The great bulk of the earth consists of the _lithosphere_, or solid
globe of rocks, with which geology properly deals. It is on the part of
this lithosphere, composing a little more than a quarter of the earth’s
whole area--55,500,000 square miles--which rises above the seas and is
called land, that mankind lives.
The central core is a globe of about 7600 miles in diameter, which is
composed of iron and other elements, probably not forming compounds, in
the gaseous state, but exposed to such tremendous pressure that it
behaves as a solid and extremely rigid body. Outside this core is a
shell of liquid matter which consists of all the rocks which we know at
the surface in a state of fusion, perhaps one hundred miles in
thickness. Upon this magma floats the solid crust, thirty or forty miles
thick, which is composed of various rocks, breaking down at the surface
into soil. Three-fourths of the surface of this crust are covered by the
water of the oceans, the hydrosphere, the rest being dry land. Outside
all comes the atmospheric mantle, chiefly composed of air, which
supports life, acts as a blanket to keep the earth warm, and as a shield
against the blows of meteorites.