WHrns-J TERTIARY PERIOD. 37 



region doubtless took place after the deposition of the Green River 

 Group, but in the district here reported on the strata of the whole group 

 are, as a rule, nearly horizontal, or vary only a few degrees from the 

 horizon. This is evidently due to the fact that they have been eroded 

 from the spaces that are now occupied by the more abrupt flexures, 

 which they doubtless once covered, but are now found to occupy only 

 or mainly the comparatively undisturbed spaces on the dropped side of 

 the flexures. 



THE BKIDGEE GROUP. 



This is one of the more important of the groups among those that, in 

 western North America, are referred to the Tertiary period, especially as 

 regards the vertebrate remains that have been obtained from its strata. 

 It is most fully and characteristically developed in the region known as 

 the Green River Basin, north of the Uinta Mountains, only the south- 

 eastern portion of the formation, so far as it is now known, extending 

 within the limits of this district. In its typical localities it is found 

 resting conformably upon the Green River Group, into which it passes 

 without a distinct plane of demarkation among the strata. Its moll uscan 

 fossil remains correspond closely with those of the Green River Group, 

 some of the species being common to both, all indicating a purely fresh 

 condition of the waters in which the strata of both groups were de- 

 posited. At the typical localities the group is composed in great part 

 of soft, variegated, bad-land sandstones, a peculiar greenish color often 

 predominating over the others, which are reddish, purple, bluish, and 

 gray. Limestone strata, marly and clayey beds, and cherty layers are 

 not uncommon, and grits aiid gravelly layers sometimes occur. 



Strata of the Bridger Group are found to occupy only a comparatively 

 minute portion of the surface in this district, the only locality at which 

 they appear being in the valley of Red Bluft' Wash, between Raven 

 Ridge and White River, in the southwestern part of the district, where 

 these strata rest upon those of the upper division of the Green River 

 Group, and are covered in turn by those of the Uinta Group. The strata 

 of the Bridger Group exposed there reach only about one hundred feet 

 in thickness, and probably represent those near the base of the forma- 

 tion. The only fossils obtained from these strata were a few fragments 

 of chelonian and mammalian bones. 



THE UINTA GROUP. 



Resting directly, but by unconformity of sequence, upon all the Ter- 

 tiary and Cretaceous groups in the region surrounding the eastern end 

 of the Uinta Mountain Range, is another Tertiary group that has received 

 the name of "Uinta Group" from Mr, King, and "Brown's Park Group" 

 from Professor Powell. It is possible {.hat this group was deposited 

 continuously, at least in part, with the Bridger Group, but at the places 

 where the junction between the two groups has been seen in this region, 

 there is an evident unconformity, both by displacement and erosion. 



The group consists of fine and coarse sandstones, with frequent layers 

 of gravel, and occasionally both cherty and calcareous layers occur.! 

 The sandstones are sometimes firm and regularly bedded, and some' 

 times soft and partaking of the character of bad-land material. The 

 color varies from gray to dull reddish-brown, the former prevailing north 

 of the Uinta Mountains and the latter south of them. 



The only invertebrate fossils that are known to have been discovered 

 in the strata of this group are some specimens of a Physa, very like a 

 recent species. Therefore, invertebrate paleontology has furnished no 



