Physiography, Geology.'] SUBANTARCTIC ISLANDS OF NEW ZEALAND. 743 



The blocks on the northern beach are of hard compact basalt. The presence of dolerite 

 and phonolite is also recorded, but it is very probable that the fine-grained basalt 

 has been mistaken for the latter, as there is no record of the microscopic examination 

 of these rocks having ever been made. 



Petrology. 



It is impossible at present to give a satisfactory accomit of the petrology of 

 the islands, but the character of the specimens collected by Captain Bollons is as 

 follows : — 



No. 1. Basalt. (From the beach near the landing on the north-east of the main 



island.) 



Macroscopic. — The rock is dark in colour, slightly vesicular, with well-marked 

 flow structure. It breaks under the hammer into irregular quadratic pieces. 

 Crystals of augite are visible in a dark groundmass. Specific gravity, 2-85. 



Microscopic. — The only phenocrysts are of brownish augite and olivine in 

 roundish crystals, usually clear, but occasionally seamed with limonite. No feldspar 

 phenocrysts were seen in the sections made. The groundmass consists of colour- 

 less glass, in which are microlites of feldspar (labradorite), magnetite in grains and 

 square-shaped sections, and much granular and short lath-shaped augites similar 

 to the phenocrysts. The rock is a basalt of a somewhat basic type. It contains 

 occasional inclusions of a basalt, containing much feldspar in the groundmass ; this 

 shows that probably the underlying rocks are of that type, the later flows being 

 more basic. 



Nos. 2 a7id 3. Basalt Glasses. (From the eastern side of Reef Point.) 



Macroscopic. — Both these specimens are dark in colour, and very scoriaceous. 

 No. 3 shows the ropy surface of a lava-flow. The rocks are glassy in appearance, 

 with no phenocrysts shomng. 



Microscopic. — The main part of both specimens is a very dark glass, with small 

 feldspar microlites showing through it. There are numerous small phenocrysts of 

 clear olivine and brownish augite. These rocks are evidently basalt glasses, the 

 fragmentary and vitreous equivalents of No. 1, and show no difference in compo- 

 sition from it, being probably the products of the same eruption. 



Dr. Marshall informs me that he has in his possession a specimen of basalt from 

 these islands which shows large crystals of augite and olivine in a very fine ground- 

 mass, with much feldspar. 



