108 



University of California Publications. [Geology 



The plateau residuals are commonly isolated, but on the large 

 schist area west of Carson the ancient plain descends to the valley 

 in a series of steps. Below the plateau remnants the fault- 

 scarps, some almost unaltered and some partly degraded, descend 

 to the valley. The zones are thus hypsometric as well as geo- 

 morphic. The elevation of the ancient plain ranges from 6,500 

 feet to 8,300 feet. The zone of the Nevada diastrophic valleys 

 averages about 4,800 feet. The fault zone lies between, bounded 

 above and below by level areas. 



Upon more careful examination of the highest or plateau 

 zone, it is found that at a few points low hills rise gently above 

 the surrounding peneplain. These hills are today the important 

 mountain peaks, though rising but a few hundred feet above the 

 ancient plain. They are few in number but important in their 

 bearing upon the geomorphic evolution of the range. Hence, 

 for clearness in discussion and to enable a better coordination 

 to be made with Professor Lawson's geomorphic zones 7 in the 

 Kern basin, they are here placed separately in a summit zone. 

 There are then, in the Carson region four zones : ( 1 ) the sum- 

 mit zone, (2) the plateau zone, (3) the fault zone, and (4) the 

 valley zone. 



I. The Summit Zone. 

 The summits rising above the plateau remnants are best rep- 

 resented by Slide Mountain, Marlett Peak and Genoa Peak, that 

 of Marlett being the best example. In the Virginia Range similar 

 elements of the geomorphy exist, and Mt. Davidson appears as a 

 low summit above the plateau to the west. (See pi. 23a). The 

 exact correlation of the Virginia Range plateau with that of the 

 Sierra will do much to elucidate the geologic history of the east- 

 ern range and perhaps settle beyond all doubt the question of 

 sequence and relations of the igneous rocks of the Comstock. 

 Here is again touched the correlation of lava flow with orogenic 

 disturbances. The slopes of the summits rising above the plateau 

 are always gentle, whether composed of granitic or schistose 

 rocks. The summits themselves are flat or nearly so, likewise 

 irrespective of the rocks composing them. A priori, these ele- 

 ments of the geomorphy might be (1), due to the control by the 



7 Univ. Calif. Pub. Bull. Dept. Geol., 3, p. 307. 



