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 694. VARIETIES OF CHERT • • • " 71

Cherts of the Upper and Lower Greensand. 71Chert of the Portland Beds. 73Carboniferous Chert • 76Chalcedony in Flint . 80Chert of the Culm Measures 82

5. IMMATURE FLINT AND CHERT 836, GENERAL SUMMARY 857 CLASSIFICATION OF SILICIOUS CONCRETIONS 93

1. INTRODUCTION.

EVE RYONE familiar with the White Chalk of England isalso familiar with the nodules of a different substance

which are scattered through it. Break one, the fracture will beconchoidal, and it will be found that within a white rind ofgreater 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 nodulesin the Chalk is a matter of some obscurity. We know thatEarly Man recognised the value of flint for offensive, defensiveand domestic purposes, and until quite recent years it played nounimportant part in the economies of mankind. The wordprobably came to us from the Saxon "flinta." It is used sixtimes by the translators of the Authorised Version of the Oldand New Testament, though it seems to be applied to rocks ofexceptional hardness, and not to be confined to what we know asflint. Virgil, Pliny, and Lucretius use the word silex in connec­tion with striking fire. According to Murray the word occursin the Epinal Glossary about A.D. 700, page 8°5; it isthere 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 con­nection with fire and fire-arms, and is certainly used to describePROC. GEOL. Assoc., VOL. XXIl, PART 2, 19I1.] 6

62 WILLIAM HILL ON

the material which occurs as nodules in the Chalk. So closelyindeed is chalk and flint connected in our minds that it is hardlypossible to think of one without recalling the other, and if ageologist were asked where flints were to be found he wouldalmost certainly refer the enquirer to the Chalk, therefore wemay safely take one of these silicious nodules as the type of trueflint.

They are, as we all know, segregations of silica, probablyderived from organisms which lived in the Cretaceous sea, butthe chemical processes which have involved the disappearance ofthe silica of sponge spicules, the tests of Radiolarians, or of thefrustules of Diatoms from the chalky deposit, and its re-appear­ance in the form of flint nodules, though not perhaps so great amystery <IS it was thirty years ago, is yet far from beingunderstood.

Flint not only occurs in the form of nodules arranged in linesor scattered promiscuously through the rock, but also in thin con­tinuous sheets of considerable extent to which the name tabularflint has been given. While the lines of nodules and sheets oftabular flint usually correspond with the bedding plane, seams offlint not infrequently occur running at various angles, sometimeseven at right angles to it, and appear to have infilled the cracksor jointings which are common in the Chalk.

The nodules, which are as a rule easily separable from thesurrounding matrix, are often very irregular and sometimesfantastic in shape. They vary greatly in size, from the hugeParamoudra, "rudely cylindrical or cup-shaped, with a centralcavity, often two or three feet in height with a width of oneor two feet,"· to the small rounded nodule of finger-shapedflint not an inch in its greatest diameter. Their form not in­frequently gives some indication of an organism around or withinwhich the flint had segregated, many taking the shape of sponges,the original outline of the sponge being closely followed, but asoften the shape of the flint is uninfluenced by anything whichcan be regarded as a nucleus. Sometimes the flint has infilledthe hollows of a calcareous organism, and when the calcitic testhas been removed, a more or less perfect cast remains. Theinterior casts of Echinodeons are common instances of this.

Flints with special characters, such as a greater or less thick­ness of rind, pinkish coloration, or zoned structure, are met withcommonly in some localities, but these characters can only beregarded as local peculiarities and are of small value as guides tothe horizons of chalk.

Scarcely less familiar than chalk flints are the silicious concre­tions known as chert, which occur in the Vectian and Selbornian(the Lower and Upper Greensands of our Home Counties), andin the Purbeck and Portlandian limestones of Dorsetshire and

* "Cretaceous Rocks of Brltain." Mem. Geol, SUYV., vol. iii, p. 259.

FLINT AND CHERT.

other counties. Like flints, chert occurs in layers of nodule-likemasses and in tabular sheets arranged parallel with the beddingplanes, and in seams and cracks at various angles to it.

But while flints occurring as well marked nodules or layersare as a rule easily separated from the chalk, the boundary ofcherts is not so well defined, and there is usually a broad crust ofpartly silicified material which passes gradually into the sur­rounding rock, from which the chert IS frequently not easilyseparable.

The term chert seems to have been used nearly 200 years ago,for in 1729, in vol. xxxvi, page 30, of the "PhilosophicalTransactions," Martyn described chert as "a kind of flint, socalled when it is found in thin strata, but in the Peak the strata ofchert may be four yards thick." Thus it seems clear that at thisdate concretionary silica differing from flint was recognised.

Silicious rock, usually called chert, occurs in many formations,and in many localities, besides those above mentioned, sometimesin solid beds of great thickness. In the Carboniferous series inNorth Wales, Dr. Hinde says, "there is a continuous series (ofcherr) 350 feet in thickness without the intervention of lime­stone," again "in some Yorkshire areas there are chert beds18 feet in thickness without a break."·

Mr. H. B. Woodward describes a "compact chert containingshells occurring in the Lower Lias at Chewton Mendip on theHarptree and Egar Hills, at East End, Emborrow and nearBinegar." It is massive bedded chert, occurring in layers one tothree feet in thickness, separated by thin ochreous clayey beds aninch or two thick.t

Dr. G. J. Hinde and Mr. H. Fox describe" Cherty" rock ofvery considerable thickness which occurs in the Lower Culmmeasures of Devonshire.j

But while it is easy to direct an enquirer where to find atypical example of flint, it is by no means so simple a matter inthe case of Chert, for, broadly speaking, the term chert has beenapplied to silicious concretions in all formations other than theChalk, though W. H. Hudleston§ has called certain compactnodules occurring in the Portlandian at Chilmark" flint."

Few geologists have given more attention to chert fromvarious formations than Dr. G. J. Hinde, in his paper II on"Beds of Sponge Remains in the Upper and Lower Greensands,"and he not infrequently refers to "true chert."

Describing the Sponge Beds of the Lower Greensand atHaslemere (p. 405), he says: "The beds in question consist of a

• G. J. Hinde, "On the Organic Origin of Chert."t " Geology of England and Wales," znd Edit. (lBB7), p. 26S.: liOn Radiolarian Rocks in the Lower Culm measures:' Quart. Journ, Geol, soe.,

vol. Ii (lB9S), p, 626.§ .. On the Geology of the Vale of Wardour," Proc, Geol, Assoc., vol. vii

(lBB,-2l,p.,Bo.II Phil. Trans. R. S., part ii (lBBS), p. 403 et seq.

WILLL\l\I HILL ON

central layer of light or dark translucent chert, with an upper andunder layer of yellowish, porous, translucent rock. The outerporous crust consists of a matrix of translucent chalcedonic silica,with, at times, an admixture of quartz-sand and glauconite grains.This matrix is filled with . . . the empty moulds of spongespicules." In the central layer of massive chert: "The silicaderived from the solution of the spicules of the outer crustappears, in part at least, to have been redeposited in this centrallayer in the form of chalcedonic chert, and the spicules of thislayer have been thus enveloped by the cherty matrix." Inreferring to the chert of the Lower Greensand of Folkestone, hesays: "There is in some beds a central layer of cherty rock, butthis is usually porous, though in places it passes down into truechert." " The cavities in the cherty portion of the Sponge Beds arefrequently lined with a smooth layer of chalcedonic silica, whichshows under the microscope a radiately fibrous structure. Whenthe cavities are infilled by successive layers of this silica ... atrue chert is produced." The term chert, therefore, may here betaken to apply to a solid, translucent, silicious material, made upin this case of a matrix consisting largely of radiately-fibrouschalcedonic silica, in which are many traces of sponge spiculesand other organisms, together with some inorganic material.

Miss Raisin * has communicated to the Association a valuablecontribution to our knowledge of the Micro-structure of JurassicChert. Incidentally, the difference between flint and chert isdiscussed, but" is not easy to define precisely;" After describingthe Micro-structure of the Cherts, she says: "Thus, amongst thedifferentiated structures in a chert, a radial development (of thesilica) is evidently one of the most important."

2. TYPES OF FLINT AND CHERT (CRETACEOUS).

Bearing these remarks in mind, let us see for ourselves whatdifferences there are between a typical example of flint andchert.

As an example of flint I take a pure black translucent speci­men from the zone of Micraster cor-anguinun; near Hitchin. Iassume you must all know what flint is like, and that I need dono more than recapitulate its salient features when a thin slice ofit is viewed under the microscope. It will be seen almost clearand colourless, or with a faint brownish tinge which Prof. Sollas tregards as " probably due to the carbonaceous pigment, the lastresiduum 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 organismscontained in the chalk may often be seen faintly but faithfullyoutlined, though details of their structure may be obscure.Sponge spicules are not abundant in such flint, though traces ofthem occur in most specimens. Viewed with a high power, say~-in. objective, it will be seen that the flint has not the homo­geneous appearance of quartz and glass, and one seems to realise,in a confusion of faint and shadowy lines, that the material isbuilt up of minute particles fused or cemented together into acoherent mass. To those familiar with the minute structure ofthe chalk there would appear grounds for thinking that the flintwas a pseudomorph of the original deposit, silica replacing thechalk particle by particle, a viP-IV supported by the remarks ofProfessor Rupert jones," Professor Judd t in his paper readbefore the Association on the" Unmaking of Flints," and also byProfessor Sollas.]

