504 Transactions.— Geology. 
of which most of the dykes on the eastern side of the Lyttelton Harbour 
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 spherosiderite. 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 polyhedrie 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 entirely of that peculiar form of acidic voleanic 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 
ihe 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 Cliffs Cove in Lyttelton Harbour, where several 
irachy-doleritie 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- 
` stream, 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 voleanie systems under consideration is the predomi- 
nating acidic character of the dykes when compared with the basic lava- 
streams. In Vesuvius and Hina 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 their 
existence to paroxysmal perturbations in the earth’s crust, distinct from 
