616 A TREATISE ON METAMORPHISM. 



It has been fully explained on pages 79-81 how important a slight increase 

 in temperature is, not only in respect to the speed with which material is 

 dissolved, but in respect to the quantity which may be taken into solution 

 before saturation is reached. This small average additional increment of 

 heat in the issuing water over that of the entering water is very favorable 

 to the presence in the issuing water of larger amounts of material than was 

 contained by the entering water. 



Therefore it appears that, in proportion as the underground journey 

 of the water in the belt of cementation is slow, in so far as it has a hori- 

 zontal course, in so far as it enters at a higher elevation than it issues, and in 

 so far as it issues at a higher temperature than it enters, it should contain 

 more material in solution per unit mass when it issues than when it enters. 

 From all these points of view it is to be expected that ground waters 

 contain more material in solution when they issue from the belt of 

 cementation than they contained when they entered that belt, and that 

 waters which have taken a journey of considerable length in a uniform 

 formation will have reached a state of almost complete equilibrium between 

 themselves and the surrounding rocks, or will be saturated. 



But another important factor enters into the matter. It is not to be 

 supposed that during the underground journey the materials which are held 

 in solution are the same. Throughout the journey there are various chemical 

 interactions. Much of the material brought from the belt of weathering is 

 deposited; there is solution of material at a certain place, and later depo- 

 sition of it elsewhere; there is interaction between the solutions and solids; 

 there is interaction between the mingled solutions from different sources. 



It is the uncertainty as to the average effect of these various chemical 

 reactions that leaves us in doubt as to the end result ; that is, whether or not 

 the solutions of all systems of circulation are, on the average, richer in 

 material when they emerge from the belt of cementation than when they 

 entered it. For instance, if a solution containing oxygen were mingled with 

 a solution in which iron was the chief base, hematite or limonite would be 

 precipitated, and the solution would be depleted in material even if the 

 temperature of the water were higher at the end than at the beginning 

 of the journey. Again, colloidal silicic acid is readily soluble even at low 

 temperatures, but the acid has a marked tendency to decompose into water 

 and silica, though the reaction is a slow one. So far as it takes place in the 



