kmm()NS] BUTTE MINING DISTRICT. 369 



and mica in a subordinate ground mass. This is evidently a later erup- 

 tion and sends out tongues or dike like masses into the surrounding 

 granite which are not always visible on the surface but are disclosed 

 hi the deep drifts of the mines. Bhyolite of the typical banded variety 

 also forms a series of low ridges about 5 miles (8 km.) west of Butte. 



The ore deposits of the district oeeur in a series of veins in the gran- 

 ites, Standing nearly vertieal and having a general east and west trend. 

 flone so far as known have been found in the rhyolite. These veins 

 arc rock fractures produced by dynamic force and show evidence of a 

 faulting movement in striated surfaces and zones of broken or crushed 

 country rock, but have comparatively little selvage matter or clay 

 walls. Bach great mine consists as a rule of several parallel fractures 

 or fissures, with minor cross-courses or fractures nearly at right angles 

 with the main fissures ; these sometimes appear to fault them, but it is 

 by no means certain that they are later fractures. The ore is largely a 

 replacement of the country rock from the original fissures outwards, it 

 being often possible to trace a gradual transition from solid ore, through 

 partially altered, into entirely unaltered granite, with no defined plane 

 of demarcation or wall. In the silver mines the ore body or vein mat- 

 ter is often six to eight feet wide. In the copper mines much greater 

 thicknesses of solid copper glance (chalcocite), sometimes 20 to 30 feet 

 ((> to m.) wide, are found. 



The most common minerals in the silver mines are pyrito, sphalerite 

 or zincblende, galena, and sulphides of silver, with manganese in the 

 form of rhodocrosite or rhodonite, and little or no copper. Gold forms 

 an important value in some ores, but it is rarely visible. 



In the copper mines chalcocite, chalcopyrite, bornite, and enargite 

 are the prevailing valuable minerals, but zincblende and the manganese 

 minerals are almost entirely absent. Quartz is the general gangue 

 material, and has apparently been derived by lateral secretion from the 

 adjoining granites. Iiarite and fluorspar are also found. The oxida 

 tion of the manganese minerals, which form a remarkably regular con- 

 stituent in the silver mines, produces very prominent outcrops or 

 gossans, stained by the black oxide of manganese. 



The larger mines have been opened to a depth of over 1,000 feet 

 (300 m.) and are still productive, though the expense of mining and of 

 treatment has in some cases led to a suspension of exploitation. The 

 increasing proportion of zincblende is very disadvantageous in the 

 amalgamation processes by which these ores are reduced. 



Distinct methods of treatment are pursued for the two different 

 classes of ore. The silver ores are pulverized in dry stamp-mills, sub- 

 jected to desulphurizing- roasting, chloridized by mixture with salt, 

 and amalgamated in revolving pans, after the Nevada system. For ores 

 that are extremely refractory, owing to a large proportion of ziuc- 

 461 ge 24 



