158 HISTORICAL PALAZONTOLOGY. 
Stratigraphically, the Carboniferous rocks usually repose 
conformably upon the highest Devonian beds, so that the line 
of demarcation between the Carboniferous and Devonian for- 
mations is principally a palzontological one, founded on the 
observed differences in the fossils of the two groups. On the 
other hand, the close of the Carboniferous period seems to 
have been generally, though not universally, signalised by 
movements of the crust of the earth, so that the succeeding 
Permian beds often lie unconformably upon the Carboniferous 
sediments. 
Strata of Carboniferous age have been discovered in almost 
every large land-area which has been sufficiently investigated , 
but they are especially largely developed in Britain, in various 
parts of the continent of Europe, and in North America. 
Their general composition, however, is, comparatively speak- 
ing, so uniform, that it will suffice to take a comprehensive 
view of the formation without considering any one area in 
detail, though in each region the subdivisions of the formation 
are known by distinctive local names. Taking such a com- 
prehensive view, it is found that the Carboniferous series is 
generally divisible into a Lower and essentially calcareous 
group (the ‘‘ Sub-Carboniferous” or ‘Carboniferous Lime- 
stone”); a Middle and principally arenaceous group (the 
“‘ Millstone Grit”); and an Upper group, of alternating shales 
and sandstones, with workable seams of coal (the ‘“ Coal- 
measures ”). 
I. The Carboniferous, Sub-Carboniferous, or Mountain Lime- 
stone 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 Scot- 
land 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 
