48 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



ments of the glassy contact phase of the dacite, and are intrusive into the Siebert 

 tuffs. Sometimes these dikes are composed mainly of glassy dacite fragments, 

 sometimes of clear sand. They often follow the exact intrusive contact and are 

 never far from it (tig. 7). Detailed study shows that these breccias are truly dikes 

 that have been injected in a plastic condition (fig. 8). This injection followed the 

 intrusion, and the intrusive material was probably a mixture of ascending hot 

 waters, consequent upon the eruption, with tuff and dacite fragments. 



Mineral composition. Microscopical \\ the Brougher dacite shows a brown, 

 glassy groundmass, which is sometimes finely crystalline and contains frequently 



Scale 



so feet 



FIG. 7. Vertical sketch section of dacite contact at a point on the east side of Butler Mountain; a, gray dacite; b, glassy 

 dacite, autociastic; c, dike of friable, partly consolidated detritul sand and angular fragments composed of material 

 derived from the tufl and from dacite glass; d, finely stratified Siebert tuffs (lake beds); e, coarser layer of tuffs; /, faults. 

 The sketch shows clastic dikes consequent upon marginal fissuring around the dacite, and dowufaulting toward the 

 contact. 



broken porphvritic crystals of quartz, orthoclase, andesine or andesine-oligoclase, 

 biotite, and occasionally hornblende and augite. Magnetite and specular iron occur. 



In the field the Golden Mountain dacite was judged to be more siliceous than 

 that of the other mountains, and this observation has been borne out by microscopic 

 and chemical analysis. It shows, indeed, a close relation to the Oddie rhyolite. 

 However, the Golden Mountain rock is distinguished as dacitic by the greater 

 abundance of porphvritic crystals, the frequent presence of elongated plagioclase 

 feldspars, the greater amount of biotite, the characteristic brown, glassy ground- 



H, and the occurrence of occasional augite. 



