Science tells us that the Earth was once a shining star, a globe of
liquid fire. As it cooled down, a crust formed over its surface,
composed chiefly of rocks and metals. This crust was rent by the
force of the gases shut up within, and thus the mountains, valleys,
gorges, and volcanoes were formed. The Earth, indeed, is still
upheaving and subsiding, but so slowly that we rarely feel it.
Through these agencies the distribution of land and water on the
surface of the earth has undergone great changes. The shape of the
Earth is that of a sphere somewhat flattened at the poles, and it
has a diameter of about 8,000 miles. The solid crust is called the
_lithosphere_--which is surrounded by an envelope of air--the
_atmosphere_--and in part by an envelope of water--the
_hydrosphere_.
[Illustration: =HOW THE EARTH WOULD APPEAR IF CUT THROUGH THE CENTER=
Beneath the rocky crust of the earth, thirty-five miles in thickness,
there is a broad belt of heavier material to a depth of nine hundred
miles. Within this shell lies the great metallic core.]