510 EKTJPTIVE KOCKS EROM NEW ZEALAND. [Aug. I9OO, 



proportion of those occurring at Caradoc ; and it does not seem 

 unreasonable to suspect that where felsitic lavas are found to be 

 comparatively devoid of fluxion-banding, spherulitic, and other 

 structures so commonly met with in rhyolites, solfataric action may 

 have been instrumental in bringing those rocks to their present 

 condition. 



EXPLANATION OF PLATE XXVII. 



Fig. 1. Mayor Island [H 26 ]. — Obsidian containing gas-inclusions, small crystals 

 of hornblende, and microlites of felspar. X 140. Ordinary transmitted 

 light. (See p. 495.) 



2. Waitekauri [H 27 ]. — Perlitic obsidian. x30. Ordinary transmitted 



light. (See p. 496.) 



3. Waihi Beach [H 31 ]. — Devitrified spherulitic obsidian, some of the 



spherulites being partly altered to felsite. X 30. Ordinary trans- 

 mitted light. (See p. 498.) 



4. The same portion of the section as that shown in fig. 3. X 30. Polarized 



light (crossed nicols). 



5. Rotorua [H 43 ]. — Spherulitic rhyolite, altered by solfataric action. X30. 



Ordinary ti*ansmitted light. In this figure only vestiges of the brown 

 spherulites are seen, but, between crossed nicols, the spherulites appear 

 entire, as if no portion of them had been altered. (See p. 506.) 



6. Rotorua [H l2 ]. — Pumice-tuff, consisting of lapilli of pumice and altered 



rhyolite ? (the latter are not shown in the figure), bordered and cemented 

 by hyalite. X30. Ordinary transmitted light. (See p. 504.) 



Discussion. 



The President said that all the Fellows present would regret 

 extremely the absence of the Author, and still more the cause 

 of that absence. He (the President) had recently examined an 

 extensive series of felsites (rhyolites) and jaspers occurring as 

 pebbles in the Torridonian rocks of Scotland, and had come to the 

 conclusion that the jaspers were in many cases silicified rhyolites. 

 He would now be able to quote the present paper as additional 

 evidence of this conclusion. 



Mr. Frederick Chapman remarked upon the occurrence of the 

 thread-like bodies in the siliceous rocks of Rotorua, which may 

 possibly be comparable with the filamentous algae described by 

 Weed from Yellowstone Park and referred by that author to 

 Leptothrix, etc. It was also of considerable interest to notice the 

 presence of diatoms in the sinters. From a general examination, 

 the frustules which had their outlines best preserved seemed to 

 belong to the genera Melosira and Epithemia, and possibly Orthosira 

 was present. 



Mr. Harker remarked on the close resemblance between the 

 rhyolites and' sinters of New Zealand and those of the Yellowstone 

 Park. The Author's suggestions might also throw light on the 

 origin of certain ancient British rocks, which have probably under- 

 gone silicification by solfataric action ; but here the results are 

 complicated by the subsequent conversion of the amorphous silica 

 into quartz. Certain rocks in Westmoreland, Caernarvonshire, and 

 Pembrokeshire, consisting essentially of microcrystalline quartz, 

 still preserve the characteristic structures of rhyolitic lavas. 



Prof. Watts also spoke. 



