140 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



are, in very large part, clays, clay shales and limestones, the last 

 serving occasionally as roof or floor to coal seams. The area must 

 have embraced not less than 2,000 square miles and its surface must 

 have been a broad mud flat during formation of the coal seams. It 

 was little above the sea-level. At 50 miles farther east, the condi- 

 tions are wholly difi:"erent, for there the coals are associated with 

 sandy deposits, as they are farther north. The relations appear to 

 give support to Gilbert's suggestion, offered more than 40 years ago, 

 that the Wasatch Mountains were the source whence the sediments 

 were derived. In that case, the conditions would be normal, for 

 the sluggish streams, carrying only fine materials, would build up 

 merely a mud flood plain, such as one sees at localities along the 

 Atlantic coast, on which peat deposits are accumulating. The de- 

 posits are largely sandstone in northeastern Arizona, where they 

 contain 3 coal seams near the base. Benton rocks in the southern 

 part of the San Juan Basin have about 66 per cent, of sandstone 

 and have 3 coal seams ; but the sandstone decreases northwardly and 

 the coal disappears. The condition is similar in the northern or 

 main portion of the basin. 



The Ferron sandstone of Castle Valley, Utah, at eastern base of 

 the Wasatch Mountains, contains many and irregular coal seams, of 

 which some are locally important ; but these are confined to the 

 southern part of the valley, where the sandstone is several hundred 

 feet thick; no trace of them remains in the northern portion, where 

 the sandstone has become thin. The Frontier sandstone contains 

 several seams, yielding excellent coal, in Uinta County of Wyo- 

 ming, but farther east the sandstone becomes thin and the coal disap- 

 pears. The Bear River formation, of fresh-water origin, has nu- 

 merous coal seams but it thins away rapidly toward the east. 



The Kootenai has no coal in the southern portions, the first ap- 

 pearance being in the Black Hills region of northeastern Wyoming; 

 there and in the Bighorn Basin of the same state the rocks are 

 chiefly sandstone and contain patches of coal, which are sources 

 for local supply ; but they are far apart in Wyoming as well as in 

 Montana, there being coal in only an insignificant part of the ex- 

 posed area. In Alberta and the adjacent portion of British Colum- 

 bia, the individual seams cover greater areas than in any part of the 



