612 A TREATISE ON METAMORPHISM. 



the liberated silica remains in situ, varies in most cases from 15 to 50 per 

 cent. If, therefore, the carbonates produced and the silica liberated be 

 deposited, there is great increase in volume. 



While in the belt of cementation carbonation unquestionably domi- 

 nates over . decarbonation, doubtless it frequently happens during- earth 

 movements that the pressure is so great that silication and decarbonation 

 take place. 



HYDRATION. 



It has been stated that the belt of cementation might almost equally 

 Avell be called the belt of saturation. Water is everywhere present. The 

 process of hydration is one which involves a great increase of volume, but 

 openings exist which may be utilized by the process without overcoming 

 the strength of the rocks or gravity. Therefore the conditions are ideal for 

 hydration, and this everywhere occurs. Indeed, it is in the belt of cemen- 

 tation that the great group of hydrous silicates form most abundantly. 

 The belt of cementation is the home of the hydromicas, of the chlorites, of 

 the zeolites, of serpentine, of the epidotes, and of limonite and gibbsite. 

 Kaolin and talc also form there, although they are more especially char- 

 acteristic of the belt of weathering. The increase in volume in these 

 processes of hydration, provided all the compounds remain in situ, varies in 

 most cases from 20 to 50 per cent- While, as already explained, oxidation 

 and carbonation are important in the belt of cementation, the process of 

 li3 T dration occurs on a vastly greater scale, and the chief change in volume 

 in this belt is due to this process. 



Just as decarbonation may take place under conditions of exceptional 

 pressure in the belt of cementation, so dehydration may, and certainty does, 

 occur, although this is so subordinate to the process of hydration as almost 

 to be negligible. 



SOLUTION AM) DEPOSITION. 



Both solution and deposition are processes of great importance in the 

 belt of cementation. We have already seen that deposition occurs on so 

 great a scale as to make the tilling of the cavities and consequent cementing 

 of the rocks the rule. While at first thought it might be supposed that 

 solution is subordinate, a close analysis shows that this process is not less 

 important than deposition, and, indeed, it may more than balance deposition. 



