40 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



found fragments, some of which are several feet in diameter, of the earlier andesite, 

 of the later andesite, and of andesite and dacite tutf, which were probably hurled 

 out of the volcanoes in blocks during these eruptions. 



We may therefore reason that the period of the formation of the Fraction dacite 

 breccia was one of considerable volcanic activity, though not necessarily prolonged. 



The volcanoes exploded repeatedly, producing 

 showers of pumice and ash and rapid subaerial 

 accumulations of these materials on and near the 

 slopes, while the flows were scanty and so mixed 

 with water as to be often nearly or quite mud 

 flows. . The upper part of the formation, as seen 

 in the New York Tonopah shaft, in the north- 

 west side of Siebert Mountain, and elsewhere, is 

 more fragmental than the lower portion. Some 

 solid lava flows appear interstratified with these 

 upper fragmental deposits, but they belong 

 rather to the Tonopah glassy rbyolite-dacite than 

 to the Fraction dacite breccia. 



Relative age. A number of data are of value 

 in the determination of the relative age of this 

 formation. It is clearly younger than the later 

 andesite, for it sometimes rests upon this forma- 

 tion and typically contains abundant inclusions 

 of it. On the other hand, it is frequently cut 

 by dikes of the Tonopah rhyolite-dacite (tig. 2). 

 As already stated, it overlies the Heller dacite 

 in the Tonopah City shaft, and is most likely 

 younger than it. Therefore it is probably imme- 

 diately between the Heller dacite and the Tono- 

 pah rhyolite-dacite. 



Microscopic characters. Microscopically the 

 rock of the Fraction dacite breccia is a biotite- 

 dacite, substantially of the same composition as 

 the Heller dacite and the Brougher dacite. The 

 groundmass is brown glass, often felty, and fre- 

 quently very vesicular. As porphyritic crystals (usually broken) it contains quartz, 

 relatively sparse biotite, and feldspar, both striated and unstriated. The striated 

 crystals are relatively considerably more abundant than in the Tonopah rhyolite- 

 dacite. One determination showed andesine-oligoclase. 



4 feet 



FIG. 1. Vertical section of shaft about 1,600 feet 

 east of Tonopah and California shaft, showing 

 Fraction dacite breccia and interbreccia tuffs. 

 (1) Finely stratified tuff; (2) sandstone com- 

 posed of angular and rounded fragments of da- 

 clteglass; (3) stratified rock, largely made up 

 of pumice fragments; (4) soft dacite, broken 

 and containing pumice fragments, probably 

 a mud flow; (n) like 4, but containing little 

 pumice. 



