196 



KNOWLEDGE. 



[October 1, 1892. 



Fig. 2. — Artificial 

 Cone of Flint. 

 (From Sir J. 

 Evans.)* 



broken flake to a series of taps from our hammer, we 

 shall find that a number of smaller flakes will be readily 

 chipped off, each leavinj; a separate conohoidally-fractured 

 surface on the original flake. It is this facility with which 

 flint can be chipped, coupled with its hardness and the 

 sharpness of its fractured edges, that induced our paheo- 

 lithic ancestors to adopt it as the material for their 

 various weapons and tools. The foregoing figure of 

 one of these implements exhibits ' in great perfection 

 the characteristic conchoidal fracture of flint. 



The extreme development of this peculiar and character- 

 istic fracture is, however, exhibited when a large flat 

 surface of flint is struck at right angles by a round-ended 

 hammer. The hammer then comes in contact with a 

 minute portion of the surface of the flint, which may be 

 represented by a small circle, and as the flint is elastic, 

 " this small circle," as Sir .John Evans 

 observes, "is driven slightly inwards into 

 the body of the flint, and the result is that 

 a circular fissure is produced between 

 that part of the flint which is condensed 

 for the moment by the blow, and that 

 part which is left untouched. As each 

 particle in the small circle on which 

 the hammer impinges may be con- 

 sidered to rest on more than one other particle, it is 

 evident that the circular fissure, as it descends into the 

 body of the flint, will have a tendency to enlarge in 

 diameter, so that the piece of flint it includes will be of 

 conical form, the small circle struck by the hammer 

 forming the slightly truncated apex." A little practice 

 will enable anyone to make these flint cones with ease. 

 The size of the cone, and the degree of steepness of its 

 sides, vary with the nature of the flint, the weight and 

 form of the hammer, and the force of the blow. 



When examined with a lens or microscope, chalk-flint 

 frequently exhibits a perfectly uniform structure throughout, 

 without the least trace of the presence of any organic body. 

 In other cases, however, traces of sponges, corals, shells, 

 echinoderms, diatoms, &c., are more or less apparent in 

 flint. Perhaps the most common of all these organisms 

 are the mushroom-shaped sponges known as ventriculites ; 

 and if we examine a flint containing one of these sponges 

 we shall frequently observe that there is a complete 

 transition from portions of the perfectly preserved sponge 

 to homogeneous flint, without the least trace of organic 

 structure. In the case of echinoderms, we shall find that 

 while in some cases the whole interior of the shell (or, as it 

 is technically called, test) is filled with flint, the shell 

 itself retaining its original calcareous structure, in other 

 instances the original shell itself has been completely 

 removed and replaced by flint. In some cases the original 

 structure of the shell has been preserved in the flint, but 

 more generally this has been completely lost, and the flint 

 is structureless. 



This replacement of a calcareous by a siliceous structure 

 is an instance of pseudomorphism. Similar remarks will 

 apply to the shells of molluscs, and likewise to corals. It is 

 important to mention that the sponges found in flint were 

 not originally of the horny nature of our bath-sponges, but 

 were themselves composed of minute spicules and fibres of 

 silica, like the so-called Venus's flower-basket of our modern 

 seas. 



With regard to the mode of occurrence of flint, we have 

 first to mention that it is by no means confined to the 

 chalk, but may occur in limestones of any age. In this 



* We are indebted to Messrs. Longman and 

 and the figure on the previous page. 



G-reen for this 



country it is, however, more abundant and purer in the 

 chalk than in any other formation, and may, indeed, be 

 considered characteristic of the upper part of that 

 formation. I Flints are found in the chalk either in the 

 form of nodules or in thin continuous lamina?. The 

 nodules are generally of very irregular shape, and may 

 vary in size from a walnut to masses of a hundred-weight 

 or so. As a rule they occur in strings at comparatively 

 regular intervals in the chalk, generally conforming more 

 or less closely to the original planes of bedding, and the 

 individual nodules being sometimes at considerable distances 

 apart, but at others closer together and more or less 

 connected by long root-like pieces. On the other hand, 

 the laminated or tabular flint may cut the bedding-planes 

 of the chalk at any angle, and is often found in joints and 

 fissures, which may emerge at the surface. As a rule, this 

 tabular flint is devoid of organisms. Not unfi'equently 

 flints may be found which near their surfaces gradually 

 become paler and paler in colour, and contain an increasing 

 amount of calcareous matter, till they pass imperceptibly 

 into hard siliceous chalk. Again, flints may be hollow, 

 and contain in their cavities either corals or sponges, or 

 masses of that variety of silica known as chalcedony. 

 The latter, it may be mentioned, is a semi-transparent 

 waxy-looking stone, generally with a more or less decided 

 pinkish tinge, and forming mammillated or botryoidal 

 masses. The milky and reddish varieties of chalcedony 

 constitute carnelian, while when it is arranged in dif- 

 ferently coloured bands it forms agate. Occasionally 

 small crystals of quartz occur in the hollows of flint. 



It has still to be mentioned that in some districts — and 

 more especially near Norwich — in addition to the hori- 

 zontal layers of nodular flints, there occur in the upper 

 chalk a number of huge cup-shaped masses of flint placed 

 one above another in vertical lines ; these masses being 

 locally known as "potstones," and presenting a remarkable 

 resemblance to certain giant sponges called Neptune's 

 cups. 



The proportion of the flint to the chalk in the upper 

 chalk of England varies, according to Prof. Prestwich, 

 from fom- to six per cent. It is important to add that the 

 masses of nodular flint may not irafrequently be found to 

 be traversed by fractures which have subsequently been 

 reunited ; thus showing either that the substance must at 

 the time have been in a semi-plastic condition and capable 

 of reunion, or that the fracture has been united by the 

 subsequent deposition of siliceous matter. It must also be 

 mentioned that, as a rule, the white coating is confined to 

 the outer surface of the nodules, and that we do not find 

 layers of pure flint overlain with white coats and then 

 again by other similar layers ; thus indicating that the 

 formation of the white coat was the final act in the 

 development of flint. 



With these observations on the structure and mode of 

 occurrence of flint, we are in a position to enter on the 

 more diflicult subject of its origin. From the very first it 

 was recognized by all geologists that such a peculiarly 

 hard and homogeneous substance as flint, occurring in 

 irregular nodules among the pure white chalk, could not 

 have been deposited in its present condition directly from 

 the waters of the cretaceous sea ; and the problem there- 

 fore presented itself to account adequately for its mode of 

 formation. The problem soon resolved itself first of all 

 into two questions, namely, whether the silica was originally 

 part and parcel of the chalk as first deposited and that it 



t It also occurs abundantly in the lower part of the Portland stone 

 series of the Isle of Fori land, but is there generally less pure, and 

 has tlie conchoidal fracture less marked. 



