140 Cretaceous:. 



sandstones, forming low, long-continued bluffs. The thickness is esti- 

 mated at 400 feet. 



The yellow and white sandstones of the Dakota Group occur in the 

 northern portion of the Sweetwater Hills. East of Elkhorn Gap they 

 are much folded and plicated. In Whisky Gap, the strata curve around 

 the Western base of the Seminole Hills, with a partiversal dip, and a 

 short distance farther west the}- take part in an anticlinal upheaval. 

 The thickness is estimated at about 700 feet. 



The Colorado Group occurs also near Elkhorn Gap, and in Whisk}' 

 Gap. At the latter place the shales are dark gra}', finely laminated, and 

 have a thickness of 650 to 700 feet. 



The Fox Hills Group, in Whisky Gap, forms sharp, lo^v ridges, par- 

 ticipates in the stratigraphical disturbances, and has an estimated 

 thickness of 1,000 feet, which increases toward the south. Near Salt 

 Wells, this^group is well developed, and occupies a prominent position. 

 A valle_y of approximately semicircular shape, lies directly north of 

 the railroad, bordered b}^ steep brown bluffs of shales and sandstones 

 of this group. Dipping off in every direction, they present a most 

 ty'pical partiversal arrangement of the strata. Near the base, they are 

 composed of thinly-bedded sandstones. These are followed b}^ yellow 

 and brown shales, more or less arenaceous and micaceous. Above these 

 there is a succession of sandstones and slniles, containing carbonaceous 

 strata. A recess in the bluffs is caused by the higher series of shales. 

 The latter are covered by sandstone strata of var^'ing thickness, sep- 

 arated from each other by shales. Some good coal is found in this 

 horizon. Near the top, massive yellow sandstones are overlaid by thin 

 beds of shale and white sandstone. On ever}- side the beds ai-e con- 

 formably overlaid by strata of the Laramie Group. The thickness is 

 from 1,200 to 1,300 feet. 



The Laramie Group has a wide distribution in the southern area of 

 this territory. On the west side of the anticlinal it can be traced nearly 

 to Whisky Gap, and probably juts against the granite of the Sweet- 

 water Hills. From the stratigraphical structure of the entire region it 

 is ascertained that this group forms a basin, upon which the younger 

 strata are conformable. It is composed of sandstones, shales, marls, 

 cla3's and coals. Near the base, heav}' sandstones set in, soon super-- 

 seded, however, by shales. These contain strata of sandstones at vary- 

 ing intervals. A number of coal-beds overlie the sandstones. The 

 coal is generally covered by a comparatively thin sti'atum of sandstone, 

 upon which follow clays, shales and arenaceous marls. Higher up a 



