Transactions of the Geological Society, 1st series, vol. 4/On the Paramoudra, and Formation of Flints in Chalk
member of the geological society,
and professor of mineralogy in the university of oxford
[Read 15th January, 1816.]
Among the organic remains of the chalk in the North of Ireland, are large siliceous bodies of a very peculiar character, which I believe have not hitherto been described, and of which the annexed drawing, copied from a sketch I made of them as they appeared four years ago in some chalk pits near Moira, will give a more correct idea than can be conveyed by any description.
These singular fossils are found in many of the chalk pits from Moira to Belfast and Larne, (see the Map, pl. 8, vol. 3, of the Geol. Trans.) but are most numerous at Moira. They are known at Belfast by the name of Paramoudra, a word which I could trace to no authentic source, but shall adopt because I find it thus appropriated. They have, I believe, never yet been found in the chalk of England, except at Whitlingham near to Norwich, and at some other places in the same neighbourhood, from whence there is a good specimen in the collection of the Geological Society, equal in size to the largest I have seen in Ireland, being about two feet long and one foot in diameter. Through the kindness of my learned friend Dr. Bruce of Belfast, a still more perfect specimen from Moira has been deposited in the Ashmolean Museum at Oxford.
The usual forms of these bodies are modifications of those drawn, more or less elongated or compressed. No two of them are to be found exactly alike in all their proportions. Their length commonly varies from one to two feet, their thickness from six to twelve inches. Their substance in all cases is flint. The termination of these siliceous bodies is distinct, and the separation of the flint from its matrix of chalk always clear and decided. Their outer covering has the appearance of a thin epidermis, smooth externally, and whiter than the mass of flint inclosed, which is usually of a dark grey colour. The whiteness of this crust is probably derived from an admixture of lime with the silex, as usually happens in the exterior part of common chalk flints.
In all cases these bodies seem to have had a central aperture passing into and generally through their long diameter. The breadth of this aperture varies in different specimens, from half an inch or less to four or five inches, but is tolerably uniform in each individual. It is usually largest in the elongated varieties; small, and sometimes almost extinct in those of a more compressed form. These cavities are always filled with chalk, of the same nature with the matrix in which the flints are imbedded; they appear to have been filled when the chalk was in a fluid state, and could accommodate itself to all the cavities of the organic body. The upper extremity of this central cavity or pipe is generally terminated by folding itself outwards, so as to form a kind of lip or scroll by its junction with the outer circumference, which is inflected inwards. The lower extremity of the siliceous body is usually contracted, terminating in a root or peduncle abruptly truncated, thick and solid on one side of the central tube, but usually open on the other, to allow the passage of water to the interior (see Pl. 24, No. 2.) This peduncle bears marks of separation by violence from the point of attachment on which it grew, and which seems not to have been the chalk in which it is now imbedded: the lower portion of the peduncle from which it was torn off has, I believe, been never yet discovered.
The position of the Paramoudræ in their matrix is irregular; (see Pl. 24. No. 1.) sometimes they lie horizontally, at other times are inclined or erect; they are generally insulated, and altogether unconnected with the thin stratra of siliceous nodules which occur in the same chalk pit with them. They often lie across these strata without producing any effect on them, or being themselves affected by them. Sometimes the extremities of two specimens are found in contact; but this seems to be the result of accidental juxta-position, not of any original connexion of the animal bodies.
The animal history of these fossils is involved in much obscurity, as they display no traces of internal organization sufficient to develope the habits and character of the original bodies, whose external features are so distinctly preserved. The central aperture or pipe was calculated to allow water to have access to the interior of the animal, as is the case in many hollow spunges, that have large single tubes passing into their center, and usually closed at their lower extremity. It is possible the Paramoudra, having a tube with two apertures, may have possessed a character intermediate between a gigantic spunge and an ascidia. I have broken very many of these fossils in search of internal organization, and in one case only found the appearance represented in Pl. 24. No. 7, and there magnified beyond its natural size. It presents a small cluster of hexagonal cells about of an inch in diameter. The substance of the septa dividing the cells does not exceed in thickness that of the finest paper, and appears to be silex much iron-shot; the cells are filled with silex of the same colour with the mass that envelopes them, and display no traces of radii or fibres traversing their interior. This small cluster of cells was decidedly inclosed in the body, and within the crust of a Paramoudra, extending inwards, not an inch from the epidermis. As this is the only specimen in which I have seen or heard of such traces, I think it more probable that they are a fragment of some extraneous body, that was accidentally attached to, and at length inclosed within the substance of the Paramoudra, than that the traces of an organization so distinct and decided, if it had ever existed generally, should have been so totally destroyed in every other specimen that has been examined, as to leave only the outward form to guide us in our conjectures as to the character and habits of the original animal.
