36 REPORT UNITED STATES GEOLOGICAL SURVEY. 



can species, they differ coDsiderably from those of that group in general 

 aspect, and in composition also. The group is, lithologically, at least, 

 separable into two divisions, but they are not regarded as severally of 

 CO ordinate value with the other recognized Tertiary groups. The lower 

 division consists mainly of silicious and sandy shales, and laminated 

 and thin-bedded sandstones, with, in some places, especially in the 

 western part of this district, frequent layers of hard, dark-colored car- 

 bonaceous shales. In some places the strata are also quite calcareous, 

 occasional layers being nearly pure, compact, finely-laminated limestone. 

 Others of the calcareous layers are sometimes oolitic in texture. The 

 general aspect of the strata as seen exposed at a distance is light 

 gray. 



The upper division consists mainly of sandstones that are coarser, as 

 well as less thinly and distinctly bedded, than those of the lower divis- 

 ion. In some parts it is shaly, and in others carbonaceous. Much of 

 its sandstone is ferruginous in aspect, instead of having the gray tint 

 that the lower division has. Sometimes certain beds of its sandstones 

 are earthy and easily disintegrated, often leaving, weathered out of the 

 mass, spherical concretions of hard sandstone that vary in size from a 

 fraction of an inch to two or three feet in diameter. Other beds some- 

 times present buttress like masses in the brow of bluffs, which form 

 conspicuous and somewhat remarkable features in the landscape. Such 

 features are very characteristic of this division in the bluffs of Green 

 Kiver in the vicinity of Green Eiver City, Wyo., and, to a less extent, 

 they also appear in the bluffs which border the caiion and valley of 

 White Eiver, in the southwest portion of this district. 



The invertebrate fossils which this group affords are similar to those 

 that are tound in the fresh-water portion of the Wasatch Group, some of 

 the species being identical, and indicate a purely fresh-water condition 

 throughout. They are almost wholly molluscan, and belong to the 

 branchiferous genera Unio, Viviparus^ and Goniohasis, beside several 

 genera of pulmonate gasteropods, including both the limnophile and 

 geophile divisions. The Green Eiver Group has become somewhat 

 noted for the fossil fishes that have been discovered in its strata in Wyo- 

 ming, and, like the Wasatch Group, it has at various localities also fur- 

 nished considerable collections of ibssil vertebrates and plants. 



In this district, the Green Eiver Group is well and characteristically 

 developed, the lower division reaching a thickness of about nine hun- 

 dred leet and the upper division about five hundred feet. The large 

 hill-masses that lie between Hogback Valley and Powell's Park are 

 composed of it, as are also those that form the bluffs of the south side 

 of the valley of White Eiver, from Powell's Park to the western bound- 

 ary of the district, except that portion of them which forms the south- 

 ern border of Eaven Park. It also occupies a considerable space ad- 

 jacent to Wnite Eiver, between Eaven Park and the western boundary 

 of tlie district, and also a narrower space along the southwestern side 

 of Eaven Eidge. 



The strata of the lower division of the Green Eiver Group differ con- 

 siderably in lithological characters in different parts of this district. In 

 the hills that lie between Eogback Valley and Powell's Park they are 

 more largely composed of ordinary sandstones than is usual in this 

 group, some of which constitute thick, heavy-bedded strata. In the 

 southwestern part of the district the strata of this division are more 

 finely laminated, and contain much more calcareous and carbonaceous 

 material than elsewhere. 



The principal flexures and other displacements of the strata of this 



