A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 



555 



Table VI. 





1. 



2. 



3. 



Si0 2 



47-41 



46-73 



47-46 



Ti0 2 





1-86 



•78 



1-14 



A1 2 3 





12-30 



1005 



14-28 



Fe 2 3 





6-99 



353 



4T6 



FeO 





5-03 



8-20 



4-97 



MnO 





•14 



•28 



•19 



MgO 





6-82 



9-27 



6-64 



CaO 





9-57 



13-22 



9-42 



(Ba, Sr)( 



) 



p.n.d. 







Na.,0 





3-06 



1-81 



3 86 



K 2 





4-05 



3-76 



465 



H 9 + 

 H 2 0- 





1-24 

 •31 



1-24 



I 1-66 



p 2 o 5 





1-62 



1-51 



102 



CI 

 F . 

 C0 2 





•07 

 •21 



nt. fd. 



T8 



1 Rest 



j" -72 





100-68 



100-56 



100-17 



Less = 



CI, F.'TO 



•04 









100-58 



100-52 



1. Shonkinite, Lamarose (III, 5', 2, 3), Ochilesa, Benguella. 



2. Shonkinite, Shonkinose (III', 6, 2, '3), Square Butte, Montana. 



Quoted from Pirsson, U.S. Geol. Surv., Bull. 237, 1905, p. 102. 



3. Average shonkinite, Shonkinose (Til, 6, 2, 3). 



The Square Butte shonkinite is richer in lime and magnesia, and poorer in alkalies, 

 than the Ochilesa rock or the average shonkinite, thereby revealing its more mafic 

 character. The analysis of the average shonkinite, however, is extraordinarily like 

 that of the Ochilesa type. It is somewhat richer in alkalies and alumina, but the 

 correspondence of the other constituents is very close. A notable feature of most 

 analyses of shonkinite, including the Ochilesa rock, is their abundance in phosphorus 

 pentoxide, which constituent commonly reaches 1*5 per cent., indicating about 3^ per 

 cent, of apatite. The Ochilesa shonkinite falls into the subrang III, 5, 2, 3 (lamarose) 

 of the American Quantitative Classification, but is intermediate towards the next 

 subrang, III, 6, 2, 3 (shonkinose), in which the typical shonkinites are found. The 

 average shonkinite falls almost centrally into the subrang shonkinose. 



Light-coloured veins of alkali-syenite-aplite (208c) occur in the shonkinite of 

 Ochilesa. On the other hand, a boulder of sodalite-syenite from, the volcanic vent 

 of Chieuca shows a vein of microshonkinite or shonkinite-aplite. It is unnecessary 

 to describe these rocks in detail, as they are merely fine-grained modifications of the 

 rocks above described. 



