258 E. C. ANDREWS. 



In the extreme case the cliff is overhanging. The stream 

 mass falls with ever increasing velocity and accomplishes 

 heavy corrasion at the cliff base. The cliff face itself in 

 this case is not corraded, but is sapped from the basal 

 attack and recedes under pressure of its own weight. But 

 whether falling or flowing over a declivity, the qualitative 

 effects of corrasion are similar and tend to produce similar 

 geometrical forms. 



In the case of the overhanging cliff we are no longer 

 dealing with homogeneous structures but with lower and 

 relatively weak structures capped by relatively strong ones. 

 The "gravitative curve" in that case is carried up in the 

 weak lower structures and reversed in passing from the 

 weaker to the stronger structures. (Fig. 12 Appendix.) 



Summary of "Stream" Corrasion. — It is a fact of obser- 

 vation that earth material will corrade live rock structures 

 over which it is dragged. So long, then, as any stream 

 has power to move the total load which overlies a particular 

 point of the channel base, it is enabled to act on the under- 

 lying rocks at that point, and thus it will cut vertically 

 into the rock structures forming its channel base and sides. 

 Moreover the corrasive power of a stream rises in some 

 high geometrical ratio with the increase of stream velocity, 

 and hence at localities which are the seat of action of 

 increased stream activity, the vertical cutting will be 

 correspondingly pronounced. This is a fixed mechanical 

 principle. The conditions producing such increase of 

 velocity are to be found in areas of channel constrictions, 

 and also on steep channel slopes; and they may be expected 

 where great stream volume acts in conjunction with either 

 of the two types of configuration just mentioned. 



Gravity aims at causing the stream and its load to deliver 

 a vertical blow, with the production of a more or less 

 cylindrical cavity possessing a vertical axis, in the ideal 



