It is hard to believe that our solid earth was once a ball of seething liquid, like the red-hot iron that is poured out of the big clay cups into the sand moulds at an iron foundry. But when a mountain like Vesuvius sets up a mighty rumbling, and finally a mass of white-hot lava bursts from the centre and streams down the sides, covering the vineyards and olive orchards, and driving the people out of their homes in terror, it seems as if the earth's crust must be but a thin and frail affair, covering a fiery interior, which might at any time break out. The people who live near volcanoes might easily get this idea.

But they do not. They go back as soon as the lava streams are cooled, and rebuild their homes, and plant more orchards and vineyards. "It is so many years," say they to one another, "since the last bad eruption. Vesuvius will probably sleep now till we are dead and gone."

This is good reasoning. There are few active volcanoes left on the earth, compared with the number that were once active, and long ago became extinct. And the time between eruptions of the active ones grows longer; the eruptions less violent. Terrible as were the recent earthquakes of San Francisco and Messina, this form of disturbance of the earth's crust is growing constantly less frequent. The earth is growing cooler as it grows older; the crust thickens and grows stronger as centuries pass. We have been studying the earth only a few hundred years. The crust has been cooling for millions of years, and mountain-making was the result of the shrinking of the crust. That formed folds and clefts, and let masses of the heated substance pour out on the surface.

My first geography lesson I shall never forget. The new teacher had very bright eyes and such pretty hands! She held up a red apple, and told us that the earth's substance was melted and burning, inside its crust, which was about as thick, in proportion to the size of the globe, as the skin of the apple. I was filled with wonder and fear. What if we children jumped the rope so hard as to break through the fragile shell, and drop out of sight in a sea of fiery metal, like melted iron? Some of the boys didn't believe it, but they were impressed, nevertheless.

The theory of the heated interior of the earth is still believed, but the idea that flames and bubbling metals are enclosed in the outer layer of solid matter has generally been abandoned. The power that draws all of its particles toward the earth's centre is stated by the laws of gravitation. The amount of "pull" is the measure of the weight of any substance. Lift a stone, and then a feather pillow, much larger than the stone. One is strongly drawn to the earth; the other not. One is heavy, we say, the other light.

If a stone you can pick up is heavy, how much heavier is a great boulder that it takes a four-horse team to haul. What tremendous weight there is in all the boulders scattered on a hillside! The hill itself could not be made level without digging away thousands of tons of earth. The earth's outer crust, with its miles in depth of mountains and level ground, is a crushing weight lying on the heated under-substance. Every foot of depth adds greatly to the pressure exerted upon the mass, for the attraction of gravitation increases amazingly as the centre of the earth is approached.

It is now believed that the earth is solid to its centre, though heated to a high degree. Terrific pressure, which causes this heat, is exerted by the weight of the crust. A crack in the crust may relieve this pressure at some point, and a mass of substance may be forced out and burst into a flaming stream of lava. Such an eruption is familiar in volcanic regions. The fact that red-hot lava streams from the crater of Vesuvius is no proof that it was seething and bubbling while far below the surface.

Volcanoes, geysers, and hot springs prove that the earth's interior is hot. The crust is frozen the year around in the polar regions, and never between the Tropics of Cancer and Capricorn. The sun's rays produce our different climates, but they affect only the surface. Underground, there is a rise of a degree of temperature for every fifty feet one goes down. The lowest mine shaft is about a mile deep. That is only one four-thousandth of the distance to the earth's centre.

By an easy computation we could locate the known melting-point for metals and other rock materials. But one degree for each fifty feet of depth below the surface may not be correct for the second mile, as it is for the first. Again, the melting-point is probably a great deal higher for substances under great pressure. The weight of the crust is a burden the under-rocks bear. Probably the pressure on every square inch reaches thousands of tons. Could any substance become liquid with such a weight upon it, whatever heat it attained? Nobody can answer this question.

The theory that volcanoes are chimneys connecting lakes of burning lava with the surface of the earth is discredited by geologists. The weight of the overlying crust would, they think, close such chambers, and reduce liquids to a solid condition.

Since the first land rose above the sea, the crust of the earth has gradually become more stable, but even now there is scarcely a day when the instruments called seismographs do not record earthquake shocks in some part of the earth; and the outbreaks of Vesuvius and Ætna, the constant boiling of lava in the craters of the Hawaiian Islands and other volcanic centres, prove that even now the earth's crust is very thin and unstable. The further back in time we go, the thinner was the crust, the more frequent the outbursts of volcanic activity, the more readily did wrinkles form.

The shores of New Jersey and of Greenland are gradually sinking, and the sea coming up over the land. Certain parts of the world are gradually rising out of the sea. In earlier times the rising or the sinking of land over large areas happened much more frequently than now.