amount of quartz and alkali-felspar present. It is almost always more
than would be the case if the rock consisted of kaolin mixed with
muscovite. Alumina is high in the finer clays (18 to 30%), and they are
the most aluminous of all sediments, except bauxite. Magnesia is never
absent, though its amount may be less than 1%; it is usually contained
in minerals of the chlorite group, but partly also in dolomite. The
alkalis are very interesting; often they form 5 or 10% of the whole
rock; they indicate abundance of white micas or of undecomposed
particles of felspar. Some clays, however, such as fireclays, contain
very little potash or soda, while they are rich in alumina; and it is a
fair inference that hydrated aluminous silicates, such as kaolin, are
well represented in these rocks. There are, in fact, a few clays which
contain about 45% of alumina, that is to say, more than in pure kaolin.
It is probable that these are related to bauxite and certain kinds of
laterite.

A few of the most important clay rocks, such as china-clay, brick-clay,
red-clay and shale, may be briefly described here.

_China-clay_ is white, friable and earthy. It occurs in regions of
granite, porphyry and syenite, and usually occupies funnel-shaped
cavities of no great superficial area, but of considerable depth. It
consists of very fine scaly kaolin, larger, shining plates of white
mica, grains of quartz and particles of semi-decomposed felspar,
tourmaline, zircon and other minerals, which originally formed part of
the granite. These clays are produced by the decomposition of the
granite by acid vapours, which are discharged after the igneous rock has
solidified ("fumarole or pneumatolytic action"). Fluorine and its
compounds are often supposed to have been among the agencies which
produce this change, but more probably carbonic acid played the
principal role. The felspar decomposes into kaolin and quartz; its
alkalis are for the most part set free and removed in solution, but are
partly retained in the white mica which is constantly found in crude
china-clays. Semi-decomposed varieties of the granite are known as
china-stone. The kaolin may be washed away from its original site, and
deposited in hollows or lakes to form beds of white clay, such as
pipe-clay; in this case it is always more or less impure. Yellow and
pinkish varieties of china-clay and pipe-clay contain a small quantity
of oxide of iron. The best known localities for china-clay