Once a Week (magazine)/Series 1/Volume 5/What is flint?

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Every Londoner who aspires to the dignity of even the very tiniest of back gardens is sufficiently familiar with gravel, and must be a fastidious person indeed if he does not feel perfectly satisfied with the clean, bright-coloured material forming his well-rolled walks. On pleasant summer evenings, when the last geranium has been planted out, or the few refractory twigs trimmed into order, when no flaws can be seen on the smooth turf, and not a single intrusive weed is visible in the well-kept beds, we doubt not many a thoughtful horticulturist, with soul serenely free from the vexations of disorder and the miseries of blight, finds occupation for a few minutes’ leisure during his last stroll in giving a fugitive thought to the pebbles which he crushes under foot, and in speculating on the questions, what they are, and whence they come. It may be that this process fails to lead him farther back in the genealogy of gravel than the pit at Clapham or at Hampstead; some few inquiring spirits there will be, however, innocent perhaps of all geological knowledge, but not content with so limited an excursion into the past, who will take the trouble to consult the oracles and study the text-books of masters in the stony science. If so, we fear they will return very little enlightened as to the original birth-place and condition of the materials which make up their garden paths. Such an inquirer would be told, ’tis true, what perhaps his own sharp eyes had already discovered, that gravel is the immediate offspring of the flint; but concerning the parent’s parentage the tidings would be extremely vague and inconclusive. He would find plenty of speculation, and much ingenious theorising, but no reliable answer to his questions. Neither bold surmise, nor fanciful theory, however, could prepare his mind to receive without astonishment the startling solution which science has recently offered to the geological enigma,—What is flint?

Without going so far as to say that the explanation to which we allude meets all the difficulties of the case, it still undoubtedly accounts for several puzzling phenomena connected with flint formation in a much more complete and satisfactory manner than any previously published theory; and is withal of so singular and interesting a character, that we confidently count on our readers’ amusement, if not attention, while we state in a more detailed and popular form the substance of Dr. Wallich’s researches and conclusions.

It seems, then, we have all been wrong; we have ransacked our brains, our experience, and our fancy to no purpose; we have fathered the flint on earth and sea, chemical action, elective affinity, and metamorphic agencies, never dreaming that the legitimate parent was an old familiar friend.

It is positively pleasant to anticipate the incredulous laughter with which many readers will probably receive our statement of the fact, that every flint in the chalk, and consequently every pebble in our garden walks, owes its origin to the long-continued labours of those great silex producers—the whales. Of course every well-regulated mind will ridicule the idea forthwith, and pooh-pooh the whole thing as absurd; and it certainly does seem at first sight presumptuous to claim for Jonah the honour of having been the first and only visitor to the genuine birthplace of London gravel, and to exalt that Hebrew gentleman’s opportunities for investigation in this field of inquiry above those of such men as Lyell, Murchison, or Buckland; yet such is the conclusion towards which the latest dicta of science, uttered in sober earnest, and supported by a great array of facts, seems likely to compel the student. When the laugh has subsided, perhaps curiosity enough may survive to insure us a hearing, while we attempt to unravel this mystery step by step. Its clear elucidation to the general reader must necessarily take us over extensive ground. Several collateral branches of inquiry will claim our attention, so that if we shall presently seem to wander into paths apparently far from parallel with our main line of investigation, let us bespeak a little patience till each of these devious excursions is shown to tend towards the true end of our discourse.

To begin, we must take a hasty glance at a portion of the chalk formation of England, where the flints from which our pebbles were manufactured most abound. There are few Londoners whose summer trip to the sea-side does not take them at least across the North Downs, and perhaps even as far as the South Downs of Brighton and its neighbourhood. These great chalk escarpments form two sides of an enormous basin, roughly elliptical in form, and including within its area the whole of Sussex, parts of Surrey, Kent, and Hampshire, a strip of the English Channel, and a small tract of country known as the Bas Boulonnais, in France.

The accompanying map exhibits the outlines of this ring of chalk, and will help to illustrate our exposition. Within its white margin lies a district comprising several strata, chiefly clays and sand, and known to the geologist under the general name of the Wealden Valley. A glance at the map, or an excursion over the ground, will make it abundantly clear that there was once a time when the space enclosed in this ellipsis was covered with the chalk which now fringes its borders only. At that period the gault and clays of the Weald were deeply buried beneath a thick white pall of carbonate of lime, and the whole lay fathoms down under a cold sea.

