ions. They may be formed in what are called dead seas,
in which case the detrital materials are commonly small in amount, for
the reason that the inflowing streams are inconsiderable; in such
basins there is normally a large share of saline materials, which are
laid down by the evaporation of the water. In ordinary lakes the
deposits which are formed are mostly due to the sediment that the
rivers import. These materials are usually fine-grained, and the sand
or pebbles which they contain are plentifully mingled with clay. Hence
lake deposits are usually of an argillaceous nature. As organic life,
such as secretes limestone, is rarely developed to any extent in lake
basins, limy beds are very rarely formed beneath those areas of water.
Where they occur, they are generally due to the fact that rivers
charged with limy matter import such quantities of the substance that
it is precipitated on the bottom.

As lake deposits are normally formed in basins above the level of the
sea, and as the drainage channels of the basins are always cutting
down, the effect is to leave such strata at a considerable height
above the sea level, where the erosive agents may readily attack them.
In consequence of this condition, lacustrine beds are rarely found of
great antiquity; they generally disappear soon after they are formed.
Where preserved, their endurance is generally to be attributed to the
fact that the region they occupy has been lowered beneath the sea and
covered by marine strata.

The great laboratory in which the sedimentary deposits are
accumulated, the realm in which at least ninety-nine of the hundred
parts of these materials are laid down, is the oceanic part of the
earth. On the floors of the seas and oceans we have not only the
region where the greater part of the sedimentation is effected, but
that in which the work assumes the greatest variety. The sea bottoms,
as regards the deposits formed upon them, are naturally divided into
two regions--the one in which the _debris_ from the land forms an
important part of the sediment, and the other, where the remoteness
of the shores deprives the sediment of land waste, or at least of
enough of that material in any such share as can affect the character
of the deposits.

What we may term the littoral or shore zone of the sea occupies a belt
of prevailingly shallow water, varying in width from a few score to a
few hundred miles. Where the bottom descends steeply from the coast,