ON A HOENBLENDE-BIOTITE BOCK PKOM NEW ZEALAND. 745 



40. 0)1 a HOENBLENDE-BIOTITE EOCK /rOm DuSKY SoUND, NeW 



Zealand. By Captain P. W. Htjtton, E.G.S. (Eead May 23, 



1888.) 



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 Archa)an 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-laminse 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 with crossed nicols the general mass shows an aggre- 

 gate polarization of rather coarse grains, almost a mosaic ; but here 

 and there distinct crystals of considerable size can be recognized, 

 without, however, retaining any of their crystalline faces. These 

 crystals seldom show cleavage, and in the few cases where it is 

 developed there is only one set of lines, the position of which I could 

 not determine ; but certainly it does not lie in the orthopinacoid. 

 Most of the crystals show twinning, often of a polysynthetic character, 

 very similar to that so commonly seen in augite. A common case 

 is for one side of a crystal to show a single twin, while the other 

 side is polysynthetic. Or a band of twin laminae may occupy the 

 centre of the crystal only. In a section taken nearly parallel to 

 the br achy pin acoid one set of laminae extinguished at an angle of 

 17° 30' from the twinning plane, while the alternate set extinguished 

 at an angle of 16° 45' on the other side of that plane. Another 

 crystal, somewhat similarly cut, gave 12^ and 16° as the two angles. 

 This proves the crystals to belong to the monoclinic system and to 

 be probably hornblende. I was fortunately enabled to test this 

 determination further by finding a crystal in which the twins ex- 

 tinguished simultaneously when the twinning plane was parallel to 

 one of the diagonals of the polarizer. This proved that the crystal 



