Limestone
a general name for all rocks the basis of which is carbonate of lime. It is not generally pure, but has the surface, bringing up from the subas Auvergne and Tuscany, in which extinct volcanoes exist, hundreds of springs, both cold and thermal, rise to the surface, bringing up from the subterranean depths carbonate of lime. The quantity thus transferred to the surface in the course of ages is enormous, and manifold streams and rivers carry it into the sea. Mollusks withdraw it for their shells, corals for their reefs, and in due time the upheaval of the sea bed converts it into a terrestrial limestone rock. Metamorphic action can destroy it, and there is little limestone in rocks which have undergone such a change. When what used to be called primitive, now often composed entirely of organic refound in such beds, it is probably of organic origin. Modern limestones are often composed entirely of organic remains or are studded thickly with them. Thus crinoidal or encrinital limestone is composed chiefly of encrinites, chalk of foraminifers, nummulitic limestone of nummulites, and shells of Saccamina, or of spindle-shaped Fusulina. Sometimes