190 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



CONTACT OF EARLIER AND LATER ANDESITES. 



Since it therefore seems necessary to distinguish between the andesite near 

 the surface and that on the 200-foot level, the question as to the line of contact 

 comes up. According to the conclusions arrived at this must exist, although it 

 is very difficult to distinguish it. From a study of the rock in the shaft and 

 from specimens taken there, the approximate boundary line has been placed at a 

 point 125 feet from the surface, where a change of formation was recognized by 

 the miners in sinking. This also would correspond fairly well with the conclusions 

 in respect to the West End, where the contact was placed between ll(i and 196 feet 

 from the surface, and with that in the Tonopah Extension, where it has been 

 placed at 184 feet from the surface. 



TONOPAH RHYOLITE-UACITE AND INCLUDED VEINS. 



At a depth of 285 feet a light-colored altered rock (Tonopah rhyolite-dacite) 

 was struck beneath the green andesite. At the contact, which strikes east and 

 west and dips north at an angle of 45, was a heavy zone of ground-up material. 

 The rock immediately beneath this breccia contained a barren quartz ledge, about 

 16 feet thick, striking and clipping nearly parallel with the contact, while beneath 

 this were numerous quartz stringers. This rhyolite-dacite proves on examination 

 to be entirely altered, chiefly to quartz and sericite. with probable kaolin. Original 

 phenocrysts consisted of small and rather sparse crystals of feldspar and biotite. 

 and in one case a small crystal of quartz. This rock is the same as that which was 

 found in the lower part of the neighboring West End shaft. 



Besides the level at a depth of 200 feet, already described, there are levels at 

 depths of 855 and 500 feet. At the 355-foot level a drift runs a short distance 

 northwest of the shaft and encounters a heavy but irregular quartz vein, having a 

 general east- north east strike, and a moderate northwest dip. This vein, as shown 

 in the section (tig. 71), lies very nearly parallel with the upper contact of the 

 rhyolite-dacite and the earlier andesite. a short distance above. It consists of 

 white quartz, and also of gray and black jiispery quartz. It is in general barren, 

 but in places small assays have been obtained. It is cut by several faults, of 

 which the chief ones strike northeast and dip steeply southeast. The effect of 

 these seems to be in general to cause a movement as if the vein had been thrown 

 down on the southeast side. One of these faults, marked by a heavy drag of 

 quartz and rock breccia, has been followed by a drift for a few hundred feet to 

 the southwest. Near the end of the drift the fault splits and both forks have 

 been followed a short distance. In one of these branch drifts a small bunch of 

 ore, carrying very good values, is reported to have been found. Specimens of 

 this ore, shown to the writer, were composed of white quartz containing argen- 



