414 PEOJT. P. MAESHALL ON THE [Aug. 1906, 



tremendous steam-explosions to which this breccia owes its formation 

 also rent the neighbouring rocks asunder, and the alkaline magma 

 that filled the rents solidified into tinguaite-dykes. 



The formation of the Port-Chalmers Breccia appears to have suc- 

 ceeded the effusion of much of the basalt and the trachyte, but it 

 must have preceded the eruption of most of the trachytoid phonolite 

 and the trachydolerite. 



Many other breccias of small thickness and limited extent are 

 found between various lava-streams, but none of them have the 

 importance of the Port-Chalmers rock. 



III. Chemical Classification. 



An attempt has been made to classify the Dunedin rocks that 

 have been analysed, in accordance with the scheme of quantita- 

 tive classification formulated by Prof. Pirsson, Dr. Washington, 

 Prof. Iddings, and Mr. Weed. The calculations necessary to 

 find the proper place in the classification are slightly involved, 

 and there is no certainty that a correct result has been achieved 

 in every case. 



The alkaline rocks of the district form so well-defined a series, 

 that they constitute a good test for any system of classification, 

 although it must be allowed that fuller (and perhaps more exact) 

 analyses, with separation of titanic oxide and other compounds, are 

 desirable in order to ensure satisfactory results. 



A tabulated statement of the classification of the rocks is here 

 appended. It is framed in accordance with the large and valuable 

 collection of analyses, calculated according to the system proposed 

 by Dr. H. S. Washington, which was published in 1903 by the 

 United States Geological Survey as Professional Paper No. 14. 

 It will be seen, in every case, that the rocks fall into classes 

 containing many similar species described from other parts of the 

 world. 



Class I. PERSALANE. 



Order 5. Perfelic. 



Bang 1. Peralkalic. 

 Subrang 4. Dosodic. 



Trachyte, Portobello. 

 Tinguaite, Acheron Point. 

 Order 6. 



Eang 1. 



Subrang 3. Sodipotassic. 



Trachytoid Phonolite, Logan's Point. 



Class II. DOSALANE. 

 Order 5. 



Rang 1. 



Subrang 4. 



Green Phonolite. 

 Rang 2. 



Subrang 4. 



Camptonite. 



