642 A TREATISE ON AIETAMORPHISM. 



small masses of dense rocks, especially those which are in the midst of 

 porous rocks, are much more altered. 



Porosity is also favorable to metasomatism because the necessary space 

 is available for the expansion reactions of the belt of cementation without 

 lifting the rocks. This necessity for more space is an important restraining 

 factor in the alteration of the dense rocks. 



In so far as the minerals are in a state of strain, this is favorable to 

 solution and redeposition, and therefore, in proportion as this condition 

 obtains, metasomatism is rapid. In so far as the mineral particles are finely 

 divided, they furnish a large area for the attacking solutions, and therefore, 

 in proportion as this condition obtains, metasomatism is rapid. Other 

 things being equal, the higher the temperature the more rapid is metaso- 

 matism. Since normally the temperature increases 1° C. for 30 meters in 

 depth, metasomatism is likely to be much more rapid in the middle and 

 lower parts of the zone of fracture than in the higher parts. The process 

 of metasomatism may be greatly promoted by increase of temperature due 

 to intrusives, as explained on pages 648-649. 



Metasomatism is rapid in proportion as the rocks are composed of 

 readily alterable minerals. Of these leucite, nephelite, olivine, etc., are 

 readily alterable; the pyroxenes, amphiboles, and micas occupy an inter- 

 mediate position; and quartz and the acid feldspars are very slowly 

 alterable under the conditions of the belt of cementation, although, as has 

 been explained in a previous chapter (see p. 519), the feldspars are some- 

 what readily attacked in the belt of weathering. 



Finally, the extent to which metasomatism has taken place is a direct 

 function of the age of the rocks. The older a formation the more likely 

 are the changes to be far-reaching. 



MIJlERALS PRODUCED. 



The dominant minerals produced by metasomatism are the same as the 

 cementing minerals (see pp. 621-627), and this is what should be expected; 

 for it is to be remembered that the solutions pass through the rocks slowly, 

 and that the minerals deposited in the openings and those deposited in the 

 body of the rock are alike the joint result of the reactions between the 

 solutions and solids. 



