are Cornwall,
Limoges (France), Saxony, Bohemia and China; it is found also in
Pennsylvania, N. Carolina and elsewhere in the United States.

_Fire-clays_ include all those varieties of clay which are very
refractory to heat. They must contain little alkalis, lime, magnesia and
iron, but some of them are comparatively rich in silica. Many of the
clays which pass under this designation belong to the Carboniferous
period, and are found underlying seams of coal. Either by rapid growth
of vegetation, or by subsequent percolation of organic solutions, most
of the alkalis and the lime have been carried away.

Any argillaceous material, which can be used for the manufacture of
bricks, may be called a _brick-clay_. In England, Kimmeridge Clay, Lias
clays, London Clay and pulverized shale and slate are all employed for
this purpose. Each variety needs special treatment according to its
properties. The true brick-clays, however, are superficial deposits of
Pleistocene or Quaternary age, and occur in hollows, filled-up lakes and
deserted stream channels. Many of them are derived from the glacial
boulder-clays, or from the washing away of the finer materials contained
in older clay formations. They are always very impure.

The _red-clay_ is an abysmal formation, occurring in the sea bottom in
the deepest part of the oceans. It is estimated to cover over fifty
millions of square miles, and is probably the most extensive deposit
which is in course of accumulation at the present day. In addition to
the reddish or brownish argillaceous matrix it contains fresh or
decomposed crystals of volcanic minerals, such as felspar, augite,
hornblende, olivine and pumiceous or palagonitic rocks. These must
either have been ejected by submarine volcanoes or drifted by the wind
from active vents, as the fine ash discharged by Krakatoa was wafted
over the whole globe. Larger rounded lumps of pumice, found in the clay,
have probably floated to their present situations, and sank when
decomposed, all their cavities becoming filled with sea water. Crystals
of zeolites (phillipsite) form in the red-clay as radiate, nodular
groups. Lumps of manganese oxide, with a black, shining outer surface,
are also characteristic of this deposit, and frequently encrust pieces
of pumice or animal remains. The only fossils of the clay are
radiolaria, sharks' teeth and the ear-bones of whales, precisely those
parts of the skeleton of marine creatures which are hardest