300 GEOLOGY OF THE EUREKA DISTRICT. 



if anything, to do with the occurrence of the deposits. They depend more 

 upon the fissuring and fracturing of the mountain uplifts and the relations 

 of the accompanying faults to the outbursts of rhyolite. 



A study of the mountain blocks and distribution of ores brings out the 

 fact that what has been designated the Prospect Mountain uplift, lying 

 between the Hoosac fault on the east side and the Sierra and Spring Valley 

 faults on the west, embraces pretty much all the valuable mineral deposits 

 which have as yet been successfully developed. Within the limits of these 

 lines of faulting are embraced all the mining properties extending from 

 Adams Hill southward to Surprise Peak, including those on the west side 

 of Prospect Mountain, together with the Dugout mine at the southwest base 

 of the peak on the west side of the anticlinal fold. In preceding chapters 

 the structural features of Prospect Mountain Ridge and relations between 

 the sedimentary beds and intrusive dikes have been described with some 

 detail. As has already been shown, the strata between these faults belong 

 to the Cambrian and Silurian periods up to and including the lower portion 

 of the Lone Mountain horizon exposed on the north side of McCoy's Ridge. 

 The principal overflows of rhyolite have been along the line of the Hoosac 

 fault, the two most powerful centers of extravasation being located at Pinto 

 Peak and Purple Mountain. Purple Mountain lies in the angle between the 

 Hoosac and Ruby Hill faults, and it is along this latter fault that rhyo- 

 lites come to the surface all the way from New York Canyon to the Jack- 

 son fault, thence crossing the latter fault, fill the fault-fissure for a consid- 

 erable distance along the north slope of Ruby Hill, but without building 

 up any accumulation of rhyolite on the surface. 



While it can not be absolutely demonstrated, all evidence bears out the 

 assumption that the dikes penetrating Prospect Mountain Ridge have a 

 deep-seated connection with the source of the rhyolite material which has 

 furnished the surface outflows all along the line of faulting. It can hardly 

 be doubted that both forms of the same eruptive rock mass have had an 

 identical deep-seated origin. It should be also borne in mind that it is only 

 in exceptional instances that dikes and off-shoots from any parent body of 

 lava can be traced to their source step by step in the field without any 

 break in continuity. 



