524 HISTORICAL PAL/EONTOLOGY. 



CHAPTER L. 



CARBONIFEROUS PERIOD. 

 ROCKS OF THE PERIOD. 



OVERLYING the great formation of the Old Red Sandstone, or 

 Devonian Rocks, sometimes unconformably but more often in 

 perfect conformity, we have the large and important series of 

 the Carboniferous Rocks, so called because workable beds of 

 coal are more commonly developed in this than in any other 

 formation. It must not be forgotten, however, that coal is not 

 exclusively a Carboniferous product, but that workable seams 

 of coal occur in several formations younger than the Carboni- 

 ferous. In all cases, too, the coal forms but a very small pro- 

 portion of the actual thickness of the Carboniferous Rocks, 

 occurring in comparatively thin beds intercalated in a great 

 series of sandstones, shales, and limestones. 



The Carboniferous Rocks are largely developed in Britain, 

 on the continent of Europe, and in North America, and are 

 known to occur in other parts of the world also. Their general 

 composition, however, is, comparatively speaking, so uniform, 

 that it will be sufficient to take a general view of the formation 

 without considering each area separately. As a general rule, 

 the Carboniferous Rocks may be divided into the following 

 three groups, from below upward : 



1. The Carboniferous Slates and Mountain Limestone, mainly 

 and most typically calcareous. Sometimes termed the Sub- 

 carboniferous group. 



2. The Millstone Grit, essentially arenaceous and conglome- 

 ratic. 



3. The Coal-measures, composed of alternating shales, sand- 

 stones, and other strata, with workable beds of coal. 



I. The CARBONIFEROUS, SUB-CARBONIFEROUS, or MOUNTAIN, 

 LIMESTONE constitutes ordinarily the base of the Carboniferous 

 system. In Ireland, however, and elsewhere, the lowest beds 

 of the Carboniferous series are slates and grits, which attain a 

 maximum thickness of 5000 feet, and have been termed the 

 Carboniferous Slates. Their fossils are partially referable to 

 good Carboniferous types, and partly to Devonian forms, so 

 that they may be regarded as passage-beds. The Carboni- 

 'ferous limestone proper in its most typical development, as in 

 Wales and the west of England, consists of a great mass of 

 nearly pure limestone, from 1000 to 2000 feet thick, with a 



