458 Mr. W. J. Sollas on the Flint 



within, and this may have been just sufficient to lead to the depo- 

 sition of the silex round the sponge in preference to some other 

 place. The frequent absence of flint within the skeleton may 

 be owing to the absence of chalk-ooze, which in these cases 

 had failed to penetrate into the interior of the sponge. The 

 mere zonal enclosure of the sponge by a ring of flint may be 

 accounted for by supposing that the sponge-spicules from 

 which the flint was derived formed a bed surrounding it at 

 the level of the ring, but were not present in sufficient quan- 

 tity to produce silicification above or below that level. 



The complete enclosure of a sponge in a more or less 

 spherical mass of flint may be accounted for by supposing 

 that silicification once started at any place would continue 

 there in preference to recommencing at a fresh centre. 



Amongst some notes I made in 1873 I find a drawing 

 which somewhat strengthens the notion that silica proceeding 

 from the sponge-skeleton may have led to deposition. It 

 shows an Ostrea seated on a Ventrieulite, which has been 

 everywhere coated with silica, except where the Ostrea is 

 attached, the oyster lying below the general surface of the 

 flint, which bulges out all round it. I feel some hesitation in 

 placing entire dependence on a note made so long ago, when 

 I was only just beginning the study of flints: but I well remem- 

 ber making the observation ; and if the fact be as represented 

 it would certainly seem as though the surrounding flint had 

 been deposited through the influence of something proceeding 

 outwards from the sponge, either silica in solution or, less 

 likely, organic matter, and that the obstruction furnished by 

 the oyster had prevented the accumulation of the silex imme- 

 diately over it. 



(v) Finally there is the supposition that the sponge skeleton 

 may have led to deposition by furnishing a free surface to the 

 siliceous solution. This is likely enough, but it is difficult to 

 prove or disprove. In the case of other organisms, such as 

 Echinoids, the tests of which have determined the deposi- 

 tion of silex, supposition iv. is excluded on chemical grounds, 

 and the last supposition appears to be the only probable one. 

 The characters of the flint urchins agree very well with it : 

 frequently the silex is found only within the test, the siliceous 

 solution having filtered through the walls, filling up the 

 ambulacral pores; occasionally the test is only half filled with 

 silex, as though it had rested half immersed in a bed of 

 sponge-spicules ; sometimes it is quite filled ; sometimes the 

 silex protrudes from the mouth and anus ; and, lastly, the test 

 is sometimes not only filled with silex but completely enclosed 

 in it. 



