368 Transactions. — Geology. 



Art. XLII. — Tridymite- Trachyte of Lyttelton. 



By P. Marshall, M.A., B.Sc, Lecturer on Natural Science, 

 Lincoln Agricultural College. 



[Read before the Philosophical Institute of Canterbury, 6th September, 



1893.] 



Plates XLIV.-XLVIII. 



The volcanic system of Banks Peninsula presents to geologists 

 an exceedingly interesting if somewhat complicated problem. 

 Since the earliest days of colonisation it has been the subject 

 of elaborate investigations by the officers of the Geological 

 Survey of Canterbury, but no one has done more able and 

 lasting work in this district than the late Sir Julius von Haast, 

 formerly Provincial Geologist of Canterbury. 



According to this observer — and his views have been ac- 

 cepted throughout the colony — the peninsula is composed of 

 the ejecta and lava of a few large volcanic foci, the boundaries 

 of which were determined by him, and laid down in the official 

 geological map of Canterbury. Of these vents, the one situated 

 in the depression now known as Lyttelton Harbour was, ac- 

 cording to him, the largest and most important. 



The harbour itself is about eight miles long by two broad, 

 and is too large, according to Sir J. von Haast, to have been 

 at any time the actual crater of a volcano, and he accounted 

 for its dimensions in the following way : — 



A large volcano, perhaps several thousand feet higher than 

 the remaining caldera walls, occupied the site of the harbour, 

 its eruptions being spasmodic and explosive. Between each 

 two eruptions the vent is supposed to have been more or less 

 choked up by congealed lava and agglomeratic accumulations. 

 As, owing to the gradual diminution of the volcanic forces, the 

 eruptions became less frequent the agglomeratic matter ac- 

 cumulated to a greater extent between two successive erup- 

 tions, and this, reacting again, caused the intervals to be still 

 longer but the eruptions more energetic. After a long period 

 of quiescence he supposes that the crater resembled that of 

 Vesuvius before the eruption of 1813 — a rocky plain, over 

 which small ash-cones were built up — and that, as in the case 

 of Vesuvius, the gases and steam generated in the volcanic 

 laboratory beneath at length reached such a bulk, and had so 

 great a tension, that they were able to overcome the resistance 

 of the superincumbent matter, and a terrific eruption took 

 place, blowing out a large quantity of rock, and leaving the 

 hollow that now forms Lyttelton Harbour. Although it may 



