Vol. 62.] GEOLOGY OF DUNEDIN (NEW ZEALAND). 419 



There are three sources of information in regard to the succession 

 of the lavas : — 



1. Actual superposition. 



2. Occurrence of pebbles and boulders in breccias and conglomerates that 



are covered by other lavas. 



3. Dykes penetrating other rocks. 



The first of these has been discussed in the pages just preceding ; 

 the second was referred to when the conglomerates and breccias 

 were described ; the third is not very suggestive, but the more 

 important points may here be mentioned : — 



Tinguaites penetrate alike the Mount-Charles Dolerite, the trachyte, and 



the trachytoid phonolite. 

 Basalt traverses Mount-Charles Dolerite and trachyte. 

 Camptonite penetrates trachyte and Mount-Charles Dolerite. 



The following is the most probable order of rock-formation that 

 can be suggested when the preceding facts are borne in mind : — 



1. Basalt and Mount-Charles Dolerite. 



2. Trachyte. 



3. Basalt. 



4. Trachytoid phonolite, highly alkaline. 



5. Basalt. 



6. Basanite. 



7. Papanui Dolerite and basalts. 



8. Andesite and St. Leonard's type of phonolite. 



9. Port-Chalmers Breccia. 



10. Trachytoid phonolite (andesite). 



11. Basalt, 



12. Trachydolerite and nephelinitoid phonolite. 



13. Basalt. 



The other lavas have not been discovered in positions which 

 allow of their relative age being gauged. 



This order of emission does not seem to follow any of those that 

 are suggested by theories of magmatic differentiation. The difficulties 

 encountered are especiallv great, when the small sections at See 

 House and North Otago Head are considered. Such an occurrence 

 as that at the top of Mount Cargill is also very complex. The 

 mountain has five peaks, large and small. The main one is composed 

 of trachydolerite ; another close to it of nephelinitoid phonolite ; 

 two others of trachytoid phonolites of different types ; and the fifth 

 of basalt. Vegetation so hides all the intervening ground, that no 

 conclusions whatever can be reached as to the relative age of the 

 rocks. 



It is believed, however, that in this district the microscopic struc- 

 ture of the rocks affords very important aid in arriving at conclusions 

 as to the origin of the numerous rock-types observed. 



In the dolerites, most of the basalts, the trachyte, and some 

 trachytoid phonolites, all the minerals seen in the rocks appear to be 

 native to them. None of the constituents show signs of corrosion or 

 resorption or marginal fringes. 



