As compared
with typical oolites, the concretions in these limestones are
usually much more irregular in shape, often lengthened out and
almost cylindrical, at other times angular, the central nucleus
being of large size, and the surrounding envelope of lime being
very thin, and often exhibiting no concentric structure. In both
these and the ordinary oolites, the structure is fundamentally
the same. Both have been formed in a sea, probably of no great
depth, the waters of which were charged with carbonate of lime
in solution, whilst the bottom was formed of sand intermixed with
minute shells 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.
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