FORMATION OF ALLUVIAL PLAINS. 39 



flowing waters carry the spoils of winter most violently along 

 in their precipitous course. Thus, in those elevated regions, 

 new surfaces of solid rock are constantly exposed to the cor- 

 roding atmosphere, and thus the first alluvial plains on which a 

 more luxuriant vegetation could arise, were formed at the foot 

 of the mountains out of the wrecks and ruins of their peaks. 



According to a universal hydrostatic law, the velocity at 

 the bottom of a stream is everywhere less than in any part 

 above it, and is greatest at the surface. The superficial particles 

 in the middle of the stream also move more swiftly than those at 

 the sides. This retardation of the lowest and lateral currents is 

 produced by friction, and when the velocity is sufficiently great, 

 the soil composing the sides and bottom gives way. A velocity 

 of three inches per second at the bottom is ascertained to be 

 sufficient to tear up fine clay ; six inches per second fine sand ; 

 twelve inches per second fine gravel ; and three feet per second 

 stones of the size of an egg. 



We can thus easily understand how mountain-torrents de- 

 scending with great velocity are able to sweep along vast 

 quantities of gravel, sand, and mud ; but a question naturally 

 arises, how the more tranquil rivers of the valleys and plains, 

 flowing on comparatively level ground, can remove the pro- 

 digious burden which is discharged into them by their 

 numerous tributaries, and by what means they are enabled 

 to convey the whole mass to the sea ? If they had not this 

 removing power their channels would be annually choked up, 

 and the valleys of the lower country and the plains at the base 

 of mountain-chains would be continually strewed over with frag- 

 ments of rock and sterile sand. But this evil is prevented by 

 a general law regulating the conduct of running water that 

 two equal streams do not, when united, occupy a bed of 

 double surface. In other words, when several rivers unite into 

 one, the superficial area of the fluid mass is far less than 

 the areas previously occupied by the separate streams. The col- 

 lective waters, instead of spreading themselves out over a larger 

 horizontal space, contract themselves into a column of which 

 the height is greater relatively to its breadth. Hence, a smaller 

 proportion of the whole is retarded by friction against the 

 bottom and sides of the channel ; and in this manner the main 

 current is often accelerated in the lower country, even when the 



