THE CARBONIFEROUS PERIOD. 161 



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 palaeontological 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, signalized 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 



