394 
ELEMENTS OF GEOLOGY. 
CHAPTER XXIII. 
THE COAL OR CARBONIFEROUS GROUP. 
Principal Subdivisions of the Carboniferous Group.—Different Thickness of 
the sedimentary and calcareous Members in Scotland and the South of 
England.—Coal-measures.—Terrestrial Nature of the Growth of Coal.— 
Erect fossil Trees.—Uniting of many Coal-seams into one thick Bed.— 
Purity of the Coal explained.—Conversion of Coal into Anthracite.—Ori¬ 
gin of Clay-ironstone.—Marine and brackish-water Strata in Coal.—Fossil 
Insects.—Batrachian Reptiles.—Labyrinthodont Foot-prints in Coal-meas¬ 
ures.—Nova Scotia Coal-measures with successive Growths of erect fossil 
Trees.—Similarity of American and European Coal.—Air-breathers of the 
American Coal.—Changes of Condition of Land and Sea indicated by the 
Carboniferous Strata of Nova Scotia. 
Principal Subdivisions of the Carboniferous Group. —The next 
group which we meet with in the descending order is the 
Carboniferous, commonly called “ The Coal,” because it con¬ 
tains many beds of that mineral, in a more or less pure state, 
interstratified with sandstones, shales, and limestones. The 
coal itself, even in Great Britian and Belgium, where it is 
most abundant, constitutes but an insignificant portion of the 
whole mass. In South Wales, for example, the tliickness of 
the coal-bearing strata has been estimated at between 11,000 
and 12,000 feet, while the various coal seams, about 80 in 
number, do not, according to Prof. Phillips, exceed in the ag¬ 
gregate 120 feet. 
The carboniferous formation assumes various characters in 
. different parts even of the British Islands. It usually com¬ 
prises two very distinct members: 1st, the sedimentary beds, 
usually called the Coal-measures, of mixed fresh-water, ter¬ 
restrial, and marine origin, often including seams of coal; 
2dly, that named in England the Mountain or Carboniferous 
Limestone, of purely marine origin, and made up chiefly of 
corals, shells, and encrinites, and resting on shales called the 
shales of the Mountain Limestone. 
In the souflh-western part of our island, in Somersetshire 
and South Wales, the three divisions usually spoken of are: 
1. Coal-measures.. 
2. Millstone-grit... 
Strata of shale, sandstone, and grit, from 600 to 12,000 
feet thick, with occasional seams of coal. 
A coarse quartzose sandstone passing into a conglom¬ 
erate, sometimes used for millstones, with beds of 
shale ; usually devoid of coal; occasionally above 600 
feet thick. 
