8. Sandstones are so called because they are formed by the cementation of particles of sand - usually quartz grains - into rock. The character of the stone depends greatly on the nature of the cementing material. When this is composed wholly of silica, the stone has a light color and is hard to work. If the grains have been cemented by fusion, or under great pressure, the stone is nearly as hard as quartz; this variety is know as quartzite, and is very strong and durable. If the cementing materials are principally iron oxides, the stone is red or brownish in color, and is sufficiently soft to work readily. With carbonate of lime as a cement, the result is a light-colored or gray stone, soft and easy to work but which does not, as a rule, weather well. Sandstone containing clay is the poorest, as it easily absorbs water, which, on freezing, rapidly disintegrates the stone.

Sandstones include some of the finest and most durable stones for outside construction. The ease of working them, and their wide distribution, cause them to be very extensively used. The stone is found in a great variety of colors - shades of gray, brown, buff, pink, red, drab, and blue being common - the color depending largely on the quantity of iron oxides contained in the stone. The presence of these is not injurious, but no sandstone containing iron pyrites should be used for outside work, as it is almost sure to become stained by rust.

Sandstones vary in texture from those in which the grains are almost imperceptible, to those having grains like coarse sand. The fine-grained stones are generally the most durable, and can be given sharp edges. Quarried sandstones usually hold considerable water, which renders them soft and easy to work; but nearly all become harder as the water evaporates, and until the water is dried out, the stone should not be subjected to heavy loads.

9. Blue shale, or bluestone, is a variety of argillaceous, or clayey, sandstone, having a bluish color, and is very hard and dense, making an excellent material for foundations, flagging, etc. It is found in large quantities along the Hudson River, in the vicinity of Kingston, N. Y."