484 A TREATISE ON METAMORPHISM. 



in volume as a consequence of the combination of these three reactions, 

 but doubtless in most cases the increase in volume, where the processes are 

 complete, is 15 to 50 per cent or even more. 



It is of importance to note that the oxygen, carbon dioxide, and water 

 added to the rocks in the belt of weathering are not directly derived from 

 other rock materials, but are in large measure derived from the atmos- 

 phere and hydrosphere. As already noted, the oxygen is directly derived 

 from the atmosphere ; the carbon dioxide comes from the atmosphere mainly 

 through the intermediary action of vegetation and other organic matter, 

 and water is derived directly or indirectly from the hydrosphere. The 

 depletion of the atmosphere and hydrosphere in oxygen, carbon dioxide, 

 and water is continuous, and through geological ages would doubtless 

 seriously encroach upon the supplies of these materials were it not for 

 the compensatory reversing reactions largely occurring in the zone of 

 anamorphism. (See pp. 366-369.) 



Concurrently with the processes of oxidation, carbonation, and hydra- 

 tion, the underground waters of the belt of weathering take compounds 

 into solution. The process of solution is not only concurrent with these 

 other processes, but is promoted by them. This follows from the fact that 

 the dominant processes of carbonation and hydration transform the com- 

 pounds into more soluble forms. The change of ferrous to ferric iron by 

 oxidation has a reverse effect, but the quantitative value of this reaction is 

 small compared with that of carbonation and hydration. 



It has already been explained that all natural compounds are soluble in 

 pure water, but the solution would be slow if this were the only solvent. It 

 has been seen that in the underground solutions of the belt of weathering 

 there are continually produced organic acids, carbonic acid, nitric acid, sul- 

 phuric acid, and hydrosulphuric acid. These produce corresponding salts — 

 carbonates, nitrates, sulphates, and sulphides. Also the presence of these 

 salts favors solution. Besides these salts chlorides are present. The active 

 acids of the solutions unite with the bases of the minerals. But in this process 

 the acids with which the bases were united in the minerals are displaced, and 

 these acids may also pass into the solutions and become active constituents. 

 The oxygen, carbonic acid, nitric acid, sulphuric acid, and hydrosulphuric 



