450 MESSRS. J. PARK AND F. RUTLET ON THE [Aug. 1 899, 



followed by periods of intense solfataric action, resulting in the 

 formation of extensive siliceous deposits, some of which have proved 

 richly productive of gold and silver. The present solfataric action of 

 'I'aupo, Rotomahana, and Rotorua has more or less affected the 

 rhyolites which occupy these regions. 



While engaged in the geological exploration of the Hauraki 

 Goldfields,^ an exploration extending over a period of seven years, 

 the author collected many varieties of rhyolite, typical examples of 

 w^hich were submitted to Mr. Eutley for examination. A few brief 

 notes on the principal characteristics of the rhyolites as they appear 

 in the field may be of some interest, when read in connexion with 

 the description of their microscopic structure. 



On the surface the rhyolites are generally grey or yellowish- 

 grey. In the neighbourhood of Waihi they are frothy or pumiceous. 

 Between Waihi and Waikino they are brecciated, with small 

 angular enclosures of black pitchstone, or partially devitrified glass, 

 which impart to the rock an ashy or tuff-like appearance, especially 

 on weathered surfaces. "When deeply quarried into, the pumiceous 

 rhyolite is seen to pass insensibly into a tough, fine-grained, drab 

 or dull purple rock, in which clear glassy crystals of sanidiue are 

 conspicuous. 



The rhyolite weathers into a soft grey rock, which breaks into 

 cuboidal masses whose sides are often 10 inches square; when 

 much decomposed it outwardly resembles a claystone, and, like 

 it, crumbles into a tenacious clay on exposure to atmospheric 

 influences. 



The rhyolites of Mercury Bay and Tairua often exhibit a strongly- 

 marked spherulitic structure, many of the spherulites ranging from 1 

 to 2 inches in diameter. In these districts, moreover, they possess a 

 finely-banded structure, in which thelaminge are apparently coincident 

 with the fluxion-planes. A beautiful example of banded structure 

 is seen at the mouth of the Tairua Eiver, on the sea-face of Te Paku 

 Island. Por a distance of | mile the laminae are extremely thin, 

 and so strongly folded that the rock, viewed a little way off, presents 

 a remarkable resemblance to a highly-contorted gneiss or mica- 

 schist. 



The rhyolites which form the lofty bare hills around the Waihi 

 Plains are highly quartzose and often granitoid in appearance. 

 They are seemingly the result of eruptions earlier than the pumi- 

 ceous rhyolites, which form the floor of the plains, and wrap round 

 the flanks of the enclosing hills of andesite on the north, and of 

 quartzose rhyolite on all other sides. Boring operations on the 

 plains disclosed the presence of two distinct streams of rhyolite, 

 with grey pumiceous upper surfaces. 



In the Mercury Bay and Tairua districts the rhyolite-flows cap 

 the higher hills, and along the vallej^s present long lines of steep 

 escarpment, the faces of which are carved into deep vertical folds or 

 corrugations, the massive stony drapery backed with evergreen 



1 Trars. N. Z. Inet. M. E. vol. i (1897) pp. 1-105. 



