YAMPA PLATEAU. 285 



distinctly traced in the open valleys of the White River by the monoclinal 

 ridges, in which the variegated clays of the Jurassic and the sandstone ridges 

 of the overlying Dakota Cretaceous could be distinctly recognized from a 

 great distance. The ridges formed by the lower of these beds curve sharply 

 in a re-entering angle to the northward at the base of Section Ridge, while 

 the sandstone ridges of the Upper Cretaceous are seen to partake in a 

 much slighter degree of this curve, and run approximately east and west, 

 at a considerable distance from the flanks of the plateau. Section Ridge rep- 

 resents a sharp anticlinal fold, whose axis runs northeast and southwest, sink- 

 ing abruptly to the southwest. Its summit is covered by the fossiliferous 

 limestones of the Upper Coal-Measure group, dipping 20° to the northwest, 

 approximately with the slope of the hill, and forming abrupt escarpments to 

 the southeast, at the foot of which the same beds are seen standing vertically, 

 showing that this side of the fold has been dislocated, and perhaps suffered 

 some faulting, The following fossils were obtained from these beds : 



Nticulana heUistriata. 



Schizodus curtiis. 



Belleroplion carlonarius. 



Ortlioceras crebrosum. 



Naidites, sp.? 

 They have somewhat of a Permian facies ; but as the general character 

 of the fossils of this horizon, which has been so distinctly traced at every 

 good exposure of the Upper Coal-Measure limestones in the Uinta Range, is 

 rather Carboniferous than Permo-Carboniferous, they have been considered 

 to mark the extreme upper bed of the Upper Coal-Measure group. Kear 

 the highest point of Section Ridge, the surface is partially covered by brown 

 pebble-like accumulations of crystals of iron pyrites, colored brown by 

 superficial decomposition. The bed, from which they probably came, is a 

 dark, reddish seam, containing Avhite fragments of calcite and quartz, the 

 main mass being an admixture of quartz grains and iron oxide. 



The interior of the Yampa Plateau north of Section Ridge was not 

 visited. As seen from this point, its surface is generally level, having been 

 formerly covered by the beds of the Wyoming Conglomerate, of which a rem- 

 nant still remains on the northern point near the Green River. Doubtless, a 

 careful examination of the ravines in this plateau might reveal other flexures 



