THE ROCKS OF THE WASHOE DISTRICT. 53 



Diabasitic character. — As tlus Tock is wliollj different from the diabase of 

 the east country, and is evidently younger tlian eitlier wall of the Lode, the 

 question naturally arose whether it niig-ht not be a peculiar form of augite- 

 andesite. This supposition, however, proves untenable on closer examina- 

 tion. The tendency of augite-andesite is to glassy forms, and this tendency 

 could scarcely fail to be developed to more than a usual degree, had it 

 been injected into so narrow a fissure as that which the black dike must 

 have filled; and any hypothesis which might be invented to account for its 

 having crystallized nuich more uniformly and thoroughly than usual would 

 seem ver}^ forced. 



The black dike, moreover, thoroughly resembles diabases from other 

 localities, and indeed rei)resents a t}'pe of diabase which is nuich more widely 

 distributed than the variety which forms the east wall of the Comstock. The 

 rock from Orange Mountain, New Jersey, for example, possesses the same 

 color, turns brown in the same w-ay, has the same luicroscopical characteristics, 

 and, in short, is indistinguishable from it except by the label. The analysis 

 of Idack dike is conclusive evidence of its diabasitic character. 



Little can be said of the weathering of this rock beyond the fact that 

 it passes into a black clay; almost the only form in which it was observed 

 in the upper levels. To some extent it has been confounded in the Gold 

 Hill mines with underlying black slates, with which, however, it has exceed- 

 ingly little in common except the color. 



Had black dike occurred in a fresh condition on the upper levels 

 former observers would assuredly have recognized its true character, and 

 the east wall would never have been supposed to be of Tertiary origin. 



EARLIER HORNBLENDE-ANDESITE. 



General character. — Tlic tlioroughly frcsli liomblende-andesites are macro- 

 scopically dark-bluish rocks, showing porphyritical crystals of hornblende. 

 The feldspars are scarcely perceptible, except as they express themselves in 

 the crystalline fracture, on account of their transparency. Where the honi- 

 blendes are small, the appearance is consequently somewhat basaltic. 



No base has been recognized in the earlier hornblende-andesite of the 



