Ants.  In the tropics the mantle of waste is worked over chiefly by ants.  They excavate underground galleries and chambers, extending sometimes as much as fourteen feet below the surface, and build mounds which may reach as high above it.  In some parts of Paraguay and southern Brazil these mounds, like gigantic potato hills, cover tracts of considerable area.

In search for its food—­the dead wood of trees—­the so-called white ant constructs runways of earth about the size of gas pipes, reaching from the base of the tree to the topmost branches.  On the plateaus of central Africa explorers have walked for miles through forests every tree of which was plastered with these galleries of mud.  Each grain of earth used in their construction is moistened and cemented by slime as it is laid in place by the ant, and is thus acted on by organic chemical agents.  Sooner or later these galleries are beaten down by heavy rains, and their fertilizing substances are scattered widely by the winds.

Earthworms. In temperate regions the waste is worked over largely by earthworms.  In making their burrows worms swallow earth in order to extract from it any nutritive organic matter which it may contain.  They treat it with their digestive acids, grind it in their stony gizzards, and void it in castings on the surface of the ground.  It was estimated by Darwin that in many parts of England each year, on every acre, more than ten tons of earth pass through the bodies of earthworms and are brought to the surface, and that every few years the entire soil layer is thus worked over by them.

In all these ways the waste is made fine and stirred and enriched.  Grain by grain the subsoil with its fresh mineral ingredients is brought to the surface, and the rich organic matter which plants and animals have taken from the atmosphere is plowed under.  Thus Nature plows and harrows on “the great world’s farm” to make ready and ever to renew a soil fit for the endless succession of her crops.

The world processes by which rocks are continually wasting away are thus indispensable to the life of plants and animals.  The organic world is built on the ruins of the inorganic, and because the solid rocks have been broken down into soil men are able to live upon the earth.

Solar energy.  The source of the energy which accomplishes all this necessary work is the sun.  It is the radiant energy of the sun which causes the disintegration of rocks, which lifts vapor into the atmosphere to fall as rain, which gives life to plants and animals.  Considering the earth in a broad way, we may view it as a globe of solid rock,—­the lithosphere,—­surrounded by two mobile envelopes:  the envelope of air,—­the atmosphere, and the envelope of water,—­the hydrosphere.  Under the action of solar energy these envelopes are in constant motion.  Water from the hydrosphere is continually rising in vapor into the atmosphere, the air of the atmosphere penetrates the hydrosphere,—­for its gases are dissolved in all waters,—­and both air and water enter and work upon the solid earth.  By their action upon the lithosphere they have produced a third envelope,—­the mantle of rock waste.