370 ON THE ALLUVIAL DEPOSITS. 



found accompanying the present or former courses of 

 streams, they occupy vallies, or plains, or the seats or 

 borders of lakes, or maritime cestuaries. 



It is almost to repeat former remarks, to say that 

 the mountain torrent commences in a rocky channel, 

 generally becoming wider and deeper in its progress, 

 that at a certain stage, this bed displays fragments, and, 

 at length, consists of rounded pebbles and gravel. The 

 declivity, and consequent velocity, diminishing, the de- 

 posit becomes finer, and, as it reaches the plain, the 

 rocks disappear, even from the banks, which finally 

 become unequal, alternately presenting deep sections 

 and flat shores. The course of the stream here becomes 

 variable and shifting; while the plain displays hillocks 

 and terraces of loose materials, the remains of the banks 

 which it has successively deserted. A section through 

 the plain, discovers a considerable, but variable depth 

 of the same materials which form its surface ; but du- 

 ring the whole of this progress, the deposited materials 

 become gradually finer, as the declivity diminishes in 

 the actual stream or in its immediate vicinity. The 

 advance of the larger fragments ceases when that has 

 been much reduced ; and that of the materials which 

 friction takes from them, if still exposed to water, 

 must therefore occupy years : the heavier permanent 

 alluvia remain only because the stream has deserted 

 them. Hence the different characters of these alluvia 

 at different points of a stream, at differents parts of a 

 plain, and at different depths ; since, at the same point, 

 under a greater rapidity, consequent on a greater de- 

 clivity, to be computed from the depth, the materials 

 must differ. And hence, in the first case, engineers have 

 computed the necessary bulk of these at several points, 

 relatively to the slope, or to the velocity of the water. 

 I must however remark, that even large masses are some- 



