d and downward movements of the land and contiguous bed of the sea
have exerted, and continue to exert, an influence on the physical
geography of many hydrographical basins, on a scale comparable in
magnitude or importance to the amount of fluviatile deposition effected
in an equal lapse of time. In the basin of the Mississippi, for example,
proofs both of descending and ascending movements to a vertical amount
of several hundred feet can be shown to have taken place since the
existing species of land and freshwater shells lived in that
region.[375]

The deltas also of the Po and Ganges have each, as we have seen (p.
257), when probed by the Artesian auger, borne testimony to a gradual
subsidence of land to the extent of several hundred feet--old
terrestrial surfaces, turf, peat, forest-land, and "dirt-beds," having
been pierced at various depths. The changes of level at the mouth of the
Indus in Cutch (see below, chap. 27), and those of New Madrid in the
valley of the Mississippi (see p. 270, and chap. 27), are equally
instructive, as demonstrating unceasing fluctuations in the levels of
those areas into which running water is transporting sediment. If,
therefore, the exact age of all modern deltas could be known, it is
scarcely probable that we should find any two of them in the world to
have coincided in date, or in the time when their earliest deposits
originated.

_Grouping of strata in deltas._--The changes which have taken place in
deltas, even within the times of history, may suggest many important
considerations in regard to the manner in which subaqueous sediment is
distributed. With the exception of some cases hereafter to be noticed,
there are some general laws of arrangement which must evidently hold
good in almost all the lakes and seas now filling up. If a lake, for
example, be encircled on two sides by lofty mountains, receiving from
them many rivers and torrents of different sizes, and if it be bounded
on the other sides, where the surplus waters issue, by a comparatively
low country, it is not difficult to define some of the leading
geological features which must characterize the lacustrine formation,
when this basin shall have been gradually converted into dry land by the
influx of sediment. The strata would be divisible into two principal
groups: the _older_ comprising those deposits which originated on the
side adjoining the mountains, where numerous deltas first began to form;
and the _newer_ gro