
mr = \ 
~ Part IT. Secv. ii. § 3.] RIVER DEPOSITS. 389° 
line on either side is lofty, and the water deep, or where the coast is 
swept by powerful tidal currents, there is no delta. In some cases, 
too, the sediment spreads out over the sea-bottom without being 
allowed by the sea to build itself up into land, as happens at the 
_ mouths of some of the rivers in the north-west of France. 
When a river enters upon the delta portion of its course it assumes 
a new character. In the previous parts of its journey it is always 
being augmented by tributaries; but now it begins to ge up into 
_ branches, which wind to and fro through the flat alluvial land, often 
coalescing and thus enclosing insular spaces of all dimensions. The 
feeble current, no longer able to bear along all its weight of sediment, 
allows much of it to sink to the bottom and to gather over the tracts 
which are from time to time submerged. Hence many of the 
channels get choked up, while others are opened out in the plain, to 
be in turn abandoned ; and thus the river restlessly shifts its channels. 
‘The seaward ends of at least the main channels grow outwards by 
the constant accumulation of detritus pushed into the sea, unless this 
growth chances to be checked by any marine current sweeping past 
the delta. These features are nowhere more strikingly displayed 
than by the great delta of the Mississippi (Fig. 129). ‘The area of 
























































































































x 7 
re =~=6 


J 
il 
af 
ih 
zs 
| 
y 
WP aw 
a 
ml 
“< 


Wit 
Fig. 129.—Mapr or DELTA oF MissIssIprr. 
this vast expanse of alluvium is given at 12,800 square miles, 
advancing at the rate of 262 feet yearly into the Gulf of Mexico at 
a point which is now 220 miles from the head of the delta? 
On a smaller scale the rivers of Europe furnish many excellent 
illustrations of delta growth. Thus the Rhine, Meuse, Sambre, 
Scheldt, and other rivers have formed the wide maritime plain of 
1-Humphreys and Abbot, op. ett. 
