Barteum. — Geology of Great Barrier Island. 123 



siliceous deposit " (? rhyolite), and forms the watershed between the 

 Kaitoke and Awana Streams. At every point where the writer examined 

 the rocks at this ridge they were of the usual rhyolite, but he admits that 

 certain portions of the ridge were not visited. Park continues, however, 

 " This breccia is flanked or overlain by the siliceous deposit on all sides of 

 the Awana Flat. It is composed of a bluish-grey, shining, siliceous rock 

 embedded in a matrix resembling a fine ash or tuff. . . . On the low 

 spurs and ridges near the fantastically carved rocks overlooking the great 

 Kaitoke basin this breccia material is seen to decompose in a deep, yellow- 

 coloured, mealy, sandy clay. The origin of this breccia is evidently connected 

 with some of the volcanic outbursts of andesitic matter which preceded 

 the deposition of the great sinter deposit around this region." 



Whilst admitting that his visit afforded him but a cursory glance at the 

 geology of this district, and that he had not read Park's statement in 

 advance of it. the writer is at a loss to understand the location of this breccia. 

 He would certainly doubt that it forms the " fantastic and grotesque ' : 

 features of the ridges west of Awana Flat, and there is some uncertainty 

 as to whether or not Park actually means that it does so in this locality, 

 since he says that " This breccia is flanked or overlain by the siliceous deposit 

 on all sides of the Awana Flat." To the east and south-east of Awana 

 Flat the topography indicates that the rhyolitic rocks give place to the 

 andesitic, which are known to outcrop a very few feet below the level of the 

 flat on the south-western slope to the Kaitoke Stream from the flat, and 

 which form a wide strip along the east coast north from the mouth of that 

 stream. 



The writer did. however, observe a most curious obsidian " breccia " — 

 possibly a true breccia, more probably only such in appearance, and in 

 reality a weathered coarsely perlitic obsidianitic flow. This is exposed, 

 lying hard upon the andesitic rocks (mentioned above) outcropping a very 

 few feet below it, in a cutting of the track from Awana Flat towards 

 Oroville, the site of the now disused mines in the Kaitoke Valley. The 

 depth of the breccia actually exposed is insignificant, but it is possible 

 that its thickness is locally considerable, and that it represents some phase 

 of the breccia described by Park. 



The obsidian discovered here explains the otherwise inexplicable occur- 

 rence of obsidian noted by McKay (1897) on Te Ahumata, which is, indeed, 

 a further argument in favour of the view that the upper portion of that 

 elevation — the sinter of McKay and Park — is largely rhyolitic. This mass 

 rests on andesitic material which is exposed on the lower slopes of the White- 

 cliffs Range, and which, in altered form — propylite — is the country of the 

 gold-silver veins {fide Park, 1897). 



Petrography of the Rhyolites. 



Petrographically the rhyolites examined from the acidic area north of 

 Te Ahumata show little variety. In hand-specimen none showed any notice- 

 able phenocrysts, and the majority, as already stated, have a very fine, 

 regular, but somewhat sinuous fluxion-banding. The colours vary from 

 greyish- white to pinkish. In thinnest section, using a microscope with 

 good resolving-power, the banded varieties show up as very minutely micro- 

 spherulitic. Phenocrysts are practically absent. Small druses of opal 

 may show up in section, and in the field a few larger opal- or chalcedony- 

 filled cavities were observed. 



