Rocks and Their Origins

by Grenville A. J. Cole

LIMESTONES FORMED OF ORGANIC REMAINS

These limestones present an immense variety, according to the nature of the originating organisms, and the amount of foreign material brought down into the water where they accumulated. The calcareous remains of Chara may form a white deposit on the floors of freshwater lakes. The part played by calcareous algæ in the formation of marine limestones has long been recognised; but the detailed exploration in 1904 of the atoll of Funafuti in the Pacific showed that Halimeda may be responsible for a considerable portion of an ordinary "coral-reef." Lithothamnium occurs in immense quantities, associated with molluscan remains, near many shores, and forms a large part of the material of the raised beaches in Spitsbergen.

Animal, not vegetable, activity, however, is responsible for the majority of our limestones, and the humbler organisms, by reason of their abundance, play a prominent part in rock-formation. Analogies between the Globigerina-ooze of deep waters and the groundwork of the soft white limestone known as Chalk have been freely pointed out. Early in the nineteenth century, Ehrenberg, in a series of researches with the microscope, proved the organic origin of the compact ground of marine limestones. The occurrence of foraminifera from the shore outwards to truly oceanic waters provides a fine-grained calcareous material which forms deposits at very various depths. The milioline types, often with a surface like that of glazed porcelain, are common in the sandy beds formed near a coast. Few rocks are more fascinating under the microscope than those in which such types are seen in section, associated with detrital grains of quartz, washed down from the land, and perhaps with bright green grains of the marine mineral, glauconite. In Ireland white chalks occur, speckled throughout with glauconite, which looks dark in the rock-mass, but which reveals its green tint when streaked out by the hammer. When formed still farther from land, pure chalk arises from the consolidation of foraminiferal ooze, and the probable depth in which it accumulated must be judged from the nature of the associated organisms. A white limestone may, however, arise in a comparatively shallow sea, where the rivers bring down little solid matter from the land. A coast formed of pure limestone, with clear streams flowing from a land of similar rock behind, may allow of the development of pure limestone on its shores. It is generally agreed that the Upper Chalk of the British Isles and of northern France was laid down in water one thousand fathoms or more in depth; yet the corresponding white limestone of northern Ireland in places follows rapidly on conglomeratic and glauconitic deposits, and seems to owe its purity to the comparative absence of rain and rivers on the highland of crystalline rocks which stretched westward from its shore.

There are two epochs of the earth's history in which foraminifera were remarkable for their size as well as their abundance. The first gave us the grey Fusulina limestone of Upper Carboniferous times, when this spindle-shaped shell spread freely from the United States through the arctic regions to the east of Asia. The second gave us, in the Eocene period, the great beds formed of Nummulites and Orbitoides, which we meet with in Europe on the Lake of Thun, but which are far more important in Lower Egypt. The disc-like forms of the nummulites in the white limestone of the Pyramids are familiar to hundreds of travellers, and forms are recorded up to four and a half inches across.

The foraminiferal origin of many compact limestones can often be appreciated on smooth surfaces with a pocket-lens. The older examples have commonly become stained and darkened, and crystallisation of calcite throughout the ground has in part destroyed the original organic structures. This tendency to crystallise affects even the larger fossils, and brachiopods and molluscs have sometimes disappeared from our Carboniferous limestones, without the intervention of "metamorphic" heat or pressure. In most limestones older than the Eocene period, the shells and other fossils, such as corals, that were originally formed of aragonite have passed into the calcite state, without the destruction of their characteristic shapes. Shells, however, have been found still preserved as aragonite in beds as old as the Jurassic period[8].

The lamellibranchs, the ordinary bivalves, came into prominence as limestone-builders with the Carboniferous period, and are now rivalled by the univalve gastropods, which displayed no widespread activity until Eocene times. The most massive existing shell, however, is a lamellibranch, the giant Tridacna of Australian seas, a single valve of which may weigh 250 lbs. The cephalopods, though lying far nearer to the crown of molluscan development, became important from the Silurian Orthoceras onwards, and nautiloids of various forms are common fossils in the Carboniferous limestone. Their large size attracts attention from our present point of view. The cephalopods, however, swell the bulk of many limestones, not by the thickness of their shells, but through their chambered character, which has prevented complete infilling of the shell, and which thus allows of cavities in the mass.

This is notably the case with the ammonites, which contribute so largely to Jurassic limestones. Crystalline calcite has often been deposited by infiltration on the septa and on the inner layer of the shell, thus reducing the hollow spaces. The massive calcite guards of the belemnites form a considerable part of many limestones.

