384 GEOLOGY OF THE COMSTOCK LODE. 



CHEMISTET. 



The chemical history of the Comstock is no doubt a very complex 

 one, nor are there by any means sufficient data to trace it in detail. All 

 that can be attempted here is to show that the results observed might 

 naturally follow from highly probable causes. 



The decomposition of the rocks shows three important features — the 

 formation of pyrite from the bisilicates and mica, the decomposition of the 

 ferro-magnesian silicates into chlorite, which is in part further altered to 

 epidote, and a partial change of the feldspar. 



Decomposition of the Fe.-Mg. silicates. — The pyntc appcars to liave formed at the 

 expense of the bisilicates or mica. The really fresh rocks contain no pyrite, 

 but minute crystals often occur in or are attached to partially decomposed 

 bisilicates. Sometimes distinct pseudomorphs of pyrite after augite or 

 hornblende are visible, but this is not common, because the average size of 

 the pyrite crystals is about one-half that of their hosts. A macroscopical 

 comparison, too, of series of rocks increasingly decomposed shows that the 

 pyrite is apparently associated with the ferro-magnesian silicates, and in 

 extreme cases replaces them with an entire correspondence of distribution, 

 so that the cumulative evidence is all in one direction. It is well known 

 that ferrous silicates in contact with waters charged with hydrogen sulphide 

 produce pyrite. 



The transformation of the bisilicates and mica to chlorite is a familiar 

 fact, and the general character of the change is not obscure, though its 

 details are far from clear. It must be accompanied by a separation of 

 all the lime, and of much of the silica and magnesia. It probably took 

 place for the most part in the absence of free oxygen. 



Epidote is very common on the surface, while under ground it seems 

 rare and confined to the neighborhood of fissures. The conversion of 

 chlorite to epidote must be accompanied by a substitution of lime for mag- 

 nesia, and by the conversion of ferrous to ferric oxide It might very 

 readily occur in the presence of solutions containing carbonic acid and free 

 oxygen, or Avhen surface waters mingled with waters rising from lower 



