604 Transactions. — Geology. 



of wliich most of the dykes on the eastern side of the Lyttelton Harboui? 

 consist, are formed generally of a peculiarly lustrous and flaky rock, some- 

 times vesicular, with small crystals of sanidine. This rock has a light 

 greyish colour, and its small cavities are lined by sphserosiderite. On both 

 sides of the dyke the rock is generally tabular — parallel to the direction of 

 the flow, and is massive in the centre with polyhedric joints, of which the 

 principal ones appear at right angles to the flow. There are also a few 

 trachytic dykes, principally small ones, where the sides, for half an inch to 

 one inch, consist of a rather brittle obsidian, doubtless the effect of rapid 

 cooling. Some very thin thread-like dykes, about one to two inches thick, 

 consist entu'ely of that peculiar form of acidic volcanic rock. 



In studying the position of the dykes it becomes manifest that they have 

 been formed at different times ; however, the altitude of their uppermost 

 portion does not indicate their age. I have no doubt that many of them, 

 which scarcely reach above high water-mark, are not older than others of 

 the same petrological nature, which reach to the very summit of the caldera 

 wall. In the present state of our knowledge it is impossible to solve this 

 interesting question in all its bearings, and I can therefore only suggest 

 that dykes containing rocks of exactly the same lithological character 

 have most probably been formed during the same eruption. It is also 

 evident that a number of dykes were formed long before the whole of the 

 caldera wall was built up, and that they were partly destroyed during one 

 of the next eruptions. One clear instance of the occurrence of such older 

 dykes is to be found near Cliff's Cove in Lyttelton Harbour, where several 

 trachy-doleritic dykes were injected when the rest of the caldera wall was at 

 least 1,000 feet lower than at present. They pass through a basaltic lava- 

 etream, which latter was afterwards partly destroyed along with them, the 

 whole possessing now nearly a straight surface, upon which a large bed of 

 agglomerate has been deposited. However, what is of the greatest interest 

 in the history of the volcanic systems under consideration is the predomi- 

 nating acidic character of the dykes when compared with the basic lava- 

 streams. In Vesuvius and Etna all the dykes are formed by the same kind 

 of rock as the lava-streams are composed of, but they are generally more 

 compact, having, as Lyell suggests, cooled and consolidated under greater 

 pressure. It is evident that they owe their existence to the same subter- 

 ranean efforts by which the lava-streams were ejected from the mouth of 

 the crater, the fissures in which they were formed being evidently filled up 

 from the same focus, and about the same time as the eruption of the lava- 

 streams took place. But such a simple process cannot be admitted for the 

 greater portion of the dykes of Banks Peninsula, which must owe theu* 

 existence to paroxysmal perturbations in the earth's crust, distinct from 



