80 Van Ilise — Metamorphism of Rocks and Rock Flowage'. 



Lower Physico-chemical Zone. 



From the surface to considerable depth below the surface, 

 the temperature ever becomes higher, and consequently the 

 temperature may become so high that the tendency for 

 reactions to take place which result in the development of 

 heat is less dominant. However, at moderate depth, under 

 ordinary conditions, say at 9,000 meters, the temperature is not 

 very high, probably in the neighborhood of 300° C. Thus the 

 tendencies for reactions to take place under the first part of 

 van't Hoff's law, rather than the second part, would generally 

 still control for a very considerable depth if it were not for 

 the enormous pressure. In the lower zone this ordinarily 

 becomes the dominating factor, especially in places of mass 

 dynamic action, and reactions take place which result in the 

 production of less volume, the chemical forces which condition 

 a diminution of volume being very active, and the chemical 

 forces which condition an increase of volume being very weak 

 or even overcome. 



Relations of the Two Physico-chemical Zones. 



In so far as energy is concerned, there are four cases : The 

 chemical reaction may (1) release energy, and result in the 

 liberation of heat: (2) may consume energy, and result in 

 absorption of heat. The change of volume may be (3) by 

 compression, and result in the liberation of heat, or (4) by 

 expansion, and result in the absorption of heat. (1) and (3) 

 will be called plus, and when they are combined the heat 

 liberated is equal to their sum ; (2) and (4) will be called 

 minus, and when they are combined the heat absorbed is equal 

 to their sum. When (1) and (4) or (2) and (3) are combined, 

 heat may be liberated or absorbed, depending upon the rela- 

 tive values of the energy of the chemical reaction and that of 

 the change of volume. 



As a case in which the reactions as to temperature and pres- 

 sure are each in opposite senses in the upper and lower physico- 

 chemical zones may be mentioned hydration and dehydration. 

 The first process occurs in the upper zone, and represents an 

 association which takes place with the great liberation of 

 heat, while the second process occurs in the lower zone, espe- 

 cially in connection with mass dynamic action, and represents a 

 dissociation which takes place with important absorption of 

 heat. The first process results in very considerable expansion 

 of volume ; the second process results in equivalent contrac- 

 tion of volume. 



A second important reaction separating the outer crust of 

 the earth into two physico-chemical zones is the mutual replace- 



