the celebrated "Nummulitic Limestone" (fig. 10),
which sometimes attains a thickness of some thousands of feet--which
are almost entirely made up of the shells of _Foraminifera_. In
the case of the "Nummulitic Limestone," just mentioned, these
shells are of large size, varying from the size of a split pea
up to that of a florin. There are, however, as we shall see,
many other limestones, which are likewise largely made up of
_Foraminifera_, but in which the shells are very much more minute,
and would hardly be seen at all without the microscope.

[Illustration: Fig. 10.--Piece of Nummulitic Limestone from the
Great Pyramid. Of the natural size. (Original.)]

We may, in fact, consider that the great agents in the production
of limestones in past ages have been animals belonging to the
_Crinoids_, the _Corals_, and the _Foraminifera_. At the present
day, the Crinoids have been nearly extinguished, and the few known
survivors seem to have retired to great depths in the ocean; but
the two latter still actively carry on the work of lime-making,
the former being very largely helped in their operations by certain
lime-producing marine plants (_Nullipores_ and _Corallines_). We
have to remember, however, that though the limestones, both ancient
and modern, that we have just spoken of, are truly organic, they
are not necessarily formed out of the remains of animals which
actually lived on the precise spot where we now find the limestone
itself. We may find a crinoidal limestone, which we can show to
have been actually formed by the successive growth of generations
of sea-lilies _in place_; but we shall find many others in which
the rock is made up of innumerable fragments of the skeletons
of these creatures, which have been clearly worn and rubbed by
the sea-waves, and which have been mechanically transported to
their present site. In the same way, a limestone may be shown
to have been an actual coral-reef, by the fact that we find in
it great masses of coral, growing in their natural position,
and exhibiting plain proofs that they were simply quietly buried
by the calcareous sediment as they grew; but other limestones
may contain only numerous rolled and water-worn fragments of
corals. This is precisely paralleled by what we can observe in
our existing coral-reefs. Parts of the modern coral-islands and
coral-reefs are really made up of corals, dead or alive, which
actually grew on the spot where we now find them; but other p