61
FLINT AND CHERT. By WILLIAM HILL. F.G.S.
(Presidential Address, delivered February Srd, 19/1.)
CONTE:'l"TS. PAGE
I. !)lTRODUCTION. 61 2. T\'PES OF FLINT AND CHERT (CRHACEOUS) 64- 3. VARIETIES OF FLINT FROM FORMATIONS OTHER THAN THE CHALK 69 4. VARIETIES OF CHERT •••" 71 Cherts of the Upper and Lower Greensand. 71 Chert of the Portland Beds. 73 Carboniferous Chert • 76 Chalcedony in Flint . 80 Chert of the Culm Measures 82 5. IMMATURE FLINT AND CHERT 83 6, GENERAL SUMMARY 85 7 CLASSIFICATION OF SILICIOUS CONCRETIONS 93
1. INTRODUCTION. VE RYONE familiar with the White Chalk of England is E also familiar with the nodules of a different substance which are scattered through it. Break one, the fracture will be conchoidal, and it will be found that within a white rind of greater or less thickness is a hard, black, translucent material, possibly with some cloudy patches, which we recognise as flint. How and when the term flint became applied to these nodules in the Chalk is a matter of some obscurity. We know that Early Man recognised the value of flint for offensive, defensive and domestic purposes, and until quite recent years it played no unimportant part in the economies of mankind. The word probably came to us from the Saxon "flinta." It is used six times by the translators of the Authorised Version of the Old and New Testament, though it seems to be applied to rocks of exceptional hardness, and not to be confined to what we know as flint. Virgil, Pliny, and Lucretius use the word silex in connec tion with striking fire. According to Murray the word occurs in the Epinal Glossary about A.D. 700, page 8°5; it is there referred to as Petra-focaria, flint. In another Glossary, A.D. 1050, it is again described as Petra-focaria, "jryslan"· "jlyllte," and thenceforward the word occurs frequently in con nection with fire and fire-arms, and is certainly used to describe PROC. GEOL. Assoc., VOL. XXIl, PART 2, 19I1.] 6 62 WILLIAM HILL ON the material which occurs as nodules in the Chalk. So closely indeed is chalk and flint connected in our minds that it is hardly possible to think of one without recalling the other, and if a geologist were asked where flints were to be found he would almost certainly refer the enquirer to the Chalk, therefore we may safely take one of these silicious nodules as the type of true flint. They are, as we all know, segregations of silica, probably derived from organisms which lived in the Cretaceous sea, but the chemical processes which have involved the disappearance of the silica of sponge spicules, the tests of Radiolarians, or of the frustules of Diatoms from the chalky deposit, and its re-appear ance in the form of flint nodules, though not perhaps so great a mystery
2. TYPES OF FLINT AND CHERT (CRETACEOUS).
Bearing these remarks in mind, let us see for ourselves what differences there are between a typical example of flint and chert. As an example of flint I take a pure black translucent speci men from the zone of Micraster cor-anguinun; near Hitchin. I assume you must all know what flint is like, and that I need do no more than recapitulate its salient features when a thin slice of it is viewed under the microscope. It will be seen almost clear and colourless, or with a faint brownish tinge which Prof. Sollas t regards as " probably due to the carbonaceous pigment, the last residuum of the living protoplasm once present in the chalk." In this translucent medium the forms of foraminifera
*' "The Formation of Chert and its tt.;icro-structure in some Jurassic Strata!' Proc; Geol, Assoc., vol. xviii (1203-4), p, 71. of "Age of the Earth, ' p. 142. FLINT AND CHEI~T. spheres, shell frngments, or other small calcareous organisms contained in the chalk may often be seen faintly but faithfully outlined, though details of their structure may be obscure. Sponge spicules are not abundant in such flint, though traces of them occur in most specimens. Viewed with a high power, say ~-in. objective, it will be seen that the flint has not the homo geneous appearance of quartz and glass, and one seems to realise, in a confusion of faint and shadowy lines, that the material is built up of minute particles fused or cemented together into a coherent mass. To those familiar with the minute structure of the chalk there would appear grounds for thinking that the flint was a pseudomorph of the original deposit, silica replacing the chalk particle by particle, a viP-IV supported by the remarks of Professor Rupert jones," Professor Judd t in his paper read before the Association on the" Unmaking of Flints," and also by Professor Sollas.] With crossed nicols the silica of the flint is seen to be in a minutely crypto-crystalline condition, the optical action of the light giving rise to a multitude of bluish specks, which, though separate, yet closely approximating to each other, give the whole a greyish-blue appearance. But this even structure is not alto gether unbroken, for at points which correspond with well-shown foraminifera, or with that occupied by a shell fragment or a sponge spicule, larger and more strongly refractive crystals map out, as it were, the area once occupied by the organism. The apparent separation of the minute crystals seems due to two facts; the first is that the axes of the crystals do not all lie in the same plane-some are presented to the eye at the extinction angle, and will appear and disappear as the stage is slowly rotated j secondly, that flint is an intimate admixture of opaline and crystalline silica, the opaline silica, which is negative to polarised light, filling minute interspaces between the silicious crystals.§ It is, however, rare to find a nodule consisting throughout of pure black flint; in almost everyone there are breaks in the homogeneity of the material, breaks which are usually referred to as "cloudy patches." Many flints, too, are greyish, others are bluish-grey, and are duller and less glassy than black flint; some have a peculiar opaline appearance, others may be entirely white. These modifications are largely due to the imperfect
* H On Quartz and Other Forms of Silica," T. Rupert Jones, Proc, Geol. A soc t \-01. iv ( 1874-6 1, p. 439· t Proc, Geol. Assoc.. vol. x (1887.8), p. 2I9. t H Age of the Earth," p. 150; and red ink, readily. It may be partially, perhaps with care entirely, restored to its original dearness by immerslon in Canada balsam. Thin slices of weathered flint, white dud opaque, are quite clear and translucent when mounted in balsam. 66 WILLIAM HILL ON
silicification of the amorphous calcareous material, to the presence of imperfectly silicified organisms or organic fragments, to dif ferences in the crystalline condition of the silica, to which reference will be made, and to the decomposition of the flint itself. They lead to endless variety in the macroscopic appear ance of flints from various localities and horizons. But the ground-mass of true flint will invariably be found to be silica in a minutely crypto-crystalline condition. Let us now examine an example of true chert. We cannot do better than take it from the Lower Greensand, for it is the chert of this formation that is most familiar to the members of the Association, and it is also the silicious concretions of this forma tion to which Dr. Hinde refers as "true chert." We will take as our specimen one from the Hythe Beds of the Lower Greensand, collected during the excursion of the Association to Gomshall last summer. Viewed as a hand-specimen, its aspect is different from that of our black flint. It is brownish-grey in colour, has a saccharoid appearance, having none of the glassy look of flint, and it is opaque. Its fracture is splintery rather than conchoidal. Though the core of the silicious matter is easily distinguished, there seems no very definite boundary between it and the sur rounding material, one passing rapidly to the other, and it is not easily separable like a flint nodule. A thin section shows that the ground-mass is radiately fibrous chalcedony, almost water-clear, full of the remains of sponge spicules j so numerous are they that it is not possible in any part of the section to find a space covered by the field of the micro scope where they are not seen. For the most part they are the residuary canals of the spicules of Tetractinellid and Hexactinellid sponges, with many indeterminable rod-like lengths. When the spicule is cut transversely, or nearly so, the walls and central canal are sometimes faintly outlined, but usually the walls have disap peared, and there remains but a cast of the axial canal which has been infilled with silica in a less soluble form. Some have been infilled with glauconite, others with silica akin to that material, but of brownish tinge. By direct light these have a whitish opaline appearance, and are semi-transparent; some are hollow and solid looking; these are frequently coloured brown by the infiltration of iron, and are then opaque. Scattered promiscuously through the section, but more abundant in some places than in others, are grains of glauconite and quartz, with irregular patches of fine matter, presumably portions of the original inorganic deposit. These, with numerous more or less opaque specks, give the chert a dirty appearance. From many points along the borders of the residuary spicules and inorganic particles rise fibrous growths of chalcedony, which spread in fan-like arrangement and intersect each other, filling all the interspaces with a confusion of radiating fibres, while in clearer spaces independent aggregates FLINT AND CHERT. of radiately-fibrous chalcedony have formed. There is in this cbert no modification of the fibrous structure of the chalcedony, but the fans and fibrous aggregates are small when compared with some examples of chert, and are not well seen with a less power than a !-inch objective. There are a few cavities which seem to be caused by the solution of spicules. With crossed nicols the structure of the silica is seen to be coarsely crystalline. The difference between our typical examples of black flint and the Gomshall chert lies chiefly in the abundance of spicules con tained in the latter, and in the crystalline condition of the silica; in flint it is crypto-crystalline, in chert it is radiately fibrous chalcedony. I t would be a simple matter to determine what should be called chert and what flint, if this difference was persistent, but it is not so. In a series of both flints and cherts it is possible to find every gradation, from the minutely crypto-crystalline condi tion of the silica to the broad, radiately fibrous structure of chal cedony, and in their proportions to each other; and this is not only in the same formation, in the same bed, but in the same concretion. It is difficult to give an adequate description in words of the different structures seen, more difficult to give reasons for these variations. The fact is, that the special char acters of so-called flints and cherts are due to the nature of the rock in which they were formed, and to the conditions under which the silica segregated into a silicious concretion. Let us consider for a moment the conditions under which these two typical examples of flint and chert have originated. The flint we know has formed in a rock made up of the detrital remains of calcareous organisms, an almost pure carbonate of lime. There is abundant evidence that scattered through this mass of calcareous debris were the remains of silicious organisms. On the fractured surface of specimens of the Middle and Upper Chalk one can sometimes see hollows, each of which, from their shape, must at one time have contained the spicule of a sponge, but the silica has disappeared, and only a cast is left. Prof. Sollas ':' says that the chalk of Pang bourne contains 3 %of such hollows, and many others have gone, leaving no trace behind, especially in the case of the minuter forms of spicules. Radiolaria were also present. Their outline can sometimes be distinguished in the chalk itself, but the silica of their tests has gone, and granular crystalline calcite has taken its place. Diatoms may also have occurred, though they have not been identified. These organic remains, all of which were of amorphous and soluble silica, were disseminated through the chalk, more abundantly perhaps in some places than in others, but in quantity not enough to form a silicious rock. Their silica has now disappeared, and there * "Age of the Earth," p. '47. 68 WILLLHI HILL (1:'-1
