Henverson.—Post-Tertiary History of New Zealand. 595 
Basaltic Rocks. 
The basaltic те range in age from the oldest Pleistocene to Recent 
times. The first eruptions appear to have been closely associa with 
the great fault-movements that ань the а from the Quaternary. 
The large cones of Pirongia and Karioi occur at crossing-points of important 
fracture-systems. 2 former fills part of e trough the drowned western 
end of which forms Kawhia Harbour. The building of these mountains 
and the cones associated with them was certainly completed after move- 
ment along the great fracture-zones of this part of New Zealand had ceased ; 
the voleanie rocks of this group rest for the most part on land surfaces; 
and the youngest Tertiary rocks of the district (the late Pliocene Kaawa 
beds) contain no trace of basaltic material. On the other hand, the high- 
level shelf (1,200-1,400 ft.) on the west flank of Pirongia shows that the 
mountain existed at the maximum depression separating the early from 
the younger Pleistocene. For these reasons Pirongia, Karioi, Kakepuku, 
Te Kawa, and various lava- flows and dykes between Raglan Harbour and 
aika 
vated during the last considerable elevation. They are connected through 
the voleanie rocks of the Tuakau with the basaltic cones and lavas of 
Auckland. But the eruptions at Auckland were later, since the cones still 
have well-preserved craters and the scoria is unweat the red, whereas the 
cones south of the Waikato show no trace of craters and consist of deeply- 
weathered rock. 
The basaltic rocks of North Auckland, which chemically and mineralogic- 
ally resemble those of Pirongia and Tuakau, are divided by Clarke into two 
epe P miim groups. They probably range through the Pleistocene to 
Rhyolitic Rocks. 
Large amounts of rhyolitic material were erupted during late Pliocene 
times, but probably the bulk of the acid rocks in the Taupo-Rotorua 
occurs : 
aerial deposition cover large areas in the centre of the North Island and 
completely — the underlying rocks. On surrounding districts ex- 
ten east and west coasts finer material, in many parts weathered 
to a characteristic sandy loam, caps the hills and upland surfaces. Sub- 
aqueous tuff and breccia are interbedded with subaerial, and, in low-lyi 
country towards ч ейде of the area of thickest deposition, almost entirely 
replaces it. Beds description occur in vast amount in the Waikato 
and Hauraki depressions, near Tauranga, and eastward in 
Plenty, and in less amount in the Waipaoa and Wairoa Valleys. In the 
Waikato district the subaqueous rhyolitic tuffs con o - 
water- fra 
ments of basalt, and wrap round and overlap the bases of several of the 
te cones. Clearly the earliest basalts are older than this rhyolitic 
terial. 
Andesitic Rocks. 
The t voleanoes of the centre of the North Island, Ruapehu and 
Tongariro, together with the small adjacent cones, are formed of andesitic 
rial, and overlie the rhyolitic rocks menti oned above. In the Waihi 
district massive andesitic dykes penetrate the later rhyolitic fragmental 
rocks (48A, p 75). 
