The Rmers and Valleys of Pennsylvania. 235 



present Siisquehanna middle course between the Pocono and the 

 Medina ridges. The small stream, B, that is gaining drainage 

 area in these lowlands, corresponds to the embryo of the present 

 Susquehanna, Sq, fig. 25, this having been itself once a branch on 

 the south side of the Swatara synclinal stream, fig. 21, from 

 which it was first turned by the change of slope accompanying 

 the Newark depression ; but it is located a little farther west 

 than the actual Susquehanna, so as to avoid the two synclinal 

 cove mountains of Pocono sandstone that the Susquehanna now 

 traverses, for reasons to be stated below (section 35). This 

 stream had to cross only one bed of hard rock, the outer wall of 

 Medina sandstone, between the broad inner lowlands of the rela- 

 tively weak Siluro-Devonian rocks and the great valley lowlands 

 on the still weaker Cambrian limestones. Step by step it must 

 have pushed its headwater divide northward, and from time to 

 time it would have thus captured a subsequent stream, that crossed 

 the lowlands eastward, and entered a Carboniferous syncline by 

 one of the lateral gaps ah-eady described. With every such 

 capture, the power of the growing stream to capture others was 

 increased. Fig. 19 represents a stage after the streams in the 

 Swatara and Wiconisco synclines (the latter then having gained 

 the Juniata) had been turned aside on their way to the Carbon- 

 iferous basins. On the other hand, the Anthracite river, rising 

 somewhere on the plains north of the Wyoming syncline and 

 pursuing an irregular course from one coal basin to another, 

 found an extremely difficult task in cutting down its channel 

 across the numerous hard beds of the Carboniferous sandstones, 

 so often repeated in the rolling folds of the coal fields. It is also 

 im-portant to remember that an aid to other conditions concerned 

 in the diversion of the upper Anthracite is found in the decrease 

 of slope that its lower course suffered in crossing the coal fields, 

 if that area took any part in the deformation that produced the 

 Newark monocline — whichever theory prove true in regard to 

 the origin of the southeastward flow of the rivers — for loss of 

 slope in the middle course, where the river had to cross many 

 reefs of hai'd sandstone, would have been very effective in length- 

 ening the time allowed for the diversion of the headwaters. 



The question is, therefore, whether the retardation of down- 

 cutting here experienced by the Anthracite was sufficient to 

 allow the capture of its headwaters by the Susquehanna. There 

 can be little doubt as to the correct quality of the process, but 



