Bartrum. — Rocks of Mount Gar gill, Dunedin. 1(53 



Art. XVI. — Some Rocks oj Mount Cargill, Dunedin. 



By J. A. Bartrum. M.Sc, 



Communicated by Dr. Marshall. 



[Bead before the Otago Institute, 3rd October, 1911.} 



In these notes an attempt will be made to describe a series of trachy- 

 dolerites and phonolites outcropping near the summit of Mount Cargill. 

 and, with them, such adjacent rocks as may be helpful in throwing light 

 on their origin. 



A difficulty in arriving at the exact relationships of the rocks in this 

 district lies in the fact that bush and debris obscure a large portion of 

 country where outcrops are probable. 



Several references to Mount Cargill rocks have been made by different 

 writers, notably Professors Ulrich and Park and Mr. C. A. Cotton, but 

 Professor Marshall's paper on the " Trachydolerites " * and that on the 

 " Geology of Dunedin "f gave the first comprehensive account of these rocks. 

 The standard types arrived at by Professor Marshall in the latter of these 

 two papers have been made full use of, and very much personal advice 

 and help has been given by Professor Marshall to the writer during the 

 preparation of this paper. 



It is hoped that some of the information brought forward may help 

 to supplement previous knowledge of the rocks described. 



General Geology. 



The Mount Cargill rocks form part of the volcanic complex of the 

 Dunedin district. In several exposures the volcanic rocks overlie uncon- 

 formably the Caversham sandstone, a member of the Oamaru series, which 

 is generally referred to a Miocene age. J From the fact that volcanic rocks 

 apparently have been unaffected by the earth-movements that caused dis- 

 turbance of the Oamaru series, these former probably first were emitted 

 at a period later than that of the disturbance of the Oamaru beds. The 

 age of the earliest volcanic outburst must thus be at earliest post-Miocene. 



That there are two main periods of volcanic activity in the Dunedin 

 area is evidenced by the occurrence of a conglomerate of volcanic material 

 overlying plant-beds that are unconformable to the Caversham sandstone. § 



Professor Park considers the trachydolerites to belong to the first 

 period. || To this period he assigns a Pliocene age.^[ They were extruded 

 probably through trachytoid phonolites that have been described by Pro- 

 fessor Marshall** and Cotton, ff and that occupy a large area on Signal 

 Hill, about two miles south-east of Mount Cargill. 



* Trans. Aust. Ass. Adv. Sci., vol. 10, 1904, p. 186 ; Dunedin. 



t Quart. Journ. Geol. Soc, vol. 62, 1906, p. 381. 



t Park, " Geology of New Zealand," p. 25. 



§ Marshall, " Geology of Dunedin," Quart. Journ. Geol. Soc., vol. 62, 1906, p. 390. 



|| " Geology of New Zealand," p. 148. 



II " On Geology of North Head, Waikouaiti," Trans. N.Z. Inst., vol. 36, 1904, p. 418. 

 ** Loc. cit. 

 ft Trans. N.Z. Inst., vol. 41, 1909, p. 111. 



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