184 Transactions. 



of greenish -blue pleochroic mineral in very minute fragments, 

 which may be either the soda amphibole or the greenish augite. 

 These would account for the small percentage of magnesia that 

 does occur. This greenish mineral with slight pleoehroism is 

 found in other rocks occurring as dykes in this series. In some 

 cases it is undoubtedly an augite of a soda-bearing variety ; 

 but in other cases where it has the bluish tinge of varying de- 

 grees of intensity it is, in all probability, a soda-iron amphi- 

 bole. 



Perhaps the most interesting occurrence of this mineral is 

 in the syenite of Onawe Peninsula, Akaroa. In his description 

 of this rock Captain Hutton says,* " The hornblende goes up to 

 005 in. in length ; when fresh it is greenish and pleochroic, 

 changing from blue-green to yellow-green, the polarisation 

 colours not brilliant." On examining this rock further with the 

 advantage of thinner sections I find the masses of iron-oxides 

 which have in most cases replaced the hornblende show not 

 merely a greenish-blue, but a deep-blue colour, aud in other 

 cases I noticed small pieces of hornblende exactly resembling 

 the amphibole of Cass's Peak. This, therefore, seems to me a 

 case of the occurrence of an arfvedsonite syenite. Just as in 

 many dykes of the Mount Pleasant series, this rock is very light 

 in colour, and shows a small proportion of iron-bearing mineral. 



The fairly wide occurrence of the rocks of the phonolitdc 

 trachyte variety so closely connected with the trachytoid pho- 

 nolites, as well as the occurrence of arfvedsonite syenite at Akaroa, 

 is of special interest when we note the existence at Dunedin of 

 the magnificent series of alkaline, rocks discovered by Ulricli, 

 and well described latterly by Marshall. The occurrence of the 

 rocks previously mentioned in the Banks Peninsula, area, shows 

 distinctly that the distribution of alkali rocks in New Zealand 

 is wider than at first supposed. 



* " The Eruptive Rocks of New Zealand." I>y Professor F. W. 

 Hutton. Read before the Royal Society of New South Wales, 7th August, 

 ISS'.I. 



