164 A TREATISE ON METAMORPHISM. 



permanent aridity dehydration may locally occur. As already noted, as a 

 result of oxidation, carbonation, and hydration, the volume of the rocks 

 would be greatly increased if all the compounds formed remained in situ; 

 but the complex process of solution is dominant. Many of the compounds 

 formed are dissolved in large quantities and transferred by the overground 

 water circulation to the sea, or by the underground water circulation to the 

 belt of cementation below. Consequently the volume of the rocks contin- 

 uously decreases in the belt of weathering; and finally the resultant material 

 may occupy but a small fraction of the original volume. 



In the belt of weathering, in addition to the characteristic chemical 

 reactions, mechanical disintegration is the rule. Thus the complex results 

 of weathei'ing may be classified into disintegration, decomposition, and 

 solution. As a final result of the various mechanical and chemical changes, 

 rocks soften and degenerate. As coherent solids they are destroyed. The 

 processes of the belt of weathering are therefore destructive. The minerals 

 which remain are usually few and simple, and ordinarily are not well 

 crystallized. In the destructive processes all of the agents of meta- 

 morphism, both inorganic and organic, are actively at work. The details 

 of these processes are fully developed in Chapter VI, on " The belt of 

 weathering." 



BELT OF CEMENTATION. 



The belt of cementation extends from the bottom of the belt of 

 weathering to the bottom of the zone of katamorphism. On the average 

 this belt is therefore much thicker than the belt of weathering. All of the 

 very important reactions characteristic of the zone to which the belt 

 belongs — viz, oxidation, carbonation, and hydration — take place. Water 

 is everywhere abundantly present in the belt, and hence hydration is the 

 most important of the three reactions. The minerals produced by meta- 

 somatic change from the original minerals and those deposited from the 

 solutions are likely to be strongly hydrated. The processes of carbonation 

 and oxidation in the belt of cementation are largely limited by the amount 

 of carbon dioxide and oxygen there contained. 



It will be seen (pp. 608-610) that carbon dioxide is derived from several 

 sources and that carbonation is usual throughout the belt, but that the 

 oxygen is limited to that derived from above, and consequently that oxidation 



