THE CARBONIFEROUS PERIOD. 159 
appear to be of great moment whether these strata be referred 
to the former or to the latter series. Perhaps the most satis- 
factory course is to regard the Coomhola Grits and Carbon- 
iferous 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 Car- 
boniferous. 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 subor- 
dinate beds of limestone, and sometimes with thin beds of 
coal or deposits of clay-ironstone. 
Il. Zhe Millstone Grit.—The highest beds of the Carbon- 
iferous 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 conglomer- 
ates, 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 Brit- 
ish equivalent, consists of coarse sandstones and grits, some- 
times 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. Zhe Coal-measures.—TYhe 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 state- 
ment is true, that coal is mostly obtained from the true Coal- 
measures—the largest known, and at present most produc- 
tive 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, 
