76 REPORT UNITED STATES GEOLOGICAL SURVEY. 



a very even altitude, we suddenly come across a deep, narrow gulf in 

 the rocks. Through this hows the Little Popo-Agie. The canon is not 

 a straight rent across the strata, but runs in a zigzag line. This feature 

 is one endowing it with additional beauty. Regarding its formation it 

 may be said that it can scarcely .be considered as the result of erosion, 

 but rather as that of seismic action. It is highly probable that other 

 passages across this chain owe their present location to the formation of 

 fissures which must be ascribed to the same cause. In case these ex- 

 tended but a short distance downward, fluviatile erosion would tend to 

 widen them, until, eventually, a broad " gate" has been produced. Con- 

 tinuing along the hills in the same direction as above, we find that Twin 

 Creek is forced to make a considerable detour along their western base 

 before it finds apoint where it can cross. Shortly after that has been 

 reached we see Tertiary strata covering the Carboniferous and shutting 

 their southeasterly continuation out of sight. 



Along the same line of travel we find two further features which 

 change gradually as we advance southward. The first of these is the 

 decline of dip-angle. While we have, west of Camp Brown, a dip of 36° 

 to 40°, we find that east of Miner's Delight this has decreased to about 

 30° to 35°. As a rule, the diminution of the angle occurs at a very reg- 

 ular ratio. Besides this fact, we notice a change in the relative position 

 of the Carboniferous strata and those belonging to younger groups. West 

 of Brown the Mesozoic beds reach upward a considerable distance upon 

 the eastern slopes of the hills. Near Miner's Delight, however, the entire 

 slope exposes Carboniferous beds. Although the ratio of recession to 

 the eastward is not so uniform as that of the decreasing dip, it is suffi- 

 ciently constant to be regarded as a striking characteristic. On the 

 surfaces of limestones thus exx>osed to atmospheric action, we observe 

 peculiar results produced by erosion. The impregnation of siliceous 

 matter is an irregular one, and its distribution is indicated by the cor- 

 rugated surfaces produced by erosion. Numerous white fossils not un- 

 frequently cause the formation of small columnar products of erosion 

 resembling stylolites. The exposure of Carboniferous rocks, so far as in 

 direct connection with the Wind River Range, ends a short distance 

 southeast of Miner's Delight. I have considered it best to treat first of 

 those portions most intimately related to the range, before describing 

 the isolated occurrences farther east. These latter owe their present 

 position to different causes from the former, and are geologically sepa- 

 rated therefrom, although geognostically and lithologically identical. 



So far as our observations extend, no strata have been found in con- 

 nection with the Carboniferous formation that can be proved to be Per- 

 mian. Immediately resting upon the limestones we find a series of 

 shales, sandstones, and some dolomitic limestones. Although I am not 

 prepared to furnish any paheontological evidence of its age, I am inclined 

 to separate the group at least provisionally. It will be spoken of in 

 subsequent pages as Permian. In case future investigations prove that 

 it should not be regarded as Permian, it must be referred to the base of 

 the Mesozoic formations. 



PALAEOZOIC FORMATIONS EAST OF THE WIND RIVER RANGE. 



iSubcarboniferous. 



East of the Wind River Range, Carboniferous rocks have been brought 

 to the surface by virtue of serious, more or less extended, stratigraphical 

 disturbances. Examining Sheep Mountain, we find that its summit is 



