The term limestone is applied to any stone the greater proportion of which consists of carbonate of lime; but the members of the class differ greatly in chemical composition, texture, hardness, and other physical characteristics.

Composition

Chalk, Portland stone, marble, and several other varieties of limestone, consist of nearly pure carbonate of lime, though they are very dissimilar in texture, hardness, and weathering qualities.

Other limestones, such as the dolomites, contain a very large proportion of carbonate of magnesia. Some contain clay, a large proportion of which converts them into marls, and makes them useless for building purposes. Many limestones contain a considerable proportion of silica, some contain iron, others bitumen.

The carbonate of lime in stones of this class is, of course, liable to attack from the carbonic acid dissolved in the moisture of ordinary air, and is in time destroyed by the more violent acids and vapours generally found in the atmosphere of large towns.

Texture

A great deal depends therefore upon the texture of the stone.

The best weathering limestones are dense, uniform, and homogeneous in structure and composition, with fine even small grains, and of a crystalline texture.

Some limestones consist of a mass of fossils, either entire, or broken up and united by cementing matter. Others are entirely made up of round grains of carbonate of lime, generally held together by cement of the same material. (See p. 56.)

The Royal Commissioners gave a preference to limestones as a class, "on account of their more general uniformity of tint, their comparatively homogeneous structure, and the facility and economy of their conversion to building purposes;" and of this class they preferred "those which are most crystalline."

Many of the most easily worked limestones are very soft when first quarried, but harden upon exposure to the atmosphere.

"This is said to arise from a slight decomposition taking place, which will remove most of the softer particles and leave the hardest and most durable to act as a protection to the remainder."1 By others it is attributed to the escape of the "quarry damp."

1 Guide to Museum of Practical Geology, by R Hunt, F.R.S.

Classification

Limestones are classed: - 1st, Scientifically from a geological point of view; or, 2d, Practically, according to their physical characteristics.

Scientific Classification

Limestones are known as Carboniferous, Lias, etc., according to the formation from which they are obtained. These formations are shown in the Tables, pp. 67 to 73, but they need not be further referred to.

Practical Classification

The terms Liver rock, Freestone, Flagstone, are applied to limestones in the same way as to sandstones (see p. 35).

The difference in the physical characteristics of limestones leads to their classification by the engineer as follows: -

Marbles.

Compact limestones. Granular „ Shelly Magnesian „

These will now be described in turn.