as mainly sculptured by 

 eolian action, and general desert leveling and lowering of the country 

 viewed is carried on largely by the wind. Out of the broadly uplifted 

 region, which had been previously profoundly faulted and folded and 

 then planed off to the condition of a peneplain perhaps, mountain and 

 plain appear to have been developed through differential eolian effects 

 upon belts of alternating hard and weak rocks. Between the original 

 plains level and the present general plains surface more than 5,000 feet 

 of rock have been removed, leaving the more resistant mountain rock 

 raised above the plains as rocky isles stand out of the sea. 



With most of the broad intermont plains of the desert being areas of 

 rapid degradation instead of aggradation, as shown in their remarkable 

 rock floors, with normal water action confined to the loftier mountains, 

 and with the plains little influenced by stream corrasion, general desert 

 leveling and lowering must find for their chief sculpturing agents some- 

 thing other than stream action. All things considered, wind scour seems 

 to be the principal erosive process in dry lands, water action very second- 

 ary. Their relative efficiencies may be roughly measured by the fact that 

 the total volume of rock waste brought down by the storm waters from 

 a desert range in a year may be removed by the wind in a single day. 

 What general erosion by means of water is in a wet climate, eolation is 



under conditions of aridity. 



1,1 ■ I ' ■ . 



Initial Physiographic Conditions of the Arid Country 

 governing factors of an arid cycle 



The statement is made by Davis*^ that "no special conditions need be 

 postulated as to the initiation of an arid cycle. The passive earth's crust 

 may be (relatively) uplifted and offered to the sculpturing agencies with 

 any structure, any form, any altitude, in dry as well as in moist regions." 

 While as a general proposition this broad assertion might be in itself 

 literally true, it needs strong qualification before it can be made accept- 

 able in its entirety. The necessary consequences in the development of 

 the arid cycle in a region like western America could hardly be as the 



Journal of Geology, vol. xiil, 1905, p. 382. 