seems nothing incongruous in the idea that the silica of flint has been derived from the solution of these organisms. That the dissolved silica could be, and was, transferred from one part of the chalk to another seems incontestably proved by the occurrence of oblique and vertical layers of flint which are so common in the chalk. The flint seems to have formed along the sides of joint-planes or cracks, occasionally expanding into bulbous nodule-like concretions. It differs in no way from the ordinary nodule, for foraminifera, spicules, and Xanthidia, occur in it. It is difficult to conceive any other way in which this transference could be effected except by the agency of water. Mr. Jukes-Browne" says: "It is quite possible, however, that the deposition of silica in these cracks took place as soon as the cracks were opened; that is to say, during the first upheaval of the mass while the occluded sea-water was passing away from it. This water must have contained much silica in solution, and the mere relief of pressure by upheaval may have been sufficient to have caused precipitation of the dissolved silica." It is only necessary to compare sections of flint side by side with some from the surrounding chalk, to show that it is a silicious presentment of the mother rock. Though the minuter particles, such as the pseudo-coccoliths, the piece sof foraminiferal tests, calcite crystals, and the like are obliterated, the foraminifera shell fragments and spicules are often distinctly outlined. (Plate XIII, Fig. I.) Thus we find that organic silica widely disseminated in finely divided calcareous debris, replacing pure detrital calcareous matter, ultimately assumes the minutely crypto-crystalline condi tion which we recognise as flint. In the case of the Chert the conditions were altogether different. It will be within the memory of many here to-night, that during our visit to Gomshall, in order to assure ourselves of the presence of the Hythe Beds, acid was used, for the overlying Bargate Beds, so like them in outward appearance, betrayed them selves by the presence of calcite; in the Bythe Beds there was none. The Hythe Beds from which our Chert was taken are in fact a fine sand curiously free from calcareous debris or subse quent calcitic infiltration, quite the antithesis of the Chalk. The area occupied by the Chert must have at one time been packed with spicules, was at one time probably a felted mass of them; from the appearance of the section they must have been so abundant in certain parts as to have almost excluded the sand and mud of the original deposit. Here, then, there is no question of. the replacement of detrital carbonate of lime by silica derived from disseminated silicious organisms. In the area occupied by the Chert there was an abundance of organic silica on the spot. and hough some may have been carried away it was probably replaced
* H Relative Age of Flints." Geol, Mag; dec. iii., vol. X, p. 31]. FLINT AND CHERT. 61) and perhaps augmented from the silica derived from the more isolated spicules in the immediate neighbourhood. Here in a non-calcareous deposit in the presence of an ample supply the silica has finally crystallised in the form of radiately fibrous chalcedony. (Plate XIII, Fig. 2.)
3. VARIETIES OF FLINT FROM FORMATIONS OTHER THAN THE CHALK. Silicious concretions which can be compared with chalk flint occur in the Carboniferous Series at Bakewell, Derbyshire, and at Richmond, Leyburn, Keld, and Harrogate, Yorkshire, and specimens from the white limestone in the Portlands from Chilmark and Wockley might also be included. In all these the silica may be regarded as crypto-crystalline. I am indebted to several friends for many examples of the silicious rocks from the Yoredale Series, and my best thanks are due to Dr. G. J. Hinde, Mr. Rhodes, and also to Mr. J. Wag staff, of Reeth, Yorkshire, who at my request sent me a large number of specimens of Chert. Of these only one, from Bakewell, Derbyshire, had any of the mother rock attached. This was a dense brownish-grey lime stone which, when seen under the microscope as a thin section, was in many respects similar to some of the more impure varieties of the Lower Chalk. It was evidently once a calcareous ooze, now finely granular calcite, but the crystals are coarser in some places than in others, and therefore the rock more transparent. It contains a few foraminifera, many fragments, probably of shells, and small pieces of an Encrinite, but the structure is nearly obliterated in the general crystallization of the rock. There are a few spicules, recognisable chiefly when cut in oblique or trans verse section, showing the canal; their silica is replaced by calcite. The silica of the concretion attached to this specimen is in a crypto-crystalline condition, closely resembling that of our example of flint. Traces of the spicules, shell fragments and foraminifera, though nearly obliterated, are shown here and there by slightly coarser and more refractive crystals, and there is a slight variation of the grain following appearances presented in the limestone. The material is not perfectly silicified, a little amorphous cal careous matter remains, and there is a distinct though very slight reaction with acid. Two other specimens, one from Leyburn and the other from the Undersett Beds of Keld, may be described as flint, for their silica is crypto-crystalline, They contain a few small, thin spicular shafts and much inorganic as well as calcareous matter. They appear to have been formed in a fine, impure, calcareous deposit, free from coarse inorganic fragments or foraminifera. WILLIAM HILL ON
Four other examples, from Richmond, Leyburn, Keld, and Harrogate, may be also mentioned as approximating closely in their nature to flint. The silica in these specimens is minutely crypto-crystalline, and they might be considered by some as examples of Hornstone. Small, thin spicular shafts are numerous in the first three, the silica of their walls is usually replaced by calcite, and there is much calcareous matter included in the silicious ground-mass. There are no large fragments of shell or foraminifera; they appear to have been formed in a fine calcareous mud, purer than the last. The specimen from Harrogate, how ever, contains a quantity of sand-grains and much fine inorganic material. Spicules of the same kind are very numerous, though their forms are easily distinguishable with crossed nicols, they have become merged in the silicious ground-mass. There is no calcite of any kind in this specimen. The silicious concretions from the Portland Beds at Chilmark and Wockley, which may be regarded as flint, are those occurring at the top of the series in a comparatively pure white limestone. This consists chiefly of amorphous matter, and there is no foraminiferal debris, pseudo-coccoliths, or the like, in this limestone, though some of the larger particles rna}' be derived from shells. Downwards, shell particles become larger, and a structure develops of semi-opaque aggregations and more or less elliptical grains. These white beds contain many silicious con cretions which, like the chalk flints; separate cleanly from the matrix, and there are also some vertical and oblique seams filled with silica. The concretions repeat the features of the rock, and, with crossed nicols, the condition of the silica is micro-crystalline Several specimens, both of the rock and the concretions, collected from this locality twenty-five years ago, but not with a view to this inquiry, show the passage from the amorphous to the pseudo oolitic structure. A specimen from the vertical layers, like those of the Chalk, also repeats the features of the rock. But here we meet with a difficulty in the determination of what is flint. I have already said that, under crossed nicols, the crypto-crystalline condition of the silica in flint is seen in the optical action of the light on multitudes of minute crystals, which, though separate, closely approximate. In different samples of flint it will be found that there is a variation in the size of the crystals. Those of our typical example are minute and rather widely separate, but it is possible to arrange a series in which they are progressively larger, more brilliant in their optical qualities, and at the same time approximate more closely to each other. This feature may well be described as greater or less coarseness of grain. Those silicious concretions from rocks other than the Chalk, which most closely resemble flint, often differ from it as a whole in having coarser grain, but in this comparative coarseness flint FLINT AND CHERT. and so-called chert may approach very closely indeed, and parts of a so-called flint may be as coarse in the grain as a so-called chert. It is indeed possible to follow this progressive coarseness of grain till one arrives at fibrous chalcedony, though some fibrous chalcedony may exhibit under crossed nicols a crystalline structure less coarse than some chalcedony in which fibres are not visible. The silica in these concretions from the Portland Beds is, as a whole, somewhat coarser in the grain than that of Chalk flint, though in some places it approaches it very closely, but as the deposit assumes the pseudo-oolitic structure the crystalline condition of the silica becomes more irregular and coarser. It will be seen that all these flint-like concretions occur in rocks in which the deposit was originally a finely-divided calcareous material, not, however, quite so pure as chalk, and that sponge spicules were intermixed with it.
