264 ECONOMIC GEOLOGY

Its age is generally Upper Pal®ozoic, and mainly Carboniferous;
 but it is found in the Bathonian rocks at Brora in
Northern Scotland, and in still later rocks that have undergone
 much disturbance. The two chief varieties of bituminous
 coal are the coking and non-coking. In coking
coal some of the material becomes plastic and agglutinates
the rest into firm cellular coke, from which the volatile constituents
 have been expelled as gas, pitch, coal-tar, and oil.
Coke is a smokeless fuel, and is indispensable for blast
furnaces as non-coking coals fall to pieces when burnt. The
cause of coking has been long discussed, and is not yet
fully understood. Many coals lose their coking quality on
exposure to the atmosphere, and by gentle heating. The
clue to this property was given by Anderson, who found that
a coal which loses its coking property when heated in air
retains it if heated in carbon dioxide. The loss of the property
 is due to the oxidation of some resinoid constituent,
on the nature of which much light has been thrown by the
researches of Prof. W. A. Bone (Proc. R. Soc., A, 1922, c,
Pp. 582-08; cv, 1024, p. 625). The coking quality of coal is expressed
 by the *‘ coking index,” which is the proportion of
sand that is cemented by a given weight of the powdered coal
when heated in a crucible. Coal with a coking index of less
than 171 is useless for coke; that with an index of 12-15 can
be mixed with better coking coal; the grades 16-19 include
good, and 19-28 the best coking coals.
Bituminous coal often consists of layers or patches of four
different types of material. The dull black, often powdery
charcoal-like material, known as mother-of-coal, or fusain,
has been regarded as charcoal made by forest fires during
the formation of the coal seam ; it appears to be derived from
material which has undergone carbon enrichment by decomposition
 before burial in the seam. Dull hard layers in
coal, which are opaque in thin sections and contain many
spores, consist of durain; the glossy translucent variety is
clarain ; and the jet-like structureless bands are vitrain.
AnTtHRACITE—Anthracite is a hard black coal which does
not stain the hand, burns slowly without smoke and with a
slight blue flame, and gives off intense heat. It cannot be
kindled with wood; but if ignited by bituminous coal or
gas it burns steadily until entirely consumed. Anthracite