180 GEOLOGY OF TONOPAH MINING DI8TBICT, NEVADA. 



VEINS IN THE MIDWAY. 



There are veinlets of calcite in the later andesite and these very often contain 

 pyrite. In the shaft at a depth of about -430 feet there are quartz stringers 

 containing pyrite. At a depth of 435 feet there is a short northwest drift, showing 

 a vein of black, jaspery quartz, which is barren and irregular. 



A fragment of a vein was cut at 475 feet in the shaft. The vein was largely 

 barren but contained a rich bunch or shoot of original sulphide ore. This ore. 



Flo. fi._ Section showing geology exposed by Midway wiirkint-s. 



when examined microscopically, shows the typical structure of the productive 

 earlier andesite veins. The quartz has the usual varied grain, ranging from 

 inicrocrystalline to medium crystalline. There is scattered pyrite seeming to have 

 no relation to the values, which consist of black silver sulphide and silver chloride, 

 both of which are relatively abundant. The relation between these two is remark- 

 able, for the black sulphide forms rims around the chloride and in some cases is 

 found along cracks, showing that it was formed later than the chloride and is 

 very probably an alteration product of it. There is occasionally a little ruby silver, 



