Vol. 4] 



Beid. — Tlie Comstock Lode. 



191 



All the facts relating to these peculiar ores tend to confirm the 

 view of their deposition resulting from the intermingling of oxi- 

 dized surface waters with deep alkaline unoxidized solutions. 

 They are known to occur nowhere else on the lode in any mine 

 workings, although the exploration within the croppings is very 

 little in amount. 



The acid surface waters which are now doing so much work, 

 have covered the walls and cross-cuts of the Andes mine with 

 from six inches to a foot of sulphates containing traces of gold 

 and silver. The chief salt is the magnesian aluminum sulphate, 

 with also large admixtures of iron and copper, which results 

 in a remarkable variety of colors. The workings of the Central 

 tunnel likewise shown these sulphates, but in general the circula- 

 tion of air is too rapid to allow of their great formation except in 

 some favorable localities. The composition of these salts is 

 shown as an average in the water analysis following. In the Cen- 

 tral tunnel in one particular spot, where the surface waters are 

 not fully oxidized, ferrous sulphate and pyrite are being de- 

 posited at the present time. Some sulphates are being formed 

 here in delicate needle-like crystals containing a large amount 

 of the ferrous salt. Bright, well-formed cubes of pyrite and 

 some few dark sulphides, too small in amount to admit of testing, 

 occur below and within the sulphates. Not infrequently a little 

 crystal of pyrite tips a needle of sulphate where the solution 

 is plentiful. Also, almost solid masses of the sulphide are found 

 within the wall in the partly decomposed country rock. The 

 water is descending along an east-west slip which dips to the 

 south. The clay, or clayey rock, upward along this slip is full of 

 pyrite, but this mineral is heaviest near the wall of the drift 

 where the solution is able to cover more ground. The clays of the 

 upper portion of the lode are all found to contain well formed 

 but small crystals of pyrite containing some value. There is 

 probably but one process responsible for all this, and a possible 

 reaction of the surface waters to produce such a result may be 

 expressed as follows : 



FeS0 4 + 2H 2 S0 4 + 7H 2 SO,=FeS 2 + 8H 2 S0 4 + ILO. 



This reaction is, of course, possible only when there is an 



