COAL AND ITS CLASSIFICATION 267 
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and hydrogenous or gas coal. In the last division he in- 
cluded 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 contain- 
ing 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 semi- 
bituminous (including dry steam coals) 100: 62; in bitu- 
minous between 100: 106 and 100: 25. 
Classifications based on the ratio of carbon to hydrogen 
have been adopted by Campbell of the United States Geolo- 
gical 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 con- 
tinuity 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 incorpor- 
ated the remains of plants. 
Some coal has been formed in situ from the vegetation