Similarly a tributary to the main stream at _b_ (Fig. 150), may by means of its tributary _h_, capture the waters of _fg_, and lead them to the synclinal valley at _b_ (compare Figs. 150 and 151). Deprived of its main source of supply (at _c_) the synclinal stream is greatly diminished above _b_, and cuts more and more slowly, while the stream _fgh_ (Fig. 151), having greater volume and working mainly in softer rock, sinks its channel faster than the stream in the synclinal axis. Under these circumstances, the stream at _f_ may cut its valley below the valley in the synclinal axis _a_ (Fig. 150). In this event, the divide between _f_ and _a_ (Fig. 150) may be pushed back until the synclinal stream is beheaded at _a_ and carried out of the syncline and over into the anticlinal valley (Fig. 151). Thus, the old anticlinal axis comes to be the course of the main stream. Similarly the stream entering the syncline at _b_ (Fig. 151) might later be captured by _i_, thus lengthening its anticlinal course. It is not to be understood that this sequence of events will take place in the degradation of every anticline, but the principles here set forth will always be operative. The result specified will be accomplished wherever hard and soft layers have the relations indicated in the diagrams; that is, where the stream in the syncline finds itself on a resistant layer as it approaches base-level, while at the same time the (original) tributary streams are working in softer beds. It is not to be understood, therefore, that streams migrate from synclines to anticlines for the sake of getting out of the former positions into the latter. If they shift their courses it is to find easier ones. That these changes are not fanciful is shown by the fact that the adjustment described corresponds with that shown in many parts of the Appalachian Mountains, and in other mountains of similar structure. If in a later stage of its history, the new main stream, _fh_, were to cut its bed down to a lower hard layer, while the original stream, _ab_, reached a softer bed beneath the hard one above, the latter would again have an advantage, and a new series of adjustments would be inaugurated which might result in re-establishing the main stream in its original synclinal position. EFFECT OF CHANGES OF LEVEL.