NATURE 



[November 21, 1912 



CavitiL'ti occur in chalk flints as in the agate of the 

 geodes qi igneous rock, and rocli crystal, as well as 

 botryoidal chalcedony, is often found lining the cavi- 

 ties of tlie flints as of the agate geodes. Occasion- 

 ally crystals of iron sulphide occur in chalk Hints. 

 W'c require some definite classification and recogni- 

 tion of the varieties of chalk flint and their probable 

 significance. The hardness, fracture, density, and 

 especially the elasticity of each kind of flint must be 

 measured and stated. 



(2) As to the origin and formation of flint, our 

 knowledge seems to be very little further advanced 

 than it was fifty years ago. The microscopic examina- 

 tion of thin sections of flint has not been applied to 

 many varieties of flint, and, so far as I can ascertain, 

 possible methods of staining thin sections and of 

 applying light, heat, and chemical agents to the detec- 

 tion of structure and dilTerentiation in the substance 

 of thin sections of flint, examined with the micro- 

 scope, have not been thoroughly and extensively used. 



It appears to be held that 'the normal chalk-flint 

 consists of extremely minute crystals of silica, 

 cemented by opaline silica, and that the white cortex 

 which every chalk-flint possesses is due to the removal 

 of the opaline cementing colloid silica from the cortical 

 region by solution. One would like to know more 

 about this as the outcome of experiment. What is 

 the solvent? Re-deposited flints in Pleistocene gravels 

 are often opaque ("decomposed," it has been called) 

 right through, and in some cases are pulverulent. 

 How has this been brought about? Broken flints 

 (flint implements) in Pleistocene gravels sometimes 

 show a curious basket-work of white bands crossing 

 and interlaced on a black ground. Is this white 

 pattern a pre-existent structure developed by the action 

 of a solvent? Can such a change be produced experi- 

 mentally? 



There is no general agreement as to the mode of 

 origin of the flints in the chalk. It is clear from 

 the existence of tabular flint in vertical and oblique 

 fissures traversing great thicknesses of chalk that 

 the flint was deposited in cavities formed after the 

 solidification of the chalk. It is also probable that 

 the silica deposited is the opaline or colloid silica of 

 the spicules and shells of marine organisms mixed 

 up with calcareous particles in the original chalk 

 ooze, and dissolved out of it by percolating water 

 containing some solvent— but what? What are the 

 circumstances which have determined (i) the solution 

 of the colloid silica of spicules, and (2) its deposition 

 in the form of cavity-filling masses consisting of 

 minute crystals cemented by colloid silica? 



The cavities in which the nodular flints were 

 formed were probably once filled bv organic lumps 

 and debris, but it is questionable whether the organic 

 matter attracted the silica and determined its deposi- 

 tion (although we know this occurs in the silicification 

 of tree-trunks in the sea), since flint is deposited freely 

 m the tabular form in the upper chalk, in vertical 

 fissures containing no organic residues. In what 

 respects (one would like to inquire) is the mode of 

 deposition of chalk-flint similar to, and different from, 

 that of chert on the one hand and of geode-agate on 

 the other? The solubility of the colloid silica of 

 organic skeletons requires investigation. The silica 

 deposited as agate in trap-rocks had probably a 

 different origin from that of flint. 



(3) Apart from these questions as to the intimate 

 structure of flint, its varieties, and its origin in the 

 chalk, there are certain more direct and simple 

 physical investigations of flint which are necessary, 

 and would hein us in distinguishing varieties of flint, 

 and perhaps throw light on other questions. They 

 certainly would render it possible for archreologists 

 NO. 2247, VOL. 90] 



to speak of facts and not merely make guesses as to^ 

 the causes of the fracturing of flints found in Pleis- 

 tocene, Pliocene, and other lertiary deposits. 



The most important of these mquiries are (i) as to 

 the ■porosity ot flint, and (2) as to the fracture of 

 flint by blows, by pressure, by heat, and by cold. 

 The two inquiries are closely related. It is well 

 known that an agate geode is porous, and will absorb 

 a large quantity of water containing colouring matters- 

 in solution. Our chalk flints are also highly porous 

 and absorbent of water. But, so far as 1 can ascer- 

 tain, this property has never been investigated quan- 

 titatively. It should be determined experimentally in 

 the case of normal black flint from the chalk (and in 

 varieties of it and in allied bodies). We require to- 

 know — 



(i) What is the difference in the specific gravity 

 of f!int fresh from the chalk, and of carefully dried 

 flint, from which all free water has been removed by 

 non-destructive methods of desiccation ? 



(2) What is the maximum amount of water which 

 such a specimen of dried flint can be made to absorb? 

 We could thus get the coefficient of absorption of 

 water by flint at various pressures and temperatures, 

 also of ilint lying naturally in the chalk, as compared 

 with flint when lying on the surface and under 

 various other conditions. 



(3) Other facts as to this porosity could be accu- 

 rately determined, as, for instance, in what way it is 

 related to structure. Coloured substances might be 

 forced into the pores, as also chemical solvents, and 

 microscopic examination of thin sections made with 

 very high powers. 



The investigation of fracture is closely related to 

 the foregoing. The most familiar and certain cause 

 of the fracture of flint is a blow with a hammer 

 wielded by a man. Many archaeologists are (I have 

 found) not aware that according to the character of 

 the blow given flint may be broken with a practically 

 flat surface of fracture, or, on the other hand, with 

 what is called "a conchoidal fracture." The flint- 

 knappers of . Brandon break the large masses of flint 

 removed from the chalk into blocks of convenient 

 size by heavy blows given with what they call "a 

 quartering hammer." The surfaces of fracture so 

 obtained are not "conchoidal." A heavy blow in a 

 direction perpendicular to the surface gives this plane 

 fracture. The lighter knapping hammer gives the 

 kind of blow which produces a conchoidal fracture, 

 and the flint w-orkers can produce complete cones of 

 flint at pleasure by giving the needful kind of blow. 



The exact quantitative features of the weight, 

 velocity, and direction of this blow must be determined 

 experimentally, as also must those of the " quarter- 

 ing " or plane-fracture blow. Apparatus to determine 

 these features could be devised. It would then be 

 possible to investigate the exact measurable char- 

 acters of the conchoidal fracture or "cone" or 

 "dome" of percussion, and to compare it in different 

 varieties of flint. It would be very important to 

 determine whether "saturated wet flint" has the same 

 fractural indices as " dry " flint^whether the one frac- 

 tures with conchoidal form as easily as the other, &c., &-c. 

 Then we could arrive at an answer to the question, 

 "What weight and velocity of blow were necessary 

 to produce the fracture (whether conchoidal or plane) 

 exhibited by a given piece of flint?" And so it would 

 be possible to arrive at a certainty as to whether the 

 fractures which give shape to some supposed human 

 flint implements could have been produced bv the 

 inter-concussion of flint nodules driven by the waves 

 of the sea. 



But in this investigation the very important fact 

 would be exactly and quantitatively determined that 



