Jfesozoic and Coenozoic Geology and Palceontology. 271 



the westward. Erosion has removed large masses of the strata, and 

 exposed the Wasatch for a number of miles up stream. Eastward 

 the Green River makes a sharp turn and passes noi"th of Essex 

 mountain. In this section the lower members of the Green River Group 

 are composed of gray and bluish shales, more or less calcareous and 

 arenaceous. Higher up the shales are yellow and light brown, mostly 

 ver}'^ sandy, but containing strata of impure argillaceous limestones. 

 Above these follow concretionary sandstones and shales, of yellow and 

 rusty brown color. The former contains one very prominent horizon 

 of silicious material, appearing in the form of chalcedony and agate. 

 JN^ear the base a thin seam of oolite occurs. West of Packer's creek, 

 the total thickness of this group is from 1,700 to 1,800 feet. Of 

 this the upper sandstones with their shales, occup3' about 800 to 900 

 feet, and the arenaceous beds near the base, about 150 to 200 feet, 

 which leaves an average thickness for the shales of 700 to 800 feet. 

 Both the shales and sandstones diminish in thickness in their north- 

 ern extension. 



The Green River Group is succeeded by the Bridger Group in this 

 section, wherever the bluffs rise high above the general level as on 

 Steamboat buttes, A thickness is preserved of about 500 feet, but 

 most of the group has been eroded. Toward the south and southwest 

 it becomes thicker. On the northern edge of the Sweetwater plateau the 

 Wasatch Group is succeeded b}' a local deposit, called, the Sweetwater 

 Group. It consists of brown, yellow and white arenaceous marls and 

 clays, and near the top some sandstones without clearh'^ defined strati- 

 fication. It is not conformable with the Wasatch. This gi'oup has 

 suffered greatly by erosion, but retains a thickness in some places of 

 1,200 to 1,400 feet. The hills south and southwest of Saint Mary's 

 ranch, the central butte in Elkhorn Gap, the Sweetwater hills and nu- 

 merous bluffs are composed of the strata of thi% group. It is of Mio- 

 cene age. 



The Sweetwater Group is succeeded by the Pliocene. Near the base 

 it consists of a very loosely aggregated sandstone, of a light gray or 

 yellowish color. Above this there is a succession of light marls and 

 indurated clays. Usually these are either vei'y light yellow or white, 

 but pink and greenish beds are not wanting. Toward the eastern ter- 

 mination of the group the strata become highly silicious. Thoroughly 

 permeated by silica, the clays become very hai'd and brittle. The 

 former occurs also in the shape of narrow veins, concretions and even 

 strata. Innumerable moss agates are strewn over about six square 



