where there are no strong off-shore setting currents, and where the
region is not near the mouth of a large river which bears a great tide
of sediment to the sea, the land waste may not affect the bottom for
more than a mile or two from the shore. Where these conditions are
reversed, the _debris_ from the air-covered region may be found three
or four hundred miles from the coast line. It should also be noted
that the incessant up-and-down goings of the land result in a constant
change in the position of the coast line, and consequently in the
extension of the land sediment, in the course of a few geological
periods over a far wider field of sea bottom than that to which they
would attain if the shores remained steadfast.

It is characteristic of the sediments deposited within the influence
of the continental detritus that they vary very much in their action,
and that this variation takes place not only horizontally along the
shores in the same stratum, but vertically, in the succession of the
beds. It also may be traced down the slope from the coast line to deep
water. Thus where all the _debris_ comes from the action of the waves,
the deposits formed from the shore outwardly will consist of coarse
materials, such as pebbles near the coast, of sand in the deeper and
remoter section, and of finer silt in the part of the deposit which is
farthest out. With each change in the level of the coast line the
position of these belts will necessarily be altered. Where a great
river enters the sea, the changes in the volume of sediment which it
from time to time sends forth, together with the alternations in the
position of its point of discharge, led to great local complexities in
the strata. Moreover, the turbid water sent forth by the stream may,
as in the case of the tide from the Amazon, be drifted for hundreds of
miles along the coast line or into the open sea.

The most important variations which occur in the deposits of the
littoral zone are brought about by the formations of rocks more or
less composed of limestone. Everywhere the sea is, as compared with
lake waters, remarkably rich in organic life. Next the shore, partly
because the water is there shallow, but also because of its relative
warmth and the extent to which it is in motion, organic life, both
that of animals and plants, commonly develops in a very luxuriant way.
Only where the bottom is composed of drifting sands, which do not
afford a foothold for those s