544 C. R. KEYES PROFILES AND STRUCTURES IN DESERT RANGES 



In accounting for the origin of the mountains of the Desert region, it 

 was long believed that they were formed by simple normal faulting on a 

 grand scale. This was the view first advanced by Gilbert,^ and it was 

 generally adopted by the early explorers of the Great Basin region of 

 western United States. At first glance this explanation appears to be 

 quite sufficient. Throughout the arid region the mountains are largely 

 devoid of vegetation and the attitude of the strata is readily made out 

 even at great distances. Around the bases of the ranges long, smooth 

 slopes extend in all directions until they merge into the central parts of 

 the intermont plains. To all appearances the slopes are composed of 

 debris washed down from the bordering highlands, and the natural infer- 

 ence is that the intermont valleys are constructional plains formed of 

 the wash from the mountains on all sides. In many cases the so-called 

 wash-plains extend up the mountain sides until the rugged bare rocks 

 seem to be all but completely buried. 



Within the past few years our notions regarding the origin and struc- 

 ture of the desert ranges have undergone some very radical changes. 

 The tendenc}'' is to modify more or less the simple normal fault hypoth- 

 esis so generally held a generation ago. There are some novel reasons 

 for the recent awakening of a new interest in the genesis of this class of 

 mountains. Prominent among these newer considerations is the idea 

 that the dominant relief features of the so-called Basin-range type of 

 mountains are probably not due so much to local and intense fault- 

 movements as they are to the differential topographic effects of general 

 desert-leveling, without notable assistance from running water, but with 

 eolic influences operating to their maximum extent. On the local prog- 

 gress of eolation the ancient faultings and flexings have a very important 

 influence, yet the rearing of the mountains throughout the region can not 

 be ascribed wholly to a single cause. In some, and in perhaps most, 

 cases the direct role of recent crustal movement seems quite secondary. 



Subdivisions of the American Arid Eegion 



As the term is usually employed, the desert region of the United States 

 is a rather vaguely defined part of our country. Geographically the 

 territory included within the limits of this region actually embraces sev- 

 eral more or less well defined provinces of which the Great Basin is only 

 a minor part. In the central portion of the region is the elevated plain 

 or dome commonly called the Colorado plateau. Only to the north of 



2 Geographical and Geological Survey West of the 100th Meridian ; Report of Trog- 

 ress for 1872, p. 48, 1874. 