Geological map of Wealden Valley after Lyell (transparent).png
Map of the Wealden Valley.—After Lyell.

We explained in a former paper,[1] that recent discoveries have proved the cretaceous formation to be itself the result of the life, death, and accumulation of minute marine animals, similar in genera, though not in species, with the Foraminiferæ forming a large portion of the floor of the Atlantic Ocean. Throughout the ages during which these Protozoa were slowly building up the white cliffs of Dover and the forelands of Beechy Head, some agency, at which we have hitherto done little more than guess, was busy at equal, or nearly equal intervals, laying down upon this living and snowy carpet those regular strata of isolated flint nodules which characterise the formation. The deposition of each layer was a work of considerable time, and their periodical appearance is not the least among the many puzzling difficulties which beset the question of their origin. We shall see presently how Dr. Wallich’s theory meets this part of the problem; but for the present we must confine ourselves to a more minute exploration of the locality from which the London flint and gravel is derived.

When the primeval Foraminiferæ had completed their great work, and the heaped-up shells of countless generations had accumulated into a stratum nearly a thousand feet in thickness, a change took place over the whole district comprised within the limits of the Wealden Valley. A gradual upheaval of the sea-floor began, and the “white cliffs of Albion” rose into the daylight, to play another and more conspicuous part in the economy of nature and the history of the world.

This upward movement was not effected with perfect uniformity, for the central portion of the ellipsis on our map was lifted first, while the hills which hem the Wealden in did not emerge till later on. The country rose, in fact, somewhat in the form of an inverted basin, the spot where Hastings stands being, perhaps, the first point which showed above the level of the ocean.

So slow was the ascent, and so nearly were the rate of upheaval and the erosive power of water matched, that foot after foot of the chalk was washed away by the waves as fast as it appeared; and this went on until the stratum was cut completely through over the apex of the basin and the beds below exposed to a similar action. Through the uppermost of these, too, the sea ate its way till the lowest member of the Wealden group, the green sand, had been uncovered. Meanwhile the North and South Downs, as a matter of course, receded farther from each other in every direction, until the progress of denudation was stopped by the increased activity of the upheaving forces which finally lifted the whole country above the destructive action of the ocean. The process may be aptly illustrated by making a series of thin successive slices at any part in the surface of an orange, the rind of which will represent the chalk, while the pulp beneath stands for the underlying clays and sand. Each cut of the knife exposes more and more of the substrata, and widens the white ring of peel just in the same manner as the denuding sea extended the area enclosed within the Downs. Since this operation ceased no geological change of importance has occurred. Man has come on the scene, and the picturesque hills and valleys of Hastings, the gently curving combes of Brighton and of Lewes, and the bold chalk escarpments which surround them, remain to delight him with their varied beauty or instruct him of their wonderful origin. Glancing again at the map, we shall be naturally struck by the enormous amount of material which has thus been washed away. The original chalk covering, some hundreds of feet thick, has been removed from whole counties and carried everywhither by the tides, the currents, and the storms. But the influences which disinterred could not so effectually distribute the huge mass of flint they gradually washed out from its matrix; hence, though the milky water holding large quantities of chalk in suspension was free to flow to any distance with its burden, the heavier nodules remained more nearly in the immediate neighbourhood to be broken, rolled, and rounded on many a beach into the smooth red and yellow pebbles which we know so well. The Wealden Valley, then, is the great original gravel-pit whence our minor local deposits are derived—the actual home of every shattered and abraded flint in London.

It would be too long a story to tell of their dispersion thence over the field they now occupy, or of the means used for their removal, though this is in itself a geological romance well worthy of a special chapter. When the winds and waves had done their best, the great ice-fields and bergs of the glacial epoch took up the business of their transportation, nor did this wonderful carrier cease his labours till tens of millions of tons of flints were scattered broad-cast over half our southern and eastern counties.

