REACTIONS IN ZONE OF KATAMORPH1SM. 161 



alteration; for the heat developed by the first reaction is retained by the 

 adjacent material, at least for a time, and this promotes further reaction, etc. 

 But this tendency, as has been seen, may be reversed if the temperature 

 becomes too high. (See p. 79.) 



Since the law of chemical reactions with the liberation of heat is the 

 dominant factor in this upper zone, alterations may take place which 

 work with or against pressure. In the first case both the chemical reaction 

 and the compression in volume result in the liberation of heat. In the 

 second case the heat liberated is that developed by the chemical reaction 

 minus that absorbed as a result of the work done in expanding the volume. 



As a matter of fact, near the surface of the earth the very important 

 reactions from the point of view of the nonmetallic elements, aside from 

 solution, are those of oxidation, hydration, and carbonation. Oxidation 

 and hydration commonly involve the addition of material, although the 

 former frequently occurs by substitution of oxygen for sulphur, and 

 therefore by desulphidation. Carbonation frequently involves the addition 

 of material, but more commonly occurs by the substitution of C0 3 for Si0 2 

 and the decomposition of silicates. Often the freed silica, or a part of it, 

 remains in situ. All of these reactions are well known to liberate heat 

 Commonly they decrease rather than increase the specific gravity of the 

 minerals. Since they usually involve addition of material, it is clear that 

 where all the residual material, or a large part of it, remains in situ the 

 volume of the rocks is considerably increased. However, it will be seen 

 that solution is also a very important reaction in parts of the zone of 

 katamorphism, and where this takes place to a sufficiently great extent 

 the volume of material may be decreased. 



The main part of the oxygen and much of the carbon dioxide for 

 oxidation and carbonation is directly or indirectly derived from the atmos- 

 phere. The water is chiefly that of the ground circulation. It is there- 

 fore clear that in the upper zone oxygen and carbon dioxide are being 

 steadily abstracted from the atmosphere and fixed in the rocks, and ground 

 water is steadily becoming fixed by hydration. The amount of oxygen 

 and carbon dioxide thus fixed is great. If it Were not for replenishment, 

 it is little short of certain that the carbon dioxide of the atmosphere 

 would have long since become exhausted. But probably the amount of 

 water fixed by hydration is even greater than that of the gases, oxygen 

 jiox xlvii — 04 11 



