196 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



and is very similar to some of the rock at Gold Mountain (4 miles south of Tonopah) 

 in the Tonopah Union shaft. It has a pyroclastic structure, with occasional pheno- 

 crysts of quartz and more common crystals of feldspar, chiefly orthoclase, which 

 are largely altered to quartz and muscovite. Small biotite crystals are also altered 

 to white mica by bleaching. The groundmass is a microfelsitic devitrified glass. 

 Some secondary adularia was observed. 



From the lower contact of this rhyolite-dacite body the shaft passes through 

 later andesite again to a depth of about 620 feet, making the thickness of this 

 body of andesite traversed somewhat over 100 feet. At this point again there 

 is a contact between the later andesite above and Tonopah rhyolite-dacite below 

 similar to that just described. A short distance from the contact a vein of quartz 

 2 feet thick, containing pyrite and otherwise having the same characteristics of 

 the upper veins, was encountered in the rhyolite-dacite. This also seems to be 

 very nearly a contact vein. The bottom of the shaft, at a depth of 800 feet, 

 is still in the same rhyolite-dacite. 



From the bottom of the shaft a drift was run due east 525 feet since the 

 visit of the writer. A specimen of the rock sent to the writer from the end of 

 the drift is rhyolite-dacite, like that at the bottom of the shaft, but a specimen 

 taken from an intermediate point in the drift is later andesite. Mr. C. E. Knox, 

 the president of the company which has conducted these explorations, reports that 

 the veins cut in the shaft were cut again in this drift in regular order. It is 

 probable, therefore, that the alternating bands of rock, striking northwest and 

 dipping southeast, were encountered in the drift also, with the exception perhaps 

 of the white rhyolite, which has not been reported as occurring in the drift. It is 

 interesting to note that the end of the drift has been carried somewhat past the 

 surface contact of the rhyolite-dacite with the later andesite perpendicularly above 

 on the slopes of Ararat Mountain. 



CORRELATION OF THE RHYOLITIC ROCKS IN THE SHAFT. 



As before stated, the contact phenomena were not observable in the mine on 

 account of alteration by circulating waters, but from what has been observed at 

 other points in the district it may be believed that here, too, the andesite' is the 

 older of the rocks exposed; that it has been cut by the Tonopah rhyolite-dacite, 

 and that the white rhyolite was the last of all and is also of an intrusive nature. 

 The form of the different igneous bodies underground must be very complex, and 

 it is difficult or impossible to even outline the connections between the similar 

 lavas. It seems likely, however, that the white rhyolite is connected with that 

 of Mount Oddie, and the Tonopah rhyolite-dacite with that around Ararat 

 Mountain. 



