h man is only just beginning to spell out and
understand. And this is only half the difficulty with which we have to
struggle.

If a large and learned book were put before you and you were set to
read it through, you would perhaps, have no insurmountable difficulty,
with patience and perseverance, in mastering its meaning.

But how if the book were first chopped up into pieces, if part of it
were flung away out of reach, if part of it were crushed into a pulp,
if the numbering of the pages were in many places lost, if the whole
were mixed up in confusion, and if _then_ you were desired to sort,
and arrange, and study the volume?

Picture to yourself what sort of a task this would be, and you will
have some idea of the labors of the patient geologist.

Rocks may be divided into several kinds or classes. For the present
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.

Crystallizati