250 A NATURALIST IN THE PACIFIC chap. 



The Hypersthene-gabbros. — These rocks also contain 

 monoclinic pyroxene, and are the plutonic equivalents of the 

 hypersthene-augite-andesites which as a rule prevail in the locali- 

 ties where these rocks occur. They are usually dark grey or steel- 

 grey in colour with a specific gravity ranging from 27 to 2-84 and 

 have a granitoid aspect. The following characters are common to 

 all the specimens. 



They display a mixture of plagioclase and pyroxene, the last 

 filling up the spaces between the felspars and apparently of later 

 formation. The plagioclase crystals, which are i to 2 mm. in 

 size, are opaque ; and since they are traversed by numerous fine 

 fissures filled with dust-like decomposition products, their appear- 

 ance is often semi-saussuritic. They are much cross-macled, are 

 at times zoned, and give lamellar extinctions of andesine-labrado- 

 rite (20° — 30°). . . . The pyroxene includes both the rhombic and 

 monoclinic forms, the last with extinction angles of over 30°. They 

 may be associated or may occur as separate crystals, the rhombic 

 prevailing in the less basic and the monoclinic in the more basic 

 rocks. The rhombic pyroxene is usually more or less converted 

 into bastite which by further change passes into a chloritic 

 material ; whilst the augite sometimes undergoes the diallagic 

 change resulting from schillerisation. 



Some special features are presented by rocks from different 

 localities. That from Nawi is most basic and looks like a diallage- 

 gabbro. That from the Valanga Range (sp. gr. 275) contains 

 some quartz, apparently secondary and filling up the interspaces. 

 The rock from the Na Kula Ridge shows traces of a groundmass ; 

 but it comes near the plutonic type. 



A HORNBLENDE-GABBRO. — This granitoid rock, which is from 

 the Na Kula Ridge, has a specific gravity of 272. Hand-speci- 

 mens display large porphyritic crystals of hornblende (7 mm. long) 

 in a base of opaque felspar and smaller hornblende. In the slide 

 we observe besides the large crystals of plagioclase and hornblende 

 a little pyroxene ; but the mass of the rock consists of greenish- 

 brown hornblende, plagioclase, and some secondary quartz, form- 

 ing a coarse mosaic with a " grain " of about a millimetre. The 

 hornblende is displayed in regular hexagonal sections, markedly 

 pleochroic, and gives extinctions up to 12°. It shows no dark 

 resorption borders ; and the larger porphyritic crystals have the 

 same characters. Almost all the plagioclase of the rock is 

 traversed by numerous fine fissures, and often acquires a semi- 

 saussuritic appearance from the presence of dust-like decora- 



