152 



THE VOYAGE OF H.M.S. CHALLENGER. 



plays only a subordinate part, being substituted by augite. The microliths in the 

 glassy base are not so numerous, but of the same species as in the preceding rocks. 



Numerous and well - defined plagioclase sections are full of vitreous inclusions. 

 Some of them show simultaneously the twinnings of pericline and of Baveno ; 

 that of albite is subordinate. The two series of polysynthetic lamellae, which 

 correspond in the principal individuals, cross at an angle of about 90°. The albitic 

 striae extinguish at 30°, a fact which indicates that we have to do with a mixture 

 approximating to labradorite. When the sections present the lamellae of pericline 

 clearly defined, the extinctions for the latter are a little smaller than for the principal 

 individual, being about 27° for the lamellae in question and 30° to 31° for the 

 polysynthetic lamellae twinned following the albite law (see fig. 26). 



Fig. 26.— Augite-andesite of Kandavu. Section of twinned plagioclase. 



I. II. ... Pericline twin. 



I.' II." ... do. 



(I. II.) (I.' II.") Baveno twin. 



III. I. . . . Pericline twin. 



IV. V. . . . Twinned with I. and II. having the face P common. 



The augite presents its usual characters in augitic andesites. It is sometimes 

 twinned polysynthetically ; in other cases the sections show a fibrous structure causing 

 them to resemble diallage. Augite contains felspar and magnetite as inclusions. Horn- 

 blende, which has a very small part to play in this rock, is represented by sections 

 often twinned, with worn angles and surrounded by magnetite. The plcochroism of 

 this hornblende is — 



yellow-brown. 



/3 

 brownish yellow. 



> 



pale yellow. 



