Appalachian area in Pennsylvania and the Virginias. 43 



Along the easterty outcrop in southern Pennsjdvania this 

 group is not merely very thick, but it contains coarse sand- 

 stones with not a few beds of conglomerate. But westward and 

 southwestward to southward, the beds become less coarse, the 

 conglomerates practically disappear and the group is repre- 

 sented even in the Draper Mountain area almost wholly by fine 

 grained sandstones interstratified with shales. The variations 

 in thickness along definite lines are probably not so great as 

 the measurements at various points appear to suggest; on the 

 contrary, a comparison suggests that the changes along the 

 southeasterly border were not abrupt until considerably beyond 

 New River. In Smyth County, of Yirginia, the change begins, 

 and thence the group thins out so rapidly that it is recognized 

 only with doubt in Washington and Lee Counties, and its exist- 

 ence in Tennessee is at least questionable. 



These conditions appear to show that the land area at the 

 east was narrower in southern Virginia than in Pennsylvania 

 and at the same time less elevated ; that the streams were 

 shorter and had less rapid fall, so that the detritus brought 

 down by them was less in amount as well as less coarse. It is 

 worth noting that the variations of this detrital group are very 

 like those of the upper Devonian. 



Throughout, the Umbral, that part of the Appalachian gulf 

 lying within the present boundaries of Pennsylvania, was prac- 

 tically shallows throughout, except for brief epochs, during 

 which thin limestones were deposited. The easterly shore-line 

 in Pennsylvania and Virginia lay along the Blue Ridge, as dur- 

 ing the Vespertine, but -doubtless much farther toward the 

 southeast ; for in the most easterly areas spared by erosion, 

 those in the Anthracite region of Pennsylvania and those in 

 Montgomery and Pulaski Counties of Virginia, the Umbral 

 rocks are fine clayey shales or argillaceous sandstones and clay 

 beds, which contrast sharply with the well marked and often 

 conglomerate sandstones of the Vespertine in Pennsylvania. 

 No limestone "is shown in Virginia along the Walker or the 

 Saltville fault eastward or northeastward from the line of 

 Smyth County; nor is any seen in northern Virginia, aside 

 from petty streaks, east from the North Fork of the Potomac 

 River. 



The Umbral limestone is shown in Fayette County, of Penn- 

 sylvania, as well as in the adjoining county, Monongalia, in 

 West Virginia. It can be followed along an almost continuous 

 outcrop through Preston, Tucker, Randolph, Pocahontas, 

 Greenbrier, Monroe and Mercer Counties, of West Virginia, 

 into Tazewell, Smyth and Washington Counties, of Virginia. 

 The mass increases along these outcrops from a few feet in 

 Fayette County, of Pennsylvania, to more than 2100 feet in 



