LIMESTONE, the popular as well RS technica. 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 epoch. Carbonate of lime is nearly insolithle in pure water, but it is ren dered 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, is shnilarly formed in running streams, lakes, and springs, by the deposition of the carbonate of lime on the beds or sides, where it incrusts and binds together shells, fragments of wood, leaves, stones, etc. So also birds' nests, wigs, and other objects become coated with Hate 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 store-house of carbonate of lime, very little of it, how ever, 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 front 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 supplying the materials for fortning 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-colored 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 rocks in the carboniferous and other formations. The rocks thus organically formed do not always occur as they were originally deposited; denudation has somethnes broken them up to redeposit them as a calcareous sediment. Great changes, too, may have taken place through metamorphic action in the texture of the rock, some lime stones being hard, others soft, some compact, concretionary, or crystalline.
The chief varieties of limestone are: chalk (q.v.); oolite (q.v.); compact lime,stone, a hard, smooth, fine-grained rock, generally of a bluish-gray color; crystalline limestone, a rock which, from metamorphic action, has become granular; fine-grained white varie ties, resembling loaf-sugar in texture, are called saccharine or statuary marble. Mayne sian limestone or dolomite (q.v.) is a rock iu which carbonate of magnesia is mixed with carbonate of lime. Particular names are given to sotne limestones from the kind of fossils that abound in them, as nummulite, hippurite, indusial, and crinoidal lime stones; and to others from the formation to which they belong, as Devonian, carbonif erous, and mountain limestones.