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).  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 parts are composed of a limestone-rock ("coral-rock"), or of a loose sand ("coral-sand"), which is organic in the sense that it is composed of lime formed by living beings, but which, in truth, is composed of fragments of the skeletons of these living beings, mechanically transported