wekd.] ORE DEPOSITS AT BUTTE, MONT. 173 
the mining of silver ores has been of relatively slight importance and 
has been carried on chiefly by leasers working in the old properties. 
The importance of Bntte as a producer of silver and gold at the pres- 
ent time is due to the fact that the copper produced contains 0.0375 
ounce of silver and $0.0025 in gold for each pound of copper produced, 
or approximately 2£ cents in the precious metals for each pound 
of copper. On this basis the Butte copper mines yielded in 1801 
8,550,000 ounces of silver, which, at 55 cents per ounce, amounted to 
$4,702,500, together with $570,000 in gold, or a total of $5,272,500 in 
precious metals. Thus we see that in the production of precious 
metals the Butte district ranks among the great producers of the 
world. The total of 2,841,791 ,572 pounds of copper has been produced 
from a tonnage which may be safely estimated as at least 100 pounds 
per ton of ore, and on this basis over 28,000,000 tons of copper ore 
have been mined in the Butte district down to the close of 1901. 
ROCKS OF THE DISTRICT. 
The* rocks of the ore-bearing area are all igneous, the district form- 
ing part of an extensive region of Tertiary igneous activity. The 
prevailing rock, and the one in which all the veins occur, is a dark 
basic-gi'anite, technically known as quartz-monzonite, which is a part 
of a great mass of granitic rock extending from the snow-capped 
Highland Peaks*, seen 20 miles south of Butte, northward to Helena. 
This great mass of intrusive igneous granite is surrounded by altered 
limestone and other sedimentary rocks, and is in part covered by 
dark-colored andesite (both massive and fragmeutal varieties) of ear- 
lier age. Neither sedimentaries nor andesite occur in the district. 
Throughout the Butte mining district the granite is remarkably uni- 
form in color, texture, and composition, and the name Butte granite 
has been applied to it. This rock -is cut by dikes and irregular intru- 
sions of the Bluebird granite, a white aplite a composed of quartz and 
feldspar, with a little mica. This rock, though intrusive in the gran- 
ite, is supposed to have separated from the same magmas as the Butte 
granite and to have penetrated fissures in the latter while it was still 
hot, as the aplite is found in all sorts of small veins and masses which 
do not show any chilling along the contact. The rock is found fre- 
quently, but in relatively small masses. In the copper-bearing area 
the_Modoc porphyry appears in lenticular dikes, traversing both vari- 
eties of granite in very irregular fissures. It is a light-colored rock, 
carrying large and distinct crystals of feldspar and quartz in a dense 
ground mass, and is technically designated rhyolite-porphyry or quartz- 
porphyry. After the intrusion of the Modoc porphyry extensive frac- 
turing occurred, with vein formation, the veins cutting the porphyry 
in many instances. After the formation of these earlier veins, renewed 
"Called "granulite 1 ' by . some writers— a name applied by German geologists to a variety of 
schist, but by French petrographers to aplite. 
