Cotton. — Geology of Signal Hill, Dunedin. 115 



Several intrusions of both phonolitic and basaltic rock are found. The 

 most striking of these occurs on the saddle north of McGregor's Hill. The 

 rock, which is nephelinitoid phonolite, is not found in situ, but fragments 

 are found on the surface over an area 40 yards by 150 yards, with its greatest 

 elongation in the direction W.N.W.-E.S.E. Great variations occurring in 

 the mineral characters of this rock in specimens collected within a few feet 

 of each other, and its isolated occurrence, lead to the conclusion that it is 

 intrusive. It is surrounded by a ring of basalt, an isolated remnant of the 

 basalt-flow, which is here cut down by the action of streams approaching 

 each other from opposite sides of the saddle. The intrusion, tlierefore, is 

 later than all the lava-flows. 



A group of dykes of a very basic basalt occur on the Main North Road. 

 One is vertical, and 5 ft. in thickness ; two others are irregularly inclined, 

 and vary from 2 ft. to 3 ft. in thickness. Lower down the hill is a vertical 

 dyke of the same character, 15 ft. in thickness. The strike of these dvkes is 

 about east and west. 



In the upper part of the North-east Valley, about 500 yards south of 

 the Junction School, there appears traversing the basalt a dyke of decom- 

 posed rock. On account of its decomposed nature, no detailed examination 

 has been made of tliis rocJc. It has a fine lamellar structure, developed to 

 such an extent that the lamelhe may be pulled apart by the hand. Its 

 colour is a creamy grey, and it shows numerous white crystals of feldspar 

 not entirely decomposed. Some of these are very large, and some of medium 

 size. Their occurrence gives the decomposed rock a micaceous sheen. On 

 account of the abundance of feldspar crystals, the rock has been tentatively 

 classed as a trachytoid phonolite. 



Sequence of the Rocks. 



From the above description the followinp' aopears to be the sequence : — 



1. Basalt of an acid type. 



2. Logan's Point phonolite. 



3. Signal Hill trachytoid phonolite. 



4. Eruptions of basaltic scoria of an intermediate type. 



5. Main basalt-flows. 



6. Dykes of extremely basic basalt. Intrusion of nephelinitoid phono- 

 lite. Dyke of rock of doubtful composition, perhaps trachytoid phonolite. 



Source of the Lavas. 



Nothing definite can l)e stated as to the position of the vents from which 

 the various lavas issued. 



In the case of the Logan's Point phonolite tlie greatest thickness of rock 

 occurs in the southern portion of Signal Hill. On the western side it thins 

 out, and to the north dips below younger rocks. In the precipitous peak 

 on the south-eastern side of Signal Hill the cleavage-planes of the rock are 

 highly inclined, while elsewhere they are horizontal. Possiblv this peak mav 

 be the neck or plug which solidified in the ancient crater ; but the surround- 

 ing rock does not show signs of having flowed from this point. Similar 

 rocks also are described from Mount Cargill, from the opposite shore of 

 Otago Harbour, and from Otago North Head, where a bedded flow occurs : 

 but such a great thickness does not elsewhere occur as that in the main 

 Signal Hill mass. 



Marshall, " Geology of Dunedin," Quart. Journ. Geol. See, Ixii. 190fi. 