With crossed nicols the silica of the flint is seen to be in aminutely crypto-crystalline condition, the optical action of thelight giving rise to a multitude of bluish specks, which, thoughseparate, yet closely approximating to each other, give the wholea greyish-blue appearance. But this even structure is not alto­gether unbroken, for at points which correspond with well-shownforaminifera, or with that occupied by a shell fragment or asponge spicule, larger and more strongly refractive crystals mapout, as it were, the area once occupied by the organism. Theapparent separation of the minute crystals seems due to twofacts; the first is that the axes of the crystals do not all lie in thesame plane-some are presented to the eye at the extinctionangle, and will appear and disappear as the stage is slowlyrotated j secondly, that flint is an intimate admixture of opalineand crystalline silica, the opaline silica, which is negative topolarised light, filling minute interspaces between the siliciouscrystals.§

It is, however, rare to find a nodule consisting throughout ofpure black flint; in almost everyone there are breaks in thehomogeneity of the material, breaks which are usually referred toas "cloudy patches." Many flints, too, are greyish, others arebluish-grey, and are duller and less glassy than black flint; somehave a peculiar opaline appearance, others may be entirelywhite. 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 <i Flint Nodules of the Trimingham Chalk," AnJ1.

ana Mag. Nat. Hist, (1880), P.447.§ If flint is boiled in a strong solution of caustic soda the opaline silica will be dis­

solved, the specimen will become white and opaque, and the crystals, as a white dust,will soil the ringers like chalk. Flint in this condition will adhere to the tongue and takein a stain, such as fuchsine or red ink, readily. It may be partially, perhaps with careentirely, restored to its original dearness by immerslon in Canada balsam. Thin slicesof weathered flint, white dud opaque, are quite clear and translucent when mounted inbalsam.

66 WILLIAM HILL ON

silicification of the amorphous calcareous material, to the presenceof imperfectly silicified organisms or organic fragments, to dif­ferences in the crystalline condition of the silica, to whichreference will be made, and to the decomposition of the flintitself. They lead to endless variety in the macroscopic appear­ance of flints from various localities and horizons. But theground-mass of true flint will invariably be found to be silica in aminutely crypto-crystalline condition.

Let us now examine an example of true chert. We cannotdo better than take it from the Lower Greensand, for it is thechert of this formation that is most familiar to the members of theAssociation, and it is also the silicious concretions of this forma­tion to which Dr. Hinde refers as "true chert." We will take asour specimen one from the Hythe Beds of the Lower Greensand,collected during the excursion of the Association to Gomshalllast summer. Viewed as a hand-specimen, its aspect is differentfrom that of our black flint. It is brownish-grey in colour, has asaccharoid 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 sur­rounding material, one passing rapidly to the other, and it is noteasily separable like a flint nodule.

A thin section shows that the ground-mass is radiately fibrouschalcedony, almost water-clear, full of the remains of spongespicules j so numerous are they that it is not possible in any partof the section to find a space covered by the field of the micro­scope where they are not seen. For the most part they are theresiduary canals of the spicules of Tetractinellid and Hexactinellidsponges, with many indeterminable rod-like lengths. When thespicule is cut transversely, or nearly so, the walls and central canalare sometimes faintly outlined, but usually the walls have disap­peared, and there remains but a cast of the axial canal which hasbeen infilled with silica in a less soluble form. Some have beeninfilled with glauconite, others with silica akin to that material, butof brownish tinge. By direct light these have a whitish opalineappearance, and are semi-transparent; some are hollow and solid­looking; these are frequently coloured brown by the infiltration ofiron, and are then opaque. Scattered promiscuously through thesection, but more abundant in some places than in others, aregrains of glauconite and quartz, with irregular patches of finematter, presumably portions of the original inorganic deposit.These, with numerous more or less opaque specks, give the cherta dirty appearance. From many points along the borders of theresiduary spicules and inorganic particles rise fibrous growths ofchalcedony, which spread in fan-like arrangement and intersecteach other, filling all the interspaces with a confusion ofradiating fibres, while in clearer spaces independent aggregates

FLINT AND CHERT.

of radiately-fibrous chalcedony have formed. There is in thiscbert no modification of the fibrous structure of the chalcedony,but the fans and fibrous aggregates are small when comparedwith some examples of chert, and are not well seen with aless power than a !-inch objective. There are a few cavitieswhich seem to be caused by the solution of spicules. Withcrossed nicols the structure of the silica is seen to be coarselycrystalline.

The difference between our typical examples of black flint andthe Gomshall chert lies chiefly in the abundance of spicules con­tained in the latter, and in the crystalline condition of the silica;in flint it is crypto-crystalline, in chert it is radiately fibrouschalcedony.

I t would be a simple matter to determine what should becalled chert and what flint, if this difference was persistent, butit is not so. In a series of both flints and cherts it is possible tofind every gradation, from the minutely crypto-crystalline condi­tion of the silica to the broad, radiately fibrous structure of chal­cedony, and in their proportions to each other; and this is notonly in the same formation, in the same bed, but in the sameconcretion. It is difficult to give an adequate description inwords of the different structures seen, more difficult to givereasons for these variations. The fact is, that the special char­acters of so-called flints and cherts are due to the nature of therock in which they were formed, and to the conditions underwhich the silica segregated into a silicious concretion.

Let us consider for a moment the conditions under whichthese two typical examples of flint and chert have originated.The flint we know has formed in a rock made up of the detritalremains of calcareous organisms, an almost pure carbonate oflime. There is abundant evidence that scattered through thismass of calcareous debris were the remains of silicious organisms.On the fractured surface of specimens of the Middle and UpperChalk one can sometimes see hollows, each of which, from theirshape, 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 inthe case of the minuter forms of spicules. Radiolaria were alsopresent. Their outline can sometimes be distinguished in thechalk itself, but the silica of their tests has gone, and granularcrystalline calcite has taken its place. Diatoms may also haveoccurred, though they have not been identified. These organicremains, all of which were of amorphous and soluble silica, weredisseminated through the chalk, more abundantly perhaps insome places than in others, but in quantity not enough to form asilicious 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 hasbeen derived from the solution of these organisms.

That the dissolved silica could be, and was, transferred fromone part of the chalk to another seems incontestably proved bythe occurrence of oblique and vertical layers of flint which are socommon in the chalk. The flint seems to have formed along thesides of joint-planes or cracks, occasionally expanding intobulbous nodule-like concretions. It differs in no way from theordinary nodule, for foraminifera, spicules, and Xanthidia, occurin it. It is difficult to conceive any other way in which thistransference could be effected except by the agency of water. Mr.Jukes-Browne" says: "It is quite possible, however, that thedeposition of silica in these cracks took place as soon as thecracks were opened; that is to say, during the first upheaval ofthe mass while the occluded sea-water was passing away from it.This water must have contained much silica in solution, and themere relief of pressure by upheaval may have been sufficient tohave caused precipitation of the dissolved silica. "

It is only necessary to compare sections of flint side by sidewith some from the surrounding chalk, to show that it is asilicious presentment of the mother rock. Though the minuterparticles, such as the pseudo-coccoliths, the piece sof foraminiferaltests, calcite crystals, and the like are obliterated, the foraminiferashell fragments and spicules are often distinctly outlined. (PlateXIII, Fig. I.)

Thus we find that organic silica widely disseminated in finelydivided calcareous debris, replacing pure detrital calcareousmatter, ultimately assumes the minutely crypto-crystalline condi­tion which we recognise as flint.

In the case of the Chert the conditions were altogetherdifferent. It will be within the memory of many here to-night,that during our visit to Gomshall, in order to assure ourselves ofthe presence of the Hythe Beds, acid was used, for the overlyingBargate Beds, so like them in outward appearance, betrayed them­selves by the presence of calcite; in the Bythe Beds there wasnone. The Hythe Beds from which our Chert was taken are infact a fine sand curiously free from calcareous debris or subse­quent calcitic infiltration, quite the antithesis of the Chalk. Thearea occupied by the Chert must have at one time been packedwith spicules, was at one time probably a felted mass of them; fromthe appearance of the section they must have been so abundantin certain parts as to have almost excluded the sand and mud ofthe original deposit. Here, then, there is no question of. thereplacement of detrital carbonate of lime by silica derived fromdisseminated silicious organisms. In the area occupied by theChert there was an abundance of organic silica on the spot. andhough 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 moreisolated spicules in the immediate neighbourhood. Here in anon-calcareous deposit in the presence of an ample supply thesilica has finally crystallised in the form of radiately fibrouschalcedony. (Plate XIII, Fig. 2.)

3. VARIETIES OF FLINT FROM FORMATIONS

OTHER THAN THE CHALK.

Silicious concretions which can be compared with chalk flintoccur in the Carboniferous Series at Bakewell, Derbyshire, andat Richmond, Leyburn, Keld, and Harrogate, Yorkshire,and specimens from the white limestone in the Portlands fromChilmark and Wockley might also be included. In all these thesilica may be regarded as crypto-crystalline.

