METAMORPHISM 



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gneiss. Gneisses may be formed either (i) by the metamorphism 

 by mashing of granite or other igneous rock; (2) by the meta- 

 morphism of sedimentary beds; or (3) when a granite magma is 

 intruded into sedimentary or schistose beds under pressure operating 

 from a distance, the molten magma spreading along the sedi- 

 mentary planes or between the folia of the schists. This intimate 

 admixture permits of extensive mineral changes, and the two types 

 of rock, very different in geological age, become welded into a com- 

 posite gneiss. 



Table Showing Metamorphic Changes 



Sediments 



Sedimentary Rocks 



Metamorphic Equivalents 



Gravel 



Conglomerate 



Gneiss and various schists 



Sand 



Sandstone 



Quartzite and quartz schist if from 

 pure quartz sand ; mica schist 

 if certain impurities are present 



Clay 



Shale 



Slate and schists, especially mica 

 schist 



Lime deposits, such as 

 chalk or shells 



Limestone 



Marbles 



Igneous Rocks 



Metamorphic Rocks 



Granite, syenite, and other rocks with 

 much feldspar 



Gneiss 



Fine-grained feldspar rocks, such as felsite 

 and tuffs 



Slate and schists 



Diorite, basalt, and other basic rocks 



Hornblende schist and other schists 



Summary of Causes of Metamorphism 



The important factors to be considered in the production of meta- 

 morphism are heat, moisture and pressure, mechanical movements, 

 and the nature of the material involved. 



Heat. — The heat necessary for metamorphism may come (1) 

 from igneous intrusions. In this way the surrounding rocks are 

 hardened and dehydrated. The process is shown in the manufac- 

 ture of bricks, in which the clay is dehydrated and is hardened to a 

 rock-like mass by partial fusion. New minerals are often developed 



