s gas-charged
water rapidly brings about a decay of the fragments. Much of the
material passes at once into solution in this water, and drains away
through the multitudinous springs which border the river. As this
matter is completely dissolved, as is sugar in water, it goes straight
away to the sea without ever again entering the alluvium. In many, if
not most, cases this dissolving work which is going on in alluvial
terraces is sufficient to render a large part of the materials which
they contain into the state where it disappears in an unseen manner;
thus while the annual floods are constantly laying down accumulations
on the surface of these plains, the springs are bearing it away from
below.

In this way, through the decomposition which takes place in them, all
those river terraces where much vegetable matter is mingled with the
mineral substances, become laboratories in which substances are
brought into solution and committed to the seas. We find in the water
of the ocean a great array of dissolved mineral substances; it,
indeed, seems probable that the sea water contains some share, though
usually small, of all the materials which rivers encounter in their
journey over and under the lands. As the waters of the sea obtain but
little of this dissolved matter along the coast, it seems likely that
the greater share of it is brought into the state of solution in the
natural laboratories of the alluvial plains.

Here and there along the sides of the valleys in which the rivers flow
we commonly find the remains of ancient plains lying at more or less
considerable heights above the level of the streams. Generally these
deposits, which from their form are called terraces, represent the
stages of down-wearing by which the stream has carved out its way
through the rocks. The greater part of these ancient alluvial plains
has been removed through the ceaseless swinging of the stream to and
fro in the valley which it has excavated.

In all the states of alluvial plains, whether they be the fertile
deposits near the level of the streams which built them, or the poorer
and ruder surfaced higher terraces, they have a great value to
mankind. Men early learned that these lands were of singularly uniform
goodness for agricultural use. They are so light that they were easily
delved with the ancient pointed sticks or stone hoes, or turned by the
olden, wooden plough. They not only give a rich return when first
subjugated, but, owi