STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 143 



of shallow water and suggests the action of floods, which dropped 

 their load on the broad surface, which was exposed during the in- 

 tervals between floods. 



3. The Form of Coal Deposits. — Cretaceous coal seams are 

 lenses. No statement to this efl^ect occurs in any of the older works, 

 as nearly all students, prior to less than 25 years ago, held in a some- 

 what hazy way, that coal seams are continuous deposits. Compari- 

 son of sections in all fields proves that this conception was errone- 

 ous. The Wealden coals of Hannover are local, present in one sec- 

 tion, absent in others, and in all cases they have small areal extent. 

 There is a rather persistent coal horizon at the base, which seems to 

 be made up of overlapping lenses. The Lower Ouader has only 

 nests of coal, which occasionally become workable ; the Hungarian 

 coals are well-defined lenses as are those of Queensland ; and the 

 detailed studies in N^ew Zealand have proved lens form in the great 

 seams. 



The condition in North America is so marked that it has been 

 noted by the great majority of observers during later years. Occa- 

 sionally, a seam has an area so extensive that the describer is un- 

 willing to commit himself as to the form. But it must be remem- 

 bered that, even though the lenses have an area of hundreds or thou- 

 sands of square miles, the general features are the same with those 

 of smaller lenses, united by transgression to form the large one. 



The Laramie coals are in lenses, usually small and thin within 

 the United States ; the great bed of the Saskatchewan in Alberta 

 becomes only a thin deposit of carbonaceous shale in its southern 

 extension. The Fox Hills seams are lenses, usually thin or impure, 

 but locally important and workable in considerable areas. This 

 feature is noteworthy in all districts along the eastern base of the 

 Front Ranges in New Mexico, as well as the southern tier of coun- 

 ties of Wyoming. The Middle Pierre (Mesaverde) is probably the 

 most productive formation with usually one or more workable 

 seams; but its seams are like those of the newer formations. They 

 are variable and uncertain in New Mexico ; in the Uinta Basin, west 

 from Grand River, portions of the section, containing workable 

 coals in one district, are wholly barren in others ; east from that 

 river the coals are local, important here, unimportant or absent else- 



