254 Transactions. ' - 



No. 46. Olivine Andesite. — Fragment from L'Esperance or French Rocl:. 



Macroscopic. — A dark-grey vesicular rock, with small feldspar pheno- 

 crysts showing. Specific gravity, 2-68. 



Microscopic. — Groundmass coarse-grained, principally of feldspar (labra- 

 dorite) laths, grains of augite, and much secondary titaniferous magnetite. 

 There are phenocrysts of labradorite, olivine, and augite, the first-named 

 being very numerous. 



Pyroclastic Roclc^. 



Besides the foregoing, a number of specimens of fragmentary volcanic 

 matter were in the collection. These vary somewhat in character. Nos. 22 

 and 25, from Boat Cove, Sunday Island, are composed of fragments of 

 andesitic pitchstone and ordinary andesite cemented by a calcareous base 

 and volcanic ash. This contains numerous fragments of reef-building coral. 

 This occurrence seems best explained by a submarine explosion, where the 

 overlying volcanic rocks wath their veneer of coral were shattered into frag- 

 ments and subsequently consolidated, perhaps below sea-level. No. 32 is 

 from a fine-grained tuS on Sunday Island, composed principally of angular 

 fragments of feldspar and augite grains. In this were rounded bombs of 

 blown glass, full of crystallites, and containing fragments of augite and 

 feldspar. Nos. 5, 10, 18, 20, from the small islands north-east of Sunday 

 Island, and Nos. 13 and 14, from Sunday Island, the latter being undoubtedly 

 submarine, are specimens of tuff beds which show the same origin, and are 

 andesitic in character, as they contain numerous fragments of augite and 

 plagioclase feldspar. 



The collection also contains sulphur and siliceous sinter from Curtis 

 Island, specimens of volcanic mud from the crater on Sunday Island, and 

 calcite incrustations from Dayrell Islet, Sunday Island. 



A general conspectus of the collection shows that the islands are built 

 up of volcanic materials which are andesitic or basaltic in origin, and that 

 acidic rocks form no part of them. The andesites have in general a strong 

 affinity to the basalts, and are sometimes characterized by the presence of 

 olivine and by a groundmass which is basaltic in character ; but the nume- 

 rous phenocrysts of feldspar, and the small number of those of augite or 

 olivine, demands that the rocks be classified with the andesites. Instances 

 of intermediate varieties between andesites and basalts are very common. 



The general facies of the specimens does not show any marked resem- 

 blance either to New Zealand tj^es or to the specimens I have in my posses- 

 sion from Tofua and Savaii. The presence of hypersthene in some suggests 

 a connection with the Ruapehu - Tongariro - White Island fine, the rocks 

 of which are characteristically hypersthene-bearing, as indeed are so many 

 of the andesites of New Zealand — e.g., those from the Clent Hills in the 

 South Island, as well as those from numerous localities in the North Island 

 (see Marshall, " Distribution of Volcanic Rocks in the North Island," 

 Trans. N.Z. Inst., vol. xl, p. 79). The presence of oUvine in the andesites is 

 also reminiscent of the later flows from Ruapehu, but suggests much more 

 strongly the olivine-bearing andesites of Banks Peninsula. This type, how- 

 ever, seems to occur in other parts of the south-central Pacific area. 



