662 A TREATISE ON METAMORPHISM. 



sphere which pass into the lower part of the lithosphere also contain some 

 mechanically mingled water, but it is rather probable that the amount is 

 very small. 



(b) It has been noted that dehydration is one of the most important 

 reactions of the zone of anamorphism. Water is thus set free within the 

 zone of anamorphism. Dehydration is therefore a certain source of water. 

 It is explained elsewhere (pp. 742-744, 895-896) that in muds the com- 

 bined water is about 8 per cent, in shales is only about 4 per cent, in slates 

 about 3 per cent, and in schists about 1.50 per cent. The process of 

 metamorplhsm b) r which a mud is transformed to a schist requires the 

 squeezing out of least 6.5 per cent of water by weight. If the conclusion 

 be true that in general in the zone of anamorphism the quantity of free 

 water is less than 1 per cent, the process of dehydration is capable of 

 producing more water than is present at any one time. Dehydration is 

 therefore an important source of water. Indeed, water of hydration is 

 probably a main reservoir of this essential agent for the recrystallization 

 of the rocks in the zone of anamorphism. 



(c) It will be explained under "Injection" that igneous intrusion is one 

 of the important processes which modify the zone of anamorphism. It is 

 well known that when a magma makes its way to the surface it may contain 

 a considerable amount of occluded water. The amount of this water is 

 greater in many cases than the amount ordinarily contained in the crystal- 

 lized deep-seated equivalents of these magmas. If this be so, the process of 

 crystallization of intruded magma in the zone of anamorphism may release 

 water within the zone, and thus furnish another important source of this 

 agent within the zone itself. How important this source of supply is, is 

 more or less conjectural, but I suspect that it is important, as will be more 

 fully explained under " Pegmatization." But it is necessary to say here 

 that where there are great complex intrusive masses in the zone of anamor- 

 phism the central masses crystallize like igneous rocks. Often peg-matites 

 and veins form peripherally to this central mass. In their crystallization 

 water is believed to be a veiy important agent, and it is rather probable 

 that this agent, or at least a large part of it, is that liberated during the 

 crystallization of the large igneous masses. 



In addition to the above sources of water, water may have passed from 

 the belt of cementation downward into the zone of anamorphism, or from 



