RESUME OF RANGES OF WESTERN CENTRAL NEVADA 233 



subsequently covered by lavas ; and deeply worn down by general ero- 

 sion, which produced anticlinal valleys and synclinal hills. 



Resume of ranges of western central Nevada. — The abundance of volcanic 

 rock in this region makes the determination of the origin of the ranges 

 still more uncertain than usual. The high degree of folding of the 

 early Tertiaries (which are not found farther east) shows that the last 

 epoch of mountain-making was Tertiary. We can not be sure how long 

 this period of disturbance lasted, nor have we any reason for believing 

 that it is yet entirely finished. But the period of greatest activity closed 

 before the epoch of the late Pliocene- early Pleistocene lake ShoshonC) 

 whose deposits are generally horizontal. In certain places, however, the 

 deposits of this lake have probably been uplifted a thousand feet or more 

 in a gentle swell, showing more recent Pleistocene movement. Whether 

 any of the pronounced folds and faults have outstripped erosion so as to 

 preserve their direct expression in the topography we can not be certain, 

 though it appears possible. This region lies in an orographic zone, whose 

 trend is defined by the direction of its mountain ranges and by their 

 parallelism to the eastern front of the Sierra ; farther south this zone 

 includes the Death Valley region, where late Tertiary and Pleistocene de- 

 formation certainly occurred, and, apparently, directly created mountains 

 and valleys.* Nevertheless, where we have opportunity to investigate 

 the amount of erosion in western central Nevada, as in the case of the 

 Excelsior mountains, we find it has been sufficient to overbalance the 

 effects of the Tertiary folding. Therefore we must conclude that the 

 effects of erosion are more important than those of direct deformation, 

 though the latter agency may be a greater factor than we are yet aware 

 in producing ranges and minor topography. 



RANGES OF SO UTIJERN NE VA DA 



Virgin range. — The Virgin range constitutes the last of the high moun- 

 tain ranges to the east and faces the Colorado plateau. According to 

 Marvine,t the east face of the range coincides with a fault which appears 

 to be one of the Colorado Plateau system, for Button J describes on the 

 eastern edge of the Grand wash (which lies directly east of the Virgin 

 range) a second fault, by which the country to the east is upheaved 

 between six and seven thousand feet. Marvine states that the main 

 fold of the range is an anticline. Toward the south the folding dies 

 out, and the rocks become horizontal in the Colorado canyon. 



If, as appears from the reports, the eastern face of the range is a simple 



* See p. 239. 



*U. S. Geol. Survey West of Hundredth Meridian, vol. iii, Geology, p. 194. 

 - t Second Ann. Kept. U. S. Geol. Survey, p. 126. 



XXXIV— Bull. Geol. Soc. Am., Vol. 12, 1900 



