and fragments of the skeletons of larger marine
animals. The excess of lime in the sea-water was precipitated
round the sand-grams, or round the smaller shells, as so many
nuclei, and this precipitation must often have taken place time
after time, so as to give rise to the concentric structure so
characteristic of oolitic concretions. Finally, the oolitic grains
thus produced were cemented together by a further precipitation
of crystalline carbonate of lime from the waters of the ocean.

[Illustration: Fig. 14.--Slice of arenaceous and oolitic limestone
from the Carboniferous series of Shap, Westmoreland; magnified.
The section also exhibit _Foraminifera_ and other minute fossils.
(Original.)]

_Phosphate of Lime_ is another lime-salt, which is of interest to
the palaeontologist. It does not occur largely in the stratified
series, but it is found in considerable beds [4] in the Laurentian
formation, and less abundantly in some later rock-groups, whilst
it occurs abundantly in the form of nodules in parts of the
Cretaceous (Upper Greensand) and Tertiary deposits. Phosphate
of lime forms the larger proportion of the earthy matters of the
bones of Vertebrate animals, and also occurs in less amount in the
skeletons of certain of the Invertebrates (_e.g._, _Crustacea_). It
is, indeed, perhaps more distinctively than carbonate of lime, an
organic compound; and though the formation of many known deposits
of phosphate of lime cannot be positively shown to be connected
with the previous operation of living beings, there is room for
doubt whether this salt is not in reality always primarily a
product of vital action. The phosphatic nodules of the Upper
Greensand are erroneously called "coprolites," from the belief
originally entertained that they were the droppings or fossilised
excrements of extinct animals; and though this is not the case,
there can be little doubt but that the phosphate of lime which
they contain is in this instance of organic origin.[5] It appears,
in fact, that decaying animal matter has a singular power of
determining the precipitation around it of mineral salts dissolved
in water. 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 "conc