368 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



since L864, when the town was first founded. The, earliest large min- 

 ing operations were in the silver mines; tlie Alice, Moulton, and Lex- 

 ington mines being tlie most prominent. The latter was purchased in 

 1SS1 by the "Societe Anonyme des mines de Lexington," of Paris, 

 France. During the last decade the copper mines have gradually 

 increased in importance, and within the past lew years the value of the 

 copper produced has far exceeded that of silver. The production for 



the year 1890 was: 



Kilograms. Value. 



Gold, '-'.*). TO 1 ounces 



Silver, 7,500,000 ounces i_*:ct. 264 B, 006, 750 



Copper, 112,700,000 pounds 51,117,204 16, ()■_':!, 2M 



800 $518,816 



The city is situated on the southern slope of rounded granite hills, 

 included within the angle of Silverbow creek as it Hows first south and 

 then west. It lies just east of a round conical hill, known as the 

 "Butte/' from which it. takes its name. The greater part of the silver 

 and copper mines lie in the hills back of and north of the city and to 

 the east of the Butte, but a few important silver mines, notably the 

 Bluebird and the Nettie, are situated a few miles to the westward, 

 beyond the Butte. 



Of reduction works for extracting the metals from their ores, the 

 stamp mills are generally located on the hills near the mines, while tlie 

 several smelting works lie in the valley of Silverbow creek. The 

 Smelting works of the largest copper mine, the Anaconda, are situated 

 27 miles (43 km.) to the northwest, where a town of the same name, of 

 several thousand inhabitants, has been built around them, which is 

 connected with the mines by a railroad. 



The geological formation of the district is almost entirely of granite 

 and rhyolite. There are two varieties of granite. The most wide- 

 spread variety, which forms the country rock of the mines around the 

 city and to the east of "the Butte,*' is an unusually basic rock, carry- 

 ing a relatively large proportion of plagioclase feldspar : its basic min- 

 erals are mica, hornblende, and augite, and much of the hornblende 

 appears to be only a paramorphic alteration of augite. The other 

 granite occurs to the west of "the Butte" ami forms the principal 

 country roek of the Bluebird, Nettie, and other mines. It is a light 

 colored rock and consists almost exclusively of quartz and orthoclase 

 feldspar, with a few minute grains of biotite. At its contact with the 

 Butte granite it is found to send veins into it and include fragments of 

 it, and is hence assumed to be of later age. The Butte itself consists 

 of rhyolite, a line-grained, granular rock consisting of quartz, samdine, 