4. VARIETIES OF CHERT. Concretions of silica which may be compared with our example of chert are to be found in the Selbornian of Wiltshire, Dorsetshire and the Isle of Wight, at Haldon Hill in Devonshire, the Portland Beds, and in the Yoredale Beds of the Carboniferous series in Yorkshire and Derbyshire, as well as in the Lower Greensand of our Home Counties. Chert of the Upper and Lower GreelZSand.-In the Cherts which I have examined from the Lower Greensand the radiate structure of the chalcedony is on a small scale. While the walls of the spicules are often lost in the fibrous chalcedony, the canals, which have become filled with silicious material which does not merge into the fibrous fans and aggregates, remain, so that the original character of the spicule can often be distinguished. The chalcedony lies in the comparatively small spaces between the residuary canals, between the patches of amorphous matter and the grains of sand and glauconite, where, in fact, there were spaces in which the fibrous growths could proceed with least interruption. (Plate XIII, Fig. 2.) In cherts from the Selbornian sands the fibrous aggregations and fans of water-clear chalcedony are developed on a larger scale than in the example described. Obscure traces of spicules, often hexactinellid mesh, may be seen in them, and the radiating fibres of the aggregates and fans frequently emanate from points clearly representing the position of a spicule. These broadly fibrous areas are interrupted by others in which the chalcedony graduates rapidly and confusedly from smaller aggregates and fan shaped growths to a granular condition which, with crossed nicols, shows a micro-crystalline structure. Within these latter areas 72 WI LLI AM HI LL ON are visibl e spicul es in which th e silica is in many stages of alt eration and dissolution, together with a variable quantity of min eral grains, a few foraminifera and shell fragm ents, aggrega tion s of silica either as discs or in globular form , and irregular patches of small masses of am orph ous matter permeated by silica. From all these, exce pt the sand-grains, rise fibrous growths of chalcedony which fill the int erspaces, save th ose occu pied by the micro -crystalli ne silica. (Plat e XIII, Fig. 3.) In th e H ald en chert there is even a greater development of clear fibrous chalcedo ny. It is needless to follo w the differences seen in th e structure of these cherts in detail and explai n how they have arisen. Certain broad features ac compan y them that lead one to infl'!r they are du e to those minor variations in th e character of the deposit which must inevitably occur during the accumulation of a form ation like the Greensands, and which affect the segregation and crystallisation of th e silica in their mass. Thus, in respect of the cherts, both of the Vectian and the Selborn ian, radiately fibrous chalcedony has developed where spicules accumulated. The smaller radiately fibrous fans and aggregat es occur whe re the spicules have been mixed and inter calated with the sand and finer po rtions of the deposit. That the Upper Greensand contained, besides mineral grains and fine inorgan ic matter, a considerable quantity of detrital carbonite of lime derived from parti cles of sh ell a nd some foram inifera is evident from the analysis of the rock. Though some of this lime may have been r emoved, some at least has been replaced by silica which, probably from the abundance available, has in som e cases assumed a more coarsely crystalline structure than in ord inary flint. . I t will be seen from the foregoing remarks that th e mass of these cherts is made up of man y ingred ients, viz. : (r ) silica, varying from fibrous chalcedony to the minutely crystalline ; (2) sponge spicules in various stages of alteration and dissolution ; (3) colloid silica in discs or minute globules ; (4) sand grains, to which ma y be added grains of glauco nite; (5) calcareous de bris, more or less silicifie d, and occasionally ca lcitic crystal s ; (6) fine organic and inorganic matt er. There is infinite variety in th e proportion of th ese ingredients in the cher t from th e sam e bed or q uarry. Thus, while som e specimens will be found packed with spicules, others contain but few, and th e material of th e rock, little altered, is simply perm eated with silica now in a crystalline condition . Some of the con cretions co ntain a large proporti on of globular coll oid silica, whilst others consist alm ost entirely of sa nd ceme nted togeth er by cha lcedo ny. Again, the rock may ass ume th e hard, silicious aspect of chert, yet, when examined, the evide nce of spicules will be found only in th eir emp ty casts, with , perhaps, the spicular canal rem aining. FUNT AND CHERT. 73 Chert of tlte Portland Beds.-How intimately the character of silicious concretions is associated with the rock in which they occur is well shown in those of the Portlands, and ill describing them I must refer to the rock as well as the concretions them selves. The highest layer of silicious concretions which we noticed in the Portland Beds during our excursion to Swanage was in a cherty or flinty bed about 7 ft. below the "Berna Bed." * I obtained examples of the concretions, as well as of the rock immediately overlying them. Sections of this were cut as clear as possible from large shell fragments. Under the microscope it is seen to consist of definite oval and elliptical grains lying, as a whole, close together, interspersed with fragments of shell and grains of quartz. Sponge spicules, relatively of somewhat large size compared with those of the Greensands, are abundant throughout the rock; they are not massed, but sometimes two or three may be in close proximity. There is a general permeation of the whole rock with crystalline calcite, which completely fills all interspaces, accumulating in some places in sufficiently large crystals to show the cleavage planes, sometimes surrounding and isolating individual grains. It passes, however, to granular calcite where the grains lie very close together. There are also areas in which the whole space between the grains is filled with amorphous material, and this may extend irregularly between them, taking the place of the crystalline calcite. This amorphous material is impurely calcareous, but the lime in it appears to have crystallised into minutely granular calcite, though the actual structure is somewhat obscure. The oval grains are not truly oolitic; they have not definite centres surrounded by calcitic layers, but appear to be rolled fragments of a previously con solidated but somewhat impure calcareous deposit. Though the calcareous matter has passed into finely granular crystalline calcite, it is possible to make out foraminifera, shell fragments, and obscure traces of some original structure in many of them. The periphery of the grain is usually the denser part, the centre being often clearer. In some the centres are clear granular calcite, but such grains are often obviously some large calcareous particle, such as a shell-fragment around which the matrix of the original deposit still adheres. One or two contain sand-grains. The rock is very calcareous, and breaks down entirely in a 20 per cent. solution of hydrochloric acid. There was a residue of 7'3 per cent.-2 per cent. of this was fine amorphous matter. and 5'3 per cent. of coarser material was separated by levigation. More than three-fourths of the coarser residue consisted of sponge spicules of one type. Their exterior was not rough and corroded like many of those in the Greensands, but most of their canals were much enlarged, some spicules being a mere shell; in others, -, See Pror, Geol, Assoc., vol. xxi (ISO;-IO), p, SID. 74 WILLIAM HILL ON however, the canal had become infilled and they were solid. The silica of the walls is now quartz. The silicious concretions obtained from the rock just de scribed were whitish-grey passing into darker grey, and to trans lucent, almost black, chert; none are black and translucent throughout, but are much seamed and marked with grey patches. In a thin section of the darker and more translucent chert all the details of the rock described are faithfully represented; sponge spicules are, however, a little more numerous than in the mother rock j nearly all the calcite is replaced by silica. The outline of the oolite grains is faintly but distinctly discernible. Those which were probably finely granular calcite are now crypto-crystalline silica. In the centres of some the chalcedonic crystals are larger and more pronounced, in others there is a closely-packed mosaic of crystalline silica (quartz ?). These were probably the grains with more or less clear centres. In those which obviously contained a particle of shell, the shell-fragment is replaced by more coarsely crystalline chalcedony than the periphery which is crypto-crystalline. The shell-fragments contained in the grains are not, however, completely silicified, some are only partly so, the silicification apparently proceeding interstitially between the prisms of the shell. Some of the grains contain minute calcite particles also. Instead of the crystalline calcite which permeated the rock there is now fibrous chalcedony, pale brownish by transmitted light, the radiating fibres standing at right angles to the surface of the grains, forming a fringe and meeting midway between them. But in the larger spaces, probably once occupied by the broader crystals of calcite, fan-like radiations of chalcedony have developed and blend with the fibres emanating from the grains; in such spaces there is nearly always a central lacuna in which the silica is clear and not fibrous. In spaces, easily distinguished, occupied by amorphous material, and others probably occupied by finely granular calcite, the silica is minutely crystalline. Where three or four spicules lie together there is always an area of fibrous chalcedony, fan-like radiations or aggregates being developed. In some cases spicules are clearly outlined, in others the energy of the crystallising chalcedony has nearly obliterated them. Isolated spicules are usually distinctly outlined and sur rounded by a fringe of fibrous chalcedony. (Plate XIII, Fig. -I.) A second section was taken from the same nodule, where there were alternations of dark and clear, with whiter and less translucent chert. The oolite grains were here not so even in size, so thickly packed, nor so evenly distributed, but there were many more sponge spicules. These were massed in irregular, disjointed, incontinuous layers alternating with oolitic grains, or else oolitic grains, amorphous matter and spicules were mixed together indiscriminately. Where massed they were feebly outlined, and FLINT AND CHERT. 