So the proof seemed complete, that the coal had been formed out of
vegetation growing where it was buried. If any further proof for
that theory was needed, it would be found in this fact, most
ingeniously suggested by Mr. Boyd Dawkins. The resinous spores, or
seeds of the Lepidodendra make up--as said above--a great part of the
bituminous coal. Now those spores are so light, that if the coal had
been laid down by water, they would have floated on it, and have been
carried away; and therefore the bituminous coal must have been
formed, not under water, but on dry land.
I have dwelt at length on these further arguments, because they seem
to me as pretty a specimen as I can give my readers of that regular
and gradual induction, that common-sense regulated, by which
geological theories are worked out.
But how does this theory explain the perfect purity of the coal? I
think Sir C. Lyell answers that question fully in p. 383 of his
"Student's Elements of Geology." He tells us that the dense growths
of reeds and herbage which encompass the margins of forest-covered
swamps in the valley and delta of the Mississippi, in passing through
them, are filtered and made to clear themselves entirely before they
reach the areas in which vegetable matter may accumulate for
centuries, forming coal if the climate be favourable; and that in the
cypress-swamps of that region no sediment mingles with the vegetable
matter accumulated from the decay of trees and semi-aquatic plants;
so that when, in a very dry season, the swamp is set on fire, pits
are burnt into the ground many feet deep, or as far as the fire can
go down without reaching water, and scarcely any earthy residuum is
left; just as when the soil of the English fens catches fire, red-hot
holes are eaten down through pure peat till the water-bearing clay
below is reached.
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