Effects produced by the Meeting oftxoo Streams. 41 



principal stream it is suddenly turned aside. Every pebble and 

 all the smaller detritus which it brings, are also hurried down- 

 wards, so that the bed of the lateral stream is abruptly termi- 

 nated by a line, coinciding with the surface and direction of the 

 principal stream. Thus by the joint action of the two streams 

 a talus is formed, similar to those beneath the margins of lakes, 

 but differing in this, that the edge of the steep declivity is in- 

 clined instead of being level. I have observed cases of this ac- 

 tion in the valley of Chamonix, where, supposing cdgh. 

 Fig. 29, to be a bank of stones and gravel forming part of the 

 channel of the Arve, ichk is part of the bed of a stream de- 

 scending through a lateral valley. The stones and other debris 

 brought down from the region of the snows and glaciers, as soon 

 as they reach the border of the principal stream ch, fall down 

 the steep decHvity, and are hurried along with the solid mate- 

 rials of the Arve. In mountain valleys we often see a terrace of 

 this description, where the waters have been nearly or altogether 

 discharged, and the only method of deciding whether they have 

 been produced by a lake or a river, seems to be by ascertaining 

 whether the Hne c A is level or inclined. 



As a case of similar action in the sea, may be mentioned the 

 entrance of the Southampton Water below Portsmouth *. The 

 sediment of this river has a tendency to form a semicircle pro- 

 jecting towards the Isle of Wight. It is cut off in a nearly 

 straight line passing from one headland to the other by the 

 strong current of the Solent, and terminates in a dechvity ana- 

 logous to cdgh in Fig. 29. In estuaries and channels of the 

 sea, still more than in rivers, the actions ot different currents 

 are infinitely modified. One portion of a stream may be con- 

 sidered as at rest relatively to another portion, which forms a 

 stream within it, and the minor streams conflict and unite with 

 endless varieties of force and direction. The general bed of the 

 whole stream is consequently varied by numerous channels, 

 slopes, and terraces, formed by the contemporaneous action of 

 the included currents, and in its shoals and banks it exhibits a 

 repetition, upon a larger and more expanded scale, of the same 

 forms which are produced by similar influences in the beds of 

 rivers. 



• G. Tr. New Series, vol. i. pi. xii. and p. 92 



