326 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 3, 



presence of carbonic acid, a material which is known always to exist 

 in sea-water. And so powerful is the effect of the presence of this 

 acid when in contact with the silicates, that it was found by Poltorf 

 and Wiegmann, that sand might be boiled in a mixture of nitric and 

 muriatic acids, and then thoroughly washed, and yet after this purifi- 

 cation, when left for thirty days immersed in water saturated with 

 carbonic acid, the water was found to contain in solution, silica, car- 

 bonate of potass, lime, and magnesia ; thus proving beyond a doubt 

 the power of carbonic acid alone sufficient to take up and retain in 

 solution this hitherto supposed to be insoluble earth*. 



The continued attrition of the material of every beach throughout 

 the world must necessarily also be reducing enormous quantities of 

 silex to the state of impalpable powder, or into such a state of com- 

 minution as to render it soluble under favourable circumstances ; and 

 we know that in the state of extreme division, arising from precipita- 

 tion by chemical action, it is readily soluble even in cold fresh water, 

 containing no more than the usual quantity of carbonic acid. 



The facility with which silex is held in solution under ordinary 

 circumstances, is equally well proved by every corn-field that we pass 

 through, in the well-known abundance of it secreted in the stalks of 

 the plants. The rain-water no sooner permeates the soil, than it 

 becomes charged with a sufficiency of that earth to affi)rd the material 

 for the secretion of a considerable coating of silex, by this as well as 

 many other similar plants. 



Thus it has ever been during our geological periods, and if we may 

 judge by the similarity of the phsenomena of fossilization, throughout 

 a long series of geological formations from the Silurian upwards. 



It is not only in plants and in the Spongiadse that these deposits 

 of silica have taken place ; it is almost equally abundant in the fossil 

 Corallidse. Everything organic appears to have an active affinity for 

 silex in solution, and no sooner have the waters of the ocean, or other 

 agents, removed the carbonate of lime from shells, than the animal 

 matter thus released from combination is immediately seized upon by 

 the silex. It is thus in the oolites, the cretaceous groups, and in the 

 clays of the tertiaries also ; for as perfectly silicified shells are found 

 in the London clay as can be obtained even from the greensand for- 

 mation. Nor is it surprising that this should be the case, when we 

 see that such delicate and ephemeral organs as the tentacula of 

 polyps are invested with silex in so rapid and perfect a manner, as 

 we observe to be the case in the polypiferous fossil that I have just 

 described. 



With these great sources continually in action in all parts of the 

 world, liberating enormous quantities of soluble silicates, and the well- 

 established soluble powers of carbonic acid, we may be content to 

 throw aside the supposititious source of the gelatinous condition of 

 silex, — to dispense even with thermal springs and high pressure in the 



* I have learned since this paper was read, that Forchhammer has found silex 

 in solution in sea-water, to the amount of 0"003, and that it has been detected by 

 Pagenstecher in the water of the Aar, near Berne, and of the Rhine at Basle (see 

 Berzelius's Jahresbericht). 



