242 GEOLOGY OF THE COMSTOCK LODE. 



ently drain but a very small area. Before mining began, however, little 

 or no water issued from the surface. When the fii'st floods were encoun- 

 tered it was supposed that there must be great accumulations of water in 

 subterranean caves, and that water-ways leading to them had been cut 

 by the workings. But no such openings were ever reached in the mines, 

 and it came to be supposed that the water had accumulated in the inter- 

 stices of shattered rock masses. Broken as the rock is, however, it is very 

 closely packed, so that the interstitial space is but small, and considering 

 the vast quantities of water which have been pumped from the mines, I 

 cannot think the explanation adequate.* The pressure under which the 

 water is frequently met is a significant feature of its occurrence. Thougli 

 there may be other workings on the same level, and though the country 

 above may be extensively opened up, a new source will sometimes show a 

 head of several hundred feet. The deeper the point at which the water is 

 struck the hotter it usually is, and there appears to be some tendency of the 

 temperature of the water from a single source to increase as it is drained. 

 But if it were accumulated in a mass of shattered rock of limited extent, tlie 

 water and the rock throughout the entire space would necessarily assume a 

 perfectly uniform temperature, and channels tapping such an accumulation 

 at different levels would emit streams of the same temperature. As has 

 been seen, the rock is commonly cooler than the water, and the general 

 reasoning in the foregoing paragraphs points 'to the rise of currents from 

 great depths. An attempt will be made to reconcile these facts. 



Hypothesis of its origin in the Sierra. lu thc Gold lUU miueS tllC foOt Wall of tllC 



LoDK in the lower levels is composed of metamorphic rocks dipping to the 

 east, as do those also on the whole which occur at the southwestern corner 

 of the map. But from the ^Jomstock west the country, excepting one or two 

 small masses of granite, is completely covered by volcanic rocks, for a 

 distance of about 12 miles, or until the main range of the Sierra Nevada 

 is reached. This grand feature of the continent is far too complex to be 

 simply characterized as an anticlinal, but the declivities opposite the Com- 

 STOCK show more or less metamorphosed strata with an easterly dip, and 

 it is fair to infer that for some distance from its vast mass, i. e., in the coun- 

 try between it and Virginia, the strata underl3'ing the fields of andesites dip 



' Seven million tons of water, tUe estimated annual discbarge, is about COO feet cube. 



