SANDSTONE is a consolidated rock built up dominantly of grains of sand (q.v.) held together by a cementing substance. Sandstones are composed mainly of quartz, but may vary in com position in the same manner as sands. By increase in the size of their constituents they pass into conglomerates (q.v.) and by de crease into arenaceous shales and clay rocks. When the grains of sand are angular, the rock is termed a grit.
The minerals of sandstones are the same as those of sands. Quartz is the commonest; with it often occurs a certain amount of felspar (as in the rock arkose) and frequently white mica. The flakes of mica may often be seen lying on the bedding-planes and may give the sandstone a fissile character (e.g., paving-stone) of much value in quarrying. The cementing material is often fine chalcedonic silica, or it may be secondary quartz, when a quartzite like rock is produced. Calcareous material (calcite), glauconite, iron oxides, carbonaceous matter and other substances also act as cements and give the sandstones characteristic colours. Glauconitic sandstones are greenish, ferruginous sandstones, red, brown and yellow or grey. When the cementing substance is clay, the rock
is often of white or grey colour and firmly compacted.
Pure sandstones may contain as much as 99% of silica. If rela tively soft they are crushed to sand for commercial purposes. If firmly cemented they are utilized, on account of the resistance of silica to heat, for the manufacture of silica-bricks, furnace-linings, hearths, etc. Of this character is the well-known rock "ganister" worked in the districts of Sheffield, South Wales, etc. Less pure, but firmly-cemented siliceous sandstones are used for the making of grindstones and millstones. Similar rocks, as well as calcareous, dolomitic (see DOLOMITE) and ferruginous sandstones are exten sively worked as building-stones, mostly by quarries but sometimes by mines. As sandstones are always porous, they do not take a good polish, and are not used as ornamental stones, but this prop erty makes them valuable storage-basins and sources of water (e.g., the Trias of the Midlands). (P. G. H. B.)