Speight. — The Geology of Banks Peninsula. 377 



the level of the plains; but in places artesian bores have struck them some 

 distance out from the foot of the hills, so that their actual extension in 

 that direction is quite uncertain. To the east of Gebbie's Pass they form 

 the ridges between Gebbie's Vallev and Kaituna Vallev, and extend there- 

 from t<> the north along the western flanks of Mount Herbert, and pro- 

 bablv continue round the western heads of the streams discharging into 

 Charteris Bay. After a considerable gap, where they are covered with 

 later rocks, they appear again at the head of the Purau Valley and form all 

 the mass of hills lying between Purau Bay and Port Levy. They may reach 

 as far eastward as the ridge between Port Levy and Pigeon Bay, since the 

 flows exposed on the eastern shore of the former have a distinct inclina- 

 tion to the east ; but as it is apparently impossible to separate in that locality 

 the flows coming from the direction of Akarca and those from Lyttelton 

 the boundaries in that part of the periphery of the volcano are uncertain. 

 No doubt at one time they extended right across the entrance to the har- 

 bour, and across the gap at Gebbie's and McQueen's Passes, but have been 

 removed by erosion in those sectors. Remnants of these covering beds 

 are to be seen on the ridge between Gebbie's Valley and McQueen's Valley. 

 Rabbit Island, or Motukarara, is an isolated fragment lying about half a 

 mile off the western bounding ridoe of Gebbie's Vallev, surrounded on all 

 sides by lake sediments, and proving by its position a former wider 

 extension of the limits of the volcano in that direction. This remnant 

 owes its preservation to the resistant character of the rock of which the 

 low hill is composed. 



With one exception, there are no apparent difficulties in interpreting 

 the structure of the locality as far as these rocks are concerned. On the 

 south side of McQueen's Pass and extending across McQueen's Valley from 

 its western side to the east side of Gold Valley is an occurrence of basic 

 reck associated with the rhyolite whose position is difficult to account for 

 except on the supposition that it is older than the rhyolite. These basic 

 rocks rest on cherts, and are apparently overlain by the acid variety. The 

 appearance may. however, be deceptive, but it should be noted. If it 

 represents an old volcanic flow subsequent to the rhyolite, then the surface 

 contours must have been extraordinarily steep when the extruded material 

 flowed over them. 



At the only places where contacts of the rocks of the Lyttelton series 

 of volcanics with underlying beds are visible they rest either on greywackes, 

 slates, and sandstones of the Trias-Jura series, or on rhyolites. their rela- 

 tions to each of these being well seen on the east and west sides of the 

 Gebbie's -McQueen's Valley depression, where the original covering beds 

 have been removed by erosion and the basement series exposed. 



The lava-flows and ash-beds of which the cone is constructed are ex- 

 clusively basic in character ; they vary from fine-grained basalts to those 

 in which feldspar phenocrysts form a considerable bulk of the rock. On 

 account of this feature and their consequent high percentage of silica (up 

 to 55 per cent.) they have been classified as andesites, but they contain 

 normally a considerable amount of olivine, so they should more properlv 

 be called basalts of andesitic habit. Haast records the presence of trachyte- 

 flows in the Lyttelton Tunnel, but this is probably incorrect, as in my 

 experience trachytes occur only in the form of intrusions, and the occur- 

 rence in the tunnel may be merely an overflow from a dyke or a sill. The 

 general high percentage of feldspar phenocrysts seems to indicate that 

 the lava-flows were fairly viscid, and therefore the inclination of the beds 

 is steeper than would be expected in the case of a normal basalt. 



