440 Mr. W. J. Sollas on the Flint 



of the silica in some form or other ; and thus, if one finds 

 flints, chalcedonized shells, or minute quartz crystals in such 

 strata, one will naturally look for the remains of the siliceous 

 organisms which supplied them, and one's search will seldom 

 be unsuccessful." 



As an objection to what we may call the theory of the 

 intrinsic source of the silex in flint we may quote the follow- 

 ing passage by Prof. Renard* : — " The details of micro- 

 structure which we have entered into prove also that it is 

 impossible to admit, in order to explain the formation of 

 Phthanites, as has been so often repeated in the case of flints, 

 that these rocks are derived from an accumulation of organ- 

 isms with siliceous envelopes. In the first place the exami- 

 nation of thin slices shows us but very seldom in these rocks 

 sections of shells which one would refer to organisms with 

 siliceous tests ; and if in some cases we do meet with them, 

 in flint for example, the siliceous envelopes are there so well 

 preserved that, admitting the entire mass of the nodule to 

 have been derived from the transformation of these remains 

 of organisms into gelatinous silica, we cannot understand 

 why some sections should have escaped this transformation 

 and should have been preserved intact in the midst of the 

 { fusionment.' " 



The Trimmingham flints appear to throw some light on 

 this difficulty ; for we find that as silicification proceeds and 

 the nodule becomes more completely a flint, the sponge- 

 spicules, which are abundant enough in the contiguous sili- 

 ceous chalk, completely disappear in the flint itself; indeed 

 one may even observe one half of a large spicule projecting 

 out of a mass of silex, while the other half, which is certainly 

 imbedded within it, is not to be distinguished from the sur- 

 rounding flint. As regards the precision with which the 

 form of some spohge-spicules is preserved in flint, my obser- 

 vations t show that this does not extend to their substance ; 

 for in such cases, though white, opaque, well defined, and 

 apparently solid, they are really nothing more than empty 

 hollow casts. When these casts become filled in with silica 

 subsequently, as they sometimes do, they lose their solid 

 appearance and become mere shadows of their former selves. 



* u Recherches lithologiques sur les Phthanites du Calcaire carbonifere 

 de Belgique," par A. Renard, S.J. Extrait des Bulletins de l'Academie 

 royale de Belgique, 2 me ser. t. xlvh nos. 9, 10 (1878). 



t Quart. Journ. Geol. Soc. xxxiii. p. 817 (1877) : — " Indeed, I may go 

 so far as to state that whenever one sees a very white and opaque solid- 

 looking spicule imbedded in clear transparent flint, one may expect to 

 find it just the very reverse as regards solidity of what it seems " (" On 

 the Genus Siphonia "). 



