COAL AND ITS CLASSIFICATION 271

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stage and the main agents in the conversion of peat or vegetable
 tissue into coal are heat and pressure in the absence
of air. Wood immersed in a peat bog becomes hard and black,
and resembles jet. The heat of a burning coal seam converts
mine timber in an open position into charcoal, but timber
buried in sodden ground becomes hard, black, and brittle
like coal.
Coal has been artificially prepared by E. Bergius (¥. Soc.
Chem. Ind., xxxil, 1913, pp. 4626) by heating peat to 584° F.
in the presence of water at the pressure of 5000 atmospheres.
The carbon percentage in the artificial coal depends upon the
temperature and length of treatment. Coal with 84 per
cent. carbon (excluding ash and moisture) required 229
hours at 500° F., but was obtained in 21 hours at 644° F.
Bergius therefore calculated that peat under pressure in the
presence of water at 50° F. would be converted into bituminous
 coal in 8 million years, and into anthracite in a still
longer period. Rogers in 1858 (Geol. Pennsylvania, ii,
pt. 2, pp. 996-7) maintained that anthracite was formed
from ordinary coal by debituminization, as he termed it.
De la Beche (1848) had adopted that view for South Wales,
since the coals are more anthracitic in the lower part of
the Coal Measures. Strahan and Pollard (*‘ Coals of South
Wales,” Mem. Geol. Surv., 1908) on the contrary hold that
anthracite cannot have been formed from bituminous coal,
as seams of both are interbedded and must have been subject
to the same physical conditions. Strahan and Pollard regard
 anthracite and bituminous coals as formed from different
 kinds of vegetation.
The general evidence is however in favour of the formation
of anthracite from bituminous coal. It is true that anthracite
 is not dependent upon igneous intrusions, which are
absent from the Pennsylvanian fields, and in South Wales
are older than the Coal Measures, nor upon faults which are
often later than the change into anthracite. Nevertheless
the deeper seams which have been under greater pressure
and subject to higher temperature, generally contain less
volatile matter than the upper seams; fields which have
undergone especially severe dislocation yield anthracite,
e.g. in Belgium, France, the Alps, and the Cretaceous coalfields
 of New Zealand, The fossil plants of the anthracitic