222 GEOLOGY OF THE COMSTOCK LODE. 



cates which has taken place throughout a large area. On the other hand, 

 minute quantities of gold and silver can be more easily and more certainly 

 determined by drv assay than by analysis, provided that pure lead reagents 

 can be procured But the selection of suitable material for the investiga- 

 tion of the gold and silver contents of the Washoe rocks was by no means 

 a simple matter. As has been seen, there is but one spot known in which 

 nearly fresh diabase can be collected, and that close to the Comstock fissure. 

 Moreover, the quantities of the precious metals to be dealt with are so minute 

 that a mere trace of infiltrating solutions of their compounds would impart 

 a comparatively important metallic contents, and that such impregnations 

 occur in some of the rocks there is very good reason to believe. This 

 occurrence of fresh diabase is therefore open to suspicion. If, however, the 

 diabase which forms the east or hanging wall of the Lode is the source of 

 its gold and silver, fresh portions of the rock will show a larger quantity of 

 the precious metals than decomposed samples; while, if the source of the 

 ore were independent of the diabase, decomposed portions of the latter, 

 being more porous, would have been more readily and fully impregnated 

 by the metalliferous solutions. Moreover, it has been shown that pyrite 

 forms at the expense of the augite of the diabase, and as pyrite is known to 

 have a very strong affinity for gold, the decomposed pyritiferous rock should 

 show a greater pi'oportion of gold to silver than the fresh diabase, if this rock 

 is the source of the metals. Were the original distribution of gold and silver 

 and their subsequent extraction nearly uniform, the composition of the ore in 

 the Lode would correspond to the contents of the fresh rock, less that of the 

 decomposed rock and the pyrite, as shown b}^ a limited number of assays. 

 The quantity of the precious metals occurring in the vein should also be 

 calcvilable from the extent of the decomposed rock. Such ideal conditions, 

 however, are not to be expected. The excessive difficulty of obtaining a 

 representative sample of any gold or silver deposit is familiar to all mining 

 men, and in the Comstock itself great vai'iations, both in the relations of 

 gold to silver and in the total tenor, are of constant occurrence. On the 

 supposition that the metals have been extracted from the diabase these 

 variations indicate great irregularity in the leaching action or in the original 

 distribution of the metals, or, more probably, in both. 



