450 Transactions. — Geology. 



much modified by the material that was thrown from the 

 volcanoes of Ruapehu and Ngauruhoe. The ridge to the east 

 of these mountains is known as the Onetapu Plain and Rangipo 

 Desert. It is made of materials thrown from the mountains, 

 and it forms the watershed between the north and south parts 

 of the Island. The drainage appears to have been towards the 

 Rangitaiki Basin and thence through the Kaingaroa Plain into 

 the great Whakatane Swamp. Subsequently it went through 

 the Wai-o-tapu Valley, and thence into the Tarawera River. 

 The Waikato of to-day is the product of the Taupo Lake ; but 

 when did the lake come into existence, and under what circum- 

 stances ? 



The whole of the western side of the lake is surrounded by 

 rliffs that rise 1,500 ft. or more in vertical height from the 

 bottom of the lake. Along the eastern and northern sides 

 pumice is the predominant material, and in some places pumice 

 cliffs rise to a height of 200 ft. or more above the surface of the 

 lake. It is evident that important physical surface - changes 

 must have taken place to bring about the conditions such as at 

 present are to be found. A map of the Taupo district shows 

 the present drainage-area of the lake. From Ruapehu and 

 Ngauruhoe proceed many streams, of which the Tongariro 

 (Waikato I) is one of the chief. It falls into Lake Taupo at the 

 south-eastern corner. At the northern end of the lake the 

 Waikato River begins. It is the only river that flows from the 

 lake, and represents the difference between the total inflow and 

 the amount of evaporation that takes place. But the history 

 of the lake is the history of volcanic phenomena along the line 

 of weakness, which is at present the line of greatest activity 

 within the volcanic zone. Thus. Tokaanu at the southern end 

 of the lake, the Terraces and the Spa at the northern end. 

 then Wairakei, Roto-kawa, Orakei-korako, Wai-o-tapu, and the 

 great Rotomahana-Tarawera rift, are included within a belt 

 drawn north-east and south-west so as to include the eastern 

 and western sides of Lake Taupo. Within this belt all the 

 phenomena traceable to volcanic action are to be found. But 

 this line of weakness is only a remnant of volcanic action. The 

 volcanic work on a huge measure has been done : and that 

 work is to be estimated not by the cones thai remain along the 

 present line of weakness, but rather by the distribution of Lavas 

 over a very large part of the middle and northern portions of 

 the Island. The volcanic cones of Egmont. Ruapehu, &c, are 

 mostly cones of accumulation made up of material that has been 

 thrown out by explosive force and by the welling-up of lavas. 

 As they grew in height, and during their earlier periods of 

 activity, probably much larger flows of lava, took place from 