75 the silica had developed in the usual fibrous fans and aggre gations, with central lacuna: of clear silica which in some places passed into a mosaic of quartz crystals. But where spicules, grains, and amorphous matter appear mixed together there is a confusion of radiately fibrous and minutely crystalline chalcedony. The oolite grains show the same features as before, but are not so perfectly silicified and contain much fine calcitic matter; there is a great deal, too, throughout the section generally, especially where oolitic grains and spicules are mixed together. Well defined crystals of calcite appear to be granulating, silica pene trating the cleavage planes and splitting them up into granules, or the crystal seems breaking into finely divided calcitic matter interpenetrated by the silica. The silica taking the place of the broad calcite crystals may be radiately fibrous or coarsely crystalline, that taking the place of the finer calcitic crystals minutely or even crypto-crystalline. A third section was taken from a whitish, evenly grained con cretion. Only a portion of the section shows oolitic structure, but the grains are not so well defined as in the former cases. The greater part of the material seems to have originally been amorphous calcareous matter crystallising as finely granular calcite, though amongst the grains are broad calcite crystals. Some of these broader crystals, as well as much granular crystal line calcite, still remain, though evidently attacked and penetrated by the silica, and there is a thick dusting of calcitic particles throughout the section. The spicules are sparsely distributed throughout the concretion, the silica of their walls being chalce donie, but there is no radiately fibrous chalcedony in the section, the whole, except that of the spicules, being minutely crystalline. Portions only of the larger shell fragments are replaced by silica, their structure remaining visible; some of the smaller ones are unaltered, but in a few the calcite is replaced by a mosaic of closely applied silicious crystals. Below this flint-bed the next series of silicious concre tions are those in the well-known" Chert Beds." From Dancing Ledge I obtained specimens from the higher and lower parts, together with examples of the adjacent rock. Downwards, at the horizon of the" Chert Beds," the rock of the Portland passes rapidly from the oolite structure described to one in which the grains are less distinctly developed. In a section from the upper part of the "Chert Beds" the deposit seems to consist of smallish semi-opaque angular and sub-angular aggregations of amorphous material; intermixed with them are a few oval grains, shell-fragments, a few spicules and a little quartz sand. These ingredients are closely compacted, the aggregations in some parts showing a tendency to blend together, their outline becoming less defined, the whole being permeated with calcite. Whether this condition is due to circumstances connected with WILLIAM HILL ON the deposition of the material, or whether it indicates the gradual development of oolitic structure, I am unable to say, but both aggregations and crystals are smaller than the oolite grains which characterise the higher Portland Beds. The structure of chert obtained from the centre of a large black specimen at this horizon is simply the reproduction of the rock above described; as most of the rock is amorphous matter the silica as a whole is micro-crystalline, a coarsely grained flint varied with patches of fibrous chalcedonic silica. There is little difference in the grain of the silica, either in that replacing the aggregations of fine matter or calcitic crystals, except perhaps some slight increase of coarseness in the case of the latter. The edge of each well-marked grain is marked by a cloud of very fine brownish matter, which, under high powers, can be resolved into minute particles. Some of these are negative to polarised light; others are opaque, so that the outline of the aggregations or crystals is quite visible when viewed with crossed nicols. A second specimen was taken of the lowest chert bed exposed and one of the rock. The structure of the rock reminds one of that from the upper part of the chert beds-there are the same angular and sub-angular semi-opaque aggregations of fine amor phous matter, though smaller, but the crystalline calcite has devoloped to a much greater extent, the crystals covering in places a considerable area and show cleavage planes. There are no sponge remains at all except the reniform spicules of Geodia, which are fairly numerous, but their silica has been replaced by calcite. In the chert we have again a representation of the rock, but there are a number of rod-like lengths of small spicules besides the reniform spicules of Geodia, which are abundant. Large areas of fibrous chalcedony showing small fans and aggregates seem to correspond to those occupied by crystalline calcite, while the pellets are minutely crystalline silica. Carboniferous Chert.-The Carboniferous Series of Yorkshire afford us examples of silicious rock which will compare with our typical example of chert as well as the flint. Though found in some localities as nodules it frequently occurs in massive beds from I ft. to 20 ft. in thickness. It is from such beds that the greater part of my specimens were obtained. They come from the upper part of the Yoredale Series above the Millstone Grit to the top of the Undersett Beds, the chief localities being Richmond and exposures between that place and Reeth, Harrogate, Leyburn, Keld, and from various other places in Swaledale and Arkengarthdale. In their external aspect Carboniferous cherts vary consider ably even in specimens from the same hed; some are dull black and earthy-looking, others equally black are more glassy and PROC. GEOL. Assoc., VOL. XXII. PLATE XIII.
x 45. FIG. I.-CHALK FLl='lT FRO~I THE TOP OF THE ZO"E Rhyllc. cutneri, NEAR HITCHIN, HERTS.
x 45. FIG. 2. -CHERT FROM THE LOWER GREENSAND, RAIKE'S LANE, NEAR GOMSHALL, SURREY.
To faa rage 76. x 45. FIG. 3.-CHERT FROM THE UPPER GREENSAND, FIDDINGTON, NEAR DEVIZES, WILTS.
x 45. FIG. 4.-CHERT FROM THE PORTLAND BEDS, ABOUT 7 FEET BELOW fHE "PERNA BEDS," WHINSPIT, NEAR SWANAGE. FLINT AND CHERT. 77 translucent; some resemble grey flint, while what is known as "Crow Chert" is pale grey with a curious opalescent appearance. Mr. J. Waastaff the manager of the Boulder Flint Co., of Reeth, most courteously sent me a series of specimens from a thick bed of chert which occurs here. It is mined for the pur pose of obtaining blocks which are used for grinding flints, the powder thus obtained being mixed with the clay in the manufac ture of certain kinds of pottery. The actual section is worth giving as showing the succession and different qualities of chert occurring in a single bed. The following particulars were kindly furnished by Mr. Wagstaff:
r. Limestone, taken within I ft. of NO.2. 2. Black Chert and Limestone I ft. 3. Grey Chert .•. I ft. 4. Grey Chert, a massive bed. 3 ft. 5. Grevish- black Chert .. 8 in. 6. Black Chert, dull and earthy-Jooking 12 ft. 7. Limestone;
The overlying rock No. I consists of shell fragments, broken encrinites and foraminifera, cemented together into a coarse limestone by crystalline calcite. There are few sponge spicules. No.2 seems to be a passage bed, not however the passage of a limestone to a silicified representative of the same material, but from a deposit consisting of the remains of calcareous organisms to one in which the organic remains are chiefly silicious. The chert consists of large sponge spicules, with which are intermixed a few fragments of shells and encrinites similar to those of the limestone j filling the interspaces is much fine brownish opaque matter, the whole being permeated with crystalline chalce dony. The spicules for the most part retain the outline of their walls and canals, their silica being crystalline. Many are, however, replaced by calcite. In one part of the micro-section there are a number of minute globular bodies, which, from their aspect and arrangement, leaves no doubt that they were once globular silica. Several spicules in the same area show a partial conversion of their walls to silica in this form, while their canals are in several instances filled with it. But the silica of the spicules in this condition is now replaced by calcite, and the globules infilled with this material The application of acid to a portion of the section destroyed a large number, though some which appear to be protected by the enveloping chalcedony remain. The ground-mass of No. 3 is clear, granular and fibrous chalcedony so intimately mixed that few definite radiately fibrous fans or aggregates can be made out. It was evidently once a felted mass of small-sized spicules; little remains but their spicular canals, though here and there faint outlines of the original form can be seen. The section is traversed by denser streaks, which seem to indicate the admixture of fine material now PROC. GEOL. Assoc., VOL. XXII, PART 2, I9I1.] 7 WILLIAM HILL ON silicified. These and the long axes of the spicules lie in the same plane, facts which suggest current action. NO.4 is very similar chert packed with spicules. The greater number are represented by spicular canals only. The silica which has filled the canals has a brownish coloration, some are hollow; all these are more or less opaque, or only semi-transparent by transmitted light, and are therefore clearly defined, but by direct light the first are opalescent, the latter white and solid-looking. The ground-mass of the silica is in the same condition as NO.3, but has a .brownish tinge and is not so clear. NO.5. There are far fewer spicules in this chert and a corresponding increase of fine opaque matter, brownish in colour. The silica as a whole is minutely crystalline, but coarser than in flint. In the section there is a large patch, filled partly by chalcedony and partly by quartz. Around its edges are large fans and aggregates of brownish coloured chalcedony, which passes insensibly into the water-clear quartz which occupies the centre. No.6. In the specimens of this bed sent me, spicules are rare. There is much fine opaque matter, thoroughly permeated by silica in a minutely crystalline condition. A few particles of shells occur in it. Underlying, and sometimes overlying, the chert beds of this district is a fine laminated shale known as .. plate," from 4 ft. to 10 ft. in thickness. Bed 6 appears to me to be very like" plate" permeated with silica. The greater number of my specimens of chert from the Yoredale Series agree with the general character of Bed 4. Viewed as thin sections under the microscope they present to the eye a tangled mass of sponge spicules embedded in crystalline chalcedony. In a long series of specimens there are, of course, differences in detail; in some the spicules are small and densely packed-in such cases the crystalline chalcedony is of fine grain. and fibrous fans and aggregates are not visible-but where the spicules are large and interstices occur between them, radiately fibrous chalcedony has developed, but in no case have I met with large areas of water-clear chalcedony such as occur in the chert of the Upper Greensand. (Plate XIV, Fig. 1.) In these cherts there is no replacement of a mother-rock; they must represent a large area of vigorous sponge growth enduring for a considerable period, and whatever may have been the deposit accumulating elsewhere in the Carboniferous sea, little else but fine amorphous matter, drifted probably by current action, found its way into this forest of silicious sponges. Carboniferous chert is not always so completely silicious. A series of specimens obtained from various localities, but which all come from the neighbourhood of Richmond, contain much calcite. The original deposit was probably a fine calcareous ooze in which spicules were very abundant. A few shell fragments FLINT AND CHERT. 79 occur but no broken ecrinites. The calcareous matter is now crystalline calcite. Silica, in the form of clear chalcedony, usually coarsely crystalline and sometimes fibrous, permeates the mass j though predominating it does not entirely replace the calcite, and the two minerals are often intimately intermingled. The silicious walls of the spicules are in nearly every case replaced by calcite. Specimens of chert from a mine called "Good Intent," between Richmond and Reeth, now unworked, are of quite a different character. The limestone above and below the chert is like that already described, full of organic fragments and foraminifera j the cementing calcite is coarsely granular, extending here and there in broad calcitic crystals. (Plate XIV, Fig. 2.) In the chert the character of the silica corresponds to that of the calcite-it is chiefly fibrous in small aggregates-but unlike that of other- somewhat similar rocks fans rarely fringe the edges of the included fragments, the aggregates seem to arise from indepen dent centres of growth. Most of the larger inorganic fragments bave resisted the invasion of the silica, only the cementing calcite is replaced j traces of it yet remain, and some stilI fringes many of the pieces of shell or encrinites. There are two thickly packed layers of spicules, nearly all of which must have their axes in the same plane, as they are cut transversely and appear only as circles. From the denser parts of these layers the larger calcareous organic fragments are excluded, only the smaller pieces being intermixed with the spicules. The general permeation of the limestone by the silica gradually passes away downwards, the rock gradually becoming more and more calcareous, and the silica at the same time crypto crystalline. Many of these concretions contain rhombohedral crystals of a carbonate of lime, probably dolomite. (Plate 'XIV, Fig. 3 ) They vary greatly in size. The concretions which occur in the Reef Knoll limestone of the Carboniferous Series near Cracoe, Yorkshire, accumulated under different conditions. They are hardly nodules, but rather an irregular impregnation of the rock by silica. The limestone in which they were formed is made up of coarse shell-fragments, encrinite stems, and other undetermined calcareous organisms, with a few foraminifera. The whole is cemented together by broadly crystalline calcite, but between the fragments there was probably a certain amount of fine calcareous matter, for the broader crystals give place here and there to granular calcite, and granular calcite occurs in the interstices of the organic fragments. In the concretion many of the larger fragments are unaltered, some are partially, others are entirely, replaced by silica. The silica, which shows a brownish coloration by transmitted light, 80 WILLIAM HILL ON
