XX HYPERSTHENE-AUGITE-ANDESITES 291 



not often exceeding "05 mm., whilst both the monoclinic and 

 rhombic kinds occur. Interstitial glass varies in amount, some- 

 times absent, sometimes scanty and viriditic, at other times 

 abundant and opaque. Magnetite abounds in the groundmass 

 much of it often being of secondary origin. 



Fourth Order, Felsitic, of the Hypersthene-Augite 



Andesites 



Formula. — Hypersth-aug, matr, fels. 



Characters. — The groundmass presents a rudely granular 

 appearance or a blurred mosaic. 



This order is capable of subdivision, as in the other orders of the 

 hypersthene-augite-andesites (see Synopsis, page 247) ; but since 

 it is only represented by six of my rock-sections, I will confine the 

 description to the general characters. 



These rocks are dark-grey, sometimes granitoid in appearance, 

 with specific gravity 2-65 to 275. They usually show some 

 alteration, arising from secondary changes within the rock-mass ; 

 and probably the felsitic or semi-mosaic appearance of the scanty 

 groundmass is the result of such a secondary change. Such rocks 

 in some respects approach the type of the gabbros. They are 

 frequent on the north coast of Natewa Bay in the vicinity of 

 Waimotu and also occur in the Valanga Ridge. They generally 

 present themselves as deeper-seated massive rocks exposed by the 

 stripping off of the superficial deposits. 



There are as a rule more or less conspicuous phenocrysts, up 

 to 3 mm. in size, of plagioclase and pyroxene, in a relatively 

 scanty micro-felsitic groundmass, displaying a blurred mosaic, in 

 which a few stout felspar-lathes can still be recognised, and com- 

 posed apparently of felspar and crystalline silica. The " grain " 

 of the mosaic may range in different rocks from "005 mm. to 

 •02 mm. The pyroxene of the groundmass is largely decom- 

 posed, and the scanty residual glass is represented by viriditic 



materials. 



The plagioclase phenocrysts, which give extinctions of medium 

 and basic andesine (iS°— 25°). are often semi-opaque and cor- 

 roded. They are traversed by numerous cracks and often contain 

 many whitish alteration products, though the lamellar structure is 

 usually well preserved. The pyroxene phenocrysts are composed 

 of brownish-yellow augite (ext. -f 30°) and pale rhombic pyroxene 



U 2 



