would show, as clearly as a
written chronicle, the following sequence of events. First, there was a
salt-water estuary peopled for many years by species of marine Testacea
identical with those now living, and into which some of the larger
Cetacea occasionally entered. Secondly, the inlet grew shallower, and
the water became brackish, or alternately salt and fresh, so that the
remains of freshwater and marine shells were mingled in the blue
argillaceous sediment of its bottom. Thirdly, the shoaling continued
until the river-water prevailed, so that it was no longer habitable by
marine Testacea, but fitted only for the abode of fluviatile species and
aquatic insects. Fourthly, a peaty swamp or morass was formed, where
some trees grew, or perhaps were drifted during floods, and where
terrestrial quadrupeds were mired. Finally, the soil being flooded by
the river only at distant intervals, became a verdant meadow.

_In delta of Ganges and Indus._--It was before stated, that on the
sea-coast, in the delta of the Ganges, there are eight great openings,
each of which has evidently, at some ancient period, served in its turn
as the principal channel of discharge.[1103] As the base of the delta is
200 miles in length, it must happen that, as often as the great volume
of river-water is thrown into the sea by a new mouth, the sea will at
one point be converted from salt to fresh, and at another from fresh to
salt; for, with the exception of those parts where the principal
discharge takes place, the salt water not only washes the base of the
delta, but enters far into every creek and lagoon. It is evident, then,
that repeated alternations of beds containing freshwater shells, with
others filled with marine exuviae, may here be formed. It has also been
shown by artesian borings at Calcutta (see above, p. 267), that the
delta once extended much farther than now into the gulf, and that the
river is only recovering from the sea the ground which had been lost by
subsidence at some former period. Analogous phenomena must sometimes be
occasioned by such alternate elevation and depression as has occurred in
modern times in the delta of the Indus.[1104] But the subterranean
movements affect but a small number of the deltas formed at one period
on the globe; whereas the silting up of some of the arms of great rivers
and the opening of others, and the consequent variation of the points
where the chief volume of their waters is discharged into