104 



GEOLOGY. 



(Fig. 90) is said to be beheaded, and its upper portion, de, diverted. 

 The beheaded stream is diminished in volume; or if its total supply 

 of water came in above the point of tapping it would disappear 

 altogether. 



The process may not end even here. If after the diversion of de 

 the point in the channel to the left is lowered faster thar«. the channel 

 of the beheaded stream /, the divide between dg ar^d the head of / 

 (Fig. 90) will be shifted down the valley of the latter, as shown in 

 Fig. 91. The shifting will go on until the divide reaches a position 

 of stability, that is, until erosion on its opposite sides is equal. 



The foregoing case may be called foreign 'piracy because the valleys 

 of different systems are concerned. Domestic piracy may also take 



Fig. 91. — Diagram showing the shifting of a divide after piracy. 



place, as illustrated in the accompanying diagrams (Figs. 92 and 93). 

 Here a tributary to a crooked river may develop, working back until 

 it taps the main at a higher point, thus straightening the course of 

 the stream. The change takes place only when the highest point in 

 the tributary valley is brought below the surface of the water in the 

 main stream at the point where the tapping takes place. This would 

 be likely to occur only after the main stream had attained a low gra- 

 dient, for so long as it is deepening its channel notably, the small amount 

 of water flowing through the tributary valley would not be likely to 

 bring it down to the level of the main. In any case the flow of water 

 from the main stream through the new valley would be likely to be 

 started during flood, and at such time the erosion in the new channel 



