160 J. J. STEVENSON CARBONIFEROUS OF APPALACHIAN BASIN 



ceding page. But it is insiiffieient. The enormous thickness in an area 

 of almost 3,000 square miles, central in the northern part of the Monon- 

 gahela area, as it now exists, can hardly be explained in this way, as 

 there is no known source whence the calcareous mud could be derived by 

 erosion or by solution. One is shut off completely from consideration of 

 the Michigan Mississippian, for the Monongahela limestones disappear 

 wholly in that direction long before the outcrop has been reached. 

 Equally impossible is the supposition that they could have been derived 

 from the West Virginia Mississippian far toward the southeast. That 

 they accumulated as marls in fresh-water areas is equally difficult to be- 

 lieve, for the immense Benw.ood limestone on the Monongahela is 

 equivalent to very nearly the same thickness of shale and sandstone 

 within a very short distance. The magnesian beds, seemingly the most' 

 -persistent, can hardly be regarded as fresh-water limestones. Marine 

 origin seems questionable, owing to absence of marine invertebrates. It 

 is true that no careful search for fossils has been made in these rocks ; 

 yet the beds have been measured at so many places that some forms 

 should have been found, if such exist. Weathered surfaces of the harder 

 layers in the Benwood occasionally show what bear close resemblance to 

 sections of branching bryozoans, but in every case the fossil is replaced 

 by calcspar and is unrecognizable. Fish remains, teeth, and spines of 

 sharks occur, the most characteristic- being Ctenacanthus marshii. These 

 are certainly marine, and the specimens obtained were of such size as to 

 show that the surroundings were not unfavorable. Additional evidence 

 that the sea was not wholly shut out is the presence of Solenomya in 

 shale above the Sewickley coal bed in Monongalia county of West Vir- 

 ginia. Naiadites occurs in the Uniontown limestone at Uniontown, 

 Pennsylvania; but the ingress of seawater does not appear to have con- 

 tinued long enough to permit an invertebrate fauna to make its way. 

 For the present, the origin of the limestones must be regarded as an 

 unsolved problem. 



Toward the close of the Monongahela the condition marking the later 

 portion of the Conemaugh was reached once more. In by far the greater 

 part of the area the deposits are fine in grain, and at the end the Waynes- 

 burg coal bed was formed, in the northern part of the basin, a bed of 

 curiously multiple structure, which is retained. Like the Pittsburg, 

 it is wanting in the interior region, but it seems to have reached irregu- 

 larly southward to a long distance on each side. 



The Washington opens with a plant-bearing shale like that overlying 

 the Pittsburg, succeeded by a great sandstone, recalling in some respects 

 the sandstones of the Eockcastle. As the area grows smaller in ascend- 



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