Marshall. — Geology of North Island. 91 



no material change within the period of my observations, and 

 they have been accurately described by many observers. 



The violent eruptions of Tarawera in June, 1886, have been 

 so fully described by Hector, Hutton, Percy Smith, and Thomas, 

 as well as a host of other writers, that it is unnecessary to refer 

 further to them here. The features of this volcanic area have 

 lately been examined by Bell,* and the changes that have oc- 

 curred since the eruption are described by him. 



An important feature of the physiography of the district 

 is described by Cussen. f This is the range of old folded sedi- 

 ments here referred to as Mesozoic, though stated by Cussen, 

 in conformity with the usual custom, to be Carboniferous. The 

 range commences ten miles to the west of Tongariro, and extends 

 throughout the country to the west of Taupo. There is little 

 doubt that, though the old sediments have actually been found 

 only in few places on this range, it is really an old denuded 

 range which has been smothered beneath the accumulation of 

 volcanic material. This range was first discovered by Hoch- 

 stetter. 



The deep dry valleys found at intervals in the pumice country 

 are deserving of some notice. They are especially frequent on 

 the north of Taupo. In many of them no water has ever been 

 known to flow, yet they are 150 ft. to 200 ft. in depth, with 

 nearly vertical sides, and 30 or 40 yards wide, and often of 

 great length. Even if heavier rainfall is assumed to have taken 

 place in the past, it is hard to account for these. The eruption 

 of Tarawera afforded a clue to their origin, for the torrential 

 downpours of condensed steam and mud which succeeded the 

 eruption caused the erosion of such channels in several places, 

 notably near the road between Rotorua and Wairoa. It seems 

 reasonable to suppose that the dry channels have generally been 

 formed in this way. 



Another physiographical feature which is most striking is 

 the steep, straight-sided form of many of the hills in this region. 

 Horohoro is a well-known example. The straight sides are 

 formed of rhyolitic lava in most cases, though Cussen states 

 that Titirangenga, in which straight sides are noticeable, is 

 formed of augite-andesite. These remarkable hill-forms have 

 been described as fault-lines along which the surrounding land 

 has fallen in. Hochstetter first held this view, and more re- 

 cently GregoryJ has adopted it, and the theory was mentioned 

 by Marshall. § Gregory describes one fault-plane along the 



* J. M. Bell, Geograph. Journal, 1906, p. 369. 



t Cussen, Trans. N.Z. Inst., vol. xx, p. 320. 



% Gregory, " Australasia," vol. 1, p. 582 ; Stanford. 



§ Marshall, " Geography of New Zealand," 1905, p. 183. 



