542 



MR G. W. TYRRELL. 



Table I. 





1. 



2. 



3. 



4. 



5. 



Quartz .... 



39-2 



19-8 



2-5 



1-7 



23 



Orthoclase . 



8-6 



43 









Oligoclase (AbgAiij) 



28'0 





... 





... 



Andesine (Ab,An 3 ) 





49 









Labradorite (Ab,Anj) . 







30-4 



324 



22-8 



Biotite .... 



10-7 



8-6 









Pyroxenes 



126 



11-8 



491 



34-5 



39-5 



Hornblende . 







16-G 



29-5 



28-6 



Magnetite 



•6 



5-9 



1-4 



1-7 



6-8 



Apatite 





•6 





•4 





Felsic/ Mafic ratio . 



/ 75-8 

 1 24-2 



731 



26-9 



32 9 

 671 



3&9?*! 

 66-1 



25-1 

 749 



Summation in each case, 100. 



1. Charnockite (177), Ochilesa. 



2. Charnockite, intermediate variety (193), N. of Etunda. 



3. Hornblende-hyperite (194a), S. of Unyamba River, Andulo. 



4. Hornblende-hyperite (1766), Ochilesa. 



5. Melanocratic hornblende-hyperite (176a), Ochilesa. 



By calculating the ratios of the felsic and mafic constituents,* it is seen that the 

 series falls into two well-marked groups, one dofelsic, consisting of the charnockites 

 proper and the intermediate varieties ; and other domafic, consisting of the hyperites. 

 No rocks intermediate between these groups occur in the collection. The first group 

 is mineralogically distinguished by the abundance of quartz and the presence of 

 orthoclase and biotite. In the domafic group, quartz, while still present, is in ex- 

 tremely small quantity, orthoclase and biotite have disappeared, and pyroxenes and 

 hornblende constitute the bulk of the rocks. 



Whilst belonging to the ancient basement of the Benguella plateau, the char- 



* The terms felsic and mafic (Cross, Iddings, Pirsson, and Washington, Journ. GeoL, xx, 1912, p. 560) are used 

 to denote two main groups of rock-forming minerals, one containing quartz, felspars, and felspathoids, and the other 

 pyroxenes, amphiboles, micas, olivine, iron-ores, etc. Variations in the relative proportions of these groups give 

 rise to the important variations in igneous rocks denoted qualitatively by Brogger's terms, leucocratic and melano- 

 cratic. A quantitative meaning may be imparted to the terms felsic and mafic by attaching the prefixes do- and per-, 

 with exactly the same connotation in respect to the mode (actual mineralogical composition) as the analogous terms 

 persalic, dosalic, etc., have in respect to the norm (standard mineral composition) of the American Quantitative 

 Classification. 



Felsic constituents 7 



Perfelsic 



Dofelsic 



Mafelsic 



Domafic 



Permafic 



Mafic constituents 1 



<?> 5 . 

 1 3 



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