IMPORTANCE OF WATER IN RECRYSTALLIZATION. 745 



during the process of metamorphism, and thus constantly furnish a supply 

 of the medium through which recrystallization can take place. The same 

 is certainly true of many other rocks the materials of which have been 

 subject to the processes of alteration in the zone of katamorphism, and 

 which therefore contain hydrous minerals. The most important exceptions 

 are the pure silica and the pure carbonate rocks. 



The steadily lessening amount of combined water with increasing 

 metamorphism is illustrative of the facts already explained on pages 

 178-180, that the deep-seated zone of recrystallization is one of dehydra- 

 tion. In opposition to this, the alterations under conditions of small depth 

 are those of hydration. Many of the rocks metamorphosed under such 

 conditions contain as high or higher percentages of water than the soils 

 and clays the water contents of which have been quoted. If later such 

 hydrated rocks, whether of igneous or of aqueous origin, were subjected 

 to alterations in the zone of anamorphism, recrystallized schists or gneisses 

 would be developed. 



In the above no account is taken of the mechanically mingled water. 

 This is undoubtedly always present in important amount in all of the 

 sedimentary and in many of the igneous rocks, and is an additional supply 

 of the agent of transformation. The amount of this mechanical water has 

 already been discussed, (bee Chapter III, pp. 124-127.) It varies from a 

 fraction of 1 per cent of the volume in the densest rocks to' as high as 50 

 or more per cent in the more open soils and clays. At the outset of the 

 process of change the mechanically included water is often an abundant 

 source of the agent of metamorphism ; but when the rocks become compacted 

 by pressure, or cemented, or both, this water is largely removed. This is 

 likely to take place in large measure in the zone of katamorphism; 

 consequently, when the rocks reach the zone of anamorphism the amount 

 of mechanical water is usually much reduced. It follows that the originally 

 mechanically mingled water, while probably of importance, can not be 

 depended upon as an adequate supply for recrystallization in the zone of 

 anamorphism. Indeed, it is believed that the water freed by dehydration 

 is the most reliable source of water for recrystallization in the zone of 

 anamorphism. (See pp. 662-663.) 



The facts presented on the preceding pages give strong support to the 

 idea that presence of water at the beginning of the process of metamorphism 



