THE LOWER COAL MEASURES. 135 
time, but it suffered a partial relapse during the later Devonian period, 
in which the Ohio Black Shale (Huron of Newberry) was formed. But 
early in the Sub-carboniferous age it had made a great and permanent 
gain, and had transformed the western half of the area of the State 
into dry land. The Berea Grit, a Sub-carboniferous formation, which 
now extends in an unbroken wall from the Ohio valley to Lake Erie, 
through the central portion of the State, is as well-marked a shore line 
as was ever left by a retreating sea. 
The Cincinnati Axis had now become connected, virtually at least, 
with the northern continental nucleus, and the subsequent history of 
the eastern half of the State depends upon the joint advance of these 
land masses, the western and northern borders of the gulf. Both seem 
to have extended themselves in the same manner by a slow and nearly 
uniform rise of the border, accompanied by a corresponding movement 
of depression in front of the advancing land. The result was the 
gradual expulsion of the sea from this northern arm of the gulf, and it 
also followed that each new and well-marked shore line must be found 
interior to that which had preceded it. Thus, for example, the western 
outcrop of the Shenango sandstone, the Waverly Conglomerate of Ohio, 
which marks a shore line, and which belongs 300 to 500 feet above the 
Berea Grit, is found between 10 and 20 miles to the east, and south of 
the western outcrop of the Grit. In like manner the lowest coal seam 
was formed around the margin of a sea which had left those earlier forma- 
tions behind it. The later coals never extended over the outside 
margins of the earlier swamps. If there had been continued subsidence 
without this corresponding elevation during the growth of the coal 
seams, the later seams would occupy constantly widening areas, but the 
contrary is true. The outcrop of the horizon of the Sharon coal passes 
through a circuit of 13 counties, and its length, exclusive of the sinuosi- 
ties due to erosion, will not fall far below 275 miles. The outcrop of 
the Pittsburgh coal passes through 9 counties, with an approximate 
length of 175 miles. At the time when the Sharon coal was forming, 
the area of the gulf in Ohio was not less than 10,000 square miles. In 
the time of the Pittsburgh coal, the area was reduced to less than 6,000 
square miles. ) 
All of the coal seams of Ohio below the Freeport horizon, and a 
number above, appear to have been formed as marginal swamps around 
the border of the sea. The earliest suggestion of this view seems to 
