The term limestone is applied to any stone the greater proportion of which consists of lime carbonate; but the members of the class differ greatly in chemical composition, texture, hardness, and other physical characteristics. Chalk, Portland stone, marble, and several other varieties of limestone, consist of nearly pure lime carbonate, though they are very dissimilar in texture, hardness, and weathering qualities. Other limestones, such as the dolomites, contain a very largo proportion of magnesia carbonate. 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 iron, others bitumen. The lime carbonate 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. 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 crystalline texture. Some limestones consist of a mass of fossils, either entire, or broken up and united by cementing matter.

Others are made up of round grains of lime carbonate, generally held together by cement of the same material. Many give 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 prefer 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. By others it is attributed to the escape of the "quarry damp." The difference in the physical characteristics of limestones leads to their classification into marbles, and compact, granular, shelly, and magnesian limestones.

Practically, the name " marble " is given to any limestone which is hard and compact enough to take a fine polish. Some marbles - such, for example, as those from Devonshire - will retain their polish indoors, but lose it when exposed to the weather. Marble is found in all great limestone formations. It consists generally of pure lime carbonate. The texture, degree of crystallization, hardness, and durability, of the several varieties differ considerably. Marble can generally be raised in large blocks. The handsomer kinds are too expensive, except for chimney-pieces, table slabs, inlaid work, etc. The less handsome varieties are used for building in the neighbourhood of the quarries. The appearance of the ornamental marbles differs greatly. Some are wholly of one colour, others derive their beauty from a mixture of accidental substances - metallic oxides, etc, which give them a veined or clouded appearance. Others receive a varied and beautiful " figure " from shells, corals, stems of encrinites, etc, imbedded in them. Marble is used in connection with building chiefly for columns, pilasters, mantelpieces, and for decoration. Its weight makes it suitable for seawalls, breakwaters, etc, when it is cheaply obtainable, but some varieties are liable to the attacks of boring molluscs.

In the absence of better material, marble may be used for road metal and paving setts, but it is brittle and not adapted to withstand a heavy traffic. Roads made with it are greasy in wet weather and dusty when dry.