56 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 

 WESTERX FLANKS OF THE GALLATIN RANGE. 



In the northwestern portion of the Gallatin Range, within the Yellow- 

 stone Park, the western slopes of the mountains are abruptly terminated 

 by a fault, bringing up the sedimentary beds against subaerial volcanic 

 breccias that probably represent a remnant of the old Electric Peak and 

 Sepulchre Mountain volcanic cone. The fault, where it crosses Fan Creek 

 and along its course down Cinnabar Creek, shows a profound displacement, 

 Its southern continuation has already been noted west of Gray Peak, but 

 it becomes of slight importance in the vicinity of Grayling Creek, west of 

 the end of Crowfoot Ridge. The andesitic breccias which form the high 

 mountain ridges west of this fault are continuous with the high range of 

 the Gallatin which stretches northward along the western side of the 

 Yellowstone River to the vicinity of Bozeman. Within the park region 

 the underlying upturned and irregularly eroded sedimentary beds are not 

 exposed. The position and dip of the strata forming the high ridge on the 

 eastern side of the Gallatin River, just within the northwestern corner of 

 the park, show a monoclinal structure which would bring the Montana 

 shales and sandstones beneath the andesite breccias and against the Fan 

 Creek fault. It is clear that the horizon west of the fault was consid- 

 erably higher than that to the east, for the latter is near the base of the 

 Colorado shales and contains laccolithic sheets, which must have been 

 intruded at considerable depths beneath the surface of the country, and 

 were contemporaneous with those intruded in the upper part of the Colo- 

 rado formation of Electric Peak, which is but 3 miles distant. Another 

 fact that is apparent is that the sedimentary strata were upturned and 

 eroded down to the Carboniferous sandstone, 7 miles to the west, before 

 the volcanic breccias were thrown out over the country. 



The only other andesitic tuff-breccia in this vicinity occurs in isolated 

 patches resting directly upon crystalline schists in the neighborhood of 

 The Crags, 5 and 10 miles south of the breccia west of Gray Peak, and 

 from 13 to 17 miles distant from the Electric Peak center of the eruption. 



From these facts it would appear that the surface of the country at 

 the time when the andesitic tuff-breccias were deposited consisted of 

 crystalline schists in the south, of Carboniferous strata in the west, and 

 of Cretaceous strata in the northeast. This indicates the uplifting of the 



