608 A TREATISE ON METAMORPHISM. 



Thus the process of deoxidation or reduction, so far as it takes place, 

 involves the passage of the oxygen abstracted to the reducing agents or 

 their oxidation. Moreover, the sum total of the heat effect, taking into 

 account both the reduction and- the oxidation, is to liberate heat. Hence 

 the reaction is one normal for the zone of katamorphism. The sulphides 

 formed may be precipitated and serve as one of the subordinate cementing 

 substances. The formation and precipitation of sulphides will be much 

 more fully considered in Chapter XII, on "Ore deposits." 



We have just seen that deoxidation, so far as it takes place, is explained 

 by oxidation of some other compound. But oxidation by the oxygen origi- 

 nally derived from the surface and present in the solution does not involve 

 the deoxidation of some other compound. It therefore follows that the 

 process of oxidation in the belt of cementation overbalances that of reduc- 

 tion in the belt by the amount of oxygen which is carried into the belt of 

 cementation by the entering waters, and is abstracted from it by the union 

 of the oxygen with the compounds encountered during its underground 

 journey. 



It has been explained on page 467 that the process of oxidation of 

 inorganic compounds involves increase of volume varying under ordinary 

 circumstances from a small amount to as much as 64 per cent. But by the 

 oxidation of the carbonates carbon dioxide is liberated, and this may give 

 a considerable decrease in the volume of the product, amounting in some 

 instances to as much as 50 per cent. 



In summary, it is clear that oxidation is a reaction characteristic of 

 the belt of cementation, but that in this belt it has no such importance 

 as in the belt of weathering. In the belt of cementation oxidation goes 

 on but to a very limited extent. While in the upper part of the belt great 

 masses of highly oxidized rocks are locally found, as in the case of iron 

 ores, for the major part of the belt the evidences of oxidation are not 

 apparent, it being evidenced perhaps to the greatest extent by magnetite. 



CARBOIVATIOJi. 



The water which passes from the belt of weathering to the belt of 

 cementation carries dissolved carbon dioxide precisely as it does oxygen. 

 The quantity thus carried varies from an exceedingly small amount to 

 sufficient to produce saturation. 



