188 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



distinguished if the rock was much decomposed or faulted. In the Tonopah 

 Extension this contact is at a depth of about 18-t feet from the surface and is 

 nearly flat. 



Similarly in the Midway mine, which is very likely in the same block, the 

 contact between the overlying later andesite and the underlying earlier andesite 

 could not be definitely located, probably on account of the great decomposition 

 of the rocks at this place. 



The earlier andesite in the Tonopah Extension, moreover, partakes very 

 largely of the characteristics of the Fraction andesite, and in many cases resem- 

 bles somewhat the later andesite, but is elsewhere quite typical, and contains 

 strong veins, which show in places high values and evidently belong to the earlier 

 andesite series of veins, so that there can be no doubt as to its identity. 



TONOPAH RHYOLITE-DACITE. 



Andesite similar to that on the 200-foot level continues down in the shaft to 

 390 feet, at which point a slight breccia is encountered, striking N. 70 C E. and 

 dipping northwest at an angle of 45. Below this a quartz vein is encountered, 

 with highly silicified Tonopah rlvyolite-dacite as its walls. 



On the 500-foot level drifts run north and south about 300 feet in all. There are 

 also crosscuts. The whole is entirely in rhyolite-dacite. The rock is intensely 

 silicitied. being in places nearly solid quartz, and contains pyrite throughout, but 

 there are no definite veins. This quartz is barren, although assays of $51 or $2 have 

 been obtained in places. The rock is characteristically intensely fractured, and in 

 places contains open fissures running in a direction somewhat east of north. These 

 fissures when cut contain the heavy gas elsewhere referred to as being probably 

 carbonic acid (see p. !)4). The probable explanation is that the gas was formed in 

 the overlying soft andesite by the reaction of acids upon the contained calcite, 

 and by its weight sank into the fissures in the underlying rigid rhyolite-dacite 

 and there accumulated. 



EARLIER ANDESITE AT BOTTOM OF SHAFT. 



At a depth of about 680 feet in the shaft there is a sharp contact between the 

 rhyolite-dacite above and a fine-grained green variety of earlier andesite below. 

 This contact is said to dip east at an angle of about 40. The bottom of the shaft 

 is at a depth of 780 feet, and specimens taken from here and from below the contact 

 show earlier andesite of a type very much like that on the 700-foot level of the 

 Siobert shaft. 



