640 A TREATISE ON METAMORPHISM. 



The material precipitated is derived from three sources — that carried by 

 the waters passing from the belt of weathering into the belt of cementa- 

 tion, that contributed by igneous emanations, and that which passes into 

 solution within the belt of cementation. While at any one time the belt of 

 weathering may not be of great thickness, as a consequence of denudation, 

 there is steady addition of new material to this belt, and therefore a con- 

 stant supply of material. If all of this material could be deposited in the 

 belt of cementation it would undoubtedly be adequate for the work. The 

 same is true of the material which passes into solution within the belt of 

 cementation itself. But doubtless if the belt of cementation had only one 

 of these sources, cementation would be very imperfect. It is only by the 

 combination of the important sources of material that an adequate supply 

 is obtained to furnish the issuing solutions the abundant materials which 

 they cany and yet leave a sufficient residuum for cementation. 



METASOMATISM. 

 DEFINITION. 



Metasomatism may be defined as the process by which original minerals 

 are partly or wholly altered into other minerals, or are replaced by other 

 minerals, or are recrystallized with or without mineral changes, or one or 

 more of these together. In the alteration of a mineral into other minerals, 

 or in its replacement by other minerals, there may be addition or subtraction 

 of certain constituents. As a result of the changes, the new rock may gain 

 or lose variety in the minerals composing it. Either by the substitution of 

 constituents, or by the loss of constituents, and therefore concentration of 

 other constituents, a resultant rock may have a predominant mineral. 



EXTENT OF PROCESS. 



In rocks changed by metasomatism under mass-static conditions all 

 stages of alteration may be seen, from comparatively fresh rocks, in which 

 the changes are incipient in the minerals most readily alterable, to those 

 rocks in which all alterable minerals have been transformed by metasomatism 

 into others which are permanent under the prevailing conditions. 



Throughout extensive areas important formations are so altered that 

 no original mineral remains. That the , recrystallization of great masses of 



