Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
LIMESTONE
Online EncyclopediaEncyclopedia Home :: LEO-LOB
del.icio.us it!
|
LIMESTONE , in petrography, a See also:rock consisting essentially of carbonate of See also:lime. The See also:group includes many varieties, some of which are very distinct; but the whole group has certain properties in See also:common, arising from the chemical See also:composition and See also:mineral See also:character of its members. All limestones dissolve readily in See also:cold dilute acids, giving off bubbles of carbonic See also:acid. Citric or acetic acid will effect this See also:change, though the mineral acids are more commonly employed. Limestones, when pure, are soft rocks readily scratched with a See also:knife-blade or the edge of a See also:coin, their hardness being 3; but unless they are earthy or incoherent, like See also:chalk or See also:sinter, they do not disintegrate by pressure with the fingers and cannot be scratched with the See also:finger See also:nail. When See also:free from impurities limestones are See also: If very pure, they may have smooth rounded surfaces, or may be covered with narrow runnels cut out by the See also:rain. In such cases there is very little See also:soil, and See also:plants are found growing only in fissures or crevices where theinsoluble impurities of the limestone have been deposited by the rain. The less pure rocks have often eroded or pitted surfaces, showing bands or patches rendered more resistant to the See also:action of the See also:weather by the presence of insoluble materials such as See also:sand, clay or chert. These surfaces are often known from the crust of hydrous oxides of iron produced by the action of the See also:atmosphere on any ferriferous ingredients of the rock; they are sometimes black when the limestone is carbonaceous; a thin layer of gritty sand grains may be See also:left on the See also:surface of limestones which are slightly arenaceous. Most limestones which contain fossils show these most clearly on weathered surfaces, and the See also:appearance of fragments- of See also:corals, crinoids and shells on the exposed parts of a rock indicate a strong See also:probability that that rock is a limestone. The interior usually shows the organic structures very imperfectly or not at all. Another characteristic of pure limestones, where they occur in large masses occupying considerable areas, is the frequency with which they produce See also:bare rocky ground, especially at high elevations, or yield only a thin scanty soil covered with See also:short grass. In mountainous districts limestones are often recognizable by these peculiarities. The chalk See also:downs are celebrated for the See also:close green sward which they furnish. More impure limestones, like those of the See also:Lias and Oolites, contain - enough insoluble mineral See also:matter to yield soils of See also:great thickness and value, e.g. the See also:Cornbrash. In limestone regions all See also:waters tend to be hard, on See also:account of the abundant carbonate of lime dissolved by percolating waters, and caves, See also:swallow holes, sinks, pot-holes and underground See also:rivers may occur in abundance. Some elevated tracts of limestone are very barren (e.g. the See also:Causses), because the rain which falls in them sinks at once into the See also:earth and passes underground. To a large extent this is true of the chalk downs, where surface waters are notably scarce, though at considerable depths the rocks hold large supplies of See also:water. The great See also:majority of limestones are of organic formation, consisting of the debris of the skeletons of animals. Some are foraminiferal, others are crinoidal, shelly or See also:coral limestones according to the nature of the creatures whose remains they contain. Of foraminiferal limestones -chalk is probably the best known; it is See also:fine, white and rather soft, and is very largely made up of the shells of See also:globigerina and other See also:foraminifera (see CHALK). Almost equally important are the nummulitic limestones so well See also:developed in Mediterranean countries (See also:Spain, See also:France, the See also:Alps, See also:Greece, See also:Algeria, See also:Egypt, See also:Asia See also:Minor, &c.). The pyramids of Egypt are built mainly of nummulitic limestone. Nummulites are large See also:cone-shaped foraminifera with many See also:chambers arranged in See also:spiral See also:order. In See also:Britain the small globular shells of Saccamina are important constituents of some Carboniferous limestones; but the upper : portion of that formation in See also:Russia, eastern Asia and See also:North See also:America is characterized by the occurrence of limestones filled with the spindle-shaped shells of Fusulina, a genus of foraminifera now See