The mineral history of the Paramoudra seems intimately connected with that of many other spungiform bodies which we find in chalk flints. In all these cases the organic bodies thus preserved, appear to have been lodged in the matter of the rock, while it was in the state of a compound, unconsolidated, pulpy fluid; and before that separation of its siliceous from its calcareous ingredients, which has given origin to the flinty nodules in chalk, and to beds and nodules of chert in other limestone rocks. The present shape of many chalk flints being that of organic bodies, demonstrates the latter to have existed before the consolidation of the former; for the fidelity with which the silex has often copied the organization, and even the accidents and irregularities of the bodies enveloped, is so accurate, that it is impossible to attribute the form of the flint to any other cause than that of the body on which it was deposited. Sometimes the organization is so delicately retained, that it seems not to have undergone the smallest derangement before the siliceous cast was taken; and the model is thus permanently preserved. In other cases the minute fibres and tubes of the animal are not expressed by the silex which has filled the spaces which they occupied, yet the external form represents with faithful accuracy that of the body which afforded to the silex its mould or nucleus. This appears to have been the casein a remarkable degree in the instance of the Irish Paramoudra.
Before the consolidation of the original compound fluid which is now hardened and separated into beds and nodules of flint and chalk, a variety of organic bodies being dispersed through its mass would afford a number of nuclei, to which, in separating itself from the chalk, the silex seems to have had a tendency to attach itself Hence the insulated nodules that occur irregularly in the chalk, out of the line of the flinty strata, do I believe very frequently bear traces of an organic nucleus; so also in many cases do those that occupy the flinty strata. But the greater number of these latter, though their form be usually that of nodules separated from each other by an intervening portion of chalk, yet indicate no traces that refer them to organic origin, and are sometimes extended into thin, continuous tabular masses. The organic bodies that afforded nuclei to these nascent flints, appear to have been dispersed pretty uniformly through the original compound mass which is now divided into beds of chalk and flints, but it is not easy to determine what cause it was that regulated the distances at which the beds of flint have been disposed, or to say why we sometimes find organic bodies preserved in flint, at other times enveloped and filled only by pure chalk. The solution of the latter question may be, that different genera of organic remains afforded centers that attracted the silex with unequal force, and that this will in some degree explain the phenomenon so common in the chalk formation, that bodies allied to the genus spunge and alcyonium, are most frequently preserved in flint and chalcedony, whilst shells and other bodies, which in their natural state were more calcareous, generally have their form retained by chalk or calcareous spar.
In cases of many of these silicified spunges and alcyonia (of which
there is in the Ashmolean Museum at Oxford, an extensive collection,
from Henley and Stokenchurch, in Oxfordshire) the outer
crust being composed of flint in its common state, represents
rudely the outline of the body inclosed. But the internal structure
retains traces of all its tubes and fibres, most delicately preserved in a reddish chalcedony. The introduction of this chalcedony appears
to have been subsequent to the first incasing of the body by the
coarse siliceous crust, and contemporaneous with the gradual decay
of the animal matter inclosed, the particles of chalcedony being successively
introduced into the space vacated by the animal particles as
they successively perished, till the result was an entire substitution of
chalcedony bearing the form of the organization of the animal.
