VOLUMES OF SILICON AND CARBON COMPOUNDS. 177 



rectly infers that when any of these carbonates occur with quartz at a 

 sufficient depth within the earth, where a temperature of 100° C. is reached, 

 this reaction may take place. He calculates that this depth will be 2,440 

 meters. He correctly infers that the presence of abundant carbon dioxide 

 in deep-seated waters is probably due to this process of silication. ffi We 

 now understand that under conditions of moderate pressure and temperature 

 not only are the carbonates which Bischof mentioned decomposed, but other 

 carbonates may be altered in a similar manner. However, it is noteworthy 

 that the carbonates which Bischof mentioned are those of predominant 

 importance. 



The substitution of silicon for carbon would result in increase of 

 volume provided silica were derived from the solutions and the carbon 

 dioxide passed into the solutions. But in the process of silication in the 

 belt of anamorphism little material is available from outside sources. 

 Therefore the most of the silica which replaces carbon dioxide in carbonates 

 must be considered as a solid. It is probable that a large part of the freed 

 carbon dioxide slowly escapes; for at temperatures prevailing- in the zone 

 of anamorphism the carbon dioxide is above its critical temperature, and 

 therefore a gas, and probably slowly makes its way through the subcapil- 

 lary spaces to the zone of katamorphism (see p. 667.) Hence the volume 

 comparison must be made between the carbonate and replacing silica 

 combined and the resultant silicate. On this basis there is a marked 

 diminution of volume. One of the simplest illustrations of the formation 

 of the silicates with condensation of volume is the development of wollasto- 

 nite from calcium carbonate and quartz. In this change the volume of the 

 solid remainder is decreased 31.48 per cent. However, this calculated 

 decrease is somewhat too great; for it will be seen (p. 667) that some 

 of the carbon dioxide does not escape, but is retained in the rocks in the 

 form of numerous inclusions. 



It appears from the foregoing that in the replacement of silicon dioxide 

 by carbon dioxide in the zone of katamorphism, the chemical law of reac- 

 tions with liberation of heat dominates over that of pressure; and that in 

 the substitution of silicon dioxide for carbon dioxide in the zone of 

 anamorphism the physical law that pressure demands decrease of volume^ 

 dominates over the chemical law of reactions with liberation of heat. 



« Bischof, cit. , vol. 1, pp. 237-241. 

 MON XLVII — 04 12 



