COAL AND ITS CLASSIFICATION 271 
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stage and the main agents in the conversion of peat or vege- 
table 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 bitu- 
minous 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 re- 
gard anthracite and bituminous coals as formed from dif- 
ferent kinds of vegetation. 
The general evidence is however in favour of the formation 
of anthracite from bituminous coal. It is true that anthra- 
cite 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 coal- 
fields of New Zealand, The fossil plants of the anthracitic