620 ALLUVIA. 



same causes, in most instances, where they form 

 those extensive plains that accompany the es- 

 tuaries of rivers, whether these terminate in the 

 sea or in an inland lake. In the former case, they 

 constitute submarine banks and low islands ; in 

 the latter, in the course of their progress, they 

 obliterate the lakes, and thus form solid and ex- 

 tensive deposits in vallies, which have sometimes 

 been imagined to originate in causes of a more 

 general nature. 



The action of the sea on its shores, is some- 

 times such as to reject on them the loose mate- 

 rials which it contains, and which it may have 

 derived, either from more distant and similar 

 sources, or from the perpetual reproduction and 

 death of the tribes of shell-fish by which it is 

 inhabited. In some cases of this nature, the 

 sea is thus caused to retire before the increasing 

 shore, and that increase is often accelerated or 

 consolidated by the growth of various plants 

 which take root in the new-formed soil. In 

 others, the winds disperse the lighter materials, 

 consisting of sand ; which thus frequently over- 

 whelms the interior land, for a certain space, 



