In a broad way the changes which rocks undergo in weathering are an adaptation to the environment in which they find themselves at the earth’s surface,—­an environment different from that in which they were formed under sea or under ground.  In open air, where they are attacked by various destructive agents, few of the rock-making minerals are stable compounds except quartz, the iron oxides, and the silicate of alumina; and so it is to one or more of these comparatively insoluble substances that most rocks are reduced by long decay.

Which produces a mantle of finer waste, frost or chemical decay? which a thicker mantle?  In what respects would you expect that the mantle of waste would differ in warm humid lands like India, in frozen countries like Alaska, and in deserts such as the Sahara?

The soil.  The same agencies which produce the mantle of waste are continually at work upon it, breaking it up into finer and finer particles and causing its more complete decay.  Thus on the surface, where the waste has weathered longest, it is gradually made fine enough to support the growth of plants, and is then known as soil.  The coarser waste beneath is sometimes spoken of as subsoil.  Soil usually contains more or less dark, carbonaceous, decaying organic matter, called humus, and is then often termed the humus layer.  Soil forms not only on waste produced in place from the rock beneath, but also on materials which have been transported, such as sheets of glacial drift and river deposits.  Until rocks are reduced to residual clays the work of the weather is more rapid and effective on the fragments of the mantle of waste than on the rocks from which waste is being formed.  Why?

Any fresh excavation of cellar or cistern, or cut for road or railway, will show the characteristics of the humus layer.  It may form only a gray film on the surface, or we may find it a layer a foot or more thick, dark, or even black, above, and growing gradually lighter in color as it passes by insensible gradations into the subsoil.  In some way the decaying vegetable matter continually forming on the surface has become mingled with the material beneath it.

How humus and the subsoil are mingled. The mingling of humus and the subsoil is brought about by several means.  The roots of plants penetrate the waste, and when they die leave their decaying substance to fertilize it.  Leaves and stems falling on the surface are turned under by several agents.  Earthworms and other animals whose home is in the waste drag them into their burrows either for food or to line their nests.  Trees overthrown by the wind, roots and all, turn over the soil and subsoil and mingle them together.  Bacteria also work in the waste and contribute to its enrichment.  The animals living in the mantle do much in other ways toward the making of soil.  They bring the coarser fragments from beneath to the surface, where the waste weathers more rapidly.  Their burrows allow air and water to penetrate the waste more freely and to affect it to greater depths.