232 J. E. SPURR — ORIGIN AND STRUCTURE OF THE BASIN RANGES 



fault-scarps, but are either simple erosion scarps or erosion fault-scarps.^ 

 The rapidity with which the Carson river has eroded its deep canyon 

 between Carson and Dayton since the disappearance of the late Pliocene 

 lake Shoshone and the deep and rugged transverse canyons in the Vir- 

 ginia range all militate against the belief that faulting should have been 

 rapid enough to have outstripped on so large a scale the erosion. 



While acknowledging, then, that it will require more careful study to 

 finally decide the origin of these volcanic ranges, we may yet point out 

 that the fault theory has only the value of an h3q)othesis, and at the 

 same time we may indicate the claims of an antagonistic hypothesis, 

 that of basal erosion. 



Although the climate is arid, this region, compared with the most of 

 the Great basin, is well watered, for it receives'streams which rise in the 

 moister Sierras. In earlier times the region was occupied by fiorded 

 lakes which fretted their mountain shores and wore them back. On 

 account of the general westerly slope of the whole western part of the 

 Great basin these lakes were deeper, and consequently possessed more 

 eroding and transporting power on the western side of the valleys ; and 

 when they shrank so that they could not occupy whole valleys, they 

 rested against the eastern face of the ranges. Lake Lahontan washed 

 the eastern side of the Walker River range, but did not touch the west- 

 ern ; it lay at the eastern base also of the range next west, but did not 

 approach its western side.f Hence it is, perhaps, that the ranges have a 

 tendency to a steep scarp on the east more than on tbe west. In places, 

 however, we find they have worn ])ack so as to present a stee]^ scarp on 

 both east and west. This is true of the Pinenut range just east of ])ay- 

 ton ; at this i)oint lake Lahontan washed both sides of the range.J The 

 Sierra Nevada, since it formed the final barrier to all these lakes, was 

 naturally attacked only on one side. § 



Excelsior range. — The Excelsior range is made up partly of igneous rock 

 and partly of a series of folded limestones, shales, and sandstones. In 

 the limestones fossils were found which are regarded as early Tertiary, 

 while the sandstones (judging from their lithology alone) appear more 

 nearly referable to the Jurassic or Triassic. These sedimentary rocks 

 have been folded and eroded so as to outcrop in synclinal hills and anti- 

 clinal valleys. The volcanic rocks, which are of later date, have also 

 been deeply worn down. The axis of the main fold is east and west. 



The Excelsior mountains, therefore, were folded during Tertiary time ; 



* See p. 259. 



t Russell : Map, Monograph xi, U. S. Gcol. Survey. 



J Russell: Ibid. 



g Tlie question of the existence, number, and nature of the Tertiary lakes of the Great basin, 

 although demanding discussion (see W. M. Davis, Proc. Am. Acad. Arts and Sciences, vol. xxxv, 

 no. 17, March, 1900), can not suitably be dwelt on in this paper. 



