BRIDGER EOCENE. 245 



greatest development in the southern portion of the Bridger Basin. Tlu-ough- 

 out the basin, they occupy an approximately horizontal position, no steeper 

 angle of dip being observed in them, even along the immediate flanks of 

 the Uinta Range, than 4°. Their aggregate thickness in this basin is 

 estimated at from 2,000 to 2,500 feet. As compared with the beds of the 

 Green River series, they are characterized by an absence of calcareous ma- 

 terial. The lower beds consist of a series of gray-drab sandstones, generally 

 rather thinly bedded, with a varying proportion of argillaceous beds. The 

 upper 1,000 or 1,500 feet of the formation coiisist of heavy beds of peculiar, 

 loosely aggregated, but homogeneous clayey sandstone, of prevailing olive- 

 green color, containing thin, interstratified beds of harder sandstones, and 

 in the upper portion of the series passing into marls, and at two horizons 

 into impure limestones. The beds abound in siliceous and calcareous secre- 

 tions ; the latter form seams, about an inch in thickness, of crystalline car- 

 bonate and sulphate of Hme, whose glistening fragments at times almost cover 

 the surface of the ridges ; from the former result the moss-agates, in which the 

 region abounds, and singular cylindrical concretions, which in places have 

 weathered out of the rusty-drab sandstones, resembling the broken trunk 

 of a small tree, whose hollow interior is lined with crystals of quartz. 



Along the line of the railroad and to the north only the lower beds have 

 escaped erosion, consisting of thin-bedded drab and greenish sandstones and 

 clays, forming a monotonous region of low flat ridges, whose surface is only 

 varied by the shallow alluvial valleys of Black's Fork and its tributaries. 

 To the south of the railroad, the country rises in a series of irregular, broad, 

 flat terraces, in which each terrace represents approximately a higher bed 

 of the series. At Church Buttes, the first exposures of the- characteristic 

 bad-land formation are found. These hills, which are isolated portions of 

 the beds, which form the line of blufis bordering the upper valley of Black's 

 Fork on the east, have been so named from the pecuHar architectural forms 

 into wliich the green argillaceous sandstones of the upper part of the Bridger 

 formation are eroded. The peculiar character of this erosion, which is simi- 

 lar to that already noticed in the Washakie Basin, and of which typical 

 views are given in the colored illustrations of volume I, is best seen in the 

 region of Grizzly Buttes, an irregularly semicircular line of bluffs, cut 



