How the Earth’s Crust Was Formed
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.