GEOLOGY. 305 



form of graphite, both in crystalline limestone and in beds of magnetic iron- 

 ore; and it is well known that these substances,' and even the vapor of 

 water, oxidize graphite at a red heat, with formation of carbonic acid or 

 carbonic oxide. I have, however, shown that solutions of alkaline car- 

 bonates, in presence of silica and earthy carbonates, slowly give rise to 

 silicates, with disengagement of carbonic acid, even at a temperature of 

 212 Fahr., the alkali being converted into a silicate, which is then decom- 

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

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

 endeavored to explain the production of the various silicates of lime, mag- 

 nesia, and oxide of iron, so abundant in crystalline rocks, and with the 

 intervention of the argillaceous element, the formation of chlorite, garnet, 

 and epidote.* I called attention to the constant presence of small portions 

 of alkalies in insoluble combination in these silicates, both natural and 

 artificial a fact which had already led Kuhlmann to conclude that alka- 

 line silicates have played an important part in the formation of many min- 

 erals; and I suggested t that, by combining with alkalies, clays might 

 yield feldspars and micas, which are constantly associated in nature with 

 the silicates above mentioned. This suggestion has since been verified by 

 Daubre'e, 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 car- 

 bonates of sedimentary deposits, of the vai'ious 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 con- 

 stantly produced by the slow decomposition of feldspathic sediments, and 

 are met with alike in the waters of the unaltered Silurian schists of Canada, 

 and of the secondary strata of the basins of London and Paris. In the 

 purer limestones, however, the feldspathic or alkaliferous elements are 

 wanting; and these strata often contain soluble salts of lime or magnesia. 

 These would neutralize the alkaline salts, which, infiltrating from adjacent 

 strata, might otherwise effect the transformation of the foreign matters 

 present in the limestones into crystalline silicates. By a similar process 

 these calcareous or magnesian salts, penetrating the adjoining strata, would 

 retard or prevent the alteration of the latter. These considerations will 

 serve to explain the anomalies presented by the comparatively unaltered 

 condition of some portions of the strata in metamorphic regions. | 



* Proceedings of the Royal Society, May 7, 1857. 



t Report Geol. Surv. Canada, 1856, p. 479. 



t De Senarmont, in his researches on the artificial formation of the minerals of 

 metalliferous veins by the moist way, has shown that by aid of heated solutions of 

 alkaline bicarbonates and sulphurets, under pressure at temperatures of 200 or 300 

 C., we may obtain in a crystalline form many native metals, sulphurets, and sul- 

 pharseniates, besides quartz, fluor-spar, and sulphate of barytes. 



Daubree has since shown that a solution of a basic alkaline silicate deposits a 

 large portion of its silica in the form of crystalline quartz when heated to 400 C. 

 "We have here, beyond a doubt, a key to the true theory of metalliferous veins. 

 The heated alkaline solutions, which are at the same time the agents of metamor- 

 phism, dissolve from the sediments the metallic elements which these contain dis- 

 seminated, and subsequently deposit them, with quartz and the various spars, in. (he 

 Ussures of the rock. 



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