SANDSTONE , in
See also:petrology, a consolidated 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:brown or
See also:colour and very impure . The minerals of sandstones are the same as those of sands .
See also:Quartz is the commonest; with it often occurs a considerable amount of
See also:felspar, and usually also some
See also:mica .
See also:Chlorite, argillaceous
See also:calcite and iron oxides, are exceedingly
See also:common in sandstones, and in some varieties are important constituents; garnet,
See also:rutile and
See also:anatase are often
See also:present though rarely in any quantity . According to their composition we may distinguish siliceous sandstones (some of these are so pure that they contain 99% of
See also:silica, e.g . Craigleith
See also:stone and some gannisters), felspathic sandstones or arkoses (less durable and softer than the siliceous sandstones); micaceous sandstones, with flakes of mica lying along the
See also:bed-ding planes; argillaceous sandstones; ferruginous sandstones, brown or red in colour with the sand grains coated with red
See also:haematite or brownish yellow
See also:limonite; impure sandstones, usually in the
See also:main consisting of quartz with a large addition of other minerals . The cementing material is often
See also:fine chalcedonic silica, and exists in such small quantity that it is difficult to recognize even with the microscope . In some of the cherty sandstones of the
See also:Greensand the chalcedonic
See also:cement is much more abundant: these rocks also contain rounded grains of
See also:glauconite, to which they owe their
See also:green colour . Crystalline silica (quartz) is deposited interstitially in some sandstones, often in
See also:regular parallel crystalline growth on the
See also:original sand grains, and when there are cavities or fissures in the rock may show the development of regular crystalline facets . By this
See also:process the rock becomes firmly compacted, and is then described as a
See also:quartzite (q.v.) .
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 French rock
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 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:barytes, fluorite and phosphate of lime, but these are only locally found . Many sandstones contain concretions which may be several feet in 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 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 lead ores (the
See also:Eifel, Germany), 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 petroleum . Many
See also:Measures sandstones contain remains of
See also:plants preserved as 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: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 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 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 bridges, crystalline rocks, such as 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 England) may be used for lining furnaces, hearths, &c . As sandstones are always porous, they do not take a
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 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 Prussia, and copper at Chessy near
See also:Lyons . (J . S .
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