I am indebted to several friends for many examples of thesilicious rocks from the Yoredale Series, and my best thanks aredue to Dr. G. J. Hinde, Mr. Rhodes, and also to Mr. J. Wag­staff, of Reeth, Yorkshire, who at my request sent me a largenumber of specimens of Chert.

Of these only one, from Bakewell, Derbyshire, had any of themother rock attached. This was a dense brownish-grey lime­stone which, when seen under the microscope as a thin section,was in many respects similar to some of the more impure varietiesof the Lower Chalk. It was evidently once a calcareous ooze,now finely granular calcite, but the crystals are coarser in someplaces 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 nearlyobliterated in the general crystallization of the rock. There area few spicules, recognisable chiefly when cut in oblique or trans­verse section, showing the canal; their silica is replaced bycalcite.

The silica of the concretion attached to this specimen is in acrypto-crystalline condition, closely resembling that of our exampleof flint. Traces of the spicules, shell fragments and foraminifera,though nearly obliterated, are shown here and there by slightlycoarser and more refractive crystals, and there is a slight variationof the grain following appearances presented in the limestone.The material is not perfectly silicified, a little amorphous cal­careous matter remains, and there is a distinct though very slightreaction with acid.

Two other specimens, one from Leyburn and the other fromthe Undersett Beds of Keld, may be described as flint, for theirsilica is crypto-crystalline, They contain a few small, thinspicular shafts and much inorganic as well as calcareous matter.They appear to have been formed in a fine, impure, calcareousdeposit, free from coarse inorganic fragments or foraminifera.

WILLIAM HILL ON

Four other examples, from Richmond, Leyburn, Keld, andHarrogate, may be also mentioned as approximating closely intheir nature to flint. The silica in these specimens is minutelycrypto-crystalline, and they might be considered by some asexamples of Hornstone. Small, thin spicular shafts are numerousin the first three, the silica of their walls is usually replaced bycalcite, and there is much calcareous matter included in thesilicious ground-mass. There are no large fragments of shell orforaminifera; they appear to have been formed in a fine calcareousmud, purer than the last. The specimen from Harrogate, how­ever, contains a quantity of sand-grains and much fine inorganicmaterial. Spicules of the same kind are very numerous, thoughtheir forms are easily distinguishable with crossed nicols, theyhave become merged in the silicious ground-mass. There is nocalcite of any kind in this specimen.

The silicious concretions from the Portland Beds at Chilmarkand Wockley, which may be regarded as flint, are thoseoccurring at the top of the series in a comparatively pure whitelimestone. This consists chiefly of amorphous matter, and thereis no foraminiferal debris, pseudo-coccoliths, or the like, in thislimestone, though some of the larger particles rna}' be derivedfrom shells. Downwards, shell particles become larger, and astructure develops of semi-opaque aggregations and more or lesselliptical grains. These white beds contain many silicious con­cretions which, like the chalk flints; separate cleanly from thematrix, and there are also some vertical and oblique seams filledwith silica.

The concretions repeat the features of the rock, and, withcrossed nicols, the condition of the silica is micro-crystallineSeveral specimens, both of the rock and the concretions, collectedfrom this locality twenty-five years ago, but not with a view tothis inquiry, show the passage from the amorphous to the pseudo­oolitic structure. A specimen from the vertical layers, like thoseof the Chalk, also repeats the features of the rock.

But here we meet with a difficulty in the determination ofwhat is flint. I have already said that, under crossed nicols, thecrypto-crystalline condition of the silica in flint is seen in theoptical action of the light on multitudes of minute crystals, which,though separate, closely approximate. In different samples of flintit will be found that there is a variation in the size of the crystals.Those of our typical example are minute and rather widelyseparate, but it is possible to arrange a series in which they areprogressively larger, more brilliant in their optical qualities, and atthe same time approximate more closely to each other. Thisfeature 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 wholein having coarser grain, but in this comparative coarseness flint

FLINT AND CHERT.

and so-called chert may approach very closely indeed, and partsof a so-called flint may be as coarse in the grain as a so-calledchert. It is indeed possible to follow this progressive coarsenessof grain till one arrives at fibrous chalcedony, though somefibrous chalcedony may exhibit under crossed nicols a crystallinestructure less coarse than some chalcedony in which fibres arenot visible.

The silica in these concretions from the Portland Beds is, asa whole, somewhat coarser in the grain than that of Chalk flint,though in some places it approaches it very closely, but as thedeposit assumes the pseudo-oolitic structure the crystallinecondition of the silica becomes more irregular and coarser.

It will be seen that all these flint-like concretions occur inrocks in which the deposit was originally a finely-dividedcalcareous material, not, however, quite so pure as chalk, andthat sponge spicules were intermixed with it.

4. VARIETIES OF CHERT.

Concretions of silica which may be compared with ourexample 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 Carboniferousseries in Yorkshire and Derbyshire, as well as in the LowerGreensand of our Home Counties.

Chert of the Upper and Lower GreelZSand.-In the Chertswhich I have examined from the Lower Greensand the radiatestructure of the chalcedony is on a small scale. While the wallsof the spicules are often lost in the fibrous chalcedony, the canals,which have become filled with silicious material which does notmerge into the fibrous fans and aggregates, remain, so that theoriginal character of the spicule can often be distinguished. Thechalcedony lies in the comparatively small spaces between theresiduary canals, between the patches of amorphous matter andthe grains of sand and glauconite, where, in fact, there werespaces in which the fibrous growths could proceed with leastinterruption. (Plate XIII, Fig. 2.)

In cherts from the Selbornian sands the fibrous aggregationsand fans of water-clear chalcedony are developed on a largerscale than in the example described. Obscure traces of spicules,often hexactinellid mesh, may be seen in them, and the radiatingfibres of the aggregates and fans frequently emanate from pointsclearly representing the position of a spicule. These broadlyfibrous areas are interrupted by others in which the chalcedonygraduates rapidly and confusedly from smaller aggregates and fan­shaped growths to a granular condition which, with crossed nicols,shows a micro-crystalline structure. Within these latter areas

72 W I LLI AM HI LL ON

are visibl e sp icul es in which th e silica is in many s tages ofalt eration and dissolution, together with a variable quantity ofmin eral grains, a few foraminifera and shell fragm ents, aggrega­tion s of silica either as discs or in globular form , and irregularpatches of small masses of am orph ous matter permeated bysilica. From all these, exce pt the sand-grains, rise fibrous growthsof chalced ony which fill the int erspaces, save th ose occu piedby the micro -crystalli ne silica. (Plat e XIII, F ig. 3.) In th eH ald en chert there is even a greater development of clear fibrouschalcedo ny.

It is needless to follo w the differences seen in th e structure ofthese che rts in d eta il and explai n how they have ar isen. Certainbroad features ac compan y them that lead one to infl'!r they aredu e to those minor var iations in th e character of the depositwhich must inevitably occur during the accumulation of a form­ation like the Greensands, and which affect the segregation andcrystallisation of th e silica in their mass.

Thus, in respect of the cherts, bo th of the Vect ian and theSelborn ian , radiately fibrous chalcedony has developed wherespicules accumulated . The smaller radiately fibrous fans andaggregat es occur whe re the sp icules have been mixed and inte r­calated with the sand and finer po rt ions of the d eposit. Thatthe Upper Greensand contained, besides mineral grains and fineinorgan ic matter, a considerable quantity of detrital carbonite oflime derived from parti cles of sh ell a nd some foram inifera isevident from the analysis of the rock. Though some of this limemay have been r emoved, some at least has been replaced bysilica which, probably from the abundance available, has in som ecases assumed a more coarsely crystalline structure than inord inary flint. .

I t will be seen from the foregoing remarks that th e mass o fthese cherts is made up of man y ingred ients, viz. : (r ) s ilica,varying from fibrous cha lcedony 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) calcareousde bris, more or less silicifie d, and occasionally ca lcitic crystal s ;(6) fine organic and inorganic matt er. There is infinite vari etyin th e proportion of th ese ingredients in the cher t from th e sam ebed or q uarry. Thus, while som e specimens will be foundpacked with spicules, others contain but few, and th e material ofth e rock, little altered, is simply perm eated with silica now in acrystall ine condition . Some of the con cretions co ntain a largeproporti on of globular coll oid silica, whilst others co nsist alm ostentirely of sa nd ce me nted togeth er by cha lcedo ny. Again, therock may ass um e th e hard, silicious aspect of chert, yet , whenexamined, the evide nce of spicules will be found only in th eiremp ty cas ts, with , perhaps, the spicular canal rem aining.

FUNT AND CHERT. 73

Chert of tlte Portland Beds.-How intimately the characterof silicious concretions is associated with the rock in which theyoccur is well shown in those of the Portlands, and ill describingthem I must refer to the rock as well as the concretions them­selves.

