460 



exerts a similar action. I have found that when artificial carbonate 

 of magnesia in excess is boiled with a solution of silicate of soda, the 

 latter is completely decomposed with the formation of carbonate of 

 soda, and a silicate of magnesia which gelatinizes with acids ; and I 

 have long since described this reaction in the evaporation of alkaline 

 mineral waters*. This mutual decomposition of carbonate of mag- 

 nesia and silicate of soda, conjoined with the power of carbonate of 

 soda to dissolve silica, leads to a curious result. If we boil for some 

 hours a mixture of ignited silica, obtained from the decomposition of 

 a silicate by an acid (and consequently readily soluble in an alkaline 

 carbonate), with a small portion of carbonate of soda and an excess of 

 hydrocarbonate of magnesia, we obtain a dense powder which contains 

 all the silica united with magnesia, and may be boiled with carbonate 

 of soda and sal-ammoniac without decomposition. It is obvious from 

 the above experiments that similar results may be obtained with 

 quartz, although the process is much slower ; it would doubtless be 

 accelerated under pressure at a somewhat elevated temperature, 

 which would enhance the solvent power of the alkaline carbonate. 



Silicates of potash and soda are everywhere present in sedimentary 

 rocks, where decomposing felspathic materials are seldom wanting, 

 and these salts in the presence of a mixture of quartz and earthy 

 carbonates, aided by a gentle heat, will serve to effect a union of the 

 quartz with the earthy bases, eliminating carbonic acid. A small 

 amount of alkali may thus, like a leaven, continue its operation in- 

 definitely and change the character of a great mass of sedimentary 

 rock. Such a process is not only a possible but a necessary result 

 under the circumstances supposed, and we cannot, I think, doubt 

 that alkaline silicates play a very important part in the metamorphism 

 of sedimentary rocks, which are composed for the most part of earthy 

 carbonates, with siliceous, aluminous, and felspathic materialsf. 



The direct action between the carbonates and silica must neces- 

 sarily be limited by their mutual insolubility, and by the protecting 

 influence of the first-formed portions of earthy silicate ; but with the 

 solvent action of a small portion of alkali which is changed from 



* Reports of the Geol. Survey of Canada, 1851-53-54. 



f It is well known that small portions of alkalies are seldom or never wanting 

 in the earthy silicates, such as serpentine, talc, pyroxene, asbestus, epidote, ido- 

 crase, aud even beryl and corundum. See the memoir of Kuhlmann already cited. 



