We only need to transport ourselves to the
islands of the Pacific, to the West Indies, or to the Indian
Ocean, to find great masses of lime formed similarly by living
corals, and well known to everyone under the name of "coral-reefs."
Such reefs are often of vast extent, both superficially and in
vertical thickness, and they fully equal in this respect any of
the coralline limestones of bygone ages. Again, we find other
limestones--such as 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_).
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