ASHLEY CEEEK BASIN. 297 



valley only 15 or 20 feet wide, marked by an overlying sandstone bed 

 containing remarkable spherical concretions. For miles along this line of 

 outcrop they stand, freed from the loose sandstone in which they were 

 enclosed, great spheres, from 6 to 10 feet in diameter, on whose surface 

 little projecting ridges, of rectangular form, are so arranged as to give them 

 roughly the appearance of parallels and meridians on a globe. Analysis 

 shows them to contain about 45 per cent, of carbonate of lime, and con- 

 siderable iron and alumina, which was, however, not quantitatively deter- 

 mined : they probably result, then, from a concentration of the limy 

 material of the arenaceous beds, in which they occur. 



The strata of the outer curved ridges on the western point of Split 

 Mountain have a steep dip of from 40° to 50°, and curve sharply round, as 

 indicated on the map. The course of Brush Creek is cut through the clay 

 beds of the Colorado group ; near the ford were found some exposures in a 

 deep ravine, showing blue plastic clays at the base, which are impregnated 

 with salts of alum and alkalies, which crystallize out on some of the exposed 

 surfaces. These clays seem to immediately overlie the spherical sandstones 

 adjoining the coal. On the flat-topped benches of yellow marly clays, close 

 to Brush Creek, the surface was found to be covered with conglomerate debris 

 resembling the material of the Wyoming Conglomerate, though entirely 

 decomposed. These curved ridges are so continuous as to make the ascent 

 of Split Mountain from the west a very difficult matter, since they are rarely 

 broken through, and the drainage follows, as will be seen by the map, their 

 monoclinal valleys. 



Split Mountain itself is a remarkable topographical feature, formed of 

 the hard beds of the Upper Coal-Measures, dipping steeply north from 40° 

 to 50° ; while at the extreme western point of the anticlinal the dip decreases 

 to 15°, and on the south becomes as high as 70°; nearly through its centre 

 the Green River has cut a V-shaped canon in a southwesterly direction, 

 bending at an acute angle to the eastward, and running for some distance 

 in the strike of the steeper beds of the southern side of the anticlinal fold, 

 through which it then suddenly breaks into the open alluvial valley of the 

 Ashley Basin. In this canon, the lower beds exposed have a distinctly red 

 color, but it was not definitely determined that they belong to the Weber 



