Boult. — Occurrence of Gold at Harbour Cone. 431 



which was probably intruded into the hot bostonite. The heat 

 of the bostonite kept this dyke in a molten condition long 

 enough for the magma to act on the first-foimed crystals, 

 as described under " Petrography." Then followed dykes of 

 tinguaite and phonolite, in what order it is almost impossible 

 to say. They were probably connected with the large erup- 

 tions of alkaline rocks on other parts of the Peninsula. 



At a later stage followed an eruption of an entirely dif- 

 ferent nature. The basalt cap now topping the mountain 

 is certainly the upper portions of a basalt-filled volcanic pipe, 

 which communicated with the vent of a basalt-emitting vol- 

 cano on the surface of the bostonite. After that enormous 

 denudation went on and entirely denuded away any outflows 

 of basalt which may have occurred. Some time during this 

 period the explosive eruptions having their focus near Port 

 Chalmers formed a deposit upon the partially denuded bos- 

 tonite, which must have had a form then approximating to 

 its present one, as a deposit of the breccia is found on the moun- 

 tain-slopes. Since then more denudation has taken place. The 

 breccia has, with the exception of the deposits shown on the 

 plan, been entirely removed. A river has cut its passage through 

 the flow, leaving the projecting hummocks now forming Quaran- 

 tine Islands, which are entirely composed of bostonite. The 

 hard pipe of basalt has resisted denudation while the bostonite 

 all round it has been denuded away, thus giving to the moun- 

 tain its present form. Thus, though the mountain has the 

 typical form of a volcano and its projecting neck, it is far from 

 probable that the materials composing its slopes have been 

 ejected from its summit, but rather the white bostonite once 

 formed a high plateau on the top of which was once a basalt- 

 emitting volcano. 



Thus, in this particular area, the order of outflow seems 

 to have been — (1) bostonite ; (2) intrusion of bostonite dykes ; 

 (3) intrusion of syenite mass, intrusion of tinguaite and phono- 

 lite dykes in unknown order ; (4) outflow of basalt ; (5) ex- 

 plosive eruptions producing breccia. 



In giving the order of flow for the whole Peninsula, Dr. 

 Marshall, in the work above referred to, writes — (1) tractate 

 (bostonite) ; (2) basalt and nepheline basanites ; (3) gieen 

 trachytoid phonolite ; (4) kenyte ; (5) Port Chalmers breccia. 

 In the neighbourhood of Harbour Cone it seems probable 

 that the phonolite and tinguaite dykes preceded the basalt, 

 since the latter is remarkably fresh, while the former is very 

 decomposed. The phonolite, however, lends itself to decom- 

 position very readily, so that arguments based on relative 

 decomposition are perhaps not reliable. 



