Bartrum. — Geology of Great Barrier Island. 125 



Quartz forms about half the bulk of the minerals, though cryptoperthite 

 is fairly plentiful along with orthoclase. The only other minerals are a 

 little plagioclase and some flakes of biotite. It is possible that this rock 

 is the " true dyke of granite " mentioned casually by Hector (1870) in 

 his paper on mining in New Zealand. 



There is some uncertainty in the writer's miud as to the exact classi- 

 fication of this rock ; the structure is typical neither of pegmatites nor 

 granophyres. 



2. Quartz-porphyry, 



Several dykes of this type are recorded by Hutton (1869), as well as 

 a number cf an allied rock called by him ' r felstone," and defined by 

 Hector (1870) as a rock lacking phenocrysts, but otherwise similar to 

 quartz-porphyry. Many of Hutton's quartz-porphyry dykes seem to be 

 in reality porphyrites with quartz. 



The quartz-porphyries so called by the present writer have rather 

 finely granular groundmass and large phenocrysts of quartz and weathered 

 orthoclase and plagioclase, with a few flakes of altered biotite. The 

 groundmass seems largely feldspar, so that the classification is perhaps 

 uncertain in the absence of chemical analvsis. Dvkes of this rock are 



V J 



rare ; only three were found, all of them close together in the headwaters 

 of Mine Bay Creek. 



Many of Hutton's " felstone " dykes are very doubtfully acidic. The 

 groundmass approaches the felsitic, but the phenocrysts, if present, are 

 rare ones of feldspar. The alteration is so intense that exact identification 

 is almost impossible. In the field these dykes may ramify in intricate 

 fashion, as can be seen from Plate XXV, fig. 2. 



The jmotomicrograph, fig. 6 of Plate XXVI, exhibits a typical quartz 

 phenocryst, along with zoned plagioclase, in a quartz-porphyry forming 

 a narrow finely banded dyke in the headwaters of Mine Bay Creek. 



3. Quartz-porphyrites and Quariz-andesite. 



The distinction between the terms " porphyrite " and " andesite " when 

 applied to dyke-rocks is at best an artificial one. Those here classed as 

 porphyrites show rather coarse feldspar phenocrysts in hand-specimen, 

 along with prominent biotite or else hornblende, or chlorite pseudomorphs 

 after those minerals ; they have a coarse groundmass lacking the usual 

 structures found in andesites. In the Great Barrier dykes these latter 

 rocks are very typical representatives of their class ; feldspar is their 

 only common prominent phenocryst. Fig. 1 of Plate XXVII illustrates the 

 coarse irregular structure of the groundmass of a type best classed as a 

 porphyrite. 



Porphyrites and andesites are by far the commonest of the intrusives ; 

 some contain quartz and some are without it. All show the prevailing 

 alteration, though in a few instances this is not intense. Two good examples 

 of comparative freshness are furnished by a quartz-biotite-porphyrite forming 

 a massive intrusion near the adit of the old copper-mine, and by a wide 

 intrusion in a small bay just south of Miner's Head, which is mapped by 

 Hutton as a diorite, but is in fact a quartz-porphyrite, rich in both 

 biotite and hornblende. 



The majority of the quartz-porphyrites are types with altered biotite, 

 usually with chloritic, sometimes with sagenitic, pseudomorphs after that 



