WHITE. J TEETIARY PERIOD. 35 



Laramie Group, whicli I Lave designated as Post-Cretaceons, and those 

 of the Wasatch Group, the earliest of the Tertiary groups proper. 

 Neither have I been able to discover any definite, stratigraphical plane 

 of demarcation bewteen the two groups. 



THE WASATCH GROUP. 



In his annual report for 1870, Dr. Hayden proposed the name " Wa- 

 satch Group" for a series of strata that are extensively developed in 

 Southern Wyoming and adjacent parts of Utah and Colorado. I regard 

 the series of strata to which Mr. King has given the name "Vermilion. 

 Creek Group," and Professor Powell, that of "Bitter Creek Group," as 

 geologically equivalent with the Wasatch Group of Dr. Hayden, and I 

 therefore use that name in this report, in accordance with the recognized 

 rule in such cases. 



The Wasatch Group is the lowest of a series of three fresh- water Ter- 

 tiary groups, all of which are intimately connected, not only by an evi- 

 dent continuity of sedimentation throughout, but also by the passage of 

 a portion of the molluscan species from one group up into the next 

 above. Not only were these three groups, aggregating more than a mile 

 in thickness, evidently produced by uninterrupted sedimentation, but it 

 seems equally evident that it was likewise uninterrupted between the 

 Laramie and Wasatch epochs, although there was then a change from 

 brackish to fresh waters, and a consequent change of all the species of 

 invertebrates then inhabiting those waters. 



The Wasatch Group in this district consists very largely of soft, varie- 

 gated bad land sandstones that reach a thickness of about 1,500 feet, 

 together with from 100 to 300 feet of the ordinary indurated sand- 

 stones, alternating with bad-land material at the base, and a similar 

 amount of similar material at top, the estimated aggregate thick- 

 ness being about 2,000 feet. The lithological characteristics vary some- 

 what in difierent parts of the district, the middle portion sometimes 

 losing its distinctive bad-land character, and the sandstones becoming 

 more indurated, but they seldom become very hard. 



The exposures of the Wasatch Group in tliis district are mostly con- 

 fined to that portion of the surface occupied by the principal flexures of 

 strata, the surfaces of Coyote Basin and Powell's Park being the broad- 

 est spaces occupied by those strata. They are upturned by tbe Midland, 

 Grand Hogback, and Kaven Eidge flexures, and are exposed in the 

 valley of White River along a great part of the whole distance from 

 Powell's Park to a point a few miles below Raven Park. Being com 

 posed of easily eroded materials, the strata of this group seldom pro- 

 duce any conspicuous features of the surface, except the valleys and 

 basins that are eroded out of them, such as Powell's Park, Coyote Basin, 

 Hogback Valley, and a part of White River Valley. 



Very few fossils were obtained from the strata of the Wasatch Group 

 in this district. Specimens of the genera Viviparus, Goniohasis,and Unto 

 were obtained from strata near the top of the group in Raven Ridge, near 

 the south western border of Raven Park. The same genera, and doubtless 

 the same species, were found in a similar horizon in the valley of White 

 River opposite Pifion Ridge, where also fragments of Physa pleromatis 

 White, were obtained. 



THE GREEN RIVER GROUP. 



Resting immediately and conformably upon the Wasatch are the 

 strata of the Green River Group. Although intimately connected with 

 the former by continuous sedimentation and specific identity of mollus- 



