2 70 A NATURALIST IN THE PACIFIC chap. 



4. Genus of the Augite-Andesites 



Formula. — An^-, matr, non-flu^ gran, non-phen, parv. 



Characters. — In the groundmass the felspar-lathes and prisms 

 are not in flow-arrangement and the augite is granular. There are 

 no plagioclase phenocrysts, and those of augite when present are 

 small (under 2 mm.). 



Description. — Two groups of these rocks occur in my 

 collection. In the one there are vesicular and scoriaceous rocks 

 forming dykes near Nukunase and near the village of Ndriti. They 

 display a plexus of felspar-lathes with abundant smoky more or 

 less devitrified glass, the augite granules not being always differ- 

 entiated. The felspar-lathes vary from •: to '2 mm. in average 

 length, and when lamellar give extinctions of basic andesine (25°). 

 There are no pyroxene phenocrysts, and the augite granules when 

 present average "02 mm. in size. 



In the other group are included some propylites from the dykes 

 of the Ndriti Basin. They are greyish or greenish rocks, have a 

 sp. gr. of 272 to 276, sparkle often with pyrites, and contain so 

 much secondary calcite that they effervesce freely with an acid. 

 Except in the rarity or absence of plagioclase phenocrysts, they 

 come near to the propylitic rocks described under genus 2. They 

 usually display a doleritic groundmass exhibiting long felspar-lathes^ 

 •2 to "33 mm. in length, which present the same pseudo-flow 

 arrangement from their being gathered into bundles. The alteration 

 corresponds precisely to that previously described, chlorite, 

 epidote, pyrites, &c., occurring in quantity as secondary products. 



II Prismatic Sub-Order of the Augite-Andesites where 

 the Felspar-lathes are not in Flow-arrangement 



Formula. — Aug, matr, non-flu, prism. 



The augite-andesites, which display in the groundmass a plexus 

 of felspar-lathes and much prismatic pyroxene, are not frequent in 

 my collection. About half of the specimens belong to agglomerates, 

 whilst the rest are of the massive type, none apparently being 

 obtained from dykes. They admit of the same classification as 

 that generally adoped for the " granular " sub-orders ; and it must 

 be not forgotten that granular pyroxene also occurs but is not pre- 

 dominant. 



