Wonders of Earth, Sea and Sky | Page 4

regards man's knowledge of it,
gained from mining, from boring, from examination of rocks, and from
reasoning out all that may be learned from these observations, we shall
allow an ample margin if we count the field of geology to extend some
twenty miles downwards from the highest mountain-tops. Beyond this

we find ourselves in a land of darkness and conjecture.
Twenty miles is only one four-hundredth part of the earth's diameter--a
mere thin shell over a massive globe. If the earth were brought down in
size to an ordinary large school globe, a piece of rough brown paper
covering it might well represent the thickness of this earth-crust, with
which the science of geology has to do. And the whole of the globe,
this earth of ours, is but one tiny planet in the great Solar System. And
the centre of that Solar System, the blazing sun, though equal in size to
more than a million earths, is yet himself but one star amid millions of
twinkling stars, scattered broadcast through the universe. So it would
seem at first sight that the field of geology is a small field compared
with that of astronomy....
With regard to the great bulk of the globe little can be said. Very
probably it is formed through and through of the same materials as the
crust. This we do not know. Neither can we tell, even if it be so formed,
whether the said materials are solid and cold like the outside crust, or
whether they are liquid with heat. The belief has been long and widely
held that the whole inside of the earth is one vast lake or furnace of
melted fiery-hot material, with only a thin cooled crust covering it.
Some in the present day are inclined to question this, and hold rather
that the earth is solid and cold throughout, though with large lakes of
liquid fire here and there, under or in her crust, from which our
volcanoes are fed....
The materials of which the crust is made are many and various; yet,
generally speaking, they may all be classed under one simple word, and
that word is--Rock.
It must be understood that, when we talk of rock in this geological
sense, we do not only mean hard and solid stone, as in common
conversation. Rock may be changed by heat into a liquid or "molten"
state, as ice is changed by heat to water. Liquid rock may be changed
by yet greater heat to vapor, as water is changed to steam, only we have
in a common way no such heat at command as would be needed to
effect this. Rock may be hard or soft. Rock maybe chalky, clayey, or
sandy. Rock may be so close-grained that strong force is needed to
break it; or it may be so porous--so full of tiny holes--that water will
drain through it; or it may be crushed and crumbled into loose grains,
among which you can pass your fingers.

The cliffs above our beaches are rock; the sand upon our seashore is
rock; the clay used in brick-making is rock; the limestone of the quarry
is rock; the marble of which our mantel-pieces are made is rock. The
soft sandstone of South Devon, and the hard granite of the north of
Scotland, are alike rock. The pebbles in the road are rock; the very
mould in our gardens is largely composed of crumbled rock. So the
word in its geological sense is a word of wide meaning.
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