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 i6° — 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. — OHv, matr, cop, non-flu, opk, non-pken, 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-lamellse 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. — OHv, 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 



