406 B. E. Rastall— Rocks of New Zealand. 



No. 67 is a grey, markedly porphyritic rock with large pbeno- 

 crysts of plagioclase felspars and a little femic material in a fine- 

 textured groundmass. The felspar is of the usual zoned type, and 

 ranges from andesine to oligoclase. The coloured minerals include 

 both oblique and rhombic pyroxenes, but the latter is dominant ; it 

 is very pale brown and slightly pleochroic, so that it must be classed 

 as hypersthene. 



The groundmass may on the whole be described as of the 

 hyalopilitic type, but the prisms are not so well defined as usual, 

 and there is a good deal of glass. This rock is a hypersthene-augite- 

 andesite, and strongly resembles the specimen from the "Waiora 

 Valley, No. 61. 



3. Huka Falls, near Taupo Lake. 



This rock has horizontal joints, and gives rise to a waterfall some 

 150 feet high. Its connection with the other rocks of the district is 

 obscured by the post-Tertiary ' cloak of pumiceous debris. 



This is another glassy rock, with rather abundant broken 

 plagioclase crystals and some quartz in a groundmass of fragments 

 of vesicular glass, showing good flow-structures. Coloured minerals 

 are very rare. It is probably more acid than those described above, 

 and may be regarded as a rhyolite tuff. 



4. Tarawera. 



No. 69 was obtained in a roadside cutting near the buried village 

 at the west end of Lake Tarawera, and is said to have been ejected 

 during the great eruption of 1886. 



It is composed of fragments of glass, with very fine flow- 

 structures and perlitic cracks, and the usual plagioclase crystals are 

 often broken. There are also several well - formed crystals of 

 a mineral with the characters of a pyroxene, pale brown and 

 slightly pleochroic, with an extinction angle as high as 45^ (cf. 

 Nos. 62 and 64). The structure of the rock is clearly pyroclastic. 



Conclusion. 



Owing to the small amount of material at my disposal it is not 

 possible to draw any very definite conclusions as to the character of 

 these rocks. On the whole there is a considerable family likeness 

 between the different types, and they seem to form a series of 

 pyroxene andesites and rather basic pitchstones. The rock from 

 Huka Falls is more acid and probably of rhyolitic composition. In 

 many cases it is possible to distinguish a more or less distinct 

 fragmental structure, and some are tuff's, or at any rate a mixture of 

 lava and tuff. 



The general mineralogical composition suggests that they belong 

 to the subalkaline family, and they contain pyroxenes of a rather 

 imusual type. 



1 See Hector's map. 



