Speight. — Soda Amphibole Trachyte. 179 



The foregoing section shows the relative position of the 

 different outpourings of volcanic rocks. It will be noted that 

 in it I have classified the lavas from Akaroa as belonging to the 

 Mount Pleasant series ; but this classification is merely tentative, 

 and for reasons just stated they should probably be marked as 



belonging to a later date. 



© © 



The Mount Pleasant series is penetrated by a remarkable 

 series of dykes, well described by Sir Julius von Haast, who 

 pointed out in the case of Lyttelton that they are oriented in a 

 somewhat striking manner. They all, with few exceptions, con- 

 verge on a point at the back of Quail Island, no matter in what 

 parts of the crater-ring they are found. The dykes of the 

 Gebbie's Pass series are not so arranged, while there are none 

 visible in the Mount Herbert series. Some of these dykes have 

 been previously described by Hutton*, Ulrich,f Marshall,! 

 Filhol,§ Kolenko,|| and the author. ^f They consist, as far as is 

 known at present, of dolerites, basalts, hornblende, and augite 

 andesites, some containing olivine, trachytoid phonolites, and 

 trachytes, the last being probably the most numerous, although 

 basaltic dykes are also common. Some of the trachytes con- 

 tain hornblende and others augite, and they are in general of 

 whitish, pale-grey, and sometimes of a greenish colour, and 

 very vesicular. Chemical analysis shows that some contain a high 

 percentage of soda. It is to this class that the rock to be de- 

 scribed belongs. 



The Soda Amphibole Trachyte. 



Its Occurrence. 



The rock is found as a massive dyke on the northern side 

 of Cass's Peak, one of the highest points on the west side of the 

 old crater-ring of Lyttelton. The dyke can be traced fully half 

 a mile from near the crest of the ridge, through the Kennedy's 

 Bush reserve, and down one of the valleys towards Lansdowne. 

 At times it is fully 60 ft. wide, but it thins out towards the top 

 of the ridge, and also when followed down the valley. At one 

 spot it was quarried as a building-stone, and several buildings 

 in Christchurch were built of it, notably the present Tourist 



* " Eruptive Rocks of New Zealand," Trans. Roy. Soc. N.S.W., 1889. 



f " Transactions of the Australasian Association for the Advancement 

 of Science," vol. 4, 1891. 



J " On a Tridymite Trachyte of Lyttelton," Trans. N.Z. Inst., vol. xxvi 

 (1893). 



§ " Mission de l'lle Campbell," Paris, 1883. 



|| " New Zealand Journal of Science," vol. ii. 



If " On a Doleritic Dyke at Dyer's Pass," Trans. N.Z. Inst., vol. xxvi 

 (1893). 



