
"Parr IL Scr. ii. §8.] RIVER DEPOSITS. | a 
the action described in the foregoing paragraph, alluvium is laid 
| down on the level tracts or flood-plain over which a river spreads in 
flood. It consists usually of fine silt, mud, earth, or sand; though 
close to the channel it may be partly made up of coarser materials. 
When a flooded river overflows, the portions of water which spread 
out on the plains, by losing velocity and consequently power of 
transport, are compelled to let fall some or all of their mud and 
sand. If the plaims happen to be covered with woods, bushes, 
scrub, or tall grass, the vegetation acts the part of a sieve, and 
filters the muddy water, which may rejoin the main stream 
comparatively clear. The height of the plain is thus increased by 
every flood, until, partly from this cause and partly, in the case of a 
rapid stream, from the erosion of the channel, the plain can no longer 
be overspread by the river. As the channel is more and more 
deepened, the river continues, as before, to be liable, from inequalities 
in the material of its banks, sometimes of the most trifling kind, to 
be turned from side to side in wide curves and loops, and cuts into its 
old alluvium, making eventually a newer plain at a lower level. Pro- 
longed erosion carries the channel to a still lower level, where the 
stream can attack the later alluvial deposit, and form a still lower 
and newer one. ‘The river comes by this means to be fringed with a 
series of terraces, Fig. 120, the surface of each of which represents a 
: RIVER 
YT 
See Tae gam 




Fig. 120.—SrcTion oF River TERRACES, 
former flood-level of the stream.’ In Britain it is common to find 
three such terraces, but sometimes as many as six or seven or even 
more may occur. On the Seine and other rivers of the North of 
France there is a marked terrace at a height of 12 to 17 metres 
above the present water level. In North America the river-terraces 
exist on so grand a scale that the geologists of that country have 
named one of the later periods of geological history, during which 
those deposits were formed, the Terrace Hpoch. The modern 
alluvium of the Mississippi from the mouth of the Ohio to the 
- Gulf of Mexico covers an area of 19,450 miles, and has a breadth of 
from 25 to 75 miles and a depth of from 25 to 40 feet. The old 
alluvium of the Amazon likewise forms extensive lines of cliff for 
hundreds of miles, beneath which a newer platform of detritus is 
being formed. 
In the attempt to reconstruct the history of the old river- 
terraces of a country, we have to consider whether they have 
1 The stages of this process in the régime of a great river are well brought out in the 
case of the Amazon. C. B. Brown, Q. J. Geol, Soc. xxxv. p. 763. 
