368 A TREATISE ON METAMORPHISM. 



the belts of weathering and cementation, and the only possible conclusion 

 is that these minerals have recombined and reproduced the heavier minerals 

 of the lower physical-chemical zone. That this is so is shown by the fact 

 that, barring- the water and the carbon dioxide which are liberated in the 

 process of alteration, the average chemical compositions of the unaltered 

 pelites and their metamorphosed equivalents are nearly the same. 



While it is held that the reactions are reversible, it is not supposed 

 that this is often exactly the case for a given rock. In order that this 

 should even approximately take place, it would be necessary that there be 

 no change of average composition in the zone of katamorphism, and this is 

 never the case. The minerals formed in the zone of anamorphism depend 

 not only upon the minerals of the zone of katamorphism present, but upon 

 their proportion and many other factors. What is meant by the reversi- 

 bility of the reactions is that, when compounds produced in the zone of 

 katamorphism from a given mineral are together in proper proportions 

 and conditions in the zone of anamorphism, the original mineral may be 

 reproduced. 



If this law of the reversibility of reactions in the two zones be true, 

 the question naturally arises why so few of the reversing reactions in 

 the zone of anamorphism have been recorded. The answer lies in the 

 difference in the readiness with which observations may be made in the 

 two zones. The reactions of the belts of weathering and cementation of 

 the zone of katamorphism have been more fully described, because they 

 are constantly taking place at or near the surface under conditions of 

 ready observation. Many of the reverse reactions have not been fully 

 described, because they occur at depth, and because in areas of strong 

 metamorphic action they have been complete. Usually gradation from 

 practically complete reactions to very incomplete reactions in the zone of 

 anamorphism is comparatively rapid. But notwithstanding the very imper- 

 fect observations of the zone of anamorphism, the general reversibility of 

 the reactions in the two zones seems as certain as if it were established by 

 observation, and it is believed that it will be established by observation. 



If the conclusions of the foregoing- paragraphs be correct it is evident 

 that there is an almost entirely neglected field of observation in metamor- 

 phism — that by which the minerals of the zone of anamorphism are produced 

 from the minerals of the zone of katamorphism. 



