BRIDGER'S PASS REGION. 159 



crystalline gypsum. It apparently filled a fissure in the sandstones about 



2 feet in width, running at right angles to the stratification. These sand- 

 stones probably belong to the upper part of the Dakota, or lower part of 

 the Colorado group. The southerly-dipping ridges extend to the westward, 

 with a trend parallel to the line of the Fox Hill Bluffs, until, under the 

 influence of the Rawlings Peak fold, they curve to the north along the 

 western flanks of that uplift. The northern ridges cross the railroad about 



3 or 4 miles east of Rawlings, curving round to the northeast so as to 

 enclose a slight synclinal crumple, and are lost under surface accumula- 

 tions. On the slightly-rising ground about 4 miles east of Rawlings Peak, 

 however, the characteristic conglomerate of the base of the Dakota group 

 is seen, a line of boulders, as much as 6 feet in diameter, marking its line 

 of outcrop, which forms a curve parallel to the base of the ridge. This 

 conglomerate is made up of small pebbles, generally not larger than a 

 filbert, in a siliceous matrix. The pebbles are mostly of black jasper or 

 chert, while the matrix is largely made up of broken and partially- 

 rounded crystals of quartz. The boulders represent portions of the con- 

 glomerate, which have become so hardened and compacted by local meta- 

 morphism, that the matrix is as unyielding as the jasper pebbles, and 

 the mass fractures with equal ease through either. The western faces of 

 these boulders present the most remarkable instance of polishing by wind- 

 driven sand we have had an opportunity of observing. The surface of 

 the otherwise rather light-colored rock has assumed a dark leaden-gray 

 hue, and a polish equal to that of glass, while the sand has drilled irregular 

 grooves and holes, often three-quarters of an inch deep, and not more than 

 an eighth of an inch in diameter, through pebbles and matrix indifi'erently. 

 In the finer-grained portions of the unaltered conglomerate are mixed, with 

 the grains of limpid quartz, white earthy particles of kaolinized feldspar, 

 generally too small to show any shape, but in some cases as large as a pea, 

 when the general outline of the original feldspar crystal can be easily 

 traced. Below the conglomerates are found a few outcrops of the Jurassic 

 limestones, showing a comparatively small thickness of beds, and dipping 

 like the conglomerates 8° to 10° to the eastward. Beyond these, to the 

 westward, are the Triassic sandstones, whose presence is indicated by the 



