Canterbury and Westland. 337 



lias 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 excercised 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 domilic 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 constructed, has 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, of 

 which, most of the dykes on the eastern side of Lyttelton Harbour 

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

 sometimes 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 entirely of 

 that peculiar form of acidic volcanic rock. 



The chemical analyses of the same dyke, ]S"o. 17, of Carl Bitter von 

 Hauer, and of 29a centre, and No. 29b side, made in the Colonial 

 Laboratory are very instructive, because they show us that there is a 

 great difference in the composition of the same dyke, if the specimens 

 to be analysed are taken from different localities. The specimen 

 analysed in Vienna in 1863 was obtained from near the summit of the 

 hill, whilst the two others were taken from the tunnel shortly after 

 this dyke had been crossed by the miners. It is interesting to 

 observe, that although the appearance of the rock is in every respect 



