276 DESCEIPTIVB GEOLOGY. 



■with some clay seams "with a llnckness of about 100 feet, making, in all, a 

 thickness of about 2,000 feet for the Triassic formation. The ridges formed 

 by these heavy-bedded sandstones are here, as elsewhere, prominent topo- 

 graphical features. Above these 150 feet of limy shales and limestone of 

 the Lower Jurassic, succeeded by several hundred feet of variegated clays," 

 whose d(^bris so covers the surface that, where examined by us, no definite 

 series, corresponding to the arenaceous and calcareous shales elsewhere 

 found, could be recognized. 



Next, a rusty, white sandstone, having at its base a conglomerate con- 

 taining pebbles of black and colored jasper, generally quite small and 

 rather angular, the characteristic bottom rock of the Dakota Cretaceous, 

 forms the commencement of that group, which also includes about ICO feet 

 of blue clays, capped by 150 feet of buff sandstones, with included thin argil- 

 laceous beds. From these sandstone ridges, to the bed of the north branch 

 of Vermillion Creek, which runs for a few miles in the strike of the forma- 

 tions, extends a series of low, rounded, clayey ridges of the Colorado Cre- 

 taceous. In this portion, no continuous section could be obtained near the 

 creek, but the general character of a series of bluish clays below and yellow 

 clays and marls above could be distinguished, the whole carrying thin beds 

 of sandstone and arenaceous shales. In the bed of this branch, and even 

 for some distance beyond, the clayey character of the soil continues ; but 

 under the benches there soon appear outcrops of the white sandstones of 

 the Fox Hill group, which was supposed, from the position of the sandstone 

 outcrops, found along the base of the Tertiary cliffs on the east of Vermil- 

 lion Creek, to occupy approximately the position given on the map. 



Of beds supposed to represent the Laramie group, there were found in 

 the bottom of Vermillion Creek, northeast of the branch above mentioned, and 

 at the base of the Tertiary benches included between its two northeastern 

 parallel forks, beds dipping 25° northeast, of impure sandstones, carrying 

 seams of coal, one of which was found to be 25 feet thick. Occasional ex- 

 posures of these sandstones are found in the bottom of these branches of 

 Vermillion Creek, where the stream has removed the thick deposit of allu- 

 vial mud. The outlines of the two upper formations, as elsewhere, could 

 not be very definitely determined, and it is probable that more outcrops 



