Cotton. — Geologij of Signal Hill, Dunedin. 113 



however, practically certain, from sections exposed elsewhere — notably 

 that at North Head, figured by Dr. Marshall* — that the oldest rock is the 

 Logan's Point type, and that the Signal Hill type, which is closely related 

 to the andesitic type of the North Head, rests upon it. The intervening 

 basalt and basanite flows are not represented at Signal Hill, although a 

 first inspection of the sections exposed along the Main North Road might 

 lead to the belief that basalt intervenes between the flows of the two types 

 of phonolite. 



Going up the road from Normanby one passes successively Logan's 

 Point phonolite. basalt, and Signal Hill trachytoid phonolite. A closer 

 inspection shows that, while the basalt overlies the Logan's Point rock, it 

 also overlies the Signal Hill type, and is continuous with the main basalt- 

 flow which covers the summit of the hill, and has the peculiar distribution 

 to be described later. This mass of basalt evidently filled a depression 

 which existed near the line of junction of the two types of phonolite, and 

 which may have been the result of erosion, or perhaps was caused by the 

 rapid solidification of the relatively acid phonolite lava. Section No. 1 

 gives the distribution of these rocks. 



In connection with both types of trachytoid phonolite it may be stated 

 that, in places where the rocks are exposed in situ, the cleavage is approxi- 

 mately horizontal. 



Above the phonolite of the Logan's Point type there occurs, on the 

 Signal Hill Road, about the 800 ft. contour-line, a bed of scoria 30 ft. in 

 thickness. From an undecomposed core in this a specimen of basalt was 

 obtained of a much more acid type than the main basalt-flow which followed 

 it. It is described as basalt No. 4. 



The main basalt-flow, described as basalt No. 1, covers the summit of 

 the hill, and on the south-eastern side extends down only a short distance. 

 It continues, however, much farther in a north-westerly direction, and to 

 the north extends in a narrow belt across the head of the eastern arm of the 

 North-east Valley towards Mount Cargill. This distribution is very peculiar, 

 and it might be suggested that the basalt covering this area consists of a 

 number of lava-flows, some of which alternate with flows of trachvtoid 

 phonolite. This explanation, however, is not upheld by field evidence. 



The sections exposed on the Main North Road, and numbered con- 

 secutively 1 to 10 on the accompanving map, are as follows : — 



(1.) Direction, N.E.-S.W. Height above sea-level, 330 ft. Basalt lies 

 horizontally upon Logan's Point phonolite. 



(2.) Direction, N.W.-S.E. Height above sea-level. 480 ft. Basalt lies 

 upon trachytoid plionolite (Signal Hill tvpe), dipping N.W. 12'. 



(3.) The section is not clear, but basalt appears to overlie Signal Hill 

 phonolite nearly horizontally. Height above sea, 520 ft. 



(4.) Direction, N.W.-S.E. Height above sea-level, 550 ft. Basalt lies 

 upon Signal Hill phonolite, dipping 12|° N.W. 



(5.) Direction. N.E.-S.W. Height above sea-level, 580 ft. Basalt lies 

 upon Signal Hill phonolite, dipping 4i° N.E. The basalt is then con- 

 tinuous for about half a mile. 



(6.) Direction, E.N.E.-W.S.W. Height above sea-level. 75<) ft. Basalt 

 lies upon Signal Hill phonolite, dipping W.S.W. 8°. 



At (7) and (8) the sections are not clear, but between these points basalt 

 is continuous. 



♦Marshal), " Geology of Duiiedin," Quart. Joum. Geol. Soe, Ixii, 190f>. 



