Vol. 62.] 



GEOLOGY OP DUNEDIN (NEW ZEALAND). 



4i; 



Unfortunately, information is not at present complete as to 

 whether the basalts occurring in the different parts of this series 

 are similar in composition ; but, leaving them out of consideration, 

 the silica-ratio increases from the dolerite to the trachytoid phono- 

 lite, then decreases in the tr achy dolerite, in the ratio 44 : 51 : 55 : 52. 



Pig. 1. — Section along the Leith Valley, from See House to tlie 

 Reservoir. 



N.20°W. 



s. 20 °E. 



[Scales, vertical and horizontal : 4 inches =1 mile.] 



l = Basalt. 2=Papanui Dolerite. 3= Andesite. 4=Trachytoid phonolite 

 (andesitic type : two flows). 5 = Trachydolerite. ^ 



The alkalies, on the other hand, constantly rise if the upper basalt 

 be not considered. The minerals of these rocks are as follows : — 



Dolerite. 



Anorthite. 

 Augite. 

 Olivine. 

 Magnetite. 



Andesite. 



Labradorite. 

 Augite. 

 Hornblende. 

 Olivine (little). 

 Magnetite. 



Trachytoid 

 Phonolite. 



Oligoclase. 



iEgirine-augite. 



Hornblende. 



Magnetite. 



Nepheline 



(microchemical 



tests). 



Trachydolerite. 



Sanidine. 



Anorthoclase. 



iEgirine. 



Augite. 



Olivine. 



Hornblende (resorbed). 



Nepheline. 



Sodalite. 



Magnetite. 



Both chemically and mineralogically, there seems to be, in this 

 case, little support for the theory of magmatic differentiation, for the 

 order is basic, intermediate, alkaline-basic, alkaline-basic. 



The minerals are different in the lavas. The heavy mineral 

 hornblende is not present in the heaviest rock. The heaviest 

 mineral olivine is present in the last rock erupted, which is a 

 comparatively-light one. 



The resorption-zones of the hornblende in the andesite show that 

 it was unstable in this rock, but was much more stable in the 

 trachytoid phonolite, and was completely resorbed in the trachy- 

 dolerite. 



The other important section (fig. 2, p. 418) is exposed at North 

 Otago Head. Here the lowest rock seen is basalt. Upon this 

 rests trachyte. It is succeeded by basalts, and they give place to a 

 trachytoid phonolite of a highly-alkaline type. Basalts succeed this 

 for 200 feet in ten distinct flows. Then comes a phonolite rich 



