EXDUcn.] EAST OF WIND EIVER RANGE COLORADO GROUP. 85 



fumisliecl will indicate the stratigrapliieal conditions ot the Dakota 

 Groui) {m). The easterly dip toward Sheep Mountain amounts to 12° 

 to 150, gradually diminishing. Before long we reach a point where 

 the dip is zero, and after that find it reversed. Near the mountain it 

 increases to about 40° westward. On the eastern side it is again 

 gentle, approaeliing the horizontal as we enter the lower country. In 

 this region a slight increase of the total thickness of the groui) may be 

 noticed. 



Colorado Group. 



Directly overlying the upper arenaceous shales of the Dakota Group 

 we find an extensive series of dark gray, shghtly calcareous shales. 

 They are thinly laminated, easily eroded, and become light gray or 

 white upon exposure. Covering the highest portions of the region lying 

 between Sheep Mountain and the base of the third chain, they present 

 comparatively steep bluffs parallel to their strike, and rounded sui'faces 

 along tlieu" dip. A few banks of argillaceous limestone may be found 

 within them. The entire group is a very characteristic one, readily rec- 

 ognized. Its thickness in our district amounts to about GOO feet. With- 

 in the upper third we observe that the shales become somewhat more 

 arenaceous than lower down. Evidently this is a deposit which accu- 

 mulated in very deep water. Unfortunately no fossils were found except 

 a few iragments of Ostrea, resembling congesta and hwceramus. The 

 position of these shales {n) may be seen from Sections I and II. As the 

 youngest member shown in the first section they appear at an elevated 

 point. From there southward they soon end. Northward they follow 

 the Dakota Group, taking part in the various disturbances of the region. 



As these grow less prominent and sliift toward the northeast, the 

 area covered by Colorado shales expands somewhat. Near Camp Brown, 

 about 6 miles distant, we find a cold sulphur spring, which seems to take 

 its rise in these shales. In this connection may be mentioned the fact 

 that the Colorado shales must be regarded as a very prolific source for 

 alkaline compounds of a highly soluble nature. Within the shales Ave 

 frequently notice smaU inclusions of pyrite. Upon decomposition of both 

 this and the shales various salts are formed. When the two are in close 

 proximity we obtaiu sulphates, otherwise mainly carbonates and some 

 chlorides. 



East of the anticlinal elevation the Colorado shales gradually diminish 

 their dip and are finally covered by the succeeding Cretaceous group. 

 They form barren bluffs, steep, usually, on the side where the edges of the 

 strata are exposed; less so along the dip. Bunch-grass, pifious, cedars, 

 and sage-brush comprise the main portion of the vegetation on such 

 bluffs in the regions of low elevation. Fluviatile erosion attacks the 

 shales very rapidly and often carves them into very beautiful miniature 

 ranges, ridges, and vaUeys. 



Fox Sills Group. 



In Section I the Fox Hills Group no longer appears. Some distance 

 north of the Little Popo-Agie it sets in, however. Eesting upon the gray 

 shales of the preceding group we find brown and yellow shiiles inter- 

 stratified with thin beds of sandstone. Some of the shales are very dark 

 and carbonaceous. In the lower portions of the Fox Hills I did not find 

 any properly developed coal-seams. Above this alternating series we 

 met with a considerable tliickness of yellow and brown shales. As a rule 

 they are arenaceous, but some of them seem quite free tiom sand. Small 

 particles of mica occur throughout. Higher up, sandstones set in again, 

 containing, together with, thm seams of shales, small deposits of coal. 

 None that I have seen were workable, but I am informed that they are 



