"218 A FLAKE OF FLINT AKD ITS HISTOET. 



sponges and tlieir gelatinous animal matter, " the soluble 

 or coUoidal silica, dispersed through the soft chalk-mud, 

 slowly segregated from out of the surrounding pulpy mass, 

 and gradually replaced pai-t or the whole of the organic 

 matter, as it decayed away. Nor has it stopped there ; 

 owing to the affinity of the particles of colloidal silica 

 amongst themselves, the segregation has not ceased with 

 the replacement of the organic body, but has continued so 

 long as any portion of silica remained in the surrounding 

 soft matrix ; whence the frequent excess of flint beyond 

 the interior or body of the shells, echinodemis, &c., and 

 whence also the irregular shape arising from this over- 

 growth of the flint nodules." JSText to sponges, echino- 

 derms seem to have afforded the most attractive centres 

 of segregation ; and while in some cases only their shells 

 have been filled A\ath flint, m other instances we find a 

 mass of thes:e shells cemented together by a nodule 

 .if flint. 



In some parts of the Continent, and also in Yorkshire, 

 we find that for some reason or other — not imjirobably a. 

 greater development of sponges and a smaller amount of 

 soluble silica — the segregating process has extended down- 

 wards to the lower chalk, where flints are then found ; and 

 we susjiect that in such cases analysis would also show in 

 these beds a corresponding absence of free soluble silica.* 



With regard to the so-called " potstones " of the 

 Norfolk chalk, some of which may be upwards of a yard 

 in height, with a diameter of a foot or so, the only 

 adequate explanation of their formation that has yet been 

 ofl'ered is that they rejiresent gigantic cujj-like sponges 

 which have grown one upon the top of the other, as they 

 were successively buried in the newly-formed chalk, and 

 that they have been subsequently silicified by the same 

 S(^gregating process. 



We conclude, therefore, that the flints of the chalk were 

 originally an integral portion of the rock itself, which was 



* Since this "vvas written, some obserTatione have been published 

 tending to throw doubt upon the constancy of the relation between 

 the absence of ilint and the presence of free soluble silica in chaJk. 



