xvin OLI VINE-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 ('05) approach the 

 semi-ophitic rocks included in genus 33. 



26. GENUS OF THE OLIVINE-BASALTS 

 FORMULA. Oliv y matr^pauc^ non-flu >gr an ^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 

 opacity of the felspar phenocrysts, look like porphyrites. They 

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

 Mount Seatura. Two different types occur in my collection 

 which may be regarded as sub-genera. In the most basic kind, 

 where the sp. gr. is 2*86 to 2*89, the plagioclase phenocrysts, 2 to 



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

 ture. They give lamellar extinctions (15 30) of andesine labra- 

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

 but a fair amount of olivine grains, less than *i mm., occur in the 

 groundmass. Pyroxene phenocrysts are scanty, but micro- 

 porphyritic pale brown augite ("I mm.) is frequent. In the 

 groundmass are found stoutish felspar-lathes, averaging *2 mm. 

 long, together with an abundance of fine augite granules ( g oi '02 

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

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



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

 dorite (10 30). There is an approach to the orthophyric struc- 

 ture in the groundmass, as is indicated by the number of short 

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

 extinctions of acid and basic andesine. The granular augite of 

 the groundmass is coarse ('04 mm.), and occasional prism-forms 

 give extinctions of 40. 



