The Rivers and Valleys of Pennsylvania. 227 



divergence of their margins demonstrates, and must for this 

 reason have carried the hard sandstones below baselevel in a 

 shorter distance and on a steeper slope than in the Catawissa 

 syncline. The further soiithwestward extension of the Pocono 

 sandstone ridges in the southern than in the northern syncline 

 gives further illustration of this peculiarity of form. Lateral 

 capture of the Juniata by a branch of the initial Tuscarora, and 

 of the latter by a branch of the Wiconisco therefore seems pos- 

 sible, and the accordance of the facts with so highly specialized 

 an arrangement is certainly again indicative of the correctness 

 of the hypothesis of consequent drainage, and this time in a 

 larger stream than before. At first sight, it appears that an 

 easier lateral capture might have been made by some of the 

 streams flowing from the outer slope of the Nittany highland ; 

 but this becomes improbable when it is perceived that the heavy 

 Medina sandstone would here have to be worn through as well as 

 the repeated arches of the Cai'boniferous beds in the many high 

 folds of the Seven Mountains. Again, as far as present appear- 

 ances go, we can give no sufficient reason to explain why posses- 

 sion of the headwaters of the Juniata was not gained by some 

 subsequent stream of its own, such as G, fig. 18, instead of by a 

 side-stream of the river in the neighboring syncline ; but it may 

 be admitted, on the other hand, that as far as we can estimate 

 the chances for conquest, there was nothing distinctly in favor of 

 one or the other of the side-streams concerned ; and as long as 

 the problem is solved indifferently in favor of one or the other, 

 we may accept the lead of the facts and say that some control 

 not now apparent determined that the diversion should be, as 

 drawn, through D and not through G. The detailed location of 

 the Juniata in its middle course below Lewistown will be con- 

 sidered in a later section. 



30. Avoidance of the Broad Top basin by the Juniata head- 

 waters. — Another highly characteristic change that the Juniata 

 has suffered is revealed by examining the adjustments that 

 would have taken place in the general topography of the Broad 

 Top district during the Perm-Triassic cycle of erosion. When 

 the basin, BT, fig. 22, was first outlined, centripetal streams 

 descended its slopes from all sides and their waters accumu- 

 lated as a lake in the center, overflowing to the east into the 

 subordinate basin, A, in the Juniata syncline along side of the 

 larger basin, and thence escaping northeast. In due time, the 



