122 Transactions. 



Comparison with similar Fragmented Series. 



There is little doubt that the andesitic mas? of Great Barrier Island 

 is coeval with that so well exhibited in rocks possessing similar mode 

 of occurrence at Coromandel and elsewhere throughout the Coromandel 

 Peninsula. These are the " Beeson's Island " or " second period " rocks 

 of earlier writers (cf. Fraser and Adams, 1907 ; Fraser, 1910), which are 

 considered Miocene in age. 



Acidic Volcanic Rocks of the " Third Period." 



In conformity with conditions on the Coromandel Peninsula, where the 

 latest volcanics are practically without exception rhyolitic lavas, breccias, 

 and tuffs, and cap an erosion-surface of the " second period " andesites, 

 there are on Great Barrier Island truly comparable acidic " third period " 

 rocks. The writer did not make as extensive an examination of them as 

 he desired, but visited them on the lower north-west slopes of Mount Young, 

 the western slopes of Mount Hobson, and examined them fairly thoroughly 

 along the ridges south-east and east of Mount Hobson which form the 

 long pinnacled divide between the headwaters of Kaitoke Stream and of 

 Awana Stream. The writer's route from the top of Mount Hobson was an 

 irregular zigzag along this ridge, and across the upper basin of the Awana 

 Stream to Awana Flat. Along his route, and particularly west of it in the 

 circle of precipitous crags surrounding a basin-like hollow in the headwaters 

 of the Kaitoke Stream, these " third period " rocks are by far the most 

 conspicuous feature of the landscape, and the bizarre pinnacles and sheer- 

 walled bluffs are scarcely to be matched even in rugged regions such as that 

 of exactly similar rocks on the main divide between the Kauaeranga and 

 Tairua Rivers, south-east of Thames. (See Plate XXIV.) 



The main portion of the mass of acidic volcanic rocks at Great Barrier 

 Island appears to consist of pinkish-grey rhyolitic lava with very fine wavy 

 fluxion-banding. There is breccia in several places, but it is apparently 

 not extensive. No important tufaceous beds were seen, but the topography 

 in the vicinity of Mount Young indicates their possible occurrence there in 

 quantity. 



Hutton (1869) and Park (1897) deal with the rocks of this series, the 

 former excusably considering them " pink slates," the latter bedded sinter. 

 Undoubtedly Park was influenced by the occurrence of siliceous sinter 

 in large quantity on the Whitecliffs Range ; McKay's able description 

 leaves no room for doubt that there is here a considerable mass of sinter 

 (McKay, 1897). Park (1897) refers to the same mass, and adds an interest- 

 ing detail, which the writer can verify from examination of specimens given 

 him, to the effect that some of this sinter is oolitic. The oolites are about 

 ^ in. in diameter, and consist of concentric shells of very fine white mud ; 

 they imperfectly resemble the celebrated oolites of the Carlsbad springs. 

 A few sections were cut from rocks forming the fringe of the Whitecliffs 

 (or Te Ahumata) mass, and tend to show that all of it is not sinter, 

 but more or less silicified rhyolite and rhyolitic tufaceous breccia. The 

 silicification is perhaps a result of the hydrothermal activity manifested 

 by sinter in other parts of the mass ; but this suggestion is at best a 

 surmise. 



Park (1897, p. 105) refers in some detail to " a remarkable breccia, 

 which has been carved by subaerial denudation into the most fantastic 

 and grotesque features," which, he says, is " immediately east of the great 



