234 National Geographic Magazine. 



greatest where the dips are least, and hence most apparent at the 

 ends of the plunging anticlines and synclines. Some of the 

 Medina anticlines of Perry and Juniata counties are not indi- 

 cated because they were not then uncovered. The country 

 between the residual ridges of Jurassic time was chiefly Cam- 

 brian limestone and Siluro-Devonian shales and soft sandstones. 

 The moderate ridges developed on the Oriskany and Chemung 

 sandstones are not represented. The drainage of this stage 

 retained the original courses of the streams, except for the 

 adjustments that have been described, but the great Anthracite 

 river is drawn as if it had been controlled by the Newark depres- 

 sion and reversed in the direction of its flow, so that its former 

 upper course on the Cambrian rocks was replaced by a superim- 

 posed Newark lower course. Fig. 26 therefore represents the 

 streams for the most part still following near their synclinal axes, 

 although departing from them where they have to enter a syncli- 

 nal cove-mountain ridge ; the headwaters of the Juniata avoid 

 the mass of hard sandstones discovered in the bottom of old 

 Broad Top lake, and flow around them to the north, and then by 

 a cross-country course to the Wiconisco synclinal, as already 

 described in detail. Several streams come from the northeast, 

 entering the Anthracite district after the fashion generalized in 

 fig. 13. Three of the many streams that were developed on the 

 great Kittatinny slope are located, with their direction of flow 

 reversed ; these are marked Sq, L and D, and are intended to 

 represent the ancestors of the existing Susquehanna, Lehigh and 

 Delaware. We have now to examine the opportunities offered to 

 these small streams to increase their drainage areas. 



The Jurassic elevation, by which the Newark deposition was 

 stopped, restored to activity all the streams that had in the 

 previous cycle sought and found a course close to baselevel. 

 They now all set to work again deepening their channels. But 

 in this restoration of lost activity with reference to a new base- 

 level there came the best possible chance for numerous re-arrange- 

 ments of drainage areas by mutual adjustment into which we 

 must inquire. 



I have already illustrated what seems to me to be the type of 

 the conditions involved at this time in figs. 19 and 20. The 

 master stream, A, ti-a versing the synclines, corresponds to the 

 reversed Anthracite river ; the lowlands at the top are those 

 that have been opened out on the Siluro-Devonian beds of the 



