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.