Thus, when any animal bodies are undergoing decay at the
bottom of the sea, they have a tendency to cause the precipitation
from the surrounding water of any mineral matters which may be
dissolved in it; and the organic body thus becomes a centre round
which the mineral matters in question are deposited in the form
of a "concretion" or "nodule." The phosphatic nodules in question
were formed in a sea in which phosphate of lime, derived from the
destruction of animal skeletons, was held largely in solution;
and a precipitation of it took place round any body, such as a
decaying animal substance, which happened to be lying on the
sea-bottom, and which offered itself as a favourable nucleus. In
the same way we may explain the formation of the calcareous nodules,
known as "septaria" or "cement stones," which occur so commonly in
the London Clay and Kimmeridge Clay, and in which the principal
ingredient is carbonate of lime. A similar origin is to be ascribed
to the nodules of clay iron-stone (impure carbonate of iron) which
occur so abundantly in the shales of the Carboniferous series and
in other argillaceous deposits; and a parallel modern example
is to be found in the nodules of manganese, which were found
by Sir Wyville Thomson, in the Challenger, to be so numerously
scattered over the floor of the Pacific at great depths.
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