he original silicates of the rock, and less of the altered
minerals.

The production of soil from sedimentary rocks involves the same
processes as alter igneous rocks; but, starting from rocks of different
composition, the result is of course different in some respects.
Sandstones by weathering yield only a sandy soil. Limestones lose their
calcium carbonate by solution, leaving only clay with fragments of
quartz or chert as impurities. A foot of soil may represent the
weathering of a hundred feet of limestone. Shales may weather into
products more nearly like those of the weathering of igneous rocks.
Silicates in the shales are broken down to form clay, which is mixed
with the iron oxide and quartz.

In some localities the soil may accumulate to a considerable depth,
allowing the processes of weathering to go to an extreme; in others the
processes may be interrupted by erosion, which sweeps off the weathered
products at intermediate stages of decomposition and may leave a very
thin and little decomposed soil.

Soils formed by weathering may remain in place as residual soils, or
they may be transported, sorted, and redeposited, either on land or
under water. It is estimated by the United States Bureau of Soils[14]
that upward of 90 per cent of the soils of the United States which have
been thus far mapped owe their occurrence and distribution to
transportation by moving water, air, and ice (glaciers), and that less
than 10 per cent have remained in place above their parent rock.
Glaciers may move the weathered rock products, or they may grind the
fresh rocks into a powder called _rock flour_, and thus form soils
having more nearly the chemical composition of the unaltered rocks.
Glacial soils are ordinarily rather poorly sorted, while wind and
water-borne soils are more likely to show a high degree of sorting.

The character of a transported soil is less closely related to the
parent rock than is that of a residual soil, because the processes of
sorting and mixture of materials from different sources intervene to
develop deposits of a nature quite different from residual soils; but
even transported soil may sometimes be traced to a known rock
parentage.

Where deposited under water, soil materials may be brought above the
water by physiographic changes, and exposed at the surface in condition
for immediate use. Or, they may become buried by other sediments and not
be exposed again until after they have been pretty