676 On Flints in the Chalk. 



On Flints in the Chalk. — There appear to be two favorite 

 theories for explaining the existence of flint beds in the up- 

 per chalk formation. The one assumes that each bed of flint 

 is the result of the subsidence of siliceous matter from a semi- 

 fluid mixture of silex and chalk ; the supernatant chalk be- 

 ing supposed subsequently to solidify, and thus form a hard 

 bed, on which a second deposite of chalky matter may occur 

 by a second subsidence, and so on successively through the 

 series. The other theory attributes the collection of silex in- 

 to flint-beds, to the attraction of the particles for each other ; 

 the position of the beds being determined by the accidental 

 formation of a nucleus in any particular part of the semifluid 

 mass. This theory does not require the assistance of subsi- 

 dence, but supposes that the flint beds may have been form- 

 ing in different parts of the chalk at the same moment. On 

 considering these theories, it appeared to me that if subsi- 

 dence were the agent, or in other words, if gravity acted so 

 powerfully as to counteract the attraction upwards to a nu- 

 cleus, of any floating silex, then we might expect that such 

 particles would be detained in the gradually solidifying chalk 

 in different quantities, being in greater proportion towards the 

 plane of subsidence, and gradually decreasing as we proceed- 

 ed upwards from that plane. If, on the contrary, attraction 

 were the principal agent, then we should find that no such 

 gradual diminution of the proportion of silex would be ob- 

 served ; but that, owing to the more rapid and complete na- 

 ture of the attraction between particles, we might expect the 

 chalk to be pretty constant in its proportion of silex at any 

 distance above or below the layers of flint. 



No. 1 was taken from the chalk 3 in. above the flint bed ; 



No. 2 at 3 ft. 3 in. ; and 



No. 3 at 6 ft. 3 in. On analysis — 



No. 1 yielded 0.5 per cent, of silex ; 



No. 2 also 0.5 per cent. ; and 



No. 3 1.0 per cent. 



From these results it is evident that no decrease occurs in 

 the proportion of silex in chalk, as we ascend from the flint 

 bed ; which I cannot help regarding as an argument against 

 subsidence, particularly whe observe that at 6 ft. 3 in. above 

 the flint-bed we have \ per cent, more silex present. 



These facts are however in no way opposed to the theory 

 of aggregation by attraction, and the small quantity of silex 

 observed in the chalk shows a completeness of action on the 

 part of the aggregating power which is a strong argument in 

 favor of its having taken effect at small distances. 



