102 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



i,200 feet from the base. Each coal group has 2 to 3 workable 

 coal seams, but the nvunber and thickness of the seams vary from 

 place to place. At the time when this field was examined, the popu- 

 lation was sparse and none but insignificant mines had been opened. 

 In the eastern part, coal seams, 4 to 10 feet thick, were exposed 

 in both the middle and the lower group ; but the upper group is ill- 

 exposed. Farther west, seams of greater thickness were seen, one 

 near Lay being 20 feet, with a parting of 15 inches midway. There, 

 the three coal groups are in a vertical space of not more than 800 

 feet. Many seams have shale roof and floor and one is clearly be- 

 tween sandstones. A faux-toit was seen in many openings and 

 either bone or dirty coal is the usual parting. A faux-mur is re- 

 corded in but one instance. 



The irregularity in thickness of the Mesaverde in the Yampa 

 field may be due to the eastward disappearance of shore conditions. 

 At 25 miles east from the boundary of the Yampa field, Beekly's^^ 

 sections on the west side of North Park show no evidence of Mesa- 

 verde, while at 25 miles farther east in the same Park, the Pierre 

 is represented by about 4,500 feet of shale, wholly like that beyond 

 the Front Ranges in Colorado and New Mexico. It is sandy on 

 top and passes into a marine sandstone, shown on east side of the 

 Park — apparently the Fox Hills. Some thin sandstones were seen 

 in the lower part of the formation but no trace of coal is reported by 

 Beekly. 



Northward in Wyoming and east from the INIedicine Bow Moun- 

 tains about 60 miles east of north from the exposures in North 

 Park, the section by C. E. Siebenthal, cited by Darton,*- shows 

 about 5,500 feet of Montana rocks, divided at about 1,300 feet from 

 the top by the Pine Ridge sandstone, 60 to 80 feet thick. The mass is 

 practically shale throughout, there being in all only 127 feet of sand- 

 stone in the upper 1,332 feet and 35 feet in the underlying 4,150 

 feet. The formation contains marine fossils at many horizons, the 

 highest being within 140 feet from the top. It is difficult to deter- 

 mine a positive plane of separation between Pierre and Fox Hills 

 in this region so that authors frequently employ " Montana " or 



81 A. L. Beekly, Bull. 596, 1915, pp. 20, 43, 45. 



82 N. H. Darton, Bull. Geol. Soc. Amcr., Vol. 19, 1908, 459, 460. 



