Haast.—On the Geological Structure of Banks Peninsula. 503 
eruptions, molten matter in a high state of fusion generally rushed up in 
the fissures which had been formed at the time, radiating from the focus 
like the spokes of a wheel. An examination of these dyke rocks will show 
at a glance that most of them are quite different in composition and 
character from those of which the lava-streams have been formed. The 
latter, as already explained, with one notable exception,* all consist of true 
basic rocks—basalts often assuming a doleritic texture, the dyke rocks 
being generally acidie, having either the composition of a trachyte or 
domite. We are able to judge of the more or less high state of fusion in 
which the molten matter ascended the open fissures from the effect pro- 
duced on the walls on both sides. The trachytic matter forming the dykes, 
which are principally developed on the eastern side of the caldera wall, has 
evidently been in such a condition that it could exercise a most powerful 
effect on both walls of the fissure, the rocks often, for several inches, being 
changed to tachylite, a peculiar basic voleanie glass, quite distinct from 
obsidian. This change in the character of the rock is most observable 
when the dykes pass along tufaceous or agglomeratic beds. Here the 
reddish orlight purple rocks have been altered to a black vitreous mass, 
containing small crystals of felspar. The domitic dykes, mostly confined 
to the western half of the caldera wall, seem not to have exercised such a 
great influence as the former, as in most instances the walls on both sides 
of the dykes are only slightly hardened. However, there is no constant 
rule; large dykes, as for instance the huge domitic dyke at Governor's 
Bay, running for a considerable distance parallel to the coast, and forming 
such a conspicuous object along the picturesque beach road lately con- 
structed, have scarcely made any alteration on either side, whilst smaller 
dykes of the same rock, only a few feet in thickness, are sometimes 
accompanied by a well-defined selvage of tachylite. The same may be said 
of the basaltic dykes, of which, however, by far the greatest part has caused 
no visible alteration along the walls on either side. The trachytic varieties, 
* This exception consists of a trachytic lava-stream of considerable size, and having 
an average thickness of eighty feet, which is interstratified between two others of a basic 
character. This peculiar stream occurs between Lyttelton and the pass to Sumner. It 
is the only trachyte lava known to me as having flowed from any of the different centres 
of eruption of Banks Peninsula, all the other acidic rocks, as I shall show in the sequel, 
having been ejected into fissures of more recent date. This lava-stream consists of a . 
white vesicular trachyte rich in quartz, resembling closely some of the domites of the 
Auvergne, from which, however, it is distinguished by its larger amount of silica, 
although it approaches it again in its considerable percentage of potash. A vertical 
dyke, about eight feet thick, of a peculiar flaky silky trachyte, passes through this lava- 
stream, narrowing, however, in its upper portion. Although this acidic lava is rather 
soft and friable in small pieces, it has nevertheless resisted the disintegrating agencies at 
work far better than the hard basaltic lavas and agglomerates in its neighbourhood, 
