ALLUVIUM, a word formerly applied to the gravel, mud, sand, etc., deposited by water subsequently to the Noachian deluge. It was op posed to diluvium, supposed to be laid down by the deluge itself, or, in the opinion of others, by some great wave or series of waves orig inated by the sudden upheaval of large tracts of land or some other potent causes, different from the comparatively tranquil action of water which goes on day by day. Now allu vium is especially employed to designate the transported matter laid down by fresh water during the Pleistocene and Recent periods. Thus it indicates partly a process of mechanical operation and partly a date or period. It should not be forgotten that the former has gone on through all bygone geological ages and has not been confined to any one time. Many of the hardest and most compact rocks were once loosely cohering debris laid down by water. The most typical example of allu vium may be seen in the deltas of the Nile, Ganges, Mississippi and many other rivers. Some rivers have alluviums of different ages on the slopes down into their valleys. The
more modern of these belong to the Recent period, as do the organic or other remains which they contain, while the older (as those of the Somme, Thames, Ouse, etc.), which are of Pleistocene age, enclose more or less rudely chipped flint implements, with the remains of mammals either locally or everywhere extinct. Though in many cases it is possible clearly to separate alluviums of different ages, yet the tendency of each new one is to tear up, redis tribute and confound all its predecessors. Vol canic alluvium is sand, ashes, etc., which, after being emitted from a volcano, come under the action of water and are by it redeposited, as was the case with the materials which entered and filled the interior of houses at Pompeii. Marine alluvium is alluvium produced by in undations of the sea, such as those which have from time to time overflowed the eastern coast of India. See DELTA; RIVER; SOIL.