also:extinct. Coral limestones are being formed at the See also:present See also:day over- a large extent of the tropical seas; many existing coral reefs must be of great thickness. The same See also:process has been going on actively since a very See also:early See also:period of the earth's See also:history, for similar rocks are found in great abundance in many See also:geological formations. Some See also:Silurian limestones are See also:rich in corals; in the Devonian there are deposits which have been described as coral reefs (See also:Devonshire, See also:Germany). The Carboniferous limestone, or See also:mountain limestones of See also:England and North America, is sometimes nearly entirely coralline, and the great See also:dolomite masses of the Trias in the eastern Alps are believed by many to be merely altered coral reefs. A See also:special feature of coral limestones is that, although they may be to a considerable extent dolomitized, they are generally very free from silt and See also:mechanical impurities. Crinoidal limestones, though abundant among the older rocks, are not in course of formation on any great See also:scale at the present See also:time, as crinoids, formerly abundant, are now rare. Many Carboniferous and Silurian limestones consist mainly of the little cylindrical See also:joints of these animals. They are easily recognized by their shape, and by the fact that many of them show a See also:tube along their axes, which is often filled up by carbonate of lime ; under the See also:microscope they have a punctate or fenestrate structure and each See also:joint behaves as a See also:simple crystalline See also:plate with See also:uniform See also:optical properties in polarized See also:light. Remains of other echinoderms (starfishes and See also:sea urchins) are often found in plenty in Secondary and See also:Tertiary lime-stones, but very seldom make up the greater See also:part of the rock. Shelly limestones may consist of See also:mollusca or of See also:brachiopoda, the former being common in limestones of all ages while the latter attained their See also:principal development in the Palaeozoic See also:epoch. The shells are often broken and may have been reduced to See also:shell sand before the rock consolidated. Many rocks of this class are impure and pass into marls and shelly sandstones which were deposited in shallow waters, where See also:land-derived sediment mingled with remains of the creatures which inhabited the water. Fresh-water limestones are mostly of this class and contain shells of those varieties of mollusca which inhabit lakes. Brackish water limestones also are usually shelly. Corallines (bryozoa, See also:polyzoa, &c.), cephalopods (e.g. See also:ammonites, beltmnites), crustaceans and See also:sponges occur frequently in limestones. It should be understood that it is not usual for a rock to be built up entirely of one See also:kind of organism though it is classified according to its most abundant or most conspicuous ingredients. In the organic limestones there usually occurs much finely granular calcareous matter which has been described as limestone mud or limestone See also:paste. It is the finely ground substance which results from the breaking down of shells, &c., by the waves and currents, and by the decay which takes See also:place in the sea bottom before the fragments are compacted into hard rock. The skeletal parts of marine animals are not always converted into limestone in the place where they were formed. In shallow waters, such as are the favourite haunts of mollusca, corals, &c., the tides and storms are frequently sufficiently powerful to shift the loose material on the sea bottom. A large part of a coral See also:reef consists of broken coral rock dislodged from the growing See also:mass and carried upwards to the See also:beach or into the See also:lagoon. Large fragments also fall over the steep outward slopes of the reef and build up a See also:talus at their See also:base. Coral muds and coral sands produced by the waves acting in these detached blocks, are believed to See also:cover two and a See also:half millions of square See also:miles of the ocean See also:floor. Owing to the fragile nature of the shells of foraminifera they readily become disintegrated, especially at considerable depths, largely by the solvent action of carbonic acid in sea water as they sink to the bottom. The chalk in very great part consists not of entire shells but of debris of foraminifera, and mollusca (such as Inoceramus, &c.). The Globigerina See also:ooze is the most widespread of See also:modern calcareous formations. It occupies nearly fifty millions of square miles of the sea bottom, at an See also:average See also:depth of two See also:thou-sand fathoms. Pteropod ooze, consisting mainly of the shells of pteropods (mollusca) also has a wide See