32 MmmG INDUSTET. 



trachyte family, its mode of geological occurrence enables the observer to 

 detect it without fail on the ground. Its appearance, and especially the ten- 

 dency to break in parallel planes, distinguish it from all other kindred rocks. 

 The prevailing color is a dark greenish-gray, inclining to purple and black. 

 The luster, although varying greatly in degree, is almost uniformly resinous. 

 It is formed chiefly of a vitreous, compact, feldspathic paste, but is not unfre- 

 quently distinctly porphyritic, and finally, by a series of gradual approxima- 

 tions, shades in to the more crystalline propylite. Oligoclase, hornblende, 

 occasional but rare occurrences of magnetic iron, are the chief constituents, 

 but the peculiar texture and parallel fracture are given to it by a prevailing 

 brilliant, glassy feldspar, M^hich being transparent, does not alter the general 

 color. The nature of this feldspar is somewhat obscure, although it is most 

 probably the so-called andesine. 



In certain instances, as is the case in most of the Gold Hill andesite 

 occurrences, the paste becomes exceedingly vitreous, approximating closely to 

 obsidian. Comparing a large number of specimens, the conclusion is almost 

 inevitable that its wide variety of texture is wholly due to the condition 

 of the feldspar. It is proposed to call this peculiar feldspar andesine, 

 without regard to its chemical origin. With some hesitation, the black dike 

 which occurs in various parts of the Comstock lode is referred to this rock. 

 Highly altered, and in some places reduced to a mere clay, there are 

 some evidences, to be hereafter discussed, which induce the belief that it 

 is andesite. If so, it belongs to a fourth zone intermediate between numbers 

 one and two. 



One very marked fact, and an important one in connection with the geology 

 of the Comstock, is, that while all of the propylites east of the foot of Mount 

 Davidson are in a highly altered condition, the andesites which directly over- 

 lie them show not the slightest trace of solfataric action. From the mineral- 

 ogical characteristics of aadesite it would seem impossible that this could have 

 been the case if solfataric action had continued after its outflow. Every fact 

 corroborates the idea that the age of general solfatarism had closed before the 

 later andesitic ejection ; and this is important as limiting the period of form- 

 ation of the Comstock lode to a point anterior to the sanidin-trachytes in 

 whose eruption Richthofen has found a cause of the Comstock solfatara. It is 



