We have shown how strata of solid rock are sometimes formed in lakes by the precipitation of lime from a state of solution. Now this process cannot take place in the sea; for though lime is present in the sea, the quantity of carbonic acid with which it is there associated, is far more than sufficient to render its precipitation impossible.[49] But Nature has another contrivance for gathering together the solid elements of her building. The depths of the ocean are teeming with life; and countless tribes of minute animals are furnished with the power of extracting the lime from the waters they inhabit, and of reproducing it under a new form. Sometimes, through this mysterious operation of organic life, the lime is converted into a calcareous shell, like that of the oyster; sometimes into a stony skeleton, as in the case of the numerous families of coral-producing animalcules. After death the soft, fleshy substance of these animals melts away and disappears; but the limestone shells and skeletons remain, accumulating during the long course of ages to an almost incredible extent. And, if we are to believe Geologists, out of these accumulated materials, sometimes preserving their original form and structure, sometimes altered more or less by chemical action, sometimes broken up into fragments by mechanical force, has been produced a very large proportion of the limestone rocks which occur so abundantly in the Crust of the Earth. No better illustration can be found than the white earthy limestone, familiar to every one under the name of chalk. An undulating stratum of Chalk Rock, attaining not unfrequently a thickness of one thousand feet, may be said, speaking roughly, to underlie the southeastern half of England. Sometimes it appears at the surface: sometimes it dips downward, and forms a kind of great basin, over which are regularly spread out various other groups of Stratified Rocks. On the southern coast it rises to a height of several hundred feet above the level of the sea in a line of perpendicular cliffs, conspicuous from a distance by their dazzling whiteness. But the White Chalk of England is only an insignificant part of a great rock-formation, which may be traced over extensive areas throughout all Europe, from Ireland to the Crimea, from the Baltic Sea to the Bay of Biscay; and which everywhere preserves in a remarkable degree the same mineral character, and presents to the eye the same general appearance.