252 DESOEIPTIVE GEOLOGY. 



inches in length, a species which, as far as our observations go, has not 

 been found higher than the Fox Hill horizon. Interstratified in these sand- 

 stones is a peculiar bed of green siliceous clay-slate, made up of fine 

 grains of quartz in a green amorphous clayey matrix, which also seems 

 somewhat characteristic of this horizon. 



The southern portion of the valley of the Little Muddy occupies a 

 synclinal fold in the rocks of the Fox Hill group. On the western side of 

 the valley, in one or two of the ravines, however, where the overlying 

 Tertiaries have been eroded off, the crest of an anticlinal fold is found, 

 whose members dip 45° east and west, with the same strike of north 30° 

 east. To indicate this structure, a strip of Laramie beds has been colored 

 on the map, although, owing to the close lithological resemblance of the 

 Fox Hill and Laramie groups and the absence of characteristic fossil 

 distinctions, it cannot be definitely determined that the Laramie beds are 

 actually exposed here. At the extreme southern end of the valley, the folds 

 become narrower and more abrupt, and the dip steeper, still preserving, 

 however, the same trend of north 30° east. At the upper end of the valley, ■ 

 the beds of the Fox Hill group, which form the synclinal, dip 60° toward 

 the centre. In a little ravine to the eastward are found a series of beds of 

 blue clay containing very perfect casts of Cardium pauperculum, a charac- 

 teristic fossil of the Colorado group. Through these beds, borings have 

 been made for oil, and a small amount of fair petroleum obtained. It 

 was impossible to determine the exact horizon of the oil-bearing rocks, 

 though they probably belong to the lower part of the Colorado series. 

 The summit of the ridge is here capped by the beds of the Vermillion 

 Creek group, dipping 2° or 3° to the eastward, and, at their eastern base in 

 Pioneer Hollow, are found sandstones dipping 20° westward, and carrying 

 a 20-foot vein of coal. These rocks have been colored as belonging to the 

 Fox Hill group, because the clays of the Colorado group on the other side 

 of the ridge dip 45° to the eastward, which would apparently carry them 

 under these sandstones. It is possible, however, that there may be faulting 

 in this region, and that the sandstones may belong to the Colorado group, 

 in which case the coal seam corresponds to that found at this horizon on 

 the south slope of the Uintas. 



