602 A TREATISE ON METAMORPHISM. 



masses by faulting, into blocks by jointing, into layers by fractures along 

 bedding planes and other planes of weakness, into narrow slices by fissility, 

 and into irregular fragments by brecciation. It has furthermore been 

 seen that deformation produces comparatively unimportant changes in the 

 size of the masses between the fractures. In these masses the original 

 structures and textures are unaffected. But it will be explained under 

 cementation, metasomatism, and injection, that the ruptures furnish condi- 

 tions very favorable for these processes. Everywhere the ruptures give 

 ready passages for the circulating ground water; everywhere they give 

 ready entrance to intrusives. In consequence, the openings are closed 

 by cementation and injection. Moreover, from the openings the water, 

 frequently heated and loaded with material, is able to make its way for 

 short distances in the subcapillary openings between the grains, and thus 

 metasomatic processes are greatly promoted. Where the spacing of the 

 fractures is small metasomatism may modify the unbroken masses through- 

 out. Where the spacing of the ruptures is somewhat wide the metasomatic 

 alterations may not extend all the way from one passage to another, and 

 thus leave the interiors of the masses unaffected. Therefore there is very 

 great variation in the amount of metasomatic alteration in passing from the 

 walls of the openings to the interiors of the large unbroken masses. This 

 has been fully worked out in connection with ore bodies, which commonly 

 form along widely spaced, simple or complex, large openings. Lindgren 

 especially has shown" how metasomatic processes greatly affect the walls 

 of the fissures. The effect gradually dies out in passing away from the 

 openings, but may be marked for some distance. 



CHEMICAL 'WORK. 



The chemical work in the belt of cementation needs consideration 

 from two points of view — the chemical changes and the resulting processes. 



CHEMICAL CHANGES. 



The chemical work within the belt of cementation is accomplished 

 mainly by water solutions. Water is here the great agent of metamor- 

 phism, but the igneous rocks are second only to water solutions. They 



"Lindgren, Waldernar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol. 30, 

 1901, pp. 578-692. 



