126 Transactions. 



mineral. An excellent example of such a sagenite pseudomorph is afforded 

 by fig. 2 of Plate XXVII. 



In a section cut from a greatly altered porphyrite-like rock there are 

 nests of chalcedony and some phenocrysts still recognizable as micro- 

 perthite in spite of their being largely replaced by calcite and quartz. In 

 the rather fine-grained groundmass there are laths of plagioclase, whilst 

 a little of the quartz appears to be primary. 



In the quartz-porphyrite from near the copper-mine the phenocrysts 

 are mainly basic andesine, with a moderate number of flakes of biotite. 

 The groundmass is fairly coarse, and consists mainly of plagioclase with 

 abundant small shreds of biotite and a certain amount of quartz ; it 

 contains occasional radial structures. Occasional pseudomorphs after horn- 

 blende are recognizable. Fig. 3 of Plate XXVII illustrates a typical portion 

 of this porphyrite, whilst fig. 4 portrays a quartz phenocryst in the 

 hornblende-biotite type which has already been mentioned. In this latter 

 rock the biotite is very fresh, and is perhaps in excess of the greenish- 

 brown somewhat chloritized hornblende ; a few large phenocrysts of quartz 

 are visible, typically corroded, but andesine is again the most abundant 

 phenocryst. There is quartz in the groundmass. Zircon, apatite, and 

 iron-ore are present in small amount. 



The only andesitic dyke-rock containing phenocrysts of free quartz 

 outcrops in the bed of Mine Bay Creek about a mile above its mouth. It 

 contains ilmenite with associated sphene. 



• 4. Porphyrites and Andesites. 



Porphyrites and andesites are amongts the commonest of the intrusive 

 rocks represented. A mica type, always greatly altered, is the most 

 prevalent of the porphyrites. The only other variety noted is a coarse 

 highly feldspathic one, almost lacking in ferro-magnesian minerals. 



The andesites are varied. Even when appearing fairly fresh macro- 

 scopically, all are found in section to be altered to a greater or less extent. 

 Some are highly feldspathic, some noticeably pilotaxitic (see photomicro- 

 graph, fig. 5, Plate XXVII). Many are altered beyond recognition of variety. 

 It is possible, however, positively to identify the following varieties : Mica- 

 andesite from a small tributary to Mine Bay Creek about half a mile above 

 its mouth, augite-andesite from a prominent dyke at the north end of 

 Harataonga Bay, and an andesite with brown hornblende from a dyke 

 near the north head of Mine Bay. In the augite-andesite the structure of 

 the groundmass is unusual, for the plagioclase laths are enwrapped pseudo- 

 poecilitically by a clear mineral resembling quartz. 



Comparison with Intrusives of Coromandel Peninsula. 



The presence of abundant porphyrites and andesites in the basement 

 rocks of Great Barrier Island is another evidence of the close similarity 

 of that area to the Coromandel Peninsula, where even greater variety is 

 shown in dykes of the same petrographic character, which are intrusive 

 especially into the Moehau series of pre-Jurassic age (Fraser and Adams, 

 1907, p. 22). Particularly on the western flank of the Moehau Bange 

 intrusions are both numerous and varied, but all are basic intermediate 

 in character (Fraser and Adams, 1907, pp. 87-93). 



It is impossible in Great Barrier Island to form any estimate of the age 

 of the intrusions, or indeed to correlate the basement rocks themselves 



