ALTERED FORMS OF MASSIVE LIMESTONE
A certain amount of magnesium carbonate is present in the skeletons of some marine organisms. This has been shown both by Forchammer and Walther[911]. A foraminifer, Nubecularia novorossica, has been found with 26 per cent. of magnesium carbonate, and a serpula with 7·64 per cent.; alcyonarian corals contain up to 9·32 per cent., while calcareous algæ, such as Lithothamnium and Halimeda, contain about 12 per cent.[12]. The magnesium salt is not, however, here combined with calcium carbonate to form the mineral dolomite; none the less it is clear that such organisms introduce magnesium in appreciable quantities into the constitution of marine limestones.
Marine limestones are very commonly "dolomitised." Dolomite, the joint carbonate, CaMg(CO3)2, contains 54·35 per cent. of calcium carbonate and 45·65 per cent. of magnesium carbonate, or carbon dioxide 47·8, lime 30·4, and magnesia 21·8. Its specific gravity is 2·85.
The occurrence of dolomite in intimate association with calcite has been proved by E. W. Skeats[13] in the case of modern coral-reefs, and the secondary deposition of the mineral has been made clear. The skeletons of the corals themselves may now consist of dolomite, while calcite has crystallised in their interstices, or remains as part of the original infilling of mud. The presence of dolomite in reefs has, of course, long been known, having been observed by J. D. Dana in 1849, and it has been realised that, by prolonged alteration, masses of Dolomite Rock become built up[14].
Commonly, the process produces a Dolomitic Limestone, in which calcium carbonate is still in excess of the 54 per cent. which is present in the mineral dolomite.
The alteration of the original limestone is, however, sufficiently profound. The ready crystallisation of dolomite as rhombohedra destroys the organic structure, and traces of corals or molluscan shells disappear from great thicknesses of rock. It is uncertain whether the process of dolomitisation proceeds most rapidly in the evaporating waters of the lagoons, or, as Pfaff believes, at considerable depths, where the pressure may reach 100 atmospheres. Magnesium carbonate, as we shall note later, may be removed from dolomite in solution under pressure at a greater rate than calcium carbonate. If this occurs in sea-water, it would seem to militate against the production of dolomite in the lower levels of a reef.
The magnesium required for dolomitisation is derived from the magnesium sulphate and chloride of sea-water, calcium being removed during the change. C. Klement in particular urges that a concentrated solution of sodium chloride at 60° C. assists the process in the case of magnesium sulphate. Aragonite, the material of coral skeletons and of most molluscan shells, is more susceptible than calcite. The temperature of Klement's experiments may be realised in lagoons or between tide-marks, and Doelter suggests that the element of time in nature may allow the reaction to take place at lower temperatures.
The intimate structure of modern dolomitic limestone, as exhibited in coral-reefs, satisfies us that many older or fossil dolomites were formed from marine calcareous deposits while these were still accumulating. In other cases we must admit that the dolomite has developed in the neighbourhood of joints after the consolidation of the rock. The view that dolomitisation results from the mere removal of calcium, the magnesium originally present in organic skeletons becoming thus more concentrated, is not borne out by recent observations.
Skeats[15] has carefully compared the dolomite-rocks of Tyrol with the materials of recent coral-reefs. In both there is a striking absence of detritus of inorganic origin, and his work goes far to show that the much-discussed Alpine dolomites were formed under conditions which occur in the neighbourhood of existing reefs. This, however, does not solve the question as to whether we are dealing in Tyrol with fossil coral-reefs, or with the calcareous type of ordinary marine sediments, which might undergo the same kind of alteration. While Skeats finds in two dolomites from recent reefs 43 per cent. of magnesium carbonate, the substitution seems usually to terminate when 40 per cent. has been introduced. In Tyrol, however, the process has gone so far as to give rise to true dolomites, with 45·65 of magnesium carbonate.
The dolomites of the Jurassic series in north Bavaria are massive rocks almost devoid of fossils, traversed by shrinkage cracks, and associated with richly fossiliferous stratified limestones. The relations of these two types of rock are those of coral-reefs to the bedded deposits on their flanks, and the dolomite seems to merge horizontally into the stratified series. As in Tyrol, fossils and corals are rare in the bosses of dolomite, but the structural evidence is strongly in favour of their having originated as steeply sided reefs.
The dolomitic facies of the Carboniferous limestone in our islands is an example of the second type of origin. The dolomite here frequently occurs in irregular veins and patches. The introduction of iron carbonate with the magnesium salt stains the dolomite brown on exposure to oxidation, and its limits are thus clearly seen in the general blue-grey mass. The dolomitisation has evidently proceeded from joint-surfaces inwards. It is often sufficiently thorough to obliterate all traces of fossils, and the shrinkage accompanying the chemical change has produced numerous cavities, in which calcite has subsequently crystallised. An expansion takes place when aragonite is altered into dolomite, unless more of the calcium carbonate is removed than is necessary to give place to the magnesium carbonate introduced. In the change from calcite, with a density of 2·72, to dolomite, with a density of 2·85, there is, on the other hand, a shrinkage of 4·56 per cent. Where the alteration, then, takes place while the aragonite organisms still remain as aragonite, and not as calcite, an expansion rather than a contraction should occur in the substance of a reef; but when an old limestone, in which all the calcium carbonate is present as calcite, becomes dolomitised, a considerable shrinkage will occur, and rifts and hollows may remain obvious.
