196 Dr. Wallich on the Origin of the 



I have shown maintains its position on the immediate surface 

 of the calcareous deposit, and gradually dissolves the silex. 

 This layer, in virtue of its inferior specific gravity, rises with 

 every increase in the thickness of the deposit, until at last 

 the supersaturation of the protoplasmic masses with silex takes 

 place, and the first step towards the consolidatiou into flint 

 is accomplished — the continuity of sponge-life, and of the 

 various other forms which tenant the calcareous areas, being 

 secured through the oozy spaces which separate the sponge- 

 beds, and thus admit of both adult and larval forms having 

 free access to the overlying stratum of water. 



" That the predisposition of silica, itself in reality a colloid, 

 to form colloidal combinations with albuminous and other 

 materials was known long before deep-sea exploration was 

 dreamt of, is a well-known historical fact ; it has been alluded 

 to by most of the writers who have attempted an explanation 

 of the mode of formation of the flints. But the various con- 

 ditions that present themselves, from the earliest elimination 

 of the silica from the sea-water to the period when it becomes 

 finally consolidated, have never, that I am aware, been con- 

 secutively followed out. 



" But that the colloidal idiosyncrasy of silica performed a 

 much more important function in the phenomena connected 

 with the flints than has heretofore been supposed, appears to 

 me to be indicated by the evidence of the almost perfect 

 incorporation of the organic silica with a colloid material, the 

 unique Amoebiform nodulation of the flints, and its homogene- 

 ousness, whether occurring in nodules, in continuous sheets 

 parallel to the stratification, or as sluggish overflows into 

 fissures in the Chalk. But for a very highly developed 

 colloidal condition of the materials, these peculiarities could 

 not, I conceive, have presented themselves so uniformly 

 throughout the formation. From a mere aqueous solution 

 the deposit of silica would have exhibited totally different 

 characters : there would have been a general infiltration into 

 the substance of the chalk, the particles of which would thereby 

 have been cemented together, so as to fonu a siliceous lime- 

 stone ; the various minute organic forms in which the silica 

 showed itself, though, no doubt, capable of solution to a 

 limited extent in water charged more or less highly with 

 carbonic acid, and aided perhaps by the stupendous pressure, 

 would have occasionally left more pronounced traces of their 

 original structure than is observable in the body of the flints ; 

 probably all the fossils would have been either infiltrated with 

 silica, or a substitution of that substance would have taken 

 place even more frequently than we find it ; there would have 



