Invasion to Regional Metamorphism. 267 



forces once more upon tliis region, it is still heated 

 to near the temperature of crystallization ; rock mashing 

 and recrystallization take place in it. The development 

 of differentiation to its final stages gives an acidic 

 magma in the upper part of the reservoir. The es- 

 cape of the gases in whose presence only this magma 

 can remain molten hastens the crystallization to great 

 depth. The crust is once more strengthened, rock 

 mashing becomes ineffective, the period of orogenic 

 activity as marked by igneous intrusion and dynamo- 

 metamorphism has come to a close. 



