Bartrum. — The Distribution of Igneous Rocks in New Zealand. 419 



The abundance of olivine and, hypersthene appear to justify the classi- 

 fication of the rock as a basalt, though only chemical analysis can establish 

 this. 



The Kuatangata rock is similar to the more feldspathic of the Auckland 

 basalts, but besides phenccrysts of olivine, augite, and plentiful coarse 

 labradorite there is also subsidiary hypersthene. The groundmass is pilo- 

 taxitic, and plagioclase predominates slightly over pyroxene. Iron-ore is 

 unimportant. There are occasional xenolites of andesitic nature. 



In several instances the augite has formed a parallel growth about a 

 central crystal of hypersthene ; the latter mineral apparently became 

 unstable during the last stage of consolidation, although sharply idio- 

 morphic unattached hypersthene is also common. The photomicrograph 

 (Plate XXVIII, fig. 2) illustrates this feature satisfactorily, the hypersthene 

 being extinguished in the centre of the field. 



Abundance of 'feldspar is more marked than in most Auckland basalts, 

 whilst olivine is less noteworthy ; but field relations and microscopic cha- 

 racters alike show that the rock is one of the series of the latest extrusives, 

 which, in the Auckland and Whangarei districts, are universally basalts. 



Troctolite, Wade, Auckland. 



In hand-specimens this is a blotched dark-greer and white coarse 

 gabbroid rock, showing serpentine, some evident fresh plagioclase with 

 albite striations, and much altered feldspathic material. 



In section serpentine shows up as almost 50 per cent, of the rock. It 

 presents the usual characters of serpentine derived from olivine, and 

 encloses here and there large kernels of unaltered olivine. A little pale 

 bladed amphibole (tremolite) is associated with the alteration products 

 both of the olivine and of the plagioclase. This latter mineral where 

 unaltered has extinction angles on albite twin lamellae of from 30° up to 

 44° (approximately), and is therefore labradorite. Its alteration presents 

 difficulties. Often the result of the alteration is a greyish product, with 

 very high refractive index and very low polarization tints, which is appa- 

 rently a saussurite consisting mainly of some mineral of the epidote group. 

 Again the feldspar is largely converted first into strings and then into larger 

 patches of a densely fibrous transparent mineral with approximately the 

 same index of refraction and the same birefringence as the feldspar itself. 

 This is probably a zeolite. 



Granodiorite with Primary Epidote, Keefton. 



The field relations of this rock are unknown. It is represented by a 

 specimen collected by Professor A. P. W. Thomas (apparently from river- 

 gravels), and now in the collections of the Auckland University College. 



Macroscopically it is a moderately coarse light-coloured granitic rock, 

 with abundant biotite (occasionally aggregated in small basic secretions), 

 and showing other pale yellowish-green crystals along with brownish rather 

 resinous ones. The microscope shows that these latter are epidote and 

 sphene respectively. 



Microscopically the rock is readily classed as a biotite-rich granodiorite. 

 The feldspars include microcline, orthoclase, microcline perthite, perthite, 

 and plagioclase which is probably acid andesine, though this latter deter- 

 mination is not exact. The lime-soda feldspar predominates slightly over 

 the alkali variety. 

 14* 



