TRANSACTIONS OF THE HULL GEOLOGICAL SOCIETY
Volume 6, pages 19-23
THE FORMATION OF FLINT.
By T. R. WILLIAMS.
(Read February 20th 1902)
Two main theories have been advanced to account for the presence of flint in chalk.
(I) That the silica was introduced into the flint-bearing bed in solution from without, subsequent to the: deposition and consolidation of the beds.
(2) That it was deposited contemporaneously with the: bed by organic means, and dissolved in situ
The first theory has been advanced by Rupert Jones, Hull, Hardman, and others, but it is difficult to see how the flint could on this theory have its tabular and nodular form, or how the chalk could at one horizon be very flinty, while above and below it is practically free. How could the sea at one time be so charged with silica in solution, and how came these conditions to vary rythmically to account for the successive bands, while at other times it was so free as to leave the resulting chalk almost absolutely devoid of silica ? The theory was based largely on a study of carboniferous cherts in Ireland, but subsequent discovery of radiolaria and sponge spicules in these beds has altered the complexion of the matter.
It is to Dr. Sorby that we probably owe the direction of research along correct lines. In the discussion on a paper before the Geological Society on " The Physical History of Cretaceous Flints " (Dr. Wallich), he pointed out " that the remains of siliceous organisms are absent from the chalk, but flints present, whilst in the deep sea deposits siliceous organisms are abundant and flints absent, probably the material of the flints had been to a greater or less extent derived from these organisms." This brilliant surmise set Prof. Sollas to work on the Trimmingham Flints, and Dr. Hinde on the Sponge Bed of the Greensand, with the result that Dr. Sorby's surmise has turned out to be nothing less than a statement of actual fact. The process of research has been essentially a microscopic one, whereas Hardman and Hull depended largely on chemistry
Direct observation shows that the deposits in which flint and chert occur did originally contain abundant siliceous organisms which have since, to a greater or less extent, disappeared. In the cavities of flints filled with chalk, great numbers of spicules and occasionally fragments of sponges occur. These spicules are corroded and pitted on account of solution. These are mere remains, like fern leaves and sigillaria in coal. There must have been great abundance of such sponges. They have mysteriously disappeared, and somehow flints have made their appearance. The inference, as Sollas says, is clear.
Dr. Hinde has shown that the sponge beds of the Green- sand are composed almost entirely of sponge spicules, some of them large enough to be visible with the naked eye. Sections of flint nodules also show the spicules imbedded in a matrix of silica, with the characteristic pitting of the outer :surfaces and the enlargement of the axial canal due to solution. Sometimes the ends of the spicules protrude into the interior chalk-filled cavity of the sponge, as may be seen by dissolving the chalk in hydrochloric acid.
It has been suggested that the spicules have been accumulated by drifting. At first sight this seems feasible, as the same nodule of flint still retaining the characteristic form of a particular sponge often contains large numbers of spicules evidently obtained from other sponges of a different species. That there was a gentle washing is certain from the presence of quartz granules in the chalk; but the spicules are not sorted out from the coccoliths and foraminifera, nor separated into "large" and "small" as would have been the case if there had been true drifting.
The difficulty of accounting for the heterogeneous mixture of spicules is removed by the Challenger Reports. A sponge bed is shown to be inhabited by a large variety of sponges, many of them leading a parasitic life upon others. Seven different species have been found growing on a small fragment of Lophohelia not an inch square. During life these sponges regularly cast their spicules, and after death the remaining spicules, with the decomposition of the fleshy tissues, would fall on the ocean floor, and in this way the chalky ooze would become crammed full of sponge remains. Occasionally a sponge which possessed a coherent skeleton would retain its form and all its cavities would be filled with a mass of spicules derived from itself and neighbours. There is a striking resemblance between this process and that which resulted in the formation of a seam of coal. The trees shed their leaves, and the ferns their fronds and spores; in the coal itself the structure has generally been obliterated, yet a few leaves and an occasional Sigillaria indicate the former existence of a whole forest. So in the chalk an occasional sponge found preserved is the last representative of a host of associates, vanished or turned into flint.
