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



2. The Relative Age oe Formation. 



The flint, relative to the Chalk, may have been formed either — 



(a) Contemporaneously by original deposition, either organically as 

 advocated by Bulman, 1 or chemically as held by Tarr. J 



(b) Penecontemporaneously by segregation of silica disseminated in 

 the ooze of tbe Cretaceous sea, as Van Tuyl 3 seems to think. 



(c) At the time of uplift by diffusion, precipitation, and replacement, 

 as advocated by Cole and Liesegang. 



(d) Subsequent to uplift and complete consolidation. With regard 

 to (d) there is no direct evidence. On the other hand there is- 

 something to be said for each of the other relative times. Setting 

 aside for the moment the difficulties that arise when the regular 

 vertical spacing of the bands is taken into account, the bearing of 

 other field evidence may be mentioned. 



The general mode of occurrence simulates bedding, but as 

 W. Hill 4 and others have pointed out there is such a close corre- 

 spondence between the grain, micro-structure, and faunal content of 

 the flint and Chalk that these observers have expressed their belief 

 that the flint is replacive in its attitude to the Chalk. This is quite 

 consistent with penecontemporaneous formation. 



On the other hand, the well-known occurrence of exactly similar 

 flint in vertical or highly inclined planes 5 is difficult to reconcile 

 with any other view except that according to which the flint was 

 formed later than the complete deposition of the Chalk. There is 

 no difference between the two types of flint. Sometimes the line of 

 the crack can be seen running through the flint, but where the 

 nodules bulge out they show the same replacive features as do the 

 horizontal flints. This "vein" flint is most commonly observed in 

 anticlinal areas, and the cracks conform in direction to the other 

 tectonic features. They are thus directly related to uplift, and 

 point to the formation of the flint at that time, so supporting 

 Liesegang's contention. Moreover, there are certain puzzling types 

 of nodule (paramoudras, pot-stones, flint rings) which are by no 

 means easy to understand as original deposits. And it may be 

 mentioned that although carefully sought for in both oceanic and 

 shallow water, no flint either partially or completely formed has 

 been dredged up. 6 On the other hand, in both the Upper and 

 Middle Chalk masses of chalk of flint-like form have been found in 

 all stages from white silicified chalk, with perhaps just a small 

 central core of black flint, to the completely black nodule — facts 

 strongly supporting a replacement hypothesis. 



The evidence, 7 so far as it goes, seems to me to be decidedly in 



1 G. W. Bulman, Sci. Prog., No. 41, 1916, p. 154. 



2 W. A. Tarr, Amer. Journ. Sci., ser. IV, vol. xliv, p. 428, 1917. 



3 F. M. Van Tuyl, Amer. Journ. Sci., ser. IV, vol. xlv, p. 449, 1918. 



4 W. Hill, Proc. Geol. Assoc, vol. xxii, p. 62, 1911. 



5 A. D. Kowe and C. D. Sherborn, Proc. Geol. Assoc, vol. xvi,p. 170, 1900- 

 W. J. Sollas, The Age of the Earth, London, 1905, p. 135. 



6 J. Murray and J. Hjort, Depths of the Ocean, London, 1912, pp. 183-5. 



7 See also E. Eay Lankester and otbers, discussion in Nature, 1917,. 

 reprinted in Trans. Geol. Pbysics Soc, 1917. 



