138 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



plains of China. The drainage appears to have been irregular and 

 shifting, the deposits are variable in form and composition, and 

 except in a few localities, widely separated, the coal seams are thin. 

 The periods, during which coal accumulation was possible in any 

 locality, were usually brief ; but in the northern part of the San Juan 

 Basin, one seam attains the thickness of lOO feet and in the Edmon- 

 ton district of Alberta the seams are not only thick but, unlike the 

 seam in the San Juan, they yield coal of excellent quality. 



The Fox Hills, underlying the fresh-water Laramie, is recogniz- 

 able as a persistent sandstone with intercalated shales and coal 

 seams. It resembles a low-lying strand of vast extent, frequently 

 invaded for considerable periods by the sea, so that it has an off- 

 shore fauna, which is of strangely persistent type. This is passage 

 from the continental conditions of the Laramie to the marine con- 

 ditions of the Pierre. The coal seams, yielding better fuel than that 

 from the Laramie seams, are thin and variable at most localities, 

 but at times in considerable areas, some of them become thick and 

 of great economic importance. Merely insignificant seams occur in 

 the San Juan Basin except at the north, where two, 4 and 12 feet 

 thick, are present in the shales immediately overlying the Pictured 

 Cliffs sandstone. In the Green River Basin, the Adaville seam has 

 a maximum thickness of 84 feet, but the seams become thin east- 

 wardly and there are great spaces in which the formation seems to 

 be barren. In central and eastern Wyoming as well as in Montana 

 and Alberta, only occasional exposures of coal have been reported 

 and those are unimportant. In the basins along the eastern foot of 

 the Front Ranges in New Mexico, the seams are numerous and 

 some horizons are extremely productive along this line of more 

 than 300 miles ; but the individual seams are variable to the last de- 

 gree in thickness and quality, there being many spaces where the 

 coal is either wanting or worthless. 



The Pierre at the west and southwest is, for the most part, a 

 mass of sandstone and sandy shale ; toward the east, it becomes 

 shale at top and bottom, while Middle Pierre o"r Mesaverde persists 

 as a wedge of sandstone and shale thinning eastwardly until it be- 

 comes replaced wholly with fine shales and irregular limestones. 

 This wedge thins away unbroken in Colorado and New Mexico but 



