These features, together with those predicted by Darwin, have been recognised by von Richthofen and Mojsisovics in the Tyrol dolomites, and have afforded Austrian geologists good evidence that large parts of these limestones originated as coral-reefs. Faulting, however, has undoubtedly taken place in this region, producing here and there a subsidence of the limestone blocks among the surrounding more normal sediments. Rothpletz, Ogilvie Gordon[10], and other critics of von Richthofen's view have seen in this faulting the cause of the abrupt change from a facies of massive dolomite to one of normal sedimentation on the same horizontal level. They have also urged that shell-banks may accumulate locally so as to simulate reefs by their contrast with their surroundings, while the change to dolomite has obliterated their original features (see [p. 30]). It cannot be denied, however, that coral-reefs and their associated detrital deposits must exercise a very important influence in the formation of solid limestone.
Even small knots and local groups of compound corals are seen in ordinary limestones to serve as a mesh in which other organic remains have become entrapped. The ease with which the aragonite of their skeletons becomes silicified causes them often to stand out on weathered surfaces with all the delicacy of structure displayed upon a modern reef.
Where limestones and shales are associated together, a "knoll structure" may be found, the limestone occurring in masses of a somewhat hemispherical form, with the shales fitted against and round them. In some cases this may be due to the local distribution of patches of growing coral on the old sea-floor; but in other cases the structure has arisen from compression and brecciation of the strata, the original beds of limestone becoming broken up and the more yielding beds flowing round them. This structure is well seen on a small scale in many "crush-conglomerates," where the limestone appears as knots and eyes, resembling pebbles. Yet near at hand the true bedding may be traced, bands of limestone alternating with shale, and a few cross-joints indicating the possibility of a separation of the limestone into blocks. These blocks become rounded in the general rock-flow; but Gardiner and Reynolds[11] suggest solution by infiltering water as an explanation of certain remarkable examples studied by them.
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.