Marshall. — Geology of North Island. 93 



explosion cavities, as Lake Rotomahana actually does. It is 

 perhaps advisable in connection with this part of the subject 

 to state that there is every reason against the supposition that 

 the pumice was derived from any of the present volcanic cones. 

 Without any known exception, all the cones of the district are 

 formed of andesitic rocks from top to base. 



So far as the nature of the rocks is concerned, I am able 

 to make a few additions to the descriptions given, and, in view 

 •of the large amount of literature now available, to generalise 

 rather more widely as to the distribution of various rock-types. 



Rhyolites of many types are found throughout the dis- 

 trict. The purely glassy type, obsidian, is found at Mayor Island 

 and near Tarawera ; spherulitic obsidians are common at Roto- 

 rua and near Wairakei. The glassy base is usually trichitic. 

 Spherulitic rhyolites are very abundant. The coarse types, from 

 the Cape Colville Peninsula, contain nests of angular quartz 

 grains and some tridymite. I am quite unable to agree with 

 either Rutley or Sollas as to the origin of the spherulites. While 

 being somewhat diffident in this matter, I cannot regard them 

 -as either due to refusion or to decomposition. They appear to 

 be essentially original, though the exact conditions necessary 

 to their formation cannot at present be defined. They are the 

 last objects to form during consolidation of the rock. At Lake 

 Taupo and in many other places there is a banded rhyolite. 

 When examined microscopically the darker bands are found 

 to consist of axiolitic structures of indefinite length, and the 

 other portion consists chiefly of microscopic spherulites, and 

 sometimes the micropcecillitic structure of Sollas is distinct. 

 The rhyolites in the eastern portion of the district, in the valley 

 of the Ongaruhe, have a groundmass in which there is little 

 individualisation of minerals, and the rock has markings that 

 somewhat resemble the damascened patterns on a gun-barrel. 

 Tridymite is common in this type of rock, but quartz is absent. 

 The minerals which have crystallized out are not very numerous. 

 Quartz occurs quite infrequently, but its place is generally taken 

 by tridymite in very small aggregates. In the spherulitic rhyo- 

 lites of Tairua quartz is found in nest-like aggregates, and dis- 

 tinct grains are found in some Taupo rhyolites and in the silicified 

 tuff of the Huka Falls. Feldspar is found in all but the more 

 glassy varieties. Often it is confined to minute radially arranged 

 microlites in the spherulitic types, but distinct crystals are 

 found in the rocks that are not particularly glassy. It is most 

 abundant from rocks in the south and west of the district. 

 Sanidine is relatively infrequent, for nearly all the crystals 

 belong to triclinic forms, apparently between albite and oligo- 

 ■clase. 



