COAL AND ITS CLASSIFICATION 267

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and hydrogenous or gas coal. In the last division he included
 cannel, torbanite, and * asphaltic coal” or albertite,
a fossil bitumen. Rogers’ classification was based on the
ratio of fixed carbon to volatile matter—the bituminous
coals containing from 52 to 84 per cent. of fixed carbon and
12 to 48 per cent. of volatile matter, and anthracite containing
 84 per cent. of fixed carbon and 7} per cent. of volatile
constituents. This system was developed by Persifor Fraser
(2nd Geol. Surv. Pennsylvania, MM, 1879, pp. 143-4), who
adopted the same divisions and based them on the ratio of
fixed carbon to volatile hydrocarbons, the ratios being in
anthracite 100: 12, in semi-anthracite 100: 67, in semibituminous
 (including dry steam coals) 100: 62; in bituminous
 between 100: 106 and 100: 25.
Classifications based on the ratio of carbon to hydrogen
have been adopted by Campbell of the United States Geological
 Survey; and C. H. Seyler for the coals of South Wales
(Proc. S. Wales 1.E., 1900, xxi, p. 483 ; xxii, p. 112). Seyler’s
five divisions are based on the percentage of hydrogen—
namely perbituminous with hydrogen more than 5-8 per cent.,
bituminous 5% to 5:8 per cent., semi-bituminous 4} to 5
per cent., carbonaceous 4 to 4} per cent., and the anthracitic
less than 4; each division is subdivided according to the
carbon percentage. This classification well illustrates the continuity
 of the coal series from lignite to anthracite, but is too
elaborate for general commercial use. A classification based
on both physical and chemical properties has been adopted
in the volumes on the Coal Resources of the World, issued
by the International Geological Congress, 1913, i, pp. Xi-xiii.
Tue Origin oF Coar—Coal is generally regarded as the
fossilized debris of ancient forests; most coal contains so
little wood that many authorities, such as Jeffreys, Lomax,
Hickling and Murray Stewart, have returned to the view of
Hutton in the eighteenth century, that coal was deposited
as a fine-grained carbonaceous silt carried by rivers to lakes
or the sea, or as a humic jelly or as sapropel, an organic mud.
Another explanation—of historic interest as it was supported
by Darwin and Huxley—was that coal is an injection along
some permeable layer of liquid bitumen, which has incorporated
 the remains of plants.
Some coal has been formed in situ from the vegetation