xtures, and chemical composition. They are collectively
known as _katamorphic_ alterations, meaning destructive changes. The
zone in which these changes are at a maximum is called the _zone of
weathering_. This general zone is principally above the surface or level
of the ground-waters, but for some rocks it extends well below this
level. In some regions the ground-water level may be nearly at the
surface, and in others, especially where arid, it may be two thousand or
more feet down. Disintegrated weathered rocks form a blanket of variable
thickness, which is sometimes spoken of as the residual mantle, or
"mantle rock."

[Illustration: FIG. 1. Graphic representation of volume change
in weathering of a Georgia granite.]

Mineral products formed by weathering from common igneous rocks include
soils, clay, bauxite, and certain iron, chromite, and nickel ores. Again
the commercial importance of this group is not large, as compared with
products formed in other ways described below.

The same weathering processes described above for igneous rocks cause
considerable changes of economic significance in deposits formed as
igneous after-effects. In some cases they result in removing the less
valuable minerals, thus concentrating the more valuable ones, as well as
in softening the rock and making it easier to work; and in other cases
they tend to remove the valuable constituents, which may then be
redeposited directly below or may be carried completely out of the
vicinity. The _oxide zones_ of many ore bodies are formed by these
processes.

SEDIMENTARY PROCESSES

Sedimentary rocks are formed by the removal and deposition of the
weathered products of a land surface. Air, water, and ice, moving under
the influence of gravity and other forces, all aid in this transfer. The
broken or altered rock materials may be merely moved down slopes a
little way and redeposited on the surface, forming one type of
_terrestrial_ or _subaerial deposits_, or they may be transferred and
sorted by streams. When deposited in streams or near their mouths, they
are known as _river_, _alluvial_, or _delta deposits_. When carried to
lakes and deposited they form _lake deposits_. Ultimately the greater
part of them are likely to be carried to the ocean and deposited as
_marine sediments_.

Part of the weathered substances are carried mechanically as clay and
sand, which go to make up the _shale_ and _sandstone_ sediments. Part
are carried in solu