LINEAMENTS OF THE DESERT 23 



wind-scouring, the very subordinate, local and sporadic character of 

 water-action, and the remarkable plains-forming tendency which de- 

 flative erosion effects. 



Since most of our conceptions of landscape genesis are derived from 

 our experiences in a normal moist or wet climate, the erosion agency 

 with which we are best acquainted is running water. In the desert 

 regions actually and necessarily water plays but small part in erosion. 

 With less than ten inches of annual rainfall, most of which sinks into 

 the earth as soon as it touches it, as in the dry regions of southwestern 

 United States and the northern part of the Mexican tableland, or less 

 than one inch as in the Nubian and Libian deserts of north Africa, 

 the erosive influences of water must be all but a negligible quantity. 



With water-action reduced to relative impotency in the desert 

 region, wind-scour assumes a role the denuding power of which has 

 been heretofore little considered. Its action is general and constant. 

 Its effects are probably even more vigorous than the work of water 

 under normal climatic conditions. In the effort to reduce the land 

 surface to a low-lying plain the belts of hard and soft rocks are brought 

 into somewhat stronger contrast than in the case when water is the 

 chief agency of planation. The geologic structures are more sharply 

 accentuated. The rock-floors are cleaner swept. The belts of weak 

 rocks are faster removed. At all times the plain is more character- 

 istically the main relief feature. 



Contrary to popular belief the great desert tracts of the earth are 

 mountainous regions. The mountain character has many novel and 

 instructive peculiarities. Yet so dominant is the plains feature locally 

 that the mountain ranges, bold and lofty as they often are, rise sharply 

 from the level expanse as do volcanic isles out of the sea. So charac- 

 teristic is this aspect that it is, in the South African deserts, appro- 

 priately denominated the " Inselgeberglandschaft." 



In regard to the manner of their development the salient linea- 

 ments of the desert deserve much more attention than ever has been 

 given them. They acquire new meanings when their peculiarities are 

 considered in the light of an origin eolian in nature. Notwithstanding 

 the fact that some of us, whose lifelong experiences have been mainly 

 with the workings of the geologic processes in the moister parts of the 

 globe, may find it a little difficult to fully appreciate at first the direct 

 significance of many of the details of the relief features, a visit to the 

 desert soon convinces us of their verity. There are at least a score 

 of these physiographic characteristics of the diy lands that are espe- 

 cially striking. 



The dominant feature of such desert regions as the western part 

 of our own country and of Mexico is the interrupted plain the general 

 surface of which is 5,000 to 7,000 feet above the sea. Out of it rise 



