"218 A FLAKE OF FLINT AND ITS HISTORY. 



sponges and their gelatinous animal matter, " the soluble 

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

 slowly segregated from out of the surrounding pulpy mass, 

 and gradually replaced part 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, echinoderms, <fcc., and 

 whence also the irregular shape arising from this over- 

 growth of the flint nodules." Next 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 with flint, in other instances we find a 

 mass of these shells cemented together by a nodule 

 of flint. 



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

 we find that for some reason or other not improbably 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 suspect 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 

 offered is that they represent gigantic cup -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 

 segregating process. 



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

 originally an integral portion of the rock itself, which was 



* Since this was written, some observations have been published 

 tending to throw doubt upon the constancy of the relation between 

 the absence of flint and the presence of free soluble silica in chalk. 