Here we will pause for a moment before our subject leads us into other climates, and among fresh creations, to review what we have advanced, and consider what conditions have been established with which any hypothesis explanatory of flint formation must of necessity agree. We have traced the materials of our garden-paths back a tolerably long way in their history, and marked both the locality in which they were elaborated and the agencies that first dug them from their chalky bed. We have further seen that they were deposited in a sea still in its profounder depths and differing in none of its essential constituents from the waters washing our shores to-day. There is no evidence whatever that the cretaceous ocean was highly charged, as some have assumed, with silex in a state either of fine division or solution; nothing indicates the existence of a larger per centage of any mineral matter in the seas of that period than is found in modern salt water. The Foraminiferæ, we may be certain, needed the same element then they want now, and the theory of gradual accumulation from a fluid surcharged with siliceous particles must be pronounced untenable. The chief remaining condition to be met is the periodical nature of the deposit; and this is a stumbling-block over which almost every previous explanation hopelessly breaks its neck. We shall see presently how the cetacean hypothesis stands this crucial test, and we now approach the history and evidence in favour of Dr. Wallich’s suggestions.

Sitting at our window writing these lines we have but to lift our eyes from the paper to be made powerfully sensible of the seemingly hopeless nature of the task we have undertaken.

In view of the numberless large and small stones, all fragments of chalk flints, which cover the causeways on either side of the road, and remembering of what a countless multitude these are the representatives, it seems impossible to make the reader clearly grasp the fact that every pebble among them, together with every larger nodule in cliff or pit, are but masses of the collected valves or coverings of marine plants, each in itself so inconceivably minute that the highest powers of the microscope are taxed to detect and analyse their forms, gathered, aggregated, and moulded into shape by the all-powerful influence of affinity, and the marvellous chemistry of digestion—yes, digestion! For we are about to prove, or at least attempt to prove, that the chalk flints are nothing other than the rejected materials of many a primeval cetacean banquet—perhaps the strangest remains of the hugest meal the world has ever seen.

If we pick up a few flints at random, take them home and examine them carefully, we shall discover several noteworthy things.

Selecting a dark specimen, and breaking it with a smart blow into fragments, we shall readily procure several bits thin and transparent enough to bear investigation under the microscope. From these pieces it will be easy to select many which exhibit white dots; others, dark points, or yellowish streaks and patches. Under the lens these little marks resolve themselves into well-preserved and very beautiful fossils, the white dots will turn out to be Foraminiferæ of precisely similar form and species with the chalk builders; the yellowish discolorations will be manifestly due to the presence of silicified sponge tissue, while the remaining organisms will most likely comprise spicula of various kinds and several Xanthidia. We give below a figure of each of these as they appear when highly magnified.

Fossils in Wealden flint - OAW (transparent).png
a. Xanthidium vestitum. b. Xanthidium. c. Foraminiferæ.
d. Spicules of Sponge, greatly magnified.

But besides these fossiliferous flints which we shall come across in any heap of ballast, a good hunt among the larger nodules is sure to reward us by other curious specimens. If we are lucky we shall find here and there a funnel-shaped stone with a serrated rim, looking something like a mushroom; or another, having the external form of some inhabitant of the cretaceous sea, perhaps a star-fish or sea-urchin printed deep and clear upon its surface, as in wax or clay.

The first of these two specimens is a fossil-zoophyte, originally similar in appearance and family to the sea anemones of our aquaria, but now permeated with silex; the second tells its own tale, and is simply a print or cast of urchin, or star, taken in plastic flint; both, however, point unmistakeably to a time in their history when the hard material of which they are composed was soft and impressible, ready to take the form of any firmer substance with which it might happen to come in contact. Bearing this quality in mind let us return to the microscopic fossils of our illustration. These three forms, Foraminifera, Xanthidia, and spicules of sponge represent the chief varieties of organic remains found in the flint, each of them abounded in the waters of the chalk ocean, and the problem before us therefore narrows itself to the consideration how they were first collected and compacted into the plastic masses of siliceous matter which have hardened into the pebbles we daily tread upon. A little study of the life history of one of these organisms, the Xanthidium vestitum, may help us towards an answer.

