The Rivers cmd Valleys of Penvisylvcmia. 225 



where the anticlines are still ridges and the longitudinal streams 

 still follow the synclinal troughs ; while the transverse streams 

 cross from one synclinal valley to another at points where the 

 intervening anticlinal arches are lowest.* We could hardly ask 

 for better illustration of the deductive drainage system of our 

 early Appalachians than is here presented. 



27. Development and adjustment of the Perm,ian drainage. — : 

 The problem is now before us. Can the normal sequence of 

 changes in the regular course of river development, aided by the 

 post-Permian deformations and elevations, evolve the existing 

 rivers out of the ancient ones ? 



In order to note the degree of comparison that exists between 

 the two, several of the larger rivers of to-day are dotted on the 

 figure. The points of agreement are indeed few and small. 

 Perhaps the most important ones are that the Broad Top region 

 is drained by a stream, the Juniata, which for a short distance 

 follows near the course predicted for it ; and that the Nittany 

 district, then a highland, is still a well-marked divide although now 

 a lowland. But there is no Anthracite river, and the region of 

 the ancient coal-basin lakes is now avoided by large streams ; con- 

 versely, a great river — the Susquehanna — appears where no con- 

 sequent river ran in Permian time, and the early synclinal streams 

 frequently turn from the structural troughs to valleys located on 

 the structural arches. 



28. Lateral water gaps near the apex of synclinal ridges. — One 

 of the most frequent discrepancies between the hypothetical and 

 actual streams is that the latter never follow the axis of a descend- 

 ing syncline along its whole length, as the original streams must 

 have done, but depart for a time from the axis and then return 

 to it, notching the ridge formed on any hard bed at the side 

 instead of at the apex of its curve across the axis of the syncline. 

 There is not a single case in the state of a stream cutting a gap 

 at the apex of such a synclinal curve, but there are perhaps hun- 

 dreds of cases where the streams notch the curve to one side of 

 the apex. This, however, is precisely the arrangement attained 

 hj spontaneous adjustment from an initial axial course, as indi- 

 cated in figure 13. The gaps may be located on small transverse 

 faults, but as a rule they seem to have no such guidance. It is 

 true that most of our streams now run out of and not into the 



* This is beautifully illustrated in the recent monograph by La Noe 

 and Margerie on " Les Formes du Terrain." 



