Invasion to Regional Metamorphism. 175 



of the rocks retain rather closely their original composi- 

 tion. Upon entering a rock formation, the passing 

 emanations seem to establish rather quickly a chemical 

 equilibrium, and thence to carry forward a heating and 

 crystallizing effect. But although this is true in a large 

 way, it is far from being a universal principle. There 

 is more or less transfer of magmatic material into the 

 w^all rocks, and reaction with them, with resulting meta- 

 somatism: amphibolite, garnet, and albite, as well as 

 other minerals, may be largely developed as a result. 

 The acidic magmas are much more important in this 

 respect than the basic, but wide individual variation is 

 to be observed. 



It is thought that the emanations from the same magma 

 at different stages are of highly different character. The 

 first gases are probably eliminated by supersaturation 

 before any crystallization. They include especially the 

 volatile constituents, chlorine, fluorine, boron, and water, 

 but most of the non-volatile oxides remain behind in 

 mutual solution. Another stage seems to carry off chiefly 

 silica and iron, and a following stage is rich in water- 

 vapor holding in solution the quartz and feldspar which 

 make the pegmatites. 



The metasomatic zone is to be expected, and is in fact 

 found, on the inner side of the metamorphic zone, devel- 

 oping in sequence following the first or metamorphic 

 recrystallization. It is apt to be very irregular in 

 boundary, following the zones of easiest passage through 

 the rocks. In very deep-seated masses, as in the Gren- 

 ville limestones involved in Laurentian granite, the result 

 is very much more pervasive than in more shallow zones. 

 Surrounding plateau batholiths (those intruded without 

 relation to orogenic pressures and rock mashing), lime- 

 stones offer the most favorable channels for the exten- 

 sion of the metasomatic zone, since they are subject to 

 volume changes, shattering, and ready chemical altera- 

 tion. Where non-uniform mountain-making pressures 

 have operated, however, giving rise to foliated struc- 

 tures,^^ the schists become the most favorable channels 

 for conducting aAvay the solutions and an intimate inter- 

 foliation of country rock and magma may result, known 



^^ [See John Johnston and Paul Niggli, The general principles underlying 

 metamorphic processes, Jour. Geology, 21, 599, 1913. This paper calls atten- 

 tion to the fact that non-uniform compression has a much more important 

 metamorphic effect than uniform pressure.] 



