*5 6 HOLLAND: CHARNOCKITE SERIES. 



A partial analysis of No. 9/660 by my colleague Dr. T. L. Walker 

 gave the following results :— 



^°* 5338 



Al=O a .... 19 . 38 



Fe 2 3 andFeO , 5 - 39 



Ca O • 768 



M fiO 279 



Augite-Norite. 



The nearest approach to normal norite is the rock which 

 forms the only associate of the type-mass of charnockite near 

 St. Thomas' Mount. Charnockite forms the central mass of the hill 

 whilst augite-norite forms the north-eastern (No. 9-660) and south- 

 western (No. 9*657) ends. 



This norite is uniform in structure throughout the mass exposed 

 and specimens have a specific gravity seldom varying by '05 from 

 3*025. The foliation is scarcely noticeable except on weathered sur- 

 faces. 



The principal constituents are hypersthene, augite, plagio- 

 clase and opaque iron-ores with accessory apatite, quartz and a fel- 

 spar that shows no lamellar twinning. 



The hypersthene crystals are devoid of idiomorphic outline 

 although they generally show a well marked prismatic cleavage and 

 the normal pleochroism, with straight extinction. 



The bluish-green colour of the rays vibrating parallel to the 

 axis of minimum optical elasticity C in the augite is so precisely 

 similar to that of the rays vibrating parallel to the corresponding axis 

 in the hypersthene, that without moving the polariser the two 

 minerals might very easily be confused. The feeble pleochroism 

 and the wide angle of extinction of the augite, however, form a ready 

 means of distinction in polarized light. The colours of the rays vi- 

 brating parallel to b and C only differ from those parallel to & by a 

 yellowish tinge in the former and a bluish tinge in the latter. The 

 angle of extinction on the clinopinacoid is about 44°. The crystals 

 very frequently show the characteristic lamellar twinning of augite, 



( 38 ) 



