14 MINING INDUSTEY. 



identical with the similar rocks which predominate in the majority of Cordil- 

 lera ranges; but we have positive proof of this in the fact that in Eldorado 

 canon, one of the ravines of the Pine Nut hills, Professor Whitney has found 

 Triassic fossils. 



With few exceptions, then, the range is built up by successive outpour- 

 ings of volcanic rocks, whose mode of occurrence, although simple and evident 

 in general plan, is very complicated in detail. 



In r^sumd, it may be said that this range is one of the old Jurassic folds 

 of stratified rocks, through whose fissures granite and syenite have obtruded; 

 that after a very long period of comparative repose, from the early Cretaceous 

 to the late Tertiary, the old range was riven in innumerable crevices and deluged 

 by floods of volcanic rocks, which have buried nearly all its older mass, and 

 entirely changed its topography. During this period of vulcanism the present 

 valleys were in great part filled with fresh water lakes; and near the base of 

 the Virginia Range we have evidence, in the tufa deposits, that a considerable 

 quantity of volcanic material was both ejected under water and flowed down 

 into it. Water penetrating the fissured range and meeting melted rock gave 

 rise to the solfataras and hot springs, whose traces are everywhere apparent. 

 Following this age of lava and steam eruptions came the Glacial epoch, with 

 its sequel of torrents and floods, and finally a great desiccating period, intro- 

 ducing our present condition. 



Atlas-Plate II is a geological and topographical map of the region 

 immediately about the Comstock lode, on a horizontal scale of three inches 

 to one mile, and constructed with grade curves of fifty feet vertical interval. 

 The distance embraced from north to south is a trifle less than six miles, and 

 from east to west a little more than four miles. This ground was chosen 

 with the purpose of illustrating the geological mode of occurrence of the Com- 

 stock lode. It is, therefore, almost wholly in the eastern slope of the range, 

 extending longitudinally a sufficient distance to embrace the silver lode and 

 such developments of rocks as afiect it, with a breadth from east to west 

 which includes the summit peaks, and reaches in one place to the eastern base 

 of the range. It would have been interesting to print, also, a full map of the 

 range, but a wider area would have introduced no facts which are not suffi- 

 ciently well shown in the limits of Atlas-Plate II. The topographical survey 



