xix AUGITE-ANDESITES 279 



A rock, almost holocrystalline and 274 in density, which was 

 obtained from the Tongalevu district in Wainunu Bay, approaches 

 the orthophyric type in the character of the ground mass. The 

 felspars are short ( - o6 mm.) and stout, and yield lamellar extinc- 

 tions of oligoclase (5° — io°). The plagioclase phenocrysts are of 

 basic andesine. Amongst the granules ('025 mm.) of pale brown 

 augite occur prismatic forms giving oblique extinctions of + 30 . 



14. Genus of the Augite-Andesites 

 FORMULA. — Aug, matr, flu, gran, pJien, opac. 



CHARACTERS. — In the groundmass the felspar-lathes are in 

 flow-arrangement and the augite is granular. The plagioclase 

 phenocrysts are opaque. 



Description. — Only two rocks are referred to this genus. 

 One which is dark grey with a specific gravity of 272 is exposed 

 in the gorge of the Mbutu-mbutu River below the falls of Na 

 Savu. Flow-arrangement is displayed both by the felspar pheno- 

 crysts and lathes. The phenocrysts, 2 to 3 mm. in size, owe their 

 opacity to the abundance of inclusions of brown glass. They are 

 corroded and give extinctions of acid labradorite (26 — 32 ). 

 Pyroxene phenocrysts are scanty and small. The groundmass 

 has a characteristic " pilotaxitic " appearance, the densely packed 

 felspar-lathes averaging only *05 mm. in length, whilst the pyroxene 

 granules are -oi mm. in size. Residual glass scanty. 



The other rock is from the range behind Sueni. It shows large 

 porphyritic crystals (5 or 6 mm.) of medium andesine which 

 contain magma-inclusions in abundance. The average length of 

 the felspar-lathe is '08 mm. and the size of the augite granules is 

 •02 mm. There is but little glass. The rock is somewhat altered. 



16. Genus of the Augite-Andesites 

 FORMULA. — Aug, matr,fiu, gran, non-phen, parv. 



Characters. — In the groundmass the felspar-lathes are in 

 flow-arrangement and the augite is granular. Plagioclase 

 phenocrysts are absent, or if present very scanty and not usually 

 over 1 mm. in size. When present the augite phenocrysts are 

 under 2 mm. 



DESCRIPTION. — This is a very extensive genus, admitting 

 considerable variation and including most of the aphanitic augite- 