The highest layer of silicious concretions which we noticed inthe Portland Beds during our excursion to Swanage was ina cherty or flinty bed about 7 ft. below the "Berna Bed." *I obtained examples of the concretions, as well as of the rockimmediately overlying them. Sections of this were cut as clearas possible from large shell fragments. Under the microscope itis seen to consist of definite oval and elliptical grains lying, as awhole, close together, interspersed with fragments of shell andgrains of quartz. Sponge spicules, relatively of somewhat largesize compared with those of the Greensands, are abundantthroughout the rock; they are not massed, but sometimes two orthree may be in close proximity. There is a general permeationof the whole rock with crystalline calcite, which completely fillsall interspaces, accumulating in some places in sufficiently largecrystals to show the cleavage planes, sometimes surrounding andisolating individual grains. It passes, however, to granularcalcite where the grains lie very close together. There are alsoareas in which the whole space between the grains is filled withamorphous material, and this may extend irregularly betweenthem, taking the place of the crystalline calcite. This amorphousmaterial is impurely calcareous, but the lime in it appears to havecrystallised into minutely granular calcite, though the actualstructure is somewhat obscure. The oval grains are not trulyoolitic; they have not definite centres surrounded by calciticlayers, but appear to be rolled fragments of a previously con­solidated but somewhat impure calcareous deposit. Though thecalcareous matter has passed into finely granular crystallinecalcite, 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 centrebeing often clearer. In some the centres are clear granularcalcite, but such grains are often obviously some large calcareousparticle, such as a shell-fragment around which the matrix of theoriginal deposit still adheres. One or two contain sand-grains.

The rock is very calcareous, and breaks down entirely in a20 per cent. solution of hydrochloric acid. There was a residueof 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 spongespicules of one type. Their exterior was not rough and corrodedlike many of those in the Greensands, but most of their canalswere 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 in filled and they were solid.The silica of the walls is now quartz.

The silicious concretions obtained from the rock just de­scribed were whitish-grey passing into darker grey, and to trans­lucent, almost black, chert; none are black and translucentthroughout, but are much seamed and marked with grey patches.

In a thin section of the darker and more translucent chert allthe details of the rock described are faithfully represented; spongespicules are, however, a little more numerous than in the motherrock j nearly all the calcite is replaced by silica. The outline of theoolite grains is faintly but distinctly discernible. Those whichwere probably finely granular calcite are now crypto-crystallinesilica. In the centres of some the chalcedonic crystals arelarger and more pronounced, in others there is a closely-packedmosaic of crystalline silica (quartz ?). These were probably thegrains with more or less clear centres. In those which obviouslycontained a particle of shell, the shell-fragment is replaced bymore coarsely crystalline chalcedony than the periphery which iscrypto-crystalline. The shell-fragments contained in the grainsare not, however, completely silicified, some are only partly so,the silicification apparently proceeding interstitially between theprisms of the shell. Some of the grains contain minute calciteparticles also.

Instead of the crystalline calcite which permeated the rockthere is now fibrous chalcedony, pale brownish by transmittedlight, the radiating fibres standing at right angles to the surfaceof the grains, forming a fringe and meeting midway betweenthem. But in the larger spaces, probably once occupied by thebroader crystals of calcite, fan-like radiations of chalcedony havedeveloped and blend with the fibres emanating from the grains;in such spaces there is nearly always a central lacuna in whichthe silica is clear and not fibrous. In spaces, easily distinguished,occupied by amorphous material, and others probably occupiedby finely granular calcite, the silica is minutely crystalline.Where three or four spicules lie together there is always an areaof fibrous chalcedony, fan-like radiations or aggregates beingdeveloped. In some cases spicules are clearly outlined, in othersthe energy of the crystallising chalcedony has nearly obliteratedthem. Isolated spicules are usually distinctly outlined and sur­rounded by a fringe of fibrous chalcedony. (Plate XIII, Fig. -I.)

A second section was taken from the same nodule, where therewere alternations of dark and clear, with whiter and less translucentchert. The oolite grains were here not so even in size, so thicklypacked, nor so evenly distributed, but there were many moresponge spicules. These were massed in irregular, disjointed,incontinuous layers alternating with oolitic grains, or else ooliticgrains, amorphous matter and spicules were mixed togetherindiscriminately. Where massed they were feebly outlined, and

FLINT AND CHERT. 75

the silica had developed in the usual fibrous fans and aggre­gations, with central lacuna: of clear silica which in some placespassed into a mosaic of quartz crystals. But where spicules,grains, and amorphous matter appear mixed together there is aconfusion of radiately fibrous and minutely crystalline chalcedony.The oolite grains show the same features as before, but are notso perfectly silicified and contain much fine calcitic matter; thereis a great deal, too, throughout the section generally, especiallywhere oolitic grains and spicules are mixed together. Well­defined crystals of calcite appear to be granulating, silica pene­trating the cleavage planes and splitting them up into granules,or the crystal seems breaking into finely divided calciticmatter interpenetrated by the silica. The silica taking the placeof the broad calcite crystals may be radiately fibrous or coarselycrystalline, that taking the place of the finer calcitic crystalsminutely or even crypto-crystalline.

A third section was taken from a whitish, evenly grained con­cretion. 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 beenamorphous calcareous matter crystallising as finely granularcalcite, though amongst the grains are broad calcite crystals.Some of these broader crystals, as well as much granular crystal­line calcite, still remain, though evidently attacked and penetratedby the silica, and there is a thick dusting of calcitic particlesthroughout the section. The spicules are sparsely distributedthroughout the concretion, the silica of their walls being chalce­donie, 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 areunaltered, but in a few the calcite is replaced by a mosaic ofclosely applied silicious crystals.

Below this flint-bed the next series of silicious concre­tions are those in the well-known" Chert Beds." From DancingLedge 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 ofthe Portland passes rapidly from the oolite structure described toone in which the grains are less distinctly developed. In asection from the upper part of the "Chert Beds" the depositseems to consist of smallish semi-opaque angular and sub-angularaggregations of amorphous material; intermixed with them are afew oval grains, shell-fragments, a few spicules and a little quartzsand. These ingredients are closely compacted, the aggregationsin some parts showing a tendency to blend together, their outlinebecoming 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 gradualdevelopment of oolitic structure, I am unable to say, but bothaggregations and crystals are smaller than the oolite grains whichcharacterise the higher Portland Beds.

The structure of chert obtained from the centre of a largeblack specimen at this horizon is simply the reproduction of therock above described; as most of the rock is amorphous matterthe silica as a whole is micro-crystalline, a coarsely grained flintvaried with patches of fibrous chalcedonic silica. There is littledifference in the grain of the silica, either in that replacing theaggregations of fine matter or calcitic crystals, except perhapssome slight increase of coarseness in the case of the latter. Theedge of each well-marked grain is marked by a cloud of very finebrownish matter, which, under high powers, can be resolved intominute particles. Some of these are negative to polarised light;others are opaque, so that the outline of the aggregations orcrystals is quite visible when viewed with crossed nicols.

A second specimen was taken of the lowest chert bed exposedand one of the rock. The structure of the rock reminds one ofthat from the upper part of the chert beds-there are the sameangular and sub-angular semi-opaque aggregations of fine amor­phous matter, though smaller, but the crystalline calcite hasdevoloped to a much greater extent, the crystals covering inplaces a considerable area and show cleavage planes. There areno sponge remains at all except the reniform spicules of Geodia,which are fairly numerous, but their silica has been replaced bycalcite.

In the chert we have again a representation of the rock, butthere are a number of rod-like lengths of small spicules besidesthe reniform spicules of Geodia, which are abundant. Largeareas of fibrous chalcedony showing small fans and aggregatesseem to correspond to those occupied by crystalline calcite, whilethe pellets are minutely crystalline silica.

Carboniferous Chert.-The Carboniferous Series of Yorkshireafford us examples of silicious rock which will compare with ourtypical example of chert as well as the flint. Though found insome localities as nodules it frequently occurs in massive bedsfrom I ft. to 20 ft. in thickness. It is from such beds that thegreater part of my specimens were obtained.

They come from the upper part of the Yoredale Series abovethe Millstone Grit to the top of the Undersett Beds, the chieflocalities being Richmond and exposures between that place andReeth, Harrogate, Leyburn, Keld, and from various other placesin Swaledale and Arkengarthdale.

In their external aspect Carboniferous cherts vary consider­ably even in specimens from the same hed; some are dull blackand 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, NEARDEVIZES, WILTS.

x 45.

FIG. 4.-CHERT FROM THE PORTLAND BEDS, ABOUT 7 FEET BELOWfHE "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., ofReeth, most courteously sent me a series of specimens from athick bed of chert which occurs here. It is mined for the pur­pose of obtaining blocks which are used for grinding flints, thepowder thus obtained being mixed with the clay in the manufac­ture of certain kinds of pottery. The actual section is worthgiving as showing the succession and different qualities of chertoccurring in a single bed. The following particulars werekindly furnished by Mr. Wagstaff:

r. Limestone, taken within I ft. of NO.2.2. Black Chert and Limestone3. Grey Chert . • .4. Grey Chert, a massive bed.5. Grevish- black Chert . .6. Black Chert, dull and earthy-Jooking7. Limestone;

I ft.I ft.3 ft.8 in.

12 ft.

The overlying rock No. I consists of shell fragments, brokenencrinites and foraminifera, cemented together into a coarselimestone by crystalline calcite. There are few sponge spicules.

No.2 seems to be a passage bed, not however the passage ofa limestone to a silicified representative of the same material, butfrom a deposit consisting of the remains of calcareous organismsto one in which the organic remains are chiefly silicious.

