"Vol. 62.] GEOLOGr OF DTJNEDIN (NEW ZEALAND). -415 



Order 6. 



Eang 1. 



Subrang 3. 



Nephelinitoid Phonolite, Mopanui. 

 Leucitophyre. 

 Trachydolerite. 

 Subrang 4. 



Camptonitic Tinguaite. 

 Traclrytoid Phonolite. 

 Eang 2. 



Subrang 3. 

 Andesite. 



Subrang 4. 



Trachytoid Phonolite. 



Class III. SALFEMANE. 



Order 5. 



Eang 3. 



Subrang 5. 



Dolerite, Mount Charles. 

 Dolerite, Papanui. 

 Order 6. 



Eang 2. 



Subrang 4. 

 Basanite. 



In Class I it is seen that the trachyte from Portobello and the 

 tinguaite from Acheron Point fall into the same subrang. It is 

 evident that a classification in which this happens is hardly satis- 

 factory from a petrographical standpoint, for the mineralogical 

 composition, structure, and geological occurrence of the rocks are 

 all different. On the other hand, both rocks belong to the alkaline 

 series, and the slight chemical difference, although sufficient to 

 disturb the mineralogical formation, is insufficient to allow of 

 separation of the rocks according to this system, which is based 

 on chemical characters alone. 



This class includes a lestiwarite, an andesite, a porphyrite, and 

 a sblvsbergite ; therefore, the two rocks here included do not 

 correspond to the extremes that are possible. In the same class, 

 but in a different order and subrang, is the Logan' s-Point phonolite. 

 There is no doubt as close a relationship geologically and petro- 

 graphically as in this classification. 



The majority of the alkaline rocks find their places in the second 

 class, and in closely-related divisions of that class. The andesite 

 and green phonolite are in slightly-different divisions. The classi- 

 fication here seems to agree satisfactorily with the relationships 

 as expressed by mineralogical composition. However, the rocks in 

 Order 6, Rang 1, Subrang 3 are very different in many particulars, 

 and hardly any petrographers or mineralogists could be content with 

 such a result, for mineral constituents, structure, and occurrence 

 are all different. It is true that, at present, our knowledge is too 

 scanty to allow of any formulation of a natural or genetic system 



2e2 



