488 A TREATISE ON METAMORFHISM. 



dominates over the reverse set, all the above sets of reactions take place 

 on an important scale in the belt of weathering, although the belt is 

 characterized by the one set rather than the other. 



CONTACT METAMORPHISM. 



Contact metamorphism is a term used to cover the mutual effects of 

 intrusive and intruded rocks. The effect upon the intruded rocks is known 

 as exomorphic and that upon the intrusive rock as endomorphic. The first 

 is of greater consequence. In considering the exomorphic effect, ordinarily 

 there has been no well-defined attempt to estimate the relative importance 

 of the direct and indirect action of the igneous rocks. The direct effect is 

 due to the heating of the intruded by the intrusive rock. The indirect 

 effect is due to the increased activity of the gaseous and liquid solutions 

 caused by the intrusive rocks. 



The gaseous and liquid solutions adjacent to igneous rocks differ from 

 ordinary solutions in two respects. First, at the time of the crystallization 

 of magmas, gases and liquids emanate from the igneous rocks. The chief 

 constituent of these emanations is, of course, water, but associated with the 

 Avater are other compounds. By the circulation of the gaseous and liquid 

 solutions the material emanating from the igneous rocks may pass into the 

 surrounding rocks. A second and still more important indirect effect is 

 that of heating the solutions of the surrounding rocks. In Chapter III, on 

 "The agents of metamorphism," it has been explained that solutions gain 

 marvelously in their metamorphic power by heat, and adjacent to igneous 

 rocks the temperature may reach or even surpass that of the critical 

 temperature of water. While by conduction high temperature progresses 

 from the magma, by convection, where the circulation is vigorous, solutions 

 having a high temperature are carried over wide areas. In this treatise 

 contact metamorphism is considered in three places, for the nature of the 

 contact action, like that of other forms of metamorphism, is very different 

 in the different belts and zones. Therefore the subject of contact metamor- 

 phism is considered under the belt of weathering, the belt of cementation, 

 and the zone of anamorphism. In this chapter only the first part of the 

 subject is considered; the other parts may be found under the heading 

 "Injection," on pages 646-652. 



