464 Transactions. — Geology. 



ground -mass. A large fibrous mass of yellowish-brown 

 pleochroic bastite appears in the centre of the section. It 

 gives straight extinction along the planes of schillerisation. 

 Under a high power it is seen to be a very pale-yellow mass, 

 with deep-brown prisms arranged in parallel lines, and giving 

 the fibrous appearance. Sometimes little prisms are arranged 

 crosswise, giving a scalariform appearance. 



Fig. 3: Cheltenham. — This is ahypersthene-andesite. The 

 ground-mass is hyalopilitic, but the feldspar laths are not very 

 numerous. Grains of magnetite are thickly distributed. The 

 phenocrysts consist chiefly of large striped feldspars, whose 

 extinction angle shows them to be labradorite. Augite also 

 occurs, associated with a rhombic pyroxene, which is probably 

 hypersthene. The colour is paler, however, than in the 

 drawing. 



Fig. 4: Onchunga. — This is a very amygdaloidal rock. In 

 the hand specimen it is, black, spotted with white amygdules 

 of chabasite, which show under the microscope beautiful 

 fibrous forms. The ground-mass is augitic, with feldspar 

 laths and a little magnetite. The phenocrysts are numerous, 

 especially large striped crystals of labradorite or andesine. 

 Augite is the usual pyroxene, forming very large green six- 

 sided prisms; but pleochroic brownish crystals of hypersthene 

 are also present. 



Fig. 5 : Wairau Greek. — The ground-mass is typically 

 hyalopilitic, with numerous elongated laths of feldspar, show- 

 ing flow-structure. Augite is also present in the ground- 

 mass in abundant pale-yellowish grains. Magnetite grains 

 are numerous. The rest of the ground-mass consists of a 

 deep-brown glass, perfectly isotropic. 



The phenocrysts consist of plagioclase, but, though binary 

 twinning is common, multiple twinning is very rare, and some 

 of the feldspars are untwinned. Zonal structure is very 

 general. Augite phenocrysts are not very numerous, occur- 

 ring as pale-green six-sided prisms. The augite and the 

 feldspar contain numerous inclusions of glass and magnetite 

 grains. Magnetite also occurs as large three-sided crystals. 



The rock closely resembles an " andesitic basalt," from 

 the Waitakerei Eange, which is in the collection of the Uni- 

 versity College laboratory, the only difference being that 

 multiple twinning is more common in that rock. It also 

 somewhat resembles an "andesitic basalt" from Eskdale- 

 muir, described by Sir A. Geikie.* 



Sir James Hector has always distinguished between the 

 Parnell grit and the Cheltenham breccia, but he placed the 

 latter above the former on the strength of a supposed uncon- 



*Proc. Roy. Phy. Soo. Edin., vol. v., 1880. 



