On a Hornblende-biotite Rock from Dusky Sound, New Zealand

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Abstract
This rock was collected in Dusky Sound by Mr. W. Docherty, and given to the late Sir J. von Haast, who gave it to me. I do not know its field relations, but undoubtedly it is of eruptive origin and is associated with the Archœan gneisses and schists of that district. As I am not aware of any similar rock having been described, I think that some account of it may be interesting. The rock is compact, crystalline, of a dark green colour, weathering reddish brown, and the specific gravity varies between 3·00 and 3·07. With a lens it is seen to be composed of two minerals in nearly equal proportions. One is a black mica, the plates of which are sometimes collected into masses 0·1 to 0·2 inch in diameter, but generally scattered through the other mineral. Cleavage-flakes of this mica can be easily detached, and, under the polariscope, prove to be biotite, in which the two optic axes nearly coincide. When thin, these cleavage-laminæ have a greenish tinge by transmitted light Under the microscope, in thin sections of the rock, the biotite has the usual brown colour and strong dichroism. It often contains crystals of apatite, which is, I think, not usual. The other mineral, in thin sections, is of a pale bluish-green colour and dichroic, but not strongly so, passing from a pale brownish green to a pale bluish green, some portions being more strongly dichroic than others. With ordinary light very little structure is apparent, but

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