256 A NATURALIST IN THE PACIFIC chap. 



millimetre in size ('2 — '8 mm.), which are hematised in the fissures 

 and at the borders. The felspar-lathes, which display a few twin- 

 lamellae giving extinctions of l6° — 18° (medium andesine), vary 

 greatly in size. The smaller are *i to -3 mm. and the larger -3 to 

 •5 mm. long ; but the two are connected by felspars of intermediate 

 length. The abundant augite granules average -037 mm. in 

 breadth. Pyroxene phenocrysts are not represented in the slide. 

 From its coarsely crystalline texture this rock merits the field-name 

 of a grey doleritic basalt ; and except in the arrangement of the 

 felspar-lathes it does not differ from the grey doleritic basalts of 

 genus 16. 



12. Genus of the Olivine-Basalts 

 FORMULA. — Oliv, matr, cop, non-flu, opJi, non-phen, parv. 



CHARACTERS. — Olivine abundant. Felspars of the ground- 

 mass not in flow-arrangement. Pyroxene of the groundmass 

 ophitic or semi-ophitic. No plagioclase phenocrysts. Small 

 phenocrysts (under 2 mm.) of olivine and occasionally a few of 

 pyroxene. 



DESCRIPTION. — A dark greenish-brown rock, with sp. gr. 2-91, 

 showing abundant micro-porphyritic olivine in a groundmass con- 

 sisting of ophitic pale-brown augite inclosing the felspar-lathes, 

 together with small olivines, patches of magnetite, and a little 

 altered interstitial glass. The olivine-phenocrysts are about -5 mm. 

 in average size and are more or less hematised. The felspar-lathes, 

 which average -15 mm. in length, often show twin-lamellae that give 

 extinctions of 23 — 30 (andesine-labradorite). 



The species with felspar-lathes -i — -2 mm. long is alone 

 represented. 



13. Genus of the Olivine-Basalts 

 FORMULA. — Oliv, matr, cop, flu, gran,phen, vitr. 



Characters. — Olivine abundant. Felspars of the groundmass 

 in flow-arrangement. Pyroxene of the groundmass granular. 

 Phenocrysts of glassy plagioclase. 



DESCRIPTION. — Dark grey or dark brown rocks with sp. gr. 

 2'88 to 2-99. Phenocrysts of olivine, pyroxene, and plagioclase 

 occur in a groundmass showing partially parallel felspar-lathes 

 abundant pyroxene grains, and fine magnetite, residual glass being 

 scanty or absent. The olivine-phenocrysts do not exceed 3 or 



