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Young Folks' Library: Wonders of Earth, Sea and Sky

What The Earth's Crust Is Made Of
"Stand still and consider the wondrous works of God."
What is the earth made of—this round earth upon which we human beings live and
move?
A question more easily asked than answered, as regards a very large portion of it. For the
earth is a huge ball nearly eight thousand miles in diameter, and we who dwell on the
outside have no means of getting down more than a very little way below the surface. So
it is quite impossible for us to speak positively as to the inside of the earth, and what it is
made of. Some people believe the earth's inside to be hard and solid, while others believe
it to be one [pg 2] enormous lake or furnace of fiery melted rock. But nobody really knows.
This outside crust has been reckoned to be of many different thicknesses. One man will
say it is ten miles thick, and another will rate it at four hundred miles. So far as 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 [pg 3] 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
 
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