Sandstones are composed of grains of sand held together by a cementing substance to form a compact rock. The cementing medium may be silica, alumina, carbonate of lime or an oxide of iron. Those stones that have a siliceous cement are the most durable. Sandstones vary more in colour than limestones, the colour being largely due to the presence of iron. Cream, brown, grey, pink, red, light and dark blue, and drab are common colours. Typical British sandstones are Corsehill (red) from Dumfries shire, the Yorkshire sandstones (brown), Pennant stone and For est of Dean (blue and grey) from Gloucestershire. In America sandstones are quarried in many States, principally Connecticut (brown stone), New York (Potsdam red stone), Ohio (Amherst, Berea and other quarries, light brown or grey stone) and Massa chusetts (Longmeadow brown stone). The heaviest, densest, least porous and most lasting stones usually have a fine grain.
Granites are igneous rocks formed by volcanic action and are of all geological ages. Granite is composed of quartz, felspar and mica. Stones with a large proportion of quartz are hard and diffi cult to work. Earthy felspar is opaque and liable to decay, it should be almost transparent. The colour of the granite is gen erally due to this substance, but may be affected by the mica, whether light or dark. Granite is the hardest, strongest, and most durable of building-stones. When polished, many varieties present a beautiful and lasting surface. They are used for founda tions, bases, columns, curbs and paving where strength is required.
The granites from Peterhead and Aberdeen in Scotland and from Cornwall and Devonshire in England are much used. In the United States good granites are quarried in Connecticut, Massa chusetts and Minnesota. Canada, especially the eastern provinces, supplies excellent granite. So do Norway and Sweden. Syenitic granite contains hornblende in addition to quartz, felspar and mica. In true syenite hornblende takes the place of mica. It is named from a stone found at Syene in Egypt, but it has since been discovered that this stone actually contains more mica than hornblende. These rocks are very hard and are used more for paving and road-metalling than for building purposes.
Slate is composed of sandy clay metamorphosed by heat and pressure. Such rocks began as sediment by the sea or river. Owing to some sliding motion having taken place, slaty rocks can be split into thin sheets. A good slate will give out a metallic ring if struck, and when trimmed will not splinter. Slates range in colour from purple to grey and green. The best known British slates are those of the Welsh and Westmorland quarries. In America good slate is found in the States of New York, Pennsyl vania and Maine. (See also ROOFS.) There are several kinds of artificial stone on the market, con sisting of fine cement concrete placed to set in wooden or iron moulds. Although from an artistic point of view it must be used with great discretion, it is prepared with such care that its cheap ness, strength and uniform character have led to its wide employ ment. One of the best-known varieties is composed of finely crushed granite and Portland' cement, carefully mixed by ma chinery in the proportions of three to one, and filled into moulds of the required shape. When the blocks are set hard the moulds are loosened and the blocks placed in a solution of silicate of soda for about two weeks for the purpose of indurating and hardening them. Many manufacturers turn out a material that is prac tically non-porous and is able effectually to resist the corroding influence of sea air or the impure atmosphere of large towns. Artificial stone is exceedingly useful for paving purposes, and blocks can readily be cast of any size or thickness to suit the pavements of streets or buildings; their regularity greatly reduces labour costs in laying.