122 Cincinnati Society of Natural History. 
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 passes without a distinct plane of 
demarkation among the strata. Its molluscan 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 
they 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 nearly 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 the 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 tre 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 formerly 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 
unevenly eroded surface of a portion of this group, at Fortification 
Butte. That portion occupies a higher level than does the principal 
portion of the group, and the trap rests unconformably upon the Lara- 
mie and Cretaceous strata, in the immediate vicinity, as well as upon 
the Brown’s Park strata, in such a manner as to show that little, ifany, 
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 from 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. 
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