Classification and Description of Building Stones

CLASSIFICATION AND DESCRIPTION OF BUILDING STONES.

Building stones are variously classified ac cording to geological position, physical structure, and chemical composition.

Geological Classification.

The geological position of rocks has but little connection with their properties as building materials. As a general rule, the more ancient rocks ate the stronger and the more durable; but to this there are many exceptions. According to the usual geological classification, rocks are divided into igneous, metamorphic, and sedimentary. Greenstone, basalt, and lava are examples of igneous rocks; granite, marble, and slate, of meta morphic; and sandstone, limestone, and clay, of sedimentary. Al though clay can hardly be classed with building stones, it is not entirely out of place in this connection, since it is employed in making bricks and cement, which are important elements of masonry.

Physical Classification.

With respect to the structural character of large masses, rocks are divided into stratified and unstratified.

In their more minute structure the unstratified rocks present, for the most part, an aggregate of crystalline grains, firmly ad hering together. Granite, trap, basalt, and lava are examples of this class.

In the more minute structure of stratified rocks, the following varieties are distinguished: 1. Compact crystalline structure; accom panied by great strength and durability, as in quartz-rock and marble. 2. Slaty structure, easily split into thin layers; accom panied by both extremes of strength and durability, clay-slate and hornblende-slate being the strongest and most durable. 3. The granular crystalline structure, in which crystalline grains either adhere firmly together, as in gneiss, or are cemented into one mass by some other material, as in sandstone; accompanied by all degrees of compactness, porosity, strength, and durability, the lowest extreme being sand. 4. The compact granular structure, in which the graitrs are too small to be visible to the unaided eye, as in blue limestone; accompanied by considerable strength and durability. 5. Porous granular structure, in which the grains are not crystalline, and are often, if not always, minute shells cemented together; accompanied by a low degree of strength and durability. 6. The conglomerate structure, where fragments of one material are embedded in a mass of another, as graywacke; accompanied by all degrees of strength and durability.

A study of the fractured surface of a stone is a good means of determining its structural character. The even fracture, when the surfaces of division are planes in definite positions, is characteristic of a crystalline structure. The uneven fracture, when the broken

surface presents sharp projections, is characteristic of a granular structure. The slaty fracture gives an even surface for planes of division parallel to the lamination, and uneven for other directions of division. The conchoidal fracture presents smooth concave and convex surfaces, and is characteristic of a hard and compact structure. The earthy fracture leaves a rough, dull surface, and indicates softness and brittleness.

Chemical Classification.

Stones are divided into three classes with respect to their chemical composition, each distinguished by the earth which forms its chief constituent, viz.: siliceous stones, argillaceous stones, and calcareous stones.

Siliceous Stones are those in which silica is the characteristic earthy constituent. With a few exceptions. their structure is crystal line granular, and the crystalline grains contained in them are hard and durable; hence weakness and decay in them generally arise from the decomposition or disintegration of some softer and more perish able material, by which the grains are cemented together, or, when they are porous, by the freezing of water in their pores. The prin cipal siliceous stones are granite, syenite, gneiss, mica-slate, green stone, basalt, trap, porphyry, quartz-rock, hornblende-slate, and sandstone.

Argillaceous or Clayey Stones are those in which alumina, although it may not always be the most abundant constituent, exists in suf ficient quantity to give the stone its characteristic properties. The principal kinds are slate and graywacke-slate.

Calcareous Stones are those in which carbonate of lime pre dominates. They effervesce with the dilute mineral acids, which combine with the lime and set free carbonic acid gas. Sulphuric acid forms an insoluble compound with the lime. Nitric and muriatic acids form compounds with it, which are soluble in water. By the action of intense heat the carbonic acid is expelled in gaseous form, and the lime is left in its caustic or alkaline state, when it is called quicklime. Some calcareous stones consist of pure carbonate of lime; in others it is mixed with sand, clay, and oxide of iron, or combined with carbonate of magnesia. The durability of calcareous stones depends upon their compactness, those which are porous being disintegrated by the freezing of water, and by the chemical action of an acid atmosphere. Such stones are, for the most part, easily wrought. The principal calcareous stones are marble, limestone, granular limestone (the calcareous stone of the geological classification), and magnesian limestone or dolomite.