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Stone Masonry

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STONE MASONRY Rocks in Their Natural State. Rocks are divided into three general classes, determined by the manner of their formation in nature's great laboratory. These classes are: (1) Igneous, (2) Sedimentary or Aqueous, and (3) Meta morphic.

(1) Igneous rock

is that which, in some remote geological era, has been thrown up from the earth's interior in a molten state as volcanic lava. Such rock varies greatly in texture and appearance, from the well-known pumice-stone, which is porous and light in weight, to the dense and crystalline granite, so valuable as building material. Some of this lava has been left at or near the surface of the ground, and so has cooled quickly; while some has been covered to a great _ depth and has remained warm for centuries. The latter condition tends to the formation of large crystals and solid masses, while the former results in fine-grained and glassy stones.

Igneous rock is otherwise divided according to chemical composition, into acid and basic varieties. In the acid stone, of which granite 1 may be taken as a type, silicon is always present in large proportion. In the basic variety, of which trap is typical, potassium, magnesium, and iron are predominating features.

(2) All exposed rock is subject to slow dis integration, due to many causes. The action of water and of air are potent in causing rocks to crumble; alternate heat and cold and impact from particles blown about by the wind, produce similar results. The fine pieces thus formed may be deposited, by the action of water, in layers. The addition of pressure for great periods of time transforms this debris into new rock, which is called aqueous or sedimentary. The most notable example of rock of such forma tion is the sandstone in such common use.

Most of the limestones have been formed in water by animal life of a very low order. These structures are either in the form of coral reefs or masses of shells, both composed of carbonate of lime extracted from the water of the ocean.

(3) Rock that lies deep in the earth's crust may be subjected to enormous pressure, which, in some cases, is sufficient to alter materially its structure and appearance without changing its composition. Thus the beautiful marbles are

formed from ordinary limestone; deposits of peat may become anthracite; and deposits of clay may be changed into slate. This change of character gives rise to the name metamorphic.

Characteristics of Building Stones. Any variety of stone is valuable according as its char aster suits the requirements of the occasion; and the following brief statements regarding the qualities of some of the more common stones should prove of value in choosing building mate rial. In a general way, the requirements of a good building stone are: (1) Durability; (2) Strength; (3) Beauty; and (4) Cheapness. Sometimes more than one of these requirements are necessary, and all are desirable. Usually a compromise is effected between the best, regard less of cost, and the cheapest, if nothing else be considered.

Durability. Many agencies are constantly active in destroying the integrity of building stone. The air in the vicinity of large cities and manufacturing plants is laden with fumes, both acid and alkaline, which attack stone of all kinds. Stones which contain large proportions of lime are especially susceptible to the sulphuric and carbonic acid of smoke. Ammonia and nitric acid of rain water have marked effects upon the surface of exposed stone. All stone absorbs more or less moisture, which changes volume with each change of temperature and so sets up strains between the particles. When the range of temperature is from above to below freezing, the change of volume of the moisture is very marked, and the resulting strains are corre spondingly large.

Other destructive or disintegrating agents are fire, friction, and pressure. Fire is especially damaging to granite, which is otherwise, per haps, the most desirable stone. Friction is se rious on stairways and on door-sills. The pres sure of superimposed loads is seldom sufficient to crush stone unless it be applied so as to cause bending. This result is usually unavoidable; and many vertical cracks are seen, even in expensive buildings.

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