would boil at a depth of eight thousand feet below the surface; iron
would melt at a depth of twenty-eight miles; while at a depth of forty
or fifty miles no known substance upon earth could remain solid.

The force of this proof is, however, weakened by the fact that the
rate at which the heat increases differs very much in different
places. Also it is now generally supposed that such a tremendous
furnace of heat--a furnace nearly eight thousand miles in
diameter--could not fail to break up and melt so slight a covering
shell.

Many believe, therefore, not that the whole interior of the earth is
liquid with heat, but that enormous fire-seas or lakes of melted rock
exist here and there, under or in the earth-crust. From these lakes
the volcanoes would be fed, and they would be the cause of earthquakes
and land-upheavals or land-sinkings. There are strong reasons for
supposing that the earth was once a fiery liquid body, and that she
has slowly cooled through long ages. Some hold that her centre
probably grew solid first from tremendous pressure; that her crust
afterwards became gradually cold; and that between the solid crust and
the solid inside or "nucleus," a sea of melted rock long existed, the
remains of which are still to be found in these tremendous fiery
reservoirs.

The idea accords well with the fact that large numbers of extinct or
dead volcanoes are scattered through many parts of the earth. If the
above explanation be the right one, doubtless the fire-seas in the
crust extended once upon a time beneath such volcanoes, but have since
died out or smouldered low in those parts.