144 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



where; the Mesaverde coals of Green River Basin attain commer- 

 cial importance in only one county ; in Montana the lenses are 

 usually small and thin ; in Alberta, the coals are present in a great 

 area, and often workable, but available details merely suggest, they 

 do not prove that the seams are lenses. 



Benton coals are present in only a small part of the Cretaceous 

 area, but, wherever they have been studied, the lens form is charac- 

 teristic. In southwestern Utah, in Castle Valley of that state, in 

 Gunnison Valley of Colorado and in Uinta County of Wyoming, 

 they are distinctly lenticular. The Dakota coals are merely insig- 

 nificant lenses. The Kootenai is without coal south from northern 

 Wyoming. There, within the Black Hills districts, coal lenses of 

 typical form are present but they are all small, nowhere embracing 

 more than a score of square miles. An occasional lens has been 

 found in the Bighorn Basin. The lenses are few and unimportant 

 in southwestern Montana ; they become numerous and some attain 

 workalble thickness in Lewistown and Great Falls fields ; but in 

 Teton County, on the northern border, there are only insignificant 

 nests. In Alberta, on the contrary, as well as in the adjacent part 

 of British Columbia, the seams are numerous and the quantity of 

 coal is enormous. Comparison of sections leaves no room for doubt 

 respecting the lenticular form of the seams. 



The lenses ordinarily show increase of foreign matters toward 

 the borders, the coal is broken by fine partings and very often it be- 

 comes at last merely carbonaceous shale with laminae of coal. Some- 

 times the lenses are connected by a stretch of black shale, but com- 

 monly no such bond exists and a barren space intervenes. These 

 lenses, great and small, are similar to peat deposits on broad river 

 plains and even more strikingly to those on coastal plains; at times, 

 these are separated by broad spaces, forested ; at others they are 

 united by carbonaceous muds, while at still others, the peat of sev- 

 eral lenses has become continuous by transgression. 



4. Contemporaneous Erosion. — The effects of contemporaneous 

 erosion are conspicuous. The curious intermingling of coal and 

 debris, observed at one locality in the Loewenberg area of Silesia, 

 seems to be explicable only by the supposition that it represents a 

 washed out swamp. The presence of coal grains in sandstone may 



