Tlie Rivers and Valleys of Pennsylvania. 253 



ture to mature courses in the several cycles of development are 

 so numerous and extensive that, as Lowl truly says, the initial 

 drainage has almost disappeared. The larger westward-flowing 

 streams of the plateau are of earlier. Carboniferous birth, and 

 have suffered little subsequent change beyond a loss of head- 

 waters. The lower courses of the Atlantic rivers are younger, 

 having been much shifted from their Permian or pre-Permian 

 courses by Newark and Cretaceous superimposition, as well as by 

 recent downward deformation of the surface in their existing estu- 

 aries. ISTo recognizable remnant of rivers antecedent to the Per- 

 mian deformation are found in the central part of the State ; and 

 with the exception of parts of the upper Schuylkill and of the Sus- 

 quehanna near Wilkes-Barre, there are no large survivors of Per- 

 mian consequent streams in the ordinary meaning of the term 

 " consequent." The shifting of courses in the progress of mature 

 adjustment has had more to do with determining the actual loca- 

 tion of our rivers and streams than any other process. 



Harvard College, June, 1889. 



