Mr. J. S. Bovverbank on the Siliceous Bodies of the Chalk. z67 



derable degree, so as to render it evident that the surface of the 

 water was elevated by its power the tenth or the eighth of an 

 inch, and some hght dust shaken over the spot was dispersed in a 

 circle with great rapidity. I have seen the same phsenomenon 

 in vivid action with Grantia botryoides in a closed cell filled with 

 sea-water beneath the microscope, when at Weymouth in the year 

 1845. Need we wonder then, with such powers inherent in the 

 Spongiadse, that minute animal or vegetable organisms should be 

 found in such positions as those alluded to by Mr. Smith ? But 

 there is yet another way in which the filling of the pulp-cavity 

 and the space intervening between the tooth and its socket may 

 be accounted for in the small jaw figured by the author, and it is 

 simply this : that as the whole of the fragment of the jaw has been 

 built over by the sponge originating the flint, it is quite natural 

 that it should have insinuated its fibres into those spaces in thin 

 plates, and such thin plates of single layers of reticulated fibre, 

 not exceeding the five -hundredth of an inch in diameter, may be 

 frequently seen by the aid of a lens in the sponges of commerce, 

 especially at the termination of the excurrent canals of the West 

 Indian species. But in reality Mr. Smith's specimen needs none 

 of these conditions to account for the presence of the silex in any 

 one part of it more than in another, as the whole substance of the 

 jaw is more or less silicified, which fact was not observed by the 

 author at the time of the publication of his paper. There is no- 

 thing more surprising in this replacement of carbonate or phos- 

 phate of lime in bone by silex, than there is in like replacements 

 in the shells of the greensand formation and of the London clay, 

 Voluta luctator and other shells. The same phsenomenon takes 

 place in the corals of the mountain limestone of Derbyshire, 

 Ireland and elsewhere ; and this I believe to take place without 

 the presence of any degree of heat above the ordinary mean tem- 

 perature of the earth, and for this reason ; that in almost every 

 flint that I have examined, I have found evidence of chalcedonic 

 crystallization wherever there has been a small space originally 

 not occupied by spongeous substance. And in almost every moss 

 agate it may be seen that the fibres are the prevailing nuclei of 

 crystallization ; from these they constantly radiate until the va- 

 rious crops of crystals meet at their apices and form ultimately 

 the solid mass of the agate. In fact, the process of siliceous de- 

 posit in these organized fossils appears to be precisely the same 

 in principle as in the deposit of siliceous matter in hollow spaces 

 in rocks of igneous origin, only that in the first case the place of 

 crystallization is determined by the presence of the organic fibre 

 of the sponge, and in the latter case simply by the sides of the 

 cavities in the rock. We find also in chalk flints, that where there 

 has happened to be a large central cavity, the sides are often coated 



