QQQ A TREATISE ON METAMORPHISM. 



plastic rocks, and "in large part above the critical temperature of water. It 

 is a characteristic of the zone of anamorphism that the rocks are not 

 sufficiently strong to sustain themselves, and that consequently no large 

 and continuous openings are present; therefore the imprisoned water is not 

 subject to the hydrostatic pressure of a column of water reaching to the 

 surface only, as is that of the belt of cementation, but to the weight of 

 the rock column to the surface, which is approximately 2.7 times as much as 

 the pressure of an equivalent column of water. It has been seen that in 

 much of the zone of anamorphism the water is above its critical temperature, 

 and therefore that the enormous expansive force of this gas is tending to 

 drive the water from the zone of anamorphism to the belt of cementation. 

 In the experiment of Daubrde a the water gas traveled from a place of greater 

 pressure to a place of less pressure. So far, therefore, as this experiment is 

 applicable to the case under discussion it indicates that the water would 

 be driven from the zone of anamorphism to the belt of cementation. 

 From a consideration of physical principles, therefore, we conclude that 

 water does not pass from the belt of cementation to the zone of anamor- 

 phism, but probably is driven by the superior pressure and the expansive 

 force of the gas from the zone of anamorphism to the belt of cementation. 

 This conclusion, reached by physical reasoning, is fully confirmed by 

 the facts of observation. It has already been pointed out that, in the zone 

 of anamorphism, in the transformation of a shale to a schist the amount of 

 combined water is reduced from 4 to 1.50 per cent. But the amount of 

 free water in the zone of anamorphism is only a fraction of 1 per cent. 

 This liberated water must have escaped either above or below. That the 

 relative pressures in the zone of anamorphism and the belt of cementation 

 are such as to make the movement upward has already been seen, and there 

 can be little doubt that much or most of the water freed by dehydration 

 goes in this direction. This seems to me highly probable from the fact that 

 the amount of combined water in the recrystallized rocks lessens as the 

 intensity of the metamorphism increases. Where the rocks have been 

 metamorphosed under moderately deep-seated conditions, such minerals as 

 chlorite may be found; where the metamorphism was deeper seated, biotite 

 and muscovite, containing less water, are abundant; but where the 

 metamorphism is most profound, and where apparently the pressure has 



«Daubree, cit, p. 236-241. 



