The Rivers and Valleys of Pennsylvania. 23Y 



coal basins are now occupied by the large stream that originally 

 followed them. The reason for this is manifestly to be found in 

 the great depth of the Wyoming basin, whereby the axial portion 

 of its hard sandstones are even now below baselevel, and hence 

 have never yet acted to throw the river from its axial course. 

 Indeed, during the early cycles of denudation, this basin must 

 have been changed from a deep lake to a lacustrine plain by the 

 accumulation in it of waste from the surrounding highlands, and 

 for a time the streams that entered it may have flowed in mean- 

 dering courses across the ancient alluvial surface ; the lacustrine 

 and alluvial condition may have been temporarily revived at the 

 time of the Jurassic elevation. It is perhaps as an inheritance 

 from a course thus locally superimposed that we may come to 

 regard the deflection of the river at Nanticoke from the axis of 

 the syncline to a narrow shale valley on its northern side, before 

 turning south again and leaving the basin altogether. But like 

 certain other suggestions, this can only be regarded as an open 

 hypothesis, to be tested by some better method of river analysis 

 than we now possess ; like several of the other explanations here 

 offered, it is presented more as a possibility to be discussed than 

 as a conclusion to be accepted. 



I believe that it was during the earlier part of the great Jura- 

 Cretaceous cycle of denudation that the Susquehanna thus be- 

 came the master stream of the central district of the state. For 

 the rest of the cycle, it was occupied in carrying off the waste 

 and reducing the surface to a well finished baselevel lowland that 

 characterized the end of Cretaceous time. From an active youth 

 of conquest, the Susquehanna advanced into an old age of estab- 

 lished boundaries ; and in later times, its area of drainage does 

 not seem to have been greatly altered from that so long ago 

 defined ; except perhaps in the districts drained by the West 

 and North Branch headwaters. 



34. Homologies of the Susquehanna and Juniata. — Looking 

 at the change from the Anthracite to the Susquehanna in a broad 

 way, one may perceive that it is an effect of the same order as 

 the peripheral diversion of the Broad Top drainage, illustrated in 

 figures 22, 23 and 24 ; another example of a similar change is 

 seen in the lateral diversion of the Juniata above Lewistown 

 and its rectilinear continuation in Aughwick creek, from their 

 original axial location when they formed the initial Broad Top 

 outlet. They have departed from the axis of their syncline to 



