SUMMARY. 337 



vein, where tliey would have been dei)osited on rehef" ot" pressure and dimi- 

 nution of temperature. It is by no means unlikel}' tliat, as at Steamboat 

 S])ring-s, evaporation aided in inducing jn-ecipitation 



Substitution. — It lias been claimed that the ore and (piartz have been 

 deposited by substitution for masses of country i-ock. This hypothesis is 

 exceedingly doubtful on chemical grounds, but there is also at least one in- 

 superable physical objection to it. In all proccs.ses involving the solution 

 of angular bodies it is a matter of common observation that points and 

 corners, which expose a greater sui-face than planes, are first attacked; conse- 

 quently masses exposed to solution, substitution, weathering, and the like, 

 always tend to spheroidal forms. Now, nothing is more common than to 

 find masses of country rock included in the ore-bearing quartz. I'hese 

 masses, in all cases which liave come under my observation, are angular 

 fragments, in form precisely such as result from a fresh fractui'e; not a 

 single instance has been observed in which a spheroidal rock was sur- 

 rounded by moi"e and more polyhedral concentric shells of quartz and ore. 



HEAT PHENOMEiSrA OF THE LODE. 



High temperatures met. — One of the famous pecullarities of the Comstock 

 Lode is the abnormally high temperature which prevails in and near it. 

 This manifested itself in the upper levels, and has increased with the depth. 

 The present workings are intensely hot. The water which flooded the lower 

 levels of the Gold Hill mines dui-ing the winter of 1880-1881 had a tem- 

 perature of 170° F. This water will cook food, and will destroy the human 

 epidermis, so that a partial immersion in it is certain death. The air in the 

 lower levels more or less nearly approaches the temperature of the water 

 according to the amount of ventilation. The rapidity of the ventilation 

 attained in the mines is something unknown elsewhere, yet deaths in venti- 

 lated workings from heat alone are common, and there are drifts wdiich, 

 without ventilation, the most seasoned miner cannot enter for a moment. 

 Except where circulation of air is most rapid, and in localities not far 

 removed from downcast shafts, the air is very nearly saturated with moist- 



