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.

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