194 National Geographic Magazine. 



It may be concluded with fair probability that the folds began 

 to rise in the southeast, where they are crowded closest together, 

 some of them having begun here while coal marshes were still 

 forming farther west ; and that the last folds to be begun were 

 the fainter ones on the plateau, now seen in Negro mountain and 

 Chestnut and Laurel ridges. In consequence of the inequalities 

 in the force of compression or in the resistance of the yielding 

 mass, the folds do not continue indefinitely with horizontal axes, 

 but vary in height, rising or falling away in great variety. 

 Several adjacent folds often follow some general control in this 

 respect, their axes rising and falling together. It is to an unequal 

 yielding of this kind that we owe the location of the Anthracite 

 synclinal basins in eastern Pennsylvania, the Coal Measures being 

 now worn away from the prolongation of the synclines, which 

 rise in either direction. 



8. Perm-Triassic denudation. — During and for a long time 

 after this period of mountain growth, the destructive processes 

 of erosion wasted the land and lowered its surface. An enormous 

 amount of material was thus swept away and laid down in some 

 unknown ocean bed. We shall speak of this as the Perm-Triassic 

 period of erosion. A measure of its vast accomplishment is seen 

 when we find that the Newark formation, which is generally cor- 

 related with Triassic or Jurassic time, lies unconformably on the 

 eroded surface of Cambrian and Archean rocks in the southeastern 

 part of the State, where we have concluded that the Paleozoic 

 series once existed ; where the strata must have risen in a great 

 mountain mass as a result of the Appalachian deformations ; and 

 whence they must therefore have been denuded before the depo- 

 sition of the Newark beds. Not only so ; the moderate sinuosity 

 of the southeastern or under boundaiy of the Newark formation 

 indicates clearly enough that the surface on which that portion 

 of the formation lies is one of no great relief or ineqxiality ; and 

 'such a surface can be carved out of an elevated land only after 

 long continued denudation, by which topographic development 

 is carried beyond the time of its greatest strength or maturity 

 into the fainter expression of old age. This is a matter of some 

 importance in our study of the development of the rivers of 

 Pennsylvania ; and it also constitutes a good part of the evidence 

 already referi*ed to as indicating that there must have been some 

 earlier deformations of importance in the southeastern part of 

 the State ; for it is hardly conceivable that the great Paleozoic 