Very few dolomites, except those found in association with rock-salt and other products of the evaporation of lagoons, can now be attributed to direct chemical deposition from the sea.
Daly[7] has argued that the first Palæozoic and the pre-Cambrian dolomites were formed by precipitation, since the calcium salts in those early days were completely removed from the sea-water. Ammonium carbonate, though effective in precipitating the calcium salts, does not act on those of magnesium until the calcium salts have been brought down. But, under the conditions postulated for the river-waters that reached the sea from the earliest continental lands, conditions involving the presence of only small quantities of salts of calcium, the decay of organisms on the sea-floor might lead to a deposition of all the magnesium salts, following on those of calcium, both coming down in the form of carbonates.
The experimental work of Pfaff[16] should be considered in connexion with Daly's suggestions, since means are there indicated whereby basic magnesium carbonate, precipitated from sea-water, may associate itself with calcium carbonate to form dolomite; shallow-water conditions, with concentration by evaporation, are required.
Daly compares analyses of river-waters now running over pre-Cambrian rocks with analyses of pre-Cambrian limestones, and the ratio of the carbonates of magnesium and calcium is shown to be the same in both series.
From what we have said, it now seems probable that the great majority of dolomitic limestones owe their magnesium to substitution from without. Direct precipitation of dolomite has, however, been invoked to account for several cases of Permian age, such as the Magnesian Limestone of the county of Durham. Near Sunderland, this rock is greatly modified, containing ball-like and other concretions, associated with frequent cavities. Traces of the original bedding remain, running through the concretions, and marine fossils are abundant. Conybeare and Phillips, so far back as 1822, stated that the nodules were devoid of magnesia, though formed in a magnesian rock. In spite of this, these objects long appeared as dolomite in collections. E. J. Garwood[17] showed conclusively that they resulted from the concentration of calcium carbonate in a concretionary form. The process whereby a dolomite may thus revert towards the ordinary limestone condition, with removal of magnesium in most cases, has been styled "dedolomitisation." Water containing calcium sulphate after passing through a dolomite is found to carry magnesium sulphate by a chemical exchange. Skeats[18], moreover, points out that, under a pressure of five atmospheres the magnesium carbonate of dolomite becomes more soluble than the calcium carbonate in fresh water containing carbon dioxide. The ordinary relations are thus reversed under pressure, and a cause of dedolomitisation may be indicated.
Under the influence of contact-action from igneous rocks, dolomite may separate into calcium carbonate, magnesium oxide, and carbon dioxide. The magnesium oxide takes up water and yields the flaky colourless mineral brucite. Where silica is present, either as an impurity in the dolomite, or introduced from an invading siliceous magma, magnesium and calcium silicates may be built up[19]. Olivine thus arises, and, on becoming hydrated and passing into serpentine, stains the rock in various shades of green. The calcium carbonate crystallises as a ground of granular calcite, and the whole mass becomes a handsome Ophicalcite, or serpentinous marble. The famous rock of Connemara, used in polished slabs, has arisen through contact with intrusive diorite.
Dolomitic limestones are liable to decay rapidly in towns, owing to the formation of magnesium sulphate, which, as shown above, is even more soluble in water than is the accompanying calcium sulphate. In the country, the crystals of dolomite resist ordinary weathering by the carbon dioxide of the rain-water better than those of calcite; and the rock thus becomes loosened through the loss of one constituent, and crumbles into a dolomite sand[20]. Compact dolomites, however, have furnished some excellent building-stones for country use, since here the more resisting mineral forms the bulk of the rock.
The Phosphatic Limestones are commercially even more important. Tricalcium orthophosphate, derived, perhaps, in the first instance from the decay of bones of fishes and the excreta known as coprolites, tends to become aggregated in certain limestones, as in the chalk of Mons in Belgium and of Taplow in Buckinghamshire. The phosphate replaces foraminiferal and other shells, and frequently forms internal casts of fossils. In the latter case, it has replaced the calcareous mud that first occupied the shells. The observations of the "Challenger" expedition show that concretionary calcium phosphate is forming among the calcareous and glauconitic oozes of existing oceans, nodular masses collecting, in which foraminiferal shells are united and even replaced by calcium phosphate. Where deposits of guano are formed by sea-birds on surfaces of coral limestone, as at Christmas Island to the south of Java and at Sombrero in the Windward Islands, calcium phosphate becomes washed downwards and replaces part of the calcium carbonate of the rock. The resulting phosphatic limestone is quarried on a commercial scale, and the very existence of Christmas Island is said to be threatened by the energy of excavators. The "phosphorites du Quercy," well known to agriculturists in France, are accumulations in hollows and fissures of Jurassic limestone, and are associated with the bones of fossil mammals. But in this and in other cases there is much doubt as to whether the phosphate is derived from the bones, or is locally concentrated, with other impurities, such as sand and clay, through solution of the adjacent limestone.