The chert consists of large sponge spicules, with which areintermixed a few fragments of shells and encrinites similar to thoseof the limestone j filling the interspaces is much fine brownishopaque matter, the whole being permeated with crystalline chalce­dony. The spicules for the most part retain the outline of their wallsand canals, their silica being crystalline. Many are, however,replaced by calcite. In one part of the micro-section there are anumber of minute globular bodies, which, from their aspect andarrangement, leaves no doubt that they were once globular silica.Several spicules in the same area show a partial conversion oftheir walls to silica in this form, while their canals are in severalinstances filled with it. But the silica of the spicules in thiscondition is now replaced by calcite, and the globules infilledwith this material The application of acid to a portion of thesection destroyed a large number, though some which appear tobe protected by the enveloping chalcedony remain.

The ground-mass of No. 3 is clear, granular and fibrouschalcedony so intimately mixed that few definite radiately fibrousfans or aggregates can be made out. It was evidently once afelted mass of small-sized spicules; little remains but theirspicular canals, though here and there faint outlines of the originalform can be seen. The section is traversed by denser streaks,which seem to indicate the admixture of fine material nowPROC. GEOL. Assoc., VOL. XXII, PART 2, I9I1.] 7

WILLIAM HILL ON

silicified. These and the long axes of the spicules lie in the sameplane, facts which suggest current action.

NO.4 is very similar chert packed with spicules. The greaternumber are represented by spicular canals only. The silica whichhas filled the canals has a brownish coloration, some are hollow;all these are more or less opaque, or only semi-transparent bytransmitted light, and are therefore clearly defined, but by directlight 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 acorresponding increase of fine opaque matter, brownish in colour.The silica as a whole is minutely crystalline, but coarser than inflint. In the section there is a large patch, filled partly bychalcedony and partly by quartz. Around its edges are largefans and aggregates of brownish coloured chalcedony, which passesinsensibly into the water-clear quartz which occupies the centre.

No.6. In the specimens of this bed sent me, spicules arerare. There is much fine opaque matter, thoroughly permeatedby silica in a minutely crystalline condition. A few particles ofshells occur in it.

Underlying, and sometimes overlying, the chert beds of thisdistrict is a fine laminated shale known as .. plate," from 4 ft. to10 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 theYoredale Series agree with the general character of Bed 4.Viewed as thin sections under the microscope they present to theeye a tangled mass of sponge spicules embedded in crystallinechalcedony. In a long series of specimens there are, of course,differences in detail; in some the spicules are small and denselypacked-in such cases the crystalline chalcedony is of fine grain.and fibrous fans and aggregates are not visible-but where thespicules are large and interstices occur between them, radiatelyfibrous chalcedony has developed, but in no case have I met withlarge areas of water-clear chalcedony such as occur in the chertof 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 growthenduring for a considerable period, and whatever may have beenthe deposit accumulating elsewhere in the Carboniferous sea,little else but fine amorphous matter, drifted probably by currentaction, 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 whichall come from the neighbourhood of Richmond, contain muchcalcite. The original deposit was probably a fine calcareous oozein which spicules were very abundant. A few shell fragments

FLINT AND CHERT. 79

occur but no broken ecrinites. The calcareous matter is nowcrystalline calcite. Silica, in the form of clear chalcedony,usually coarsely crystalline and sometimes fibrous, permeates themass j though predominating it does not entirely replace the calcite,and the two minerals are often intimately intermingled. Thesilicious walls of the spicules are in nearly every case replaced bycalcite.

Specimens of chert from a mine called "Good Intent,"between Richmond and Reeth, now unworked, are of quite adifferent character. The limestone above and below the chertis like that already described, full of organic fragments andforaminifera j the cementing calcite is coarsely granular, extendinghere and there in broad calcitic crystals. (Plate XIV, Fig. 2.) Inthe chert the character of the silica corresponds to that of thecalcite-it is chiefly fibrous in small aggregates-but unlike that ofother- somewhat similar rocks fans rarely fringe the edges of theincluded fragments, the aggregates seem to arise from indepen­dent centres of growth. Most of the larger inorganic fragmentsbave resisted the invasion of the silica, only the cementingcalcite is replaced j traces of it yet remain, and some stilI fringesmany of the pieces of shell or encrinites. There are two thicklypacked layers of spicules, nearly all of which must have theiraxes in the same plane, as they are cut transversely and appearonly as circles. From the denser parts of these layers the largercalcareous organic fragments are excluded, only the smallerpieces being intermixed with the spicules.

The general permeation of the limestone by the silicagradually passes away downwards, the rock gradually becomingmore and more calcareous, and the silica at the same time crypto­crystalline.

Many of these concretions contain rhombohedral crystals ofa carbonate of lime, probably dolomite. (Plate 'XIV, Fig. 3 )They vary greatly in size.

The concretions which occur in the Reef Knoll limestone ofthe Carboniferous Series near Cracoe, Yorkshire, accumulatedunder different conditions. They are hardly nodules, but ratheran irregular impregnation of the rock by silica.

The limestone in which they were formed is made up ofcoarse shell-fragments, encrinite stems, and other undeterminedcalcareous organisms, with a few foraminifera. The whole iscemented together by broadly crystalline calcite, but between thefragments there was probably a certain amount of fine calcareousmatter, for the broader crystals give place here and there togranular calcite, and granular calcite occurs in the interstices ofthe organic fragments.

In the concretion many of the larger fragments are unaltered,some are partially, others are entirely, replaced by silica. Thesilica, which shows a brownish coloration by transmitted light,

80 WILLIAM HILL ON

probably replacing that which seems to have been broadlycrystalline calcite, is in large fans and aggregates, the fibresas usual emanating from the outer edges of the organic particles,but in places no fibres are visible. There are some largespicules but as a whole their outline has merged in the fibrouschalcedony, and they are difficult to distinguish.

A curious elongate nodule was found in the shelly beds of theMillstone Grit when the Association visited Klint, near Harrogate,last August. This has proved to be a spicular concretion. Itconsists chiefly of spicules, with shell-fragments and pieces ofencrinite mixed promiscuously together and embedded in amor­phous material probably partly calcareous and partly inorganic.Some of the shell-fragments are still calcite, others are partly orwholly silicified. The spicules, most of which are cut trans­versely in the section, are all chalcedonic silica; their walls andcanals still remain visible. The cementing material is partlyfibrous chalcedonic and partly coarsely granular silica.

Chalcedony in Flint.-In the so-called chert wehave examined,fibrous chalcedony has always occurred in close association withsponge spicules. To everyone who has examined flints from theChalk it must be evident that sponges flourished in the Chalksea. Are aggregations of spicules never found in the Chalkwith the resultant fibrous silica, were all the spicules disseminatedthrough the chalky ooze, and have all flints been formed by thesilica derived therefrom?

In some sponges the spicules are separated bodies embeddedin the soft parts of the organism, and on its death would losecoherence and become spread over the sea-floor. But otherswhen growing become packed with spicules shed by themselvesand their neighbours, and some have a rigid skeleton, the formof which we know has been preserved j both these would becomecovered with calcareous ooze which might penetrate into theirinterior in an irregular manner. There are many flints whoseexterior shape suggests that of a sponge, but there are manyothers whose shape carries with it no such suggestion, yet containmuch fibrous chalcedony, though it" may be encrusted or sur­rounded by flint.

It must be remembered that the water of the Chalk sea wascomparatively quiet. Though there may be evidence of currentaction in the Totternhoe Stone, the Melbourn Rock, and in themarly bands which occur at various horizons, flints certainlyoccur where there is least evidence of it, and whatever movementthere may have been in the water it did not sift out the spiculesfrom the fine mud in layers or aggregations as in the Greensands.The sponges with rigid skeletons, or those which may have beenpacked with spicules, were buried where they grew with littledisturbance of their original form.

FLINT AND CHERT. 8r

If thin slices of flint from the Chalk are examined it will befound that they sometimes contain much fibrous chalcedony,which often takes the form of an indefinite pattern, and whichmay be referred to as sponge-structure; sometimes it takes a formsuggesting 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 Hexactinellidsponges and possibly of the sponge body itself. Frequently alltraces of the spicules is nearly lost, they may be vaguely seen,.sometimes the mesh is preserved. This is no mere change ofthe organic silica of the mesh to chalcedony; it does not alwaysrepresent the area occupied by the original spicules, it is larger.Intervals in the chalcedony are filled in with flint in which thegeneral character of the surrounding deposit may be traced, andthe space occupied by the flint and chalcedony could not havebeen a hollow.

In these cases the silica seems to me to have followed itsusual course, and in the presence of much organic silica on thespot has crystallised as fibrous chalcedony within the area thatthe extra amount of organic silica could make itself felt, theenergy of the crystallising chalcedony being sufficient toobliterate nearly all traces of spicules, and possibly some of theadjacent detrital carbonate of lime, but it was not enough alwaysto obliterate the outlines of a shell fragment, e.e., an Inoceramusprism, which may remain visible. But the fibrous chalcedonyoccurring in flints is not always in direct association with thatwhich rna}' be taken as the skeleton of a rigid silicious sponge.

The spicules which may be found in most of the ordinarysections of black flint are usually small, thin shafts, oftenresiduary canals only; the cladome of the Tetractinellid spiculeis not often seen. They appear to have been mixed with thecalcareous mud, but occasionally may be found aggregatedtogether, though in small numbers. Here, as usual, the silica isnot minutely crystalline but fibrous, and there often seems arelation between the number of spicules present and thecoarseness in the grain of the flint. Sometimes spicules of largesize occur, similar to those common in flint meal; these arealways fibrous chalcedonic silica, the fibres sometimes seem toextend slightly beyond the area actually occupied by the spicule.

