300 A NATURALIST IN THE PACIFIC chap. 



are broad and often rectangular and may give lamellar extinctions 

 of oligoclase-andesine. The pyroxene in the groundmass of the 

 porphyrites is often partly decomposed. It is as a rule prismatic. 

 A little interstitial glass, altered in the porphyrites, is generally 

 present. 



Fourth Order of the Hornblende-Hypersthene- 

 Andesites 



{Groundmass felsitic, displaying a granular mosaic structure) 



These are light and dark grey rocks showing u.^ually macro- 

 scopic pyroxene and hornblende. They vary considerably in 

 appearance from the open-textured rock to that with a granitoid 

 coarsely crystalline aspect. They generally carry brown horn- 

 blende phenocrysts, but frequently these are represented by 

 pseudomorphs ; and they all have a felsitic groundmass. They 

 are only separated by the absence of porphyritic quartz from the 

 dacites of Vanua Leva, which are treated in the next sub-class. 

 They present all stages from the crypto-crystalline to the holo- 

 crystalline condition, but all show a groundmass which may be 

 scanty in the more coarsely crystalline rocks. 



These rocks are characteristic of some of the hills of the 

 Ndrandramea district and of the isolated peaks of Vatu Kaisia 

 and Na Raro. They include a large proportion of the acid 

 andesites of the island, and all belong to the prismatic sub-order 

 with prismatic pyroxene in the groundmass, and to the section 

 with plagioclase phenocrysts. Their specific gravity ranges usually 

 from 2'5S in the more acid and less crystalline types to 274 in the 

 most crystalline and basic kinds. 



In the typical slides they display phenocrysts of plagioclase, 

 pyroxene, and brown hornblende in a microfelsitic groundmass 

 formed evidently of felspar and quartz together with much prism- 

 atic pyroxene. They may be conveniently divided into three 

 species according to the size of the " grain " of the groundmass. 



Of the first species, where the " grain " is less than -oi mm., 

 the rocks of Mount Ndrandramea are typical. They have a 

 crypto-crystalline groundmass where the felsitic structure is in 

 process of development and where the pyroxene prisms or micro- 

 liths are very minute. The plagioclase phenocrysts (i mm. in size) 

 give extinctions of acid and medium andesine (10° — 20°), and are 

 tabular, zoned, and contain abundant pale inclusions. The horn- 

 blende phenocrysts, except in the case of the rocks at the foot of 



