94 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



are lenses and are the marked features of the Book CHffs ; the lower 

 members contain Halyuienites major and brackish-water forms are 

 present at many horizons. The coal seams of economic importance 

 are confined to the lower 700 feet but Richardson's section makes 

 clear that the importance in each case is confined to a small area 

 and that the seams must be lenses. Near Thompson, Utah, at the 

 southern point of the field, there are 5 seams, beginning at 490 feet 

 from the base ; near Price canyon farther north, are 7 beds, begin- 

 ning at 340 feet, while near the Colorado line 6 seams were seen in 

 the basal 275 feet, the lowest being only 95 feet from the bottom. 

 On the Grand River the section shows 10 seams in the lower 519 

 feet. No coal seam has been traced for more than a few miles ; one, 

 21 feet 6 inches thick, where mined, proved to be a mere lens, which 

 disapeared quickly toward the west. Seams important at the east 

 disappear toward the west. There are coal horizons, not continuous 

 beds. 



The Grand Mesa coal field and smaller fields farther east have 

 been discussed by Lee,'^ who has made the relations clear for the 

 region east from Grand River. The Upper Mancos is rich in Pierre 

 fossils and the Mesaverde is 600 to 2,500 feet thick, the variation 

 being due to erosion preceding deposition of newer formations. The 

 upper part or undififerentiated Mesaverde, about 2,000 feet thick, is 

 of fresh-water origin, mostly sandstone and contains little coal. It 

 rests on the Paonia shale, closely allied to it lithologically, and about 

 400 feet thick. This has plant remains, fresh-water mollusks and 

 important coal beds. Underlying this and separated from it in a 

 considerable area by an unconformity, are the Bowie shales, o to 425 

 feet thick, with important coal seams and brackish-water as well as 

 marine invertebrates. The basal deposit is the Rollins sandstone, 

 usually about 100 feet thick, white, massive, with Halyuienites major 

 and marine invertebrates — evidently the basal white sandstone ob- 

 served by Richardson in the Book Cliffs field. 



Lee recognized a distinct unconformity l^elow the Paonia ; ordi- 

 narily, that formation rests on the Bowie, but for a considerable 

 space in one portion of the region it overlies the Rollins. This leads 



'^ W. T. Lee, Bull. 510, 1912, pp. 19, 27, 45- 81, 82, 86, 92, 95, 98, 106-109, 

 182, 188. 



