Vol. 62.] GEOLOGY OF DTJNEDIN (NEW ZEALAND). 407 



The analysis is repeated here (C) : — 



A. 



Si0 2 5312 



TiO" 2 0-25 



ALO3 20-48 



Fe 2 3 5-13 



FeO 1-50 



CaO 429 



MgO 1-88 



Na 2 6-20 



K 2 4-88 



H;o 2-25 



Ci; 0-28 MnO 



P 5 0-43 



B. 



C. 



53-80 



51-86 



0-31 



— 



18-46 



19-87 



6-22 



6-30 



0-40 



311 



2-53 



3-77 



1-05 



2-33 



7-09 



4-88 



5-46 



6-20 



4-39 



1-48 



0-33 



Cl 2 0-51 



0-53 



0-36 



Totals 100-69 100-57 10Q-67 



A = Trachydolerite (tephritic trachyte), Colurnbretes (Spain). Rosenbusch, 

 'Elemente der Gesteinslehre ' 2nd ed. (1901) p. 355, No. 3 a. 



B = Kenyte, from Mount Hohnel (Mount Kenya). G-. T. Prior, Min. Mag. 

 vol. xiii (1903) p. 247. 



C = Leith Valley, Dunedin (N.Z.). Anal. P. Marshall. 



The Dunedin rock differs but little in composition from the others 

 quoted, although the higher percentage of potash is noticeable. 

 Chemically the rock is more nearly related to the phonolites than 

 to the basalts, and this is emphasized by a consideration of its 

 mineralogical composition. No analysis was made of any of the 

 specimens that show marked trachytoid or marked nephelinitoid 

 structures. 



The extent of the trachydolerite was fairly definitely outlined in 

 my paper quoted above. The main area is found on the south side 

 of Flagstaff, and on the south and west side of Pine Hill and Mount 

 Cargill. Petrographically, the rock is closely related to the green 

 phonolite of Mopanui and Blueskin, and in places where it has 

 cooled very rapidly the same green colour is conspicuous. In no 

 place has this rock been found interbedded with other lavas. In 

 every one of its known occurrences it covers the surface, and no 

 pebbles of it have been found in breccias or conglomerates. On 

 the eastern side of Flagstaff the rock is certainly found near the 

 base and half way up the slopes. It is, however, not likely that 

 this position is due to a subsequent outpouring and covering by 

 the rocks nearer to the summit of the hill, for no traces of the 

 trachydolerite are to be found on the western side. 



This occurrence is best explained by the supposition that the 

 trachydolerite flowed downwards from Mount Cargill into a 

 previously-eroded valley, wherefore some of it occupies a lower level 

 than the previously-emitted and eroded lavas. This explanation 

 is rendered more probable, when the fact is stated that the 

 trachydolerite is in part at a lower level than the calcareous 

 sandstone on the eastern flanks of Flagstaff, where it is exposed in 

 Morrison's Creek. 



