462 MESSRS. J. PAKE AND F. EUTLEY ON THE [Aug. 1 899, 



Apart from porphyritic crystals, the rock is seen under the micro- 

 scope to consist almost exclusively of very small spherulites. When 

 viewed in ordinary transmitted light the section appears mottled 

 with feebly translucent flocculent matter, while in reflected light 

 these mottlings look nearly snow-white, and they are apparently 

 due to partial kaolinization. 



The porphyritic crystals are chiefly sanidine, quartz, and biotite ; 

 while diminutive crystals of magnetite are sprinkled throughout the 

 rock as a fine dust. The porphyritic crystals are more or less 

 corroded. The crystals of biotite vary considerably in dimensions, 

 and occasionally present ragged boundaries (PI. XXXIII, fig. 7). 

 Eiotite-rhyolites from other localities in New Zealand have been 

 described by Capt. P. W. Hutton,^ the late Mr. R. Daintree,^ and 

 others. Curious crystallites, similar to those described by Vogelsang 

 as occurring in a rock from Theresienhiigel, at Tarczal, near Tokay, 

 are to be seen in this biotite-rhy elite from Waihi, and some of 

 them are represented in PI. XXXIII, fig. G. 



H^^. Mat aura. — A deep violet to blackish rock with very small 

 crystals of vitreous lustre. Delicate, rudelj-parallel, but irregular 

 bands of pale greyish to slightly reddish colour traverse the speci- 

 men. The cut surface exhibits a good lustre, almost a polish. 

 The specimen is weakly but distinctly magnetic, and is so bard that 

 the point of a knife-blade makes no impression upon it. The rock 

 might pass for jasper. 



Under the microscope, in thin section, which, however, is not 

 very translucent, the rock seems to be possibly a rhyolite with 

 fluxion-banding. The less translucent bands appear so dark be- 

 tween crossed nicols that the presence of much almost unaltered 

 glass is suggested, while in ordinary transmitted light, that is, in 

 as much light as thin parts of the section will transmit, the bands 

 seem to consist of densely-packed globulites. 



In a paper read before this Society in 1891,^ I had occasion to 

 allude to a specimen of obsidian collected by my friend, Mr. G. P. 

 E-odwell, from the crater of Yulcano, at a point whence a strong 

 jet of steam was then issuing, and which had apparently com- 

 pletely or almost completely devitrified the obsidian through the 

 development of globulites. It seems quite possible that this rock 

 from Mataura has undergone a like devitrification, but that in this 

 instance a silicifying process has also taken place, which would 

 account for the hardness and jasper-like character of the rock. The 

 section contains numerous porphyritic crystals, which from their 

 extinction-angles appear to be sanidine, oligoclase, andesine, and 

 possibly still more basic plagioclase. These felspars seem fresh, and 

 are in no instance represented by vitreous matter, hjalomorphs, 

 if one may venture to coin the name for vitrified felspars, such as 

 those from Waihi, described on pp. 459-60. It would seem that 



1 ' The Eruptive Rocks of New Zealand,' Proc. Eoy. Soc. N.S.W. vol. xxiii 

 (1889) p. 116. 



2 Trans. N.Z. Inst. vol. vii (1874) p. 459. 



3 ' On a Spheruhtie & Perlitic Obsidian from Pilas, Jalisco,' Quart, Journ, 

 Geol. Soc. vol. xlvii (1891) p. 532. 



