Vol. 56.] ERUPTIVE ROCKS FROM NEW ZEALAND. 495 



a porphyritic felspar-crystal shows zonal banding in polarized light, 

 and the extinction -angles of the twin lamellae indicate that it 

 is bytownite. The pumiceous portions of this section show the 

 characteristic fibrous structure, which is, however, sometimes hard 

 to distinguish from the fluxion-banding of rhyolitic lavas. Apart 

 from the fragments of crystals, the general mass of the rock is 

 isotropic. 



H 24 . Tauranga Bridge. — A pale pinkish-grey rock with 

 minute crystals of glassy lustre, small dark specks, and pale fibrous- 

 looking fragments. 



In thin section the rock is seen, under the microscope, to be 

 essentially a pumice-tuff, containing a few small fragments, 

 apparently of andesite, fragments of plagioclastic felspar, corroded 

 quartz, with small sporadic scales of biotite and specks of pyrites. 



H 03 . Tahua, Mayor or Obsidian Island. — A black 

 obsidian, appearing as a very pale brownish glass on thin edges, 

 and showing very perfect conchoidal fracture. 



Under the microscope, the section is seen to contain numerous 

 gas-inclusions, often elongated and of very irregular shape, some- 

 times sharply angular in form, at others oval or fusiform. A few 

 colourless microlites, which for the most part give approximately 

 straight or very low extinction-angles, are no doubt felspars. 

 There are also a few very pale-green microlites, which in some 

 instances give an extinction- angle of 19° to the direction of elon- 

 gation. The latter appear to be extremely narrow in proportion to 

 their length. 



Prof. P. W. Hutton a has described an obsidian from this locality, 

 in which he detected similar hornblende-microlites, and adds that 

 there are ' no trichites nor microvesicles.' He also describes a 

 reddish-brown pitchstone from Mayor Island, which exhibits a 

 pumiceous character. 



H 26 . Tahua or Mayor Island. — A black obsidian with some 

 small greyish-white specks. It has a less marked conchoidal frac- 

 ture and a less perfect vitreous lustre than the preceding specimen. 

 This is due to the presence of the very numerous gas-pores, which, 

 with a pocket-lens, may be seen pitting the fractured surfaces. 



Under the microscope these gas-inclusions are seen to vary greatly 

 in form and dimensions, and it is only here and there that, when 

 fusiform or tubular, they indicate the direction of flow (PI. XXVII, 

 fig. 1). Very small crystals, mostly mere microlites of felspar, are 

 plentiful in this section. They sometimes form little aggregates, 

 which are apparently the white specks visible in the hand-specimen. 

 There are also many microlites of hornblende present : these occa- 

 sionally show extinction-angles up to 19° or more with the axis of 

 elongation. The section also shows part of a pyroxene-crystal, 



1 'The Eruptive Eocks of New Zealand' Journ. Roy. Soc. N.S.W. vol. xxiii 

 (1889) p. 121. 



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