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Volume 17, 1884
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– 340 –

Art. XLIII.—Analysis of Slate in contact with Granite from Preservation Inlet, New Zealand.

[Read before the Philosophical Institute of Canterbury, 27th November, 1884.]

Analysis of a specimen of slate and granite in contact was read before the Otago Institute in November, 1877 (Trans. N.Z. Inst., 1877, p. 505). Since then a further analysis of another portion of the slate has been made with the following results:—

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Hygroscopic moisture .480
Silica 53.350
Alumina 19.889
Iron sesquioxide 2.294
" " protoxide 5.241
Manganese protoxide 1.522
Lime 3.025
Magnesia 5.060
Potash 3.904
Soda 3.652
Undetermined, combined water, etc. 1.583
Specific gravity 2.72

The above results, in common with the first analysis, show that on the whole there is no very great similarity in composition between the granite and slate, such as might be expected were the granite merely a metamorphosed or crystalline form of the slate; it would rather appear that the granite is distinctively intrusive, and not derived from the slate by metamorphic action.

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This specimen was collected by me in Isthmus Sound, Preservation Inlet, during March, 1874, when I was examining the west coast Sounds as geologist to the Provincial Government of Otago. My observations seemed to show that the granite was intrusive and much younger than the gneiss with which it is associated in Preservation and Chalky Inlets, and in my report to the Provincial Government of Otago** I considered that it had pierced rocks belonging to the Maitai (=Kaikoura) System, and consequently that it was much younger than the gneiss which was considered as of Archæan age. Fragments of the slate are found in the granite, and consequently the slate is the younger of the two. The analysis by Professor Liversidge shows that the granite cannot be metamorphosed slate, so consequently it must be intrusive. The age of the slate is however doubtful, as Dr. Hector in his last geological map of New Zealand has coloured it as belonging to his “foliated schists” of uncertain age. The actual rock in question, however, is not a foliated schist, but an argillite, that is an uncleaved slate, and the alteration produced by the granite does not penetrate very far into it.

[Footnote] * Geology of Otago, by Hutton and Ulrich. Dunedin, 1875, p. 40.