The occurrence of fibrous silica, then, in flint can hardly besaid to be the result of the accidental inclusion of these forms,but is due in fact to the conditions under which the Chalk and itssilicious contents accumulated.

That chalcedony and quartz fill up hollows in flints is ofcourse well known, but I prefer to regard these hollows as anaccident in the formation of a concretion, rather than part ofthose 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 becalled flints or cherts?

Chert of the Culm Measures.-The Chert Beds of the CulmMeasures, as they are seen in the Codden Hill Quarry nearBarnstaple, are described by Messrs. Hinde and Fox" as con­sisting mainly of hard, dark or light grey, sometimes bandedcherty silicious rocks with intermediate beds of soft grey or whitesilicious shale, a description which seems to he applicable to mostexposures of this part of the series in Devonshire and Cornwall.The most silicious portion of these Radiolarian rocks "occurs inevenly stratified beds usually from 2 to 4 inches in thickness,though they range from I to 9 inches, and in very exceptionalinstances to a fool in thickness. t

The microscopic characters of the silicious rock are thusdescribed d They "show under the microscope a generalresemblance in consisting of a silicious ground-mass, in somecases clear and transparent, in others dark and turbid from thepresence of fine particles of carbonaceous or ferreous materials,and minute crystal-needles of rutile and zircon. The siliciousground-mass, in polarised light, between crossed nicols, usuallyexhibits the faint speckled aspect of cryptocrystalline silica similarto that of a section of flint from the Chalk; sometimes it is almostentirely dark, in other instances certain bands of the rock in whichthe radiolaria are very crowded show the lively tints of chalcedony.The radiolaria in the rock generally have been infilled withclear, nearly transparent silica, free to a great extent from therutile crystals and the dark substances disseminated in the ground­mass. . Within the radiolarian casts the silica has notinfrequently a radiate fibrous arrangement."

The specimens of the silicious rock of the Culm Measureswhich I have examined come from Drewsteignton, Mullion Island,and Ilsington. The first two, when seen by transmitted light, showa silicious ground-mass somewhat turbid by the admixture of fineopaque matter. The similarity as a whole between it and that ofconcretions from the radiolarian rocks of Barbados is striking.Though particles of carbonaceous matter, together with minutecrystal needles of rutile and zircon, as well as flakelets of mica,are present, this ground-mass seems to me largely made up ofminute particles, which, though altered by time and circumstances,compare favourably with such as can be seen in the concretionsfrom Barbados. The turbidity of the Devon specimens seems tome to be due to some admixture of fine inorganic matter-in fact,mud-from which many of the silicious rocks of Barbados arecomparatively 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 itis radiately fibrous, but in the former case especially they oftencontain what I consider to be particles of the original deposit,and in this particular they also compare with the organismscontained in the rocks of Barbados.

A specimen of radiolarian rock from the Arenig Beds,Scotland, also compares with the silicious rock of Barbados, it isa purer silicious deposit than that of Devon or Cornwall, but itis in all other respects very similar. But while the silica in therocks of Devon and Cornwall is minutely crystalline, that inthe Barbados concretions is only incipiently so. Two otherspecimens from the Culm Measures deserve attention; both arepermeated with silica in a minutely crystalline condition, so thatthey are now beds of so-called chert. They are from a cutting ina new road at Ilsington. One seems to have been a laminatedshale of greyish colour, which may have been a fine inorganicmud, containing much detrital quartz in grains of small size; theother 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 spongespicules do not occur in some localities in the Culm, it is a fairinference to say that a large part of the silica in these rocks wasderived from the Radiolaria, for in the softer beds their silica hasdisappeared and they are sometimes represented merely by emptycasts. A point worth noting is that where the Radiolaria arecrowded the silicious rock shows the lively tints of chalcedony.Here again it would seem that in the presence of much organicsilica a well-marked form of chalcedony is produced.

5.-IMMATURE FLINT AND CHERT.

In the Chalk, the Greensands, and in the Portlandian, thereoccur concretions in which the conversion of the organic colloidinto crystalline silica and the general silicification of the rockseem to have been arrested. I regard these concretions as im­mature forms of flints and cherts.

In such concretions the original ingredients of the rock areonly partly silicified, but are permeated with silica apparently inthe transitional state between the colloid and the crystallinecondition.

These concretions usually contain many sponge spicules, thewalls of which may still be in the amorphous colloid state, whileothers are of crystalline chalcedony. In the first case the generaloutline of the spicule, with its canal, is preserved; in the secondthe outline is generally rough and corroded, and frequently thereremains but a cast of the canal which has become infilled withsilica of more stable nature.

WILLIAM HILL ON

The most striking feature of them all is the large proportion ofsoluble silica in the form of discs or globules which they in­variably contain, and which, like the spicules, exhibits manyinteresting phases in its gradual conversion to crystalline chal­cedony. The cementing silica which permeates the wholeconcretion, filling small interspaces, the cells of foraminifera orthe like, is only incipiently crystalline and shows but faint reactionwith crossed nicols. (Plate XIV, Fig. 4.)

Soluble silica in the torrn of discs or glubules has already beenwell described by Dr. G. J. Hinde in his paper on the SpongeRemains of the Upper and Lower Greensands, but its occurrencein other formations in conjunction with sponge spicules, and itsconnection with the formation of flint and chert, do not seem tobe generally recognised.

In 1889, in a communication to the Geological Society,Mr. Jukes-Browne and myself " drew attention to the occurrenceof silicious concretions in the Lower Chalk of Wiltshire. Theseare hard bluish-grey nodules with an earthy fracture quite differentfrom that of flint. Examination of thin slices shows that thematerial of the nodule is the same as the surrounding Chalk, fromwhich it differs only in being permeated with silica, largely in anamorphous globular or incipiently crystalline condition.

Since 1889 my attention has been called to concretionsoccurring in the Lower Chalk to the south-westward, in Dorset­shire and Devonshire, from Axminster, Chard, Chardstock,Crewkerne, and Warren Hill. These nodules undoubtedly showa progress from the immature concretions of the Wiltshire Chalkto more perfectly developed flint.

In their exterior aspect they differ little from the ordinary flintnodule; they have the same form, and separate easily from thesurrounding matrix, which is a purer chalk than that at the samehorizons 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 siliciousground-mass is seen to be crypto-crystalline, sometimes onlyincipiently crystalline when the nicols are crossed. Traces ofsponge spicules are numerous, but the material, especially nearthe rind, is crowded with minute opaque spherical bodies, whichbecome more separated and ultimately disappear in the clearerparts of the flint. By direct light these bodies appear white,many are certainly hollow, but the majority are either filled withwhite granules or crystals, or there is an internal coating of minuteparticles within the sphere which gives them an appearance ofsolidity. They would hardly be connected at first sight with theclear globules or discs described by Dr. Hinde. But any doubton 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 immatureconcretions of the Wiltshire Chalk; the passage of the clearglobules to the opaque stage and their ultimate disappearance inthe clear flint can be followed.

Concretions of a very similar nature can be found in theUpper Greensand; the globules or discs are, however, larger as awhole than in the flints just described. I would refer those whoare interested in the changes to be observed in the condition ofglobular silica to Vol. I of the" Survey Memoir dn the CretaceousRocks of Britain," pages 359 et seq.

From Dancing Ledge, in the Isle of Purbeck, I obtained otherexamples of these immature concretions from the Chert Beds inthe Portlands, In these the cementing silica is only incipientlycrystalline. The globules of silica are of large size; most of themare opaque, others are clear, and, as before, the increasing opacityand ultimate disappearance can be followed.

Though I have not found similar concretions in the examplesof Carboniferous Chert sent me, it is interesting to note that theglobular silica occurs, and I have little doubt that if the speci­mens were selected much the same phenomena would beobserved. Concretions in which a large proportion of the silicais in the condition of opal, readily soluble in caustic alkali, maybe called opalite,

6.-GENERAL SUMMARY.

It will be seen that in all the silicious concretions we havenow reviewed there is in each case a correspondence betweenthem and the rocks in which they occur; they are in fact siliciouspresentations of the rock in which they were formed. Fromtheir association with the remains of silicious organisms, one canhardly avoid the conclusion that the silica was derived fromorganic sources. There can be no doubt as to the solution ofthese organisms, and in many instances there seems to be adirect connexion between the character of the crystalline silicaand the quantity of organic silica which was once immediatelyavailable. Both in spiculiferous and radiolarian deposits, wherethere is evidence of an excess of organic silica, the crystallinesilica of the flint or chert is invariably of coarse grain or ofradiately fibrous structure. But where the silica of the organismshas been dissolved and transferred from some other stratum tothat in which it is now found, its final crystalline condition isgoverned by the nature of the rock in which it was redeposited.

Thus, in the Chalk, one of the most finely divided as well asone of the purest deposits of detrital carbonate of lime known,the silica of flint presents in its corresponding purity andminutely crystalline condition the character of the mother rockwhich it replaces.

