oal (the
"Coal-measures").

I. The _Carboniferous, Sub-Carboniferous_, or _Mountain Limestone
Series_ constitutes the general base of the Carboniferous system.
As typically developed in Britain, the Carboniferous Limestone
is essentially a calcareous formation, sometimes consisting of a
mass of nearly pure limestone from 1000 to 2000 feet in thickness,
or at other times of successive great beds of limestone with
subordinate sandstones and shales. In the north of England the
base of the series consists of pebbly conglomerates and coarse
sandstones; and in Scotland generally, the group is composed
of massive sandstones with a comparatively feeble development
of the calcareous element. In Ireland, again, the base of the
Carboniferous Limestone is usually considered to be formed by
a locally-developed group of grits and shales (the "Coomhola
Grits" and "Carboniferous Slate"), which attain the thickness
of about 5000 feet, and contain an intermixture of Devonian with
Carboniferous types of fossils. Seeing that the Devonian formation
is generally conformable to the Carboniferous, we need feel no
surprise at this intermixture of forms; nor does it appear to be
of great moment whether these strata be referred to the former
or to the latter series. Perhaps the most satisfactory course
is to regard the Coomhola Grits and Carboniferous Slates as
"passage-beds" between the Devonian and Carboniferous; but any
view that may be taken as to the position of these beds, really
leaves unaffected the integrity of the Devonian series as a distinct
life-system, which, on the whole, is more closely allied to the
Silurian than to the Carboniferous. 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