38 GEOLOGY OF THE COMSTOCK LODE. 



to make certain of any ilmenite, while spliene, in small and irregular masses, 

 is frequent. Apatite is not specially plentiful, and is of the ordinary color- 

 less variety. Many small but beautiful zircons are visible with the higher 

 objectives, mostly in eight-sided prisms terminated by the fundamental 

 pyramid. 



The granitoid diorites resist decomposition better than any other rocks 

 in the district. On the surface erosion evidently proceeds with greater 

 rapidity than decomposition. Slides from beneath the surface, but near the 

 Lode, show that the hornblende is replaced by chlorite and epidote, and the 

 feldspars by calcite and quartz. 



Dark varieties. — Tlic dark fiue-graincd diorite presents a much stronger con- 

 trast to the ordinary gray variety macosci'opically than microscopically. The 

 difference in its appearance seems to depend simply on the fineness of the 

 grain, and on the percentage of fibrous hornblende, which is greater in this 

 modification. 



The dark coarse-grained diorite from the lower levels is very peculiar 

 in appeai-ance, and some of that from the Union shaft might be more readily 

 confounded with specimens of Scotch foundry-pig than with any other rock 

 occurring in the District. This variety also differs from the ordinary gray 

 diorite, principally in respect to the hornblende, which is more abundant. 

 It is not fibrous as a rule, and has consolidated in grains simultaneously with 

 the feldspar. As in the freshest gray diorites, the granules which show 

 no evidences of alteration are brown, and incipient alteration seems to be 

 accompanied by a change to a green fibrous mass. The hornblendes also 

 contain numerous black inclusions, probably ilmenite. The feldspars are 

 very fresh and clear, and the black color of the rock is the natural conse- 

 quence of such a mineral composition. Although the difference in appear- 

 ance between the three varieties of diorite is very marked, it thus depends 

 on a variation of unes.sontial characteristics. 



Structure of the granular diorites. — Tlic grauular diorito Is exceediuglv hard and 

 tough, so much so that before the introduction of nitro-glycerine exjjlosives 

 it was almost impossible to penetrate it where decomposition had not loosened 

 the texture. In the Chollar mine, many years since, when black powder 

 only was in use, an attemjjt to drive a gallery into this rock was abandoned 