The most common substance that replaces calcium carbonate in limestones is silica, in the form of Flint. The nodules of this material, white on the outside and richly black within, mark bands of stratification in the Cretaceous chalk, and are among the best known materials in south-east England. Their fantastic forms have given rise to many speculations. Sometimes, however, when fractured, they are clearly seen to include the remains of fossil sponges. The sponges may be represented merely as hollow casts; but there is abundant evidence in other cases that they belong to genera which secreted skeletons of amorphous (non-crystalline) silica during life.
The nodular flint has collected round the sponge, while the sponge itself has often disappeared. G. J. Hinde[21] has shown how readily the spicules of siliceous sponges go into solution. Even at the bottom of existing seas they become rounded at the ends, while their canals become enlarged. In some fossil instances, they are replaced by calcite. W. J. Sollas[22], emphasising this point, remarks that "it may be taken as an almost invariable rule that the replacement of organic silica by calcite is always accompanied by a subsequent deposition of the silica in some form or other." This subsequent deposition is frequently at the expense of calcite in some other part of the rock. The solid flint is a replacement of the limestone in which it occurs.
The pocket-lens will often show traces of sponge-spicules, as dull little rods, in the translucent substance of a flint. But the microscope shows that the mass of the flint has the structure of the limestone in which it lies. The foraminifera and other small structural features of the original rock are perfectly preserved in chalcedonic (that is, minutely crystalline) silica. Larger fossils, such as thick molluscan shells and the tests of sea-urchins, may escape alteration, while the chalk mud, the original ooze, with which they are infilled has become completely silicified. This explains the internal moulds of fossils in brown oxidised flint that are found in gravel-pits on the surface of the Chalk, and also the tubular hollows, representing stems of crinoids, that often occur in flint from the Carboniferous Limestone. In the latter case, the fossil remained calcareous while the ground became silicified, and the fossil was removed by subsequent solution.
Where great thicknesses of strata, as may happen in the Carboniferous Limestone, have become thus silicified, it may be presumed that siliceous skeletons were unusually abundant in the mass. But, as L. Cayeux[23] observes, such skeletons may be in one case entirely removed, and in another represented by massive flints; in yet another case, the silica may remain disseminated through the rock. The irregularity of its segregation is shown by the growth of flints in branching or hook-like forms, running from one bed to another in a limestone.
Oolitic limestones and the skeletons of corals, both having been originally made of aragonite, are often replaced by flint, forming conclusive instances, appreciable by the naked eye, of the secondary origin of this form of silica. Traces of diatoms are comparatively rare, though they probably contributed to the silicification of the freshwater Calcaire de la Brie of the Paris basin. Radiolaria, however, have now been well recognised as flint-formers, even in dark "cherts" of Silurian age. Radiolarian cherts have been taken as an indication that the beds in which they occur were formed in oceanic depths.
It is difficult to determine the stage in the history of a rock at which silicification has set in. As A. Jukes-Browne[24] remarks, solution of the silica skeletons may be accelerated by pressure, i.e. by the depth of water in which the bed accumulated. Yet, in comparison with the calcareous shells of foraminifera, radiolarian and diatomaceous remains are only slowly soluble, and are found in the deepest spots reached by soundings. H. B. Guppy[25], on the other hand, has observed silicification of modern corals in reefs in the Fijis, and believes that the process went on during the elevation of the area, when waters containing silica became concentrated, and parts of the mass were exposed to evaporation.
The instability of the non-crystalline siliceous skeletons in geological time makes it probable that a rock cannot long retain them when buried among other strata in the earth.
It is clear that there is no support for the view, current from the time of James Hutton onwards, that nodular flints are formed by matter in hot solutions entering pre-existing cavities in limestone rocks. But there must be cases where the silicification of limestone has arisen through its penetration by hot springs. The presence of tabular flint in joints of the Chalk shows that water has imported silica along easy lines of passage from some other portion of the rock. Just as stems of trees become replaced by chalcedonic silica, so may beds of limestone be converted into flint, especially in volcanic areas. A. W. Rogers[26] records that recent limestones formed in the Cape province by the evaporation of ascending waters have already become silicified. These flinty rocks have been found in the Kalahari Desert and elsewhere, though not south of the Orange River; the chemical change is probably due to the character of local water rather than to temperature. Yet it is remarkable how, in the vast majority of instances, the partial or complete silicification of a limestone may be traced to an intermediate resting stage of the silica in the form of skeletons of the vegetable diatoms or the animal sponges or radiolarians.
The decay of flint itself, by the removal of part of its substance in solution, is the cause of the white surface on specimens from the Chalk, and of the crumbling white residues found in certain gravels. This process has been fully discussed by J. W. Judd, who believes that the material removed is silica in the opaline condition[27].