XVIII OLIVINE-BASALTS 261 



the interstitial glass which is present in small quantities in most 

 rocks, being greenish or brownish and showing fibrous devitrifi- 

 cation. 



In some localities semi-vitreous rocks referable to this genus are 

 frequent. This is especially the case in the Naivaka peninsula, 

 where the rocks show a fair amount of glass in the groundmass, 

 the porphyritic augite being well developed, whilst the pyroxene of 

 the groundmass is only in part differentiated. Three of the four 

 species are here represented. Those with large felspar-lathes 

 ("2 — '3 mm. long) and coarse augite granules ("OS) approach the 

 semi-ophitic rocks included in genus 33. 



26. Genus of the Olivine-Basalts 

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



Characters. — Olivine scanty. In the groundmass the fel- 

 spar-lathes and prisms are not in flow-arrangement and the 

 pyroxene is granular. The plagioclase phenocrysts are opaque. 



Description. — Dark grey porphyritic rocks, which, from the 

 Dpacity of the felspar phenocrysts, look like porphyrites. They 

 ire not very frequent and occur mostly on the northern slopes of 

 Mount Seatura. Two different types occur in my collection 

 A^hich may be regarded as sub-genera. In the most basic kind, 

 vhere the sp. gr. is 2'86 to 2'89, the plagioclase phenocrysts, 2 to 

 5 mm. in size, owe their opacity chiefly to their aggregate struc- 

 ure. They give lamellar extinctions (15° — 30°) of andesine labra- 

 lorite. Porphyritic olivine is scanty and more or less hematised ; 

 )ut a fair amount of olivine grains, less than 'i mm., occur in the 

 ; jroundmass. Pyroxene phenocrysts are scanty, but micro- 

 ])Orphyritic pale brown augite ("i mm.) is frequent. In the 

 iToundmass are found stoutish felspar-lathes, averaging '2 mm. 

 long, together with an abundance of fine augite granules ('Oi — "02 

 I im.) and fine magnetite, the residual glass being scanty. ... In 

 1 tie other type the sp. gr. is 275 ; and the plagioclase phenocrysts 

 i to 6 mm. in size give extinctions of andesine and acid labra- 

 ( orite (10° — 30°). There is an approach to the orthophyric struc- 

 t are in the groundmass, as is indicated by the number of short 

 \ road felspars, averaging '2 mm. in length and giving lamellar 

 e xtinctions of acid and basic andesine. The granular augite of 

 t le groundmass is coarse ('04 mm.), and occasional prism-forms 

 % ive extinctions of 40°. 



