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



There yet remains one further matter which I think has not 

 hitherto been discussed, namely, the history of the carbonate set 

 free if the flint is a replacement of the Chalk. There are one or two 

 interesting facts in this connexion, though no special emphasis should 

 perhaps be laid on them. Jukes-Browne states that calcite is 

 abundant in the gracilis and planus zones of Yorkshire, and that it is 

 not uncommon in the Chalk Rock. 1 It has also been observed in the 

 lower zones of the south. There are no quantitative observations, but 

 it is significant that it should be several times reported in the lower 

 zones. However, it is by no means necessary to suppose that the 

 replaced carbonate is represented by calcite. It may in part be 

 piesent as a cementing material in the Chalk, for there is usually 

 about 30 per cent of pore space. It is therefore curious to note that 

 Avhilst the Upper Chalk of the South is usually described as soft and 

 friable, the bulk of the building stones are quarried in the Middle and 

 Lower Chalk. 2 



4. The Rhythm of the Flint Bands. 



The regularity of the recurrence of bands of flint in the Chalk 

 attracted early attention. Many theories have been framed in the 

 past to account for this periodicity of flint, and indeed no theory 

 which neglects to take into account this obvious feature is likely to be 

 generally accepted. Most of the earlier theories looked to periodic 

 organic growth or to periodic precipitation from the sea. But there 

 is no adequate external cause for this periodicity except a seasonal 

 one, and Cole and others have shown the incompetence of this. 3 It is 

 the simplicity of the Liesegang explanation of the rhythm that makes 

 a re-examination of the evidence for possible support desirable. 

 Before discussing the matter further, it is necessary to examine 

 Liesegang's results and to summarize some recent work on precipita- 

 tion in gels. 4 



If a gel, in which silver chromate has been precipitated rhythmically, 

 or one of Liesegang's figures, be examined, it will be seen that near 

 the entrance of the inward-diffusing solution the bands are so close 

 together that they appear as a continuous precipitate, though they 

 can usually be resolved by magnification. Passing outwards the 

 banding is obvious to the naked eye. By measurement of one of 

 Liesegang's 6 figures and plotting the amount separating the bands 

 against their distance from the drop, as in Fig. 2a, a picture of the 

 rhythm is obtained. It will be seen from this curve that the rhythm 

 can be divided into two stages — 



AB = near the origin. The bands so close as to appear continuous. 



BC = Bands with the separation increasing approximately 

 according to a linear law. 



1 A. E. Eowe and C. D. Sherborn, Proc Geol. Assoc, vol. xvii, p. 229. 

 A. J. Jukes-Browne, Geol. Mag., 1893, p. 317. 



2 J. A. Howe, Geology of Building Stones, London, 1910, p. 259. 



3 G. A. J. Cole, loc. cit. 



4 For a simple and clear account of precipitation in gels see E. Hatschek, 

 Introduction to the Physics and Chemistry of Colloids, 2nd ed., London, 1916. 



5 R. Liesegang, loc. cit., chap. x. 



