Pakk. — On the Geology of North Head, Waikouaiti. 425 



the parent volcanic rocks and marine sandstones near Dun- 

 edin. 



Source of Basanite and Phonolite in Gravels. — The gravels 

 underlying the basalt-flow at Mount Cronin are, as we have 

 seen, principally composed of basanite and phonolite. 



In his excellent paper on the lithology of Dunedin, Pro- 

 fessor Ulrich pointed out that the great mass of the volcanic 

 material in that region belonged to the phonolite group of 

 rocks, in which nepheline forms an essential component ; 

 while some, he said, through accession of plagioclase, and 

 either absence or presence of olivine, graduate respectively 

 towards tephrite and basanite/'' Since no basanite or phono- 

 lite is known in the neighbourhood of Waikouaiti, we may 

 conclude that the Mount Cronin gravels were derived from 

 the erosion of the basanites and phonolites described by 

 Professor Ulrich at Mount Cargill and Purakanui. 



Age of Volcanic Rocks around Dunedin. — I have just 

 shown that the materials forming the gravel and sand beds 

 at Mount Cronin a^'e principally volcanic, and in all proba- 

 bility derived from the denudation of the volcanic area lying 

 between Waitati and Dunedin. Before, therefore, discussing 

 the age of the leaf beds it will be necessary, in the first 

 place, to consider the probable age of the volcanic rocks m 

 that area. 



The volcanic rocks around Dunedin rest on a deeply 

 eroded surface of the Caversham sandstone and underlying 

 quartz sands and grits. 



The Oamaru series, to which these rocks belong, is con- 

 sidered by Captain Hutton to be of Oligocene age ; but the 

 proportion of fossils belonging to living species in that for- 

 mation would indicate, from European standards, rather a 

 Miocene age. If, however, we accept the Oligocene age of 

 Captain Hutton, it is evident that the earliest volcanic out- 

 bursts could not have connnenced until some time in the 

 Miocene period, for we find that the Caversham sandstone, 

 one of the higher members of the Oamaru series, was 

 deposited, consolidated, elevated, and partly denuded before 

 the first outbursts began. 



There is no evidence of contemporary volcanic activity 

 during the deposition of the Oamaru series anywhere south 

 •of Palmerston. There is, we know, abundant evidence that 

 volcanic outbursts took place during the deposition of the 

 middle members of the Oamaru series in the Oamaru basin. 

 But these outbursts were purely local, and there is nothing 

 to show that they reached north of the Waitaki or south of 

 the Shag Eiver. At any rate, south of Palmerston the 



• Proc. Aust. Assoc. Adv. Science, vol. iii., 1891, p. 14.5. 



