MONTANA TONOPAH VEIN SYSTEM. 179 



MIDWAY WORKINGS. 



The Midway lies a short distance northwest of thte Siebert shaft, and 

 almost in line with and halfway between the Montana Tonopah and the Tonopah 

 Extension. 



LATER ANDESITE IV SHAFT. 



The surface at this point is composed of tlie typical later andesite. A 

 specimen taken a short distance from the Midway shaft has the characteristic 

 relatively fresh appearance, dark color, and large feldspars of this rock. Under 

 the microscope it is also typical, showing numerous phenocrysts crowded 

 together, these phenocrysts being mainly feldspars, often large and compound, 

 with pseudomorphs of serpentine after pyroxene. 



The contact of this rock with the underlying earlier andesite is an obscure 

 one. This is a condition similar to that noted in other workings, such as the 

 West End and MacNamara, where, as described, the contact between the two 

 andesites could not be located in the shafts. 



TYPICAL EARLIER ANDE8ITE IN SHAFT. 



In the case of the Midway, as shown in the section (fig. 68), the contact 

 has been perhaps rather arbitrarily drawn at a depth of about 425 feet. From 

 this point to a point just below 475 feet in the shaft the formation is regarded as 

 probably all typical earlier andesite. 



GLASSY TONOPAH RHYOLITE-DACITE IN SHAFT. 



At a point in the shaft below 475 feet there is a change in the formation, 

 and the rock is quite uniform and of the same hard, siliceous nature and light- 

 green color as that at the main level of the Ohio Tonopah. 



This rock contains jaspery quartz veinlets and fine quartz lines some of the 

 cavities left by the removal of pyrite and other crystals. 



FORMATIONS EXPOSED BY DRIFTING. 



The workings of the Midway consist of two levels at depths of 535 and 685 

 feet, the former having a north drift over 400 feet long and a south drift about 

 150 feet long, while the latter has a north drift nearly 700 feet long and a south 

 drift of about 150 feet. The formation in the upper level is entirely Tonopah 

 rhyolite-dacite, except at the end of the north drift, which passes through the 

 same contact as that encountered in the shaft and enters the earlier andesite. 

 The shaft passes downward through the body of rhyolite-dacite and enters earlier 

 andesite beneath it, of a type like that found on the 700-foot level of the Siebert 

 shaft. Similar andesite is encountered on the south drift of the (>35-foot level, 

 while the whole of the north drift on this level lies in the rhvolite-dacite. 



