5. Limestone is a term which includes all stones containing lime, although they may differ from one another in almost every other respect. Those used for building purposes have, besides lime, one or more of the following constituents: silica, clay, talc, hornblende, mica, carbonate of magnesia, and iron. Fossil remains, as shells, coral, etc., are often found in limestone, usually in a more or less pulverized condition.

The principal limestones used for building purposes are the common kinds, the fine-grained crystalline ones, usually called marbles, and the magnesian varieties. Limestones containing 10 per cent. or more of magnesia are called mag-nesian, and those having over 45 per cent. of it are termed dolomites; these are crystalline and granular in structure, and usually have a white or yellowish tinge.

In color, limestones are generally a light gray, blue, cream, or buff. The light-gray varieties frequently resemble the light, fine-grained granites. Examples of these are the Bedford, Ind., and the Bowling Green, Ky., limestones, which are both very durable and make excellent building stone.

The best limestones have a fine grain and weigh about 145 pounds per cubic foot.

6. Marble is a variety of crystallized limestone, and is much valued on account of its beautiful colors and its capability of taking a high polish. Nearly all kinds can be quite easily worked, and the fine-grained varieties are especially adapted for carving, and are particularly suitable for interior decoration. As it resists frost and moisture well, marble is also a valuable material for exterior construction.

Some of the finest varieties of white American marbles are found at Lee, Mass., and in the vicinity of Rutland, Vt. The dark-blue marble, from the Vermont quarries, is very durable and has a very close grain. A fine black marble is quarried at Glens Falls, N. Y. Colored marbles, including gray, light and dark pink, buff, chocolate, etc., are also found in Tennessee, Georgia, and elsewhere.

The strength of marble varies from 5,000 to 22,000 pounds per square inch.

7. The term onyx is applied to some kinds of marbles for the reason that their banded appearance somewhat resembles that of the true onyx. They have the same general composition as the common varieties of marble, but are formed in a purely chemical manner, instead of in ordinary sedimentary beds. Their variegated colors are due to the presence of metallic oxides and other impurities. Onyx can be worked and polished very readily, and is considered the handsomest of building stones, owing to its translucency and great variety of colors. It is used almost entirely for interior decoration, in wainscoting, mantels, etc. The stone presents the best appearance when cut across the grain, but this impairs the strength, and it is necessary to use backing of stronger marble. Most of the onyx used in this country comes from Mexico and California.