The Rivers and Valleys of Pennsylvania. 243 



antecedent river ; and yet there is no reason to think that it could 

 have been brought into its present special position by any process 

 of shifting divides. The processes that have been suggested to 

 account for its special location, as departing slightly from a loca- 

 tion due to slow adjustments following an ancient consequent 

 origin, call for the occurrence of certain additional peculiarities 

 in the courses of its tributary streams, entirely .unforeseen and 

 unnoticed until this point in the inquiry is reached ; and on 

 looking at the map to see if they occur, they are found with 

 perfect distinctness. The hypothesis of superiraposition may 

 therefore be regarded as having advanced beyond the stage of 

 mere suggestion and as having gained some degree of confirmation 

 from the congelations that it detects and explains. It only remains 

 to ask if these correlations might have originated in any other 

 way, and if the answer to this is in the negative, the case may be 

 looked upon as having a fair measure of evidence in its favor. 

 The remaining consideration may be taken up at once as the first 

 point to be examined in the Tertiary cycle of development. 



37. Events of the Tertiary cycle. — The elevation given to the 

 region by which Cretaceous basele veiling was terminated, and 

 which I have called the early Tertiary elevation, offered oppor- 

 tunity for the streams to deepen their channels once more. In 

 doing so, certain adjustments of moderate amount occurred, 

 which will be soon examined. As time went on, much denudation 

 was effected, but no wide-spread baselevelling was reached, for 

 the Cretaceous crest lines of the hard sandstone ridges still exist. 

 The Tertiary cycle was an incomplete one. At its close, lowlands 

 had been opened only on the weaker rocks between the hard beds. 

 Is it not possible that the flood-plaining of the Susquehanna and 

 the down-stream deflection of its branches took place in the 

 closing stages of this cycle, instead of at the end of the previous 

 cycle ? If so, the deflection might appear on the branches, but 

 the main river would not be transferred to the Pocono ridges. 

 This question may be safely answered in the negative ; for the 

 Tertiary lowland is by no means well enough baselevelled to 

 permit such an event. The beds of intermediate resistance, the 

 Oriskany and certain Chemung sandstones, had not been worn 

 down to baselevel at the close of the Tertiary cycle ; they had 

 indeed lost much of the height that they possessed at the close of 

 the previous cycle, but they had not been reduced as low as the 

 softer beds on either side. They were only reduced to ridges of 



