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CONCRETION

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Originally appearing in Volume V06, Page 841 of the 1911 Encyclopedia Britannica.
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CONCRETION , in See also:

petrology, a name applied to nodular or irregularly shaped masses of various See also:size occurring in a See also:great variety of sedimentary rocks, differing in See also:composition from the See also:main See also:mass of the See also:rock, and in most cases obviously formed by some chemical See also:process which ensued after the rock was deposited. As these bodies See also:present so many See also:variations in composition and in structure, it will conduce to clearness if some of the commonest be briefly adverted to. In sandstones there are often hard rounded lumps, which See also:separate out when the rock is broken or weathered. They are mostly siliceous, but sometimes calcareous, and may differ very little in See also:general See also:appearance from the bulk of the See also:sandstone. Through them the bedding passes uninterrupted, thus showing that they are not pebbles; often in their centres shells or fragments of See also:plants are found. Argillaceous sandstones and flagstones very frequently contain " See also:clay See also:galls " or concretionary lumps richer in clay than the See also:remainder of the rock. Nodules of See also:pyrites and of See also:marcasite are See also:common in many See also:clays, sandstones and marls. Their See also:outer surfaces are tuberculate; internally they commonly have a radiate fibrous structure. Usually they are covered with a dark See also:brown crust of See also:limonite produced by weathering; occasionally imperfect crystalline faces may See also:bound them. Not infrequently (e.g. in the See also:Gault) these pyritous nodules contain altered fossils. In clays also siliceous and calcareous concretions are often fund. They present an extraordinary variety of shapes, often grotesquely resembling figures of men or animals, fruits, &c., and have in many countries excited popular wonder, being regarded as of supernatural origin (" See also:fairy-stones," &c.), and used as charms.

Another type of concretion, very abundant in many clays and shales, is the " septarian nodule." These are usually flattened disk-shaped or ovoid, often lobulate externally like the See also:

surface of a See also:kidney. When split open they prove to be traversed by a network of cracks, which are usually filled with See also:calcite and other minerals. These See also:white infillings of the fissures resemble partitions; hence the name from the Latin septum, a See also:partition. Sometimes the cracks are partly empty. They vary up to See also:half an See also:inch in breadth, and are best seen when the nodule is cut through with a saw. These concretions may be calcareous or may consist of carbonate of See also:iron. The former are common in some beds of the See also:London Clay, and were formerly used for making See also:cement. The clay-ironstone nodules or sphaerosiderites are very abundant in some Carboniferous shales, and have served in some places as iron ores. Some of the largest specimens are 3 ft. in See also:diameter. In the centre of these nodules fossils are often found, e.g. See also:coprolites, pieces of plants, See also:fish See also:teeth and scales. Phosphatic concretions are often present in certain limestones, clays, shelly sands and marls. They occur, for example, in the See also:Cambridge See also:Greensand, and at the See also:base of certain of the See also:Pliocene beds in the See also:east of See also:England.

In many places they have been worked, under the name of " coprolite-beds," as See also:

sources of artificial See also:manures. Bones of animals more or less completely mineralized are frequent in these phosphatic concretions, the commonest being fragments of See also:extinct reptilia. Their presence points to a source for the phosphate of See also:lime. Another very important See also:series of concretionary structures are the See also:flint nodules which occur in See also:chalk, and the patches and bands of chert which are found in limestones. Flints consist of dark-coloured cryptocrystalline See also:silica. They See also:weather See also:grey or white by the removal of their more soluble portions by percolating See also:water. Their shapes are exceellingly varied, and often they are studded with tubercules and nodosities. Sometimes they have See also:internal cavities, and very frequently they contain shells of echinoderms, molluscs, &c., partly or entirely replaced by silica, but preserving their See also:original forms. Chert occurs in bands and See also:tabular masses rather than in nodules; it often replaces consider-able portions of a See also:bed of See also:limestone (as in the Carboniferous Limestones of See also:Ireland). See also:Corals and other fossils frequently occur in chert, and when sliced and microscopically examined both flint and chert often show silicified See also:foraminifera, See also:polyzoa &c., and sponge spicules. Flints in chalk frequently See also:lie along See also:joints which may be See also:vertical or may be nearly See also:horizontal and parallel to the bedding. Hence they increase the stratified appearance of natural exposures of chalk.

It will be seen from the details given above that concretions may be calcareous, siliceous, argillaceous and phosphatic, and they may consist of carbonate or sulphide of iron. In the red clay of the deep See also:

sea bottom concretionary masses See also:rich in See also:manganese dioxide are being formed, and are sometimes brought up by the dredge. In clays large crystals of See also:gypsum, having the shape of an arrow-See also:head, are occasionally found in some See also:numbers. They See also:bear a considerable resemblance to some concretions, e.g. crystal-See also:line marcasite and pyrite nodules. These examples will indicate the great variety of substances which may give rise to concretionary structures. Some concretions are amorphous, e.g. phosphatic nodules; others are cryptocrystalline, e.g. flint and chert; others finely crystalline, e.g' pyrites, sphaerosiderite; others consist of large crystals, e.g. gypsum, See also:barytes, pyrites and marcasite. From this it is clear that the formation of concretions is not closely dependent on any single inorganic substance, or on any type of crystalline structure. Concretions seem to arise from the tendency of chemical compounds to be slowly dissolved by interstitial water, either while the See also:deposit is unconsolidated or at a later See also:period. Certain nuclei, present in the rock, then determine reprecipitation of these solutions, and the deposit once begun goes on till either the See also:supply of material for growth is exhausted, or the See also:physical See also:character of the bed is changed by pressure and consolidation till it is no longer favourable to further See also:accretion. The process resembles the growth of a crystal in a See also:solution by slowly attracting to itself molecules of suitable nature from the surrounding See also:medium. But in the See also:majority of cases it is not the crystalline forces, or not these alone, which attract the particles. The structure of a flint, for example, shows that the material had so little tendency to crystallize that it remained permanently in cryptocrystalline or sub-crystalline See also:state.

That the concretions See also:

grew in the solid sediment is proved by the manner in which lines of bedding pass through them and not See also:round them. This is beautifully shown by many siliceous and calcareous nodules out of See also:recent clays. That the sediment was in a soft See also:condition may be inferred from the purity and perfect crystalline See also:form of some of these bodies, e.g. gypsum, pyrites, marcasite. The crystals must have pushed aside the yielding See also:matrix as they gradually enlarged. In deep-sea dredgings concretions of phosphate of lime and manganese dioxide are frequently brought up; this shows that concretionary See also:action operates on the sea See also:floor in muddy sediments, which have only recently been laid down. The phosphatic nodules seem to originate around the dead bodies of fishes, and manganese incrustations frequently enclose teeth of sharks, See also:ear-bones of whales, &c. This recalls the occurrence of fossils in septarian nodules, flints, phosphatic concretions, &c., in the older strata. Probably the decomposing organic See also:matter partly supplied sub-stances for the growth of the nodules (See also:phosphates, See also:carbonates, &c.), partly acted as reducing agents, or otherwise determined See also:mineral precipitation in those places where organic remains were mingled with the sediment. (J. S.

End of Article: CONCRETION

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