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THE QUESTION OF WORKABLE
1. Deep-sea marine formations, consisting for the most part of
limestones with marine animal remains, known as the Carboniferous
Limestone, passing up into Yoredale shales and sandstones. In some
areas (as in Devon) this formation changes its character to the Culm,
the marine fauna being feebly represented along with a considerable
development of a land-flora.
2. Shore-formations indicating a gradual shallowing of the sea,
consisting of grits and sandstones, known in English geology as the
Millstone-grit.
3. Fresh-water marsh and lagoon formations, broken by insigni-
ficant oscillations and occasional shallow sea deposits, consisting of
the Coal-bearing strata with land-plants in abundance, the vegetation,
where sufficiently concentrated (chiefly in the middle and upper
strata), being mineralised into seams of coal, which together with the
sandstones and shales form the productive Coal-measures.
[The maximum thickness of the Millstone-grit and Coal-measures
in Britain exceeds 15,000 feet.]
4. Shore formations, consisting chiefly of conglomerates and
sandstones, bearing workable coal-seams in some parts of Europe,
and indicating gradual subsidence (of more limited areas than in t)
generally with a stratification quite discordant with that of the older
formations. This is the Rothliegendes, or Lower Dyas of German
geologists.
5. Deep-sea marine formations, consisting for the most part of
limestones with marine animal-remains, the Magnesian Limestone of
the north of England, the Zechstein of Germany.
The storage of carbon in the lithosphere of the earth in the form
of the mineralised vegetable-matter of our coal seams is not confined
to one stage of the earth's history, but it is to be found so concen-
trated at that stage as to exceed the aggregate amount of the work
done of this nature through all other periods put together ; and no
name in geology has, perhaps, been more felicitously chosen than that
which is universally given to the great Carboniferous System. How
was this brought about ? There must have been a cause, and recent
researches have brought the cause to light. We must recollect that
the small quantity of carbonic acid gas (about four parts in 10,000)
in our present atmosphere is no measure of what existed in the
atmosphere of palaeozoic times, for the simple reason that all the
carbon of our coal-seams, all the carbon of the later (Tertiary) Brown
Coal deposits, all the carbon locked up in the present vegetation and