On Some Toints in Chemical Geology. 415 



very different degrees ; so that fossiliferous beds but little altered 

 are sometimes found beneath crystalline schists, or even interca- 

 lated with them. 



We cannot admit that the alteration of the sedimentary rocks has 

 been effected by a great elevation of temperature, approaching, as 

 many have imagined, to that of igneous fusion ; for we find un- 

 oxidized carbon, in the form of graphite, both in crystalline lime- 

 stone and in beds of magnetic iron-ore ; and it is well known that 

 these substances, and even the vapour of water, oxidize graphite at a 

 red heat, with formation of carbonic acid or carbonic oxide. I have 

 however shown that solutions of alkaline carbonates in presence of 

 silica and earthy carbonates slowly give rise to silicates, with disen- 

 gagement of carbonic acid, even at a temperature of 212° Fahr., — 

 the alkali being converted into a silicate, which is then decomposed 

 by the earthy carbonate, regenerating the alkaline salt, which serves 

 as an intermedium between the silica and the earthy base. I have 

 thus endeavoured to explain the production of the various silicates 

 of lime, magnesia and oxide of iron so abundant in crystalline 

 rocks, and with the intervention of the argillaceous element, the 

 formation of chlorite, garnet and epidote.f I called attention to 

 the constant presence of small portions of alkalies in insoluble com- 

 bination in these silicates, both natural and artificial — a fact which 

 had already led Kuhlmann to conclude that alkaline silicates have 

 played an important part in the formation of many minerals ; and I 

 suggested^ that, by combining with alkalies, clays might yield feld- 

 spars and micas, which are constantly associated in nature with the 

 silicates above mentioned. This suggestion has since been verified 

 by Daubree,§ who has succeeded in producing feldspar by heating 

 together for some weeks to 400° C. mixtures of kaolin and alkaline 

 silicates in the presence of water. 



The problem of the generation from the sands, clays and earthy 

 carbonates of sedimentary deposits, of the various silicious minerals 

 which make up the crystalline rocks, may now be regarded as solved, 

 and we find the agent of the process in waters holding in solution 

 alkaline carbonates and silicates, acting upon the heated strata. 

 These alkaline salts are constantly produced by the slow decomposi- 

 tion of feldspathic sediments, and are met with alike in the waters of 



fPrcceeding3 of the Royal Society, May 7, 1857. 

 JReport Geol. Surv. Canada, 1856, p. 479. 

 §Bull. Soc. G6ol. de France (2) vol. xv. p. 103. 



