158 J. J. STEVENSON CARBONIFEROUS OF APPALACHIAN BASIN 



ness of the movement are shown by the structure of this great bed, per- 

 sisting in such minute details as partings ia tracts of thousands of miles 

 and reappearing even in isolated patches within West Virginia. 



The area of greatest subsidence during the Monongahela did not coin- 

 cide with that of the earlier formations, as appears abundantly from com- 

 parison of sections along several lines. The deepest deposits of Alle- 

 gheny and Conemaugh were at the north and east; not so in the Monon- 

 gahela. Going west-northwest from the eastern outcrop in Maryland, 

 the measurements are 250 feet in the Potomac basin, 316 in the Connels- 

 ville, 375 to 380 in Greene county of Pennsylvania, and 213 in Belmont 

 county, on the western outcrop in Ohio. Along a south-southwest line 

 near the middle of the basin one finds 150 feet in Allegheny county of 

 Pennsylvania, 225 in northern Washington, 355 in southern Washington,- 

 360 to 375 in Greene, and 428 in western Marion of West AT'irginia. 

 Similar variations appear along a west-northwest line farther south in 

 West Virginia as well as along a south-southwest line farther west in 

 the basin. The greatest subsidence was in north central West Virginia, 

 whence the thicloiess decreases in all directions. The change is due to 

 differential subsidence, as the intervals between the important members 

 of the section become thinner. The change bringing this about began 

 after deposit of the Pittsburg coal bed, for that is not overlapped toward 

 the outcrop by the higher beds; it certainly extends farther north than 

 do some of the later beds. The absence of that coal bed in so large an 

 area may be due merely to slight irregularities of the swampy surface, 

 for in some parts of that area the underclay with a black streak above it 

 marks the place of the bed. 



With this change in place of chief subsidence there came clearly a 

 farther contraction of the basin, while elevation at the north led to spread- 

 ing out of sandstone along much of the northern border. This Pittsburg 

 sandstone is not present in the eastern localities of Pennsylvania and 

 Maryland, but is persistent in the Chestnut Ridge area of Payette and 

 Westmoreland counties, in that state, as well as southward along the 

 eastern outcrop in West Virginia to the last exposure near Charleston, 

 where Doctor White found it 70 feet thick. Evidently it prevailed along 

 the western outcrop in Ohio, for it is present on the northwestern out- 

 crop and also in the central counties along that line, where one is again 

 much farther west than in the intervening counties. This sandstone be- 

 comes more and more indefinite from all sides toward the interior of the 

 basin. The Sewickley sandstone, underlying the Upper Sewickley coal 

 bed, is fairly persistent on the east side, but is wholly insignificant in 

 Ohio. There, however, an important sandstone overlies the Upper Se- 



