682 A TREATISE ON METAMORPHISM. 



It has been seen (pp. 663-664) that the migration of water through the 

 discontinuous minute openings is exceedingly slow. Therefore the transfer 

 of material in any considerable amount and for great distances is very 

 slow. It follows that the process of cementation is confined to the depo- 

 sition of material in the minute microscopic spaces, and, ignoring injec- 

 tions, that this material is mainly if not wholly derived from the body 

 of the adjacent rock. In this respect there is a marked contrast between 

 the zone of anamorphism and the belt of cementation. It has been pointed 

 out on pages 617-619, 656, that in the latter belt the material deposited 

 may be great in quantity and derived from points remote from deposition. 

 It follows that, so far as the process of cementation is concerned, the chemical 

 composition of the rocks is little changed in the zone of anamorphism; 

 whereas, as has been shown on pages 655-656, in the belt of cementation 

 the chemical composition may be greatly changed by this process. 



MBTASOMA TIS3 r . 



Metasomatism in the zone of anamorphism may take place in various 

 ways, the same as in the belt of cementation. A mineral may recrystallize 

 without change in chemical composition, as, for instance, the alteration of 

 tridymite to quartz. Two or more minerals may unite to form a single 

 mineral, as, for instance, the union of, calcite and quartz, producing wollas- 

 tonite. A mineral may change into two or more minerals, as, for instance, 

 kaolinite into andalusite and quartz, but this class of reactions is much 

 more common in the belt of cementation. One mineral may be replaced 

 by another, as, for instance, the substitution of magnetite for quartz. Two 

 adjacent minerals may react upon each other, producing a third mineral, 

 as, for instance, bytownite and olivine, producing amphibole." But more 

 frequent than any of these simple reactions are complex changes by which 

 the materials from a number of minerals rearrange themselves to produce 

 more than one new mineral. These various reactions between the minerals 

 are mainly accomplished, as usual, through the medium of the water 

 solutions. 



There are important differences between metasomatism in the zone of 

 anamorphism and metasomatism in the belt of cementation. In this zone 



"Williams, Geo. H., The gabbros and associated hornblende rocks occurring in the neighborhood 

 of Baltimore, Md. : Bull. U. S. Geol. Survey No. 28, 1886, p. 52. 