probably replacing that which seems to have been broadly crystalline calcite, is in large fans and aggregates, the fibres as usual emanating from the outer edges of the organic particles, but in places no fibres are visible. There are some large spicules but as a whole their outline has merged in the fibrous chalcedony, and they are difficult to distinguish. A curious elongate nodule was found in the shelly beds of the Millstone Grit when the Association visited Klint, near Harrogate, last August. This has proved to be a spicular concretion. It consists chiefly of spicules, with shell-fragments and pieces of encrinite mixed promiscuously together and embedded in amor phous material probably partly calcareous and partly inorganic. Some of the shell-fragments are still calcite, others are partly or wholly silicified. The spicules, most of which are cut trans versely in the section, are all chalcedonic silica; their walls and canals still remain visible. The cementing material is partly fibrous chalcedonic and partly coarsely granular silica.
Chalcedony in Flint.-In the so-called chert wehave examined, fibrous chalcedony has always occurred in close association with sponge spicules. To everyone who has examined flints from the Chalk it must be evident that sponges flourished in the Chalk sea. Are aggregations of spicules never found in the Chalk with the resultant fibrous silica, were all the spicules disseminated through the chalky ooze, and have all flints been formed by the silica derived therefrom? In some sponges the spicules are separated bodies embedded in the soft parts of the organism, and on its death would lose coherence and become spread over the sea-floor. But others when growing become packed with spicules shed by themselves and their neighbours, and some have a rigid skeleton, the form of which we know has been preserved j both these would become covered with calcareous ooze which might penetrate into their interior in an irregular manner. There are many flints whose exterior shape suggests that of a sponge, but there are many others whose shape carries with it no such suggestion, yet contain much fibrous chalcedony, though it" may be encrusted or sur rounded by flint. It must be remembered that the water of the Chalk sea was comparatively quiet. Though there may be evidence of current action in the Totternhoe Stone, the Melbourn Rock, and in the marly bands which occur at various horizons, flints certainly occur where there is least evidence of it, and whatever movement there may have been in the water it did not sift out the spicules from the fine mud in layers or aggregations as in the Greensands. The sponges with rigid skeletons, or those which may have been packed with spicules, were buried where they grew with little disturbance of their original form. FLINT AND CHERT. 8r
If thin slices of flint from the Chalk are examined it will be found that they sometimes contain much fibrous chalcedony, which often takes the form of an indefinite pattern, and which may be referred to as sponge-structure; sometimes it takes a form suggesting branching passages, or there may be no form at all. This fibrous chalcedony certainly represents, in many cases, the position of the sponge mesh of one of the Hexactinellid sponges and possibly of the sponge body itself. Frequently all traces of the spicules is nearly lost, they may be vaguely seen, .sometimes the mesh is preserved. This is no mere change of the organic silica of the mesh to chalcedony; it does not always represent the area occupied by the original spicules, it is larger. Intervals in the chalcedony are filled in with flint in which the general character of the surrounding deposit may be traced, and the space occupied by the flint and chalcedony could not have been a hollow. In these cases the silica seems to me to have followed its usual course, and in the presence of much organic silica on the spot has crystallised as fibrous chalcedony within the area that the extra amount of organic silica could make itself felt, the energy of the crystallising chalcedony being sufficient to obliterate nearly all traces of spicules, and possibly some of the adjacent detrital carbonate of lime, but it was not enough always to obliterate the outlines of a shell fragment, e.e., an Inoceramus prism, which may remain visible. But the fibrous chalcedony occurring in flints is not always in direct association with that which rna}' be taken as the skeleton of a rigid silicious sponge. The spicules which may be found in most of the ordinary sections of black flint are usually small, thin shafts, often residuary canals only; the cladome of the Tetractinellid spicule is not often seen. They appear to have been mixed with the calcareous mud, but occasionally may be found aggregated together, though in small numbers. Here, as usual, the silica is not minutely crystalline but fibrous, and there often seems a relation between the number of spicules present and the coarseness in the grain of the flint. Sometimes spicules of large size occur, similar to those common in flint meal; these are always fibrous chalcedonic silica, the fibres sometimes seem to extend slightly beyond the area actually occupied by the spicule. The occurrence of fibrous silica, then, in flint can hardly be said to be the result of the accidental inclusion of these forms, but is due in fact to the conditions under which the Chalk and its silicious contents accumulated. That chalcedony and quartz fill up hollows in flints is of course well known, but I prefer to regard these hollows as an accident in the formation of a concretion, rather than part of those changes which have brought about the birth of the nodule. Are the silicious nodules of the Chalk which contain much WILLIAM HILL ON fibrous chalcedonic silica-and some contain a great deal-to be called flints or cherts?
Chert of the Culm Measures.-The Chert Beds of the Culm Measures, as they are seen in the Codden Hill Quarry near Barnstaple, are described by Messrs. Hinde and Fox" as con sisting mainly of hard, dark or light grey, sometimes banded cherty silicious rocks with intermediate beds of soft grey or white silicious shale, a description which seems to he applicable to most exposures of this part of the series in Devonshire and Cornwall. The most silicious portion of these Radiolarian rocks "occurs in evenly stratified beds usually from 2 to 4 inches in thickness, though they range from I to 9 inches, and in very exceptional instances to a fool in thickness.t The microscopic characters of the silicious rock are thus described d They "show under the microscope a general resemblance in consisting of a silicious ground-mass, in some cases clear and transparent, in others dark and turbid from the presence of fine particles of carbonaceous or ferreous materials, and minute crystal-needles of rutile and zircon. The silicious ground-mass, in polarised light, between crossed nicols, usually exhibits the faint speckled aspect of cryptocrystalline silica similar to that of a section of flint from the Chalk; sometimes it is almost entirely dark, in other instances certain bands of the rock in which the radiolaria are very crowded show the lively tints of chalcedony. The radiolaria in the rock generally have been infilled with clear, nearly transparent silica, free to a great extent from the rutile crystals and the dark substances disseminated in the ground mass. . Within the radiolarian casts the silica has not infrequently a radiate fibrous arrangement." The specimens of the silicious rock of the Culm Measures which I have examined come from Drewsteignton, Mullion Island, and Ilsington. The first two, when seen by transmitted light, show a silicious ground-mass somewhat turbid by the admixture of fine opaque matter. The similarity as a whole between it and that of concretions from the radiolarian rocks of Barbados is striking. Though particles of carbonaceous matter, together with minute crystal needles of rutile and zircon, as well as flakelets of mica, are present, this ground-mass seems to me largely made up of minute particles, which, though altered by time and circumstances, compare favourably with such as can be seen in the concretions from Barbados. The turbidity of the Devon specimens seems to me to be due to some admixture of fine inorganic matter-in fact, mud-from which many of the silicious rocks of Barbados are comparatively free. The radiolaria in these specimens are in the
* "Radiolarian Rocks in the Lower Culm Measures," by Dr. G. J. Hinde and H. Fox. Quart. [our», Geol, Soc., vol. Ii (1895), p. 6'7. t iu«, p. 6'7. t tu«, pp. 630-631. FLINT AND CHERT. condition described, some filled with clear silica, and in others it is radiately fibrous, but in the former case especially they often contain what I consider to be particles of the original deposit, and in this particular they also compare with the organisms contained in the rocks of Barbados. A specimen of radiolarian rock from the Arenig Beds, Scotland, also compares with the silicious rock of Barbados, it is a purer silicious deposit than that of Devon or Cornwall, but it is in all other respects very similar. But while the silica in the rocks of Devon and Cornwall is minutely crystalline, that in the Barbados concretions is only incipiently so. Two other specimens from the Culm Measures deserve attention; both are permeated with silica in a minutely crystalline condition, so that they are now beds of so-called chert. They are from a cutting in a new road at Ilsington. One seems to have been a laminated shale of greyish colour, which may have been a fine inorganic mud, containing much detrital quartz in grains of small size; the other is a black shale, almost opaque, containing no sand-grains. The former contains two or three Radiolaria, the latter none. Though the authors of the paper referred to say that sponge spicules do not occur in some localities in the Culm, it is a fair inference to say that a large part of the silica in these rocks was derived from the Radiolaria, for in the softer beds their silica has disappeared and they are sometimes represented merely by empty casts. A point worth noting is that where the Radiolaria are crowded the silicious rock shows the lively tints of chalcedony. Here again it would seem that in the presence of much organic silica a well-marked form of chalcedony is produced.
