THE CARBONIFEROUS SYSTEM. 169 
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 water 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. 8. 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. 
THH PARALLELISM OF COAL SEAMS. 
In the “Conclusions” appended to his report contained in Volume L., 
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 
shali confine myself to a brief statement of some of these which appear 
