OF THE APPALACHIAN CHAIN. 495 



accompanying this paper. Sections D and E represent (at a) the 

 conditions prevailing in the prolonged fault on the northwest side 

 of the axis of the Sweet Spring Valley. This axis, in its normal 

 state, brings up the great lower Appalachian limestone, flanked 

 on the northwest by the overlying slate and sandstone, which, 

 together with the northwestern half of the limestone, have a steep 

 northwestern dip. More towards the southwest, this dip aug- 

 ments ; the strata on the northwest side of the axis soon become 

 vertical, and thence quicldy pass into the inverted position. At 

 this point, the fault begins, being marked, at first, by the disap- 

 pearance of a portion of the slates (For. Ill) and variegated 

 shales (For. V), adjoining the thick-bedded sandstone (For. IV), 

 which forms the framework of the ridge, that bounds the anticli- 

 nal valley on the northwest. It presents, as it extends southwest- 

 ward, a continually augmenting hiatus in the geological series, 

 ingulfing in succession nearly all the strata between the limestone 

 of the axis, and the carboniferous limestone, and exhibiting an 

 inversion of the latter, for some distance across to the northwest 

 of the line of fault. The inversion of the strata near the disloca- 

 tion on its northwest side, giving them a sovitheasterly dip, be- 

 comes less as we recede from the fault. By a gradual transition, 

 the dips become perpendicular, then steeply inclined to the north- 

 west, and eventually, at no great distance, very gently so ; after 

 which, a few broad and -feeble undulations succeed, as we pass 

 into the coal region. Tracing this line of fault, in a southwest- 

 erly du-ection, for a distance of upwards of one hundred miles, we 

 encounter, at various points, portions of the ingulfed strata, which 

 occasionally reappear to form isolated knobs or short ridges, in- 

 closed between the two great limestone formations (F. II, and 

 F. XI), the crushed edges of which, however, are usually not thus 

 separated. The detached masses, so curiously wedged into the 

 chasm of the fault, consist of small remnants of the thick slate 

 group, which underlies, at some interval, the carboniferous lime- 

 stone, and of the hard white sandstone (F. IV, Shawungunk 

 grit), which constitutes^ as it were, the bony skeleton of our prin- 

 cipal Appalachian ridges. 



Sections C, D, and E, show (at c) the conditions usually 



