r deposits are
diffused continually over a wider area, and are consequently more
horizontal than the older. When at first there were many independent
deltas near the borders of the basin, their separate deposits differed
entirely from each other; one may have been charged, like the Arve where
it joins the Rhone, with white sand and sediment derived from
granite--another may have been black, like many streams in the Tyrol,
flowing from the waste of decomposing rocks of dark slate--a third may
have been colored by ochreous sediment, like the Red River in
Louisiana--a fourth, like the Elsa in Tuscany, may have held much
carbonate of lime in solution. At first they would each form distinct
deposits of sand, gravel, limestone, marl, or other materials; but,
after their junction, new chemical combinations and a distinct color
would be the result, and the particles, having been conveyed ten,
twenty, or a greater number of miles over alluvial plains, would become
finer.

In those deltas where the tides and strong marine currents interfere,
the above description would only be applicable, with certain
modifications. If a series of earthquakes accompany the growth of a
delta, and change the levels of the land from time to time, as in the
region where the Indus now enters the sea, the phenomena will depart
still more widely from the ordinary type. If, after a protracted period
of rest, a delta sinks down, pebbles may be borne along in shallow water
near the foot of the boundary hills, so as to form conglomerates
overlying the fine mud previously thrown into deeper water in the same
area.

_Causes of stratification in deltas._--The stratified arrangement, which
is observed to prevail so generally in aqueous deposits, is most
frequently due to variations in the velocity of running water, which
cannot sweep along particles of more than a certain size and weight when
moving at a given rate. Hence, as the force of the stream augments or
decreases, the materials thrown down in successive layers at particular
places are rudely sorted, according to their dimensions, form, and
specific gravity. Where this cause has not operated, as where sand, mud,
and fragments of rock are conveyed by a glacier, a confused heap of
rubbish devoid of all stratification is produced.

Natural divisions are also occasioned in deltas, by the interval of time
which separates annually the deposition of matter during the periodical
rains, or melting of snow upo