NOETHERN BEAR RIVER REGION. 337 



extends along the line of its strike a little east of north, for about 4 miles 

 north of the creek, and 3 miles to the south. To the east, north, and south, 

 it is bounded by the overlying Tertiary sandstones, which extend to Bear 

 River. Upon the same strike, about 8 miles to the north of the point of its 

 northern disappearance under the field of the Vermillion Creek Tertiary, 

 outcrops a local development, known as the "Needles", of similar, though 

 coarser, conglomerate, having a vertical position and a strike of a little west 

 of north. This is, no doubt, a continuation of the same bed, and, from the 

 position of the former, conformably under the black shales and yellow sand- 

 stones, they may be considered to be the bottom of the Cretaceous, or the 

 Dakota group. This, therefore, represents the bottom of the western half 

 of the Cretaceous fold, or a point very near the anticlinal axis; the upper 

 part of the eastern member being represented perhaps by the Evanston beds. 



Northern Bear River Region. — Medicine Butte is, as has already 

 been mentioned, formed of upturned Cretaceous sandstones, which are 

 exposed on the western slopes and in the ravines to the north and south. 

 On the east, coarse red conglomerates of the Vermillion Creek series rest 

 unconformably on these sandstones, coming up to within a few hundred feet 

 of the summit of the butte. To the north, the summit of Aspen Plateau is 

 formed of the Tertiaries lying- almost horizontal, or inclined 1° or 2° to 

 the eastward. They consist of the same dull-red striped sandstones, with 

 some interstratified beds of marl and argillaceous limestones. The under- 

 lying Cretaceous beds are exposed in some of the deeper-cut valleys, along 

 the valley of the Little Muddy, as already mentioned, in Map II. In the 

 valley of Bear River, about 7 miles north of Evanston, on the eastern side, 

 the heavy compact sandstones of the Laramie group are exposed, dipping 

 eastward into the foot-hills of the Aspen Plateau, and at one locality is a 

 narrow seam of coal, accompanied by bituminous shales and clays. 



About midway between Evanston and the Narrows of Bear River 

 occurs an outburst of rhyolitic tufa, rising above the foot-hills of Aspen 

 Plateau, which is of interest, since it is far separated from any other known 

 occurrence of this rock. In its physical habit, it is a uniform fine-grained 

 rock, of an arenaceous, pumice-like texture, crumbling readily, and having 

 a prevailing lavender tint. The groundmass is an intimate admixture of 

 22 D G 



