th the mud. In a few places a red colour prevails, the iron
being mostly oxidized; elsewhere the muds are green owing to abundant
glauconite. Traced landwards the muds become more sandy, while on their
outer margins they grade into the abysmal deposits, such as the
globigerina ooze (see OCEAN AND OCEANOGRAPHY). Near volcanoes they
contain many volcanic minerals, and around coral islands they are often
in large part calcareous.

Microscopic sections of some of the more coherent clays and shales may
be prepared by saturating them with Canada balsam by long boiling, and
slicing the resultant mass in the same manner as one of the harder
rocks. They show that clay rocks contain abundant very small grains of
quartz (about 0.01 to 0.05 mm. in diameter), with often felspar,
tourmaline, zircon, epidote, rutile and more or less calcite. These may
form more than one-third of an ordinary shale; the greater part,
however, consists of still smaller scales of other minerals (0.01 mm. in
diameter and less than this). Some of these are recognizable as pale
yellowish and white mica; others seem to be chlorite, the remainder is
perhaps kaolin, but, owing to the minute size of the flakes, they yield
very indistinct reactions to polarized light. They are also often
stained with iron oxide and organic substances, and in consequence their
true nature is almost impossible to determine. It is certain, however,
that the finer-grained rocks are richest in alumina, and in combined
water; hence the inference is clear that kaolin or some other hydrous
aluminium silicate is the dominating constituent. These results are
confirmed by the mechanical analysis of clays. This process consists in
finely pulverizing the soil or rock, and levigating it in vessels of
water. A series of powders is obtained progressively finer according to
the time required to settle to the bottom of the vessel. The clay is
held to include those particles which have less than 0.005 mm. diameter,
and contains a higher percentage of alumina than any of the other
ingredients.

As might be inferred from the differences they exhibit in other
respects, clay rocks vary greatly in their chemical composition. Some of
them contain much iron (yellow, blue and red clays); others contain
abundant calcium carbonate (calcareous clays and marls). Pure clays,
however, may be found almost quite free from these substances. Their
silica ranges from about 60 to 45%, varying in accordance with the