Tlie Rivers and Valleys of Pennsylvania. 197 



It is not to be supposed that the Jurassic deformation was 

 limited to the area of the Newai'k beds ; it may have extended 

 some way on either side ; but it presumably faded out at no 

 great distance, for it has not been detected in the history of the 

 Atlantic and Mississippi regions remote from the Newark belt. 

 In the district of the central folds of Pennsylvania, with which 

 we are particularly concerned, this deformation was probably 

 expressed in a fui'ther folding and over-pushing of the already 

 partly folded beds, with rapidly decreasing effect to the north- 

 west ; and perhaps also by slip-faults, which at the surface of the 

 ground nearly followed the bedding planes : but this is evidently 

 hypothetical to a high degree. The essential point for our sub- 

 sequent consideration is that the Jurassic deformation was prob- 

 ably accompanied by a moderate elevation, for it allowed the 

 erosion of the Newark beds and of laterally adjacent areas as 

 well. 



11. Jiira- Cretaceous denudation. — In consequence of this ele- 

 vation, a new cycle of erosion was entered upon, which I shall 

 call the Jura-Cretaceous cycle. It allowed the accomplishment of 

 a vast work, which ended in the production of a general lowland 

 of denudation, a wide area of faint relief, whose elevated rem- 

 nants ai*e now to be seen in the even ridge-crests that so strongly 

 characterize the central district, as well as in certain other even 

 uplands, now etched by the erosion of a later cycle of destructive 

 work. I shall not here take space for the deliberate statement of 

 the argument leading to this end, but its elements are as follows : 

 the extraordinarily persistent accordance among the crest-line 

 altitudes of many Medina and Carboniferous ridges in the central 

 district ; the generally corresponding elevation of the western 

 plateau surface, itself a surface of erosion, but now trenched by 

 relatively deep and narrow valleys ; the generally uniform and 

 consistent altitude of the uplands in the crystalline highlands of 

 northern New Jersey and in the South Mountains of Pennsyl- 

 vania ; and the extension of the same general surface, descending 

 slowly eastward, over the even crest-lines of the Newark trap 

 ridges. Besides the evidence of less continental elevation thus 

 deduced from the topography, it may be noted that a lower stand 

 of the land in Cretaceous time than now is indicated by the 

 erosion that the Cretaceous beds have suffered in consequence of 

 the elevation that followed their deposition. The Cretaceous 

 transgression in the western states doubtless bears on the problem 



