CH. xviii OLIVINE-BASALTS 



253 



scanty olivine, the proportion of ophitic rocks is much higher, 

 probably about 20 per cent. The pyroxene in the olivine-basalts 

 is nearly always augite, intergrowths with rhombic pyroxene being 

 only occasionally observed. 



I. Division of the Olivine-Basalts 

 Characters.— Abundant olivine. 

 Formula. — OHv, matr, cop. 



These rocks are characterised by abundant olivine usually as 

 phenocrysts but sometimes represented in the groundmass. When 

 a basalt presents much of this mineral in an ordinary hand-speci- 

 men and displays at least five or six phenocrysts in a slide, it is 

 placed in this division. Olivine-basalts of this character are well 

 exhibited in the hills around Solevu Bay and in the neighbouring 

 Seatovo Range. They are also fairly represented on the northern 

 slopes of Mount Seatura, on the coast between the Wainunu River 

 and Nandi Bay, and on the Wainunu basaltic tableland. In the 

 Ulu-i-ndali Range, which lies east of the Wainunu estuary, they 

 are especially frequent. Whilst confined mostly to the portion of 

 the island west of Savu-savu Bay, they occur sporadically in other 

 localities to the eastward, as in Na Suva-suva Hill and in some 

 parts of the Natewa Peninsula. The grey basalts, which form one- 

 third of the total, are chiefly characteristic of the hill of Ulu-i- 

 ndali, of the Solevu district, and of the northern slopes of Seatura. 

 Whilst the blackish basalts usually compose the flows, the grey 

 basalts form dykes and volcanic necks. 



Two-thirds of these olivine-basalts belong to the order present- 

 ing flow-structure and almost all (28 out of 29) are included in the 

 sub-order exhibiting granular augite in the groundmass. The 

 ophitic structure is displayed in only one case ; and the prismatic 

 form of the augites is never a predominant feature. 



I. Genus of the Olivine-Basalts 

 Formula. — OHv, matr, cop, non-flu, gran, phen, vitr. 



Characters. — Abundant olivine. Felspar-lathes of the 

 groundmass not in flow arrangement. Pyroxene of the ground- 

 mass granular. Phenocrysts of glassy plagioclase. 



Description. — Dark-brown or blackish rocks. Sp. gr. 2'88 

 to 2'93. Phenocrysts of pyroxene occur in fair quantity in addition 



