COMMON ROCKS 



27 



CLASSnaCATlON OF IGNhOUS ROCKS, (atter Tirrson) 



I Grained, constituent grains recognizable. Mostly intrusive. 



Non- 

 porphyritic 



Porphyritic 



Feldspathic rocks, usually 

 light in color 



With quartz 



Granite 



Granite- 

 porphyry 



With little or 

 no quartz 



Syenite 



Syenite- 

 ]X)rphyry 



Ferromagnesian rocks, general- 

 ly dark to black 



With subordi- 

 nate feldspar 



Dolerite 



])olerite- 

 porphyry 



Without 

 feldspar 



Peridotite 

 (rare) 



II Dense, constituents wholly or partly unrecognizable. Mostly extrusive rocks. 



Non-_ 

 porphytitic 



Porphyritic 



Light-colored, white, light gray, 



red, yellow, brown, light and 



medium green. Usually feld- 



Tipathic 



Felsite 



l-'elsite-porphyry 



Dark-colored to black, very dark 

 green or very dark gray. Usual- 

 ly ferromagnesian 



Basalt 



Basalt-porphyry 



III Rocks composed wholly or partly of glass. Extrusive. 



Obsidian, pitchr.tone, pumice, rarely porphyries of thes;. 



I'\' Fragmertal volcanic material, tuff, volcanic ash. 



F;gukk 4. — Granite from the Booth Quarry, Waterford. Show- 

 ing granitic texture. 



