648 A TREATISE ON METAMORPHISM. 



Not only do igneous intrusions follow the various openings of rocks, 

 such as those of joints, faults, fissility, brecciation, etc., and thus close them, 

 but they produce a metainorphic effect upon the injected rocks. This 

 contact effect may be confined to a centimeter or two adjacent to the 

 intrusive or it may extend to a distance of several kilometers, depending 

 upon many factors considered below. Furthermore, the contact effect, 

 whether confined within narrow limits or affecting an extensive area, may 

 be either slight or profound. 



As already stated on page 488, the contact effect of the injecting upon 

 the injected rock is known as the exomorphic effect, and that of the injected 

 rock upon the injecting rock as the endomorphic effect. As has been fully 

 explained on pages 489-494, the exomorphic effect is produced in two 

 ways — by the direct contact action of the igneous rocks due to rise of 

 temperature, and by indirect action through heated solutions. The exomor- 

 phism is ordinarily at a maximum immediately adjacent to the intrusive, 

 and it decreases in amount, either rapidly or gradually, with distance from 

 the intrusive. The change immediately adjacent to the injecting rock is 

 often very mai'ked. The rocks may be greatly indurated; the colors may 

 be changed, etc.; and these physical changes are commonly accompanied 

 by changes of chemical and mineral compositions. 



The modifications of the intruded rocks immediately adjacent to the 

 intrusive rocks are commonly attributed to direct contact action, but the 

 change in chemical composition show's that solutions are also an important 

 factor in the result, and therefore that there is indirect action. Indeed, in the 

 belt of cementation the two are never independent, and it is impracticable 

 to separate direct and indirect exomorphism and measure their quantitative 

 importance. While this is true, it will be seen that contact metamorphism 

 in the belt of cementation is only in small measure direct, and in very large 

 measure indirect. 



As fully explained on pages 490-494, in the belt of weathering the 

 exomorphic effect is accomplished through gaseous solutions; in the belt 

 of cementation the exomorphic effect is accomplished mainly through liquid 

 solutions. In the latter belt, as in the belt of weathering, the solutions are 

 modified in two ways — by the actual emission of water and associated 

 materials from the igneous rocks to the solutions, and by the simple 



