ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxix 



in which that gentleman, after adverting to fluoride of siHcon heing 

 decomposed by contact with water, part of the sihca being deposited 

 as a jelly, and silicated hydrofluoric acid produced, and to the solu- 

 bility of the deposited sihca in water, remarks that he had always 

 observed that the separated silicated hydrofluoric acid contained un- 

 combined silica, deposited after the lapse of some months in minute 

 crystals of quartz. Upon microscopically examining these siliceous 

 bodies, Mr. Bowerbank found not only crystals of quartz, but also 

 chalcedonic deposits in the form of filmy plates, composed of two 

 and even three layers of characteristic acicular crystals. 



Our colleague then proceeds to inquire whence the abundance of 

 silica was obtained which we observe amid deposits of various geolo- 

 gical dates. After adverting to the action of high pressure and tem- 

 perature in aiding the solution of silica, Mr. Bowerbank is disposed 

 to think that the supply of this substance to fossils has been more 

 from the waters of the ocean than has been supposed, being there 

 greatly diffused. He adduces the infusoria, abounding in all parts 

 of the seas of the world, as proving this diffusion, and remarks on the 

 amount of soluble silicates borne into the ocean from the decomposi- 

 tion of felspathic minerals, and observes on the appropriation of silica 

 by animals and by plants. He also adverts to the possibility of 

 animal and vegetable matter exercising an attractive influence on 

 silica. 



With respect to the dissemination of silica by means of solution in 

 the crust of the earth, we have sufficient evidence that it has been 

 most abundant and extensive. Not only does it occur as the chief 

 cementing substance uniting the grains of so many sandstones, but it 

 is also found disseminated in clays and in a variety of other rocks, 

 even certain limestones, far more than, without minute investigation, 

 might be supposed. When previously mentioning the labours of 

 Berzelius, we had occasion to notice the abundance of silica among 

 the various mineral masses known to us. Silica exists in large quan- 

 tities in the igneous rocks (indeed the mass of them is formed of little 

 else than various silicates), and from these rocks the detrital deposits 

 have been chiefly formed*. 



It would be scarcely necessary to remind you, that among other 

 sources, the decomposition of that abundant mineral family, the fel- 

 spars, readily affords the means of throwing silica into conditions for 

 solution. Nor need we advert to the facilities afforded by solutions 

 for the transport of silica from one situation to another by geological 

 means, nor of the ease with which it would pass through the pores 

 of rocks, even of many supposed to be veiy compact. We may, how- 

 ever, recall your attention to the experiments of Sir James Hall, who 

 many years since (twenty-four) showed that by mingling common 

 salt (chloride of sodium) with grains of sand and exposing the mix- 

 ture to suflicient heat, even beneath water, that consolidation was 



* Taking a general view of the relative abundance of the metanic bases of the 

 earths and alkalies, they would stand, as we had occasion formerly to notice 

 ('Researches in Theoretical Geology,' 1834, p. 24), as follows: — 1. Silicium, 

 2. Aluminium, 3. Potassium, 4, Sodium, 5. Magnesium, and 6. Calcium, 



