Second Order of the Hornblende-Hypersthene-Andesites
(Felspar-lathes in flow-arrangement)
This order is only represented by three rocks, all of which belong to the prismatic sub-order where the pyroxene of the groundmass is prismatic and not granular.
Two of these rocks are very similar in appearance and character, though coming from different localities on the opposite sides of Savu-savu Bay, one from the agglomerate of Vatu-ndamu in the Kumbulau peninsula (page [91]), the other from an intrusive mass in the vicinity of Urata (page [184]). They are dark grey, with specific gravity 2·6 to 2·7, and display macroscopic crystals of hornblende and pyroxene. In the slide they exhibit in addition numerous phenocrysts of plagioclase, 1 to 2 mm. in size, in a groundmass showing small felspar-lathes (less than ·1 mm. in length) in partial flow-arrangement and numerous pyroxene prisms (·05 mm. long) giving straight extinctions, together with a little residual glass.... The plagioclase phenocrysts, which give extinctions of medium and basic andesine, are often tabular and display zone-lines. They contain abundant pale inclusions arranged zone-wise.... The hornblende phenocrysts are dark brown, markedly pleochroic, and give extinctions up to 12 degrees. They have dark resorption borders and are sometimes deeply corroded. They show in various stages the remarkable conversion at the borders into fine pyroxene, which is described on page [306].... The pyroxene phenocrysts are more numerous in the Urata rock. They are for the most part of the pale yellow feebly pleochroic rhombic type that prevails in the island. A few phenocrysts of pale augite (ext. 35°) may occur in the same slide; whilst the two pyroxenes may be associated as intergrowths.
A crypto-crystalline variety of these rocks, where the felspar-lathes and rhombic pyroxene prisms of the groundmass are only in part differentiated, is found on the hills of Ndreke-ni-wai on the shores of Natewa Bay (page [201]). It is a pale-grey open-textured rock, displaying numerous small macroscopic crystals of hornblende.