THE ORIGIN OF FERTILE SOIL

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“Fertile or arable soil,” resumed Uncle Paul, “constitutes only the surface layer of earth, that which is worked by the farmer’s implements and yields nutriment to the roots of plants and promotes their development. In one place you will see bare rocks and utter barrenness; in another you find fertile soil to a depth of an inch or two, scantily carpeted with grass; and again, in a third, you come upon rich earth so deep as to maintain abundant vegetation. But nowhere does this fertile layer have an indefinite thickness: at a depth never very considerable a subsoil having the qualities of the neighboring mountains is sure to be found. How then has there come to be formed this layer of earth whence is derived all the nutriment required by plants, animals, and men?