264 A NATURALIST IN THE PACIFIC chap. 



of olivine and pyroxene in a groundmass of parallel felspar-lathes, 

 augite granules, and magnetite, with very scanty, if any, inter- 

 stitial glass. . . . The plagioclase phenocrysts, 2 to 3 mm. in size, are 

 often aggregates of smaller crystals. They contain colourless 

 granular inclusions and are sometimes zoned, giving extinctions of 

 medium andesite(i5° — 18°), and of andesine labradorite (25° — 29°). 

 . . The pyroxene phenocrysts are pale-brown, scanty, 2 to 3 mm. in 

 size, often twinned and give the large extinctions of augite. . . . The 

 olivine phenocrysts, which do not exceed 2 or 3 mm., are deeply 

 eroded by the magma and are hematised and schillerised. Small 

 grains also occur in the groundmass. . . . The felspar-lathes, which 

 in the species here represented average "15 mm. in length, are stout 

 and give lamellar extinctions of andesine (18° — 22°). . . . The augite 

 granules are pale-brown and usually "02 — "03 mm. in diameter. 



The only species represented in my collection is that with the 

 felspar-lathes "i — '2 mm. long. 



40. Genus of the Olivine-Basalts 

 Formula. — Oliv, matrypauc,flu, gran, non-phen, parv. 



Characters. — Olivine scanty. In the groundmass the 

 felspar-lathes are in flow-arrangement and the pyroxene is granular. 

 No plagioclase phenocrysts ; but there are a few small phenocrysts 

 of olivine and sometimes of pyroxene under 2 mm. in size. 



Description. — Compact-looking non-porphyritic blackish- 

 brown rocks, sp. gr. about 2*9. Occasionally a little vesicular. For 

 the most part dyke-rocks. 



In the slide are displayed a few small phenocrysts of olivine 

 and pyroxene in a groundmass formed of more or less parallel 

 felspar-lathes, augite granules, magnetite, sometimes in rods, and a 

 little greenish devitrified residual glass. . . . The pyroxene pheno- 

 crysts are of pale-brown augite and are generally less than a milli- 

 metre in size. They may be single crystals or they may be formed 

 of an aggregate of a few smaller crystals. . . . The olivine pheno- 

 crysts rarely exceed 2 mm. in size and are in part serpentinised. . . . 

 The augite granules vary usually from 'Oi to '03 mm. in 

 diameter. . . . The felspar-lathes of the rocks in this collection are 

 large, often exceeding "2 mm. in length, giving the rock a doleritic 

 texture in the slide. In a single slide they may range from "i to '6 

 mm. When lamellar they give extinctions of 15° to 25° (basic 

 andesine). 



