CHARACTERISTICS OF THE ARID COUNTRY 567 



PowelFs deductions^ regarding denudation in the dry regions of west- 

 ern United States led to the great generalization of the baselevel of ero- 

 sion. Not less important and far reaching was the later deduction of 

 Passarge^ of desert-leveling without baseleveling. For the development 

 of these basic ideas into a general^ definite, and systematic working 

 scheme chief credit, of course, belongs to Prof. W. M. Davis. This au- 

 thor has developed the normal cycle of land degradation;* he has pre- 

 sented a plan of a cycle of erosion under glacial conditions/ and more 

 recently he has discussed a geographic cycle in an arid climate. ° 



It is to the operation of certain processes of general denudation under 

 conditions of aridity that attention is here called. Two features in par- 

 ticular, the relative efficiencies of the different erosive agencies in a dry 

 climate and the probable character of the general surface relief of the 

 American desert region at the beginning of the present arid period, have 

 not as yet been accorded the critical consideration that they seem to de- 

 mand. For obvious reasons specific illustration is drawn here mainly 

 from the desert regions of southwestern United States and from northern 

 Mexico. In a general treatment of the subject later on, facts from other 

 desert districts of the globe will be more fully considered. 



The published descriptions of American arid features relate principally 

 to the Great Basin. The desert region of this country is, however, a far 

 greater expanse. Of it the Great Basin is only a minor part. Its major 

 distinctive provinces are known as the Great Basin, the Colorado plateau, 

 the Californian Gulf basin, and the Mexican tableland. For purposes 

 of present discussion the approximate boundaries of these provinces are 

 indicated on the accompanying sketch map (figure 1) of western United 

 States. 



General Characteristics of the arid Country 



Among the most distinctive peculiarities of the desert region, and those 

 which present the strongest contrasts to the relief expression of moist 

 countries, may be mentioned the following characteristics : 



1. Vastness and evenness of the intermont plains;^ 



2. Complete encirclement of the mountains by plains;^ 



3. General isolation of the mountain ranges ; 



4. Characteristic absence of foothills around mountain ranges; 



5. Great resistance to erosion of the mountain rocks ; 



2 Exploration of the Colorado River of the West, 1875, p. 207. 



3 Zeltsch. d. deut. geol. Gesellschaft, LVI Bd., Protokol, 1904, p. 193. 



* National Geographic Magazine, vol. i, 1888, pp. 11-26. 



s Proceedings of the Boston Society of Natural History, vol. xxix, 1900, pp. 273-322. 



« Journal of Geology, vol. xill, 1905, pp. 381-407. 



' Keyes : Bulletin of the Geological Society of America, vol. 19, 1908, p. 63, 



• Journal of Geology, vol. xvi, 1908, p. 434. 



