Vol. 56.] ERUPTIVE ROCKS PROM NEW ZEALAND. 509' 



alteration, but also whether the latter was in any way due to solfa- 

 taric action; in other words, whether hot springs and geysers 

 originally existed in the areas where British rhyolites are found. 



It cannot at this moment be positively affirmed that we have any 

 evidence of this kind ; but a comparison of thin sections of our 

 older rhyolites with those of New Zealand lead to the belief that 

 careful study in this direction will eventually prove that some, at 

 least, of our older rhyolites have been more or less 

 affected by solfataric action. So far I have failed to find 

 any positive evidence upon this point ; and I can only mention 

 one or two instances in which sections examined in reflected, as 

 well as in polarized and ordinary transmitted, light have aroused 

 some suspicion. 



One case is that of a rock from the northern end of Dufton Pike 

 in Westmoreland. A section of somewhat similar rock has already 

 been described by Mr. A. Harker 1 under the heading i Rhyolitic 

 Bocks,' in Appendix I to Marr & Nicholson's paper on ; The Cross 

 Fell Inlier.' This section when examined in reflected light, seemed 

 to show a very close resemblance to some siliceous sinters ; but 

 this similarity of appearance is hardly to be trusted, since formerly 

 vitreous acid lavas, now devitrified, often present a like aspect, 

 when viewed in reflected light. On submitting part of this speci- 

 men to Mr. Philip Holland, he informed me that the total silica in 

 it amounts to only 69 per cent. : a circumstance which strongly 

 supports Mr. Harker' s opinion that the rock is a rhyolite. 2 



Another specimen from Carneddau, near Builth, has a brecciated 

 aspect, and so closely resembles H 42 from Rotorua (p. 504) that a 

 comparison of the sections seemed desirable. Under the micro- 

 scope, the Carneddau rock is seen to consist of angular fragments of 

 felsite, held together by a brownish cement which still contains 

 much isotropic matter, possibly to be regarded as siliceous sinter 

 somewhat altered. The felsitic fragments show none of the struc- 

 tures characteristic of rhyolites, although such structures may 

 originally have been present. 



As to the causes which may convert a glassy into a lithoidal rhyo- 

 lite, we still seem to lack information ; but, as I have suggested in 

 a former paper read before this Society, it is possible that the action 

 of steam may be instrumental in such a change. This, however, is 

 probably only an occasional agent, and the more general cause of 

 such changes must be sought elsewhere. 



In microscopic characters the rhyolitic rocks of New Zealand 

 represent, structurally, everything met with in our ancient rhyolites,. 

 apart from the alteration which some of the latter have undergone. 



The obliteration of original structures by the development of a 

 felsitic condition, as illustrated in the spherulitic rhyolite of Waihi 

 Beach, may account for the comparatively structureless character 

 of some of our old rhyolites, such, for instance, as a considerable 



1 Quart. Journ. Geol. Soc. vol.xlvii (1891) p. 518. 



2 Mr. Harker expresses very guardedly his opinion concerning the ground- 

 mass of this rock, which deserves further examination. 



Q. J. G. S. No. 223. 2 m 



