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SANDSTONE
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SANDSTONE , in See also:petrology, a consolidated See also:sand See also:rock built up of sand grains held together by a cementing substance. The See also:size of the particles varies within wide limits and in the same rock may be See also:uniform or irregular: the coarser sandstones are called grits, and See also:form a transition to conglomerates (q.v.), while the finer grained usually contain an admixture of mud or See also:clay and pass over by all stages into arenaceous shales and clay rocks. Greywackes (q.v.) are sandstones belonging to the older See also:geological systems, such as the See also:Silurian- or See also:Cambrian, usually of See also: A calcareous cement is almost equally common: it may be derived from particles of shells or other calcareous fossils originally mixed with the sand and subsequently dissolved and re-deposited in the spaces between the other grains. In See also:Fontainebleau sandstone and some See also:British Secondary rocks the calcite is in large crystalline masses, which when broken show See also:plane cleavages mottled with small rounded sand grains; in the See also:French rock See also:external See also:rhombohedral faces are present and the crystals may be of consider-able size. Many of the British See also:Jurassic and Cretaceous sandstones (e.g. Kentish Rag, Spilsby Sandstone) are of this calcareous type. In ferruginous sandstones the iron oxides usually form only a thin pellicle coating each See also:grain, but sometimes, in the greensands, are more abundant, especially in concretionary masses or segregations. In argillaceous sandstones the fine claye y material, compacted by pressure, holds the sand grains together, and rocks of this See also:kind are soft and break up easily when exposed to the See also:weather or submitted to crushing tests. Among other cementing materials may be mentioned, See also:dolomite, See also:barytes, fluorite and phosphate of See also:lime, but these are only locally found. Many sandstones contain concretions which may be several feet in See also:diameter, and are sometimes set See also:free by weathering or when the rock is split open by a See also:blow. Most frequently these are siliceous, and then they interfere with the employment of the rock for certain purposes, as for making grindstones or for buildings of fine dressed stone. Argillaceous concretions or clay See also:galls are almost equally common, and nodules of See also:pyrites or See also:marcasite; the latter weather to a brown rusty See also:powder, and are most undesirable in See also:building stones. Phosphatic, ferruginous, barytic and calcareous concretions occur also in some of the rocks of this See also:group. We may also mention the presence of See also:lead ores (the See also:Eifel, See also:Germany), See also:copper ores (Chessy and some British Triassic sandstones) and See also:manganese oxides. In some districts (e.g. See also:Alsace) bituminous sandstones occur, while in N. See also:America many Devonian sandstones contain See also:petroleum. Many See also:Coal-See also:Measures sandstones contain remains of See also:plants preserved as See also:black impressions. The See also:colours of sandstones arise mostly from their impurities; pure siliceous and calcareous sandstones are white, creamy or See also:pale yellow (from small traces of iron oxides). Black colours are due to coal or manganese dioxide; red to haematite (rarely to copper See also:oxide) ; yellow to limonite, green to glauconite. Those which contain clay, fragments of shale, &c., are often grey (e.g. the See also:Pennant Grit of S. See also:Wales). Sandstones are very extensively worked, mostly by quarries but sometimes by mines, in all districts where they occur and are used for a large variety of purposes. See also:Quarrying is facilitated by the presence of two systems of See also:joints, See also:developed approximately in equal perfection, nearly at right angles to one another and perpendicular to the bedding planes. Sometimes this jointing determines the weathering of the rock into square See also:pillar-like forms or into mural scenery (e.g. the Quader Sandstein of Germany). As building stones sandstones are much in favour, especially in the Carboniferous districts of See also:Britain, where they can readily be obtained. They have the See also:advantage of being durable, strong and readily dressed. They are usually laid " on the bed," that is to say, with their bedding surfaces See also:horizontal and their edges exposed. The finer kinds of sandstone are often sawn, not hewn or trimmed with See also:chisels. Pure siliceous sandstones are the most durable, but are often very ex-pensive to See also:dress and are not obtainable in many places. Sandstones are also used for grindstones and for millstones. For See also:engineering purposes, such as dams, piers, docks and See also:bridges, crystalline rocks, such as See also:granite, are often preferred as being obtainable in larger blocks and having a higher crushing strength. Very pure siliceous sandstones (such as the gannisters of the See also:north of See also:England) may be used for lining furnaces, hearths, &c. As sandstones are always porous, they do not take a See also:good See also:polish and are not used as ornamental stones, but this See also:property makes them absorb large quantities of See also:water, and consequently they are often important See also:sources of water See also:supply (e.g. the water-stones of the Trias of the See also:English Midlands). See also:Silver is found in beds of sandstone in See also:Utah, lead near Kommern in See also:Prussia, and copper at Chessy near See also:Lyons. (J. S. Additional information and CommentsThere are no comments yet for this article.
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