They are said to form the FLOOR on which the coal rests; and
some of them have a slight admixture of carbonaceous matter, while others are
quite blackened by it.
All of them, as Sir William Logan pointed out, are characterised by inclosing a
peculiar species of fossil vegetable called Stigmaria, to the exclusion of other
plants. It was also observed that, while in the overlying shales, or "roof" of
the coal, ferns and trunks of trees abound without any Stigmariae, and are
flattened and compressed, those singular plants of the underclay most commonly
retain their natural forms, unflattened and branching freely, and sending out
their slender rootlets, formerly thought to be leaves, through the mud in all
directions. Several species of Stigmaria had long been known to botanists, and
described by them, before their position under each seam of coal was pointed
out, and before their true nature as the roots of trees (some having been
actually found attached to the base of Sigillaria stumps) was recognised. It was
conjectured that they might be aquatic, perhaps floating plants, which sometimes
extended their branches and leaves freely in fluid mud, in which they were
finally enveloped.
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