214 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



these shales are, and that species is therefore known to have as great a 

 vertical range as any of the other species of the group. 



The Laramie Group of this district is made up of sandstones and 

 sandy shales, with occasional layers of clayey shales, which toward the 

 base are reddish in color, indurated, fine-grained, and fissile. There are 

 also several beds of coal or lignite in the group as developed in this re- 

 gion, one being near the horizon of the fossils of the foregoing list and 

 another near that of the fossils collected from the upper strata of the 

 group in Yampa Valley. Besides this, comminuted fragments of plants 

 are very common in almost all the strata from base to top of the group, 

 and in some of the layers they are very abundant. 



In the Danfortk Hills, and also extending far down White Eiver Val- 

 ley, and over to Yampa Valley, the strata of nearly the whole group 

 have a decided reddish aspect, which often appears as if produced by 

 heat. The fossils of the foregoing list were all collected from the reddish 

 fissile shales before mentioned, about fifteen miles northwestward from 

 White Eiver Indian Agency, mostly about 400 feet above the base of the 

 group, and I found fragments of a part of the same species three miles to 

 the southward of that locality, some 300 feet lower in the series, or within 

 100 feet of the base. About 200 feet beneath these lowest Laramie fossils 

 I obtained some characteristic fossils of the Fox Hills Group, which will 

 be discussed on following pages. The plane of division between these 

 two groups is of course between these two fossiliferous layers, which are 

 only 200 feet apart. No unconformity of any of these strata could be 

 detected, nor is there any abrupt change from the lower to the higher 

 group ; yet viewing the strata as they are abundantly exposed in the 

 hillsides of the district, one has little difficulty in tracing a sufficiently 

 distinct line of division by the aspect of the strata. This difference of 

 aspect is due mainly to the greater thinness of the layers that compose 

 the strata which are referred to the Laramie Group, and also to a per- 

 ceptible difference of color. 



The remaining investigations of the Laramie Group for this season, 

 (except those of the peculiarly developed portion of the group at the 

 western part of Green Eiver Basin) were made near its close in the valley 

 of Bitter Creek, in Southern Wyoming, from 100 to 125 miles north- 

 westwardly from the Yampa and White Eiver localities. As the faunae 

 of the two districts are intimately related, I shall depart from the course 

 of my journey to discuss, in immediate connection with the foregoing, 

 the Laramie Group and its fossils as developed in the valley of Bitter 

 Creek, and then return to the Fox Hills Group, in the valley of White 

 Eiver, and follow the line of my journey to its close. 



In the valley of Bitter Creek, a tributary of Green Eiver, the whole 

 series of Laramie strata is twice exposed, once on each side of a broad 

 anticlinal flexure, called by Professor Powell the Aspen Mountain Up- 

 lift. Fossils were obtained from them on both sides of the fold, but they 

 all come apparently from the upper half of the series, although they occur 

 at several distinct fossiliferous horizons. My examinations began in the 

 valley of Bitter Creek, just west of the village of Eock Springs, where 

 the uppermost strata of the group occur, the dip being to the westward. 

 I made careful examination here, as well as formerly in Yampa Valley, 

 for a plane of demarkation between the Laramie and Wasatch Groups, 

 but without success. I rode back and forth, on the western side of the 

 valley, over the upturned edges of the strata between two fossiliferous 

 horizons containing characteristic fossils of each group respectively, and 

 I could nowhere find a plane of demarkation or any particular layers 

 suggestive of a plane of separation between the two groups. 



