CHAPTER II. 



GENEEAL GEOLOGY. 



In the district examiued during 1876 the variety of geological forma- 

 tions is very limited. It may be well to state that on the east side con- 

 nection is made with the work of the late Mr. Marvine, which he accom- 

 plished during 1874, and on the south with Dr. Peale, 1876; on the west 

 I nearly join with Major Powell, and on the north my northern border 

 was the southern one of Professor White. Fortunately for the peace 

 and happiness of the formations involved, these latter are at all points 

 very characteristic, so that no misapprehensions exist regarding their 

 age. 



On the east we find the oldest group, that of the "Eed Beds.'/ Tbey 

 are sufficiently well developed, and, as usual, marked in so decided a 

 manner that no mistake regarding their identity could occur, in spite of 

 an entire absence of fossils. To a certain extent they have been dis- 

 turbed by local plications and faults, although their general trend is in 

 conformity with that of overliving younger beds. In canons they appear, 

 rarely reaching to the summits of any of the more elevated points, 

 although farther east tbey no doubt can be found higher up. Superin- 

 cumbent upon them are the Jurassic and Cretaceous beds. The latter 

 formation is represented by three groups, the Dakota, Colorado, and 

 Fox Hills groups. Of these the first forms prominent hills south and 

 southwest of the agency, showing a considerable thickness. Its distri- 

 bution is one following an approximate north to south line, parallel to 

 the strike of the strata. Upon this series of sandstones, that retain 

 their well-known characteristics, are deposited the Colorado shales. As 

 usual, these have aftbrded an opportunity for the formation of deep, 

 narrow valleys, bordered on the east by the Dakota beds and on the 

 west by the Fox Hills group. Owing to their peculiarly " tough " nature, 

 the shales do not permit the water to sink down to any considerable 

 depth, but as soon as the superficial strata are sufficiently saturated the 

 water is forced to tiow oft, thus producing, in these valleys, streams that 

 are not dry even during the hottest season of the year. At some points 

 the Colorado shales form the base of the long hogback ridge running 

 from the White to the Grand. Above them Fox Hills beds close the 

 Cretaceous. Their vertical development is but inconsiderable as com- 

 pared to that of Southern Colorado. Throughout the entire formation 

 fossils are rare, save the most common species. This is to be regretted 

 all the more in the upper members of the Fox Hills, as a liberal supply 

 of characteristic fossils would greatly facilitate the accurate definition 

 of the boundary-line between the Cretaceous and the succeeding Post- 

 Cretaceous. 



Of the Tertiary formation two groups are represented, the Wasatch 

 and Green River. They follow each other in the regular succession, and 

 are perfectly conformable with each other, as well as with the under- 

 lying formations. Although palseontological evidence in these groups 

 is sadly wanting, it has still been possible, from correlation partly, 

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