also:distribution, especially in See also:northern latitudes. Consolidation may to a considerable extent be produced by pressure, but more commonly cementation and See also:crystallization See also:play a large part in the process. See also:Recent shell sands on beaches and in See also:dunes are not unfrequently converted into a soft, semi-coherent rock by rain water filtering downwards, dissolving and redepositing carbonate of lime between the sand grains. In coral reefs also the mass soon has its cavities more or less obliterated by a See also:deposit of calcite from See also:solution. The fine interstitial mud or paste presents a large surface to the solvents, and is more readily attacked than the larger and more compact shell fragments. In fresh-water marls considerable masses of crystalline calcite may be produced in this way, enclosing well-preserved molluscan shells. Many calcareous fragments consist of See also:aragonite, wholly or principally, and this mineral tends to be replaced by calcite. The aragonite, as seen in sections under the microscope, is usually fibrous or prismatic, the calcite is more commonly granular with a well-marked network of See also:rhombohedral cleavage cracks. The replacement of aragonite by calcite goes on even in shells lying on modern sea shores, and is often very See also:complete in rocks belonging to the older geological periods. By the recrystallization of the finer paste and the introduction of calcite in solution the interior of shells, corals, foraminifera, &c., becomes occupied by crystalline calcite, sometimes in comparatively large grains, while the See also:original organic structures may be very well-preserved. Some limestones are exceedingly pure, e.g. the chalk and some varieties of mountain limestone, and these are especially suited for making lime. The majority, however, contain admixture of other substances, of which the commonest are clay and sand. Clayey or argillaceous limestones frequently occur in thin or thick beds alternating with shales, as in the Lias of England (the marlstone See also:series). Friable argillaceous fresh-water limestones are called " marls," and are used in many districts for See also:top dressing soils, but the naMe " See also:marl " is loosely applied and is often given to beds which are not of this nature (e.g. the red marls of the Trias). The " See also:cement stones " of the Lothians in See also:Scotland are argillaceous limestones of See also:Lower Carboniferous See also:age, which when burnt yield cement. The See also:gault (Upper Cretaceous) is a lalcareous clay, often containing well-preserved fossils, which lies below the chalk and attains considerable importance in the See also:south-See also:east of England. Arenaceous limestones pass by See also:gradual transitions into shelly sandstones; in the latter the shells are often dissolved leaving cavities, which may be occupied by casts. Some of the Old Red See also:Sandstone is calcareous. In other cases the calcareous matter has recrystallized in large plates which have shining cleavage surfaces dotted over with grains of sand (See also:Lincolnshire limestone). The See also:Fontainebleau sandstone has large calcite rhombohedra filled with sand grains. Limestones sometimes contain much plant matter which has been converted into a dark coaly substance, in which the original woody structures may be preserved or may not. The calcareous petrified plants of Fifeshire occur in such a limestone, and much has been learned from a microscopic study of them regarding the See also:anatomy of the plants of the Carboniferous period. Volcanic ashes occur in some limestones, a697 See also:good example being the calcareous schalsteins or tuffs of See also:Devon-See also:shire, which are usually much crushed by earth movements. In the Globigerina ooze of the present day there is always a slight admixture of volcanic materials derived either from See also:wind-blown dust, from submarine eruptions or from floating pieces of See also:pumice. Other limestones contain organic matter in the shape of See also:asphalt, See also:bitumen or See also:petroleum, presumably derived from plant remains. The well-known Val de Travers is a bituminous limestone of lower Neocomian age found in the valley of that name near See also:Neuchatel. Some of the oil beds of North America are porous limestones, in the cavities of which the oil is stored up. Siliceous limestones, where their See also:silica is original and of organic origin, have contained skeletons of sponges or See also:radiolaria. In the chalk the silica has usually been dissolved and redeposited as See also:flint nodules, and in the Carboniferous limestone as chert bands. It may also be deposited in the corals and other organic remains, silicifying them, with preservation of the original structures (e.g. some See also:Jurassic and Carboniferous limestones).
