dry basins are drained or filled, and their deposits, if above sea
level, resume their journey to their final goal. It is only when
carried below the level of the sea that they are indefinitely
preserved.

On reaching this terminus, rivers deliver their load to the ocean.
In some cases the ocean is able to take it up by means of strong
tidal and other currents, and to dispose of it in ways which we
shall study later. But often the load is so large, or the tides
are so weak, that much of the waste which the river brings in
settles at its mouth, there building up a deposit called the
DELTA, from the Greek letter of that name, whose shape it
sometimes resembles.

Deltas and alluvial fans have many common characteristics. Both
owe their origin to a sudden check in the velocity of the river,
compelling a deposit of the load; both are triangular in outline,
the apex pointing upstream; and both are traversed by
distributaries which build up all parts in turn.

In a delta we may distinguish deposits of two distinct kinds,--
the submarine and the subaerial. In part a delta is built of waste
brought down by the river and redistributed and spread by waves
and tides over the sea bottom adjacent to the river's mouth. The
origin of these deposits is recorded in the remains of marine
animals and plants which they contain.

As the submarine delta grows near to the level of the sea the
distributaries of the river cover it with subaerial deposits
altogether similar to those of the flood plain, of which indeed