The marbles are generally harder than the limestones, having a crushing strength of from 10,300 to 16,000 pounds per square inch. They are the most showy and ornamental of all building stones, and have been very popu lar since the earliest times. A great many beautiful varieties are quarried in various parts of the United States, but about 60 per cent of the total amount is quarried in the State of Vermont, the principal centres of the industry being Dorset, West Rutland, Middlebury, Wal lingford, Brandon and Pittsford. These marbles are of all varieties of texture, and range in color from pure white to dark green, and dark blue, the white stones often being veined and mottled with the darker colors. Very beautiful marbles are also quarried in Tennessee. They are particularly noticeable on account of their variegated colors, which in clude many shades of chocolate and red, and lemon yellow, olive and green, which form an endless variety of color combinations of strik ing effect. The distribution of these Appa lachian marbles extends from Vermont to Georgia, and they are extensively quarried in all of the States bordering that mountains ys tem. On the other hand, although very fine marble deposits exist in many of the States in the Rocky Mountain region, they have not been worked to any great extent, up to the present time, on account of lack of transporta tion, and the limited local demand.
The most notable of the foreign marbles are those of Italy, the French Pyrenees and Belgium, although Germany, Austria, Spain, Portugal and Ireland also furnish many varieties of fine texture and color. Among the most beautiful of these European productions are the "Brocatelle° marbles, having a light yellow body marked with veins and blotches of dull red, and the Languedoc, having a bril liant scarlet body color blotched with white, both of which are obtained from the Pyrenees; the °Black and Gold," a black limestone veined with yellow; the pure white stone of Carrara, and the °Giallo a yellow marble, all three of which are obtained from Italy; and the Saint Anne marble having a deep blue black body color marked with white veins, and the pure black marble known as °Belgian Black," which is obtained from Belgium.
Of the fragmental rocks, a great variety of sandstones are used for facing, lintels and gen eral structural purposes, while slate is used for roofing, and for floor tiles, flagging and mantels. Sandstones are composed of rounded and angular grains of sand, bound together by such cementing materials as silica, oxide of iron and carbonate of lime, into the form of solid rocks. The presence of silica gives a white colored stone of durable quality, but very difficult to work. Cementation by oxide of iron gives a reddish or brownish stone of medium durability, fairly easy to work. Carbonate of lime cement gives a gray-colored stone much softer than the other two varieties, and much easier to work, but much less durable. Sand stones vary in texture from those having a very fine grain to those composed of pebbles. The latter are divided into two classes — the °conglomerates° composed of rounded pebbles, and Threccias* composed of angular pebbles. Some sandstones have a clayey cement which makes them unfit for building purposes, while others, although they contain hardly any cement, and owe their tenacity to the pressure under which they were consolidated, make good build ing stone. As a general rule, sandstones are
softer when first quarried than after a period of seasoning by exposure to the air. They vary in color from light gray, buff, drab and blue, through shades of brown, pink and red. Their resistance to compression ranges from 7,000 pounds per square inch for the Oregon stone to 16,000 pounds for the Connecticut sand stone; while the stone and the blue stone of New York have a Crushing strength of 14,000 pounds per square inch. Other well known and extensively quarried varieties are the *Berea' stone of Ohio, and the °Portland* stone of Massachusetts and Connecticut. The Lake Superior *Red Sandstone' of Keweena wan Age, and the *Brownstone* of the Con necticut Valley and New Jersey, of Triassic Age, were formerly very popular but are now in much less demand.
Slate consists of an indurated clay which may be easily split into sheets of various sizes and considerable thinness. The principal quar ries in this country are located in Maine, Ver mont, Pennsylvania and New York, while those of Ardennes in France, and of Wales in England, are the greatest producers in Europe. Slate is used most extensively as a roofing material.
The methods employed in quarrying build ing-stone vary with the character of the stone, but the ultimate object of all of them is to obtain large and well-shaped blocks free from incipient fractures. Therefore, explosives are used as little as possible for that purpose, and the work of dressing is very largely done by hand. A great deal of machinery, however, is employed for the purposes of sawing, planing and polishing, and for splitting slate.
The durability of a building stone is one of the most important factors in its value as a material of construction. Durability is the ability of a stone to withstand the deteriora tion induced by its exposure, to the action of changing weather and temperature conditions, to the chemical agencies in the moisture of the atmosphere and to the disintegrating ac tion of growing organisms. The normal strengths of the softest building stones are much greater than is necessary for structural purposes, but under the action of the natural elements and agencies just stated they disin tegrate more or less rapidly according to their structure and the materials of which they are composed. Granites suffer disintegration chiefly from changes of temperature and are affected but little by the expansion and contraction due to the absorption of water and its subse quent freezing in cold climates, and are almost entirely unaffected by the chemicals ordinarily held in the atmosphere or carried by rain. Limestones suffer even less by expansion and contraction, but deteriorate much more quickly under the action of the chemicals in the air and rain; while the sandstones, on account of their porous structure, suffer chiefly from the effects of expansion and contraction and disin tegrate so rapidly from the effects of frost that they are unsuitable for building purposes in countries with cold climates.