ON THE ALLUVIAL DEPOSITS. 373 



need only add, that the existence of marl beds always, 

 and of peat and organic remains generally, will de- 

 monstrate the former places of water at points whence 

 it has long been excluded ; that the shoaling of a lake 

 is comparatively accelerated, and also modified, by the 

 vegetation on the margins, often producing peat bogs; 

 and that other modifications result from the shoaling 

 sometimes proceeding at many points, in consequence 

 of the entrance of lateral streams. 



On Maritime Alluvia. 



I have shown that these are generally produced by 

 a concurrence of the action of the sea with that of 

 rivers; nor can the former often produce extensive 

 deposits of this nature, derived from any other source. 

 The chief exception is in the fragments detached from 

 cliffs or rocky shores, which, having been rolled 

 or pulverized by the waves, are rejected on declivities 

 where they are enabled to rest. On the spits and 

 banks produced by the ordinary course of currents or 

 tides, or by gales of wind, I have formerly treated: as 

 this is the point at which the alluvia of rivers become 

 confounded with the present. In all such cases, the 

 nature of the alluvia is complicated : but however the 

 hind may have been modified,, the progress and the 

 causes may be ascertained, by finding marine remains 

 beneath ; or by the alternation of terrestrial and ma- 

 rine deposits. The common linear exposed shore re- 

 quires no remarks; yet Bute and Arrau offer striking 

 examples of entire flat belts at the foot of the 

 high lands, produced by the action of the sea on allu- 

 via of descent. When this occurs in bays, consi- 

 derable tracts of useful land are sometimes generated. 

 I must yet remark, that the action of the sea in these 

 cases, is, like that of rivers, twofold ; and that its ten- 



