CORRASION BY GRAVITY STREAMS. 217 



Definition of Terms — Channel constrictions. — A con- 

 striction is that portion of a channel where the cross- 

 section is relatively small. 



Constricted channel confluence. — This is the point of 

 junction of two or more channels, the sum of whose cross- 

 sections exceeds the cross-section of the main channel at 

 the point of junction. 



Channel "Broads." — These imply valley cross-sections 

 passing immediately upstream into areas of less cross- 

 section. 



Channel " DiffLuences." — Here the sum of two or more 

 channel cross-sections is considered as less than the cross- 

 section of the channel into which they discharge. As in 



the case of a channel "Broad" so here velocity will be 

 decreased. 



Transportation of material. — No stream moves as a rigid 

 bolt, except possibly when falling freely. As the volume 

 increases the friction between its textural units becomes 

 of relatively less importance and greater mobility is 

 attained. This is true whatever the stream material con- 

 sidered. This increasingly prevents the stream holding its 

 tools stiffly to their work, and causes it increasingly to 

 depart from the action of a plane. In proportion to the 

 mobility and strength of a stream so will the transported 

 material take on a saltatory motion. 



Stream motion round small obstacles. — Upon the slope 

 facing upstream the stream has a greater action, while 

 upon the downstream slope a less percentage of the stream 

 velocity takes effect (Fig. 1). The work of passing the 

 small obstacle represents a net loss to the stream power. 



Stream motion around rock masses large as compared 

 with the passing stream. — The upstream portion of the 

 mass acts as a dam and destroys the individuality of the 



