254 A NATURALIST IN THE PACIFIC chap. 



to those of the olivine and plagioclase. The groundmass displays 

 a plexus of felspars and augite-granules with much magnetite in 

 grains and irregular patches. The interstitial glass is scanty or 

 almost absent. The olivine phenocrysts, of which the larger are 

 3 to 4 mm. in size, are as a rule hematised at the borders and 

 in the fissures, and are sometimes partially serpentinised. In 

 some cases small crystals of olivine are enclo.sed in the pyroxene- 

 phenocrysts. The plagioclase phenocrysts do not usually exceed 

 2 mm. in size. They give lamellar extinctions of 1 5° — 28°, and are 

 often cross-macled. They generally contain magma-inclusions, 

 which may be arranged in zones, and they sometimes inclose small 

 pyroxene crystals. Their borders are often eroded. The pyroxene- 

 phenocrysts, which frequently are 3 to 4 mm. in size, give 

 extinctions of 30° and over, and may be described as composed of 

 brown augite. It is only at times that intergrowths of rhombic 

 pyroxene occur. They are often twinned and are sometimes eroded 

 and may contain magma and other inclusions. The felspars of the 

 groundmass, which for the most part form a plexus, are small and 

 stout, their average length varying from 'oS to '13 mm., whilst they 

 frequently display lamellar twinning and give extinctions of about 

 15° (oHgoclase-andesine). The pyroxene-granules of the ground- 

 mass, which are of brown augite, vary in average size from '02 

 to -04 mm. 



Two of the four species, where the felspar-lathes are less than 

 "I mm. and between 'i and '2 mm. in length, are represented in this 

 collection. 



2. Genus of the Olivine-Basalts 

 Formula. — Oliv, matr, cop, non-flu, gran, phen, opac. 



Characters. — Abundant olivine. Felspar-lathes of the 

 groundmass not in flow-arrangement. Pyroxene of the ground- 

 mass granular. Phenocrysts of plagioclase opaque white. 



Description. — Grey compact-looking rocks ; sp. gr. 2'83 to 

 2'9. Interstitial glass scanty. The olivine phenocrysts, which 

 range up to 5 mm. in size, are more or less hematised ; and in 

 extreme cases of this alteration, where schiller-planes are formed, 

 the hand-specimen appears to carry brown mica. There are some- 

 times small grains of olivine ('i mm.) in the groundmass. The 

 plagioclase-phenocrysts, varying in size from 2 to 4 mm., owe 

 their opacity partly to their composite character, when they present 

 an aggregate of smaller clear crystals, and partly to multiple 



