JEFFERSON COUNTY. 2m 
limestones, diminish rapidly in thickness towards the north, the inter- 
vals between the different limestones and coal seams becoming less, and 
the limestone strata conspicuously thinning. This shows conclusively 
that the margin of the basin in which the members of the Upper Coal 
Group were deposited was not far from the northern line of Jefferson . 
county. 
Coal No. 8.—This, as has been stated, is the great Pittsburgh seam, 
which is, on the whole, the most extensive and economically important 
coal bed in the Allegheny coal field. Its chief development hes east of 
the Ohio. It is the main coal seam worked at Pittsburgh, on the 
Youghiogeny and Monongahela Rivers, at Connellsville, Wheeling, and 
many other places. In western Pennsylvania it attains a maximum 
thickness of fourteen feet, and is estimated to underlie six to seven 
thousand square miles. It is alsoa widespread and important coal seam 
in Ohio, where it occupies three thousand to four thousand square miles, 
and here maintains a thickness of from four to six feet. The basin 
throughout which the Pittsburgh coal was originally continuous is 
deeply cut by the Ohio River, the valley of which shows parallel lines 
of outcrop on the opposite sides from near the north line of Jefferson 
county to the southern border of Belmont, where, by its southward dip, 
the coal is carried below the river, and the two outcrops join to form an 
unbroken sheet. This descends beneath the overlying strata of the 
Upper Coal Group and the Upper Barren Measures that form the surface 
rocks in south-eastern Ohio. Rising towards the west, it comes out in 
the vicinity of Pomeroy, and is the coal so extensively mined in that 
vicinity. 
The western line of outcrop of Coal No. 8 passes north-easterly through 
the eastern part of Gallia, Meigs, Athens, Morgan, Noble, Guernsey, and 
Harrison, to the southern part of Carroll, and the northern of Jefferson ; 
thence it curves round into the valley of the Ohio, as has already been 
described. In the most northern localities, where it is found, it occupies 
the summits of the hills, and forms isolated patches separated by the 
valleys of the draining streams. It is here thinner than, further south, 
as are also its associated limestones, and it is evident that we have 
reached nearly to the margin of the basin in which it originally aceu- 
mulated. If it had not been removed by surface erosion in this region, 
it 18 probable we should find it running toa feather edge within a few 
miles of what is now its most northerly outcrop. 
The quality and thickness of the Pittsburgh coal varies considerably,in 
different parts of the county. In the hills about Knoxville and Rich- 
mond it is from thirty inches to four feet in thickness, has generally 
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