A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 



547 



subordinate alkali felspar, mostly microcline, with a little orthoclase. The ferro- 

 magnesian constituents are biotite, and a green hornblende, which, in the position 

 of greatest absorption, shows a distinct tinge of blue. Magnetite, cassiterite, and 

 apatite occur sparingly, and in one place are associated in a small intergrowth like 

 those in the hornfelsed porphyry from Bailundo (see p. 544). Cassiterite also occurs 

 in the rock from kilometres 238, Benguella Railway, where it is associated with little 

 clots of biotite. Epidote and chlorite occur sparingly as secondary products derived 

 from the alteration of biotite. The quantitative relations of the minerals, as 

 determined by the Rosiwal method, are set out in Table II. A typical Benguella 

 granodiorite therefore contains about 25 per cent, of quartz, 15 per cent, of alkali 

 felspar, 50 per cent, of oligoclase-andesine, and 10 per cent, biotite, hornblende, and 

 iron-ores. The typical American granodiorites are somewhat richer in the mafic 

 constituents. # 



Table II shows the contrast between the two types of granitic rocks in Benguella. 

 The granites proper are rich in microcline and albite, whereas the granodiorites are 

 poor in alkali felspar, and contain nearly 50 per cent, of oligoclase-andesine. Along 

 with this they are decidedly poorer in quartz, and richer in the mafic constituents. 

 The granodiorites contain cassiterite, but this mineral is in too small quantity to 

 come under the Rosiwal measurement, and its amount must therefore be considerably 

 less than 1 per cent. 



Table II. 





L . 



2. 



Quartz . . 



Microcline and orthoclase . 



Albite (Ab) . 



Andesine (Ab 7 An 3 ) . 



Biotite and chlorite . 



Hornblende 



Magnetite 



Epidote .... 



Felsic/Mafic 



359 

 473 

 10 5 



3-9 



2-4 



( 93-7 

 1 63 



26-5 

 15-4 



48-4 

 5-5 

 16 

 1-5 

 11 



903 

 9-7 



1. Biotite-granite, E. of Quingenge, Benguella (248). 



2. Biotite-granodiorite, kilometres 238, Benguella Railway (123). 



Of especial interest in this connection is the occurrence of a stanniferous selvage 

 to the Kunhungamua granite. This rock (139) is an even-grained microgranite con- 

 sisting of quartz and orthoclase, with the latter distinctly in greater quantity. The 

 orthoclase frequently has the mottled and moire appearance which indicates a con- 

 siderable content of the albite molecule. The dominant ferro-magnesian constituent 

 is a greenish-brown biotite, but there are also a few large ragged patches of a bleached 



* "Geology and Ore Deposits of Butte, Mont.," U.S. Geol. Surv., Prof. Paper No. 74, 1912, p. 36. 



