40 Remarks on the Formation of Alluvial Deposites. 



and quantity of detritus. The deposites thus formed tend 

 continually to remove the point of junction lower down, and to 

 reduce both streams nearer to a state of parallelism. Thus if 

 A B, Fig. 28, be any stream flowing from A to B, and if it be 

 met in the point C by any other stream flowing from D, the ac- 

 cumulation of detritus in the angle A C D, accompanied by a 

 wearing away of the bank in the adjoining angle BCD, causes 

 the point C to move lower down towards B, the channel D C 

 revolving round the point D. This constant tendency of two 

 rivers uniting into one to sharpen the angle of their confluence, 

 was remarked by Guglielmini, and striking instances of it, as 

 exhibited along the course of the Ganges, are mentioned by 

 Eennell and Colebx'ooke in their accounts of the bed of that 

 river *. 



Not unfrequently the principal stream, being overpowered 

 by the tributary, is for a time hemmed in by the force of the 

 latter, and, rising to an unusual height, begins to work with great 

 power upon the bank opposite to the entrance of the tributary. 

 Of this, several .remarkable instances are mentioned in Sir Ti 

 Lauder''s late work on the Floods of Morayshire. In such 

 cases D C, Fig. 28, becomes for a while the principal stream, 

 and A C, bearing to it the relation of a tributary, begins to re- 

 volve round the point A. 



It is obvious that this action can only be rendered manifest 

 in alluvial plains, or where the banks are soft enough to be ea- 

 sily eroded. In descending from Rosen-laui to Meiringen, I 

 saw the results of a similar concurrence of two streams, where 

 the banks "were hard and rocky. A lateral torrent, almost dry 

 when I crossed it, was suddenly swollen by a thunder-storm, 

 and brought down an enormous load of rocks, sand, and mud. 

 On reaching the principal stream, the Reichenbach, it threw 

 the largest rocks, two yards or more in thickness, across its 

 channel. The smaller fragments, with the semi-fluid masses 

 of more comminuted detritus, were diverted from their course 

 by the Reichenbach, and carried downwards to the Aar. 



If a lateral stream brings down a large quantity of detritus, 

 the frequent consequence is the raising of its bed, so that it flows 

 at a higher level than the principal stream. On meeting the 



• Phil. Trans, for 1781. As. Researches, vol. vii. 



