178 A TREATISE ON METAMORPHISM. 



The third important case in which the reactions occur in the opposite 

 sense in the zones of katamorphism and anamorphism are hydration and 

 dehydration. 



Hydration is a characteristic reaction of the zone of katamorphism, 

 only less important than that of carbonation; moreover, hydration occurs 

 on a great scale both in the belt of weathering and in that of cementation. 

 That hydration occurs extensively deep in the belt of cementation is 

 evidenced by the hydrated minerals which develop in the cavities of the 

 rather deeply buried rocks, such as the amygdules of amygdaloids. 

 Hydration represents, in the words of the first part of van't Hoff's law, "an 

 association which takes place with great liberation of heat." This process 

 also results in very considerable increase of volume, provided all or nearly 

 all of the products formed remain in situ. 



Dehvdration is a characteristic reaction of the zone of anamorphism, 

 only less important than that of silication. When the hydrated minerals 

 formed in the belt of katamorphism pass into the zone of anamorphism by 

 deep bui-ial they are dehydrated. The pressure, or the high temperature, 

 or the two combined, unite to drive off a large part of the water. Dehy- 

 dration, in the words of the second part of van't Hoff's law, represents " a 

 dissociation which takes place with great absorption of heat" and it takes 

 place with decrease of volume. 



Therefore, so far as hydration and dehydration are concerned, in the 

 upper zone the first part of van't Hoff's law, that of chemical reactions with 

 the liberation of heat obtains, but in the lower zone the law of diminution 

 of volume controls, regardless of the heat effect. The first part of this 

 statement is sufficiently evident; the second possibly needs further expla- 

 nation. To drive off the combined water of rocks at ordinary pressure 

 usually requires a temperature above 110° C. This temperature under 

 mass-static conditions would not be found until a depth of 3,300 meters had 

 been reached. It is certain that at depths much less than this, and at 

 temperatures lower than this, dehydration takes place on an important scale; 

 for it will be shown (p. 744) that in the transformation of mudstones to 

 shales there is a loss of about one-half of the combined water. I conclude 

 that under many circumstances the increase in temperature is not suffi- 

 cient to reverse the reaction of hydration, and therefore the reversal must 



