THE GLACIAL PERIOD 77 



recognized by their easy separation into thin layers like roofing 

 slate. Sedimentary rocks composed of the disintegration of 

 granites are by metamorphosis altered to schists and gneiss. 

 The latter contains the same minerals as granite, but there is 

 evidence of stratification and the crystals or grains of the com- 

 pressed minerals are arranged with their long axes in one plane. 

 If the compression has been so great as to flatten the component 

 grains or crystals into scales, the rock becomes a schist, easily 

 broken down by the fingers, especially when it is somewhat 

 weathered. Schists are commonly named from the mineral that 

 is so predominant as to give them their chief character, as mica 

 schist, chlorite schist, etc. 



Granites seem also to have been transformed directly by 

 metamorphic agencies into gneiss and the schists, so that the 

 end result of metamorphism on such igneous rocks and on sedi- 

 mentary rocks derived from them may be identical. 



