BY D. MAWSON. 465 



Class ii., dosalane. 



Order 5, perfelic, Gernianare. 



Rang 4, docalcic, Hessase. 



Subrang 3, presodic, Hessose. 



Oli-salglomero])hi/ro-pilotaxiti-hessose. 



8. Olivine Basalt Porphyrite. 



L o c a li t y. — From the numerous dykes and sills intruding 

 the agglomerates in the vicinity of Fatmalapa, Efate. 



Macroscopic Characters. — Colour dark grey with 

 scattered reddish-stained plienocrysts. Fracture fairly even with 

 a rough surface. Texture microcrystalline porphyritic. Con- 

 stituents : porphyritic olivines stained reddish by iron oxide are 

 very noticeable. Less frequent are large pyroxenes which appear 

 darker than the fine-grained base. 



Microscopic Characters. — Texture : Crystallinity, 

 holocrystalline porphyritic. Grain-size : two distinct crystallisa- 

 tions : (a) An elder generation of scattered individuals averaging 

 1 mm. diam.; (b) A younger generation represented by the micro- 

 crystalline pilotaxitic groundmass, equal to 65 % of the rock by 



Minerals present (in approximate proportions by area) 



Felspar — labradorite ( Ab 2 An 3 ) 

 Pyroxene — light yellow augite 



Magnetite 



Olivine , 



Apatite and decomposition products 



Felspar of both generations is labradorite (Aba An 3), in 

 crystals more or less rectangular in habit and exhibiting the 

 usual cleavage. The earlier crystallisations average about 2mm. 

 diam., and though usually single, the crystals are often aggregated 

 into small bunches containing several; these are the felspar 

 glomerules noted in the Mau basalt. In this rock, however, the 

 glomerules are distinct in that tbey are neither so well developed 

 nor do they show evidence of rejuvenescence in the same d( 

 30 



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