Now the business of the geologist is to read the history of the past
in these rocks of which the earth's crust is made. This may seem a
singular thing to do, and I can assure you it is not an easy task.

For, to begin with, the history itself is written in a strange
language, a language which 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,