100 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



near Rock Springs, but most of the coal has been taken from 

 numbers i and 7, at 481 and 743 feet from the top of the group. 



Number i has many " rock-shps " or " horsebacks," long, slim 

 wedges of white sandstone, protruding usually from the floor. 

 They are smooth on one side, rough on the other and the coal is 

 unchanged even at the contact. The roof and floor are brownish to 

 white sandstone. The coal, at times, is 10 feet thick, but changes 

 are abrupt. Partings thicken and the coal becomes worthless. In 

 one mine the coal is 11 feet thick and clean, but in another, ad- 

 joining, the coal suddenly became worthless and, at a little distance 

 beyond, it pinched out. Seam 3 shows similar complications. A 

 band of shale appeared in one mine at 2 feet from the floor; within 

 a short distance it thickened upward until the top bench became too 

 thin for working; but within 200 feet the foreign matter almost 

 disappeared and the upper bench was again more than 5 feet thick. 

 Schultz's description shows that here is a channel originating during 

 growth of the swamp and filled up before the growth ceased, so that 

 the swamp covered it. Seam 7 is less inconstant than the others 

 but it is far from free from troubles. The roof and floor are 

 shale, the former black. One important mine was abandoned be- 

 cause the good coal was replaced with worthless stuff in an area of 

 evidently great extent. The Rock Springs coal seams become unim- 

 portant southwardly and none has been discovered in the extreme 

 southern portion of the field. 



Tertiary deposits conceal the Cretaceous from the Rock Springs 

 field to near Rawlins in Carbon County, where Smith"' recognized 

 the Lewis, Mesaverde and the shales of Lower Pierre. The Mesa- 

 verde, consisting of sandstones, shales and coal seams, is still dis- 

 tinct but is much thinner than in fields farther west. It consists of 

 two massive sandstones separated by a mass of soft brown sand- 

 stones and white to gray shale. The Almond and Rock Springs 

 coal groups have become insignificant. The coal seams in this area 

 are on top and at base of the upper sandstone and just above the 

 lower sandstone : four or more seams were seen in the upper zone, 

 few were observed in the middle and 4 to 6 in the lower zone. The 



'•' E. E. Smith, Bull. 341, pp. 220-242. 



