THE CAEB05TIFEE0US SYSTEM. ] 69 



periods of subsidence when more or less of the Alleghany coal basin was 

 an arm of the sea opening into the Gulf of Mexico, and broadening and 

 deepening toward the south. This could not have been the case, how- 

 ever, as we should have found the limestone sheets becoming wider and 

 thicker in that direction. It therefore seems necessary to suppose that 

 the sea water had access to our coal basin from the south, through a long, 

 narrow channel or strait, or, what is quite as probable, by some lateral 

 inlet. In either case the broadest space of open Avater in periods of 

 submergence was as far north as southern Ohio, and while the lime- 

 stones were forming the northern part of. the trough was a land-locked 

 bay something like that of San Francisco, rather than an arm of the sea. 

 Prof. Stevenson informs me that in the " oil break " of West Virginia 

 and southern Ohio, where the Barren Measures are extensively exposed, 

 they contain no coals, and that the lower coal group is apparently ab- 

 sent, with the exception, perhaps, of the upper Freeport seam. This 

 gives farther illustration of the great diversity which prevailed in the 

 physical condition of different portions of the Alleghany basin, and it 

 may indicate, as Prof. S. suggests, that our lower coals were deposited in 

 a series of circles around the margins of the basin, only the upper coals 

 stretching across. This question can only be settled, however, by a long 

 series of patient and careful observations. 



THE PARALLELISM OF COAL SEAMS. 



In the " Conclusions " appended to his report contained in Volume I., 

 Prof. Andrews advances the theory that our important coal seams are 

 parallel to each other ; and that where, among seams supposed to be con- 

 tinuous, an absence of such parallelism is discoverable, this is evidence 

 of a want of continuity and identity in one or the other of those com- 

 pared. Prof. Andrews accounts for this claimed parallelism by supposing 

 that the different coal seams were formed at or near the line of water 

 level, and that the subsidences which have caused the accumulation of 

 successive layers of carbonaceous matter were continental and uniform. 

 To these views I have been unable to subscribe, inasmuch as I have 

 failed to detect the parallelism claimed, and, on the contrary, have, as it 

 seems to me, in numerous instances, discovered very marked inequality 

 in the distances that at different localities separate coal seams which are 

 unmistakably continuous. 



This matter is plainly one for observation, not for theory or argument, 

 and as the question will be inevitably settled by an appeal to facts, I 

 shall confine myself to a brief statement of some of these which appear 



