The thickness of the mantle.  In any locality the thickness of the mantle of rock waste depends as much on the rate at which it is constantly being removed as on the rate at which it is forming.  On the face of cliffs it is absent, for here waste is removed as fast as it is made.  Where waste is carried away more slowly than it is produced, it accumulates in time to great depth.

The granite of Pikes Peak is disintegrated to a depth of twenty feet.  In the city of Washington granite rock is so softened to a depth of eighty feet that it can be removed with pick and shovel.  About Atlanta, Georgia, the rocks are completely rotted for one hundred feet from the surface, while the beginnings of decay may be noticed at thrice that depth.  In places in southern Brazil the rock is decomposed to a depth of four hundred feet.

In southwestern Wisconsin a reddish residual clay has an average depth of thirteen feet on broad uplands, where it has been removed to the least extent.  The country rock on which it rests is a limestone with about ten per cent of insoluble impurities.  At least how thick, then, was that portion of the limestone which has rotted down to the clay?

Distinguishing characteristics of residual waste. We must learn to distinguish waste formed in place by the action of the weather from the products of other geological agencies.  Residual waste is unstratified.  It contains no substances which have not been derived from the weathering of the parent rock.  There is a gradual transition from residual waste into the unweathered rock beneath.  Waste resting on sound rock evidently has been shifted and was not formed in place.

In certain regions of southern Missouri the land is covered with a layer of broken flints and red clay, while the country rock is limestone.  The limestone contains nodules of flint, and we may infer that it has been by the decay and removal of thick masses of limestone that the residual layer of clay and flints has been left upon the surface.  Flint is a form of quartz, dull-lustered, usually gray or blackish in color, and opaque except on thinnest edges, where it is translucent.

Over much of the northern states there is spread an unstratified stony clay called the drift.  It often rests on sound rocks.  It contains grains of sand, pebbles, and bowlders composed of many different minerals and rocks that the country rock cannot furnish.  Hence the drift cannot have been formed by the decay of the rock of the region.  A shale or limestone, for example, cannot waste to a clay containing granite pebbles.  The origin of the drift will be explained in subsequent chapters.

The differences in rocks are due more to their soluble than to their insoluble constituents.  The latter are few in number and are much the same in rocks of widely different nature, being chiefly quartz, silicate of alumina, and iron oxide.  By the removal of their soluble parts very many and widely different rocks rot down to a residual clay gritty with particles of quartz and colored red or yellow with iron oxide.