There is no doubt now that this peculiar organism belongs to the important family called Diatomaceæ, the most minute and extensive group throughout the whole marine and fluviatile world. The general character of these singular creations was sketched in the paper to which we have already referred, but a few further particulars concerning them will be useful. They are best described as free, single-celled vegetable organisms, each enclosed by a siliceous valvular envelope, whose form may be aptly illustrated by a pill-box or two watch-glasses meeting rim to rim; the surface of both cover and bottom of the box being beautifully sculptured with elaborate network like designs, while the whole is so extremely minute as seldom to measure more than from three to five thousandth parts of an inch in diameter. The valves are filled with a brown endochrome, and endowed while living with a marked though limited power of locomotion. In favourable localities recent Diatomaceæ exist in such profusion as quite to baffle description, while their numbers and geologic influence must once have been far greater than at present, whole strata in various districts, often more than a hundred feet in thickness, being entirely composed of their indestructible remains, sometimes forming a light loose soil, such as is found in parts of Bohemia, America, Australia, and Algeria, sometimes compacted into a homogeneous mass, having all the hardness and coherence of the hardest flint, like the semi opal of Bilin, which is nothing other than a cohering mass of coverings from fossil Diatomaceæ. Now this curious and prolific family to which the Xanthidia of the flint belong, formed almost the sole food of certain molluscoid tribes of the cretaceous ocean. Precisely the same thing occurs in our own seas, but we fear it will be impossible to give the general reader more than the vaguest notion of the enormous numbers in which marine diatoms exist, or convey any adequate idea of the amount of nutriment thus elaborated for the sustenance of animal life. Perhaps an extract from one of Dr. Wallich’s descriptions may help to set the facts in a striking light. Speaking of the Atlantic between the Cape and St. Helena, he says, “It was here that for many degrees, and in bright weather, the ship passed through vast layers of sea-water so thronged with the bodies of a species of Salpa, as to present the consistence of jelly. What their vertical limits were it was impossible to discover, owing to the speed at which the ship was moving. They appeared to extend deep, however, and in all probability were of a similar character to what is called ‛whale food’ in higher latitudes. Each of these Salpæ measured about half an inch in length, but so close was their aggregation, that by a sudden plunge of an iron-rimmed towing-net, half the cubic contents, from which the water had percolated, generally consisted of nothing but one thick glutinous mass of pulp. Each individual presented a minute yellow digestive cavity the size of a millet-seed, which contained Diatomaceæ, Foraminiferæ, and other organic particles.” Another equally good instance of the enormous production and consumption of siliceous matter daily and hourly going on in the ocean is furnished by the guanos which we import for use as manures. The birds whose excreta compose this useful material live chiefly if not entirely on fish; the fish in their turn have fattened upon smaller marine fry, many of whom are wholly microphagous, or in other words, depend on the Diatomaceæ for their subsistence, and the imperishable flinty valves of these minute organisms, therefore, after passing safely through the stomach first of the mollusc and then of the fish, are found at last forming no inconsiderable percentage in the constituents of almost all the guanos. It has been calculated that the ships visiting the Cincha islands remove no less than 500 tons of pure Diatomaceæ yearly in their cargoes of manure, and the estimate is probably within the mark. With these aids we shall be able to credit this apparently insignificant family with a fecundity and indestructibility sufficient for the production of very great geological results, perhaps also to conceive of their having been able, so far as quantity is concerned, to furnish all the siliceous matter for the manufacture of the chalk flints; but we have still to learn how the small and separate organisms become compacted into a homogeneous and apparently structureless mineral mass.

We have already discovered in the stomachs of the Salpæ and their relatives, precisely similar collections of organisms as those which the flint exhibits in a fossil condition, and if we could find a plastic siliceous cement, or some similar agent present in the molluscoid digestive organs, we could readily imagine a flint factory at work on a small scale in the intestines of every “school” of whale food in the ocean. Dr. Wallich believes he has detected such a cementing power in the well-known tendency of the diatom valves to cohere when in a state of comparative purity, and in the solvent action of animal alkalies upon the silex which composes them. Both in the guanos and in all other rich earths, the diatomist frequently meets with little masses consisting entirely of closely aggregated valves, sometimes so perfectly combined as to have lost almost all their original external appearances, and only to be distinguished from amorphous bits of silex by some few ill-preserved markings and sculpturings which have not altogether disappeared. The purer the state of the valves, the stronger this affinity is developed, and the contents of the salpean stomachs, therefore, are singularly well prepared for the aggregative action by reason of the perfect separation of mineral and organic matter brought about in the process of digestion.

