JEFFERSON COUNTY. 721 



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 lies 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 ako a widespread and important coal eeam 

 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 Jefferton 

 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 

 descriised. In the most northern localities, where it is found, it occupies 

 the summits of the hills, and forms isolated patches separated by the 

 vallej^s 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 accu- 

 mulated. If it had not been removed by surface erosion in this region, 

 it is probable we should find it running to a 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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