242 National Geograjptiic Magazine. 



streams that once joined the main stream on the line of some of 

 the softer northeast-southwest beds, leaving the stronger beds as 

 faint hills on either side, must have forgotten such control after 

 it was baselevelled and buried ; as the flood plain grew, they 

 properly took more and more distinctly downward deflected 

 courses, and these deflections should be maintained in subsequent 

 cycles as superimposed courses independent of structural guid- 

 ance. Such I believe to be the fact. The downstream deflection 

 is so distinctly a peculiarity of a number of tributaries that join 

 the Susquehanna on the west side (see figure 1) that it cannot be 

 ascribed to accident, but must be referred to some systematic 

 cause. Examples of deflection are found in Peon's creek. Middle 

 creek and North Mahantango creek in Snyder county ; West 

 Mahantango between the latter and Juniata county ; and in the 

 Juniata and Little Juniata rivers of Perry county. On the other 

 side of the Susquehanna, the examples are not so distinct, but the 

 following may be mentioned : Delaware and Warrior runs, Chil- 

 lisquaque creek and Little Shamokin creek, all in Northumberland 

 county. It may be remarked that it does not seem impossible 

 that the reason for the more distinct deflection of the western 

 streams may be that the Susquehanna is at present east of its old 

 course, and hence towards the eastern margin of its flood plain, 

 as, indeed its position on the Pocono synclinals implies. A reason 

 for the final location of the superimposed river on the eastern side 

 of the old flood plain may perhaps be found in the eastward tilting 

 that is known to have accompanied the elevation of the Cretace- 

 ous lowland. 



It follows from the foregoing that the present lower course of 

 the Susquehanna must also be of superimposed origin; for the 

 flood plain of the middle course must have extended down stream 

 to its delta, and there have become confluent with the sheet of 

 Cretaceous sediments that covered all the southeastern lowland, 

 over which the sea had transgressed. McGee has already pointed 

 out indications of superimposed stream courses in the south- 

 eastern part of the State ;* but I am not sure that he would regard 

 them as of the date here referred to. 



The theory of the location of the Susquehanna on the Pocono 

 synclinal ridges therefore stands as follows. The general position 

 of the river indicates that it has been located by some process of 

 slow self-adjusting development and that it is not a persistent 



* Amer. Journ. Science, xxxv, 1888, 121, 134. 



