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.