COMPONENTS OF THE EARTH'S CRUST, MINERALS AND ROCKS 123 



Rocks 



These are the materials of the strata and other masses which 

 form integral parts of the earth's crust. They may be classified 

 a® massive or igneous, sedimentary and metamorphic. The re- 

 lations of these three natural groups may be shown by a trian- 

 gular diagram, as follows : 



Massive 



or 

 Igneous 



Sedimentary 

 or 

 stratified 



bj heat and pressure >■ 



•< by erosion and deposition 



Metamorphic 



This is meant to show that an igneous rock may, by erosion, 

 be reduced to sediment and laid down in beds, or by heat and 

 pressure may be metamorphosed from its original massive con- 

 dition and become schistose. A sedimentary rock may also pass 

 through the metamorphic condition, become fused and enter the 

 igneous state. A metamorphic rock may arrive at the igneous 

 condition by heat and pressure, or may become sedimentary 

 through erosion and deposition. 