Now we know from microscopic examination of the chalk itself, that the ocean in which it was deposited swarmed with Diatomaceæ, and these doubtless formed the food, then as now, of microphagous mollusca, similar in their general characteristics to the Salpæ and their congeners. By this means the supplies of silex which the sea was ever producing, became collected, and separated from the organic matter with which it was associated, thus acquiring a strong tendency to coherence, and becoming fully prepared for the next great change in its circumstances and condition. The mollusc, like the diatom, was destined not merely to eat, but to be eaten. The great whale family, living solely on such dainty fare as jelly-fish and Salpæ, destroy daily hosts of these organisms far more numerous than it is possible for us to realize, and it was in the complex stomachs of these marine monsters that the aggregation of siliceous matter was finally completed. Here it was first accumulated in large quantities, and became subjected to the solvent agency of alkaline animal fluids. These combining chemically with the silex would form soluble hydrates, which, while they effectually obliterated all the delicate sculpturings of the valves, bound the whole closely together by a natural cement till the masses voided at intervals, but still in a viscous state, sank to their resting place to harden and consolidate slowly upon the white and living carpet spread for their reception by the busy Foraminiferæ below. Here a new chapter in flint history opens, and new conditions supervene, to add fresh complexities to this already intricate biography. The floor of the chalk sea boasted other and higher forms of life than the humble foraminifer, and was thickly scattered with sponges, zoophytes, sea-urchins, and starfishes, not very different in form from those of recent oceans. On this surface, strewn with various animals, the plastic silex fell, enclosing here a sponge and there an echinite, and burying each in a flinty sarcophagus, over which it was destined that wise men should speculate and wonder when the long geologic æons were past away, and the white bed was lifted within the reach of quick eyes, diligent hands, and reasoning brains.

Such is the last published life-history of the chalk flints. The theory is certainly startling, but it meets the difficulties which beset the question more fully than any previous hypothesis. By referring the substance of the nodules to aggregated and partially dissolved diatom valves, it explains the presence and peculiarities of the inclosed microscopic fossils, and satisfactorily accounts for the perfect preservation of the delicate calcareous shells of Foraminiferæ, as well as the complete amalgamation of all save the largest siliceous particles, (such as Xanthidium), into a homogeneous mass, and the destruction by alkaline re-agents of their characteristic markings.

It further dispenses with the necessity for assuming any peculiarities in the composition of the cretaceous sea, and gives a reason for the nodular and isolated form in which chalk flints occur, while last, not least, it makes a vantage ground of the peculiar weakness of other theories by ascribing the periodical deposition of flinty strata to the intermittent nature of the agency at work. It is well known that the cetacea are both gregarious and migratory animals, frequenting certain latitudes at particular seasons, and leaving them again, under other circumstances, with almost as much regularity as swallows; and to this habit we may fairly refer the most puzzling phenomenon of the formation.

We have tracked the pedigree of our garden pebbles backwards, until it has led us to strange conclusions; some will still possibly think the means suggested inadequate to produce the results which call for explanation, and the lapse of time demanded by the hypothesis greater than can be granted. But we must remember that almost every new geological discovery teaches us to ascribe a wider influence and more important position to minute vital agencies, while the claims upon the ages made by the chalk builders themselves should make us hesitate in refusing a liberal allowance of centuries. On another occasion we may, perhaps, find some interest in examining further into the first of these questions, and ascertaining the kind and extent of the stratigraphical influences which have been exerted by apparently insignificant organic causes.

Among all the revelations of the microscope, none are more curious and surprising than those which have been made in connection with geological phenomena. The story we have told to-day is but one of many others almost, if not quite as strange, all owning the lens for author, and each illustrating with equal aptness the truth of Goethe’s great dictum: “God doth hang the heaviest weights upon the thinnest wires.”

D. P.


  1. Once a Week. No. 64, p. 333.