TRANSACTIONS OF THE HULL GEOLOGICAL SOCIETY
THE FORMATION OF FLINT.
Abstract of a paper by the late Canon H. E. Maddock, M.A. (Read February 20th, 1890)
Flint is one of the commonest stones met with in east Yorkshire. It occurs plentifully in the glacial deposits, and also in great abundance in the form of nodules and layers, in the middle division of the Yorkshire Chalk, a formation which in the form of a broad irregular crescent stretches from Flambro' Head to the Humber.
Chemical analysis has shown that flint is almost entirely composed of silica, a combination of oxygen and silicon, which probably forms not less than two-thirds of the earth's crust.
There is great variety in the structure of flint. Some specimens exhibit their original structure and appear to be simply silicified chalk- mud, others show signs of crystallization and consequent more or less complete obliteration of primary structure. In the former case the outlines of foraminifera, sponge spicules and other organisms can often be clearly seen; in the latter, and especially is this the case with black flint, traces of organic forms can seldom be detected.
Flint occurs in the following forms :--
(1) Potstones or "Paramoudra.'' Flint in this form was observed by Sir Chas. Lyell in the White Chalk near Norwich. He describes the potstones, as pear-shaped masses of flint, 3 feet in height by foot in diameter, occurring 20 feet to 30 feet apart, in rows, like a series of pillars. On breaking them open the flinty mass was found to contain a cylindrical core, of pure white chalk, of exceptional hardness. Mr. J. R. Mortimer records similar potstones at Flambro' and Speeton. He describes them as stump-like columns of flint, with an admixture of chalk, measuring from 2 feet to 5 feet in diameter, having a dish-shaped hollow or cavity in the centre of the almost flat top, filled with hard chalk. The term “paramoudra" appears to have come from Ireland.
(2) Nodules. Flint occurs commonly in the form of nodules of various shapes and sizes, from five inches in diameter downwards. The nodules though sometimes scattered, generally run in lines, from two feet to eight feet apart, parallel with the bedding. These lines can sometimes be traced many miles.
(3) Beds or seams. Flint also occurs in continuous seams or beds varying from one inch to eight inches in thickness. These beds also, like the lines of nodules, exist over large areas.
(4) Fissures. Flint is also found in oblique or vertical fissures in the chalk. The fissures are generally titled with calcite, but occasionally with flint from top to bottom.
It is also worthy of note that the Middle Chalk of Yorkshire, leaving its characteristic flint bands and nodules out of consideration, contains less silica than the Upper and Lower divisions of the Chalk, which in Yorkshire contains no flints.
Many geologists think that flint of every description was formed under the waters of an extensive sea or ocean. Microscopic examination reveals remains of marine organisms in flint, and there is no doubt that certain siliceous rocks are almost entirely made up of sponge spicules still unchanged. The "Challenger" reports prove conclusively that at the bottom of the Atlantic and Pacific oceans arc vast quantities of sponges bearing siliceous spicules. These extend to all depths, but attain their maximum development between 500 fathoms and 1000 fathoms. The forms represented by the Ventriculidae and allied families in the Chalk and Greensand are found in plenty at that depth in all parts of the ocean. Nearly all the deep sea sponges of all orders are stalked, or are provided with beards of fringes of radiating spicules, or are otherwise suppled with means of supporting themselves above the surface of the soft ooze in which they grow. These sponges must have swarmed on the floor of the Cretaceous seas. In all the beds of chalk their siliceous spicules are very abundant, and entire individuals are not uncommon. There is a very interesting point in connection with sponge spicules brought to light by Professor Sollas, in his report on the Tetracinnelid sponges of the Challenger Expedition. He has shown that certain species have the power of periodically shedding immense quantities of spicules. In some specimens he found that the passages were nearly always closely packed with a substance resembling cotton-wool. On examination this was found to consist of spicules densely matted together, everyone similar to those forming the sponge. Some of the Chalk sponges are closely allied to this species, and in all probability possessed the same power of shedding spicules during their growth. Hence we find that to account for the silica in flint, we have to take into consideration not only the spicules contained in dead sponges, but also a large number produced by the sponges during their life-time. In addition to this Radiolaria and possibly Diatoms assisted.
