272
ECONOMIC GEOLOGY
and bituminous seams are the same. Of a series of neigh-
bouring bituminous seams some may be altered to anthracite
if the volatile constituents are able to escape, while other
seams may remain bituminous.
The distribution of anthracite in the more deeply buried
seams in especially disturbed fields, and in the older rocks
supports its formation from bituminous coal. This con-
clusion is supported by J. Roberts who has altered bitu-
minous coal to anthracite by heating at 9oo® F., and by the
analytic studies of G. Hickling (Tr. I.M.E., Ixxii, 1927, pp.
261-76) which show the continuity of the series from peat
to anthracite.
Coal RESOURCES
Coal is not only of supreme importance as fuel, but as an
indispensable chemical agent in the reduction of iron ores.
The former industrial hegemony of the British Isles was
largely due to cheap coal; and it has to share that position
with the United States and Germany owing to their still
greater coalfields. The discovery that Germany, instead of
being poor in coal, had more than all the rest of Europe, was
the dominating factor in European politics from 1890-1914.
The output of coal is greater than that of any other mineral.
In 1924 the United States had an output of 518 million
metric tons, Great Britain of 271-4, and Germany of 118-8.
After the three great coal-producing countries followed
France with 58:8, Poland 32, and Belgium 23. The output
from the United States first exceeded the British in 1809.
The coalfields of the United States have an area of 335,000
square miles, while that of the British is only 12,000 square
miles; hence the output per square mile of coalfield is 16
times as great in Britain as in the United States.
The vast annual output has often lead to fears as to how
long this supply can be maintained. Fortunately geological
research has discovered new coalfields faster than the old
fields have been worked out. The world's proved coal re-
sources are enormous. The estimates of the supplies assume
that coal will not be worked at a.greater depth than 4000 feet
owing to the rise of temperature; but 5000 feet is certainly
practicable, and is assumed as the limit on the Continent.
Seams less than I foot in thickness are excluded as too thin:
