404 R. H. Rastall — Rocks of New Zealand. 



The specimens were obtained near the middle of the North Island 

 of New Zealand, and in the area marked " acidic volcanic rocks " in 

 Sir James Hector's geological map of New Zealand, published in 

 1883. They all occur in the Taupo zone, the hot-lake district which 

 lies roughly between Lake Taupo on the south and Lake Eotorua 

 on the north. 



The following descriptions are written from thin slices cut from 

 Mr. Ferrar's specimens, which were necessarily small, owing to 

 difficulties of transport. The numbering adopted is purely arbitrary, 

 and has no reference to sequence or supposed relative age. 



1. Waiora Valley. 



The Waiora Valley commences in a crater-like depression the 

 walls of which consist of rock No. 61. This depression is about 

 half a mile in diameter, and the walls rise in places to a height of 

 300 feet. In the centre of the depression there are abundant 

 fumaroles, which deposit sulphur ; also pools of boiling muddy 

 water and steam vents ; these are causing rapid disintegration of 

 the surrounding rocks. 



No. 61. Waiora Valley, Wairaki. This is a dark bluish-grey 

 and rather vesicular rock with distinct porphyritic structure. The 

 conspicuovis porphyritic elements are a plagioclase felspar and 

 prisms of pyroxene in a fine-textured groundmass. 



The felspar crystals are prismatic or tabular in form and about 

 4 mm. in length. They are sometimes slightly rounded and show 

 zones of glass inclusions. Zonary banding and twinning on the 

 albite law are conspicuous, and the extinction angles measured on 

 the twin lamella are as high as 30° in the interior of the crystals, 

 and nearly 0° in the outer zones, so that the felspar ranges from 

 labradorite to oligoclase. On the average it is of an intermediate 

 composition. The pyroxenes include both colourless augite and pale 

 brown hypersthene witli slight pleochroism. The augite is the more 

 abundant. The groundmass consists of a felted aggregate of small 

 irregular prisms of felspar, with a little interstitial glass, the typical 

 hyalopilitic structure of Rosenbusch. It also contains very numerous 

 minute cubes of magnetite. 



This rock is therefore a very typical hypersthene-augite-andesite. 



2. Aratiatia Eapids. 



On the Waikato Eiver a series of volcanic rocks crop out through 

 the covering of pumice fragments which cloaks the land. They 

 strike approximately east and west, and the dip is about 60° to the 

 south. The river crosses the strike perpendicularly, and has cut 

 a narrow gorge with sides some 200 feet high. The series is over 

 1,000 feet thick, and the specimens were collected from the harder 

 bands, which are usually each about 100 feet thick. The foot of the 

 rapids seems to be also the base of the series, and the specimens are 

 described in ascending order. 



Nos. 64 and 62 are parts of the same mass of rock near the north 



