266 ECONOMIC GEOLOGY 
coal or clay. In some cannel coals the volatile material is of 
animal origin, and may be derived largely from fossil fish. 
Cannel coals are of three types—ordinary or black cannel 
with a ratio of carbon to hydrogen of 10 to I, and is usually 
coking; brown cannel, torbanite or boghead coal, the 
variety richest in volatile oil-producing constituents, has 
a ratio of carbon to hydrogen of 10 to 14, contains 20 to 30 
per cent. of ash and does not coke; and earthy cannel or 
oil shale which contains up to 80 or 85 per cent. of ash. 
Torbanite was the first material used on a large scale for the 
distillation of mineral oil, of which it yielded 120 gallons 
to the ton. It is named from Torbane Hill near Bathgate 
in Scotland, where the first large oil shale works were estab- 
lished. Microscopic sections show that it consists of well- 
bedded layers of brown coaly material, enclosing numerous 
yellow bodies which have been regarded as Algae by Bertrand 
and Renault, and as spores by Jeffrey ; some of those bodies 
have been formed by the re-arrangement of organic material 
during the consolidation of the coal, and are similar in appear- 
ance to the spherocrystals of inulin found in plants. The 
boghead coal of Torbane Hill contained from 60 to 66 per 
cent. of carbon, 8 to 9 per cent. of hydrogen, 4 to 8 per cent. 
of oxygen, % to 1% per cent. of nitrogen, and 20 to 26 per cent. 
of ash. 
Earthy cannel or oil shale differs from torbanite by the 
higher proportion of ash. It yields usually between 15 and 
50 gallons of oil per ton. There is a gradual increase in ash 
from 20 per cent. in torbanite to 82 per cent. in the lowest 
worked grade of oil shale (cf. p. 204). 
CLASSIFICATION AND ORIGIN oF COALS 
The arrangement of the series from wood to graphite is 
simple as the chemical, physical, and commercial charac- 
teristics agree; but it is difficult to express in one classification 
of coals the factors of practical value and the history and 
composition of the material. The modern classification of 
coals was founded in 1858 by H. D. Rogers, then Professor 
in the University of Glasgow (Geology Pennsylvania, vol. ii, 
pt. 2, pp. 988-95). He divided coals into five groups— 
anthracite, semi-anthracite, semi-bituminous, bituminous,