164 Transactions. 



In describing a section exposed at the North Head, Otago Harbour, 

 Professor Marshall, in his paper " Geology of Dunedin," shows that the 

 phonolite was one of the earliest volcanics of the district. No definite 

 statement as to the period during which the trachydolerite eruption took 

 place can be made from the field evidence afforded by the outcrops of the 

 Mount Cargill area, beyond the fact that the trachydolerites are nowhere 

 overlaid by other lava-flows. Professor Marshall, remarking on this, and 

 on the additional fact that no pebbles of trachydolerite are found in con- 

 glomerates formed in the interval between the two peroids of eruption, 

 considers that the trachydolerites are amongst the latest of the Dunedin 

 rocks. 



Physiography. 



Mount Cargill lies about five miles north-east of the Town of Dunedin. 

 and forms a moderately well-rounded spur, rising to a series of sharp knobs 

 in the Main Peak (2,232 ft.), Butter's Peaks (2,040 ft.), and Mount Holmes. 

 There are several other less-prominent peaks at a lower elevation than 

 these. The main spur or ridge runs south-west from the Main Peak to- 

 wards Pine Hill, and on the south-east there is a gradual drop to the 

 saddle between Mount Cargill and Signal Hill. On the north there is a 

 steep bush-clad face leading down into the watershed of a stream draining 

 towards Waitati. On the west flattish slopes lead out to the Leith Valley. 

 From the south face of the Main Peak the North-east Valley Stream 

 drains, and has cut a well-rounded valley between the Mount Cargill and 

 Signal Hill ridges. 



Occurrence of Eock Types. 



From the Main Peak, looking north-east and east, three knobs are 

 prominent. The nearest one — an abrupt rocky knoll called Butter's 

 Peaks — is composed of a basic variety of trachydolerite and of a probable 

 nephelinitoid phonolite dyke. The Main Peak itself, a steeply cleft ridge, 

 running for about 10 chains in a north-east by east and south-west by 

 west direction, is composed chiefly of the general lava trachydolerite. 



A far rocky peak to the north-east — Mount Holmes, or Remarkable 

 Rocks, by name — shows a splendid example of columnar structure in the 

 basalt of which it is formed. A good illustration of this outcrop is given 

 in Park's " Geology of New Zealand," p. 150. A quarter of a mile to the 

 south-east of this basalt peak is a knob called Mount Zion, with a lofty 

 precipitous face edging the Main North Road, and composed of a type 

 of trachytoid phonolite called by the type-name " Logan's Point." This 

 phonolite outcrops in a series of knolls for about a quarter to half a mile 

 in a south-west direction from Mount Zion. 



In a south-west and west direction from the Main Peak the rounded 

 and flattish slopes stretching towards Pine Hill and the Leith Valley show, 

 in scattered outcrops, a comparatively unvaried type of trachydolerite. 

 On these slopes occasional rough hexagonal jointing is seen, and the dis- 

 position seems to add strength to the view of Professor Marshall that the 

 flow of the trachydolerite was from Mount Cargill towards Mount Flag- 

 staff.* 



About a mile and a half south-west by south from the Main Peak, on 

 the upper portion of the Pine Hill slopes, is a profusion of large blocks 

 of rubble showing abundant large feldspathic and ferro-magnesian minerals 



* " Geology of Dunodin," Quart. Journ. Geol. Soc, vol. «2. 190(i, p. 407. 



