on may take place either slowly or rapidly, and either
in the open air or far below ground. The lava from a volcano is an
example of rock which has crystallized rapidly in the open air; and
granite is an example of rock which has crystallized slowly
underground beneath great pressure.

Stratified rocks, on the contrary, which make up a very large part of
the earth's crust, are not crystallized. Instead of having cooled from
a liquid into a solid state, they have been slowly _built up_, bit by
bit and grain upon grain, into their present form, through long ages
of the world's history. The materials of which they are made were
probably once, long, long ago, the crumblings from granite and other
crystallized rocks, but they show now no signs of crystallization.

[Illustration: SECTION OF STRATIFIED ROCKS.

_a._ Conglomerate. _b._ Pebbly Sandstone, _c._ Thin-bedded Sandstone,
_d._ Shelly Sandstone, _e._ Shale. _f._ Limestone.]

They are called "stratified" because they are in themselves made up of
distinct layers, and also because they lie thus one upon another in
layers, or _strata_, just as the leaves of a book lie, or as the
bricks of a house are placed.

Throughout the greater part of Europe, of Asia, of Africa, of North
and South America, of Australia, these rocks are to be found,
stretching over hundreds of miles together, north, south, east, and
west, extending up to the tops of some of the earth's highest
mountains, reaching down deep into the earth's crust. In many parts if
you could dig straight downwards through the earth for thousands of
feet, you would come to layer after layer of these stratified rocks,
one kind below another, some layers thick, some layers thin, here a
stratum of gravel, there a stratum of sandstone, here a stratum of
coal, there a stratum of clay.

But how, when, where, did the building up of all these rock-layers
take place?

[Illustration: THE BEACH IN THE FOREGROUND IS A ROCKY SHELF, THE
REMNANT OF THE CLIFF WHICH ONCE EXTENDED OUT TO THE ISLAND.]

People are rather apt to think of land and water on the earth as if
they were fixed in one changeless form,--as if every continent and
every island were of exactly the same shape and size now that it
always has been and always will be.

Yet nothing can be further from the truth. The earth-crust is a scene
of perpetual change, of perpetual struggle, of perpetual building up,
of perpetual wearing away.

The work may go on slowly,