676 A TREATISE ON METAMORPHISM. 



CHEMICAL CHANGES. 



The chief chemical changes in the zone of anamorphism comprise 

 deoxidation, silication, dehydration, and solution and deposition. 



It is commonly true that the reactions of deoxidation, silication, 

 dehydration, and solution and deposition do not occur separately, but two 

 or more together; yet, in order to clearly understand their effect, each is 

 considered separately. 



DEOXIDATION. 



I 



Many of the sedimentary rocks contain organic material. In some 

 cases, at least, the deep-seated igneous rocks contain metallic iron, and 

 often they contain sulphides, among which bisulphide of iron is the most 

 important. The zone of anamorphism is deep below the surface, so that 

 oxygen can not get into it from the surface of the earth. Indeed, it has 

 been shown that the oxygen which passes downward with the surface 

 waters is usually exhausted before it has gone far into the belt of cementa- 

 tion. It follows that the solutions of the zone of anamorphism are reducing, 

 and the conditions those of deoxidation. Therefore highly oxidized com- 

 pounds are likely to be partly reduced. Ferric oxide is the most important 

 of such compounds. This compound may be reduced to the ferrosoferric 

 state, forming magnetite, or to the ferrous state, in which form it is in a 

 favorable condition for silication. The amount of reducing agents which 

 are present in the zone of anamorphism is very variable. For instance, 

 the sediments may have almost no organic matter or they may have a 

 great, even a predominant, quantity. Where reducing agents are abun- 

 dant little or none of the iron is likely to remain as ferric oxide. Where, 

 on the other hand, organic material is sparse and there are large quantities 

 of ferric oxide this may permanently remain in this state. Where deoxi- 

 dation occurs there is a decrease in volume, but in most cases the amount 

 is small. 



In the above paragraph it is supposed that deoxidation takes place 

 only if a reducing agent be present. There is no evidence that the pressure 

 may be so great that oxygen is squeezed out because of the demand for 

 decreased volume, although such a reaction is theoretically possible if the 

 pressure were high enough. 



