TlbS Petrography of the, Intrusiv, Diabase. 



9^ 



proportions of the enstatite-molecule with tlie aut,ate one. The presence 

 of enstatite is not uncommon in the rocks ; but the mineral itself is 

 never there in the proportions indicated by the analyses. Hence its 

 constituents must be in the augite. 



The following are the mineralogical compositions calculated from 

 the analyses given on page 95 : — 



tiuartz 



( Irthocliise 



Bytownite (Abj, An..,) 



Bytownite (Abi, Ang) 



Pyroxene ... 



Magnetite... 



Ilmenite ... 



Apatite 



Water 



Minor constituents 



Arakaka 



DKscrict, 



Barima 



River. 



G-3 



49-8 



31-1 



6-0 



0-9 



1-2 



100-0 



Cuyuni 

 River. 



1-.5 



6-7 

 46-1 



4-2-7 

 1-9 

 0-3 

 0-1 

 0-5 

 0-2 



100-0 



Mazaruni 

 River. 



9-0 



0-6 



44-8 



35-4 



2-.-! 

 0-3 



0-4 



100-0 



Essequiljo River. 



Mountain ^^^^^ 

 Masses. ■' 



3-7 

 0-5 



47-1 



41-2 

 6-5 

 0-6 



0-4 



100-0 



The diabase from the Barima, Cuyuni and Demerara districts is 

 Auvergnose, that from the Essequibo and Potaro districts Hessose, 

 and that from the Mazaruni district Vadose. The olivine-diabase is an 

 Auvergnose. 



H 



