20 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



we may assume that it did not reach the surface of the earth, but was 

 covered by the sedimentary rocks it displaced. The same kind of por- 

 phyry occurs in two small bodies about 3 miles farther north, along the 

 Crowfoot fault line, west of Three River Peak. Here they have broken 

 into Carboniferous limestone, which otherwise in this region is quite free 

 from intrusions of igneous rock. 



Still another small intrusion occurs along the northern border of the 

 Indian Creek laccolith, but is confined to the upper horizon of the Cam- 

 brian rocks. It is a dark basic porphyry of an unusual character, with 

 occasional phenocrysts of hornblende, mica, and feldspar. It forms a small 

 sheet, 50 to 75 feet thick, exposed on the divide south of Bighorn Pass and 

 along' the south base of Bannock Peak. It was not found in contact with 

 the laccolith, and the relative times of their intrusion were not made out. 

 There is no rock similar to it in the region explored, except a small sheet 

 in Three River Peak, and nothing approaching it in composition occurs 

 nearer than Electric Peak. 



ANTLER PEAK. 



The sedimentary rocks OA^erlying the Indian Creek laccolith, as already 

 noted, form the summits of Antler and Three River peaks; stratigraphic 

 sections were made at both these localities. Antler Peak is the prominent 

 summit lying between The Dome and the flat-topped mass of Quadrant 

 Mountain. (See PI. VI.) The greater part of the mountain is formed of 

 the intrusive mass of the Indian Creek laccolith, as just described. The 

 sedimentary rocks are best exposed on the southern side of the mountain 

 and at the eastern slopes, where the laccolithic rock passes beneath the 

 limestones. 



The gneiss is exposed on the low wooded hill at the southeast base of 

 the peak, indicated on the map by the 8,000-foot contour. This hill and the 

 slopes back of it are covered heavily with drift, which usually conceals the 

 gneiss and the overlying stratified rocks; there being no exposures except 

 near the base of the great limestone ledge which forms such a prominent 

 feature of the valley. The strata were examined where the ledge has been 

 cut through by a small stream channel from the summit, the de'bris which 

 elsewhere conceals the foot of the wall having here been washed away. 

 The lowest strata exposed were thinly bedded limestones and rather heavy 



