The finer beds of clay or sand will all be arranged in thicker or
thinner layers or laminae; and if there are any beds of pebbles
these will all be rounded or smooth, just like the water-worn
pebbles of any brook-course. In all probability, also, we should
find in some of the beds the remains of fresh-water shells or
plants or other organisms which inhabited the lake at the time
these beds were being deposited.
In the same way large rivers--such as the Ganges or
Mississippi--deposit all the materials which they bring down
at their mouths, forming in this way their "deltas." Whenever
such a delta is cut through, either by man or by some channel of
the river altering its course, we find that it is composed of a
succession of horizontal layers or strata of sand or mud, varying
in mineral composition, in structure, or in grain, according to
the nature of the materials brought down by the river at different
periods. Such deltas, also, will contain the remains of animals
which inhabit the river, with fragments of the plants which grew
on its banks, or bones of the animals which lived in its basin.
Nor is this action confined, of course, to large rivers only,
though naturally most conspicuous in the greatest bodies of water.
On the contrary, all streams, of whatever size, are engaged in
the work of wearing down the dry land, and of transporting the
materials thus derived from higher to lower levels, never resting
in this work till they reach the sea.
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