The solution of the siliceous spicules and sponges, and the redisposition of the silica was a very interesting process. Dr. Hinde has paid great attention to this matter. It will be well within the experience of members that fossil sponges obtained at Bridlington and elsewhere are sometimes merely casts, retaining the form and markings of the original sponge, but having lost all the silica. In many cases the organic silica has been replaced by calcite, and where this has taker place it may be taken as an invariable rule that the replaced silica can be found not far off in the form of flint, chert chalcedonized shells, or minute quartz crystals; and conversely if these forms of silica are discovered, a close search will result in the casts of the siliceous organism being found. The silica dissolved was in an organic state, and in that condition it was more or less readily soluble in the presence of alkaline carbonate, but when the re-deposition took place either pari passu, or at a subsequent period, the silica may well have been freed from all organic combination. Simple depositions from solution might then have taken place. This has actually been the case; Dr. Hinde has discovered silica in a globular form in flints and cherts, and where these: globules have coalesced a flinty structure has been the result. Sometimes they occur singly or in groups in the neighbour- hood of flint and chert, at other times they may be seen in the outer portions of the flint itself. In the commonest form they have a marginal ring striated in structure, while the inner portion has a granular appearance with a shadowy unclear spot. They are spherical or discoidal in form, negative under crossed Nicols, and easily soluble in caustic potash, average size about 0.02mm. in diameter. At first it was suggested that they were silicified coccoliths ; but coccoliths are always elliptical or sleeve-link in form, and never coalesce, while the globules frequently coalesce, and are never elliptical, moreover they are larger than any coccoliths, and vary in size. We have here, I believe, the actual form in which the colloid silica is deposited from solution. After deposition in this colloid form, the silica gradually changes through a series of crystallographic gradations, through chalcedony to holo-crystalline quartz. This is a characteristic of all flints and cherts, flom Silurian to upper Cretaceous rocks. It is only in some particular circumstances that the silica has retained its colloidal state, that is, negative in polarized light, and soluble in caustic alkali.
So far I have dealt only with the simple deposition of colloidal silica. The process of replacement or pseudomorphism is very much more interesting. The process may be defined as that by which any liquid holding mineral matter in solution deposits a less soluble mineral in order to take up a portion of a mineral which it can more easily retain in solution. It therefore follows hat a pseudomorph is less soluble than what it replaces. It is possible that there may be rare exceptions to the rule, due to special circumstances. In the case of flint there is evidence that the colloid silica has replaced the more soluble organic silica of the spicules themselves. The process seems to have been, first, a solution of the spicule particle by particle; and secondly, a replacement of .the particles by the inorganic silica from the adjacent water. In some slides prepared by Dr. Hinde the pseudomorphism is shown very clearly. The form of the spicule is retained, but it substance consists of a chain-like succession of globules of silica, with clusters forming at the nodes. Here there is a replacement of silica, more or less easily soluble, by silica in a less soluble form. In some cases the axial canals of the spicules have been filled by silica which has been chalcedonized, and the body of the spicule subsequently .dissolved with or without replacement by colloid silica, leaving the spicule on the one hand, with a chalcedonized canal surrounded by a body of colloid silica (the difference between the two being well shown under the microscope), and on the other as a hallow cast with its silicified canal running down the middle.
In a less complex manner calcareous shells, echinoderms, and tests of foraminifera, &c., have been replaced by silica. This process has been artificially imitated by Prof. Church, who succeeded in converting a coral into silica by the replacement of carbonate of lime in a dilute solution of colloid silica.
I have now given a brief, but I hope fairly clear and accurate view of the origin of the silica which forms the flint, and also of the manner in which the formation took place. The flint was formed while the chalk was still in the state of ooze, or at least in a pasty condition, otherwise it would be difficult, if not impossible, to account for the smooth rounded outline of both nodular and tabular flints. A theory has been advanced that the formation took place after the consolidation and induration of the chalk; that the silica was held in solution by percolating water, and was deposited in cracks and joints and hollows left after some of the chalk had been dissolved. A moment's reflection will show the improbability of this theory. Cracks and joints are seldom or never found in the bedding planes, and they are irregular, .and ranfify. The tabular flints are always parallel to the bedding planes, and never ramify, and the same bed is approximately of uniform thickness. The nodular flints also .occupy well marked zones, and have probably been formed around detached or solitary sponges, or where the amount of silica present was not sufficient in quantity to form a .continuous layer. Once the aggregation of silica began, the process would go on, and all the silicia in the adjacent water would be deposited where the nucleus had been formed. Very rarely flint, no doubt, does occur in veins very much after the fashion of true mineral veins. These were formed after the consolidation of the rock, and the silica was deposited from solution along the free surfaces of the fissures. The first process would be a replacement of the chalk on the walls of the fissure, and then a simple deposition would cement the opposite walls together.
The most recent researches of Prof. Sollas show that the chalk in the immediate neighbourhood of flint is crammed with the calcareous casts of sponge spicules, quite sufficient in quantity, to have provided the material out of which the flints were formed.
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