2 78 A NATURALIST IN THE PACIFIC chap. 



•15 mm. long, are often rather stout, showing a few lamellae that 

 give extinctions of medium and basic andesine. Interstitial glass 

 occurs in fair amount. 



Species C. — Felspar-lathes '2 — '3 mm. in average length. 



Blackish rocks with sp. gr. 275 — 284. The description of 

 Species B applies here. The plagioclase phenocrysts are for the 

 most part microporphyritic. The size of the augite granules is as 

 above given. 



2. PORPHYRITIC Sub-genus. — This group of rocks is mostly 

 confined to the slopes and vicinity of Mount Seatura in the western 

 part of the island, being prevalent in the Mbua and Ndama plains, 

 and occurring also as dyke-rocks in the Nandi Gorge leading into 

 the Ndriti Basin, and at and near the coasts of Wainunu Bay 

 between the Tongalevu and Wainunu rivers. They come near in 

 appearance to the porphyritic forms of the blackish olivine-basalts 

 belonging to genera 13, 25, and 37 of the olivine rocks; but they 

 differ in the absence of that mineral, in their lower density, and in 

 other characters. They are the type to which the term " porphy- 

 ritic basaltic andesite " is most frequently applied in the text when 

 the ophitic structure is not displayed. 



They are blackish rocks having a specific gravity of 271 to 2*8 1 

 and exhibiting large porphyritic crystals of plagioclase, but they 

 vary in their minute structure on account of the different size of 

 the felspars of the groundmass. Those forming dykes in the 

 Nandi Gorge are often more or less propylitic in character. The 

 felspar-lathes, which have an average length of '2 to -3 mm., some- 

 times show a few lamellae giving extinctions of medium andesine 

 (12° — 20°). The plagioclase phenocrysts of the same andesine are 

 3 to 5 mm. in size. They are eroded and contain abundant 

 magma inclusions. There are a few small phenocrysts of pale 

 brown augite. The augite granules are "03 or '04 mm. in diameter, 

 and there is a little dark opaque residual glass. 



The rocks of the Mbua and Ndama plains have a specific 

 gravity of 2'8i. The plagioclase phenocrysts, which yield extinct- 

 ions of basic andesine (21 — 27°), are sometimes a centimetre in 

 length. They are traversed by cracks filled with dark altered glass or 

 occupied by brownish films. The felspar-lathes, which average "ii 

 mm. in length, are often stout and lamellar and give extinctions like 

 the phenocrysts. Augite phenocrysts are either absent or scanty ; 

 whilst the granules average '02 — '03 mm. in size. There is usually 

 a little interstitial glass. 



