Devonian Era in the Ohio Basin. — Claypole. 39 
sylvania, parts of Maryland and Virginia, with an unknown 
extension into the now folded region beyond these limits. Its 
amount varied but along the most easterly line of outcrop 
where the strata are seen for the last time, it cannot have been 
less than several hundred feet — the Marcelhis black shale 
ranging from 250 feet in the middle counties to 800 feet on the 
Maryland line,* if the whole of the mass of black shale is 
rightly assigned to it. Consequently the southern bight of the 
gulf was considerably deepened into the land, regaining some- 
what of the dimensions which it had possessed in Silurian days. 
At the same time, the fineness of the Marcellus deposit 
confirms the view already presented that the former elevation 
of the land had been but slight. But during, or, perhaps, at the 
beginning of the period now under discussion, the inner por- 
tion of the Appalachian gulf subsided to the above extent, 
forming a deep basin in which were laid down the finer wash- 
ings of the eastern and southern lands so saturated with or- 
ganic matter as to become of an almost coaly blackness. 
In another direction a somewhat similar hypsographic 
change is indicated. The long but undefined Laurentian chan- 
nel to the northeast, whereby communication was probably 
maintained with the European seas, and which had been ap- 
parently somewhat contracted even in Corniferous days, seems 
to have been now completely closed by an elevation which affect- 
ed a considerable part of Xew England and of Canada adjoin- 
ing it. This led to the erosion which continued into succeeding 
periods. The movement which had prevailed in the south dur- 
ing the closing ages of the Silurian era was now transferred 
to the northeast and there it persisted until its effects became 
as marked and as extensive as those already described. 
The middle part of the Laurentian channel probably first 
felt the elevating force and was raised above the water-level, 
leaving the two ends as deep gulfs running one to the northeast 
from the present Hudson valley and the other to southeast 
from what is now the gulf of St. Lawrence. Continued and 
extending elevation increased the area of the new land and, by 
developing rivers, converted the two gulfs into estuaries which 
were gradually filled with the deposit of the streams. In later 
times even this has been again removed, and now only traces 
* Stevenson, Geol. Sur. of Penna., 12, p. 82. 
