176 Transactions. 



Art. XIV. — On a Soda Amphibole Trachyte from Cass's Peal; 



Banks Peninsula. 



By K. Speight, M.A., B.Sc. 



\[Read before the Canterbury Philosophical Institute, 6th November, 1907.] 



The oldest rocks found on Banks Peninsula consist of slates, 

 cherts, and greywackes of uncertain age ; but the last show a 

 marked lithological resemblance to Lower Mesozoic greywackes 

 that occur at the Malvern Hills. The only exposure of these 

 rocks on Banks Peninsula is near Gebbie's Pass, where they 

 occupy a considerable portion of the main ridge, and extend 

 down on both sides of it, but especially towards the head of 

 Lyttelton Harbour. Here they form a large part of the solid 

 floor of the valley in which Teddington lies. Over them lie 

 solid flows of rhyolite and beds of agglomerate penetrated by 

 dykes of rhyolite and pitchstone. The age of these beds is also 

 uncertain, but they resemble very closely in lithological charac- 

 ter the garnet-bearing rhyolites of Mount Somers, Rakaia Gorge, 

 and the Malvern Hills, which are certainly of Cretaceous age, 

 as rhyolite pebbles are found in conglomerates forming the 

 lower members of the coal-bearing series, which, as well as the 

 rhyolites, overlie unconformably Jurassic sedimentaries. At 

 Mount Somers, too, rhyolite tuffs, according to S. H. Cox, are 

 interstratified with coal - bearing beds. It is therefore highly 

 likely that the Gebbie's Pass rhyolites are of Cretaceous age. 



After a considerable lapse of time, during which the rhyolites 

 were heavily eroded, the main mass of Banks Peninsula was 

 formed, consisting chiefly of andesites of basic type and basalts. 

 These were poured out as subaerial lava-flows, and thrown out 

 as scoria and ashes from two craters which now form Lyttelton 

 and Akaroa Harbours. Onawe Peninsula probably marks the 

 centre of the latter volcano, as the extremity of the peninsula 

 is composed of a syenite, and this is the only occurrence of a 

 plutonic rock in the locality. The remaining part of this small 

 peninsula near the narrow isthmus is principally formed of 

 intercrossing dykes ; it thus shows the structure which cha- 

 racterizes the neighbourhood of the pipe of an old volcano. 

 Sir Julius von Haast suggests that a third centre of eruption, 

 belonging to this period, occurs in the valley of Little River. 

 He is not very definite about it, and says that the remains of 

 the lavas that were poured out from it are not very extensive. 

 I believe, however, that he modified his views somewhat at a 



