296 DESOEIPTIVE GEOLOGY. 



To the east of Upper Ashley Creek, as far as Brush Creek, the ridges 

 have a general strike of east 20° north, while the dip flattens out in places 

 to 5°, which gives a great width of outcrop. The actual line of the 

 sandstone cliffs is much more varied than either the topography or the 

 geological colors of the map express. Between the stream-beds, this line 

 runs rapidly up toward the hills, in re-entering curves, bending out again 

 in the deeper erosion of the valleys of the stream, giving a wavy line to 

 the actual outcrops, which have necessarily been generalized on the map 

 in consequence of the small scale on which it is drawn. To the east of 

 Brush Creek, the sandstones of the Triassic and the variegated clays of 

 the Jurassic formations are seen to run under the conglomerates of the 

 Ti-ra-kav Plateau, appearing again at its eastern end. The outlines of 

 the formations in the synclinal valley toward Island Park have been traced 

 by' observations made from the overlooking points of Ti-ra-kav Plateau 

 and Split Mountain, as time did not permit the examination of this por- 

 tion of the, basin. Near the point where the Indian trail crosses Brush 

 Creek, a coal seam is found, which probably corresponds to that already 

 mentioned on Ashley Creek. Cretaceous fossils, such as Ostrea congesta, 

 characteristic of the Colorado series, are found above these beds, and Pro- 

 fessor Marsh mentions the discovery below it of cycloidal fish-scales, such 

 as were found in the Flaming Gorge Ridge, Cretaceous turtles, and teeth of 

 a Dinosaur; near this point also was found by Professor Marsh's party a 

 fragment of the unique Cretaceous crinoid, the Uintacrinus socialis, Grinnell. 

 We are here on the southern side of the synclinal, or near the point of the 

 Split Mountain anticlinal, and the beds enclosing coal have a dip of 45° to 

 50° to the northeast. The coal seam here is also about 10 feet in thick- 

 ness, but contains several dividing seams of sandstone and slate. There 

 was the appearance of a second coal seam near this point, about 200 feet 

 lower geologically, which, judging merely from the lithological character of 

 the beds, would be within the Dakota group, perhaps near its base. No 

 outcrops were found, howcA^er, sufficiently definite to determine its exist- 

 ence beyond a doubt. 



In the singularly-curved ridges which form the western base of Split 

 Mountain, the line of outcrop of the main coal seam is a narrow monoclinal 



