Page:Popular Science Monthly Volume 8.djvu/183

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171
ON A PIECE OF LIMESTONE.

of the Eocene formation, the earliest of the tertiaries. We find this limestone forming a heel of considerable thickness on the flanks of the Pyrenees, and extending from the shores of the Atlantic along the south of France to the Alps, in some parts of which it shows a thickness of fifteen hundred feet, thence across to Asia Minor, Northern India, and probably to the Pacific shore; while another division of it ranges along Northern Africa, and is especially noteworthy in Egypt, where it rises into the hills that border the Nile for a loner distance above Cairo, and furnishes the stone of which the Pyramids are built, and out of which the Sphinx is carved. This stone not merely contained nummulites, which are Foraminiferal shells much larger than Globigerinæ (sometimes attaining the size of a half-crown), but is entirely made up of them, and of the fragments of those which have been ground down by the action of the waves, as well as of other shells inhabiting the same sea; all cemented into a solid mass by the process I shall presently describe. Another limestone of more limited extent, belonging to the Eocene age, is found in the neighborhood of Paris, and has furnished the material of which that beautiful city is built. This is entirely made up of the minute Foraminiferal shells termed Miliolæ, from their resemblance in size to grains of millet, and is known as "miliolite limestone." So in Malta and in the neighborhood of Vienna, there are limestones entirely composed of shells, corals, and Foraminifera, which were formed in the Miocene or Middle Tertiary period. And we have on the coast of Suffolk the calcareous bed known as the "coralline crag," to which equivalents are found in various parts of Europe, that belongs to the Pliocene or Later Tertiary period. The material of this bed is chiefly contributed by the calcareous skeletons of composite animals that formerly ranked as zoöphytes, but are now distinguished as Polyzoa. Although individually extremely minute, in fact microscopic, they have a very complicated structure, allied to that of the lower Mollusks; and they extend themselves like trees by continuous budding, so that the fabrics they form often have a stony solidity. They abound in our own seas, and, as we shall presently find, they extend very far back in geological time.

Thus, then, we see that, in the case of the Secondary and Tertiary limestones, there can be no question of their production by the agency of animals, which separated carbonate of lime from its solution in sea-water, and formed it into corals, shells, etc., just as similar animals are doing at the present time. And we have in these calcareous deposits many instances of local "metamorphism," which show that the existence of a sub-crystalline, or even of a complete crystalline, arrangement in the particles of carbonate of lime is no proof that the materials of these deposits were not originally drawn from their solution by the agency which formed those whose organic origin is obvious. Thus in the neighborhood of the Giant's Causeway, where volcanic