Cotton. — Geology of Signal Hill, Duncdin. Ill 



Art. XXI. — Geology of Signal Hill, Dunedin. 

 By C. A. Cotton, M.Sc. 



[Rend before the Otagn Institute, lOth November, 1908.] 



General Geology. 



The locks of Signal Hill, being all volcanic, contain no internal evidence 

 of their geological age. They form, however, a part of the Dunedin group 

 of volcanics, which are known to overlie unconforraably at Caversham and 

 at Sea View a calcareous sandstone of the Oamaru system.* The Oaniaru 

 system was placed by Captain Hutton in 1875 in the Lower Miocene, but 

 in his later works he classed it as Oligocene.f Hector includes the 

 Oamaru rocks in his Cretaceo-tertiary system ;t Park places them in the 

 Miocene.§ 



The volcanic rocks are not in any case involved in the disturbances of 

 the Oamaru rocks, nor have faults of any magnitude been observed in them. 

 A considerable amount of elevation and depression has, however, since 

 occurred, for the main valleys have been eroded far below the present base- 

 level, and at Sandy Mount and elsewhere marine terraces are found at a 

 considerable elevation. 



Most observers have supposed the first volcanic outbursts at least to 

 have occurred during the same period in which the Caversham sandstone 

 was deposited. Professor Park, however, taking into account the erosion 

 of the Caversham sandstone, suggests the beginning of the Pliocene as the 

 first period of volcanic activity. || There was either a prolonged period of 

 volcanic activity, or there were two shorter periods separated by a period 

 of quiescence, for conglomerate to a thickness of 100 ft., containing pebbles 

 derived from the earlier volcanic rocks, lies in places upon the sedimentary 

 rocks, and is covered by the later volcanic lavas. In places, also, fine 

 shales with leaf-impressions occur above the sandstone and below the 

 conglomerate. If, as suggested by Professor Park,|| these are of Pliocene 

 age, the later eruptions which followed cannot be older than Pliocene. The 

 occurrence of the conglomerate and the evidence of two periods of volcanic 

 activity explains the unconformity between the earlier and later lavas of 

 Signal Hill. 



Physiography. 



Signal Hill is a flat-topped elevation, extending as a ridge from Logan's 

 Point in a north-easterly direction for a distance of three miles. The 

 ridge then bends to the north and north-west, towards Mount Cargill. The 

 greatest elevation of Signal Hill is 1, 21-1 ft. Less than half a mile to the 



* Marshall, " Geology of Dunedin," Quart. Journ. Geol. Soc, Ixii, 1906. 



t Hutton, " Geology of Otago," 1875; " Sketch of the Geology of New Zealand, ' 

 Quart. .Joum. Geol. Soc. 1885; '" (Teological History of New Zealand," Trans. N.Z. 

 Inst., 1899. 



X Hector, " Outline of New Zealand Geology," 1886. 



§Park, Trans. N.Z. Inst., 1904. 



II Park, ■' On the Geology of North Head, Waikouaiti," Trans. N.Z. Inst., 1903. 



