ORES OF TOE DEVONIAN. 297 



much disturbed and faulted and rliyolite has i-eached the surface only a 

 shoii; distance from the mining- property. 



In the Eureka (piartzite the only instances kno^A'n of inineral de])osi- 

 tion are those found on Hoosac Mountain, a descriittion of wliicli is given 

 elsewhere. They have been worked extensively and have yielded consider- 

 able ore. Here they are intimately associated with intrasive dikes of l)oth 

 andesite and rhvolite otishoots from the great Ixxlies which forced their 

 way upward along the Hoosac fault. 



Throughout the luireka Mountains the Lone Mountain horizon has 

 here and there shown evidences of mineral dei)osits when found in the 

 neighborhood of rhyolite outbursts, but over the greatei- part of tlie area 

 they exhibit no surface signs of ore-bearing material. An interesting- 

 example of ore in the Lone Mountain horizon m;iv be found at tlie 

 Seventy-six mine, in hard, flinty limestone on the northwest side of 

 McCoy'.s Ridge. While it can not be looked upon as remunerative property, 

 from the ]ioint of view of the present description it serves as an iustnjctive 

 link in the chain of facts bearing upon the geological position of the 

 Km-eka ore bodies. This is the only liody of Lone Mountain limestone 

 lying in close proximity to the Hoosac fault, and, in conse(iuence, ])artially 

 explains the occurrence of ore. 



Ores of the Devonian.— Passing u])ward, without any intervening litlu)logical 

 break, the Nevada limestones ai-e in like miumer frequently found to cany 

 oxidized, argentiferous lead ores in fissures and crevices in the regions of 

 ])rofound foults. It by no means follows that rhyolites necessarily accom- 

 ])any tlie ore at the surface, and still less that the latter occurs wherever 

 rhyolite penetrates the Nevada limestone throug-h fissure planes. Instances 

 may l)e cited in the case of the Keese and Hen-y mine, just north of the 

 canyon of tlie same name, and again on the summit of Newark Jlountain, 

 both localiti(^s indicvating distm-bances of strata without any assignable 

 cause on the surtace. The dislocation of beds may be due to intrusive 

 rocks which fixiled to penetrate the surface, the fissuring- being filled by 

 mineral matter. 



Along Rescue Canyon, Avhere there is such a continuous and powerful 

 mass of rhyolite under geological conditions similar in many res])ec.ts to 



