i^40 National Geographic Magazine. 



in the beginning the theory that the Susquehanna was an antece- 

 dent river ; but as I have said at the outset of this inquiry, it 

 seems to me that such a method is not freer from assumption, 

 even though shorter than the one here adopted; and it has the 

 demerit of not considering all the curious details that follow the 

 examination of consequent and adjusted courses. 



The sufficient reason for the assumption that the embryonic 

 Susquehanna lay farther west than the present one in the neigh- 

 borhood of the Pocono synclinals is simply that — in the absence 

 of any antecedent stream — it must have lain there. The whole 

 explanation of the development of the Siluro-Devonian lowlands 

 between the Pocono and Medina ridges depends simply on their 

 being weathered out where the rocks are weak enough to waste 

 faster than the enclosing harder ridges through which the streams 

 escape. In this process, the streams exercise no control whatever 

 over the direction in which their headwaters shall grow ; they 

 leave this entirel}" to the structure of the district that they drain. 

 It thus appears that, under the postulate as to the initial location 

 of the Susquehanna as one of the many streams descending the 

 great slope of the Kittatinny (Cumberland) highland into the 

 Swatai-a syncline, its course being reversed from northward to 

 southward by the Newark depression, we are required to suppose 

 that its headwater (northward) growth at the time of the Jurassic 

 elevation must have been on the Siluro-Devonian beds, so as to 

 avoid the harder rocks on either side. Many streams competed 

 for the distinction of becoming the master, and that one gained 

 its ambition whose initial location gave it the best subsequent 

 opportunity. It remains then to consider the means by which 

 the course of the conquering Susquehanna may have been subse- 

 quently changed from the lowlands on to the two Pocono synclines 

 that it now traverses. Some departure from its early location 

 may have been due to eastward planation in its advanced age, 

 when it had large volume and gentle slope and was therefore 

 swinging and cutting latei'ally in its lower course. This may 

 have had a share in the result, but there is another process that 

 seems to me moi'e eifective. 



In the latter part of the Jura-Cretaceous cycle, the whole 

 country hereabout suffered a moderate depression, by which the 

 Atlantic transgressed many miles inland from its former shore- 

 line, across the lowlands of erosion that had been developed on 

 the litoral belt. Such a depression must have had a distinct effect 



