APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 239 



base above which the hypothetically restored folded section could be 

 measured. But the sea-level undoubtedly did not remain stationary 

 through such a long period of time. Such a ponderous series of 

 mountain-masses must have been subject to some settling. Possibly 

 a mathematical analysis of the mechanics involved might furnish a 

 theoretical figure for the possible isostatic changes, but the problem 

 is quite beyond the reach of the present paper. 



Some rise of ocean-level is also to be expected on account of the 

 filling of the ocean-basin with the material removed from the moun- 

 tains in the process of peneplanation, and obviously sedimentation in 

 other portions of the oceanic area would be equally effective in chan- 

 ging the water-level; and hence the great ramification of the problem. 



Some crustal warping also appears to have occurred at the east 

 during the early Mesozoic. The Newark series of sediments, which 

 were laid down in local depressions in southeastern Pennsylvania 

 during the Triassic, seem to imply a certain amount of downwarping 

 in that region.' Later, at the close of the Triassic, or early in the 

 Jurassic, the deposition was stopped by the reversal of the conditions 

 which started it.^ A moderate elevation with tilting affected the 

 Newark beds. But to what extent these warpings in the eastern part 

 of the state affected the mountain-section under consideration is 

 uncertain, and whether the sum-total of the movements should be 

 regarded as upward or downward must remain, for the present, 

 largely a matter of conjecture. 



But in general the period of erosion and base-leveling which was 

 inaugurated by the mountain-building at the close of the Paleozoic 

 and which resulted in the Kittatinny plain was one of comparative 

 quiescence and, on account of its freedom from the more important 

 dynamic movements, the present case is probably as favorable as any 

 other which could be selected. 



It will therefore be assumed that when the folding commenced 

 the formations were practically horizontal and the upper surface of 

 the youngest beds essentially at sea-level. It will also be assumed 

 that the Kittatinny base-plain in turn represents somewhat approxi- 



• I Chamberlin and Salisbury, Geology, Vol. Ill, pp. 7-9. 

 2 W. M. Davis, "The Rivers and Valleys of Pennsylvania," Nat. Geog. Mag., 

 Vol. I (1889), p. 196. 



