226 GEOLOGY OF THE COMSTOCK LODE. 



Nature of the solvents. — As has been seeii, there is reason to suppose that the 

 active reagents in the decomposition of the minerals of the diabase were 

 sulphhydric and carbonic acids. These acids so usually reach the surface in 

 volcanic regions that there seems no necessity for examining their origin 

 here, but it may be pointed out that solutions of sulphates rising through 

 graphitic slates, such as form in part the foot wall of the Gold Hill mines, 

 would necessarily be reduced to sulphides. Both augite and plagioclase 

 would yield to the attack of carbonic and hydrosulphuric acids; carbonates 

 and sulphides of the alkalies and alkaline earths would be formed, and these 

 are solvents for quartz and sulphides of the heavy metals. There is no 

 difficulty, therefore, in accounting for the solution of the materials filling 

 the CoMSTOCK Lode. It is somewhat less easy to trace the precipitation .of 

 the ore with certainty. Solutions of silica in water containing alkaline car- 

 bonates deposit silicic acid only on evaporation, not on cooling; but when 

 sulphides of the alkalies are also present a reduction of temperature is fol- 

 lowed by the precipitation of a portion of the silica. Solutions percolating 

 from the east country into the main fissure, where communication with 

 the outer air was less impeded, may have deposited some of the quartz in 

 consequence of cooling. This possibility, however, seems scarcely adequate 

 to explain the phenomena. Vast quantities of the solvent must have been 

 necessary to carry all the silica occurring on the Lode ; and it is difficult to 

 understand how any great amount of cooling can have taken place. If hot 

 solutions are supposed to have issued as springs along the croppings, the 

 influence of exterior conditions on the temperature of the water below the 

 surface must have been insignificant, and Sandberger has found that copious 

 mineral springs deposit sinter about their orifices, but not in the channels 

 leading to them. Even if the solutions may be supposed not to have over- 

 flowed, being, as they must have been, in communication with an active 

 source of heat, they would have been maintained at a nearly constant tem- 

 perature by convection. 



Precipitation. — Sihca is vcry readily precipitated from solution, and it is 

 well known that when both silica and carbonate of calcium are dissolved 

 in the waters of hot springs, the acid is deposited near the source and calcite 

 at a greater distance. Sandberger states that when such solutions become 



