638 A TREATISE ON METAMORPHISM. 



part of the column, because of its contact with the rocks for a greater time, 

 would be more likely to be near the point of saturation than the higher 

 parts; and thus there is a tendency for upward diffusion against the current. 

 Diffusion is so slow that this movement might be thought to be of so little 

 consequence as to be negligible; but it has been seen that the average 

 downward movement of water in the belt of cementation is exceedingly 

 slow, the rate of movement frequently not being more than a meter per 

 annum. Therefore it is rather probable that upward diffusion against the 

 downward-moving currents is a matter of consequence in maintaining the 

 approximate equilibrium of the underground solutions, and therefore in 

 promoting continuous and uniform precipitation throughout the belt. 



During the upward movements which are characteristic of the later 

 parts of the journey of the water currents diffusion may also work against 

 the movement. This would be true if at the place where the upward 

 movement began saturation had not been attained. At some higher point, 

 as a result of the lessening temperature and pressure, saturation would be 

 reached, and thus the coefficient of saturation would be higher. Hence 

 there would be diffusion downward, or from places where there is less 

 material in solution to places where there is more material in solution. 



So far as the lateral movement of water is concerned, if variations in 

 temperature and pressure be ignored, there is diffusion from areas of greater 

 concentration to areas of less concentration. But the farther the water has 

 gone, and the longer therefore it has been in contact with the rocks, the 

 nearer are the solutions to saturation; hence there is a tendency for diffusion 

 to take place against the currents. But where the temperature and the 

 pressure are unequal, due to igneous rocks or orogenic movements or both, 

 this condition of affairs may be reversed. 



This factor of diffusion in connection with movements of the under- 

 ground currents apparently explains some of the anomalous features of the 

 belt of cementation. The downward movement may be presumed to be 

 generally so slow that diffusion keeps the solutions approximately at 

 equilibrium, and the process of cementation goes on subject to the laws of 

 the expansion reactions and selective precipitation. But at places where 

 the downward currents move so fast that diffusion does not maintain 



