346 Transactions. — Geology. 



sides of the Waikato Eiver is remarkable for its vast number 

 of hot geysers, &c. The mountain has a large crater on top 

 (Hochstetter stated that there was no crater), and its upper 

 part is thickly covered with forest. I found on top of Tau- 

 hara a sprinkling of pumice, with small fragments of lithoidite 

 quite different from the rock of which the mountain consists. 

 The lavas, as far as I observed, are all of acid composition, 

 but contain a rather large amount of hornblende, a mineral 

 not found, so far as I have seen, in the pumice-deposits. 



Hochstetter supposed that the formation of Lake Taupo 

 was due to subsidence of its area, a supposition which is 

 strengthened by the abrupt, precipitous character of part of its 

 shores and the fairly uniform depth of its waters. A more 

 popular theory supposes Lake Taupo to have been an immense 

 crater ; but evidence of this is wanting, though it is quite con- 

 ceivable that the sunken area was marked by vents from 

 which perhaps a portion of the pumice which covers the dis- 

 trict was derived. 



The Waikato leaves Lake Taupo at the north-east end of 

 the lake, and for the first part of its course flows to the north- 

 east in a gorge through rhyolitic tuffs. Three miles from the 

 lake its course is broken by the Huka Eapids and Fall. The 

 tuffs here are locally hardened by the deposit of a siliceous 

 cement from the hot springs, which have attained a consider- 

 able development at one time, and are indeed still represented 

 by one or two warm streams close to and above the rapids. 

 The river on reaching the harder rock becomes suddenly nar- 

 rowed to less than a quarter of its usual breadth, and, confined 

 to a narrow channel with vertical sides and sloping bottom, 

 rushes through it with an arrowy swiftness which the eye can 

 scarcely follow, until at length it plunges downwards some 

 30ft. into a wide basin of water eaten out of softer rocks. 

 Some of the finer pumiceous sands here contain such an 

 abundant siliceous cement of siliceous sinter that to the naked 

 eye they have the half-glassy appearance of a pitchstone, and 

 have, indeed, been described as lavas. 



A few miles further on, however, the Waikato crosses a 

 series of lava-streams, and the harder rocks have led to the 

 formation of the beautiful rapids known as the Aratiatia Falls. 

 The lavas are rhyolites, chiefly of a glassy character, spheru- 

 litic obsidians being well developed. 



Volcanic Fissures in the Taupo Zone. — We have already 

 seen that the Tongariro-Euapehu group of volcanoes are 

 arranged in a straight line, which doubtless represents a 

 fissure in the earth's crust by which the molten rocks have 

 forced their way to the surface. It affords, indeed, a re- 

 markable instance of a number of volcanic vents arranged 

 close together on the same fissure. We have the huge mass 



