264 Tertiary. 



separable into a lower division, having a thickness of about 900 feet, 

 and an upper division having a thickness of about 500 feet. 



The Bridger Group in the typical localities rests conformably upon 

 the Green River Group, into which it passe^ without a distinct plane of 

 demarkation among the strata. Its raolluscan fossil remains corre- 

 spond 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 deposited. In the 

 valley of Red Bluff Wash, between Raven ridge and White river, where 

 Ihey are covered by the Brown's Park Group, the thickness is only 

 about 100 feet. 



The Brown's Park Group is unconformable with the Bridger Group, 

 but it can not be of later date than Pliocene, for the following reasons: 

 In many places the strata still remain in a nearl}^ horizontal position, 

 but in others they have been considerably displaced, as, for example, 

 by being flexed up against the flanks of the Uinta mountains, and also, 

 in a similar manner, against the Dry mountains, northeastward from 

 Brown's Park. This shows that, although much movement of dis- 

 placement took place before the deposition of the Brown's Park strata, 

 as shown by their unconformity with those of the older groups, a con- 

 siderable amount of movement, even of mountain elevation, has taken 

 place since their deposition. Beside this, a large proportion of tlie im- 

 mense denudation which the strata of that region have suffered, is 

 known to have taken place since the deposition and partial displace- 

 ment of the Brown's Park Group, because these strata are involved with 

 the others in that denudation. Furthermore, a .remarkably extensive 

 outflow of basaltic trap, covering a large region which lies mainly to 

 the eastward, but which former!}^ extended much within the limits of 

 this district, took place after the deposition of this Group, and also 

 after it had suffered displacement and erosion to some extent, at least. 

 This is known to be the case, because the trap is found resting upon the 

 unevenl}' eroded surface of a portion of this group, at Fortification 

 Butte. That portion occupies a liigher level than does the principal 

 portion of the group, and the trap rests unconformable^ upon the Lara- 

 mie and Cretaceous strata, in the immediate vicinit}-, as well as upon 

 the Brown's Park strata, in such a manner as to show that little, ifan^-, 

 movement has taken place since the trap outflow. The denudation of 

 the rocks of that region has been so great since the trap outflow, that 

 the latter rock has been removed fi-om a large part of the surface it 

 once occupied, leaving only here and there mere shreds of the once mas- 

 sive and extensive sheet upon the higher hills. 



