50 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



and contains large veins, filled chiefly with calcite, which do not extend into the 

 Tonopah rhyolite-dacite. That this brecciated and veined white rhyolite is 

 intrusive is shown by the fact that it includes large blocks of the later andesite, 

 where it comes in contact with that rock on the southwest side of the mountain. 

 The brecciation of the white rhyolite near its contact with the Tonopah rhyolite- 

 dacite is of a nature between a flow breccia and a friction breccia. It indicates 

 clearly that movement continued in this uprising column of lava after hardening 

 and stiffening had begun, so that the cooled portions were broken and dragged 

 onward in a jumbled mass by the still viscous upward-flowing lava. The upward 

 strain, continued after further hardening, resulted in marked sheeting, and even 

 in gaping fissures, which were filled with calcite and other minerals by the 

 waters which circulated through them after the eruption. 



Smaller necks. The small areas of this rhyolite near the base of Brougher 

 Mountain are also probably necks. They are circular or roughly elliptical and 

 of relatively small size. One just northeast of Brougher Mountain is about 400 

 feet by 150 feet in dimensions, and a shaft has been sunk 200 feet in it without 

 encountering any change in the character of rock. 



Relative age of Oddie rhyolite. The later rhyolite is intrusive into the later 

 andesite at many points into the Fraction dacite breccia near Brougher Mountain, 

 into the Tonopah glassy rhyolite-dacite breccia at Ararat Mountain and Brougher 

 Mountain, and into the Siebert tuffs on the east side of Rushton Hill. 



The faults near Mount Oddie and Rushton Hill, which sometimes show great 

 displacement, seem to cease on reaching the rhyolite, like the faults that reach 

 the dacite necks. At the West End shaft a column of this rhyolite has appar- 

 ently ascended the fault plane which runs through the shaft. Therefore the 

 rhyolite is younger than all the other formations excepting the Brougher dacite, 

 and is also younger than the faulting. It is of apparently about the same age 

 as the Brougher dacite, and, as has been explained, is of the same nature and 

 origin. It is probable that the rhyolite and the siliceous Brougher dacite vol- 

 canoes were contemporaneous, and that adjacent vents gave outlet to slightly 

 differing lavas. The petrologic relationship of the rhyolites to the dacites will 

 presently be pointed out. 



Mineral coiiipositi{>n.~ Examined microscopically, the rhyolite shows scattered 

 jx>rphyritic crystals in a generally fine-grained microgranular groundmass con- 

 sisting mainly of quartz and feldspar. The porphyritic crystals consist of quartz, 

 orthoclase, and occasional plagioclase, one determination of which shows andesine. 

 Biotite is a sparse accessory. Original magnetite and sphene have been noted. 

 On decomposition the rocks yield as secondary minerals quartz and sericitc. 

 sometimes kaolin. 



