way, and the latter only requires consolidation to become actually converted into chalk.  Both are fundamentally organic deposits, apparently requiring a great depth of water for their accumulation, and mainly composed of the remains of Foraminifera, together with the entire or broken skeletons of other marine animals of greater dimensions.  It is to be remembered, however, that the ooze, though strictly representative of the chalk, cannot be said in any proper sense to be actually identical with the formation so called by geologists.  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.  The varieties of limestone are almost innumerable, but the great majority can be clearly proved to agree with chalk in being essentially of organic origin, and in being more or less largely composed of the remains of living beings.  In many instances the organic remains which compose limestone are so large as to be readily visible to the naked eye, and the rock is at once seen to be nothing more than an agglomeration of the skeletons, generally fragmentary, of certain marine animals, cemented together by a matrix of carbonate of lime.  This is the case, for example, with the so-called “Crinoidal Limestones” and “Encrinital Marbles” with which the geologist is so familiar, especially as occurring in great beds amongst the older formations of the earth’s crust.  These are seen, on weathered or broken surfaces, or still better in polished slabs (fig. 9), to be composed more or less exclusively of the broken stems and detached plates of sea-lilies (Crinoids).  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.”