The oolitic limestones See also:form a special group distinguished by their consisting of small rounded or elliptical grains resembling See also:fish See also:roe; when coarse they are called pisolites. Many of them are very pure and highly fossiliferous. The oolitic grains in See also:section may have a See also:nucleus, e.g. a fragment of a shell, See also:quartz See also:grain, &c., around which concentric layers have been deposited. In many cases there is also a radiating structure. They consist of calcite or aragonite, and between the grains there is usually a cementing material of lime-See also: They are often impure, and their iron may be present as haematite or as See also:chalybite. Oolitic limestones are known from many geological formations, e.g. the Cambrian and Silurian of Scotland and See also:Wales, Carboniferous limestone (See also:Bristol), Jurassic, Tertiary and Recent limestones. They are forming at the present day in some coral reefs and in certain petrifying springs like those of See also:Carlsbad. Their See also:chief development in England is in the Jurassic rocks where they occur in large masses excellently adapted for See also:building purposes, and yield the well-known freestones of See also:Portland and See also:Bath. Some hold that they are chemical precipitates and that the concentric oolitic structure is produced by successive layers of calcareous deposit laid down on fragments of shells, &c., in highly calcareous waters. An alternative See also:hypothesis is that See also:minute cellular plants (Girvanella, &c.), have extracted the carbonate of lime from the water, and have been the principal agents in producing the successive calcareous crusts. Such plants can live even in hot waters, and there seems .much See also:reason for regarding them as of importance in this connexion. Another group of limestones is of inorganic or chemical origin, having been deposited from solution in water without the intervention of living organisms. A good example of these is the " stalactite " which forms pendent masses on the See also:roofs of caves in limestone districts, the calcareous waters exposed to evaporation in the See also:air of the See also:cave laying down successive layers of stalactite in the places from which they drip. At the same time and in the same way " stalagmite " gathers on the floor below, and often accumulates in thick masses which contain bones of animals and the weapons of See also:primitive cave-dwelling See also:man. Cale sinters are porous limestones deposited by the evaporation of calcareous springs; travertine is a well-known See also:Italian rock of this kind. At Carlsbad oolitic limestones are forming, but it seems probable that minute See also:algae assist in this process. Chemical deposits of carbonate of lime may be produced by the evaporation of sea water in some upraised coral lagoons and similar situations, but it is unlikely that this takes place to any extent in the open sea, as sea water contains very little carbonate of lime, apparently because marine organisms so readily abstract it; still some writers believe that a considerable part of the chalk is really a chemical precipitate. See also:Onyx marbles are banded limestones of chemical origin with variegated colours such as•white, yellow, green and red. They are used for ornamental See also:work and are obtained in See also:Persia, France, the See also:United States, See also:Mexico, &c. Limestones are exceedingly susceptible to chemical changes of a metasomatic kind. They are readily dissolved by carbonated waters and acid solutions, and their place may then be occupied by deposits of a different kind. The silification of oolites and coral rocks and their replacement by iron ores above mentioned are examples of this process. Many extensive hematite deposits are in this way formed in limestone districts. Phosphatization sometimes takes place, amorphous phosphate of lime being substituted for carbonate of lime, and these replacement products often have great value as See also:sources of natural fertilizers. On ocean rocks in dry climates the droppings of birds (See also:guano) which contain much phosphate, percolating into the underlying limestones change them into a hard white or yellow phosphate rock (e.g. See also:Sombrero, See also:Christmas See also:Island, &c.), sometimes known as rock-guano or mineral guano. In the north of France beds of phosphate are found in the chalk; they occur also in England on a smaller scale. All limestones, especially those laid down in deep waters contain some lime phosphate, derived from shells of certain brachiopods, fish bones, See also:teeth, See also:whale bones, &c. and this may pass into solution and be redeposited in certain horizons, a process resembling the formation of flints. On the sea bottom at the present day phosphatic nodules are found which have gathered See also:round the dead bodies of fishes and other animals. As in flint the organic structures of the original limestone may be well preserved though the whole mass is phosphatized. Where uprising heated waters carrying mineral solutions are proceeding from deep seated masses of igneous rocks they often deposit a portion of their contents in limestone beds. At See also:Leadville, in See also:Colorado, for example, great quantities of rich See also:silver See also:lead ore, which have yielded not a little See also:gold, have been obtained from the limestones, while other rocks, though apparently equally favourably situated, are barren. The lead and fluorspar deposits of the north of England (See also:Alston See also:Moor, See also:Derbyshire) occur in limestone. In the See also:Malay States the limestones have been impregnated with See also:tin oxide. See also:Zinc ores are very frequently associated with beds of limestone, as at Vieille Montagne in See also:Belgium, and See also:copper ores are found in great quantity in See also:Arizona in rocks of this kind. Apart from ore deposits of economic value a great number of different minerals, often well crystallized, have been observed in limestones. When limestones occur among metamorphic See also:schists or in the vicinity of intrusive plutonic masses (such as See also:granite), they are usually recrystallized and have lost their organic structures. They are then known as crystalline limestones or marbles (q.v.). (J. S. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] LIMERICK |
[next] LIMINA APOSTOLORUM |