Nodules of the Trimmingham Chalk. 457 



ooze, so that it might lose its sarcode by diffusion all round 

 before silicification took place, then we are involved in the 

 admission that a considerable time had elapsed between the 

 death of the sponge and its silicification, since chalk accumu- 

 lates slowly ; and during this lapse of time the sarcode would 

 have become decomposed. 



(ii) We dismiss, then, the notion that protoplasm itself can 

 have had any direct influence in determining deposition ; but 

 peidiaps the products of its decomposition may have been more 

 effectual, and we might attempt to substitute for Dr. Wallich's 

 hypothesis a supposition of Alexis A. Julien, who says : — 

 " I would therefore modify Sollas's theory by suggesting 

 that during the decomposition of the sarcode of both animal 

 and vegetable organisms, after death, gelatinous or colloid 

 substances are generated, resembling glairine, which are 

 soluble in sea-water, which combine with silica, and may 

 therefore convey and concentrate it, dissolving its particles 

 disseminated through submarine sediments, and which may 

 in certain forms, produced by gradual oxidation, act also as 

 acid solvents of lime, oxides of iron and manganese, &c. To 

 this idea, in part, an early opinion of Bischoff approaches : 

 ' Silicifications are nothing else than the result of combinations 

 between the crenic acids (Quellsauren) formed through decom- 

 position of organic matter (e. g. of mussels and oysters) and 

 silica, which in aqueous solution, e. g. as in the water of 

 springs, comes into contact therewith ' " (p. 3G4). 



Julien, however, does not attribute the form of flints to the 

 organic matter furnished by them, but speaks of the dissolved 

 silica being deposited "... around the undissolved siliceous 

 organisms or particles as nuclei." 



We reach now the explanations based on the supposition 

 that the skeleton of the sponge has had the chief part in 

 determining the deposition of the surrounding flint ; and (iii) 

 with regard to the first notion, that silica has been attracted by 

 the sponge-skeleton and so deposited, one may point out that, 

 if true, silicification should have commenced from the surface 

 of the fibres of the skeleton and proceeded outwards, while 

 observation shows that this has not been the case, the skeleton 

 remaining an empty porous network after being completely 

 enclosed in flint, (iv) With regard to the suggestion that 

 silica has been contributed by the sponge-skeleton to the 

 pervading solution of silica, and so rendered the latter con- 

 centrated enough to bring about a replacement in the sur- 

 rounding chalk ooze, there seems much more in it. The 

 chief part of the silex enclosing the skeleton has clearly been 

 gathered from without ; but some silica has disappeared from 

 Ann. & Mag. N. Hist. Ser. 5. Vol. vi. 32 



