288 A NATURALIST IN THE PACIFIC chap. 



Description. — This is a group of rocks that comes near the 

 basaltic andesites represented in genera i and 13 of the augite 

 sub-class ; and to the more basic kinds the terms of basaltic 

 andesite is equally applicable. These rocks, however, differ in the 

 prevalence of rhombic pyroxene, which occurs as phenocrysts, but 

 always accompanied by monoclinic pyroxene, whether as separate 

 crystals or as inter-growths. Such rocks are intermediate between 

 those of the augite and rhombic pyroxene classes. They are 

 particularly characteristic of the Savu-savu peninsula, but they are 

 also found in other scattered localities. Sometimes they appear to 

 form ancient flows, and at other times intrusive masses and dykes ; 

 but they are rarely scoriaceous. 



Almost all the rocks in my collection referred to this genus be- 

 long to the species where the felspar-lathes of the groundmass are 

 •02 — -I mm. long. They are generally blackish or dark-brown, and 

 the specific gravity ranges usually from 272 to 2-83. They display 

 in the slide a fair number of plagioclase and pyroxene phenocrysts 

 in a groundmass of felspar-lathes, pyroxene granules, and mag- 

 netite, the interstitial glass being scanty or moderate in amount. . . 

 The plagioclase phenocrysts are rarely as large as 3 mm., so that 

 most of the rocks belong to the non-porphyritic group of the 

 genus. These phenocrysts, which are often zoned, give extinctions 

 of basic andesine (15° — 25°). They contain magma-inclusions, 

 sometimes in abundance, which are arranged in zone-lines. , . The 

 pyroxene phenocrysts are small, the two kinds being always 

 represented in the same slide. In some cases separate crystals 

 occur, and in others the two are associated as intergrowths, but in 

 most cases separate and compound crystals occur in the same 

 section. Not infrequently the phenocryst is an aggregate of lesser 

 crystals of the two pyroxenes. The monoclinic form is a brownish 

 yellow augite with large extinctions and often twinned. The 

 felspar-lathes of the groundmass, which usually average "05 or '06 

 mm. in length, are either narrow and untwinned, or they may be 

 stouter and display simple and at times lamellar twinning, giving 

 extinctions of medium andesine. . . The granules of pyroxene are 

 generally 'Oi — -02 mm. in size ; but occasional prism-forms occur 

 which give sometimes the straight extinction of rhombic pyroxene 

 and at other times the large oblique extinctions of the augite type. 



