GEOLOGICAL OHSF.KVATION'S IN' FIJI. 121 



The phenocrysts are large augite crystals, zoned feldspars, and less 

 numerous olivine crystals. 



The feldspar phenocrysts are beautifully zoned, showing an outer 

 zone of acid labradorite, AbisAuso, an intermediate zone of andesine, 

 AbygAnji, and an inner zone of labradorite, Ab42Anr,8. 



The diabasic ground-mass, composed of plagioclase, olivine, and 

 augite, is much more coarsely crystalline than that for the average 

 basalt and the rock for this reason has been referred to as a porphyrite. 



VI. Rocks from the Lau Islands. 



Volcanic rocks were collected from the following islands of the Lau 

 Group. The specimens will be described from the islands in the 

 order given: 



Vanua jMbalavu, Tuvutha, 



Yanu Yanu, Lakemba, 



Susui, Ono-i-lau, 



Munia, Kambara, 



Thikombia-i-lau, Moala. 

 Kanathea, 



Throughout, the order of description will be first andesites, and 

 then basalts. No vesicular types of the andesite were found on 

 Vanua JMbalavu. In general, the andesites and basalts belong to two 

 different periods. Though basalts may occur in the andesitic period, 

 such basalts are not known to contain olivine. The andesites or 

 andesitic basalts were extruded before the deposition of the elevated 

 limestones; the olivine basalts after their deposition. 



Vanua Mbalavu. 



1) Andesiie from southwcsiern side. The andesite was found as a 

 small, laccolithic mass injected into tuflFs and agglomerates and ex- 

 posed in a sea-cliff at the southern side of a point of land just south of 

 Ndaku-i-Loma Loma. 



The rock is oily, dark gray in color, holocrystalline and without 

 noticeable phenocrysts in the hand specimen. In thin section a very 

 few small phenocrysts of andesine (Ab59An4i) may be seen. These 

 phenocrysts are set in a very fine paste of augite and feldspar, showing 

 a pilotaxitic texture. There is an abundant sprinkling of fine bits 

 of magnetite and secondary epidote has developed. 



