376 Transactions. 



the basic rocks, feldspathoid minerals are often more abundant than the 

 feldspars. Such facts as these, though possibly in the future they may 

 require qualification or revision, suggest at least in the present state of our 

 knowledge that the composition of sub-Pacific material is, on the whole, 

 more basic than that of the other areas, and to that extent favours the 

 idea of the isostatic equilibrium of the Pacific basin. 



Summary. 



1. The distribution of plutonic rocks in the Papenoo Valley, in the centre 

 of Tahiti, is somewhat different from that stated in Lacroix' paper, which 

 was based on the field-work of Seurat. 



2. So far as the observations of the author showed, the outcrop of these 

 rocks is almost confined to the low hill Ahititera, which occupies the centre 

 of the large circular basin of the Upper Papenoo Valley. 



3. The hill Ahititera is the centre of the island, and from the crest of 

 the mountain rampart which surrounds the Upper Papenoo the level of the 

 land falls outward somewhat evenly on all sides to the sea-level, though 

 the slopes are deeply seamed by the stream- valleys which radiate outwards 

 in large numbers. 



4. The situation of Ahititera is therefore the position of the central 

 conduit of the original great volcano of Tahiti. 



5. The series of volcanic plutonic rocks discovered by Lacroix is ex- 

 tended by the discovery of a highly feldspathic syenite as well as a peridotite 

 in situ in addition to some of the types already described as Lacroix. 



6. This series is regarded as the result of the differentiation of an 

 essexitic magma. 



7. Parallels of these plutonic types are found in the dykes and the 

 volcanic rocks found in the island. 



8. It is suggested that the constantly occurring association of basic 

 and alkaline volcanic rocks in other Pacific islands, and at Dunedin, New 

 Zealand, has resulted in a similar manner. 



9. No evidence was found in support of Daly"s theory that the alkaline 

 are due to the solution of calcium-carbonate rock by a basic magma. 



10. It is suggested that the frequent highly basic character of the 

 volcanic rocks of the Central Pacific islands supports the theory that the 

 floor of the Pacific Ocean has its low-lying position as a result of isostatic 

 equilibrium. 



EXPLANATION OF PLATE Vil. 



Pig. 1. View of upper basin of Papenoo Valley, Tahiti, from Tetiari Pass (800 m.), look- 

 ing south. Rounded conical hill in middle distance, Ahititera (850 m.), formed 

 of plutonic rock. Tetufera Peak (1,800 m.) in background, in the centre. 



Pig. 2. View of Orohe Peak from base of Ahititera Hill, Upper Papenoo Valley, Tahiti, 

 looking north. Orohe Peak is formed of volcanic lava and breccia. 



