MONONGAHELA FORMATION 31 



At one time this bed was thought to be a practically continiious sheet 

 underlying the whole remaining area of Monongahela, somewhat less 

 than 20,000 square miles; but studies by Professor E. B. Andrews dur- 

 ing the Second Geological Survey of Ohio proved its absence from a con- 

 siderable part of the field in that state and Doctor White's study of oil 

 well records in West Virginia, almost 20 years later, led to a similar 

 conclusion for much of that state. The northern boundary of this bar- 

 ren area is in Guernsey county of Ohio and the coal is to all intents 

 .wanting in Noble, Monroe, east Morgan, and most of Washington in 

 Ohio, most of Wetzel, Tyler, Pleasants, Wood, Eitchie, Doddridge, Gil- 

 mer, Eoane, Calhoun, Jackson, Clay, Kanawha, Putnam, and Mason 

 counties of West Virginia, all within the area in which formerly the 

 bed was supposed to exist. Yet, even when thus restricted, the coal 

 underlies a vast area, 7,000 or 8,000 square miles, in which it exhibits 

 for the most part such regularity in variation as to both quantity and 

 quality as to render it, from an economic point of view, the most im- 

 portant member of the formation and probably of the whole bituminous 

 coal measures. Within the barren area itself it is represented frequently 

 by black shale and occasionally it reappears abruptly as a bed of work- 

 able thickness. 



The Pittsburg coal bed in the ordinary condition, as seen in most of 

 Pennsylvania and Ohio, is double, showing a "roof" division separated 

 by an "over-clay" from the "Main" coal below. The "roof," varying 

 in thickness from a few inches to 10 or 12 feet, is composed sometimes 

 of laminations of coal and clay; at others, the coal and clay are segre- 

 gated into beds, each 6 inches to a foot, while again it is almost wholly 

 coal or almost wholly carbonaceous shale. At the best the coal is in- 

 ferior and is not mined. This division is wanting in the Salisbury 

 basin of Somerset county, Pennsylvania, and is seen rarely along the 

 eastern outcrop in West Virginia; but it reappears suddenly on the 

 Kanawha, where at one locality it attains to almost the extreme thick- 

 ness. 



The "over-clay," varying from a few inches to 2 feet, is thoroughly 

 persistent, though sometimes so carbonaceous as to be almost a bony 

 coal; even in West Virginia it seems to be present always, though the 

 "roof" is wanting. 



The "Main" coal may be regarded as in three benches — "Breast," 

 "Bearing-in," and "Bottom" — ^but in a great part of the field the 

 '^Breast" as well as the "Bottom" is divided by a thin parting, so that 

 five benches are distinct. The parting slates are ordinarily very thin, 

 rarely exceeding half an inch, often much less, and in considerable areas 



