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



recent that its expression in the surface relief has not had time to be 

 entirely mastered by erosional effects. Lauterback's idea of a mountain- 

 block and a valley-block no doubt holds good for a multitude of instances 

 other than the few which he cites in northwestern Nevada. For a broad 

 generalization its main qualifications are that the valley-block be also a 

 block of soft rocks and that the time of faulting greatly antedate the 

 present expression of relief. So, too, Davis' theory of extensive dissec- 

 tion of mountain masses is more directly applicable to the loftier moun- 

 tain ranges, where there is notable approach to normal humid conditions 

 of climate, while aridity prevails on the plains about. My own views 

 have, been that there is much greater complexity in the genesis of the 

 deseri ranges than is generally supposed; that while flexing and profound 

 faulting have gone on rhythmically, perhaps ever since Jurassic times, 

 the greater part of the crustal movements had already taken place before 

 the extensive planing off in Tertiary times, and that the differential ero- 

 sion of such an even surface composed of alternating belts of resistant 

 and weak rocks have resulted in the marked contrasts of relief now ex- 

 hibited by abrupt mountain and even plain, the extensive erosion being 

 mainly eolic in character. 



General Tectonics of the Desert Region ^ 



Singularly enough, controversy relating to the geologic structures of 

 the Desert ranges has been confined almost wholly to a relatively minor 

 portion of the arid region. The Great Basin, which is scarcely one-sixth 

 of the whole, has received nearly all of the attention. Were it not for 

 this fact, the generally accepted notions concerning desert tectonics and 

 the rearing of the mountains would be no doubt quite different from 

 what they now are. In their bearing on the general tectonic conceptions 

 the geologic structures of the several provinces are equally as important 

 and equally as illuminating as those of the Great Basin. 



Strangely enough, the ideal type of Basin-range structure appears to 

 be, so far as recent observations go, almost entirely wanting in the Great 

 Basin district itself. Spurr's lately expressed views^ regarding the 

 mountain ranges of Nevada are without much question in the main cor- 

 rect, at least in so far as they relate to the rare occurrence of typical 

 block-mountains. However, that these mountain ranges are formed 

 chiefly by flexing of the strata, after the manner of the Appalachians, is 

 a statement which must be substantiated by additional evidence before it 

 can be accepted fully. 



» Bulletin of the Geological Society of America, vol. 12, 1901, p. 217. 



