Vol. 56.] ERUPTIVE ROCKS FROM NEW ZEALAND. 503 ; 



The silica in this rock, when powdered and dried at 100° C, has- 

 been found by Mr. Philip Holland to amount to 81*99 per cent. 



H 39 . Rotorua. — A dark-grey rock with weak vitreous lustre, 

 having dull pinkish-white patches and mottlings. The darker 

 portion of the specimen shows small, irregularly-shaped cavities,, 

 and in colour, lustre, and fracture somewhat resembles a baked slate 

 or porcellanite. 



Under the microscope it is seen to be a fine pumiceous tuff, 

 cemented by siliceous sinter or geyserite. The fragments of pumice- 

 are for the most part extremely small, little more than pumice-dust, 

 but the section contains some larger fragments. The finely shredded 

 pumice-fragments are suggestive of Mugge's bogenstruktur, or 

 what Lane describes as ' concave ash.' 1 A few fragments of felspar 

 are present. Mr. Holland has kindly estimated the silica in this 

 specimen, and finds that it amounts to 81*22 per cent., the rock 

 having previously been powdered, and the powder dried at 100° C. 



The geyserite, which constitutes a considerable portion of the 

 rock, is not perfectly isotropic, but feebly transmits a milky bluish- 

 white light between crossed nicols. This is also to be noticed in a 

 section taken from a specimen of Icelandic geyserite, 2 which has a 

 weak milky appearance where the section is thin, but where thicker 

 or less translucent the appearance is almost snow-white. 3 That 

 the milky or snow-white aspect between crossed nicols is due to 

 transmitted polarized light may be demonstrated : 



(a) By cutting off all extraneous light which could be reflected from the 



upper surface of the section, when its aspect remains unaltered ; 



(b) By placing the principal sections of the nicols parallel, when the appear- 



ance closely resembles that seen when ordinary transmitted light is 

 employed ; 



(c) By using ordinary reflected light, the section having then nearly the same 



appearance as when viewed between crossed nicols. 



H 40 . Eotorua. — A white to yellowish-white siliceous deposit 

 with a conchoidal fracture, and in most parts showing a waxy lustre, 

 in other parts dull and pulverulent. 



Under the microscope, the section exhibits a rough and somewhat 

 irregularly-banded structure. The different bands vary in character,, 

 some appearing like vitreous matter speckled with small translucent 

 bodies, at first sight suggestive of globulites, but they are not 

 spherical as a rule. In other bands these small bodies are so 

 densely packed that the bands are far less translucent. Some of 

 the darker bands seem to have been rendered quite spongy through 

 the presence of numerous irregular cavities, now filled with colour- 

 less doubly-refracting matter which, on rotation between crossed 

 nicols, appears to be hyalite : the globular forms which line the 



1 • Geological Eeport on Isle Royale ' by Alfred 0. Lane, Geol. Surv. Mich, 

 vol. vi, pt. i (1898) pp. 168, 171, 175. 



2 The specimen is one of several collected by Mr. Gr. F. Eodwell, to whose 

 generosity I am indebted for these and other Icelandic specimens. 



3 A very similar appearance is also to be noted in some de vitrified obsidians, 

 when viewed in reflected light. 



