A great lapse of time separates the two, and though composed of the remains of representative classes or groups of animals, it is only in the case of the lowly-organised _Globigerinoe_, and of some other organisms of little higher grade, that we find absolutely the same kinds or species of animals in both.
[Illustration: Fig.

8 .-- Organisms in the Atlantic Ooze, chiefly _Foraminifera_ (_Globigerina_ and _Textularia_), with _Polycystina_ and sponge-spicules; highly magnified.

(Original.)] [Illustration: Fig.

9 .-- Slab of Crinoidal marble, from the Carboniferous limestone of Dent, in Yorkshire, of the natural size.

The polished surface intersects the columns of the Crinoids at different angles, and thus gives rise to varying appearances.
(Original.)] _Limestone_, like chalk, is composed of carbonate of lime, sometimes almost pure, but more commonly with a greater or less intermixture of some foreign material, such as alumina or silica.