96 MINING INDUSTET. 



charges of metal-bearing quartz. The latest flows of andesite poured out over 

 the decomposed propylite; and since they are themselves unaltered, their 

 appearance marks the period when solfataric action over wide areas had ceased- 

 While it no longer maintained its energy through the broad zone of propylite, 

 it still continued intensely active within the chambers of the Comstock lode. 

 Metallic contents were introduced into the the quartz, the clay-seams were 

 formed by a rapid decomposition of the neighboring propylite materials, the 

 horses reduced to a spongy, semi-plastic condition, and at last the final solidi- 

 fication of the quartz took place. Outside of the vein two events of geological 

 interest have occurred: first, the period of trachyte eruptions, when from the 

 ruptures of the crust, parallel to the Comstock lode, vast volumes of sanidin- 

 trachyte overflowed the country; and, secondly, the less powerful but still 

 important outpouring of basaltic rock, which marked the close of the volcanic 

 era. Within the vein, and probably caused by one or both of these latter 

 volcanic disturbances, a pressure has been exerted which has crushed and 

 ground the masses of quartz into minute fragments. It is interesting to 

 observe that while this force was great enough to crush quartz-masses one hun- 

 dred and fifty feet in breadth into mere angular pebbles, the disturbances were 

 insufficient to cause any actual faulting of importance. Both within and with- 

 out the vein the solfataras gradually came to a close. The heated currents of 

 water which even yet ascend into the lower levels of the mines, are evidence 

 that at no very great depth a considerable temperature is still maintained ; but 

 this is only a faint relic of a once intense action. 



No chemical theory will be advanced as to the origin of the quartz, nor 

 of those delicate questions of magneto-chemical introduction and subsequent 

 transmutations of the metallic minerals. They belong rather to an abstruse 

 study of the theory of veins in general than to an investigation of a particular 

 district. The writer has endeavored to present the most important facts that 

 came within his observation, in the hope that a patient reader may gain a 

 general view of the structure and chemistry of this celebrated lode. 



