Wonders of Earth, Sea and Sky | Page 5

moment it will be enough to consider the two grand
divisions--Stratified rocks and Unstratified rocks.
Unstratified rocks are those which were once, at a time more or less
distant, in a melted state from intense heat, and which have since
cooled into a half crystallized state; much the same as water, when
growing colder, cools and crystallizes into ice. Strictly speaking, ice is
rock, just as much as granite and sandstone are rock. Water itself is of
the nature of rock, only as we commonly know it in the liquid state we
do not commonly call it so.
[Illustration: UNSTRATIFIED ROCK.--A VOLCANIC BLOCK.]
"Crystallization" means those particular forms or shapes in which the
particles of a liquid arrange themselves, as that liquid hardens into a
solid--in other words, as it freezes. Granite, iron, marble, are frozen

substances, just as truly as ice is a frozen substance; for with greater
heat they would all become liquid like water. When a liquid freezes,
there are always crystals formed, though these are not always visible
without the help of a microscope. Also the crystals are of different
shapes with different substances.
If you examine the surface of a puddle or pond, when a thin covering of
ice is beginning to form, you will be able to see plainly the delicate
sharp needle-like forms of the ice crystals. Break a piece of ice, and
you will find that it will not easily break just in any way that you may
choose, but it will only split along the lines of these needle-like crystals.
This particular mode of splitting in a crystallized rock is called the
cleavage of that rock.
Crystallization 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, but it does go on. The sea is always
fighting against the land, beating down her cliffs, eating into her shores,
swallowing bit by bit of solid earth; and rain and frost and inland
streams are always busily at work, helping the ocean in her work of
destruction. Year by year and century by century it continues. Not a
country in the world which is bordered by the open sea has precisely
the same coast-line that it had one hundred years ago; not a land in the
world but parts each century with masses of its material, washed
piecemeal away into the ocean.
Is