Flint and Chert

61 FLINT AND CHERT. By WILLIAM HILL. F.G.S. (Presidential Address, delivered February Srd, 19/1.) CONTE:'l"TS. PAGE I. !)lTRODUCTION. 61 2. T\'PES OF FLINT AND CHERT (CRHACEOUS) 64- 3. VARIETIES OF FLINT FROM FORMATIONS OTHER THAN THE CHALK 69 4. VARIETIES OF CHERT •••" 71 Cherts of the Upper and Lower Greensand. 71 Chert of the Portland Beds. 73 Carboniferous Chert • 76 Chalcedony in Flint . 80 Chert of the Culm Measures 82 5. IMMATURE FLINT AND CHERT 83 6, GENERAL SUMMARY 85 7 CLASSIFICATION OF SILICIOUS CONCRETIONS 93 1. INTRODUCTION. VE RYONE familiar with the White Chalk of England is E also familiar with the nodules of a different substance which are scattered through it. Break one, the fracture will be conchoidal, and it will be found that within a white rind of greater or less thickness is a hard, black, translucent material, possibly with some cloudy patches, which we recognise as flint. How and when the term flint became applied to these nodules in the Chalk is a matter of some obscurity. We know that Early Man recognised the value of flint for offensive, defensive and domestic purposes, and until quite recent years it played no unimportant part in the economies of mankind. The word probably came to us from the Saxon "flinta." It is used six times by the translators of the Authorised Version of the Old and New Testament, though it seems to be applied to rocks of exceptional hardness, and not to be confined to what we know as flint. Virgil, Pliny, and Lucretius use the word silex in connec tion with striking fire. According to Murray the word occurs in the Epinal Glossary about A.D. 700, page 8°5; it is there referred to as Petra-focaria, flint. In another Glossary, A.D. 1050, it is again described as Petra-focaria, "jryslan"· "jlyllte," and thenceforward the word occurs frequently in con nection with fire and fire-arms, and is certainly used to describe PROC. GEOL. Assoc., VOL. XXIl, PART 2, 19I1.] 6 62 WILLIAM HILL ON the material which occurs as nodules in the Chalk. So closely indeed is chalk and flint connected in our minds that it is hardly possible to think of one without recalling the other, and if a geologist were asked where flints were to be found he would almost certainly refer the enquirer to the Chalk, therefore we may safely take one of these silicious nodules as the type of true flint. They are, as we all know, segregations of silica, probably derived from organisms which lived in the Cretaceous sea, but the chemical processes which have involved the disappearance of the silica of sponge spicules, the tests of Radiolarians, or of the frustules of Diatoms from the chalky deposit, and its re-appear ance in the form of flint nodules, though not perhaps so great a mystery <IS it was thirty years ago, is yet far from being understood. Flint not only occurs in the form of nodules arranged in lines or scattered promiscuously through the rock, but also in thin con tinuous sheets of considerable extent to which the name tabular flint has been given. While the lines of nodules and sheets of tabular flint usually correspond with the bedding plane, seams of flint not infrequently occur running at various angles, sometimes even at right angles to it, and appear to have infilled the cracks or jointings which are common in the Chalk. The nodules, which are as a rule easily separable from the surrounding matrix, are often very irregular and sometimes fantastic in shape. They vary greatly in size, from the huge Paramoudra, "rudely cylindrical or cup-shaped, with a central cavity, often two or three feet in height with a width of one or two feet,"· to the small rounded nodule of finger-shaped flint not an inch in its greatest diameter. Their form not in frequently gives some indication of an organism around or within which the flint had segregated, many taking the shape of sponges, the original outline of the sponge being closely followed, but as often the shape of the flint is uninfluenced by anything which can be regarded as a nucleus. Sometimes the flint has infilled the hollows of a calcareous organism, and when the calcitic test has been removed, a more or less perfect cast remains. The interior casts of Echinodeons are common instances of this. Flints with special characters, such as a greater or less thick ness of rind, pinkish coloration, or zoned structure, are met with commonly in some localities, but these characters can only be regarded as local peculiarities and are of small value as guides to the horizons of chalk. Scarcely less familiar than chalk flints are the silicious concre tions known as chert, which occur in the Vectian and Selbornian (the Lower and Upper Greensands of our Home Counties), and in 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-like masses and in tabular sheets arranged parallel with the bedding planes, and in seams and cracks at various angles to it. But while flints occurring as well marked nodules or layers are as a rule easily separated from the chalk, the boundary of cherts is not so well defined, and there is usually a broad crust of partly silicified material which passes gradually into the sur rounding rock, from which the chert IS frequently not easily separable. The term chert seems to have been used nearly 200 years ago, for in 1729, in vol. xxxvi, page 30, of the "Philosophical Transactions," Martyn described chert as "a kind of flint, so called when it is found in thin strata, but in the Peak the strata of chert may be four yards thick." Thus it seems clear that at this date concretionary silica differing from flint was recognised. Silicious rock, usually called chert, occurs in many formations, and in many localities, besides those above mentioned, sometimes in solid beds of great thickness. In the Carboniferous series in North Wales, Dr. Hinde says, "there is a continuous series (of cherr) 350 feet in thickness without the intervention of lime stone," again "in some Yorkshire areas there are chert beds 18 feet in thickness without a break."· Mr. H. B. Woodward describes a "compact chert containing shells occurring in the Lower Lias at Chewton Mendip on the Harptree and Egar Hills, at East End, Emborrow and near Binegar." It is massive bedded chert, occurring in layers one to three feet in thickness, separated by thin ochreous clayey beds an inch or two thick.t Dr. G. J. Hinde and Mr. H. Fox describe" Cherty" rock of very considerable thickness which occurs in the Lower Culm measures of Devonshire.j But while it is easy to direct an enquirer where to find a typical example of flint, it is by no means so simple a matter in the case of Chert, for, broadly speaking, the term chert has been applied to silicious concretions in all formations other than the Chalk, though W. H. Hudleston§ has called certain compact nodules occurring in the Portlandian at Chilmark" flint." Few geologists have given more attention to chert from various formations than Dr. G. J. Hinde, in his paperII 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 at Haslemere (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 and under layer of yellowish, porous, translucent rock. The outer porous 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 sponge spicules." In the central layer of massive chert: "The silica derived from the solution of the spicules of the outer crust appears, in part at least, to have been redeposited in this central layer in the form of chalcedonic chert, and the spicules of this layer have been thus enveloped by the cherty matrix." In referring to the chert of the Lower Greensand of Folkestone, he says: "There is in some beds a central layer of cherty rock, but this is usually porous, though in places it passes down into true chert." " The cavities in the cherty portion of the Sponge Beds are frequently lined with a smooth layer of chalcedonic silica, which shows under the microscope a radiately fibrous structure. When the cavities are infilled by successive layers of this silica ... a true chert is produced." The term chert, therefore, may here be taken to apply to a solid, translucent, silicious material, made up in this case of a matrix consisting largely of radiately-fibrous chalcedonic silica, in which are many traces of sponge spicules and other organisms, together with some inorganic material. Miss Raisin * has communicated to the Association a valuable contribution to our knowledge of the Micro-structure of Jurassic Chert.

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