The Petrography of the Granitic Rocks. 



81 



Quartz-mica-diorUe. 



North-West- 

 ern. 



Quartz 

 Orthoclase ... 

 Seritito 



Albite 



Labradorite (Abj Anj) 



15ytownite (Ab.^ An,.,) 



Biotite 



Augite 



Hornblende... 



Ilmonite 



Magnetite ... 



Sphene 



Apatite 



Minor constituents 



23-6 



38-4 

 11-0 



24-6 

 0-9 



0-2 

 0-8 



Ma zaruni 

 River. 



100-0 



11-8 



63-3 

 2-6 



20-8 

 0-6 



0-7 

 0-2 



100-0 



QiKirtz- 

 tiiorilc. 



Essequibo 

 River. 



22-2 

 2-0 



36-0 



34-6 

 2-1 

 2-1 



0-5 

 0-5 



100-0 



Barima 

 River. 



11-2 



30-1 

 4-0 



52-7 

 0-8 



0-3 

 0-9 



100-0 



The rocks may be classified according to their ultimate magmatic 

 composition as follows : — 



Aplite, Barima River 

 ,, Mazaruni River 

 ,, Essequibo River 



Granite, Mazaruni River. 



Yukanose. 

 Varingose. 

 Alsbachose. 

 Tehamose. 



