160 GEOLOGY. 



was presumably subject at the time of folding, the tributary streams 

 might cut through the hard layer sooner than the main stream which 

 they join. This done, they would enlarge their valleys rapidly in 

 the softer rock beneath, and secondary tributaries would be developed 

 (Fig. 149). When the condition of things represented in Fig. 149 

 is reached, the streams c and d, tributary to the synclinal stream, comp 

 into competition. The former has the advantage over the latter, 

 because it joins the main stream at a lower level. Stream c will there- 

 fore be likely to capture d. The incipient stages of the capture are 

 stealthy, and the later bold. At first the divide between their head 

 waters is shifted northward inch by inch, because the gradient toward 

 g is higher than that toward e. The capture of the head waters of e 

 is as slow as the migration of the divide, until the divide reaches the 

 point where e joins /. The stream / is then diverted promptly into the 

 valley of g, and is at once led away to c (see Fig. 150). Strengthened 

 by its increased volume, the stream c (Fig. 150) lowers its valley across 

 the hard layer more rapidly than before, and so holds the advantage it 

 has gained. Not only this, but the beheaded stream d (Fig. 150), be- 

 cause of its diminished volume, sinks its valley into the hard layer less 

 rapidly than before, and its decrease in power also works to the ad- 

 vantage of the stream leading to c. The result is that the divide be- 

 tween jg and d does not remain constant, but is driven back step by 

 step toward a. 



Similarly a tributary to the main stream at h (Fig. 150), may by 

 means of its tributary h, capture the waters of jg, and lead them to 

 the syncHnal valley at h (compare Figs. 150 and 151). Deprived 

 of its main source of supply (at c) the synclinal stream is greatly 

 diminished above h, and cuts more and more slowly, while the stream 

 Jgh (Fig. 151), having greater volume and working mainly in softer 

 rock, sinks its channel faster than the stream in the svnclinal axis. Under 

 these circumstances, the stream at / may cut its valley below the valley 

 in the synclinal axis a (Fig. 150). In this event, the divide between 

 / and a (Fig. 150) may be pushed back until the syncHnal stream is 

 beheaded at a and carried out of the syncline and over into the anti- 

 clinal valley (Fig. 151). Thus, the old anticlinal axis comes to be 

 the course of the main stream. Similarly the stream entering the 

 syncline at h (Fig. 151) might later be captured by ^, thus lengthening 

 its anticlinal course. 



