7o DE. WALLICn ON THE THYSICAL 



discussing the properties of silica, says : — " But whence come the 

 enormous quantities of silica which have entered into the structure 

 of fossils during the geological period, and which still continue to 

 be separated from the ocean ? Various opinions have been offered 

 to account for these phenomena, such as extreme heat, great pres- 

 sure, thermal springs, and a peculiar gelatinous condition of silica, 

 produced by chemical manipulation, but of which we have no 

 authentic record in nature." " None of these," he continues, 

 " appear satisfactorily to account for the vast deposits of silica that 

 we have to deal with in connexion with organic matter. Great 

 pressure and high temperature, there is no doubt, are active agents 

 in promoting the solution of silica in excess, with which some 

 mineral springs are charged, and these causes are perhaps power- 

 fully effective in the formation of certain mineral products in the 

 interior of the earth ; but as regards the supply of silica in the pro- 

 duction of fossils, and in its appropriation by living organisms, I 

 believe them to have infinitely less to do with these phenomena than 

 has hitherto been supposed. Much weight has been attached by 

 some writers to the probability of the spicules of the Spongiadae 

 acting as nuclei for the attraction of silica in the process of their 

 fossilization ; but it is a remarkable fact that the true Halichondria, 

 in which the siliceous spicules abound, are exceeding rare in the fossil 

 state ; while the remains of the true Spongia, in which the animal 

 fibre predominates, are very abundant." 



Now, to my mind, this looks very much like begging the entire 

 question, inasmuch as it had long before been regarded as an estab- 

 lished fact that, apart from all submarine sources of silica, there 

 must through all geological time have been an inexhaustible supply 

 of that material, derived from the disintegration of felspathic and 

 other rocks, carried down to the ocean by rivers and currents — more, 

 indeed, than the water can take up, judging from the barely appre- 

 ciable quantity found to exist in it. Of the sufiiciency of this supply 

 there could therefore be no doubt, nor of the greatly augmented 

 solubility of the silica under the operation of increased pressure and 

 temperature, and, notably, the increased charge in the sea-water of 

 carbonic acid. Neither can there be any doubt as regards the suffi- 

 ciency of the organic agencies by which the silica held in solution 

 was being perpetually removed, nor of the general tendency of 

 silica, when undergoing solution, or in a colloid state, to seize upon 

 some foreign body and become aggregated around it. These are all 

 facts beyond dispute, the point requiring demonstration being : — 

 Through what special agency does all this enormous amount of silica 

 (after having been first eliminated from sea- water, and then secreted 

 by living creatures in the shape of shells, reticular frameworks, or 

 spicules, which constitute, as it were, the bones of the silica-secreting 

 Protozoa) become once more transformed, en hloc, into such masses 

 of mineral as the flints ? 



Of these questions Mr. Bowerbank's observations furnish no ap- 

 proach towards a solution. 



The only arguments I think it requisite to answer in detail are 



