ALLUVIAL DEPOSITS, 
15 
Alluvium. 
The geological changes effected by agents which 
are still in active operation form the subject of this 
section : these appear to have proceeded, with but 
little modification, from the ])eriod when our con- 
tinents and islands assumed their present form. The 
encroachments of the sea upon the shore, by which 
the cliffs are undermined, and their ruins swept 
away ; the overflowing of low districts, and their 
conversion into marshes subject to periodical in- 
undations ; or, on the contrary, the formation of 
downs along the coast by the drifting inland of 
sand and gravel ; the production of alluvial tracts 
by rivers, and the accumulation of estuaries at 
their mouths ; and the disintegration of the ex- 
posed surface of the strata, by atmospherical causes ; 
are the principal changes which are now taking 
})lace in the strata, of the south-east of England. 
1 . Blue Clay or Silly deposited by RiverSy 
Lakes y <^'c. 
The numerous tributary rills and streams which 
flow through the sand and clay districts into the 
rivers, bring with them particles of mud, sand, &c. ; 
these are carried on by the current, towards the 
sea ; but, as the motion of the waters becomes less 
rapid, the larger particles subside, and, by degrees, 
the greater portion is deposited at the bottom of 
the river or on its banks. It is clear that, by a 
process of this kind, Lewes and Arundel levels 
have been produced. If the operation be con- 
tinued without interruption, the bed of the river 
