Page:Popular Science Monthly Volume 8.djvu/182

170 you have a splendid illustration of the great Oolitic formation, which is almost entirely made up of calcareous deposits that can be clearly traced to an animal origin, although their condition is now very different. The Coral Rag of Oxfordshire is an old coral-reef that has undergone very little change, consisting of fossil corals, and of the shells, crinoids, etc., that lived on the reef. And the "freestones" of Bath and Portland are mainly composed of the fine sand which was formed by the wearing-down of similar reefs, of which the remains are found here and there. The name "oolite" or roe-stone, is given to the whole formation, on account of the resemblance in texture borne by some of its characteristic members to the roe of a fish; but this "oolitic" structure is not peculiar to the Oolitic formation, being found in other limestones, as I shall presently point out to you. A very curious example of the "metamorphic" action by which the texture of a calcareous rock may be so completely altered as to conceal its origin is afforded, by the fact that the beautiful Carrara marble, which is used for statuary, belongs to the Oolitic formation. If this metamorphism, the nature of which I shall presently explain, proceeds further, it will produce large crystals of calc-spar; and I remember that Mr. Baily, the sculptor of the beautiful statue of "Eve at the Fountain," which is in your Fine Arts Gallery, was greatly embarrassed by a vein of calc-spar that ran through the block from which he cut it, and had to let a patch of marble into Eve's back. The next great calcareous formation above the Oolite is the Chalk, the material of which is exactly the same as that of limestone, although its texture is so different. Our deep-sea researches have entirely confirmed the opinion which had been previously formed on the basis of microscopic research, that the whole of the enormous mass of Chalk now raised up into the cliffs and downs of the southern portion of England was formed on the bed of the ocean, by the agency of animals—chiefly the minute Foraminifera, which separate carbonate of lime from the seawater as the material of their shells; just as successive generations of fresh-water mussels living in a lake form a bed of calcareous marl on its bottom by the decay of their shells, which sets free in a solid form the lime they have taken from the water that poured it into the lake in solution. We have brought up by the hundred-weight, from depths of three miles in the Atlantic, a white mud, which, when dried, exactly resembles chalk; and this, when examined with the microscope, is found to consist partly of perfect shells of minute Globigerinæ, of which many hundreds would only weigh a grain, and partly of what we call Globigerina ooze, which is obviously the product of the decay of former generations of similar shells.

In the Tertiary or Neozic (modern life) series, we find many limestone deposits of considerable importance, but none so vast as those to which I have previously drawn your attention. The most extensive is the "nummulitic limestone," which is one of the oldest members