November 21, 191 2] 



NATURE 



io'. 



the vibration tending to set up a conclioidal fracture 

 may produce a " flaw " wlien ttie blow causing it was 

 not of sufficient power to cause an actual fracture, 

 -and the subsequent history of such "flaws" would be 

 experimentally studied. 1 have found that one of the 

 most certain ways of obtaining a fine "dome of per- 

 cussion" in black chalk-flint is to strike a "staccato" 

 blow with a light hammer. No fracture results, but 

 subsequent " tapping " with a heavier hammer causes 

 the flint to yield along the dome-like plane of " flaw " 

 set up by the first blow. 



The fracture of flint by blows due to other agents 

 than man has been rarely observed. At a few points 

 on the sea-coast large flint pebbles may be picked up 

 with one, or even six or seven, irregularly placed con- 

 choidal fractures of the size of a haricot 

 bean at most. Observations of the fracture of flint 

 by torrents or by heavy wave-action are not forth- 

 coming. The delicate pitting and granulation of the 

 surface of flint-pebbles on the seashore is due to the 

 action of the sea-waves causing the pebbles to knock 

 against one another, and is a very different thing 

 from large and uniformly "directed" fracture. 



Leaving for a moment the question of the fracture 

 of flint under graduated pressure, we must cite the 

 action of cold and of heat in fracturing flint as 

 demanding careful and quantitative investigation. 

 There is no doubt that in this country the greatest 

 "breaker of flint" is frost. In the Egyptian desert 

 a chert-like substance allied to flint is constantly 

 fractured by the heat of the sun. It is most important 

 to determine whether "wet" and "dry" flint are 

 equally subject to fracture by cold and also by heat. 

 Has the water absorbed by porous flint any important 

 part in its thermal fracture? The artificial fracturing 

 of flint by the heat of camp-fires is well known as a 

 mere fact. But the very curious structure of flint 

 revealed by it has never been investigated. 



I do not know whether anyone else has ever deter- 

 mined the simple fact experimentally that sudden 

 exposure to cold will cause flint to fracture — to " fly," 

 as the expression is in the case of glass. But last 

 July Sir James Dewar kindly placed some large flakes 

 of Brandon flint (prepared bv the flint-knappers for 

 breaking into gun-flints), which I brought to his 

 laboratory, into liquid air in my presence. An exten- 

 sive fracture of peculiar form, its edge having a deeply 

 undulated margin like that of an oak-leaf, was the 

 result. Obviously the whole subject of the fracture 

 of flint by cold and by heat requires experimental 

 investigation, and must yield results of great import- 

 ance. I am not in a position to carry out this in- 

 vestigation myself, nor have I the necessarv training 

 in such determinations. My hope is that some 

 physicist may be attracted by the subject. 



.\n important point which I should wish to deter- 

 mine as bearing on the appearances presented by 

 broken flints in Tertiary strata and gravels is whether 

 ^rost can, in any circumstances, produce a conchoidal 

 fracture in flint. It seems to me not improbable that 

 a flint may by natural (i.e. non-human) blows, or a 

 single blow — insufficient to break it — have acquired 

 conchoidal " flaws," or a single conchoidal flaw, which 

 would be developed as a conchoidal "fracture" when 

 the flint was caused to break by sudden frost. We 

 do not even know whether " suddenness " is an 

 element in the causation by lowered temperature of 

 the fracture of flint. The flints on the surface of 

 chalk downs and in many of our later gravels are 

 one and all broken into irregular angular fragments. 

 This is probablv correctly attributed to frost, but it 

 would be ]iossible to gain more precise information 

 as to the • onditions and determining causes of that 

 fracture, '/he exact temperature at which, under 



NO. 2247, VOL. go] 



varying conditions, fracture occurs and the possible 

 extent and form of frost-fractures could be determined. 

 The same is true with regard to the fracture of flint 

 by heat. 



The investigation of pressure as causing fracture 

 of flint can be accurately investigated. What kind 

 of fractures can be produced by pressure? And what 

 kind of pressure can produce fracture? We have been 

 asked to accept the statement that the pressure of 

 sandy strata overlying flints can fracture them. By 

 many this is considered an impossibility. We are then 

 told of some mysterious kind of rolling or sliding 

 pressure as producing such effects. Its action should 

 be experimentally demonstrated. 



Lastly, in regard to fracture, there seems to be a 

 possibility that vibrations produced by very slight 

 blows may, in special circumstances (such as great 

 cold or heat or dryness), start large fractures in an 

 elastic bodv like flint. The possibility requires experi- 

 mental investigation. 



There remain yet to be mentioned some other 

 matters for experimental investigation in regard to 

 flints. The acquirement of green, of yellow, 

 brown, and rich red, as well as of black colora- 

 tion, both deeply and superficially, by flint 

 nodules and pebbles when deposited in Tertiary 

 strata is one of these. This subject is part of the 

 general subject of the porosity of flint. It has an 

 important bearing on the study of the flint imple- 

 ments found in gravels. Of more peculiar import- 

 ance is the classification of the different states of 

 polish which broken flints, whether implements or 

 not, present in different gravels. And with this has 

 to be associated the study of the chemical and mole- 

 cular changes of the surface of broken flints, and 

 their curious laminar and vermicular sculpturing. 

 Further, the deposition upon those broken surfaces of 

 chemical material requires precise investigation. The 

 " eflaze " of the fragments of bone and teeth in the 

 bone-bed at the base of the Red Crag is usually attri- 

 buted to the deposition on them of phosphate of lime. 



It is not certain that this is a correct conclusion. 

 Is the peculiar glaze of most of the broken flints from 

 that deposit due to chemical action, or are all the 

 glazes supposed to be present on broken flints really 

 onlv different degrees of sand polish effected by wind 

 or by water? 



The wonderful flints found in small number 

 in the Savernake gravel, which look as though 

 they had just received a wet coat of spirit 

 varnish, have never yet been satisfactorily dealt with. 

 Some geologists have supposed that they owe their 

 appearance to a chemical glaze deposited on them. 

 But microscopical sections are absolutely contradictory 

 of that view. Their wonderfully brilliant surface is 

 almost certainlv a water-made sand-polish. But one 

 would like to see such polishing of an irregular surface 

 of flint produced experimentally. And it would be 

 important to know what were the conditions at work 

 at Savernake to produce this polish on small Acheul- 

 lian flint implements, as well as on unbroken flint 

 pebbles of large size, and upon one and not all the 

 surfaces of irregular fragments. 



A detailed knowledge of the causes -,f colour and 

 colour patterns, and of the glazing and polishing of 

 flint implements, would enable prehistorians to give 

 a more complete account of the historical vicissitudes 

 of this and that implement than is at present possible. 

 The most urgently needed of the investigations above 

 sugp'ested appears to me to be the experimental and 

 quantitative determination of the causes and condi- 

 tions of the different kinds of fracture of which flint 

 is susceptible. E. Ray Lankester. 



November 0. 



