OF THE APPALACHIAN CHAIN. 491 



vicinity of Cumberland, and continues, as the mountain aug- 

 ments in breadth and height, in its extension to the southwest. 

 Still further in that direction, beyond the intersection of the axis 

 mth the North Fork of the Potomac river, in Pendleton county, 

 Virginia, the dips on the northwestern side of the arch become 

 either perpendicular, or slightly inverted ; and this attitude they 

 retain for a further distance of about forty miles. Traced from 

 its first appearance, a little southeast of Cumberland, to its termi- 

 nation in the anticlinal valley of Crab Bottom, this axis offers a 

 beautiful illustration of the prevailing regular gradation, in the 

 amount of inflection which the sti-ata have undergone, in different 

 portions of the line, as dependent on the varying intensity of the 

 elevating and bending force. At first, the lowest rocks, which the 

 axis exposes, are the red and calcareous shales (F. V,) or Clinton 

 group. Here the flexure, though more abrupt on the northwest 

 than on the .opposite side, does not exceed a moderately steep 

 normal curvature. Further to the southwest, where the next 

 inferior formation (F. IV, Shawungunk gi-it) emerges to the 

 surface, and expands, as we advance, giving an imposing breadth 

 and elevation to* the ridge, we find the northwestern part of the 

 arch so increased in steepness, that its dips are nearly vertical. 

 The axis, becoming still more developed as we proceed, the next 

 inferior formation (F. Ill, Hudson slates) now makes its appear- 

 ance, and rapidly expands into an anticlinal valley, which sepa- 

 rates the broad and lofty mountain range into two distinct ridges. 

 The strata of the northwestern of these crests have a vertical, and 

 even, sometimes, an inverted dip. Still further, in the same line, 

 a yet lower formation rises, the gi-eat lower Appalachian lime- 

 stone (F. II), and occupies a large portion of the breadth of this 

 anticlinal valley. The dip of the rocks in the northwestern ridge 

 now becomes, as might be anticipated, very frequently inverted. 

 Passing this culminating portion of the axis, its further prolonga- 

 tion to the southwest is marked by the foregoing phenomena, in 

 a converse order, until finally, near the head-waters of Back 

 Creek, the divided strata of the higher groups once more unite, 

 to form a gentle normal flexure, in the inconspicuous ridge at the 

 southwestern termination of the axis. 



