Sandstones are essentially grains of quartz cemented together. Iron oxide, silica-, carbonate of lime, or clay may he the cementing medium. The character of the stone varies with that of the cement binding the sand grains.
Sometimes other minerals than quartz are present in sandstones, as feldspar, mica, or pyrite, thus modifying the character of the stone, usually rendering it less durable. Sandstones in which silica is the cementing material are usually the most durable. They are com monly light in color. When considerable silica is present, the stone is very hard and difficult to work, while some stones containing less cement work easily and remain gritty under wear.
Sandstone in which the cement is iron oxide is usually of a red or brown color. Thee stones usually work easily, and are often durable in use as building stones. When the cementing material is carbonate of lime, the stone usually possesses fair strength, but is not often so durable as that with silica or iron oxide. These stones are usually light colored, :oft and easy to work. Clay as a cement in sandstone is usually less desirable than the others; the stone containing it is not so strong: it absorbs water and may be liable to injury from frost. When present in small amount and uniformly distributed through the stone, clay may make the stone easier to work without otherwise injuring it.
" Sandstones, as a rule, show good durability. Some of the softer ones may disintegrate under frost action. Those with clay seams are liable to split with continued freezing. Mica scales, if abundant along the bedding planes, are also likely to cause trouble, and this is aggravated if the stone is set on edge instead of on bed. A striking example of this is the Connecticut brown stone so extensively used in former years for fronts in many of the Eastern cities. In order to get a smooth surface it was rubbed parallel with the bedding, and the stone set in the building on edge. The result is that hundreds of buildings put up more than fifteen or twenty years ago are scaling badly, and in many cases the entire front has been redressed." 1 Sandstones are of sedimentary origin and are more or less in layers. They should always be laid on their natural beds, and are apt to scale off if placed on edge. They vary in texture from grains of
powdery fineness to those in which the grains are of course sand. The fine-grained stones are usually the strongest and most durable.
Sandstone is quite widely distributed over the United States, and is one of the most desirable and most extensively used building stones. Many quarries are in use throughout the country for local purposes, while a few quarries supply stone for wider distribution. The Berea stone of Oliio is frequently shipped to considerable dis tance. The Brownstone of Connecticut, :Medina sandstone of western New York, Nettle River sandstone of :Minnesota are examples of well-known stones in common use.
Slate "is a metamorphic rock produced from clay or shale. It is characterized by a tendency to split into thin sheets with smooth surfaces. The direction of this cleavage is not parallel to the bedding and has probably been caused by heavy lateral pressure. These sheets of slate are strong under transverse loading and quite imper vious to water. They therefore make good roof covering, or may be used as flags for spanning openings. They are also commonly used for blackboards, school slates, etc. The color of slate is com monly Clark blue, gray, or black, although green and red slates are also common.
Good slate should be dense and tough and not corrodible by atmospheric gases. When loaded transversely, it should bend appreciably before breaking, and should show a modulus of rupture from 7000 to 10000 Most of the slate now in use conies from the New England and Middle Atlantic States, notably from Vermont and eastern Pennsyl vania. Important quarries have also been opened in Arkansas and California.
41. Strength of Building Stone.—The loads brought upon masonry structures are rarely sufficient to tax the strength of the stone in compression. The strength of masonry is not directly dependent upon that of the stone used in its construction. The strength of the mortar, thickness of joints, and the care and accuracy used in bedding the stones have important effects upon the strength of the inasonry. It is desirable that building stone should be strong and capable of resisting heavy loads, and tests of the strength of the stone may show whether the stone is of good quality and fit for use.