ENDLicH.] DAKOTA GROUP COLORADO GROUP. 75 



111 every respect are the Dakota sandatoues conformable with the 

 underlying and overlying beds, participating only to a slight extent, 

 however, in the local faulting disturbances of the former. It is highly 

 probable that we have bat the extreme western traces of the extensive 

 disturbances of the Elk Mountain region. In that case we can assume 

 that the force was no longer sufficient to produce any serious disloca- 

 tions so far removed from the central point of acting force. Although the 

 general structural arrangement was no doubt occasioned by the agency 

 of this force, it had already been so far spent before reaching the local- 

 ities in question, that its manifestation there is not a very decided one, 

 regarding local features of structural changes. Along its western edge 

 the Dakota Group is everywhere overlaid by Colorado shales in that 

 region, which latter dip off conformably with them. 



This line of outcrop is the only one where Dakota sandstones were 

 met with in our district, if we except a limited occurrence on the White 

 about five miles below the agency. There a slight upward folding of 

 the strata has taken place, and the river has cut through to a sufficient 

 depth to reach the sandstones. They soon disappear, however, being 

 covered by the Colorado shales. To the extreme regularity of strati- 

 graphical structure farther west this complete covering of older beds is 

 due. There has been no opportunity aftbrded them to appear on the 

 surface within our district either by disturbances or by sufficiently deep 

 erosion where the results of stratigraphical changes farther north have 

 afiected the beds in our district. 



Colorado Groujp. — As indicated above, the shales of this group occupy 

 a normal position with reference to the underlying sandstones. They 

 contain, within their area, the valley in which the agency is located, and 

 skirt the hill of Station 1, reaching from the sides of the latter down to 

 the White, thus forming the grassy, gentle slopes which descend toward 

 that river. True to their usual characteristics they here, too, are found 

 developed in a valley. Separating the higher easterly hills from the 

 Grand Hogback Eidge is a valley Jrom one to three miles in width. It 

 is very uniform, scarcely rising perceptibly even at the divide between 

 the White and Grand, near Station 2, and continuing from there down- 

 ward with Eifle Creek. This entire depression is formed by Colorado 

 shales. It is due mainly to erosion, which must have commenced with 

 the initiative upheaval of the Hogbacks. Along the eastern edge of the 

 valley the shales lorm low, rounded hills, varying from a dark grey to 

 yellowish and white color. Near the exit of the valley into that of the 

 Grand this feature is especially noticeable. Numerous fragments of 

 Inoceravd and Ostrew are scattered oil over the hills. 



On this eastern side the dip of the shales is generally a steep one, con- 

 formable with that of the Dakota sandstones; but as we gradually ap- 

 proach the base of the Hogback Eidge, we find that it diminishes, and 

 the shales dip under the younger beds at an angle of about 0° to 10°. 

 That they, mainly, have given rise to the formation of the valley by 

 more rapidly yielding to erosion, is shown by the fact that along the 

 entire Hogback they reach just about to its eastern base, so that in 

 reality the width of the valley is determined primarily by the relative 

 dip-angle of the Dakota sandstones. Wherever this angle diminishes, 

 the Colorado shale area widens, and thus affords the possibility of a 

 wider valley. It seems highly probable that a number of the low hills 

 composed of these shales were covered by sandstones belonging to the 

 succeeding group, but the cappings have been eroded away, and now 

 only their influeuce in shaping the horizontal outlines of these bluffs 

 remains as proof of a former existence there. One small bluff" was found 



