coal--the thickness of the whole
series, when well developed, varying from 1000 to 5000 feet. In
North America, the Millstone Grit rarely reaches 1000 feet in
thickness; and, like its British equivalent, consists of coarse
sandstones and grits, sometimes with regular conglomerates. Whilst
the Carboniferous Limestone was undoubtedly deposited in a tranquil
ocean of considerable depth, the coarse mechanical sediments
of the Millstone Grit indicate the progressive shallowing of
the Carboniferous seas, and the consequent supervention of
shore-conditions.

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. In Britain, and in the Old World generally, the
Coal-measures are composed partly of genuine terrestrial
deposits--such as the coal--and partly of sediments accumulated
in the fresh or brackish waters of vast lagoons, estuaries, and
marshes. The fossils of the Coal-measures in these regions are
therefore necessarily the remains either of terrestrial plants
and animals, or of such forms of life as inhabit fresh or brackish
waters, the occurrence of strata with marine fossils being quite
a local and occasional phenomenon. In va