154 THE FORMATION OF FLINTS 



present, offering free egress to the siliceous solutions 

 wandering through the pores of the chalk. But if the 

 flint of the joints has been formed since the chalk became 

 dry land, there would appear to be no good reason why 

 this should not have been the case with the other forms 

 of flint. The nodular layers may possibly mark horizons 

 in the chalk where the growth of sponges was unusually 

 profuse ; sponge spicules and whole skeletons of sponges 

 are much more richly distributed among them than else- 

 where, and there is even an almost sudden increase in 

 these remains as we pass from the immediately adjacent 

 chalk into the nodular layer. The even sheets, sometimes 

 of very wide extent, as in St. Margaret's Bay, near Dover, 

 where a layer about an inch and a half in thickness has 

 been traced for a distance of two miles, are possibly the 

 result of gentle current action, spreading out the debris 

 of a sponge bed in a more or less uniform manner. 



It has sometimes been imagined that the sponge 

 skeletons, since they consist of silica, may have exerted 

 an attractive influence on the silica in solution, but this 

 will not accord with the fact already mentioned, that 

 a considerable interval sometimes separates a sponge 

 skeleton from the surrounding flint. The presence of 

 sponge skeletons probably indicates regions in the chalk 

 where silica was unusually abundant, and where, conse- 

 quently, solutions of this material attained their highest 

 degree of concentration. 



We have now traced the building up of a flint, and 

 have seen how the scattered silica in the chalk passes 

 into solution, and is then gathered together into another 

 place, where it accumulates in a solid form ; but after the 

 completion of this process another ensues, and, strange 

 to say, in the opposite direction. No sooner has Nature 

 made the flint with seeming care and deliberation than 

 she proceeds to unmake it, taking it to pieces with the 



