Vol. 4] Smith. — Upper Region of Main Walker River. 



•1'.) 



there are a few veins that do not conform to either system. 



That the mineral veins have preceded the present topography 

 seems quite clear, and this is thought to be so for two principal 

 reasons, which are, the absence of the veins from the rhyolite 

 and the fact that the veins often reach to the highest summits. 

 The latter condition would indicate that there is not now suffi- 

 cient hydrostatic head under present conditions of surface con- 

 figuration to bring the solutions from which the veins were 

 formed to their present height. It may be remembered in con- 

 nection with the first of these reasons that previous to the advent 

 of the rhyolite the region was reduced to one of tolerably low- 

 relief. This evidently indicates that the vein formation was not 

 active at the surface, and it was not until the later Tertiary, 

 when the present ranges had been lifted and erosion had begun 

 its work of uncovering them, that they became exposed. 



The period of formation of the veins closed with the making 

 of the present mountains in the Tertiary, but when it began is 

 not so clear. Most likely it was after the effusion of the early 

 andesite, for in one instance an andesite dyke forms one wall 

 of a vein. It appears that no andesite remains residual near 

 the metamorphics in which the strongest deposits of copper 

 occur. 



The Deposits in Particular. — In a few instances the mining 

 has progressed beyond the prospect stage. At the Ludwig Cop- 

 per Mine a depth of about four hundred feet has been reached. 

 The ore occurs in chambers in limestone. These chambers when 

 taken together occur each more or less below the other, and when 

 considered as a whole the idea of an ore chute nearly expresses 

 their character. The chambers are along the contact between 

 the iron ore deposit and limestone, which two occur as a simple 

 outcrop having considerable length and a width of less than a 

 hundred feet. They do not meet squarely in this outcrop, but 

 somewhat diagonally, hence there is a longer contact zone for the 

 chutes. The rest of the country rock is granitic. The ores are 

 sulphide, oxide and carbonate. At the Douglas Mine the ore is 

 chalcopyrite and chalcocite; the gangue is garnetiferous. About 

 half of the ore is sulphide. It was apparently along the lines 

 of fracture that the ore solutions found their way. The country 