86 WILLIAM HILL ON

Though in theory we may account for the solut ion of thesilica of silicious organisms and its distribution through a cal­careous ooze, it is still to be explained why it should have formedin nodules arranged in more Or less orderly lines as well as incracks and jointings,

Professor SolIas, in his paper on the nodules of theTrimmingham Chalk , has ably discussed the probl em of theformation of flints. With regard to the solution of spicules hesays, though possibly sea-water under pressure may have been asufficiently powerful solvent, Alex. A. JuJien suggests that thealbuminoid or glairy matters and acids akin to the Azohumic ofThenard, produced during the submarine decomposition oforganic matter, may have been the agents which have accom­plished this solution."

In his remarks on the formation of flint around the partiallypreserved forms of sponges, he also discusses at some length theeffect of decaying animal matter, the sarcode or protoplasm ofthe sponge body, on the organic silica of the skeleton and thatcontained in the ooze, and again quotes A. A. Julien, t who says,"I would therefore modify SolIas's theory by suggesting thatduring the decomposition of the sarcode of both animal andvegetable organ isms, after death, gelatinous or colloid substan cesare generated resembling glairine, which are soluble in sea-water,which combine with silica, and may therefore convey andconcentrate it, dissolving its part icles disseminated throu gh marinesediments, and which may, in certain forms produced by oxida­tion, act also as solvents of lime."

Mud that I have seen brought up from the sea-botto m wherespicules abound app eared to me full of glairy matter, apparentlythe con comitant of sponge growth and decay, and it seemedpossible that in such material a combination of decaying an imalmatter and silica could occur, form ing a slimy or gelatinouscompound that would become incorporated with the ooze. Thoughspicules in the mud were unaltered, Mr. Carter has recorded thathe found some showing signs of incipient solution in com­paratively shallow water.

But what happens when the glairy matter and sponge spiculeshave become a deposit of considerable thickness? Noth ingwhich suggests the beginn ing of a silicious concretion has beenfound on the sea-floor of the present day, but our knowledge isonly superficial, extending downwards but a foot or so beneaththe surface. If, however, the embryo flint originated on the sea­floor as a combination of silica and mud rich in decayingprotoplasm, 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 accumulatingooze, as oxidization progressed the finer calcareous matter wouldbecome gradually eliminated, and one can follow Sollas in his briefsumming up :-" Replacement of the calcerous material of theooze then ensued, small shells and many large ones too beingconverted into silex, and silicious chalk, not flint, was the result.?"

The time occupied in the solution of the organic silica is stilla matter of conjecture; some spicules were probably more quicklyaffected than others. Mr. Jukes-Brownet believes "that they (thesilicious organisms) were evidently dissolved in the sea-waterbefore the ooze was raised and compacted into anything likechalk" ; on the other hand Prof. Sollasj remarks "that the chalkmust have acquired a considerable degree of consistency beforeit could preserve in such a remarkable state of perfection theseperishable cavities (i.e., the casts of spicules)." But such consist­ency might have been acquired in the deeper layers beforeupheaval and desiccation.

We may now imagine a partly consolidated calcareous oozesaturated with silica in solution, perhaps also containing some ina colloid state, the result of its combination with decaying animalmatter. For reasons yet unexplained some of the silica hadbecome redeposited at definite centres along certain horizons,slowly eliminating the fine calcareous matter by a process ofreplacement. The next phase was the upheaval of the Chalk andthe draining away of the occluded sea-water, a process whichinvolved some circulation of that containing silica in solution.The relief of pressure or contact with carbonated rain-water mayhave led to the precipitation of the dissolved silica (seeante, page 68), and may have led to the last stage in theformation of flint. To continue the quotation from the paper ofProf. Sollas: "The chambers of the foraminifera and the inter­stices of the chalk were now filled up by a simple deposition ofsilica, and the silicious chalk becomes converted into black flint."

As flints are now, so they have been from a time not long afterthe elevation of the Chalk. Flints crushed in the fault at BallardCliff were already formed, and no silica has since been depositedaround the splintery fragments. I think, however, that asecondary deposit of silica has occured in some cases, morenoticeable in some localities than in others. This may be seenin a dense white layer, sometimes coloured pinkish, which occurson the outside of the black flint just beneath the crust. Thisview has, indeed, been anticipated by M. Cayeux,§ who describedat length a secondary deposition of silica on flints of the zone ofInoceramus 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 concretionsseem to have been formed by the impregnation and free precipi­tation of silica in the interstices of the rock.

The Selbornian Sands contain a certain proportion ofcalcareous material in the form of shell fragments, foraminifera,and amorphous matter, which has influenced the character of theconcretionary silica to some extent.

During the deposition of both these formations sponge growthmust have been vigorous, and spicules, generally distributedthrough their mass, have frequently been sorted out by current­action and arranged in more or less defined layers, which, thoughmixed with some sand and fine inorganic matter, have given riseto definite bands rather than to nodules of silicious or siliceo­calcareous rock.

Silica in globular colloid form occurs in these deposits, and isoften an important constituent of the concretionary silica found inthem. In discussing this form of silica which occurs in the LowerChalk of Wiltshire, Mr. Jukes-Browne and I expressed the opinion"that the globular silica was precipitated from solution before theconsolidation of the beds, and while they were still permeated bysea-water," and" that it is very difficult to understand the precipi­tation of any of this dissolved silica without the intervention oforganic agencies," and, further, that "it seemed clear to us thatthe precipitation of the chalcedonic silica of the cherty noduleswas a secondary and subsequent operation." *

These remarks have been criticised by M. Cayeux, who thinksthat both the globular and crystalline silica were formed at thesame time. He does not seem, however, to realise the action ofdecomposing animal matter on the soluble silica of the spicules,the effects of which must have taken place during the existence ofsuch matter, and while the mud was yet soft, and before thesilica could crystallise. Moreover, M. Cayeux is referring to well­formed chert, while our remarks are confined to those noduleswhich I regard as immature concretions in which the chalcedonyis only incipiently crystalline.

With a vigorous growth of sponges, decaying protoplasmicmatter must have been abundant, and we may imagine that thesolution of the spicules was effected in much the same way as in thechalk. It is conceivable that some of this protoplasmic matterbecame enclosed and incorporated with masses of spicules, andby combining with dissolved silica may have produced a materialsufficiently solid to have excluded detrital matter. There is evenground to support this suggestion: in sections of chert are areasof clear chalcedony in which neither sand-grains nor fine in­organic 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 insuch areas occur (often Hexa ctinellid mesh) but are not closelypacked, and it seems possible that some fairly solid substancemay have filled the space. This may nave been a colloid similarin nature, if it did not actually take a globular form. The globulespart with their soluble silicious cont ents easily, and, like spicules,are frequently seen to he hollow. They afterwards become infilledwith crystalline chalcedony and become lost in the jnass of it.

This leads me to remark in passing that while the minuterforms of spicules, the microscleres, disappear, I have some doubtwhether the larger spicules (megasc1eres) are often so utterlyobliterated in silicious concretions that no trace whateverremains. Even in the coarsest radiately fibrous chalcedony ofspiculiferous rock traces of them can be seen. In thin slices offlint spicules may seem absent when viewed by transmittedlight, but by polarised light their form is distinctly shown by thelarger and more brilliantly refractive crystals. These may beresiduary canals only, or spicular casts, again infilled by crystallinesilica. Sometimes they are most delicately outlined, and some­times, both in flint and chert, there only remains an irregular lineof minute spheres (usually opaque), once globular silica whichinfilied the canal. In one way or another I think the presenceof spicules is nearly always manifest.

It will be conceded that the greater part, if not. the whole. ofthe silica of the cherts of the Vectian and Selborni an depo sitswas derived from sponge spicules. There was here a great erconcentration of organi c silica than in the Chalk , the deposititself was more porous, enabling an easier interchange of siliciousmatter. Under these conditions the silica, thoroughly permeatingthe rock, has crystallised in coarse-grained or radiately fibrouschalcedon y.

Again, the final stage of crystallisation is conjectural: probablyit began belore the raising of the Greensands into- dry land, buthaving regard to the nature of the rock and the solubility of aportion of its cont ent s, it is possible that the interchange of thesilicious material continues to the present day.

Nowhere is th e replacement of the leading features of acalcareous rock more strikingly manifest than in the concretionsfrom the Portland Beds. Here, as usual, the aggregation s ofspicules are associated with fi brous chalcedony, while the silica inits crystalline conditions follows closely the different forms ofcalcite which it replaces . The oolitic grains where they occur,formed of amorphous calcareous material. a chalk in fact, thoughnot so pure, are, like our chalk flint, replaced by minut ely crystalline silica, the broad calcitic crystals make way for radiatelyfibrous chalcedony, and the finer port ions of the limestone arereplaced 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 format ion ofthese concretions is difficult to follow. To be logical one mustinfer that the solution of their silica was brought about by thesame chemical processes as those in the Chalk and Greensands,viz., the action of sea-water under pressure, or of decaying animalmatter. But their action affected the spicules in a different way,for in all the specimens examined, either of the limestone or ofchert, 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 containingthe concretions could hardly have been a soft ooze, for it is madeup of grains which had attained a considerable degree of hardness.In my opinion these have not been developed in the rock afterits deposition as a soft mud, though it is possible they may havebeen formed on the sea-floor by the crystallisation of calcite andsubjected to a slight rolling by the action of a current.Fragments of shell and indications of other calcareous organismsto be seen in them appear to have acted as a nucleus, but I havebeen unable to detect a single fragment of a spicule occupyingsuch a position. Spicules are not as a rule massed or sorted,they are fairly distributed through the material of the rock, thoughthey are rather more abundant in the concretions.

