264 ECONOMIC GEOLOGY 
Its age is generally Upper Pal®ozoic, and mainly Carboni- 
ferous; but it is found in the Bathonian rocks at Brora in 
Northern Scotland, and in still later rocks that have under- 
gone much disturbance. The two chief varieties of bitu- 
minous 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 con- 
stituents 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 pro- 
perty 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 ex- 
pressed 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 de- 
composition 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