It must be borne in mind that the colloid or gelatinous form of silica can be dissolved with no great difficulty. Indeed it is by no means uncommon to find sponge spicules, once siliceous, which have lost every particle of silica. In many cases also we find that silica, has taken the place particle by particle, of structures which were once calcareous. Hence we are forced to the conclusion that at certain periods the floor of the Chalk-ocean was covered by an ooze of colloid silica dissolved from the remains of sponge spicules, Radiolaria etc. It is a well ascertained fact that where the decomposition of any organic substance is going on in the proximity of colloid silica there is a tendency for the particles of decaying matter to be replaced by particles of silica, or else the hollows left by decomposition are filled in with that material. Professor Sollas has shown that when such animal matters as albumen and gelatine are treated with silicic acid a definite chemical compound is formed. In process of time this complex organic substance would decompose, the organic constituents would be evolved, and the silica would remain behind. In other cases the silica would fill the cavities left by decomposition or the natural cavities of the sponges, whilst in still others it would form an entire covering, enveloping the animal or vegetable organisms. All these processes seem to have taken place during the formation of flint. In some instances the colloid silica seems certainly to have replaced the organism, molecule by molecule. In others it seems to have filled the hollow left in the interior of a sponge, shell, or urchin, whilst occasionally it has formed a deposit around the object. Perhaps the best example of the first of these are the paramoudra or Potstones. Sir Chas. Lyell pointed out how clearly the resembled in shape ad size the large sponges called Neptune's Cups, which grow in the seas of Sumatra. In these cases it is evident that the flint must have been deposited before that consolidation of the Chalk took place by which the smaller fossils were mostly destroyed, and must have taken the place of the part of the sponges, leaving unchanged the cretaceous mud in the interior.
With regard to the parallel layers of nodules of flint, there can be little doubt that each layer represents an ancient sea floor. From the plentiful occurrence of perfect sponges in these nodules, it is obvious that they owe their form in many cases to the growth of sponges in the exact place in which they now occur. The alternate beds of chalk and flint, however, requires some explanation. In my opinion we must look for the solution of this difficulty in the phenomena observed in the manufacture of porcelain in our Pottery districts. For the production of certain of the finer kinds of china, flints are ground up and mixed in definite proportions with clay. If this mixture is well stirred up and used at once, the particles of silica and alumina remain intermingled, but if it is allowed to stand for any time the siliceous particles separate from the clay and form into nodules. I believe some such process took place in the formation of flint nodules, and that when a certain amount of ooze had been deposited over the area where the layer now occurs, the law of aggregation of similar particles took effect. Wherever there was a decaying organism, there would be a nucleus for the aggregation of siliceous particles, and thus a layer of flint nodules would be formed. This would also account for the scarcity of silica in the Chalk with flints, as compared with the Chalk without flints.
The third form of flint, viz., tabular layers, has probably a similar origin to the beds of nodules, only representing a more extensive deposit. The "Challenger reports" contain much interesting information in this connection.
The fourth and last form of flint to which I have referred is of comparatively
recent origin. The fissures would no doubt be caused by the upheaval and
displacement of the Chalk strata. Water continually acting on the layers of
flint would in course of time dissolve some of the colloid silica. This would be
carried down and re-deposited on the sides of the crack, and eventually fill it
with a mass of flint.
In conclusion it may be not uninteresting to refer to the evidence of the time which must have been necessary for the deposition of the flint. Professor Sollas, in the report already referred to, has carefully estimated the proportion of silica found in the sponges most resembling those of the Cretaceous ocean. He finds that it would take eight full-sized sponges to furnish sufficient silica for an ordinary flint nodule. It would therefore appear that the time required to form an ordinary nodule, equals the period necessary for the growth of eight successive generations of sponges. It has to been found by experiment carried on in the Adriatic Sea and the Gulf of Florida, that it takes from five to seven years for a sponge to grow to the size of an average flint nodule. Hence it is argued that it would take about fifty years to form a single nodule, or, (as the process would probably go on all over the sea-bottom at once,) a layer of nodules. Is not a fact like this cause for wonder and admiration, as we look up at our chalk cliffs, remembering that to form one single layer of one or two inches, half a century has passed away, and that to form a single fragment of flint, such as we see on the roadside, and are apt to pass by quite unnoticed, there has been such a wealth of animal and vegetable life?
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