238 National Geographic Magazine. 



the softer beds on its southern side ; FE of flg. 17 has been 

 diverted to FD of fig. 1 8. 



All of these examples are truly only special cases of the one 

 already described in which the Juniata left its original syncline 

 for others to the south. The general case may be stated in a few 

 words. A stream flowing along a syncline of hard beds (Carbon- 

 iferous sandstones) developes side streams which breach the adja- 

 cent anticlines and open lowlands in the underlying softer beds 

 (Devonian and Silurian). On these lowlands, the headwaters of 

 side streams from other synclines are encountered and a contest 

 ensues as to possession of the drainage territory. 'The divides 

 are pushed away from those headwaters whose lower course 

 leads them over the fewest hard barriers ; this conquest goes on 

 until the upper course of the initial main stream is diverted to a 

 new and easier path than the one it chose in its youth in obedience 

 to the first deformation of the region. Thus the Juniata now 

 avoids the center and once deepest part of the old Broad Top 

 lake, because in the general progress of erosion, lowlands on soft 

 Devonian beds were opened all around the edge of the great 

 mass of sandstones that held the lake ; the original drainage 

 across the lake, from its western slopes to its outlet just south of 

 the Jack's mountain anticline, has now taken an easier path along 

 the Devonian beds to the west of the old lake basin, and is seen 

 in the Little Juniata, flowing along the outer side of Terrace 

 mountain and rounding the northern synclinal point where Terrace 

 mountain joins Sideling hill. It then crosses Jack's mountain at 

 a point where the hard Medina sandstones of the mountain were 

 still buried at the time of the choice of this channel. In the 

 same way, the drainage of the subordinate basin, through which 

 the main lake discharged eastward, is now not along the axis of 

 the Juniata-Catawissa syncline, but on the softer beds along one 

 side of it ; and along the southern side because the easier escape 

 that was provided for it lay on that side, namely, via the Tus- 

 caroi'a and Wiconisco synclines, as already described. The much 

 broader change from the Anthracite to the Susquehanna was only 

 another form of the same process. Taking a transverse view of 

 the whole system of central, folds, it is perceived that their axes 

 descend into the Anthracite district from the east and rise west- 

 ward therefrom ; it is as if the whole region had received a slight 

 transverse folding, and the transverse axis of depression thus 

 formed defined the initial course of the first master stream. 



