W. A. Richardson — The Origin of Cretaceous Flint. 539 



A comparison of these curves will bring out the following 

 points : — 



(a) The flinty chalk and the flintless chalk above it are both poor in 

 soluble silica. 



(b) Increased values in the Lower Chalk appear just where the 

 flint is dying out. 



The curves suggest that there is an " inverse relation " between the 

 relative amounts of flint and soluble silica present, and that it is 

 a very striking one. Jukes-Browne compared only flinty and flintless 

 zones of the Upper Chalk, but it is obvious that the whole of the 

 Chalk must be considered, because in this case the comparison 

 suggests that the flint represents the silica once spread out in its own 

 region and in the flintless beds above it. 



If we now test quantitatively the hypothesis that the silica 

 required for flint formation was obtained from that disseminated in 

 the Chalk itself, it will be found that the analyses in Table No. 1 

 give the following averages : — 



Below the Melbourn Rock, mean silica . . 2-9 per cent. 



Above the ,, ,, ,, . 0*3 ,, ,, 



There is thus a balance of some 2*5 per cent in the lower part, which 

 might reasonably be assumed to represent the minimum amount of 

 silica originally available for the formation of flint. It may be noted 

 that this amount is of the same order as the 2 per cent estimated by 

 Mortimer 1 as available for the Chalk of Yorkshire, and as the 

 observation of Sollas that the Upper Chalk may contain as much as 

 3 per cent of hollow casts of spicules. 2 The exceedingly siliceous 

 beds of the Lower Chalk are here left out of account, but it must be 

 remembered that such rich beds may originally have been present in 

 the Upper Chalk. Indeed, the occasional occurrence of desilicified 

 sponge beds rather points that way. Moreover, if modern oozes are 

 tiny criterion as much as 10 per cent of silica in organic form is a not 

 unreasonable figure for the original amount, and it might become 

 necessary to account for the disappearance of a portion. In that case 

 part of the original silica, instead of forming concretions, may easily 

 have been carried off in solution, or have been partially dissolved on 

 the sea-floor, as Murray has recorded for modern deposits. 3 



Taking, however, the 1\ per cent above as a conservative figure, 

 my estimate for the 557 feet of flinty chalk in Kent gives a mean 

 percentage flint content of 6 per cent. And, although there may be 

 elsewhere a greater thickness of flinty beds, there is not, I think, 

 a greater concentration of flint. The amount of flint to be accounted 

 for is not, therefore, enormous, and if disseminated through the whole 

 thickness of existing Chalk would be not much more than 2 per cent. 

 It would, to put it in another way, require about 1,300 feet of Chalk 

 to supply the Kent section, that is less than 700 feet above the flinty 

 region. And if Sollas's figure of 5,000 feet for the minimum 

 thickness of Chalk is correct there was ample for the purpose. 4 



1 R. Mortimer, Proc. Geol. Assoc, vol. xxi, p. 96, 1910. 



2 W. J. Sollas, The Age of the Earth, London, 1905, p. 147. 

 s Ibid., p. 160. 



* J. Murray and J. Hjort, loc. cit., pp. 183-5. 



