582 REPORT UNITED STATES GEOLOGICAL SURVEY. 



Creeks (branches of Salt River), and also the country south and west of 

 the southern end of the Preuss Range. This latter region was not vis- 

 ited, but was worked topographically from the surrounding high country. 

 From sections made south and north of it, it has been considered as 

 having the Jurassic as the surface formation, and it has been so colored 

 on the map. 



On the west side of Thomas' Fork, however, north of the Sublette 

 Range, we have Station 41, located on a coarse, gray sandstone, which 

 is conglomeritic in places, and dips to the westward at an angle of 

 about 45°. Just east of the station the dip is to the eastward, so that 

 the station is located on or near an anticlinal axis. This is, as I have 

 said before, probably the fold of Coketown Butte. Following the axis 

 to the north, the beds of the station are seen diverging as the elevation 

 increases, so that when the divide to Beaver Creek is reached, under- 

 lying gray beds (probably Jurassic limestones) come to the surface and 

 form the centre of the fold. At the point of a ridge, which is about a 

 mile east of the station, the following fossils were obtained : 



TJnio vetustus. 



Corbula pyriformis. 



Corbicula ( Veloritina) durlteei. 



Pyrgtdiferalmmerosa, 



Goniobasis ehrysaloidea. 



Goniobasis clebumi. 



Ostrea — % 



These prove the beds to be the Laramie Group, and equivalent to the 

 Bear Biver Laramie. 



Between the point where these fossils were found and the station there 

 must be at least 3,000 feet of sandstones, in which occurs a band of 

 black shales, like those of the section on the east side of Smith's Fork. 

 East of the outcrop containing the fossils the principal outcrops show 

 gray and greenish sandstones. If the conglomeritic sandstone of Sta- 

 tion 41 is the same as the conglomerate (No. 6) of the section east of 

 Smith's Fork, as I am disposed to think, then the coaly, black shales 

 above are also identical, and these beds should, therefore, in all proba- 

 bility, be referred to the Cretaceous ; but this division is made with- 

 out any fossils having been found to warrant it. In an accompanying 

 plate the section from Station 41 eastward is given. 



West of the station another synclinal fold is seen, beyond which the 

 gray Jurassic layers appear to outcrop in a bluff on the east side of the 

 middle fork of Thomas' Fork. 



Thomas' Fork joins the Bear in a beautiful broad valley which is from 

 two to four miles in width, and from the eastern side, the Sublette Bange 

 rises nearly 4,000 feet above it. 



This valley is apparently a monoclinal in red sandstones. On the east 

 side they rise inclining steeply from the range, and on the west they dip 

 westward, outcropping in low bluffs. The folds that are found in the 

 rocks west of Thomas' Fork will be noted under the head of Bear Lake 

 Plateau. 



BEAK LAKE PLATEAU. 



Geologically the Bear Lake Plateau is the northern continuation of 

 the Bear Biver Plateau of King's map (Atlas, Exploration of Fortieth 

 Parallel). Topographically the latter ends west of the south end of 

 Bear Lake, where the Wahsatch beds that form it are much more in- 

 clined, while on the east side the plateau is continued by the same 



