The Rivers mid Vcdleys of PeQinsylvania. 185 



fully persuaded of the gradual and systematic evolution of topo- 

 graphic forms, it is now desired, in studying the rivers and 

 valleys of Pennsylvania, to seek the causes of the location of the 

 streams in their present courses ; to go back if possible to the 

 early date when central Pennsylvania was first raised above the 

 sea and trace the development of the several river systems then 

 implanted upon it from their ancient beginning to the present 

 time. 



The existing topography and drainage system of the State will 

 first be briefly described. We must next inquire into the geo- 

 logical structure of the region, follow at least in a general way 

 the deformations and changes of attitude and altitude that it has 

 suffered, and consider the amount of denudation that has been 

 accomplished on its sui'f ace. We must at the same time bear in 

 mind the natural history of rivers, their morphology and devel- 

 opment ; we must recognize the varying activities of a river in 

 its youth and old age, the adjustments of its adolescence and 

 maturity, and the revival of its decrepit powers when the land 

 that it drains is elevated and it enters a new cycle of life. 

 Finally we shall attempt to follow out the development of the 

 I'ivers of Pennsylvania by applying the general principles of 

 river history to the special case of Pennsylvania structure. 



2. General description of the topography of Pennsylvania. — 

 The strongly marked topographic districts of Pennsylvania can 

 hardly be better described than by quoting the account given 

 over a century ago by Lewis Evans, of Philadelphia, in his 

 " Analysis of a map of the middle British colonies in America " 

 (1755), which is as valuable from its appreciative perception as it 

 is interesting from its early date. The following paragraphs are 

 selected from his early pages : 



" The land south westward of Hudson's River is more regularly 

 divided and into a greater number of stages than the other. 

 The first object worthy of regard in this part is a rief or vein of 

 rocks of the talky or isinglassy kind, some two or three or half a 

 dozen miles broad ; rising generally some small matter higher 

 than the adjoining land ; and extending from New York city 

 southwesterly by the lower falls of Delaware, Schuylkill, Susque- 

 hanna, Gun-Powder, Patapsco, Potomack, Rapahannock, James 

 river and Ronoak. This was the antient maritime boundary of 

 America and forms a very regular curve. The land between this 



rief and the sea and from the Navesink hills southwest 



may be denominated the Lower Plains, and consists of soil washt 

 down from above and sand accumulated from the ocean. Where 



