lY.— GEOLOGY. 



Art. LXIV. — Notes on some Rocks from the Kermadec 



Islands. 



By R. Speight, M.A. 



\^Rcad before the Philosophical Institute of Canterbury, 2nd October, 



1S95.] 



By the kindness of Captain Hutton I have been allowed to 

 examine some specimens of rock in the Canterbury Museum 

 which were brought from the Kermadec Islands by Mr. Park. 

 The specimens are all small, and I have been unable to do 

 more than examine them microscopically. I have been unable 

 to identify them completely with any of those described by 

 Professor Thomas in the " Transactions of the New Zealand 

 Institute," vol. xx.,"^' though some of them correspond in pare. 

 They are all volcanic, and belong to the intermediate division, 

 with the exception of the first, which is a doubtful tachylyte. 

 They all exhibit rather higher specific gravities than is usual 

 in such rocks. 



Tachylyte (Macaulay Island). 



This specimen is from Macaulay Island. It is black in 

 colour, and has the lustre of pitchstone. Its specific gravity 

 is 2-49, which is rather low for a basalt-glass, but corresponds 

 to the value for a tachylyte. It fuses under the blowpipe, but 

 does not dissolve to any great extent in hydrochloric acid ; 

 though after digesting the powdered rock for several days 

 with this acid a quantity of iron was dissolved out. It thus 

 corresponds with tachylyte as regards specific gravity and 

 fusibility, but differs as regards solubility. However, the 

 last does not appear to be at all constant in the case of rocks 

 which are undoubtedly tachylytes. 



Under the microscope in ordinary light it is brown in 

 colour, w^ith numerous small grains of magnetite and micro- 

 litlis of feldspar, the arrangement of which shows well marked 

 fluxion structure. It is rather opaque, and only very thin 

 sections transmit light. 



With polarized light crystals of augite, feldspar, and mag- 

 netite are visible, though none of them are large enough to be 



* See Trans. N.Z. Inst., vol. xx., arb. xli., "Notes on the Rocks of the 

 Kermadec Islands," by Professor A. P. W. Thomas, M.A., F.L.S., &c. 

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