Even freshwater lakes possess molluscan deposits, producing a white limestone of their own. Where streams flow over pure pre-existing limestone, there is no alluvial mud to choke the basins. In the hard lake-waters, gastropods such as Limnæa and Planorbis, and a few bivalves, can then flourish freely, and a "shell-marl" accumulates at the bottom, unmixed with sediment. Limestone of this type is conspicuous in hollows in the Dinaric Alps, which were once occupied by lakes, and is often found beneath peat in the limestone lowland of central Ireland.

In older days, two groups of organisms, now relatively unimportant, had a powerful place. The brachiopods, including in early Palæozoic times an interesting series of thin shells largely composed of calcium phosphate, were for long the predominant shell-bearing organisms. The stout Spiriferidæ and the well-known Productus giganteus of the Carboniferous period illustrate their dominance. The group became much restricted in variety in Jurassic times; but even then Terebratula and Rhynchonella occurred so abundantly that they now fall out of many rock-faces like pebbles from a loose conglomerate.

The sea-lilies have similarly lost their place as limestone-builders, though their "ossicles," notably from their stems, furnish crinoidal or "encrinital" masses from Silurian to Carboniferous times. The broken portions of their stems, resembling tubes of tobacco-pipes, are conspicuous when they are weathered out on rock-surfaces or revealed in polished slabs of marble. The fact that each joint or ossicle, as is the universal case in the echinodermata, consists of a single crystal of calcite causes the fragments to break with the characteristic cleavage of that mineral. The smooth glancing surfaces thus seen on fractured specimens readily call attention to them in a rock.

Those humble colonial organisms, the compound corals, have so special a place as limestone-formers that they have been reserved for more detailed treatment. The accumulation of their skeletons, and the fact that they may form large continuous masses by their very mode of growth, promotes the formation of solid rock at an unusual rate. Von Richthofen long ago pointed out how foraminifera and other drifted material became caught in the interstices of coral, producing even a stratified structure in the hollows of a reef; and subsequent research has shown the composite character of reefs in various portions of the tropic seas. Calcareous algæ as already remarked, and the massive and often encrusting skeletons of hydrozoa, such as Millepora, are freely associated with the products of true corals.

Charles Darwin, in his famous theory of the formation of atolls and barrier-reefs, showed how, in a subsiding area, corals might keep pace with the downward movement. Hence reefs might arise of great vertical thickness, although the polypes themselves could flourish only in the upper twenty fathoms or so of water. This conclusion, which appears strictly logical, has met with much opposition from Karl Semper, Alexander Agassiz, and Sir John Murray. Murray in particular urges the importance of banks of calcareous organisms in building up platforms on which corals may ultimately dwell. The extension of reefs outward into deep water has been attributed to the rolling down of wave-worn coral debris over submarine mountain-slopes. From this point of view, an apparently thick atoll may be formed as a comparatively thin mass of limestone at the summit of a volcanic cone that fails to reach the sea-level.

The opponents of the view that thick coral-limestones are formed at the present day in the Pacific have been unwilling to accept the results even of the deep boring in the atoll of Funafuti[9], which penetrated materials like those of the superficial layers of the reef to a depth of 1114 feet. They have also refused to see in the huge dolomitic rocks of Tyrol the remains of Triassic reefs four thousand feet in thickness. None the less, most geologists regard the Funafuti boring as a strong support for Darwin's contention. Whatever may be proved as to the origin of this or that atoll at the present day, it is clear that the possibility of subsidence leads us to expect considerable coral-limestones among our ancient rocks. The same problem arises wherever we have a rich molluscan fauna continuously represented in two or three thousand feet of limestone, or where we find shore-deposits of any kind accumulated to an unusual thickness. Darwin, at the end of the fifth chapter of his work on "The structure and distribution of Coral-Reefs," gives a vivid account of the features that would appear in a section of an atoll that has grown large through subsidence of its inorganic floor, and he emphasises the occurrence of conglomerates of broken coral-rock on the outer zone. The stratification of material by wave-action in this zone, and the horizontal deposition of finer material in the lagoon, would give to the dissected mass a general sedimentary aspect. Darwin concluded that the ring of solid coral, the true reef, might be denuded away during an epoch of elevation, and that only stratified portions might remain. He does not seem to have discussed the contemporaneous deposition of pelagic material from foraminiferal and other sources against the outer surface of the reef whereby an interlocking of two facies of limestone might arise.

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.