retion" 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. In
accordance with this mode of origin, it is exceedingly common
to find in the centre of all these nodules, both old and new,
some organic body, such as a bone, a shell, or a tooth, which
acted as the original nucleus of precipitation, and was thus
preserved in a shroud of mineral matter. Many nodules, it is
true, show no such nucleus; but it has been affirmed that all of
them can be shown, by appropriate microscopical investigation,
to have been formed round an original organic body to begin with
(Hawkins Johnson).

[Footnote 4: Apart from the occurrence or phosphate of lime in
actual beds in the stratified rocks, as in the Laurentian and
Silurian series, this salt may also occur disseminated through
the rock, when it can only be detected by chemical analysis. It
is interesting to note that Dr Hicks has recently proved the
occurrence of phosphate of lime in this disseminated form in
rocks as old as the Cambrian, and that in quantity quite equal to
what is generally found to be present in the later fossiliferous
rocks. This affords a chemical proof that animal life flourished
abundantly in the Cambrian seas.]

[Footnote 5: It has been maintained, indeed, that the phosphatic
nodules so largely worked for agricultural purposes, are in
themselves actual organic bodies or true fossils. In a few cases
this admits of demonstration, as it can be shown that the nodule
is simply an organism (such as a sponge) infiltrated with