174 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
and very violent volcanic activity began, resulting in the intrusion 
and eruption of rhyolite, forming dikes cutting across the veins, and 
also great sheets and masses of fragmental material. 
The Big Butte is formed of rhyolite, both fragmental and massive, 
and this rock occurs in dikes cutting both the granite and veins in the 
silver area, while the fragmental form covers a large extent of coun- 
try west of the mines. These rocks are the product of volcanic action, 
and the Butte is the eroded remnant of a small volcano. 
The granites are of Tertiary age, for at the borders of the batholith 
late Cretaceous strata are cut by the intrusion, and, moreover, included 
fragments of the early Tertiary andesites occur in the granite. The 
rock is cut by rhyolite dikes, and as rhyolite ash-showers form lake 
beds containing Miocene vertebrate remains, the granite and the 
veins are of earlier age, probably Eocene or early Tertiary. West of 
the district the lake beds appear, formed in a great Tertiary lake that 
filled a long and relatively narrow valley extending from south of Dillon 
in southern Montana to (4arrison, a valley which was warped by later 
earth movements that drained it and carried the continental divide 
across its floor. 
STRUCTURAL FEATURES. 
The Butte Mat, a level valley bottom south of the city, contains no 
lake beds; it was formerly a normal erosion valley formed by the con- 
vergence of streams from east, west, and south of Butte, and was 
subsequently depressed by faulting along the base of the mountains 
east of Butte, which reversed its principal tributary and resulted in 
the filling of the valley by torrential debris and wash from the 
adjacent slopes. This faulting altered the ground-water level of the 
ore-bearing area and played an important part in concentrating the 
ores. The district is thus shown to be one of deep-seated igneous 
rocks, subjected to fracturing at various periods, the resulting frac- 
tures being in part filled by dikes, in part by veins, and in part 
displacing the veins; it is a region of continued and continuing 
crustal adjustment. 
The veins occur in an area showing few outcrops, the rocks being 
altered by decomposition and disintegration and forming smooth 
slopes; only rarely do the granite bowlders characteristic of the 
western part of the district show in the copper area. A few of the 
copper veins outcrop, but most of them, even the largest., are recog- 
nizable at the surface only by inconspicuous debris or do not show 
at all, a fact which has led to many lawsuits to determine ownership 
of ore bodies. 
The district embraces a well-defined area of copper lodes surrounded 
by silver veins with transition ores at the borders. Though the veins 
of these two areas present a strong contrast in mineralization and 
charaeter, the vein systems appear to be similar, so that the area may 
be described as a whole. 