5.-IMMATURE FLINT AND CHERT. In the Chalk, the Greensands, and in the Portlandian, there occur concretions in which the conversion of the organic colloid into crystalline silica and the general silicification of the rock seem to have been arrested. I regard these concretions as im mature forms of flints and cherts. In such concretions the original ingredients of the rock are only partly silicified, but are permeated with silica apparently in the transitional state between the colloid and the crystalline condition. These concretions usually contain many sponge spicules, the walls of which may still be in the amorphous colloid state, while others are of crystalline chalcedony. In the first case the general outline of the spicule, with its canal, is preserved; in the second the outline is generally rough and corroded, and frequently there remains but a cast of the canal which has become infilled with silica of more stable nature. WILLIAM HILL ON
The most striking feature of them all is the large proportion of soluble silica in the form of discs or globules w hich they in variably contain, and which, like the spicules, exhibits many interesting phases in its gradual conversion to crystalline chal cedony. The cementing silica which permeates the whole concretion, filling small interspaces, the cells of foraminifera or the like, is only incipiently crystalline and shows but faint reaction with crossed nicols. (Plate XIV, Fig. 4.) Soluble silica in the torrn of discs or glubules has already been well described by Dr. G. J. Hinde in his paper on the Sponge Remains of the Upper and Lower Greensands, but its occurrence in other formations in conjunction with sponge spicules, and its connection with the formation of flint and chert, do not seem to be generally recognised. In 1889, in a communication to the Geological Society, Mr. Jukes-Browne and myself" drew attention to the occurrence of silicious concretions in the Lower Chalk of Wiltshire. These are hard bluish-grey nodules with an earthy fracture quite different from that of flint. Examination of thin slices shows that the material of the nodule is the same as the surrounding Chalk, from which it differs only in being permeated with silica, largely in an amorphous globular or incipiently crystalline condition. Since 1889 my attention has been called to concretions occurring in the Lower Chalk to the south-westward, in Dorset shire and Devonshire, from Axminster, Chard, Chardstock, Crewkerne, and Warren Hill. These nodules undoubtedly show a progress from the immature concretions of the Wiltshire Chalk to more perfectly developed flint. In their exterior aspect they differ little from the ordinary flint nodule; they have the same form, and separate easily from the surrounding matrix, which is a purer chalk than that at the same horizons further to the eastward. They have usually a thick rind, and sometimes there is only a small central core of black flint, the boundary of which is rugged and not clearly defined. Viewed in thin sections by transmitted light, the silicious ground-mass is seen to be crypto-crystalline, sometimes only incipiently crystalline when the nicols are crossed. Traces of sponge spicules are numerous, but the material, especially near the rind, is crowded with minute opaque spherical bodies, which become more separated and ultimately disappear in the clearer parts of the flint. By direct light these bodies appear white, many are certainly hollow, but the majority are either filled with white granules or crystals, or there is an internal coating of minute particles within the sphere which gives them an appearance of solidity. They would hardly be connected at first sight with the clear globules or discs described by Dr. Hinde. But any doubt on this point is set at rest, for in portions of these nodules the • Quart, fourn, Geol, Soc., vol. xlv (,889), p. 403. FLINT AND CHERT.
silica is in precisely the same condition as those of the immature concretions of the Wiltshire Chalk; the passage of the clear globules to the opaque stage and their ultimate disappearance in the clear flint can be followed. Concretions of a very similar nature can be found in the Upper Greensand; the globules or discs are, however, larger as a whole than in the flints just described. I would refer those who are interested in the changes to be observed in the condition of globular silica to Vol. I of the" Survey Memoir dn the Cretaceous Rocks of Britain," pages 359 et seq. From Dancing Ledge, in the Isle of Purbeck, I obtained other examples of these immature concretions from the Chert Beds in the Portlands, In these the cementing silica is only incipiently crystalline. The globules of silica are of large size; most of them are opaque, others are clear, and, as before, the increasing opacity and ultimate disappearance can be followed. Though I have not found similar concretions in the examples of Carboniferous Chert sent me, it is interesting to note that the globular silica occurs, and I have little doubt that if the speci mens were selected much the same phenomena would be observed. Concretions in which a large proportion of the silica is in the condition of opal, readily soluble in caustic alkali, may be called opalite,
6.-GENERAL SUMMARY. It will be seen that in all the silicious concretions we have now reviewed there is in each case a correspondence between them and the rocks in which they occur; they are in fact silicious presentations of the rock in which they were formed. From their association with the remains of silicious organisms, one can hardly avoid the conclusion that the silica was derived from organic sources. There can be no doubt as to the solution of these organisms, and in many instances there seems to be a direct connexion between the character of the crystalline silica and the quantity of organic silica which was once immediately available. Both in spiculiferous and radiolarian deposits, where there is evidence of an excess of organic silica, the crystalline silica of the flint or chert is invariably of coarse grain or of radiately fibrous structure. But where the silica of the organisms has been dissolved and transferred from some other stratum to that in which it is now found, its final crystalline condition is governed by the nature of the rock in which it was redeposited. Thus, in the Chalk, one of the most finely divided as well as one of the purest deposits of detrital carbonate of lime known, the silica of flint presents in its corresponding purity and minutely crystalline condition the character of the mother rock which it replaces. 86 WILLIAM HILL ON
Though in theory we may account for the solution of the silica of silicious organisms and its distribution through a cal careous ooze, it is still to be explained why it should have formed in nodules arranged in more Or less orderly lines as well as in cracks and jointings, Professor SolIas, in his paper on the nodules of the Trimmingham Chalk, has ably discussed the probl em of the formation of flints. With regard to the solution of spicules he says, though possibly sea-water under pressure may have been a sufficiently powerful solvent, Alex. A. JuJien suggests that the albuminoid or glairy matters and acids akin to the Azohumic of Thenard, produced during the submarine decomposition of organic matter, may have been the agents which have accom plished this solution." In his remarks on the formation of flint around the partially preserved forms of sponges, he also discusses at some length the effect of decaying animal matter, the sarcode or protoplasm of the sponge body, on the organic silica of the skeleton and that contained in the ooze, and again quotes A. A. Julien,t who says, "I would therefore modify SolIas's theory by suggesting that during the decomposition of the sarcode of both animal and vegetable organ isms, after death, gelatinous or colloid substan ces are generated resembling glairine, which are soluble in sea-water, which combine with silica, and may therefore convey and concentrate it, dissolving its part icles disseminated throu gh marine sediments, and which may, in certain forms produced by oxida tion, act also as solvents of lime." Mud that I have seen brought up from the sea-botto m where spicules abound app eared to me full of glairy matter, apparently the con comitant of sponge growth and decay, and it seemed possible that in such material a combination of decaying animal matter and silica could occur, form ing a slimy or gelatinous compound that would become incorporated with the ooze. Though spicules in the mud were unaltered, Mr. Carter has recorded that he found some showing signs of incipient solution in com paratively shallow water. But what happens when the glairy matter and sponge spicules have become a deposit of considerable thickness? Nothing which suggests the beginn ing of a silicious concretion has been found on the sea-floor of the present day, but our knowledge is only superficial, extending downwards but a foot or so beneath the surface. If, however, the embryo flint originated on the sea floor as a combination of silica and mud rich in decaying protoplasm, its first condition would be little better than a jelly, which would certainly be lost in the mass of material brought up
