. RELATIONS OF OXIDATION AND DEOXIDATION. 473 



It has been shown that in the belt of cementation oxidation is much 

 less prevalent than in the belt of weathering, and indeed that, on the whole, 

 deoxidation, especially where organic material is prevalent, is the rule. 

 Therefore, the reversal of the ordinary process of oxidation in the belt of 

 weathering usually takes place only where this belt is grading into or is 

 under conditions which are approaching those of the belt of cementation 

 and below. 



It has already been stated that the direct source of the oxygen for the 

 process of oxidation is the atmosphere. It is apparent from the foregoing 

 that a vast amount of oxygen is now being demanded for this process, and 

 the amount of oxygen which has thus been consumed during geological 

 time is beyond computation. But it will be seen under "The zone of 

 anamorphism" that there are also processes which restore oxygen to the 

 atmosphere. Which of these processes, oxidation or deoxidation, is, on 

 the whole, preponderant and what are the possible sources of supply for 

 oxygen can best be considered after all the reactions in each of the belts are 

 considered, and therefore this subject is taken up in Chapter XL 



CARBONATION. 



The process of carbonation ma)' - be defined as the union of carbonic 

 acid with bases, forming carbonates. Since the carbonates are not known 

 as original minerals in the igneous rocks, we must look in other directions 

 for the source of the carbon dioxide for the process of carbonation. The 

 immediate reservoir for this carbon dioxide^is undoubtedly the atmosphere, 

 but the amount in the atmosphere is very small, only 0.045 per cent by 

 weight, and the quantity now in the atmosphere is, as will be seen subse- 

 quently, insignificant as compared with that which must have been abstracted 

 from the atmosphere during past geological ages by the process of carbona- 

 tion. Either the atmosphere mrist have once contained vastly more carbon 

 dioxide than at present, or else it must have been continuously replenished 

 in this compound, or partly both. These questions can be best discussed 

 after the reactions of both the zone of katamorphism and the zone of ana- 

 morphism have been considered, and their consideration is deferred to 

 Chapter XL 



It has been explained that from the atmospheric reservoir the plants 

 absorb carbon dioxide, reduce it, and build it into their bodies. Further- 



