Their bark was surrounded by a dense mass of air-roots,
which often constituted a great addition to the original stem, so as to double
or quadruple its diameter. The same remark holds good in regard to certain
living extra-tropical arborescent ferns, particularly those of New Zealand.
Upon the whole, it is evident that the Permian plants approach much nearer to
the Carboniferous flora than to the Triassic; and the same may be said of the
Permian fauna.
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.
Origin of Clay-ironstone.
Marine and brackish-water Strata in Coal.
Fossil Insects.
Batrachian Reptiles.
Labyrinthodont Foot-prints in Coal-measures.
Nova Scotia Coal-measures with successive Growths of erect fossil Trees.
Similarity of American and European Coal.
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