GEOLOGY. 463 



THE COMSTOCK LODE. 



G. F. Becker has given, in the reports of the United States Geological 

 Survey, a preliminary account of a more extended study of the geology of 

 the Washoe district and the Comstock lode, while for a detailed account 

 of the lithology of the region we must await the final report. It is well 

 known that this immense quartz lode, which has yielded over $315,000,000 

 of bullion in the last twenty-five years, is in great part included in erupted 

 rocks of Tertiary age. The study of these, and of their changes under 

 the influence of the heated waters, which have doubtless brought in the 

 materials of the lode, is full of interest, and shows the frequent produc- 

 tion of epidote and of chlorite by the transformation of the original 

 mineral species. The hypothesis which has been advanced, that the 

 high temperature of the mine-waters and their inclosing rocks is due to 

 the chemical changes in these, connected with kaolinization of the feld- 

 spars, is discussed and rejected. The feldspars are not kaolinized, nor 

 do the waters contaiu the large amount of dissolved silica and alkalies 

 which would be derived from such a process. Furthermore, there is no 

 reason for believing that such a transformation of the feldspars would 

 generate heat. Analogy would suggest that heat should be rendered 

 latent in such a process. It is suggested that the source of the abun- 

 dant water of the mine is to be found in the precipitation on the high 

 crests of the Sierras, to the west, and that it reaches the lode through 

 deep-seated channels, in which it becomes heated. The mine-waters are 

 charged with carbonic acid, and that encountered at a depth of 3,000 

 feet, having a temperature of 1 70° F., was found to be charged with hy- 

 drogen-sulphide. Such waters have probably been the efficient agents 

 in filling the lode with quartz and precious metals. The well-known 

 Steamboat Springs, at a lower level, a few miles distant, still discharge 

 waters at a boiling heat, along a fissure parallel to the great lode, and 

 near to the contact of ancient massive rocks and andesites. The waters 

 of these springs, impregnated with hydrogen sulphide, still deposit silica 

 and cinnabar on the walls of the fissures, all of which facts are recalled 

 by Becker in this connection. It may here be mentioned that evidence 

 collected by the present writer at the locality shows that the discharge 

 of water and vapor at the Steamboat Springs has diminished consider- 

 ably since the opening of the deep levels of the Comstock lode. 



The theory that the filling of mineral veins, in many cases at least, 

 comes from lateral secretion, is considered by Becker, and it is shown 

 that the unchanged diabase of the Comstock lode contains, chiefly in 

 the augite, a noteworthy amount of both gold and silver; and, moreover, 

 that the diabase which has been modified by water has lost one-half of 

 this amount. It is also shown that the total exposure of diabase is suffi- 

 cient to account for far more bullion than has been extracted from the 

 mines. It is in connection with this rock that the richest ore- bodies have 

 been found, while those in contact with the ancient diorite are compara- 



