remembered, that it is not only
during inundations, when the muddy sediment is apparent, that rivers are
busy in conveying solid matter to the sea, but that even when their
waters are perfectly transparent, they are annually bearing along vast
masses of carbon, lime, and silica to the ocean.

CHAPTER XVII.

REPRODUCTIVE EFFECTS OF RIVERS.

Lake deltas--Growth of the delta of the Upper Rhine in the Lake of
Geneva--Computation of the age of deltas--Recent deposits in Lake
Superior--Deltas of inland seas--Course of the Po--Artificial
embankments of the Po and Adige--Delta of the Po, and other rivers
entering the Adriatic--Rapid conversion of that gulf into
land--Mineral characters of the new deposits--Marine delta of the
Rhone--Various proofs of its increase--Stony nature of its
deposits--Coast of Asia Minor--Delta of the Nile.

DELTAS IN LAKES.

I have already spoken in the 14th chapter of the action of running
water, and of the denuding power of rivers, but we can only form a just
conception of the excavating and removing force exerted by such bodies
of water, when we have the advantage of examining the reproductive
effects of the same agents: in other words, of beholding in a palpable
form the aggregate amount of matter, which they have thrown down at
certain points in their alluvial plains, or in the basins of lakes and
seas. Yet it will appear, when we consider the action of currents, that
the growth of deltas affords a very inadequate standard by which to
measure the entire carrying power of running water, since a considerable
portion of fluviatile sediment is swept far out to sea.

Deltas may be divided into, first, those which are formed in lakes;
secondly, those in island seas, where the tides are almost
imperceptible; and, thirdly, those on the borders of the ocean. The most
characteristic distinction between the lacustrine and marine deltas
consists in the nature of the organic remains which become imbedded in
their deposits; for, in the case of a lake, it is obvious that these
must consist exclusively of such genera of animals as inhabit the land
or the waters of a river or a lake; whereas, in the other case, there
will be an admixture, and most frequently a predominance, of animals
which inhabit salt water. In regard, however, to the distribution of
inorganic matter, the deposits of lakes and seas are formed under very
analogous circumstances.

_Lake of Geneva._