284 



NATURE 



[June 7, 1917 



water containing CO^ in solution, and thus gives rise 

 to the white crust of "decomposed" flint which forms 

 the outer "cortex" of all chalk flints. 



(i) A fact of capital importance, which must affect 

 any theory as to the origin of flint, is that 

 in many localities where a chalk escarpment 

 can be studied it is found that extensive 

 fissures traverse the stratified layers of chalk and 

 flint nodules at a sharp angle, and are filled with a con- 

 tinuous sheet of black, tabular flint. Such fissures may 

 be seen in the cliffs at Rottingdean, near Brighton, 

 often cutting through a thickness of 40 ft. or 50 ft. of 

 the stratified chalk obliquely to the plane of the strata, 

 and from these fissures sheets of flint 3 ft. square and 

 5 in. to I in. in thickness can be readily removed. 

 The size of these sheets of flint in situ is apparently 

 limited merely by the vertical height and inward ex- 

 tension of the fissure. The occurrence of these 

 deep and extensive fissures — mere cracks in so 

 far as their width is concerned — ^filled with 

 a continuous deposit of black flint, makes it 

 certain that the flint was deposited after the fissur- 

 ing of the chalk, and therefore, almost certainly, after 

 the elevation of the chalk, and probably through the 

 operation of fresh-water of atmospheric origin pene- 

 trating the porous mass of chalk after its elevation. 

 It is improbable that the nodules of flint in the chalk 

 have an origin different from that of the " tabular " 

 flint of the fissures. I am not able myself to bring 

 forward any parallel case of the filling of extensive 

 cavities and fissures in a sedimentary rock by a dense 

 chemical deposit. The formation of " septaria " in 

 clay is a parallel on a very small scale. The student 

 of mineral veins and deposits may perhaps be able to 

 throw sorne light on the matter. 



(2) A further fact of importance to any theory of 

 the origin of flint is that the black colour of flint — 

 yellowish- or greenish-brown in thin splinters — is prob- 

 ably due to carbon, though no explanation has been 

 offered of the uniform association of this element with 

 flint. The existence in the Upper Chalk of oblong 

 cylindrical nodules of perfectly colourless transparent 

 quartz, occasionally showing blue or orange-brown 

 patches or "floating clouds" scattered in the clear 

 colourless silica, is also well known. A fine collection 

 of these has been bequeathed to the Woodwardian 

 Museum, Cambridge, by the Rev. Marmaduke Lang- 

 dale. They are deposited in cavities once occupied by 

 peculiar sponges (Choanites and Ventriculites). But 

 why they are free from carbon — if carbon is the cause 

 of the black colour of black flint — is not explained. 



In the spring of 1916 a combustion analysis of black 

 flint was made in the laboratory of the Royal Institu- 

 tion at my request, under Sir James Dewar's direction. 

 The result pointed to the presence of minute quantities 

 of carbon in the flint. But a very remarkable result 

 was also obtained which it is necessary to re-examine 

 by employing black flint from various localities in 

 such combustions. A definite quantity of arsenic was 

 deposited in the combustion tube in the form of the 

 well-known arsenical mirror. Care was taken to 

 check this startling result by exclusion of the possi- 

 bility of accidental impurity in the material used. But 

 I have not been able myself to pursue the matter 

 further, and mention it now under all reserve, in the 

 hope that some expert chemist will inquire into 

 the subject. I am afraid that Sir James Dewar, to 

 whose kindness I owe this initial examination of the 

 chemical constituents of black flint, will not be able 

 to give the necessary time to it. 



I may add that another matter inquired into at my 

 suggestion was as to the amount of removable water 

 present .in normal chalk flint as quarried, and the per- 

 centage of its own weight of water, which carefully 

 dried black flint pan absorb, and the rate at which 



NO. 2484, VOL. 99] 



the water is absorbed ; further, the variation in these 

 amounts caused by variation of temperature, and the 

 question as to whether a sudden raising or lowering of 

 temperature causes the fracture of wet flint more 

 readily than of dry flint. Experiments were also made 

 as to the form of fracture caused by thermal changes 

 in flint, with the view of determining whether the 

 conchoidal fracture can be produced in flint by thermal 

 change alone, without the previous creation of struc- 

 tural strain by a blow. Although I am not able to 

 report the results of these experiments, I wish to 

 bring the desirability of a thorough chemical and 

 physical examination of black flint to the notice of 

 others who may have facilities for carrying through 

 such an examination which I do not possess. I trust 

 that some such fortunate experimentalist will take up 

 the chemical and physical investigation of flint, with- 

 out delay, as a serious task. It will take perhaps years 

 to complete, but will yield results of the highest value 

 to geology and to other branches of science. 



The occurrence of arsenic in chalk flints may be -due 

 to its presence in minute quantities in sponges, the 

 peculiar, smell of which in the living state is suggestive 

 of the presence of an organic compound similar to the 

 strongly odorous gas known as diethylarsine. 



May 25. E. Ray Lankester. 



Plated Teeth of Sheep. 



The subject of Mr. Beeby Thompson's letter 

 (Nature, May 31, p. 264) has been noted 

 in various parts of Great Britain. Writing in 

 1684, Andrew Symson, minister of Kirkinner, 

 records in his " Large Description of Gallo- 

 way " that " in this parish [Glasserton] there is a hill 

 called the Fell of BaruUion, and I have been told, 

 but I give not much faith t.o it, that the sheep that 

 feed there have commonly yellow teeth, as if they were 

 guilded." 



In this matter the worthy minister was unduly 

 sceptical. The Fell of Barhullion is on my property, 

 and jaws of sheep fed thereon have been brought to 

 me with the teeth thickly plated with iron pyrites. 

 The rock of the 'district is Lower Silurian ; in the softer 

 parts (Moffat Shales) large nodules of iron pyrites 

 are found. As there is wet peaty soil on parts of the 

 fell there is no lack of humic acid. 



Herbert Maxwell. 



Monreith, June 2. 



The Stability of Lead Isotopes from Thorium. 



Prof. Soddy's view (Nature, May 24, p. 244) in- 

 volves the disappearance from the 20 kilos of thorite 

 (with which he worked) of some 150 grams of unstable 

 lead and its conversion into (orobably) thallium. There 

 should be present in the thorite thallium to the 

 amount of about 0012 gram per gram of thorium. 



Such a quantity should be easily measurable. If 

 found to be present, support would be given to Prof. 

 Soddy's suggestion. We are told that the thallium 

 was present in amounts " that sufficed for chemical as 

 well as spectroscopic identification." 



There is some difficulty in understanding how two* 

 thirds the ionisation of a ray, additional to the seven 

 a rays which go to generate the thorium halo, 

 can have left no trace upon the halo. But 

 the range may have been such as to render this pos-! 

 sible. It is improbable, however, that a furthef 

 a-ray transformation of thallium can have occurred, 

 without affecting the ionisation curve to an extent 

 which would be detectable when the halo is compared 

 with the curve as determined from the seven known 

 rays. J. Joly. 



Trinity College, Dublin, May 29. 