It is probable that in cases where the body perished rapidly, there was not time for this process of gradual substitution, and that flinty matter of nearly the same coarse quality with the outer crust, was introduced hastily into the void spaces that were left unoccupied by the rapid decay of the animal nucleus. This coarser process is that which appears to have taken place in a vast plurality of instances, amongst which we must reckon that of the Irish Paramoudra, whose history has led us to the present discussion on chalk flints, among which they attain a size unusually gigantic, and often a weight of nearly two hundred pounds.
With respect to the general history of flinty nodules in chalk, whether insulated irregularly or disposed at certain distances in horizontal lines, I must observe that they seem to have originated from causes not dissimilar to those which have produced both nodules and horizontal beds of chert in the calcareous strata of many other secondary formations; e.g. in the freestone of Portland, in the mountain lime of Mells in the Mendip hills, and in the oolitic limestone near Pickering in Yorkshire, and near Poligny, on the north-west edge of the Jura mountains; at which latter place are extensive strata of chert disposed altogether in small nodules resembling chalk flints, as to their shape, size, and position, and without any organic nuclei. The chief difference appears to be, that in the case of the chalk formation, the nodular arrangement of the siliceous strata very much predominates, and in the other cases the siliceous strata, though occasionally nodular (as at Poligny), yet most frequently are disposed in continuous or nearly continuous flat masses; though even these sometimes pass into imperfectly lenticular and tuberous concretions. The existence of insulated siliceous concretions irregularly disseminated through limestone, is common to almost all calcareous strata in which there is any admixture mixture of silex; and both these concretions and the flinty strata appear to have originated during the fluid state of the matrix in which we find them imbedded, and to have proceeded with it through a nearly contemporaneous process of consolidation; the separation of the siliceous from the calcareous ingredients having been modified by attractions which drew to certain centres the particles of the siliceous nodules as they were in the act of separation from the original compound mass, and the distances of the siliceous strata having probably been regulated by the intervals of precipitation of the matter from which they were derived, each new mass as it was discharged forming a bed of pulpy fluid at the bottom of the then existing ocean, which being more recent than the bed produced by the last preceding precipitate, would rest on it as a foundation similar in substance to itself, but of which the consolidation was sufficiently advanced to prevent the ingredients of the last deposit from penetrating or disturbing the productions of that which preceded it.
The result of a succession of such deposits as are here supposed would be the accumulation of a formation of homogeneous strata, each containing in a fossil state such organic remains as happened to be entangled in the successive precipitates. The identity of these remains in that immense succession of beds which constitutes the mass of the chalk formation, is consistent with the identity of the matrix containing them; there being no reason to believe in any change of circumstances in the then existing condition of our globe, from the commencement to the completion of the deposition of the beds of chalk, since we find no admixture of sand or pebbles (the wreck of older strata), nor any symptom of interruption or irregularity in the processes from which has resulted that enormous mass of strata which usually attains a thickness exceeding 500 feet in most parts of the English chalk formation.
With respect to the stratified arrangement of argillaceous geodes and concretions, we have abundant examples of it in the horizontal beds of septaria found in the Sheppy clay, in the Kimmeridge clay, the lias, and indeed almost all secondary argillaceous strata; and we have beds of lenticular concretions both siliceous and calcareous, equally abundant in many of our sandy strata, e.g. in the green sand of the Isle of Wight, in the Stonesfield sand near Woodstock, and the sand of the inferior oolite near Bath.
As to such of these concretions (whether insulated or disposed
in strata) to which no extraneous nucleus can be discovered, it is
not easy to say what determined their centres of attraction. But it
does not appear possible that they could have been formed by infiltration
into pre-existing cavities, like the irregularly disseminated
geodes of the trap rocks; since this hypothesis in the case of chalk
would imply the anomaly of there having once existed, extending
uniformly over many hundred square miles, as many strata of air
bubbles as there are of flint alternating with the chalk; and of
which air holes not one was left empty or partially filled up;
whilst on the other hand many of the nodules could not have been
formed in such air holes, as they entirely derive their shape from
some extraneous bodies affording a nucleus to the silex that has
Again; the absence in chalk flints of that arrangement of the silex in concentric or parallel plates, which geodes of an agate-like origin would possess, adds weight to the arguments that have been offered against the existence of any identity in the manner of formation of agates and chalk flints.
