Limestone, the popular as well as technical name for all rocks which are composed in whole, or to a large extent, of carbonate of lime. Few minerals are so extensively distributed in nature as this, and, in some form or other, limestone rocks occur in every geological system. Carbonate of lime is nearly insoluble in pure water, but it is rendered easily soluble by the presence of carbonic acid gas, which occurs in a variable quantity in all natural waters, for it is absorbed by water in its passage through the air as well as through the earth. Carbonate of lime in solution is consequently found in all rivers, lakes, and seas. In evaporation water and carbonic acid gas are given off, but the carbonate of lime remains uninfluenced, becoming gradually concentrated, until it has supersaturated the water, when a precipitation takes place. In this way are formed the stalactites which hang icicle-like from the roofs of limestone caverns, and the stalagmites which rise as columns from their floors. Travertine (Tiber-stone), or Calcareous Tufa (q.v.), is similarly formed in running streams, lakes, and springs, by the deposition of the carbonate of lime on the beds or sides, where it encrusts and binds together shells, fragments of wood, leaves, stones, &c. So also birds' nests, twigs, and other objects become coated with lime in the so-called petrifying wells, as that at Knaresborough. From the same cause pipes conveying water from boilers and mines often become choked up, and the tea-kettle gets lined with 'fur.'
While water is thus the great storehouse of carbonate of lime, very little of it, however, is fixed by precipitation, for in the ocean evaporation does not take place to such an extent as to permit it to deposit; besides, there is five times the quantity of free carbonic acid gas in the water of the sea that is required to keep the carbonate of lime in it in solution. Immense quantities of lime are nevertheless being abstracted from the sea, to form the hard portions of the numerous animals which inhabit it. Crustacea, mollusca, zoophytes, and foraminifera are ever busy separating the little particles of carbonate of lime from the water, and solidifying them, and so supply the materials for forming solid rock. It has been found that a large portion of the bed of the Atlantic between Europe and North America is covered with a light-coloured ooze, composed chiefly of the perfect or broken skeletons of foraminifera, forming a substance, when dried, which in appearance and structure closely resembles chalk. In tropical regions corals are building reefs of enormous magnitude, corresponding in structure to many of the limestones met with in various geological systems.
The chief varieties of limestone are Chalk (q. v.); Oolite (q. v.); Compact Limestone, a hard, smooth, fine-grained rock, generally of a bluish-gray colour; Crystalline Limestone, a rock which, from metamorphic action, has become granular; fine-grained white varieties, resembling loaf-sugar in texture, called Saccharine or Statuary Marble. Particular names are given to some limestones from the kind of fossils that abound in them, as Nummulite, Hippurite, and Crinoidal limestones; or the presence of impurities or admixtures of other mineral matter may give rise to varieties, as argillaceous, ferruginous, siliceous, carbonaceous, and magnesian limestones. Hydraulic limestones contain a certain proportion of silica and alumina which forms a mortar that sets in water. Many limestones, again, derive their name from the system to which they belong, as Silurian, Devonian, Carboniferous, Jurassic, &c.