e limestones are composed of an aggregate of minute
but perfectly distinct crystals, but that minute organisms of
different kinds, or fragments of larger fossils, are often present
as well. Other magnesian limestones, again, exhibit no striking
external peculiarities by which the presence of magnesia would be
readily recognised, and though the base of the rock is crystalline,
they are replete with the remains of organised beings. Thus many
of the magnesian limestones of the Carboniferous series of the
North of England are very like ordinary limestone to look at,
though effervescing less freely with acids, and the microscope
proves them to be charged with the remains of _Foraminifera_
and other minute organisms.

_Marbles_ are of various kinds, all limestones which are sufficiently
hard and compact to take a high polish going by this name. Statuary
marble, and most of the celebrated foreign marbles, are "metamorphic"
rocks, of a highly crystalline nature, and having all traces
of their primitive organic structure obliterated. Many other
marbles, however, differ from ordinary limestone simply in the
matter of density. Thus, many marbles (such as Derbyshire marble)
are simply "crinoidal limestones" (fig. 9); whilst various other
British marbles exhibit innumerable organic remains under the
microscope. 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