Bartrhm.— TAe Distribution of Igneous Rocks in New Zealand. 421 



Counties,* and is in fact a basalt neck breaking tbrougb the massive con- 

 cretionary greensands of the Waitemata series, which are well exposed 

 near by. The basalt is a fairly typical holocrystalline t5^pe, rich in coarse 

 idiomorphic phenocrysts of oUvine and pale-greenish augite, with numerous 

 smaller intermediate crystals of plagioclase showing good fluxional arrange- 

 ment. The groundmass is largely of small feldspar laths enclosing a 

 moderate number of pyroxene grains and small magnetite crystals. 



BASALT, 



Ti Point, Whangateau Harbour, Kodney County. 



This occurrence is referred to as a dyke and also mapped by Cox in the 

 paper just quoted. Several sections were cut from various parts of the 

 mass forming the small peninsula of Ti Point. There is often good fluxional 

 banding shown in the field, but the differences in section are unimportant, 

 except that flow structure is better shown in some sections than others. 

 There are resemblances to the x\uckland basalts, but the chief differences 

 lie in the more sharply idiomorphic nature of the large very abundant 

 olivine phenocrysts, and the greater proportion of augite in the finely 

 holocrystalline groundmass (see photomicrograph, Plate XXVIII, fig. 3). 

 Feldspar scarcely occurs in the first generation, and augite is unimportant. 

 The olivine is almost invariably oxidized on its margins, and stained thereby 

 a deep reddish. This phase of oxidation accompanying extrusion is even 

 better exemplified by a basalt from Ohakune, next to be described. 



Mr. H. G. Cousins, Director of the Teacherb' Training College, Auckland, 

 closely studied this rock some years ago, and very kindly has allowed me 

 to read his unpublished thesis thereon. He shows that the eruption has 

 burst through the Waitemata beds, and concludes, mainly from consider- 

 ations of denudation, that the period of extrusion was probably Upper 

 Miocene. I entirely agree with him that it was probably not contempo- 

 raneous with that of the basalts of Auckland, but much earlier. 



BASALT, 

 Public Works Department Ballast-pit, Ohakune. 



I am indebted to Mr. F. E. Mason, of the Seddon Memorial Technical 

 College, for specimens of this rock. My sections are decidedly interesting, 

 and deserve comment. Much of the material is highly scoriaceous, and 

 stained a deep chocolate-red through oxidation. 



The phenocrysts are coarse glomeroporphyritic groups of colourless 

 augite and a few olivine crystals. In the groundmass are but a few augite 

 crystals, with subequal parts of plagioclase laths and another mineral 

 comprising the main portion. This other mineral is also lath-like, is greatly 

 stained by haematite as a result of oxidation of contained ferrous compounds 

 during the final phases of the eru^ption, and seems certainly to be olivine, 

 for it has straight extinction, very high refraction and birefringence, and 

 a high optic axial angle. The optical character was not satisfactorily deter- 

 mined. Fine iron-ore is moderately abundant. Occasionally, as the photo- 

 micrograph (Plate XXVIII. fig. 4) depicts, the olivine phenocrysts are 

 enwrapped more or less celyphitically by a rim of augite. 



' ^^__ \ 



* S. H. Cox, Geology of the Rodney and Marsden Counties, Rep, Geol. Explor. 

 during 1879-80, pp. 13-39, 1881. 



