tion, as for example lime carbonate and magnesium
carbonate, which go to make up _limestone_ and _dolomite_. Some of the
dissolved substances are never redeposited, but remain in solution as
salts in the sea, the most abundant of which is sodium chloride. Some of
the dissolved substances of weathering, such as calcite, quartz, and
iron oxide, are carried down and deposited in openings of the rocks,
where they act as cements.

The sediments as a whole consist of three main types,--_shales_ (kaolin,
quartz, etc.), _sandstones_ (quartz, feldspar, etc.), and _limestones_
or _dolomites_ (carbonates of lime and magnesia). Of these, the shale
group is by far the most abundant. There are of course many sediments
with composition intermediate between these types. There are also
sediments made up of large undecomposed fragments of the original rocks,
cemented to form _conglomerates_, or made up of small fragments of the
original rocks cemented to form _arkoses_ and _graywackes_. These,
however, may be regarded as simply stages in the alteration, which in
repeated cycles of weathering must ultimately result in producing the
three main groups,--shales, sandstones, and limestones.

Mineral products formed by sedimentary processes include sandstones,
limestones, and shales, used as building stone and road materials;
certain sedimentary deposits of iron, like the Clinton ores of the
southeastern United States and the Brazilian ores; important phosphate
deposits; most deposits of salt, gypsum, potash, nitrates, etc.;
comparatively few and unimportant copper deposits; and important placer
deposits of gold, tin, and other metals, and precious stones. With the
aid of organic agencies, sedimentary processes also account for the
primary deposition of coal and oil.

WEATHERING OF SEDIMENTARY ROCKS

After sedimentary rocks are formed, and in many cases covered by later
sediments, they may be brought again by earth movements and erosion to
the surface, where they in turn are weathered. The weathering of
sedimentary rocks proceeds along lines already indicated for the igneous
rocks. Residual mantles of impure clay and sand are commonly formed.
The mineral composition of sedimentary rocks being different from that
of igneous rocks to start with, the resulting products are in slightly
different proportions; but the changes are the same in kind and tend
merely to carry the general process of alteration farther in the same
direction,--that is