230 REPORT UNITED STATES GEOLOGICAL SURVEY. 



but they are unmistakable in their lithological characteristics, and yet 

 the latter does not here form a hogback as it usually does elsewhere. 



Proceeding westward from Ashley's Fork my journey, after the first 

 three or four utiles, was over the Uinta Group until we reached Luke 

 Fork. This group is much more extensively developed in this region 

 than I have anywhere seen it before. It is many hundred feet in thick- 

 ness, and it is quite as regularly stratified as any of the other fresh water 

 Tertiary deposits of the West. In some places, as for example, in the 

 vicinity of Green River, south of the Uinta Mountains, it is largely 

 composed of soft, bad-land sandstones, having a general reddish color. 

 But further westward it assumes a somewhat darker hue and character 

 of quite regularly bedded sandstones, some of which are soft, but many 

 of the strata are firm and even massive. At Wonsitz Eidge, four miles 

 west of Dodds's Ranch, it rests unconformably upon the Laramie Group, 

 and at Lake Fork on the Uinta and Salt Lake trail, some forty miles 

 farther west, it is found to rest upon the Bridger Group, as it was shown 

 to do near White River, in my report for 1876. 



No fossils of any kind were found by myself in the Unita Group this 

 year, but in 1875, 1 obtained a Physa from it in the valley of Snake Biver ? 

 a few miles above Junction Mountain. The Uinta Group, as already 

 shown, is regarded as equivalent with the Brown's Park Group of 

 Powell 5 and it is probably also equivalent with the Lake Beds of Middle 

 Park. 



Three or four hundred feet in thickness of the strata of the Bridger 

 Group are exposed in the valley side of Lake Fork, which have there all 

 the peculiar lithological characteristics which they possess at the typical 

 localities north of the Uinta Mountains, even including the various tints 

 of coloration and the style of weathering of the bad-land sandstones of 

 which the formation is largely composed. From these strata I made a 

 A r ery good collection of vertebrate fossils, consisting of Ganoid, Chelonian 

 and Mammalian remains, but the only invertebrate form I discovered 

 was the well known Planorbis Utahensis Meek (~ P. spectabilis Meek.) 



Following the trail, the course of which lies south of the wagon-road, 

 to the east fork of the Duchesne, I found near the crossing some limited 

 exposures of Bridger strata in its valley side, with those of the Uinta 

 Group resting upon them, but I obtained no fossils there. Still follow- 

 ing the obscure trail before mentioned, our journey from the east fork 

 of the Duchesne to the main stream was over sanely barrens with here 

 and there an exposure of sandstone. Upon reaching the latter stream 

 I found it to run in a canon or deep monoclinal valley excavated out of 

 the Green River Group. The dip of these strata at the valley where I 

 examined it is three or four degrees to the northward or northeastward, 

 but as they are seen in the high hills south of the valley the dip seems 

 to be considerably more there. This dip seems to be part of a broad 

 accessory fold, subordinate to the Uinta Mountain uplift, by which these 

 Green River strata have risen from beneath those of the Bridger Group 

 that were seen on Lake Fork and the east fork of the Duchesne. No 

 fossils were found in these Green River strata although diligent search 

 was made, but the lithological characteristics are the same as they are 

 at the typical localities north of the Uinta Mountains, the upper and 

 lower divisions of the group being plainly recognizable. The thickness 

 of the group as seen iu this region is estimated at about 1,000 feet, but 

 as the base was not visible where the upper division was observed, the 

 entire thickness of the group here is probably equal to that which it 

 attains at the typical localities. The canon of the Duchesne, where I 

 observed it, has high precipitous sides, and is in all respects like that 



