When the fire went out for lack of fuel the mass began to cool at the
surface, and a solid crust was finally formed, which with the lapse of
time became thicker and thicker. This crust shut in the steam and gases
generated in the fiery ocean underneath; and these, acting upon the
crust with enormous pressure, heaved it into ridges. At times the strain
caused the crust to crack, and forced the melted mass up through it, and
in this way hills and mountains were formed. The thicker the crust the
greater the strain it would bear before it gave way, and the greater the
amount of molten matter driven out through the rent. The highest
mountains, then, are the last that were uplifted. In some cases the
openings thus made in the crust were never completely closed, and thus
volcanoes were formed. These act like safety-valves, and prevent the
forces within from accumulating sufficiently to cause fresh rents. But
notwithstanding the relief thus given to the pent-up forces, they still
manifest themselves in earthquakes.