ain, North America and elsewhere, are very
important beds of ironstone, fire-clay, terra-cotta clay, and
occasionally oil shale and alum shale. Oil and gas are of importance
in the Lower Carboniferous Pocono sandstone of West Virginia and in
the Berea grit of Ohio, where brine also occurs.

In the Carboniferous Limestone series, the purer kinds of limestone
are used for the manufacture of lime, bleaching powder and similar
products, also as a flux in the smelting of iron; some of the less
pure varieties are used in making cement. The beds of chert are
utilized in the pottery industry, and some of the harder and more
crystalline limestones are beautiful marbles, capable of taking a high
polish.

The sandstones are used for building, and for millstones and
grindstones. Within the Carboniferous rocks, but due to the action of
various agencies long after their deposition, are important ore
formations; such are the Rio Tinto ores of Spain, the lead and zinc
ores and some haematite of the Pennine and Mendip hills and other
British localities, and many ore regions in the United States.

REFERENCES.--For a good general account of the Carboniferous system,
see A. Geikie, _Text Book of Geology_, vol. ii. (4th ed., 1903); and
for the American development see T.C. Chamberlin and R.D. Salisbury,
_Geology_, vol. ii. (1906). These two works give abundant references
to the literature of the subject. See also, _Recent Additions to
Geological Literature_, published annually by the Geological Society
of London since 1893; and _Neues Jahrbuch fur Mineralogie_
(Stuttgart). (J. A. H.)

CARBORUNDUM, a silicide of carbon formed by the action of carbon on sand
(silica) at high temperatures, which on account of its great hardness is
an important abrasive, and also has possible applications in the
metallurgy of iron and steel. Its name was derived from _carbon_ and
_corundum_ (a form of alumina), from a mistaken view as to its
composition. It was first obtained accidentally in 1891 by Acheson in
America, when he was experimenting with the electric furnace in the hope
of producing artificial diamonds. The experiments were followed up in an
incandescence furnace, which on a larger scale is now employed for the
industrial manufacture of the product. A full description of the process
has been given in the _Journ. Soc. Chem. Industry_, 1897, vol. xvi. p.
863. The furnace is rectangular, a