Black marbles owe their colour to the presence of
very minute particles of carbonaceous matter, in some cases at
any rate; and they may either be metamorphic, or they may be
charged with minute fossils such as _Foraminifera_ (_e.g._, the
black limestones of Ireland, and the black marble of Dent, in
Yorkshire).
[Illustration: Fig. 13.--Slice of oolitic limestone from the
Jurassic series (Coral Rag) of Weymouth; magnified. (Original.)]
"_Oolitic_" _limestones_, or "_oolites_," as they are often called,
are of interest both to the palaeontologist and geologist. The
peculiar structure to which they owe their name is that the rock
is more or less entirely composed of spheroidal or oval grains,
which vary in size from the head of a small pin or less up to
the size of a pea, and which may be in almost immediate contact
with one another, or may be cemented together by a more or less
abundant calcareous matrix. When the grains are pretty nearly
spherical and are in tolerably close contact, the rock looks very
like the roe of a fish, and the name of "oolite" or "egg-stone"
is in allusion to this. When the grains are of the size of peas
or upwards, the rock is often called a "pisolite" (Lat. _pisum_,
a pea). Limestones having this peculiar structure are especially
abundant in the Jurassic formation, which is often called the
"Oolitic series" for this reason; but essentially similar limestones
occur not uncommonly in the Silurian, Devonian, and Carboniferous
formations, and, indeed, in almost all rock-groups in which
limestones are largely developed.
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