- Plate 24, No. 1, 2, 3, 4, 5, 6.
- Pl. 24. No. 2.
- I mention this because an idea used to prevail at Belfast that they are occasionally found linked together in a kind of chain. For my acquaintance with this fact and many others relating to the history of the Paramoudra, I beg to acknowledge my obligations to my kind friend Dr. M'Donnel of Belfast, to whose ardent love of science, and extensive knowledge of the natural history of the north of Ireland, I am indebted for much valuable information on the Geology of that district.
- It happens occasionally that very narrow fissures, traversing the chalk and cutting two of three of its siliceous strata, are filled with tabular plates of black flint. Such fissures are rather rare, and were probably of high antiquity, nearly contemporaneous with the consolidation of the rock they traverse, and before the separation of its siliceous from its calcareous portions had been fully and finally completed. I have given in the drawing (Pl. 24, No. 8) an example of these veins, from a chalk pit on the south side of the London road, at the western extremity of Hurley Bottom, at the first ascent of the hill towards Henley., and about four miles east of that town; here the lower termination of the veins as covered by rubbish. A similar appearance may be seen near Brighton, at Rottingdean, where both the lower and upper terminations of the siliceous veins are distinctly laid open by a vertical section of the cliff. I have not seen the spot, but copy the section in Pl. 24, No. 9, from a drawing and description of it that were lately sent to me. The lines represent beds and veins of plated flint; the dots express siliceous nodules.
- Although in the present compact state of the matter of flint it is not easy, though possible, to force a fluid slowly through its pores, it is probable that before its consolidation was complete it was permeable to a fluid whose particles were finer than its
own, and that the particles of chalcedony, whilst yet in a fluid state, being finer than those of common flint, did thus pass through the outer crust to the inner station they now occupy, where they also allowed a passage through their own interstices to the still purer siliceous matter which is often crystallized in the form of quartz in the centre of the chalcedony, and so entirely surrounded by it, that it could have had no access to
its present place, except through the substance of the chalcedony and flint inclosing it.
Perhaps the same illustration may he offered, to explain the formation of quartz crystals in the centre of many agates, as well as of their concentric chalcedonic zones, the substance of which appears often to increase in purity in proportion to its distance from the outer circumference. I allude particularly to those agates in which there are no traces of any funnel through which the matter of the concentric zones could have been introduced; and to those chalcedonic geodes in the basalt of the Giant's Causeway, in which also no sign of a funnel can be discovered, but the component laminæ are disposed in parallel lines, crossing horizontally the cavity in which they are contained, and sometimes filling only the lower region of it. In such cases, the upper and void portion of the cavity is lined with an uniform thin film or arch of mammillated chalcedony, so exactly conformable to the irregularities of the hollow within which it is deposited, that we can only suppose it to have been introduced by a slow and uniform infiltration through every pore of the cavity that is now lined by it, and which, had the process been continued further, might intirely have filled it up.
This seems indeed to have happened in the case of those solid geodes, of which the lower part is composed of parallel flat plates of chalcedony, and the upper part made up of curved zones of the same substance concentric with each other, and bearing the form of the arch that overhangs the horizontal laminæ of the lower region.
Those geodes in which the cavity of the upper region is open, and merely lined by a thin vaulting of chalcedony, are known at the Giant's Causeway by the appellation of Box Agates, and Dr. MacDonnel has assured me, that in countless instances, when he has broken the laminated geodes from their matrix, with a view to examine the position of their parallel plates, they lie always horizontally.
- It is important to distinguish between cavities in the chalk itself, and those within the body or shell of the siliceous concretions contained in chalk; and whilst I contend that in the latter case the cavity of the flint has been sometimes filled in a manner analogous to the infiltration of geodes in the trap rocks (see Note, p. 419) yet the presence of the organic body, to the decay of which alone these last named cavities in the flints are indebted for their existence, proves that there was no cavity antecedent to that decay, and that the silex was originally deposited in the form of a mould round the organic body from which it derives its shape.