418 Transactions. 



Art. XXXII.- — Additional Facts concerning the Distribution oj Igneous 



Rocks in New Zealand. 



By J. A. Bartrum, Auckland University College. 



[Read before the Auckland Institute, 13th December, 1916; received by Editors, 

 30th December, 1916 ; issued separately, 30th November, 1917.] 



Pklo XXVIII. 



The following notes record a few facts which have come under the notice 

 of the writer, and which may be of use to other workers in New Zealand 

 geology. Some of the facts doubtless are known to other New Zealand 

 geologists, but the author believes that the majority have not yet been 

 published. If they have, he pleads the lack of time for full perusal of 

 literature, and the inaccessibility of some of the papers bearing on New 

 Zealand petrography, as his excuse for including them in this paper. 



In some cases considerable work, particularly with regard to the 

 chemical characters of the rocks described, is required before the knowledge 

 of them can be regarded as at all satisfactory, and the writer hopes to 

 be able to continue this work later. 



Hypersthene Basalt, Ruatangata, near Whangarei. 



In slides made from a flow basalt of Pleistocene or Recent age, which, 

 macroscopically, is similar exactly to the general ophitic basalts of the 

 Whangarei district, the mineral hypersthene has been detected in interesting 

 circumstances. 



It is believed that this is the first olivine-hypersthene basalt described 

 from New Zealand, though it must be remarked that some of the hyper- 

 sthene and other andesites approach closely to basalts. Professor Thomas 

 has pointed out, for example, that some of the andesites of the Tarawera 

 district approximate basalts, some carrying scanty olivine.* The majority, 

 however, are very evidently andesites. Professor Thomas notes also, in 

 connection with lavas from Tongariro, that " Most of them are free from 

 olivine ; but others are rich in this mineral, and must be termed basalts. "f 



In the Auckland University College collection is a light-grey rock, with 

 well-marked fluxion banding, showing numerous phenocrysts (up to 5 mm. 

 in diameter) of olivine, with smaller ones of hypersthene, a few fragments 

 of xenolitic quartz, and somewhat inconspicuous feldspar. It is labelled 

 " Otouku, near Tongariro," and evidently represents one of Professor 

 Thomas's basalts. Small partings between plates parallel with the fluxion 

 bands show very minute crystals of some zeolite, apparently cubic, though 

 in the absence of an exact goniometer this could not be established. 



In section, besides the coarse olivine phenocrysts there are numerous 

 smaller ones showing a corrosion border of iron-ore. Many highly corroded 

 coarse plagioclase phenocrysts occur, and idiomorphic hypersthene is abund- 

 ant. Augite frequently forms a fringe to the hypersthene, and there are 

 one or two small phenocrysts apparently of augite. The groundmass is 

 pilotaxitic, and the feldspar mesh is crowded by hypersthene prisms and a 

 very little iron-ore. 



* A. P. W. Thomas, Report on the Eruption of Tarawera and Rotomahana, N.Z., 

 Wellington, Government Printer, 1888, p. 58. 

 f hoc. cit., p. 21. 



