308 Transactions. — Geology. 



they decay, yield a richer soil than barren pumice, now covering 

 so large an area. 



Augite-andesites. — The most interesting of these rocks are 

 undoubtedly those which occur at the great volcanic cones to the 

 south of Lake Taupo. The most recent lava stream on 

 Ngauruhoe, said to be that of the eruption of 1869, is a black 

 scoriaceous rock of somewhat resinous lustre. A fresh fracture 

 shows to the naked eye small crystals of white felspar and dark 

 augite. Thin sections for the microscope show that the rock 

 contains numerous crystals of felspar and augite, which measure 

 up to -16 inch in diameter; but olivine is wholly absent. The 

 felspars are almost entirely plagioclases, but one or two sanidines 

 can be detected. The former are remarkably 1'ich in inclusions 

 of brown glass, sometimes so numerous as to obscure all the 

 clear substance of the felspar, except a narrow rim which is free 

 from inclusions. In some crystals all the glass of the inclusions 

 is devitrified by the appearance of globulites ; in others the glass 

 is still pure. Sometimes the inclusions are arranged in zones, 

 showing the stages of growth of the felspar crystal. A few fel- 

 spars contained numerous vapour cavities, and augite grains, in 

 addition to the glass. The augite crystals also contain glass in- 

 clusions, but they are far freer from them than the felspar. The 

 ground-mass of the rock is a yellowish-brown glass, containing 

 microliths. The glass is abundant, and the microliths of all 

 stages of development, but most of them whilst appearing as 

 simple rods are sufficiently large to polarise slightly. The 

 ground-mass also contains a few grains of magnetite. 



Other specimens from Ngauruhoe belong to the same type of 

 rock. In one of these the glass of the ground-mass is colourless, 

 but contains a fine black dust, which renders the ground-mass 

 very dark, except in extremely thin sections. The glass inclu- 

 sions of the felspar crystals, however, are many of them of the 

 same brownish colour as in the previous rock, showing that the 

 glass was originally of the same colour in both rocks. A rock 

 obtained from a considerable altitude on the western slope of 

 Ruapehu is nearly of the same variety ; the ground-mass of the 

 rock, however, is more finely microlithic. 



Other augite-andesites were obtained from near the crossing 

 of the Whanganui River, a stream flowing into the west side of 

 Lake Taupo. In one of these the ground-mass was composed of 

 microliths, cemented by a very small amount of colourless 

 glass. It remained dark between crossed nicols, save for a few 

 small crystals of felspar and augite, and some scattered micro- 

 liths sufficiently developed to polarise. In another variety no 

 glass could be detected in the microlithic base, but it included a 

 considerable number of felspars, augite grains, and magnetites. 

 An augite-andesite from Whaugamata Bay has a ground-mass 

 intermediate between the last two varieties. 



