184 Mr. BowERBANK on the Siliceous Bodies of 



of the flint stand boldly in relief from the more solid portion of the white crust, 

 which surrounds the dense and semi-transparent mass. If we give but a slight 

 consideration to the circumstances under which these bodies have been probably 

 buried, we shall at once perceive that the characters exhibited by the surface of 

 the flints, and the state of the tissues at this part, are exactly such as we should 

 naturally expect to be the case ; for if a modern sponge of a close texture be im- 

 mersed in water, in a tranquil situation, and a gradual deposit of calcareous silt, of 

 small specific gravity, were to take place upon it, the deposited matter would 

 penetrate the substance of the sponge to a very slight extent only, while the interior 

 of the space would remain filled with pure or nearly pure water, and the portions of 

 its structure thus embraced by the calcareous deposit would be protected to a great 

 extent from the operation of other agents ; and we might reasonably expect that, 

 although the minute hollow tubuli might be assailed from the interior of the mass 

 by a second and more fluid matter, and thence become silicified, yet that the space 

 between them would be occupied by the matrix in which they were imbedded : and 

 this is exactly what appears to have taken place during the process of the fossili- 

 zation of the sponge of the common flint. The structure, both internal and exter- 

 nal, and the peculiarities thus described, are common to the great mass of the 

 tuberous flint nodules of all the localities of the chalk formation of England from 

 which I have been able to procure specimens, and under circumstances where we 

 should have scarcely expected them to have been preserved. 



The structure and other characters of the layers of tabular flint are in perfect ac- 

 cordance with those of the nodular flints. Xanthidia, spicula, and foraminated shells 

 are dispersed with equal abundance in its substance. The upper surface resembles 

 in every respect that of the flint nodules, while the under one presents, even 

 to the unassisted eye, a still more strongly marked spongeous aspect. Its general 

 appearance is that of a sponge which has been built upon an irregular surface, and 

 has followed the whole of the sinuosities. The great ex-current canals are larger 

 and more numerous than usual ; and the spicula are more abundant, frequently 

 projecting from the surface, while the Foraminifera are attached so abundantly to 

 it, that upon a superficies of an inch and a half square, I have counted upwards of 

 twenty specimens of various species, some of which were attached only by a minute 

 portion to the mass, precisely in the manner in which small bodies are frequently 

 attached to recent sponges. It would thus appear, that the only difference be- 

 tween the tabular and other forms of flint was, that at the period of the formation 

 of the tabular flint the original sponge was built upon a firmer and less change- 

 able surface than usual ; and that under these circumstances it had obeyed its 

 natural instinct, that of coating over any mass upon which its gemmule might have 

 chanced to settle, in a manner precisely analogous to the habits of the freshwater 



