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SAND
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SAND . When rocks or minerals are pulverized by any agencies, natural or artificial, the products may be classified as gravels, sands and muds or See also:clays, according to the See also:size of the individual particles. If the grains are so See also:fine as to be impalpable (about r-tha in. in See also:diameter) the See also:deposit may be regarded: as a mud or See also:clay; if many of them are as large as peas the See also:rock is a See also:gravel. Sands may be See also:uniform when they have been sorted out by some agency such as a See also:gentle current of See also:water or the See also:wind blowing steadily across smooth arid lands, but usually they vary much both in the coarseness of their grains and in their See also:mineral See also:composition. The See also:great source of natural sands is the See also:action of the See also:atmosphere, See also:frost, See also:rain, See also:plants and other agencies in breaking up the surfaces of rocks and reducing them to the See also:condition of fine See also:powder; in other words sands are ordinarily the product of the agencies of denudation operating on the rocks of the See also:earth's crust. Not all, however, are of this See also:kind, for a few are artificial, like the crushed tailings produced in the extractions of metals from their ores; there are also volcanic sands which have originated by explosions of See also:steam in the craters of active volcanoes. A great See also:part of the See also:surface of the globe is covered by sand. In fertile regions the See also:soil is very often of a sandy nature; though most soils are mixtures of sand with clay or stones, and may be described as loans rather than as sands. Pure sandy soils are found principally near See also:sea-coasts where the sand has been blown inwards from the See also:shore, or on formations of soft and friable See also:sandstone like the See also:Greensand. The soil of deserts also is often arenaceous, but there the finer particles have been lifted and See also:borne away by the wind. Accumulations of sand are found also in some parts of the courses of our See also:rivers, very often over wide stretches of the seashore, and more particularly on the sea bottom, where the water is not very deep, at no great distance from the See also:land. Of the rock-making minerals which are See also:common on the earth's crust only a limited number occur at all frequently in sand deposits. For several reasons See also:quartz is by far the commonest ingredient of sands. It is a very abundant mineral in rocks and is comparatively hard, so that it is not readily worn down to a very fine muddy See also:paste. It also possesses practically no cleavage, and does not split up naturally into thin fragments. If we add to this that it is nearly insoluble in water and that it does not decompose, but preserves its freshness unaltered after See also:long ages of exposure to weathering, we can see that it has all the properties necessary for furnishing a large portion of the sandy material produced by the detrition of rock masses. With quartz there is often a small amount of See also:felspar (principally micro-cline, See also:orthoclase and See also:oligoclase), but this mineral, though almost as common as quartz in rocks, splits up readily on See also:account of its cleavage, and decomposes into fine, soft, scaly aggregates of See also:mica and See also:kaolin, which are removed by the sifting action of water and are deposited as muds or clays. Small plates of See also: Around See also:coral islands there are often extensive deposits of comminuted coral (coral sands), mixed with which there is a varying proportion of broken skeletons of calcareous See also:algae, sponge-spicules and other debris of organic origin. The Greensands which are widely distributed over the See also:floor of the oceans, in places where the See also:continental shelf merges into the greater depths, owe their See also:colour to small rounded lumps of See also:glauconite. Among the See also:accessory ingredients of sands which are of great value and See also:interest are the See also:precious metals, especially See also:gold and See also:platinum. These are found usually in the See also:lower parts of the sand deposits resting on the See also:bed-rock, because of their high specific gravity, and have been derived from the destruction of the rocks in which they originally occurred either in quartzose See also:veins or as disseminated particles. Tinstone occurs also in this way (" stream-See also:tin "), and in See also:Ceylon, See also:Burma, See also:Brazil, See also:South See also:Africa, &c., precious stones such as the See also:diamond, See also:ruby, See also:spinel, See also:chrysoberyl and tourmaline are found in beds of sand and gravel (See also:gem sands). In See also:general the sand grains have a rounded or oviform shape due to mutual See also:attrition during transport. Those which have been carried farthest are most rounded; sands deposited at no great distance from their parent rock often consist largely of angular grains. The smaller fragments may be carried along in suspension in water, and may travel for many See also:miles without being sensibly worn; but coarse sands and fine gravels are swept along the bottom and are subjected to an intense grinding action. Something depends also on the hardness of the minerals present in the sands, yet even the diamonds and other gems found in sand deposits have often their corners worn and smoothed. Minerals with very perfect cleavage, such as mica, split up into thin plates under the See also:shock of impact with adjacent grains, and are never rounded like quartz or tourmaline. In deserts the transport of the sands is effected by the wind, and owing to the See also:low viscosity of See also:air even the smallest See also:grain" are not held in suspension but are rolled along the ground; hence very fine quartzose sands are sometimes met with in arid regions with every particle smoothed and polished. These sands flow almost like a liquid and are used in See also:hour-glasses. Similar" See also:desert sands " occur among the sandstones of the Trias and were doubtless formed in the manner described. In addition to river sands, shore sands, marine sand deposits and desert sands, there are many other types of sand deposits. Blown sands are usually found near the seashore, but occur also at the margin of some great lakes like those of N. See also:America; desert sands belong in great part to this See also:category. These sands have been blown into their present position by the wind, and unless fixed by vegetation are constantly though slowly in See also:movement, being in consequence a menace to agricultural land on their leeward sides. They may be shell sands, quartz sands or mixed sands, and often show very marked oblique stratification or " current bedding." The surface of blown sand deposits is generally marked by See also:dunes. Glacial sands are common in districts like See also:Britain and those parts of N. America which have been covered by an See also:ice-See also:sheet. They are really water-borne and have been deposited by streams, though they occur in situations where rivers no longer flow. The See also:waters produced by the melting of the ice-sheets flooded extensive tracts of See also:country, laying down sand and mud deposits in temporary lakes. These sands are usually angular, because they have not been transported to any great distance. The old high-level terraces which border the lower courses of many rivers, though usually consisting of gravel, are often accompanied by considerable sand deposits. Many of the See also:Tertiary and some of the Secondary sandstone rocks are so incompletely consolidated by cementation that they are essentially sand rocks, and especially when weathered may be used as See also:sources of sand. Thus in Britain there are See also:Pliocene sands (St Erth, Cornwall, &c.), See also:Eocene sands (Bagshot sands and See also:Thanet sands); and the Lower and Upper Greensand (Cretaceous) are often dug in pits, though sometimes firmly coherent and more properly described as sandstones (q.v.). The economic uses of sands are very numerous. They are largely employed for polishing and scouring both for domestic and manufacturing purposes. " See also:Bath bricks " are made from the sand of the river Parrett near See also:Bridgwater. Sand for See also:glass-making was formerly obtained at See also:Alum See also:Bay in the Isle of See also:Wight and at See also:Lynn in See also:Norfolk, but must he very pure for the best kinds of glass, and crushed quartz or See also:flint is often preferred on this account. One of the See also:principal uses of sand is for making See also:mortar and See also:cement: for this any See also:good clean quartzose sand See also:free from salts is suitable; it may be washed to remove impurities and sifted to secure uniformity in the size of the individual grains. Moulding sands, adapted for foundry purposes, generally contain a small admixture of clay. Sands are also employed in See also:brick-making, in filtering, and for See also:etching glass and other substances by means of the sand blast. (J. S. Additional information and CommentsThere are no comments yet for this article.
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