CORRASION BY GRAVITY STREAMS. 243 



constriction, the basin formed below base level or the 

 general channel over deepening will extend along the whole 

 of the constriction or " narrow " and gradually merge into 

 the grade upstream, if the time factor be great enough. 

 But it must be remembered that it was the initial basin 

 which expressed the utmost strength of the stream at that 

 point, and that also at a time when the heavy stream action 

 was determined by the conditions existing which permitted 

 relative smallness of cross-section upstream. But with 

 the progressive enlargement of cross-section upstream, the 

 increase of velocity became less pronounced, and the lower 

 stream then lost its original strength, and could not move 

 its load as a whole over the basin depths. Aggradation 

 therefore would already have set in there : this is an 

 important point. 



(ii) Confluence of streams to form a channel constriction. 



Firstly, along channel grades of negligible slope. Here 

 the basins will recede headwards along the tributaries 

 until the conditions favouring local increase of stream 

 velocity cease. If several marked constrictions occur at 

 channel confluences, and if other channel "narrows" be 

 associated with them, then with great stream volume the 

 conditions are favourable here for the formation of several 

 deep basins almost or quite continuous and having " deeps" 

 which occur at points where the increase of stream velocity 

 is more pronounced. 



Secondly, constrictions at channel confluences along 

 channel grades of high slope. This case will be considered 

 under the heading "Declivities." 



(iii) Declivities. 



The initial history of a basin formed on a channel declivity 

 differs materially from that formed along a channel base 

 of negligible slope. 



P— Nov. 3, 1909. 



