508 ME. P. KTJTLET ON [Aug. 1900^ 



The analyses of it, which he cites, made by Mr. J. A. Pond, Colonial 



Analyst, Auckland, are :— 



I. II. III. 



Per cent. Per cent. Per cent. 



Si0 2 50-90 51-35 52-50 



Fe0&Fe o 3 14'10 14-50 1270 



A1,0 3 ..." 20-00 18-20 18-20 



Cat) 10-38 10-26 11-05 



MgO 2-77 3-10 4-65 



Na 9 -70 -835 -67 



K,0 -14 -16 -09 



CI -04 -05 -05 



S0 3 *22 -41 -32 



P 2 5 -16 -125 -06 



Water and organic matter '60 "60 — 



C0 2 trace. trace. trace. 



10001 9959 100-29 



I. Lapilli (augite-andesite), Wairoa. 

 II. ,, ,, „ Pareheru. 



III. „ „ „ Eastern end of Rotoiti. 



H 47 . Eotorua. — A dark-grey to black obsidian, profusely 

 speckled with yellowish-white crystalline or finely-granular spots. 



Under the microscope, occasional porphyritic crystals of felspar 

 are to be seen, sometimes entire, at others in fragments. For the 

 most part they are sanidine, but there are also a few which show 

 lamellar twinning and appear in one or two instances, from their 

 extinction-angles, to be labradorite. The extinction in these 

 felspars is often zonal, and they are more or less corroded and 

 traversed by irregular cracks. Isolated dark-brown spherulites are 

 also present in the section. These have occasionally been formed 

 about a fragment of felspar. Some grains of magnetite occur, but 

 they are not numerous. This obsidian is filled with globulites and 

 longulites. One or two delicate, yellowish-brown, rod-like sections 

 of crystals may be seen in this slide : their appearance suggests 

 biotite, but they show inclined extinction and are probably horn- 

 blendes with a low extinction-angle of about 5°. A delicate 

 fluxion-banding may be seen in some parts of the section. 



Conclusions. 



A consideration of the microscopic characters presented by the 

 rhyolitic rocks of New Zealand, and of the alterations which they 

 have experienced, especially through solfataric action, naturally 

 leads to the enquiry whether the ancient rhyolites of Great Britain 

 give evidence of similar changes. 



We may, indeed, regard the rhyolitic lavas of New Zealand 

 as one of the most recent text-books upon this subject, and it> 

 behoves geologists to make what use they can of such a source of 

 information by endeavouring to ascertain whether our own old 

 rhyolites can yield us some further clue to the conditions, which 

 not merely accompanied their eruption and brought about their 



