and can
longest survive exposure to sea-water. Their comparative abundance shows
how slowly the clay gathers. Small rounded spherules of iron, believed
by some to be meteoric dust, have also been obtained in some numbers.
Among the rocks of the continents nothing exactly the same as this
remarkable deposit is known to occur, though fine dark clays, with
manganese nodules, are found in many localities, accompanied by other
rocks which indicate deep-water conditions of deposit.

Another type of red-clay is found in caves, and is known as _cave-earth_
or _red-earth_ (_terra rossa_). It is fine, tenacious and bright red,
and represents the insoluble and thoroughly weathered impurities which
are left behind when the calcareous matter is removed in solution by
carbonated waters. Similar residual clays sometimes occur on the surface
of areas of limestone in hollows and fissures formed by weathering.

_Boulder-clay_ is a coarse unstratified deposit of fine clay, with more
or less sand, and boulders of various sizes, the latter usually marked
with glacial striations.

Some clay rocks which have been laid down by water are very uniform
through their whole thickness, and are called _mud-stones_. Others split
readily into fine leaflets or laminae parallel to their bedding, and
this structure is accentuated by the presence of films of other
materials, such as sand or vegetable debris. Laminated clays of this
sort are generally known as _shales_; they occur in many formations but
are very common in the Carboniferous. Some of them contain much organic
debris, and when distilled yield paraffin oil, wax, compounds of
ammonia, &c. In these oil-shales there are clear, globular, yellow
bodies which seem to be resinous. It has been suggested that the
admixture of large quantities of decomposed fresh-water algae among the
original mud is the origin of the paraffins. In New South Wales,
Scotland and several parts of America such oil-shales are worked on a
commercial scale. Many shales contain great numbers of ovoid or rounded
septarian nodules of clay ironstone. Others are rich in pyrites, which,
on oxidation, produces sulphuric acid; this attacks the aluminous
silicates of the clay and forms aluminium sulphate (_alum shales_). The
lias shales of Whitby contain blocks of semi-mineralized wood, or jet,
which is black with a resinous lustre, and a fibrous structure. The
laminated structure of shales, though partly due to successive very thin