LIMESTONE, in petrography, a rock consisting essentially of calcium carbonate ("carbonate of lime"). Many varieties are included within the group, some of which are very distinct, but all have certain properties in common, arising from the similar chemical composition, and mineralogical characters. All lime stones, exclusive of dolomite (q.v.) dissolve readily in cold dilute acids, giving off bubbles of carbon dioxide. Even "weak" acids like citric or acetic acid will effect this change, though the mineral acids are more commonly employed. Limestones are readily scratched with a knife-blade or the edge of a coin, their hardness being 3; but unless they are earthy or incoherent, like chalk or calcareous sinter, they do not disintegrate by pressure with the fingers and cannot be scratched with the finger-nail. When free from impurities limestones are white, but they generally contain small quantities of minerals other than crystalline calcium car bonate which affect their colour. The presence of iron oxide or clay results in creamy, yellowish or brownish coloration; iron sulphide and carbonate, or carbonaceous and bituminous impuri ties, render limestones bluish, grey or black. Crystalline lime stones or marbles, which in the process of metamorphism (see METAMORPHISM) have been mineralogically reconstituted under the action of heat or pressure, frequently contain minerals like epidote, chlorite, olivine, serpentine, garnet, spinel and augite of varying colours. The specific gravity of limestones ranges from 2.6 to 2.8 in typical examples.
In the field, limestones are usually recognizable by their charac teristic method of weathering. Where reasonably pure, they may display solution-phenomena such as smooth, rounded surfaces, or may be covered with narrow runnels cut by rain. In such cases there is little soil, and plants and trees grow only in fissures where insoluble impurities from the limestone have collected. Thus in mountainous districts limestones tend to yield bare rocky ground (e.g., the karst) or country covered by scanty soil and short grass. The rain which falls on them sinks into the earth and passes underground. All waters in such regions tend to be hard on account of the abundant carbonate of lime dissolved during percolation. Thus caves, swallow-holes, sinks, pot-holes and underground rivers are formed. By redeposition of the carbonate of lime in underground caves stalactites, stalagmites and deposits of travertine, often beautifully tinted by impurities, are produced.
The chalk downs are celebrated for their close green swards; surface-waters are often notably scarce, although the chalk in depth holds large supplies of water. Impure limestones such as those of the Cornbrash, Oolites and Lias contain enough impurities in the form of clay, sand and other mineral matter to yield thick soils of considerable agricultural value.
Most limestones are of organic origin, and are formed from the debris of the skeletons of animals. Off the coast of Florida a fine chalky deposit (drewite) is being laid down on the sea-bed, apparently as a result of the precipitating power of bacteria. Foraminifera, crinoids (sea-lilies), shells and corals build up con siderable masses of limestone. Some bands of the chalk are largely composed of foraminifera such as Globigerina. Another notable foraminiferal limestone, that made by the disc-shaped Nummulites, is well developed in Mediterranean countries. From this rock the pyramids of Egypt were built.
Crinoidal limestones, although frequently found in the older rocks, such as the Carboniferous, are not now being formed on any scale, for these organisms are not abundant at the present day. The small cylindrical joints (or ossicles) of these organisms are easily recognized ("St. Cuthbert's beads") on the Northum brian coast, washed out of the Carboniferous Limestone. Coral limestones are being formed over a large extent of the tropical seas. Ancient coral-reefs have been recognized in the older rocks of the earth's crust. They are usually composed of pure limestone (free from silt and sand), but may subsequently be altered to dolomite, as in the well-known region of the Tirol. Shelly lime stones may consist of Mollusca or of Brachiopoda, the former occurring plentifully in Secondary and Tertiary rocks, but the latter in limestones of rather earlier age. The remains are often fragmentary, and the limestones may, like shore-sands to-day, consist of a mixture of comminuted shell-fragments and muddy, silty or even sandy material. Other organisms such as corallines (bryozoa), sea-urchins, starfishes, crustaceans and sponges con tribute material to the formation of these rocks. Fresh-water lime stones are composed usually of the shells of Mollusca (e.g., Paludina limestone) which inhabit lakes; they are often marly from an admixture of clay.