Rocks and Their Origins

by Grenville A. J. Cole

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.