Similarly, other limestones are composed almost entirely of the skeletons of corals; and such old coralline limestones can readily be paralleled by formations which we can find in actual course of production at the present day.

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.