GEOLOGICAL MAPS 



169 



out the drainage, each one fed by longitudinal tributaries at 

 every intersection with a softer bed. But the maintenance of 

 this state of things would demand a wonderful balance in erosion 

 and resistance, which is never likely to occur. Suppose, for example, 

 that a single one of the transverse streams carries out exceptionally 

 rapid erosion of its bed (Fig. 60). This may result from the rainfall 

 being excessive in its basin, from the hard rocks which it traverses 

 being softer where crossed by it than elsewhere, from its primitive 

 slope being greater than that of the others, or from the stream 



FIG. 61. Capture of the head waters of one stream by those of another. 



being supplied with a greater amount of gravel and stones with 

 which to carve its bed downwards. If its gorge in this way 

 becomes deeper than those of the other streams, its tributaries 

 will be at once quickened in consequence of their steeper fall 

 into the transverse stream. Hence their erosive power will 

 increase, they will cut back their head waters more rapidly, and 

 will quickly grow longer. In doing so, they may encroach upon 

 the side of another transverse stream, and may eventually divert 

 its waters and capture that portion, the head, of the other stream 



