350 Transactions. — Geology. 



ditto from Red Crater ; No. 4, ash on slope of Tongariro ; No. 5, augite- 

 andesite, the lava of 1SG9, Ngauruhoe ; No. 6, pumice from North Grater, 

 Tongariro ; No. 7, rhyolite lava from Motutaiko ; No. 8, lithoidito from 

 Hamaria, Lake Taupo. 



Other varieties of augite-andesite from Tongariro differ 

 chiefly in the degree of devitrification of the glass, the por- 

 phyritic crystals being essentially the same. Specimens may 

 be obtained in which the groundmass is quite free from, 

 crystallites, and consists of a pm-e glass of a rich brown 

 colour. That this was the original coiidition of the ground- 

 mass in the other varieties may be seen from the inclusions of 

 brown glass in some of their larger felsj)ars. Here we must 

 include specimens of brownish pumice, which contains augite 

 and plagioclase in crystals and groups of crystals— evidently 

 only a similar rock distended with very numerous minute 

 vapour-cavities. 



On the side of Ngauruhoe which slopes down to the South 

 Crater on Tongariro, is a lava-strearn which is said to have 

 flowed from Ngauruhoe in 1869. This stream ends in an 

 irregular front just where the slope of the cone joins the flat 

 bottom of the crater on Tongariro. The lava is much fissured, 

 and the smooth surfaces of the blocks have a strong glassy 

 lustre. This lava is essentially like those on Tongariro, ex- 

 cept that tlie groundmass is rather more glassy, and is perhaps 

 richer in iron-oxides. The proportion of silica, however (see 

 analysis No.' 5), is rather lower, being only 57"0. Its specific 

 gravity is 2-82. 



Amongst other lavas of Tongariro may be mentioned that 

 which forms the blocks of lava at the highest point of Tonga- 

 riro. This contains a few crystals of olivine. The same 

 mineral occurs in crystals visible to the naked eye iii the lava 

 which reaches Otouku. This rock is shown by its analysis 

 (see No. 2) to differ but slightly in chemical composition from 

 the commoner type of Tongariro lava. Its structure, how- 

 ever, is very distinct. The porphyritic crystals are not nu- 

 merous, but include plagioclase, augite, and olivine. The 

 groundmass is almost entirely crystalline, and shows a pro- 

 nounced fluidal structure. It consists of small augite prisms 

 and minute laths of felspar, with scanty magnetite granules. 

 In parts of the rock a little glass may be traced ; in others it 

 appears holocrystalline. The fluidal structure is due partly 

 to the parallel grouping of the felspar laths, but more espe- 

 cially to the arrangement of the augite and felspar in such 

 a way that along certain lines the minute augite crystals 

 largely predominate, along others the felspar laths. The 

 specific gravity is 2-83, being doubtless a little higher on 

 account of the crystalline character of the groundmass. 



A rock which occurs in blocks around the Eed Crater is 



