766 A TREATISE ON METAMOEPHISM. 



of anamorphism. But the alterations in the zone of anamorphism where 

 intrusives are absent do not result in great changes in the chemical 

 composition. 



RELATIONS OF ZONE OF ANAMORPHISM TO ZONE OF KATAMORPHISM. 



It has already been intimated that the zones of anamorphism and 

 katamorphism are not sharply separated, and that there is a gradational belt 

 between them. This gradational belt follows of necessity from the varying 

 character of the rocks and from the varying forces and agents at work. 

 One rock is of a chemical and mineral composition which readily alters; 

 another is of a composition which alters with difficulty. One rock is weak; 

 another is strong. At one place the conditions are mass-static, or those of 

 quiescence; at another they are mass-dynamic, or those of movement. In 

 one place igneous rocks may be absent; in another, abundant. In one 

 place the temperature at a given depth may be low; at another, relatively 

 high. The amount of water has the widest range. Hence it is inevitable 

 that there should be no sharp bounding plane between the zones of 

 katamorphism and anamorphism. 



The weak or readily alterable rock may be changed by the reactions 

 of the zone of anamorphism, while a stronger and more refractory rock 

 below may be altered by the reactions of the zone of katamorphism. 

 Moreover, the same rock in the same position, because of varying condi- 

 tions, may be altered by the reactions of the zone of katamorphism and at 

 a different time by those of the zone of anamorphism. It follows that there 

 is a belt of considerable width in which we may have at one time, under 

 one set of conditions, the reactions of hydration, carbonation, and oxidation, 

 with expansion of volume; and at another time, under another set of 

 conditions, the reactions of dehydration, silication, and deoxidation, with 

 diminution of volume. Neither oxidation nor deoxidation is prominent in 

 the intermediate belt. 



We therefore have in an intermediate belt the metamorphic results of 

 the belt of cementation, or those of the zone of anamorphism, or various com- 

 binations of the two. In one place we may find the metamorphic effects of 

 the belt of cementation superimposed upon those of the zone of anamorphism; 

 in another we may find those of the zone of anamorphism superimposed 

 upon those of the belt of cementation. 



In order to understand more fully the phenomena in this transition 



