Minur ChannelaJ'urmed by Subsidence of Streams. S9 



the stream is more diffused, and its contents more evenly distri- 

 buted. It thus forms plains or gently sloping levels, termi- 

 nated by transverse ledges, (p. 18). 



The tongues and levels, thus formed, would never move un- 

 less the stream Avere to subside. But after its subsidence they 

 are intersected by numerous smaller channels. (See above, 

 p. 18, 19). Hence when the water rises again, the depth of those 

 channels being added to the depth above the intersected tongue 

 or level, the force of the water within the channels is increased 

 in proportion. Even on the surface of the flood, the effect of 

 the increased depth is visible. As we learn from the observa- 

 tions of Major Rennell in India, and of Sir T. Lauder in Scot- 

 land, when a whole valley is inundated, and thus converted in- 

 to one vast channel, the course of the former channels may be 

 traced by the greater swiftness, turbulence, and discoloration of 

 the water aliove them, which indeed forms distinct currents tra- 

 versing the flood. These powerful currents work unseen up- 

 on the sides of the minor channels, and thus in a short time 

 carry way the entire masses of detritus, in which they have 

 been excavated. 



Such appear to be the chief modifications in the action of 

 water carrying detritus over level or inclined surfaces, and such 

 the principles on which its action depends. If it be sufficiently 

 abundant, and if it rise and fall alternately, it appears capable 

 of conveying earth, stones, and rocks over the earth's surface to 

 any distance. To this action of the atmospherical waters, we 

 may refer many of the greatest plains through which rivers 

 flow. For if in low lying plains the beds of alluvium be exhi- 

 bited on a vast scale, the agents concerned in producing them have 

 been powerful in the same proportion ; and it is to be considered, 

 that, under present circumstances, when the mountains, lowered 

 by the supply of alluvium during thousands of vears, can no 

 longer furnish debacles and inundations of equal magnitude ; 

 when the ocean itself, having been to a considerable extent filled 

 up by these processes, must be supposed to have risen to a 

 higher level ; when, with the exception of regions of eternal frost, 

 the whole face of the earth is protected by its vegetable cover- 

 ing ; and when human art and labour do their utmost to curb 

 the destructive influence of the atmospherical waters ; that in- 



