68 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



upper surface being from 5 to 22 degrees ; but the top bench, also 

 bituminous, rests on the horizontal surface of the cannel and is regu- 

 lar throughout, as is also the roof, both showing only the insignificant 

 dip characterizing the region. In Pennsylvania, one rarely finds 

 cannel at the bottom of a coal bed, but that condition occurs occa- 

 sionally in West Virginia and it is not infrequent in Ohio. Some 

 coals of the Beaver within Ohio and Kentucky have considerable 

 areas of cannel and are spoken of as cannel beds; but even in those 

 the features are the same as in others, excepting as to extent. The 

 story is the same in all areas. Hull has shown that the celebrated 

 Wigan deposit in Lancashire is saucer-shaped ; Green found the 

 same condition in the Yorkshire deposits ; David, Mackenzie and 

 Wilkinson have recorded many observations showing that the Kero- 

 sene shale of New South Wales has similar distribution. The phe- 

 nomena are familiar in modern swamps. 



Distribution of Coal in Relation to the Accompanying Rocks. 



The distribution of coal seems to be related in some way to the 

 character of the associated rocks. In the southern and middle 

 anthracite fields, the coal beds are thick at the northeast, where 

 coarse rocks most abound, and become unimportant at the west, 

 where coarse rocks are less abundant. In the Pottsville of 

 those fields, there are thick coals with pebbly rock above and below, 

 though in most cases there is some shale, often very thin, above or 

 below the coal. In the bituminous region, coal beds of the Allegheny 

 and higher formations appear to have accumulated chiefly on the 

 borders of that region — not as continuous bands, but at definite 

 horizons. They thin away and the horizons become indefinite as one 

 approaches the central area, in which fine materials prevail ; yet even 

 there, coal was formed in thin irregular deposits at widely separated 

 localities ; and these petty accumulations seem to be at or near 

 horizons which are well defined elsewhere. Coal-making conditions 

 did not exist for any considerable period or in any considerable area 

 within the region of fine-grained rocks. 



The same relation has been observed in other countries. 



