RECAPITULATION 597 



desert ranges rise still another mile. The mountain ranges are sharp in 

 outline; the intermont plains broad and usually rock-floored (figure 6b). 



The Great Basin region appears to have reached the stage of arid ma- 

 turity. Eecent volcanic outpourings have greatly disguised many por- 

 tions of this vast area and have occasioned a puzzling medley of relief 

 features. The older elevations, represented by such mountains as the 

 Charleston, Yegas, Kingston, and many other ranges of Nevada, are now 

 relatively low and rounded in outlines (figure 6c). In this region it will 

 be important to separate the older general desert lowering features from 

 the later aspects imposed by local volcanic and orogenic disturbances 

 before the real physiographic expression can be clearly grasped. 



Of desert old age, Davis cites the South African region. In our own 

 country, in the vast Colorado plateau, with its greatly subdued surface 

 features, we seem to have a good example of arid senility (figure 6d). 

 Owing to the low dome-shaped structure, the higher and weaker strata 

 have been quite generally stripped off, leaving the more resistant Paleo- 

 zoic rocks exposed in great stratum planes, little modified by differential 

 erosion. The through-flowing Colorado Eiver introduces the exotic ele- 

 ment of vigorous corrasion in this area whereby the broader aspects of 

 general desert erosion and the immediate effects of eolation are greatly 

 obscured. 



Eecapitulation- 



In the foregoing consideration of the desert region two features in par- 

 ticular are emphasized: The dominancy of deflation among the general 

 erosive agencies and the probable plains character of the surface of the 

 country at the beginning of the present dry cycle. The conclusions 

 reached are that: 



1. The corrasive work of water is, in all desert regions, much less effect- 

 ive than is commonly supposed, and is of about as much importance as is 

 wind action in a humid land. 



2. The sheetflood effects are unimportant as lasting erosional phenom- 

 ena and they are largely counteracted by the action of the winds. Their 

 main function is merely the filling of wind-formed hollows in the plains 

 surface, thus making the latter all the smoother. 



3. Arroyo-running, occurring at infrequent intervals, is the nearest 

 approach to normal water action, but its main corrasive influence is con- 

 fined to the slopes of the higher mountains, its effects upon the plains 

 being very slight. 



4. The few large through-flowing rivers which traverse the arid region 

 do not form an integral part of it. They merely cross the dry country 



XLII— Bull. Geol. Soc. Am., Vol. 21, 1909 



