1849.] BOWERBANK ON A SILICEOUS ZOOPHYTE. 327 



formation of siliceous fossils, and content ourselves in endeavouring 

 to comprehend the mysteries of these natural phsenomena, by the 

 agency of the laws in continual operation, and with the ordinary 

 amount of pressure in the depths of the ocean around us. We see 

 that animals have the power of quietly effecting within their tissues, 

 that which in the operation of fossilization is effected by a species of 

 attraction, which, for want perhaps of a more accurate knowledge of 

 these natural operations, we designate elective attraction. 



May not this attraction of organic matter for silex, so abundantly 

 displayed throughout the whole field of geological science, shed some 

 light on the chemistry of animal and vegetable assimilation or secre- 

 tion ? The laws of endosmose are continually equalizing the densities 

 of the fluids within the cells of animals and vegetables ; and if the 

 views I have stated be correct, there needs in living animals, as well 

 as in dead ones, but that the earth in solution should be brought in 

 contact with organic surfaces under certain conditions, with which we 

 are but very imperfectly acquainted, to be deposited upon the parietes 

 of the cells or other organs into which the fluid has insinuated itself ; 

 layer after layer is then deposited, until the whole of the minute 

 cavities are filled up. If we examine the structure of shells as ex- 

 hibited in the prismatic cells of Pinna, Ostrea, and other allied genera, 

 we always find the earthy deposits in the form of successive layers 

 from the circumference to the centre. Such also is the form of 

 deposit in both the siliceous and calcareous spicula of recent sponges ; 

 so it is also in bone and in every similar organic tissue with which 1 

 am acquainted. And such also is the form of deposit of silex, car- 

 bonate of lime, and pyrites, in the cellular and vascular tissues of 

 fossils ; and in no case is it better displayed than in the cells of many 

 of the succulent fruits of the London clay, where we find their walls 

 remaining in the form of thin films of carbon ; upon the inner sur- 

 faces of these are deposited successive layers of pyrites ; but the 

 deposit is frequently found to have been arrested after several of these 

 depositions, thus leaving the centre of each cell empty, and then we 

 find the inner surface of the last coating of pyrites usually covered 

 with a beautiful crop of minute crystals. It is the same in the 

 fossilization of the Spongiadae : in the substance of the animal matter 

 there is no evidence of crystallization, and it is only at a short distance 

 from each fibre that the radiating crystalline needles of the calcedonic 

 form of deposit are seen to spring, and crop after crop succeeds each 

 other, until the whole of the intervening spaces are filled. 



Much weight has been attached by some writers to the probability 

 of the spicula of the Spongiadse acting as nuclei for the attraction of 

 silex in the process of their fossilization ; but it is a remarkable fact 

 that the true Halichondria, in which the siliceous spicula abound, are 

 exceedingly rare in a fossil state ; while the remains of true Spongia, 

 in which the animal fibre predominates, are very abundant. And 

 moreover, whenever I have found the siliceous spicula preserved, they 

 have always exhibited considerable erosion of the surface, as if they 

 were very much more amenable to the action of the solvents to which 

 they had been exposed, than to the chemical affinity of their kindred 



