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 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