The original deposit of these beds seems to have been madeup of hardened grains of calcareous matter, a few shell fragmentsand other remains of calcareous organisms with which spiculesare intermixed. Though the interstices between these largerfragments were in some places filled with calcareous mud, therock as a whole must have been porous, a condition favourable tothe rapid deposition of crystalline calcite, which in the limestonehas 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 weare met again by that seemingly inexpl icable problem : why itsegregated in concretions at this particular horizon.

The theory of the possible combination of silica with theglairy animal matter and the gradual elimination of theamorphous material of the ooze seems, as a whole, hardlyapplicable to this case, for one of the early changes appears tohave been the crystallisation of the calcite ill the grains and inthe interstices between them and the coarser ingredients of thedeposit. Though there may have been a redeposition of silicaaround small congeries of spicules which served as nuclei anda replacement of the finest part of the material as in the chalk,possibly coincident with the formation of crystalline calcite, theevidence in the sections suggests that the replacement of thelatter was a secondary operation brought about by the slowcirculation of water charged with silica within the rock. The factthat 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 (YOREDALESERIES), THORPE EDGE, NEAR REETH, YORKSHIRE, THE "MAIN

CHERT."

x 45.

FIG. 2.-CHERT FROM THE CARBONIFEROUS LIMESTONE (YOREDALESERIES), NEAR REETH, YOl<KSHIRE. CONTAINING MANY FRAGMENTS

OF CALCAREOUS ORGANISMS,

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 throughthem that water would find an easy passage. Moreover,crystalline calcite offers more resistance to the invasion of silicathan amorphous matter; in the larger calcareous organic fragmentsthe resistance is stilI greater. In few of these concretions is therock perfectly silicified, in fact none of my specimens show com­plete silicification. Both in the Portlands and in the chert ofthe Carboniferous Series broadly crystalline calcite seems to bereplaced by fibrous chalcedony.

The above remarks may also be applied with greater or lessforce to the concretions of the Chert Beds, but in view of thelength to which this address is extending, I feel I must curtailmy remarks on this very interesting subject.

But there is another feature which I must notice, for it appliesnot only to certain concretions from the Portland Beds, but alsoto the silicious rock of the Carboniferous and Culm Measures.

In the Chalky Portland limestone of Chilmark, in the lowerr2 ft. of the silicious rock at Reeth, and in some of the siliciousbeds in the radiolarian rocks of the Culm Measures, there islittle evidence that silicious organisms ever existed in sufficientquantity to account for the amount of the concretionary silica.

In the case of the concretions of the white limestone atChilmark the question of " where did the silica come from?" is adifficult one to answer. It is there, and in its segregation asnodules it appears to have behaved in the same way as that whoseorigin we have little reason to doubt. It has been said thatwhere concretionary silica is found in sedimentary rock, its origincan usually be found in silicious organisms not far away, but inthis case I can see no sufficient traces of organisms either in thesuperincumbent rock of the Portlands or of the Purbecks.Having regard to remarks made on page 89, I do not believe thatspicules in sufficient quantity to account for the concretions inthe bed have actually disappeared, the few that do occur show nosign of great solvent action. There are, however, in this section,a comparatively small one, vertical cracks or jointings filled withsilica (flint), which, I think, indicate the downward passage ofwater charged with silica, and some of this may have segregatedas nodules, though its actual source is obscure.

In the lower part of the silicious rock at Reeth there is alsoa dearth of sponge spicules, but a possible origin of the silica isnot far to seek.

If the action of decaying organic matter on the solublesilica of the sponge spicule is theoretically correct, nowhereshould its action be displayed in a greater degree than in themass of spicules in the upper part of the bed, for here theremust have been an enduring growth of sponges, and decayingprotoplasms and glairy matter must have been mixed with theirsilicious 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 beena general soluti on and redeposit ion of the organic contents of therock as a whole. It is not surprising therefore to find the rockimmediately beneath the spicules permeated with silica probablycarried downwards by infiltrating water.

And thi s argum ent I would extend to the radiolari an rocksof the Culm Measures . The authors of the papers on theserocks state that where radiolarians are most numerous there thesilica of the chert displays the lively tints of chalcedony. It isjust here that the amount of animal matt er would be found ingreatest abundance mixed with the radiolari an ooze, and theeffects of its decay on the silicious debris would be most marked.I infer that the shaly rock of the Culm Measures, in which littleor no traces of silicious organism s can be seen, has been infiltratedwith the silica derived from the radiolaria.

In the Carboniferous limestone from Bakewell we find a rockwhich, under the microscope, seems to have been a depos it havingmuch resemblance to an impure chalk, and within it we find aconcretion almost identical with our typical flint; other specimensfrom this formation seem to have been formed in a similardeposit and show silica which crystallised in a minute or crypto­crystalline form.

But where the spicules are aggregated togeth er the silicaassumes either a coarsely granular form or the radiately fibrousaspe ct of chalcedony.

Many specimens of Carboniferous chert must have been analmost pure deposit of sponge spicules with but little adm ixtureof calcareous material. They are obviously not due to thesegregation of disseminated silica in a calcareous rock, but arepractically the rema ins of sponge beds. In others the spicul es,almost as numerous, appear to have been embedded in anamorphous calcareou s sediment without marked characters. Thisis now crystalline calcite, often , in part only, replaced by thesilica, the chert being an int imate mixture of the two. Th e wallsof 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 specimensfrom Thorpe Edge are due to the permeation of the limestone bysilica probably der ived from spicules distributed through thedeposit, the replacement of the calcite occurring under conditionsak in to those of the con cret ions in the Portlands, but it may benot ed in passing that the calcite of the organisms has resisted theinvasion of the silica to a greater degree than that which formsthe cementing material.

In the Reef Knoll limeston e of Cracoe there is again someapproach to the conditions of the Portland Beds, but in the place

... Radiolarian Rocks of the Culm Mea su 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 silicafollows, as usual, the variations in the character of the calciticcrystals which it replaces, but the organic fragments are onlypartly silicified.

The radiolarian chert from the Culm Measures presents uswith new features. From the analysis given, the rock appears tocontain little or no calcite, and therefore there is no replacementof the original sediment. Silica derived in all probability fromthe radiolarian debris has permeated the rock and has concen­trated, not in nodules, but in well-defined layers. Though thesilica is described as minutely crystalline and like that of flints,yet, as soon as sufficient organic silica is present, well-markedchalcedony appears.

7.-CLASSIFICATION OF SILICIOUS CONCRETIONS.

From amongst the various forms of silicious concretionswhich have now been described, the difficulty of separating chertfrom flint on the ground of optical structure will be recognised.If each variety of silicious concretion is taken as a whole it willbe evident that a large proportion of radiately fibrous chalcedonydoes not necessarily constitute chert, for we have seen that manyof those of the chalk contain much of it, while on the otherhand the so-called chert may contain a large proportion of silicain a condition which can be compared with our typical section ofblack flint.

Nevertheless, if the gradations are acknowledged, there is atone extreme silica in a minutely crystalline condition-which wemay recognise as flint-at the other radiately fibrous chalcedony-which we may recognise as chert-silicious concretions fromvarious formations being usually a mixture of the two and theirgradations in various quantities, the so-called chert having, as arule, the larger and more constant proportion of radiately fibroussilica. The presence of this, together with a considerable amountof inorganic matter in those with which the term "chert" has longbeen associated (the concretions of the Selbornian and VectianSands), gives the so-called chert a rougher and more uneventexture than the flint. Thus I am in agreement with MissRaisin, who says: "Notwithstanding these gradations thesedistinctions (of chert and flint), especially that of texture, mightperhaps be used if we apply' them to any mass or fair-sizedspecimen, 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 fromimpurities 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 atest as any; but even then, if it has been freshly quarried and wet,and the silica chiefly in a minutely crystalline condition, thefracture may be conchoidal.

Leaving out those intermediate varieties which graduate fromone to the other, and taking the extreme forms of siliciousconcretions found in the sedimentary rocks, they may perhaps beroughly 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 fibrouschalcedony, graduating to granular crystalline... ... Chert.

3. Spiculiferous sands and rock, the cementing materialcoarsely 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 spiculessoluble in alkali ... ... ... •.. ••• ... Opalite,

7. Fine inorganic detrital matter, permeated with silica in aminutely crystalline condition. Hornstone.

Though these divisions may indicate the general structure ofsilicious concretions, variation in the quantity and kind of theseveral ingredients is the rule rather than the exception, and itwould be often difficult to say to which type any particularspecimen belongs.

Thus while flint and chert, from long usage, may be convenientnames to distinguish certain forms of silicious concretions in arough-and-ready way, no definite scientific value can be attachedto these terms, for chert and flint may pass insensibly the one tothe other.

As I have already said: "The special characters of flint andchert are due to the nature of the rock in which they were formedand the conditions under which the silica segregated into asilicious concretion."