EEACTIONS IN BELT OF CEMENTATION. 165 



is usual in only a very limited part of the belt. Not only are the processes 

 of carbonation and oxidation subordinate to hydration, but the process of 

 oxidation not infrequently is stopped or reversed in all but the upper part 

 of the belt of cementation. This anomaly is due to the fact that many of 

 the rocks contain organic materials or sulphides or both which have a 

 strong affinity for oxygen. When the oxygen is exhausted from the water 

 derived from the belt of weathering the reducing compounds may act 

 directly as reducing agents or may produce reducing solutions. The 

 demands of these reducing agents for oxygen may abstract this material 

 from highly oxidized compounds, such as ferric oxide, basic ferric sulphate, 

 etc. Deoxidation in the belt of cementation is most commonly the result of 

 the burial of the higher oxide of iron and sulphates with a considerable 

 amount of organic material in the presence of abundant water. Under 

 these circumstances the ferric compounds may be reduced to ferrous 

 compounds and the sulphates to sulphides. 



But it is to be noted that the reduction of these compounds involves 

 simultaneous oxidation of the organic compounds, the resultant products 

 being C0 2 and water. The carbon dioxide may escape from the belt or 

 enter into other combinations. For instance, as explained fully in another 

 place, the ferrous compounds largely unite with the carbon dioxide, pro- 

 ducing carbonates. Similar reactions may take place with reference to 

 other less abundant metals, as, for instance, manganese, and some metals 

 may even be reduced to the metallic condition, for instance, copper, silver, 

 and gold. These reducing reactions in the belt of cementation, except in 

 the case of iron, are of small consequence from a geological point of view, 

 but they have a most important bearing upon the deposition of ores. (See 

 Chapter XII.) It thus appears that oxidation and deoxidation are both 

 rather important in the belt of cementation. 



The changes in the belt of cementation ordinarily produce crystalline 

 minerals. Minerals which were partly altered by processes in the belt of 

 weathering may be regenerated. This applies only to those minerals which 

 are adapted to the belt of cementation. The average specific gravity of the 

 rocks is usually lessened. 



It has been noted that the most characteristic reaction of the belt 

 of weathering is solution. In contrast with this the most characteristic 

 reaction of the belt of cementation is deposition in the openings of the 



