CHAPTER XVIII 



THE VOLCANIC ROCKS OF VANUA LEVU (continued} 



OLIVINE CLASS 

 SUB-CLASS II 



THE OLIVINE BASALTS (Flag, oliv, matr.) 



THIS sub-class includes the plagioclase-olivine-basalts. 

 Although these rocks are not the most numerous of the basic 

 rocks, they are well represented in the island, being in great part 

 confined to the western half, and being especially characteristic of 

 the districts of Wainunu and Solevu and of the mountains of 

 Seatura and Naivaka. It will be seen from the Synopsis that 

 this sub-class is split up into two divisions, according to the 

 relative abundance of the olivine. Many of the rocks are grey 

 basalts with the olivine more or less hematised ; but the majority 

 are blackish with the olivine usually more or less serpentinised. 

 In the typical blackish rocks there is a little dark opaque inter- 

 stitial glass. In the grey basalts the groundmass is as a rule 

 holo-crystalline. The specific gravity ranges generally from 

 2-8 to 3. 



It will be noticed in the scheme that the " prismatic " sub- 

 orders, where the pyroxene of the groundmass is for the most part 

 in prisms, are scarcely represented. The " ophitic " sub-orders are 

 poorly represented, since they only include about 10 per cent, of 

 the total. The ophitic olivine-basalts are indeed mostly confined 

 to the division where the olivine is scanty, and the felspars of the 

 groundmass are for the most part not parallel, the plexus arrange- 

 ment, as will be often pointed out, being almost essential for the 

 development of the ophitic structure. With the basaltic andesites, 

 which cannot always be sharply separated from the basalts with 



