90 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



A narrow spur or ridge connects the peak with the mountains to the south- 

 west, while a broad, high ridge on the north carries the gently dipping 

 Cretaceous strata to the abrupt synclinal fold south of Cinnabar Mountain. 

 On the east and northeast of the summit deep gulches have been carved 

 into the heart of the mass, reaching depths of 4,000 and 5,000 feet below 

 its highest point. The valley of Reese Creek marks the line of faulting- 

 between Electric Peak and Sepulchre Mountain. 



Only the eastern half of Electric Peak is involved in the consideration 

 of the series of volcanic eruptions here discussed, the central stock or 

 conduit of eruption being located in the middle of the deep gulch east of 

 the peak and in the rocky spur north of the gulch, and the apophyses and 

 dikes extending only short distances across the southeastern and north- 

 eastern spurs. The accompanying view of the eastern face of the mountain 

 (PI. XIII) shows the jagged northeastern spur on the right, with its steeply 

 sloping base, and the deep east gulch in the middle, with light-colored 

 morainal accumulations of rock fragments covering the bottom like a glacier. 

 The long southeastern spur is on the left, with its short, steep branch imme- 

 diately south of the gulch. The barren slopes, partly covered with slide- 

 rock, are easily recognized. 



The southeastern, spur is formed of upturned beds east of the synclinal 

 axis. The black shales which constitute the greater part of the spur have 

 been baked and indurated in the vicinity of the stock, so that they 

 have withstood erosion sufficiently to form the pyramidal mass bounding 

 the east gulch on the south. The main mass of Electric Peak and the 

 greater portion of the northeast spur consist of less disturbed strata dipping 

 toward the northeast. The structure of the mountain is shown in the walls 

 of the deep gulches draining east and northeast, as well as in the bare 

 slopes on the south and west sides. The differences in the topography of 

 the two halves of the mountain are due to the influence of the vertically 

 intruded rocks, which have metamorphosed the neighboring sandstones and 

 shales, rendering them hard and resisting, and leading to the production of 

 rugged and pinnacled ridges, with precipitous walls hundreds of feet in 

 height. 



The east gulch forms an amphitheater at the base of the peak, which 

 surmounts a precipice of nearly 1,500 feet. The walls of this amphitheater 

 are shown in the panorama, PI. XIV. The gulch crosses the synclinal axis 

 and the stock of igneous rock, part of which is covered by de'bris. The cen- 