• " On Flint Nod ules of the Trlmll1lngh am Chalk: ' Ann. & Milg. Hilt. Hist., .BBo, P·4«. t Ibid., P.457. FLINT AND CHERT. by a modern dredge. But bury such a mass in accumulating ooze, as oxidization progressed the finer calcareous matter would become gradually eliminated, and one can follow Sollas in his brief summing up :-" Replacement of the calcerous material of the ooze then ensued, small shells and many large ones too being converted into silex, and silicious chalk, not flint, was the result.?" The time occupied in the solution of the organic silica is still a matter of conjecture; some spicules were probably more quickly affected than others. Mr. Jukes-Brownet believes "that they (the silicious organisms) were evidently dissolved in the sea-water before the ooze was raised and compacted into anything like chalk" ; on the other hand Prof. Sollasj remarks "that the chalk must have acquired a considerable degree of consistency before it could preserve in such a remarkable state of perfection these perishable cavities (i.e., the casts of spicules)." But such consist ency might have been acquired in the deeper layers before upheaval and desiccation. We may now imagine a partly consolidated calcareous ooze saturated with silica in solution, perhaps also containing some in a colloid state, the result of its combination with decaying animal matter. For reasons yet unexplained some of the silica had become redeposited at definite centres along certain horizons, slowly eliminating the fine calcareous matter by a process of replacement. The next phase was the upheaval of the Chalk and the draining away of the occluded sea-water, a process which involved some circulation of that containing silica in solution. The relief of pressure or contact with carbonated rain-water may have led to the precipitation of the dissolved silica (see ante, page 68), and may have led to the last stage in the formation of flint. To continue the quotation from the paper of Prof. Sollas: "The chambers of the foraminifera and the inter stices of the chalk were now filled up by a simple deposition of silica, and the silicious chalk becomes converted into black flint." As flints are now, so they have been from a time not long after the elevation of the Chalk. Flints crushed in the fault at Ballard Cliff were already formed, and no silica has since been deposited around the splintery fragments. I think, however, that a secondary deposit of silica has occured in some cases, more noticeable in some localities than in others. This may be seen in a dense white layer, sometimes coloured pinkish, which occurs on the outside of the black flint just beneath the crust. This view has, indeed, been anticipated by M. Cayeux,§ who described at length a secondary deposition of silica on flints of the zone of Inoceramus labiatus in the Vallee du Cher, in the Paris Basin. In the case of the Vectian Sands the deposit was chiefly sandy * ..Flint Nodules ofthe Trlmmingham Chalk," Ann. <5- Mag. Nat. Hist., IS8o. t Disseminated Silica In Chalk considered, Geo], Mag., Dec. 3, vol. x, p, 54'. : "Age ot the Earth," p, '53. § " L'Etude Mlcrographlque des Terrains Sedimentalres,' p. 362. 88 WILLIAM HILL ON
inorganic material mingled with innumerable sponge spicules, there was little or nothing to be replaced, and the concretions seem to have been formed by the impregnation and free precipi tation of silica in the interstices of the rock. The Selbornian Sands contain a certain proportion of calcareous material in the form of shell fragments, foraminifera, and amorphous matter, which has influenced the character of the concretionary silica to some extent. During the deposition of both these formations sponge growth must have been vigorous, and spicules, generally distributed through their mass, have frequently been sorted out by current action and arranged in more or less defined layers, which, though mixed with some sand and fine inorganic matter, have given rise to definite bands rather than to nodules of silicious or siliceo calcareous rock. Silica in globular colloid form occurs in these deposits, and is often an important constituent of the concretionary silica found in them. In discussing this form of silica which occurs in the Lower Chalk of Wiltshire, Mr. Jukes-Browne and I expressed the opinion "that the globular silica was precipitated from solution before the consolidation of the beds, and while they were still permeated by sea-water," and" that it is very difficult to understand the precipi tation of any of this dissolved silica without the intervention of organic agencies," and, further, that "it seemed clear to us that the precipitation of the chalcedonic silica of the cherty nodules was a secondary and subsequent operation." * These remarks have been criticised by M. Cayeux, who thinks that both the globular and crystalline silica were formed at the same time. He does not seem, however, to realise the action of decomposing animal matter on the soluble silica of the spicules, the effects of which must have taken place during the existence of such matter, and while the mud was yet soft, and before the silica could crystallise. Moreover, M. Cayeux is referring to well formed chert, while our remarks are confined to those nodules which I regard as immature concretions in which the chalcedony is only incipiently crystalline. With a vigorous growth of sponges, decaying protoplasmic matter must have been abundant, and we may imagine that the solution of the spicules was effected in much the same way as in the chalk. It is conceivable that some of this protoplasmic matter became enclosed and incorporated with masses of spicules, and by combining with dissolved silica may have produced a material sufficiently solid to have excluded detrital matter. There is even ground to support this suggestion: in sections of chert are areas of clear chalcedony in which neither sand-grains nor fine in organic matter occurs, though both may be abundant near by. • .. Colloid Silica in the Lower Chalk of Berks. and Wilts." Quart, [ourn. Geol, Soc., vol. xlv (,889), p. 4'9. FLINT AND CHERT.
Such areas can hardly have been hollows, for traces of spicules in such areas occur (often Hexa ctinellid mesh) but are not closely packed, and it seems possible that some fairly solid substance may have filled the space. This may nave been a colloid similar in nature, if it did not actually take a globular form. The globules part with their soluble silicious cont ents easily, and, like spicules, are frequently seen to he hollow. They afterwards become infilled with crystalline chalcedony and become lost in the jnass of it. This leads me to remark in passing that while the minuter forms of spicules, the microscleres, disappear, I have some doubt whether the larger spicules (megasc1eres) are often so utterly obliterated in silicious concretions that no trace whatever remains. Even in the coarsest radiately fibrous chalcedony of spiculiferous rock traces of them can be seen. In thin slices of flint spicules may seem absent when viewed by transmitted light, but by polarised light their form is distinctly shown by the larger and more brilliantly refractive crystals. These may be residuary canals only, or spicular casts, again infilled by crystalline silica. Sometimes they are most delicately outlined, and some times, both in flint and chert, there only remains an irregular line of minute spheres (usually opaque), once globular silica which infilied the canal. In one way or another I think the presence of spicules is nearly always manifest. It will be conceded that the greater part, if not. the whole. of the silica of the cherts of the Vectian and Selborni an depo sits was derived from sponge spicules. There was here a great er concentration of organi c silica than in the Chalk , the deposit itself was more porous, enabling an easier interchange of silicious matter. Under these conditions the silica, thoroughly permeating the rock, has crystallised in coarse-grained or radiately fibrous chalcedon y. Again, the final stage of crystallisation is conjectural: probably it began belore the raising of the Greensands into- dry land, but having regard to the nature of the rock and the solubility of a portion of its cont ent s, it is possible that the interchange of the silicious material continues to the present day. Nowhere is th e replacement of the leading features of a calcareous rock more strikingly manifest than in the concretions from the Portland Beds. Here, as usual, the aggregation s of spicules are associated with fi brous chalcedony, while the silica in its crystalline conditions follows closely the different forms of calcite which it replaces . The oolitic grains where they occur, formed of amorphous calcareous material. a chalk in fact, though not so pure, are, like our chalk flint, replaced by minut ely crystal line silica, the broad calcitic crystals make way for radiately fibrous chalcedony, and the finer port ions of the limestone are replaced by silica of a corresponding crystalline character. Sponge spicules, as before, are probably the chief source of the WILLIAM HILL ON
silica, but the sequence of events which led to the formation of these concretions is difficult to follow. To be logical one must infer that the solution of their silica was brought about by the same chemical processes as those in the Chalk and Greensands, viz., the action of sea-water under pressure, or of decaying animal matter. But their action affected the spicules in a different way, for in all the specimens examined, either of the limestone or of chert, there is little evidence of corrosion of their exterior surface, but their canals are greatly enlarged. In the case of the Portland Flint Bed 'the limestone containing the concretions could hardly have been a soft ooze, for it is made up of grains which had attained a considerable degree of hardness. In my opinion these have not been developed in the rock after its deposition as a soft mud, though it is possible they may have been formed on the sea-floor by the crystallisation of calcite and subjected to a slight rolling by the action of a current. Fragments of shell and indications of other calcareous organisms to be seen in them appear to have acted as a nucleus, but I have been unable to detect a single fragment of a spicule occupying such a position. Spicules are not as a rule massed or sorted, they are fairly distributed through the material of the rock, though they are rather more abundant in the concretions. The original deposit of these beds seems to have been made up of hardened grains of calcareous matter, a few shell fragments and other remains of calcareous organisms with which spicules are intermixed. Though the interstices between these larger fragments were in some places filled with calcareous mud, the rock as a whole must have been porous, a condition favourable to the rapid deposition of crystalline calcite, which in the limestone has filled the inters paces. In the limestone, as well as in the area occupied by the chert, there must have been water carrying silica in solution, and we are met again by that seemingly inexpl icable problem : why it segregated in concretions at this particular horizon. The theory of the possible combination of silica with the glairy animal matter and the gradual elimination of the amorphous material of the ooze seems, as a whole, hardly applicable to this case, for one of the early changes appears to have been the crystallisation of the calcite ill the grains and in the interstices between them and the coarser ingredients of the deposit. Though there may have been a redeposition of silica around small congeries of spicules which served as nuclei and a replacement of the finest part of the material as in the chalk, possibly coincident with the formation of crystalline calcite, the evidence in the sections suggests that the replacement of the latter was a secondary operation brought about by the slow circulation of water charged with silica within the rock. The fact that the silica was taken from the canals and not from the PROC. GEOL. Assoc., VOL. XXII. PLATE XIV.
x 45. FIG I.-CHERT FROM THE CARBONIFEROUS LIMESTONE (YOREDALE SERIES), THORPE EDGE, NEAR REETH, YORKSHIRE, THE "MAIN CHERT."
