NATURE OF COAL HORIZONS. l8l 



margin of the coal horizon, which at one time must have stood 

 near the sea level for a considerable period, are innumerable 

 basins separated from one another perhaps, yet to all appear- 

 ances formed contemporaneously. Now they may thicken into 

 sharply defined lenticular beds ; now thin out to mere films, or 

 disappear altogether ; and again further on they assume the 

 form of extensive lens-shaped sheets. During deposition, as 

 subsidence became too rapid or the sea too deep for the proper 

 accumulations of vegetable material, sediments were carried in 

 covering the plant beds. Or, if elevation -took place the old 

 swamps, already shut off from free access to the sea, were sub- 

 ject to the agencies of denudation and were partially or entirely 

 removed. As favorable physical conditions again set in the 

 same course of events might be repeated. 



In considering the relations of the different coal horizons to 

 one another an approximate parallelism may be made out. Not 

 a strict parallelism of the nature which Andrews^ claimed t© be 

 true in Ohio, and which Newberry^ subsequently stated to be 

 entirel}^ unsubstantiated by facts, but an approximate parallel- 

 ism in a broad way. 



There was apparently a germ of truth in the idea of the 

 first named author, though he was probably unfortunate in the 

 choice of a name for his theory. Moreover, none of his writings 

 indicate that he understood the problem in the way that recent 

 investigations reveal it. His statements all seem to show that, 

 while he was manifestly on the right path, only one side of the 

 subject had been presented to him, just as, quite recently, the 

 question has been discussed from the opposite extreme. 

 Andrews' views are perhaps best expressed in the following 

 paragraph taken from his papers on the subject: 



"I have never found the slightest proof of the formation of 

 a seam of coal over hills or high grounds. The parallelism of 

 the seams, of which further mention will be made, forbids it. 



'Geol. Sur. Ohio, Vol. I, p. 348. Columbus, 1873. 

 ^Geol. Sur. Ohio, Vol. I, p. 169. Columbus, 1874. 

 3Geol. Sur. Ohio, Vol. I, pp. 348-350. Columbus, 1873. 



