18 GEOLOGY OF THE COMSTOCK LODE. 



The yield of the ore has decreased in general from the surface down- 

 ward. The deposits of the OiMr and Mexicnn and of the Go^dd dt Curry 

 were the richest. The former yielded on an average Sl07 per ton; the 

 latter $70 and $80, notwithstanding the imperfect j^rocesses of extraction 

 which were formerly applied. Ores of $600 to the tun were then no rarity, 

 and considerable shipments could be made of such as yielded from $2,000 

 to $3,000 to the ton. It would now scarcely be possible to collect one ton 

 of such ore. The general average of all the ores extracted in 18fi '; will not 

 be more than $37 to the ton. The proportion of gold to silver decreased 

 during the early period of the working on the Comstock Lode, but is now 

 again on the increase. 



Source of the ore. — The CoMSTOCK Vein has neither been filled from above 

 nor from the sides, as none of the surrounding rocks could have yielded the 

 immense quantity of vein matter and ore; and had it been formed in this 

 way, the mass would have a banded and comby structure, which is by no 

 means observable. The eastern rock may, on account of its extensive 

 decomposition, appear to favor the assumption of lateral infiltration ; but 

 this decomposition was effected by ascending currents, which have left 

 distinct traces, and which could not have removed any matter in a lateral 

 way. Thermal springs, which are considered by man}^ authorities as the 

 agent which carried mineral matter from below into fissures and to have 

 formed every true vein, would not explain the formation of the Comstock 

 Lode. Silica, in such cases, is accumulated round the mouth of the fissures, 

 and though ordinarily removed by denudation, it could hardly be supposed 

 to be so at the Comstock vein, as since its formation the surface has under- 

 gone but slight changes. But, besides, the decomposition of the eastern 

 country for miles in extent cannot be explained by the action of thermal 

 springs. 



The Comstock fissure is, of course, of more recent origin than the 

 rocks which it traverses; and as propylite is predominant in the latter, the 

 fissure must necessarily have succeeded it in age. The only event after 

 the outbursts of propylite capable of producing such powerful action was 

 the eruption of trachyte, which accompanies the vein at a distance of two 

 miles to the east. As there is other evidence of its intimate connection with 



