462 Transactions. — Geologv. 



and small pebbly pumice showing trituration and wear. If a 

 line be drawn in the direction of the outlet of the lake and the 

 raised pumice terrace extending between Maunganamu and the 

 Crow's Nest, there will be no difficulty in understanding how the 

 deposit on the Island of Motutaiko took place. It seems to me 

 that the evidence of pumice capping the island, the existence of 

 the Horomatangi reef and the submerged forest, point to a sub- 

 sidence along the eastern side of the lake at a time when the 

 water stood at least 320 ft. above its present level. The dry 

 watercourses that are met with by the hundred surrounding the 

 lake on the eastern side, and gradually dipping towards the lake, 

 imply a rapid scouring of the pumice by a kind of rocking motion ; 

 and it was perhaps at this time that the waters of an extended 

 Taupo Lake found their way through the north-east portion of 

 the Taupo rift, forming what is now the Waikato River. Many 

 explosions must have taken place at this time, for the evidence 

 appears to be complete that the whole line extending from 

 Ruapehu to Tarawera, and thence to Edgecumbe and White 

 Island, constitutes a continuous line which is at times affected 

 in one place, and at times in another, in accordance with the 

 capacity of the overlying and ever-changing deposits to bear the 

 strains set up from beneath. The disappearance of the waters 

 to their present level in the Taupo Lake exposed hundreds of 

 hot springs, geysers, fumaroles, &c, that were pressed down 

 beneath the waters of the lake. The rilling of the Rotomahana 

 crater with water, underneath which are hundreds of intensely 

 active lines of steam-tubes, of necessity creates new possibilities. 

 If the weight of water or the pressure of the overlying water is 

 sufficiently great to overcome the steam-pressure from below, the 

 pressure will either accumulate or it will find a line of weakness 

 in the underlying rocks. Waimangu Geyser is the outcome of the 

 filling-up of Rotomahana with many millions of tons of water. 

 The same things took place in the rift that now does duty for 

 the Waikato River. In many places are geysers and other objects 

 of volcanic phenomena which broke out as the result of the Taupo 

 eruption and the subsequent filling-up of the crater-basin with 

 water. Hence the phenomena seen over the whole of the vol- 

 canic district, as illustrated by hot springs, fumaroles, solfataras, 

 and mud-volcanoes, are the outcome of unstable conditions that 

 are set up by the constant changing of pressures, known as stress 

 and strain. They are attendants of volcanic action, and they 

 have acted for many years along the great fault-line of which 

 Taupo is the centre. 



Thus we conic to view Taupo as having a similar origin to 

 the rift in Tarawera Mountain, the crater-basin of Rotomahana, 

 the subsiding areas at Parehera and Waihoru, and the crater- 



