38 REPORT UNITED STATES GEOLOGICAL SURVEY. 



evidence of its assumed Tertiary age and lacustrine conditions of its 

 deposition. Its fresh-water origin, however, seems unquestionable, be- 

 cause of its iutracontinental position, its limited extent, and the fact 

 that none but freshwater deposits are known in this part of the conti- 

 nent that are of later date than the close of the Laramie period. 



While the known unconformity of this group upon the other Tertiary 

 groups, as well as upon the still older rocks, might, in the absence of 

 other facts, be suggestive of its post-Teitiary age, the following facts 

 seem to show that it cannot be of later date than Pliocene Tertiary, to 

 which epoch Dr. Hayden, upon the occasion of his tirst visit to that 

 region, referred it. 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 displacement took place before the deposition of the 

 Brown's Park strata, as shown by their unconformity with those of the 

 older groups, a considerable amount of movement, even of mountain 

 elevation, has taken place since their deposition. Besides this, a large 

 proportion of the immense denudation which the strata of that region 

 have suffered is known to have taken place since the deposition and 

 partial displacement of the Brown's Park Group, because these strata 

 are involved with the others in that denudation. Furthermore, a re- 

 markably 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 the 

 Uinta 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 the 

 Uiuta Group at Fortification Butte. That portion occupies a higher 

 level than does the principal i)ortion of the group, at least within this 

 district; and the trap rests unconformably upon the Laramie and Cre- 

 taceous strata in the immediate vicinity, as well as upon the Uinta 

 strata, in such a manner as to show that little, if any, 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 occui)ied, leaving 

 only here and there mere shreds of the once massive and extensive 

 sheet upon the higher hills. 



Water-worn fragments of this trap, together with those of other 

 rocks, enter into the composition of the scattered drift of that region. 

 This drift was probably contemporaneous with the great northern gla- 

 cial drift, and is found not only in the valleys of that region, but also 

 scattered upon the hills many hundred feet higher than the streams. 

 These facts seem to be suflicient to |)rove that the Uinta Group cannot 

 be of later date than Pliocene Tertiary, while its relations to the other 

 Tertiary groups seem to show that it cannot be referred to an earlier 

 epoch than the Miocene. But further evidence on this point is needed. 



This group occupies that expansion of Green liiver Valley which is 

 known as Brown's Park. From there it extends eastward and around 

 the eastern end of the Uinta Uplitt, except a few miles interruption of 

 its continuity there, and thence extends westward along the southern 

 base of the Uinta Mountains a large part of the length of the range. 

 It extends northward from the eastern portion of the Uinta Mountains 

 as far as Dry Mountains and Godiva Eidge. Eemainiug patches of it 

 show that the formation formerly extended eastward as far as the foot- 



