Haast. — 0)1 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 

 hke 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 acidic, 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 eifect 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 volcanic 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 or light 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 basaltio lavas and agglomerates in its neighbourhood. 



