ature 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.

[Illustration: Fig. 5.--Diagram to illustrate the formation of
sedimentary deposits at the point where a river debouches into
the sea.]

Lastly, the sea itself--irrespective of the materials delivered
into it by rivers--is constantly preparing fresh stratified deposits
by its own action. Upon every coast-line the sea is constantly
eating back into the land and reducing its component rocks to
form the shingle and sand which we see upon every shore. The
materials thus produced are not, however, lost, but are ultimately
deposited elsewhere in the form of new stratified accumulations,
in which are buried the remains of animals inhabiting the sea
at the time.

Whenever, then, we find anywhere in the interior of the land
any series of beds having these characters--composed, that is,
of distinct layers, the particles of which, both large and small,
show distinct traces of the wearing action of water--whenever and
wherever we find such rocks, we are justified in assuming that
they have been deposited by water in the manner above mentioned.
Either they were laid down in some former lake by the combined
action of the streams which flowed into it; or they were deposited
at the mouth of some ancient river, forming its delta; or they
were laid down at the bottom of the ocean. In the first two cases,
any fossils which the beds might contain would be the remains
of fresh-water or terrestrial organisms. In the last case, the
majority, at any rate, of the fossils would be the remains of
marine animals.

The term "formation" is employed by geologists to express "any
group of rocks which have some character in common, whether of
origin, age, or composition" (Lyell); so that we may speak of
stratified and unstratified formations, aqueous or igneous
formations, fresh-water or marine formations, and so on.

CHIEF DIVISIONS OF