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.
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