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



On examining these crystals by a microscopic power of 500 linear, 

 I was much interested on finding that not only were regular crystals 

 of silex present, but that the calcedonic form of deposit was also 

 apparent in the form of filmy plates, composed of two and sometimes 

 three distinct crops of the characteristic acicular crystals of that form 

 of deposit*. 



We are familiar with the solid condition of silex, as it exists in 

 masses where the deposit has been induced by the presence of organic 

 matters, as well as in those in which the influences are purely mineral, 

 as in the agates of the igneous rocks ; but in both cases to a certain 

 extent the phsenomena are identical, as far as regards the calcedonized 

 portions of the masses. In the first instance, the solidified and en- 

 cased organic matters form the nuclei whence the first crops of 

 crystals spring. In the second, the parietes of perhaps an accidental 

 cavity in the mass of the igneous rock, furnishes a base for a similar 

 crop of crystals ; and subsequently, in both cases, the deposit appears 

 to progress uninfluenced by any other agencies than what are in ac- 

 cordance with those which are purely mineralogical. 



But whence, it may be asked, come the enormous quantities of 

 silex, which hare entered into the structure of fossils during every 

 geological period, and which still continue to be separated from the 

 ocean? Various opinions have been offered to account for these 

 pheenomena, such as extreme heat, great pressure, thermal springs, 

 and a peculiar gelatinous condition of silex, produced by chemical 

 manipulation, but of which we have no authentic record in nature. 



None of these, it appears to me, satisfactorily account for the vast 

 deposits of silex that we have to deal with in connection with organic 

 matter. Great pressure and high temperature, there is no doubt, are 

 active agents in promoting the solution of silex in excess, with which 

 some chemical springs are charged, and these causes are perhaps 

 powerfully effective in the formation of certain mineral products in 



* 7 St. Mary's Road, Canonbury, June 10, 1848. 



My dear Sir, — Your having mentioned to me that you are at present engaged 

 on some investigations respecting the deposition of siUca in a crystalline form, has 

 recalled to my mind a fact which I observed some time since, and now venture to 

 communicate to you in the hope of its proving of interest in connection with your 

 researches. 



You are aware that the gaseous body fluoride of silicon (Si F3) is decomposed by 

 contact with water, one-third of the silicic acid being deposited in the form of a 

 jelly, and silicated hydrofluoric acid (3 H F-)-2 Si F3) produced ; thus 3 (Si Fg)-}- 

 3 (HO) = Si O3 (which deposits) -j- 3 H F-)-2 Si F3 (which remains in solution). 



The deposited silicic acid is extremely soluble in water, and I have observed 

 that the separated silicated-hydrofluoric acid always retains a portion of uncom- 

 bined silicic acid in solution ; this deposits after the lapse of some months in minute 

 crystals of artificial quartz. The specimen I now send you was thus obtained : the 

 crystals vary in size from the one nine-hundredth of an inch to the one seventy- 

 fifth of an inch in length (from -00111 in. to -01333 in.) ; they depolarize light, and 

 form a very beautiful microscopic object. 



The case just cited is precisely analogous to those you were mentioning to me ; 

 in all, water is the solvent of the silica, which, when recently produced or separated 

 from its combinations by the action of the atmosphere on the earthy silicates, is 

 presented in the modification favourable for solution. 



I remain, my dear Sir, very truly yours, 

 James Scott Bowerhank, Esq., Warren Dk la Rue. 



<^'c. &(c. Sfc. 



