Firm underfoot before freezing, this surface loses its firmness after a thaw and is pushed up here and there in little finely-powdered clods. At the moment of freezing, the humidity with which the soil was impregnated turned into ice which, increasing in volume, reduced to fine particles the surface layer of the road. When the thaw comes, these particles which the ice no longer holds together form first mud, then dust. In exactly this manner arable land was formed by the disintegration of rocks of all kinds, which were reduced to particles by the action of frost. But soil suitable for agriculture contains not only powdery mineral matter, but also a little mold from the decomposition of vegetable matter. To give you an idea of the causes which from the very earliest times have little by little fertilized this rock-dust with vegetable mold, let us take the following example. Geography has taught you what a volcano is. It is a mountain whose summit is hollowed out in an immense funnel-shaped excavation called a crater. From time to time the ground trembles near a volcano and formidable noises similar to the rolling of thunder and the booming of cannon are heard from the depths of the mountain. The crater throws up into the air a lofty column of smoke, dark by day, fiery red at night. All at once the mountain is rent and vomits up through the crevices a stream of fire, a current of melted rock, or lava. Finally the volcano quiets down; the source of the terrible flood dries up. The streams of lava harden and cease running; and after a lapse of time which may be years they become quite cold. Now what is to become of this enormous bed of black stone similar in character to the slag from a forge? What will this sheet of lava covering an area of several square miles produce?