x 45. FIG. 2.-CHERT FROM THE CARBONIFEROUS LIMESTONE (YOREDALE SERIES), NEAR REETH, YOl To face page go. x 45. FIG. 3.-CHERT FROM THE CARBONIFEROFS (YOREDALE SERIES),. REETH, YORKSHIRE. CONTAINING MANY RHOMBOHEDRAL CRYSTALS· OF A CARBONATE OF LIME-PROBABLY DOLOMITE. x 75. FIG. 4.-SILICEOUS CHALK, CONTAINING MUCH GLOBULAR COLLOID' SILICA, ESTCOTT, WILTS (IMMATURE FLINT). FLINT AND CHERT. exterior of the spicules supports this hypothesis, for it is through them that water would find an easy passage. Moreover, crystalline calcite offers more resistance to the invasion of silica than amorphous matter; in the larger calcareous organic fragments the resistance is stilI greater. In few of these concretions is the rock perfectly silicified, in fact none of my specimens show com plete silicification. Both in the Portlands and in the chert of the Carboniferous Series broadly crystalline calcite seems to be replaced by fibrous chalcedony. The above remarks may also be applied with greater or less force to the concretions of the Chert Beds, but in view of the length to which this address is extending, I feel I must curtail my remarks on this very interesting subject. But there is another feature which I must notice, for it applies not only to certain concretions from the Portland Beds, but also to the silicious rock of the Carboniferous and Culm Measures. In the Chalky Portland limestone of Chilmark, in the lower r2 ft. of the silicious rock at Reeth, and in some of the silicious beds in the radiolarian rocks of the Culm Measures, there is little evidence that silicious organisms ever existed in sufficient quantity to account for the amount of the concretionary silica. In the case of the concretions of the white limestone at Chilmark the question of " where did the silica come from?" is a difficult one to answer. It is there, and in its segregation as nodules it appears to have behaved in the same way as that whose origin we have little reason to doubt. It has been said that where concretionary silica is found in sedimentary rock, its origin can usually be found in silicious organisms not far away, but in this case I can see no sufficient traces of organisms either in the superincumbent rock of the Portlands or of the Purbecks. Having regard to remarks made on page 89, I do not believe that spicules in sufficient quantity to account for the concretions in the bed have actually disappeared, the few that do occur show no sign of great solvent action. There are, however, in this section, a comparatively small one, vertical cracks or jointings filled with silica (flint), which, I think, indicate the downward passage of water charged with silica, and some of this may have segregated as nodules, though its actual source is obscure. In the lower part of the silicious rock at Reeth there is also a dearth of sponge spicules, but a possible origin of the silica is not far to seek. If the action of decaying organic matter on the soluble silica of the sponge spicule is theoretically correct, nowhere should its action be displayed in a greater degree than in the mass of spicules in the upper part of the bed, for here there must have been an enduring growth of sponges, and decaying protoplasms and glairy matter must have been mixed with their silicious remains. And this seems to have been the case, for the WILLlA:'! HILL ON larger part are spicular canals only, and there seems to have been a general soluti on and redeposition of the organic contents of the rock as a whole. It is not surprising therefore to find the rock immediately beneath the spicules permeated with silica probably carried downwards by infiltrating water. And this argum ent I would extend to the radiolari an rocks of the Culm Measures . The authors of the papers on these rocks state that where radiolarians are most numerous there the silica of the chert displays the lively tints of chalcedony. It is just here that the amount of animal matt er would be found in greatest abundance mixed with the radiolari an ooze, and the effects of its decay on the silicious debris would be most marked. I infer that the shaly rock of the Culm Measures, in which little or no traces of silicious organism s can be seen, has been infiltrated with the silica derived from the radiolaria. In the Carboniferous limestone from Bakewell we find a rock which, under the microscope, seems to have been a deposit having much resemblance to an impure chalk, and within it we find a concretion almost identical with our typical flint; other specimens from this formation seem to have been formed in a similar deposit and show silica which crystallised in a minute or crypto crystalline form. But where the spicules are aggregated togeth er the silica assumes either a coarsely granular form or the radiately fibrous aspe ct of chalcedony. Many specimens of Carboniferous chert must have been an almost pure deposit of sponge spicules with but little adm ixture of calcareous material. They are obviously not due to the segregation of disseminated silica in a calcareous rock, but are practically the rema ins of sponge beds. In others the spicul es, almost as numerous, appear to have been embedded in an amorphous calcareou s sediment without marked characters. This is now crystalline calcite, often, in part only, replaced by the silica, the chert being an intimate mixture of the two. Th e walls of the spicules have here been replaced by calcite, a fact which, however, can be parallel ed in the concretions of the Greens ands. The chert from the mine " Good Intent" and other specimens from Thorpe Edge are due to the permeation of the limestone by silica probably derived from spicules distributed through the deposit, the replacement of the calcite occurring under conditions akin to those of the con cretions in the Portlands, but it may be not ed in passing that the calcite of the organisms has resisted the invasion of the silica to a greater degree than that which forms the cementing material. In the Reef Knoll limeston e of Cracoe there is again some approach to the conditions of the Portland Beds, but in the place ... Radiolarian Rocks of the Culm Measu res," b y Dr G . J. Hinde and H . Fox {f ll a,l . JDurn . Geol. S oc. (lay S), vol. Ii, p. 617. FLINT AND CHERT. 93 of oolitic grains there are coarse organic fragments. The silica follows, as usual, the variations in the character of the calcitic crystals which it replaces, but the organic fragments are only partly silicified. The radiolarian chert from the Culm Measures presents us with new features. From the analysis given, the rock appears to contain little or no calcite, and therefore there is no replacement of the original sediment. Silica derived in all probability from the radiolarian debris has permeated the rock and has concen trated, not in nodules, but in well-defined layers. Though the silica is described as minutely crystalline and like that of flints, yet, as soon as sufficient organic silica is present, well-marked chalcedony appears. 7.-CLASSIFICATION OF SILICIOUS CONCRETIONS. From amongst the various forms of silicious concretions which have now been described, the difficulty of separating chert from flint on the ground of optical structure will be recognised. If each variety of silicious concretion is taken as a whole it will be evident that a large proportion of radiately fibrous chalcedony does not necessarily constitute chert, for we have seen that many of those of the chalk contain much of it, while on the other hand the so-called chert may contain a large proportion of silica in a condition which can be compared with our typical section of black flint. Nevertheless, if the gradations are acknowledged, there is at one extreme silica in a minutely crystalline condition-which we may recognise as flint-at the other radiately fibrous chalcedony -which we may recognise as chert-silicious concretions from various formations being usually a mixture of the two and their gradations in various quantities, the so-called chert having, as a rule, the larger and more constant proportion of radiately fibrous silica. The presence of this, together with a considerable amount of inorganic matter in those with which the term "chert" has long been associated (the concretions of the Selbornian and Vectian Sands), gives the so-called chert a rougher and more uneven texture than the flint. Thus I am in agreement with Miss Raisin, who says: "Notwithstanding these gradations these distinctions (of chert and flint), especially that of texture, might perhaps be used if we apply' them to any mass or fair-sized specimen, not to a microscope slice."· Some text-books describe chert as an impure form of flint, but flint may contain a large proportion of fine calcareous matter, besides fragments of shell, and one can hardly exclude from impurities such included material because it happens to be • Miss Raisin, "The Formation of Chert." Proc; Geol, Assoc., vel, xvill (1903'4), p. 71,72. PROC. GEOL. Assoc., VOL. XXII, PART 2, 19II.] 8 94 WILLIAM HILL ON' FLINT AND CHERT. calcareous. Perhaps the fracture of the concretion is as good a test as any; but even then, if it has been freshly quarried and wet, and the silica chiefly in a minutely crystalline condition, the fracture may be conchoidal. Leaving out those intermediate varieties which graduate from one to the other, and taking the extreme forms of silicious concretions found in the sedimentary rocks, they may perhaps be roughly divided into the following types: I. Those occurring in fine-grained limestone; the silica-crypto or minutely crystalline ...... ,...... Flint. 2. Those formed in limestone of coarse grain or consisting of oolite grains, the silica largely radiately fibrous chalcedony, graduating to granular crystalline...... Chert. 3. Spiculiferous sands and rock, the cementing material coarsely crystalline or radiately fibrous chalcedony ... Chert. 4. Sand-grains cemented by chalcedonic silica ... .,. •.• Chert. S. Radiolarian deposits in which the silica is minutely crystalline chalcedony... •.. ..• ..• ... Chert. 6. Those silicious concretions containing much opal or colloid silica, either in discs or globules, and unaltered spicules soluble in alkali ...... •.. ••• ... Opalite, 7. Fine inorganic detrital matter, permeated with silica in a minutely crystalline condition. Hornstone. Though these divisions may indicate the general structure of silicious concretions, variation in the quantity and kind of the several ingredients is the rule rather than the exception, and it would be often difficult to say to which type any particular specimen belongs. Thus while flint and chert, from long usage, may be convenient names to distinguish certain forms of silicious concretions in a rough-and-ready way, no definite scientific value can be attached to these terms, for chert and flint may pass insensibly the one to the other. As I have already said: "The special characters of flint and chert are due to the nature of the rock in which they were formed and the conditions under which the silica segregated into a silicious concretion."