Vol. 62.] GEOLOGY OF DUNEDIN (NEW ZEALAND). 403 



evidently connect this group with the andesites. An estimation was 

 made of the amount of material dissolved from these rocks, when 

 the fine powder was just brought to the boiling-point in dilute 

 hydrochloric acid. Altogether 29*98 per cent, of the rock went 

 into solution. An estimation of the substances in solution gave 

 the following result : — 



Si0 2 15-31 



A1 2 3 521 



Fe 2 3 3-54 



OaO 1-24 



Na.,0 1-53 



K 2 0-51 



H 2 2-64 



The silica was chiefly in a gelatinous condition, and had to be 

 dissolved in potassium-hydrate before estimation. This result 

 shows that there is an alkaline silicate present that is decomposed 

 by dilute acid. It is reasonable to suppose that this silicate is 

 nepheline — a result that is borne out by staining. 



There is a very large variety of these rocks in this penological 

 province. They crown the tops of many of the hills, and occur 

 as interbedded sheets in several of the cliff-exposures. 



With the exception of the green phonolite, the boundaries of 

 the different types are not distinguished in the geological map 

 (PL XXXVI) attached to this paper ; but the following statement 

 gives an idea of their relative position and abundance. 



(A) Signal-Hill type. — Forms the main mass of Signal Hill 

 and one of the small peaks of Mount Cargill. A continuation of 

 the Signal- Hill outflow occurs on the south-eastern side of Otago 

 Harbour, where it rests upon a dolerite. At Otago North Head a 

 lava-flow rests on basalt, but is overlain by basanite and other 

 basalts. The Signal-Hill rock rests upon basalt at the cemetery, 

 and is overlain by basalt on the Junction Road and at the summit 

 of the hill. 



The emission of this lava was preceded by basalts which rest, to 

 a depth of 100 feet, upon trachyte at North Otago Head. 



Prof. T. W. Edgeworth David, F.R.S., in conversation has com- 

 pared this rock with the phonolite of the Warrumbungle Mountains 

 (New South Wales). 



(B) St. Leonard's type. — No lava-flow of this has been dis- 

 covered, except at the North Otago Head. Boulders of the rock 

 are found in breccias, especially at Port Chalmers, and at North 

 and South Otago Heads. 



(C) An de si tic type. — By far the most general, forming the 

 greater part of the northern portion of the high land of Otago 

 Peninsula, the greater part of Roslyn and Maori Hill, and extending 

 from the Junction Road on Mount Cargill with little interruption 

 to North Otago Head. 



