Speight. — The Geology of Banks Peninsula. 385 



mentary matter was deposited. This layer is similar in character to the 

 layer underlying the prismatic jointed flow, and it may be connected with 

 the layer interposed between the two basaltic flows to the north-west of 

 the island. 



The last basaltic outburst has formed another markedly horizontal sheet, 

 which in all probability caps the greater part of the top of the island, and 

 extends down to sea-level on its western side. It shows almost perfect 

 columnar structure in its lower parts, especially where exposed on the cliffs, 

 but the upper layer is rudely prismatic, the line of junction between the 

 two parts being quite distinct, although there is apparently no actual break 

 in the flow. The columns are vertical, and in places the upper part of 

 the flow overhangs and large pieces of rock have broken away and have 

 fallen at the base of the cliff near sea-level. On the southern shore of the 

 island boulders from this flow form a considerable portion of the loose 

 material of the shore-line, though the flow does not appear to have 

 reached the sea-level. Professor J. P. Iddings and Dr. P. Marshall have 

 drawn my attention to the fact that some of these boulders are probably of 

 alkaline type. 



Haast considered that a hollow at the top of the island might have formed 

 the crater ;* but both the eastern and western extremities of the two upper- 

 most flows as exposed in the cliff abut against older volcanic material, 

 either solid or fragmentary, and the inclination of the flows on the shore 

 platform is generally to the north, so that the two distinct horizontal 

 sheets occupy the actual floor of the crater. It is extremely probable that 

 the northern side of the cone was breached either by the outflow of lava 

 or by inroads of the sea, or perhaps by a combination of both processes, 

 the reef which lies between the island and Lyttelton being perhaps the 

 remains of a flow which extended in that direction. There is at present 

 no evidence as to the date of these eruptions, but they probably date from 

 the latter part of the Tertiary era. 



6. Post- volcanic History. 

 (a.) Evidence of Greater Elevation. 



The subsequent history relates to the destruction of the volcano by 

 stream dissection, and to the depression of the land which allowed the sea 

 to invade the floor of the caldera. The former action was actively con- 

 tinued when the land was higher, on lines which were in all probability 

 determined when the volcano was in active operation. Besides the evi- 

 dence for former greater height deduced from the drowned valleys and 

 the sea-invaded calderas, there is proof available from the records of the 

 bores put down in the artesian area which fringes the volcanic mass 

 immediately on the west.f The land in this region was at least 700 ft. 

 higher when these beds were laid down, as is evidenced by the occur- 

 rence of layers of peat at various levels down to 700 ft. beneath sea-level. 

 These are found as far down as boring has been continued, and there 

 is no reason why they should not be found lower still. An elevation of 

 the land by even this amount would drain the caldera-floors, convert 

 the drowned valleys both inside and outside the calderas into dry land, 



* J. von Haast, loc. cit., p. 348. 



f R. Speight, A Preliminary Account of the Geological Features of the Christchurch 

 Artesim Area, Trans. N.Z. Inst., vol. 43, 1911, p. 420. 



13— Trans. 



