590 A TREATISE ON METAMORPHISM. 



not here be discussed. For the present purpose the important point is that 

 for any locality at a very moderate distance below the surface the water 

 and the rocks have a common and practically invariable temperature. 



It is highly probable that the point at which there is no appreciable 

 annual change of temperature is not deeper, on the average, than the level 

 of ground water. If this be so we may consider the ground water as 

 entering the belt of saturation at a fixed temperature. Assuming this, in 

 order to answer the question whether the water is colder or warmer when 

 it issues from the belt of cementation than when it entered the belt, it is 

 necessary to consider in what manner the water may gain or lose heat 

 during the interval. 



For the following reasons it appears highly probable that, on the 

 average, the water gains heat during its underground journey: 



(1) Earth movements are general for the lithosphere, and in many 

 regions these are of the most intense character. So far as earth movements 

 take place, to a very large extent the energy of the mechanical action finally 

 passes into heat. Where the movements are forceful the amount of heat 

 thus produced is considerable. The circulating underground water in 

 contact with the rocks necessarily absorbs some of this heat. 



(2) It has been shown that the dominant chemical reactions that take 

 place in the belt of cementation are those which liberate heat. Of all the 

 reactions characteristic of the belt of cementation it has been pointed out 

 that hydration is the most important, and this process liberates a large 

 amount of heat. The chemical reactions therefore furnish heat which the 

 percolating waters must certainly absorb, in part at least. 



In the lower zone, that of anamorphism (see p. 167), it has been 

 pointed out that the dominant chemical reactions take place with absorption 

 of heat. But where these reactions take place it is only through the 

 expenditure of mechanical energy which liberates heat to a greater extent 

 than that absorbed by the chemical reactions. That is, as a result of 

 mechanical action and chemical action together, as fully developed on 

 pages 110-113, there is a residuum of heat liberated to be absorbed by 

 the water, and therefore if the reactions of the zone of anamorphism 

 affect the temperature of the water of the belt of cementation at all it 

 must be by addition of heat. 



(3) Very large quantities of heat are brought into the belt of cementa- 

 tion by intrusive igneous rocks. The intruded rocks are partly cooled by 



