CORRASION BY GRAVITY STREAMS. 



239 



sified corrasion, while during the passage of declivities 

 there is a net gain in energy to the stream. 



Fig. 6. 



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Various stages in corrasion which is locally accentuated on a 

 channel slope. 



Let O'O represent such a declivity where O' and O repre- 

 sent such sudden changes of grade, Fig. 6 (a). Then the 

 points O and O' will furnish occasions for excavations below 

 the general channel slope. [Every channel base furnishes 

 good examples of such interrupted slope.] Upon the 

 increase of stream velocity such features will be acceotu- 



