The Rivers and Valleys of Pen^isylvania. 221 



method thus indicated and attempt to trace out the history of a 

 completely original, consequent system of drainage accordant 

 with the growth of the central mountain district. 



In doing this, it is first necessary to restore the constructional 

 topography of the region ; that is, the form that the sui'face 

 would have had if no erosion had accompanied its deformation. 

 This involves certain postulates which must be clearly conceived 

 if any measure of confidence is to be gained in the results based 

 upon them. 



24. Post-dates of the argument. — In the first place, I assume 

 an essential constancy in the thickness of the paleozoic sediments 

 over the entire area in question. This is warranted here because 

 the known variations of thickness are relatively of a second 

 order, and will not affect the distribution of high and low ground 

 as produced by the intense Permian folding. The reasons for 

 maintaining that the whole series had a considerable extension 

 southeast of the present margin of the Medina sandstone have 

 already been presented. 



In the second place, I shall assume that the dips and folds of 

 the beds now exposed at the surface of the ground may be pro- 

 jected upwards into the air in order to restore the form of the 

 eroded beds. This is certainly inadmissible in detail, for it can- 

 not be assumed that the folded slates and limestones of the 

 Nittany vailey, for instance, give any close indication of the 

 form that the coal measures would have taken, had they extended 

 over this district, unworn. But in a general way, the Nittany 

 massif was a complex arch in the coal measures as well as in the 

 Cambrian beds ; for our purpose and in view of the moderate 

 relief of the existing topography, it suffices to say that wherever 

 the lower rocks are now revealed in anticlinal structure, there 

 was a great upfolding and elevation of the original surface ; and 

 wherever the higher rocks are still preserved, there was a relatively 

 small elevation. 



In the third place, I assume that by reconstructing from the 

 completed folds the form which the country would have had if 

 vinworn, we gain a sufficiently definite picture of the form through 

 which it actually passed at the time of initial and progressive 

 folding. The difference between the form of the folds com- 

 pletely restored and the form that the surface actually reached is 

 rather one of degree than of kind ; the two must correspond in 

 the general distribution of high and low ground and this is the 



