12 E..M. MUSEUM MEMOIRS. 



die, and upper Eocene. Just south of the Uinta range is another small 

 group of rocks, of about 500 feet in thickness, which caps the Eocene and forms 

 an approach to the Miocene. The age of these beds has been determined by 

 their fauna. Their position and relation to the Uinta chain, as well as that of 

 the other groups, is made clear in the accompanying sketch-map. The history 

 of these Tertiary lakes and the present confines of the various beds can be 

 followed in this map, which affords an admirable idea, also, of the upheavals 

 which formed the great rim of the basin. It will be kept in mind that the 

 country circumscribed has been elevated since Eocene times to 6000 and 7000 

 feet above the sea by the general uplifting of the continent, and that the 

 mountain ranges of to-day rise to twice this level. 



The total thickness of the Bridger beds is about 1500 feet, presented in 

 two or three comparatively contracted exposures. These overspread two 

 main tracts : one in the Bridger basin, lying wholly east of the Green River ; a 

 second to the west of the river, situated in the Washakie basin ; a third, smaller 

 tract lies southeast of Vermillion Creek. That the Bridger beds were deposited 

 in a much smaller sheet of water than the Green River beds is evident at once 

 from the nature of the rock, and quite apart from a consideration of their 

 extent. The Green River beds are largely calcareous, fine fissile shales, some- 

 times containing carbonaceous matter. The lower members are limestones. 

 The uppermost strata of the Uinta were Mesozoic rocks ; and King suggests 

 that the earlier erosion (forming the Vermillion Creek strata) must have worked 

 these off, exposing the calcareous strata of the Carboniferous rocks, about the 

 inception of the formation of the Green River beds. The presence of lime in 

 these beds is thus ingeniously accounted for. The shales are often of delicate 

 texture, and rival the Solenhofen slates in the exquisite impressions of leaves, 

 fishes, and Crustacea which they contain. They indicate that the streams feeding 

 the Gosiute Lake were of a rather sluggish nature, and heavily charged with 

 fine silt. There are occasional calcareous strata among the Bridger beds, but 

 they are essentially a sand formation. The reader is here referred to the 

 stratigraphical section accompanying this memoir. Alternating with the fine 

 clayey and sandy beds are coarse gravelly beds, indicating streams of con- 

 siderable size and rapidity which were suited to carry such heavy materials far 

 into the centre of the lake. Rarely they contain remnants of leaves, and sel- 

 dom or never, the remains of fishes. In the coarser beds we usually found 

 only the remains of larger vertebrates, and those considerably dissociated and 

 scattered by violent aqueous and other agencies. 



There is for the above reasons no room for doubt that the Bridger beds were 

 formed in a much smaller basin than the Green River. The question remains, 

 were the later beds formed in a single basin or in a number of separate basins 



