en up, the silica is taken into solution
and carried away, and hydrous aluminum oxides remain as bauxite ores.
This extreme type of weathering is sometimes called lateritic alteration
(see pp. 172-173). Impurities of the bauxite ores are the small
quantities of iron and titanium present in the original rocks, together
with the kaolin which has not been broken up. The deposits usually form
shallow blankets over considerable areas, with irregular lower surfaces
determined by the action of surface waters--which work most effectively
where joints or other conditions favor the maximum circulation and
alteration. A certain degree of porosity in the original rock is also
known to favor the alteration. A complete gradation from the unaltered
rock through clay to the high-grade bauxite, with progressive decrease
in bases and silica, concentration of alumina and iron oxide, and
increase of moisture and pore space, is frequently evident (see Fig.
13). The bauxite is earthy, and usually shows a concretionary or
pisolitic structure similar to that observed in residual iron ores (p.
172). Near the surface there may be an increase in silica,--probably due
to a reversal of the usual conditions by a slight leaching of alumina,
thus concentrating the denser masses of kaolin which have not been
decomposed.

The Arkansas bauxite deposits, the most important in the United States,
are surface deposits overlying nepheline-syenite, an igneous rock with
a high ratio of alumina to iron content. The most valuable deposits are
residual, and some parts have preserved the texture of the original
rock, though with great increase in pore space; most of the ore,
however, has the typical pisolitic structure. Near the surface the
pisolites are sometimes loosened by weathering, yielding a gravel ore,
and some of the material has been transported a short distance to form
detrital ores interstratified with sands and gravels. The complete
gradation from syenite to bauxite has been shown.

[Illustration: FIG. 13. Diagram showing gradation from syenite
to bauxite in terms of volume. The columns represent a series of samples
from a single locality in Arkansas. After Mead.]

In the Appalachian region of Tennessee, Alabama, and Georgia, bauxite
occurs as pockets in residual clays above sedimentary rocks, chiefly
above shales and dolomites. Its origin has probably been similar to that
described.

The bauxite deposits of southern France occur in folded limeston