Sandstones consist of quartz grains cemented together by silica, lime carbonate, magnesia carbonate, alumina, iron oxide, or mixtures of these substances. In addition to the quartz grains are often other substances, such as flakes of mica, fragments of limestone, argillaceous and carbonaceous matter, interspersed throughout the mass. As the grains of quartz are imperishable, the weathering qualities of the stone depend upon the nature of the cementing substance, and on its powers of resistance under the atmosphere to which it is exposed. Sometimes, however, the grains are of lime carbonate imbedded in a siliceous cement; in this case the grains are the first to give way under the influence of the weather. Sandstones are found in great variety of colour - white, yellow, grey, greenish grey, light brown, brown, red, dark blue, and even black. The colour is generally caused by the presence of iron. Thus iron carbonate gives a bluish or greyish tint; anhydrous sesquioxide, a red colour; hydrated sesquioxides, various tints of brown or yellow, sometimes blue and green. In some cases the blue colour is produced by very finely disseminated iron pyrites, and in some by iron phosphate.

Sandstones used for building are generally classed practically, according to their physical characteristics. "Liver Rock" is the term applied, perhaps more in Scotland that in England, to the best and most homogeneous stone which comes out in large blocks, undivided by intersecting vertical and horizontal joints. "Flagstones" are those which have a good natural cleavage, and split therefore easily into the thicknesses appropriate for paving of different kinds. The easy cleavage is generally caused by plates of mica in the beds. "Tilestones" are flags from thin-bedded sandstones. They are split into layers - sometimes by standing them on their edges during frost, - and are much used in the North of England and in Scotland as a substitute for slates in covering roofs. " Freestone " is a term applied to any stone that will work freely or easily with the mallet and chisel - such, for example, as the softer sandstones, and some of the limestones, including Bath, Caen, Portland, etc. '• Grits" are coarse-grained, strong, hard sandstones, deriving their name from the " millstone grit " formation in which they are found.

These stones are very valuable for heavy engineering works, as they can be obtained in large blocks.

The recent fracture of a good sandstone, when examined through a powerful magnifying glass, should be bright clean, and sharp, the grains well cemented together, and tolerably uniform in size. A dull and earthy appearance is the sign of a stone likely to decay. Sandstones may be subjected to Smith's or to Brard's test. Recent experiments have led to the conclusion that any sandstone weighing less than 130 lb. per cub. ft. absorbing more than 5 per cent. of its weight of water in 24 hours, and effervescing anything but feebly with acid, is likely to be a second-class stone, as regards durability, where there is frost or much acid in the air; and it may be also said that a first-class sandstone should hardly do more than cloud the water with Smith's test. It is generally considered that the coarse-grained sandstones, such as the millstone grits, are the strongest and most durable; but some of the finer-grained varieties are quite strong enough for any purpose, and seem to weather better than the others. Perhaps, for external purposes, the finer-grained sandstones, laid on their natural bed, are better than those of coarser grain.

In selecting sandstone for undercut work or for carving, care must be taken that the layers are thick; and it is of course important that stones should rest in most cases on their natural beds. The hardest and best sandstones are used for important ashlar work; those of the finest; and closest grain for carving; rougher qualities for rubble; the well-bedded varieties for flags. Some of the harder Band-stones are used for sets, and also for road metal, but they are inferior to the tougher materials, and roads metalled with them are muddy in wet, and very dusty in dr\ weather.