THE COMSTOCK LODE. 19 



The district, then, is an accumulation of volcanic rocks built upon the 

 eastern slope of an earlier range. We have the evidence of an original chain 

 in the syenites and metamorphic rocks, and we have, clearly superposed in 

 their normal sequence, the propylite, andesite, trachyte, and basalt flows ; and, 

 finally, are observable the results of intense aqueous erosion, which has scored 

 the mountain slopes into sharp, deep ravines. It is an epitome of the whole 

 Great Basin, and it is doubtful whether anywhere else over the entire Cordillera 

 system can be fomid, in the same narrow limits, a representation of every 

 important geological event. The points in this geology which affect the Com- 

 stock silver lode are, first, the mass of ancient rocks which slope steeply to 

 the east; secondly, the propylites which overflowed these rocks to a given 

 height ; and thirdly, the andesites, which, in the form of an obscure, thin dike, 

 have burst out on the contact plane of syenite and propylite ; fourthly, the 

 immense solfataric activity to which the vein unquestionably owes its origin, 

 and whose influence is recorded in the decomposed propylites lying east of 

 the vein. The andesites overlying them are untouched. The general thermal 

 activity was confined to the interval between the out-flow of propylite and 

 that of the later andesite. To this period is assigned the Comstock lode. </ 

 The thermal action which began at the dawn of the volcanic period, we 

 behold at present in its last stages. It is probable that long after the great 

 solfatara had ceased altogether, the Comstock remained the theatre of great 

 activity, and that only in most recent times has intense chemical and dynamical 

 action abated. 



G-RANiTE AJSTD Metamorphics. — The granite which occurs in American 

 Caiiou is composed of opaque dull-gray orthoclase, combined with an 

 unusual proportion of translucent quartz, and containing a small and vari- 

 able percentage of green mica in half decomposed hexagonal plates. The 

 feldspar closely resembles that which occurs in the general mass of syenite, 

 and is entirely distinct from the adularia variety which is characteristic of 

 the majority of Jurassic granites in Western America. Resting upon this, 

 and upon the syenite, lies the series of metamorphic rocks which were 

 briefly alluded to in the general description of the range. Leaving out of 

 consideration the carbonaceous shales and slates of Eldorado Canon, where 

 Professor Whitney found the Triassic fossils, the metamorphic rocks belong 



