-bed, beyond the reach of our ken. The powerful rush of water in
earth's greater streams bears enormous masses of sand and mud each
year far out into the ocean, there dropping quietly the gravel, sand,
and earth, layer upon layer at the bottom of the sea. Thus pulling
down and building up go on ever side by side; and while land is the
theatre oftentimes of decay and loss, ocean is the theatre oftentimes
of renewal and gain.

Did you notice the word "sediment" used a few pages back about the
settlement at the bottom of a medicine-vial?

There is a second name given to the Stratified Rocks, of which the
earth's crust is so largely made up. They are called also _Sedimentary
Rocks_.

The reason is simply this. The Stratified Rocks of the present day
were once upon a time made up out of the sediment stolen first from
land and then allowed to settle down on the sea-bottom.

Long, long ago, the rivers, the streams, the ocean, were at work, as
they are now, carrying away rock and gravel, sand and earth. Then, as
now, all this material, borne upon the rivers, washed to and fro by
the ocean, settled down at the mouths of rivers or at the bottom of
the sea, into a sediment, one layer forming over another, gradually
built up through long ages. At first it was only a soft, loose, sandy
or muddy sediment, such as you may see on the seashore, or in a
mud-bank. But as the thickness of the sediment increased, the weight
of the layers above gradually pressed the lower layers into firm hard
rocks; and still, as the work of building went on, these layers were,
in their turn, made solid by the increasing weight over them. Certain
chemical changes had also a share in the transformation from soft mud
to hard rock, which need not be here considered.

All this has through thousands of years been going on. The land is
perpetually crumbling away; and fresh land under the sea is being
perpetually built up, from the very same materials which the sea and
the rivers have so mercilessly stolen from continents and islands.
This is the way, if geologists rightly judge, in which a very large
part of the enormous formations of Stratified or Sedimentary Rocks
have been made.

[Illustration: VIEW IN A CANON.]

So far is clear. But now we come to a difficulty.

The Stratified Rocks, of which a very large part of the continents is
made, appear to have been built up slowly, layer upon layer, out of
the gravel, sand, and mud, washed away from the la