Thomas. — Geology of Tongariro and Tauj^o. 351 



much more basic than any of the preceding rocks. It is heavy 

 and black, with all the appearance of a basalt, and shows 

 microscopic olivine. It contains, however, felspar and augite 

 crystals like the porphyritic crystals in the augite andesites 

 previously described. They are not very abundant, and to 

 them are added olivine in numerous crystals, and felspar laths. 

 The groundm^ass is in the main crystallitic, but shows far 

 more crystalline particles between crossed nicols, and glass is 

 scarcely visible. The rock thus shows a relationship to the 

 augite andesites, but at the same time approximates to the 

 basalts. Another variety collected by Mr. Cussen contains 

 chiefly olivine among the larger crystals, though a few smaller 

 augites are present. The groundmass is partly crystallitic, 

 but contains very numerous felspar laths. 



The analysis of the former variety by Mr. Pond shows (see 

 No. 3) that it contains 52-1 per cent, of silica — an amount 

 considerably higher than that usually present in rocks recog- 

 nised as augite-andesites. Its specific gravity is 2-94, which 

 also indicates a rock more basic than ordinary augite-ai:idesites. 

 The magnesia and lime are also present in much higher pro- 

 portion than in the other rocks of Tongariro. Eosenbusch, in 

 his last edition,'-' states that the augite-andesites seldom con- 

 tain less than 56 per cent, of silica, whilst some basalts attain 

 the same percentage. Teall quotes twenty-three analyses of 

 porphyrites and andesites as varying between 66-75 and 54-73 

 per cent, of silica, whilst the same number of basalts showed 

 percentages of silica varying between 53-73 and 42-65. f 

 Under the circumstances, we shall probably be justified in 

 terming the rock a basalt, though we must admit that it is 

 closely related to the augite-andesites. The system of classi- 

 fication of rocks is necessarily more or less arbitrary, and the 

 present rock is one which lies near the line of division between 

 the augite-andesites and the basalts. 



Pumice on Tongariro. — We have referred above to the 

 popular idea that the great quantities of pumice around Lake 

 Taupo are derived from Tongariro and Euapehu, and have 

 stated that the examination of the country to the south of 

 Lake Taupo lends little support to any such theory. Pumice 

 is indeed found on Tongariro, but in comparatively small 

 quantity. A small amount of pumice of an acid character, 

 containing 75-25 per cent, of silica (see analysis No. 6), was 

 found in the North Crater of Tongariro, and one or two small 

 fragments of rhyolites w^ere found elsewhere on the mountain- 

 top. But with these exceptions all the rocks high up on the 

 mountain w^ere of a more basic character. On the lower 



* " Physiographie der Massigen Gesteine," p. 702. 

 t " British Petrography," p. 49. 



