In North America, lastly,
the Sub-Carboniferous series is never purely calcareous, though
in the interior of the continent it becomes mainly so. In other
regions, however, it consists principally of shales and sandstones,
with subordinate beds of limestone, and sometimes with this beds
of coal or deposits of clay-ironstone.
II. _The Millstone Grit_.--The highest beds of the Carboniferous
Limestone series are succeeded, generally with perfect conformity,
by a series of arenaceous beds, usually known as the _Millstone
Grit_. As typically developed in Britain, this group consists of
hard quartzose sandstones, often so large-grained and coarse in
texture as to properly constitute fine conglomerates. In other
cases there are regular conglomerates, sometimes with shales,
limestones, and thin beds of 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.
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