III. _The Coal-measures_.--The Coal-measures properly so called
rest conformably upon the Millstone Grit, and usually consist of
a vast series of sandstones, shales, grits, and coals, sometimes
with beds of limestone, attaining in some regions a total thickness
of from 7000 to nearly 14,000 feet. Beds of workable coal are
by no means unknown in some areas in the inferior group of the
Sub-Carboniferous; but the general statement is true, that coal is
mostly obtained from the true Coal-measures--the largest known, and
at present most productive coal-fields of the world being in Great
Britain, North America, and Belgium. Wherever they are found, with
limited exceptions, the Coal-measures present a singular _general_
uniformity of mineral composition. They consist, namely, of an
indefinite alternation of beds of sandstone, shale, and coal,
sometimes with bands of clay-ironstone or beds of limestone,
repeated in no constant order, but sometimes attaining the enormous
aggregate thickness of 14,000 feet, or little short of 3 miles.
The beds of coal differ in number and thickness in different
areas, but they seldom or never exceed one-fiftieth part of the
total bulk of the formation in thickness. The characters of the
coal itself, and the way in which the coal-beds were deposited,
will be briefly alluded to in speaking of the vegetable life
of the period.
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