Thomas. — Geology of Tongariro and Taupo. 



349 



analyses. My own collections show that the typical rocks of 

 Tongariro are augite-andesites, though more basic rocks, 

 which may be regarded as members of the basalt group, are 

 also represented. Perhaps the commonest variety of these 

 augite-andesites is one which is found, amongst other places, 

 on the slope of the North Crater of Tongariro, where it forms 

 the lava-streams above Papakai. Similar lavas descend as far 

 as the shores of Eotoaira. The rock is a porphyritic one, of 

 medium grain and dark colour, showing when quite fresh a 

 slight resinous lustre. The porphyritic crystals are numerous, 

 but none of them reach a length of over 3mm., and they are 

 usually much smaller. Examination with the microscope 

 shows that the porphyritic crystals consist of felspar and 

 augite in about equal proportions and in well-formed crystals ; 

 there are a few smaller magnetites in irregular crystals. The 

 felspars are almost all striated, and many of them are 

 crowded with inclusions of glass, which is sometimes brownish 

 and pure, at other times is colourless, but containing 

 globulites and dark granules. The felspars also contain in- 

 clusions of augite and apatite. The augites are in eight-sided 

 prisms ; in thin section they are yellowish and only feebly 

 pleochroic, the range of tints being from greenish-yellow to 

 brownish-yellow. A few of the largest augites are completely 

 honeycombed by groundmass. The groundmass is hyalo- 

 pilitic — i.e., it consists of a felt of crystallites united by rather 

 abundant colourless glass. It contains a considerable number 

 of black granules of magnetite. The crystallites include great 

 numbers of long, slender, non-polarising longulites, and very 

 few of these incipient forms are sufficiently developed to 

 polarise. Ohvine is altogether wanting in the rock. The 

 augite and felspar not unfrequently occur in nests composed 

 of gi-eat numbers of crystals, which only show their proper 

 form on the exterior of the groups. The analysis of this rock 

 is shown as No. 1 in the following table. Its specilic gravity 

 is 2-76:— 



Table of Analyses of Volcanic Rocks by J. A. Poncl, Colonial 



Anal3'St, Auckland. 



Nos. 1-4 are from Tongariro : No. 1, an augite-andesite from the 

 lava-stream which reaches Kotoaira ; No. 2, ditto from Otouku ; No. 3, 



