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 [[18]]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?
“This desolate, blasted expanse seems destined never to be clothed with verdure. But in any such assumption one would be mistaken. After centuries and centuries a vigorous growth of oaks, beeches, and other large trees will have taken root there. In fact, you will see that air, rain, snow, and, above all, frost attack in turn the hard surface of the lava, detach fine particles from it, and slowly produce a little dust at its expense. On this dust there will spring into being certain strange and hardy plants, those white or yellow patches, those vegetable incrustations, calculated to live on the surface of stone and known as lichens. These lichens fasten themselves to the lava, gnaw it still more, and in dying leave a little mold formed from their decaying remains. On this precious mold, lodged in some cavity of the lava, there is now a growth of mosses which perish in their turn and increase the quantity [[19]]of fertilizing material. Next come ferns, which require a richer soil, and after that a few tufts of grass; then some brambles, some meager shrubs; and thus with each succeeding year the fertile soil is added to from the new remnants of lava and mold left by the preceding generation of plants that have gone to decay. It is in this way that gradually a lava-bed finally becomes covered with a forest.
“Our own arable land had a similar origin. Sterile rocks, hard as they are, contributed the mineral part by being reduced to dust through the combined action of water, air, and frost; and the successive generations of plant-life, beginning with the simplest, furnished the mold.
“Notice how admirably, in the processes of nature, the smallest of created beings perform their part and contribute as best they can to the general harmony. To produce fertile soil there is needed something more than the frosts and thaws that crumble the hardest rock: there is need of plants hardy enough to live on this sterile soil, such as tough grasses, mosses, lichens, which gnaw the stone. It is through the medium of these rudimentary plants, so pitiful in appearance and yet so hardy, that the dust of the rocks is enriched with mold and converted into a soil capable of bearing other and more delicate plants.
“It is not in cultivated fields that you will find those thick carpets of mosses and lichens, valiant disintegrators of stone; it is on the mountain-tops that they can be seen at their work of crusting over [[20]]the smooth rock in order to convert it into fertile soil. It is from these heights that this fertile soil has descended, little by little, washed down by the rain, until it has fertilized the valleys. This work is going on all the time; in hilly regions plants of the lowest order are constantly adding to the extent of arable land. The little threads of rain-water that furrow these regions carry away with them some of this humus and bear it to the plains below.
“What a worthy subject for our thoughtful study is this formation of arable soil by these legions of inferior plants, obscure workers indefatigably crumbling the rock! What immense results obtained by the simplest means!” [[21]]