OHIO TONOPAH SHAFT. 203 



more closely the character of the Fraction dacite breccia. However, as has been 

 said in discussing these formations in general, there is much admixture, and on 

 account of the intimate relation of the two lavas the tuffs often can not be, 

 properly distinguished and separated. 



The shaft is at present about 770 feet deep, and has a working level at 

 756 feet. Passing downward from the surface, the shaft passed through a con- 

 siderable thickness of the rhyolite-dacitic tuffs above referred to. These tuffs 

 continue down to about 485 feet. They are usually rather soft; under the micro- 

 scope they are plainly fragmental and are little assorted, indicating probably 

 showers of detritus from volcanic outbursts. On account of their original glassy 

 character and their subsequent decomposition (chiefly kaolinization) very few 

 definite characteristics can be distinguished. A specimen of one of the harder 

 portions, at 396 feet, however, showed, under the microscope, a glassy ground- 

 mass with phenocrysts of quartz, striated feldspar, orthoclase, and altered biotite. 

 In this slide the chief secondaiy minerals were calcite and muscovite. 



LATER ANUESITE IN SHAFT. 



From about 485 feet to 525 feet there is andesite having the appearance of 

 the later andesite. Well-marked slips near this contact indicate that it is very 

 likely a fault contact. One of these slips had a north-south strike, and a westerly 

 dip of 50. 



SOLID TONOPAH RHYOLITE-DACITE. 



Below the later andesite, from 525 feet to the bottom of the shaft, comes a 

 dense, siliceous rock, which is discussed elsewhere and is undoubtedly referable 

 to the Tonopah rhyolite-dacite. 



At the contact of this rock with the overlying andesite movement is indicated 

 by the presence of 30 to 40 feet of ground-up material, which contains fragments 

 of hard rock and occasionally of quartz. The dip of this contact is northwest, 

 at an angle of about 25 C . 



At the 756-foot level the ground has been extensively explored to the south, 

 north, and east by drifting, the main southeast drift running about 700 feet from 

 the shaft. The formation is almost entirely Tonopah rhyolite-dacite, character- 

 istically showing angular white fragments in a dense gray groundmass. The 

 brecciation indicated by these fragments took place before the cooling of the 

 rock. The only andesite shown on the level is a small patch about 150 feet 

 southeast of the shaft. This is a biotite-andesite, and may be either the earlier 

 or the later andesite. It has a sharp contact with the rhyolite-dacite, which is 

 probably intrusive into it. On the south side of the andesite patch, as exposed 

 in the drift, the contact dips north at an angle of about 55. 



