METAMORPHISM OF SHALES. 897 



By comparing the analyses of the shales, slates, and schists it is seen 

 that the chief chemical changes are those of loss of water and of carbon 

 dioxide, and transformation of the ferric iron to the ferrous form. The 

 first of these changes is evidence of the process of dehydration, so char- 

 acteristic of the zone of anamorphism. It is notable, as pointed out in 

 Chapter VIII (p. 744), that in the change from clays and soils, which 

 certainly do not contain more water than muds, to shales and slates the 

 combined water is reduced by one-half, and that in the further transforma- 

 tion to schists the combined water is again reduced by one-half or one-third. 

 The disappearance of the carbon dioxide of the carbonates is due, of course, 

 to the process of silication, the silica uniting with the bases of the carbonates 

 and forming silicates, and the carbon dioxide slowly escaping with the 

 squeezed-out water. It has already been noted that deoxidation begins 

 in the change from muds to shales. The average amount of ferric iron 

 is further decreased as the process of metamorphism goes on, as shown 

 by the fact that in the shales the ferric oxide is 4.03 per cent, in the 

 slates is 2.726 per cent, and in the schists is 1.896 per cent. Correlative 

 with this process is increase of ferrous oxide from 2.46 per cent in the 

 shales to 3.634 per cent in the slates and 3.348 per cent in the schists. (§ee 

 pp. 890, 896.) In the analyses of shales from eight different States, given in 

 the publications of the United States Geological Survey, carbon is reported 

 in six of them. The carbon in the shales is an adequate reducing agent for 

 this work. While the amount of carbon in the shales is not great, it is per- 

 sistently present. The average amount of carbon in 78 shales is 0.81 per 

 cent." (See p. 890.) It is to be remembered that this amount of carbon 

 is much more than sufficient to reduce all of the ferric iron to the ferrous 

 state, as is shown by the following equation : 



2 Fe 2 3 +C=4 PeO+CO„. 



Since the molecular weight of two molecules of Fe 2 O s is 320, and the atomic 

 weight of carbon is 12, this equation means that 12 parts of carbon is 

 theoreticalh" sufficient to reduce 320 parts of the ferric oxide to ferrous 

 oxide. It follows, therefore, that 0.81 per cent of carbon would theoret- 

 ically reduce 20 per cent of ferric iron to the ferrous state. Since the total 



aClarke, F. AV., Analyses of rocks, laboratory of the U. S. Geol. Survey, 1880-1899: Bull. U. S. 

 Geol. Survey No. 168, 1900, p. 17, col. C. 

 MON XLVII — Oi 57 



