200 PROCEEDINGS OF SECTION C. 



A comparison of these analyses with those quoted by Rosen- 

 bush (" Elemente de Gesteinlehre," 2nd edition, p. 292) shows that 

 the South Sea phonolites are somewhat higher in silica and lower 

 in alumina than the average phonolite ; the difference, however, is 

 not great. 



Raratonga. — I have previously recorded a nephelinite from Muri 

 Point and from Black Rock. (PI. xvii. fig. 1, xviii. fig. 1). A visit to the 

 island showed that the alkaline rock occurs over a wide area. It was 

 actually found in situ on the summit of Maungatea, and here it contains 

 a large quantity of an isotropic mineral, which is probably sodalite. 

 All the lower slopes of Maungatea are formed of a much decomposed 

 basalt which has weathered into a red soil. This rises to 500 feet 

 above sea level. On it there is a thick mass of nephelinite breccia 

 200 feet thick. (PI. xvii., fig. 2). On this rests the nepheUnitelava, 

 which, however, on this hill is not thick. The nephelinite at the 

 west side of Maungatea is associated with the porphyritic basalt that 

 is found so frequently in the island, and is very similar to the 

 common rock type of the Societj^ Islands. The dense covering of 

 vegetation on the summit of the hill prevented me from arriving 

 at any definite conclusions as to the relative age of the rocks, though 

 it appeared probable that the porphyritic rock was intrusive into 

 the nephelinite. This is rendered likely by the occurrence of the 

 porphyritic basalt in situ at the bottom of the neighbouring valley of 

 the Avatiu. It also forms the crest of Hikurangi, the sharp spur of 

 which is I believe a dyke of this rock that penetrates the 

 nephelinite breccia, which is well exposed on the north-east face 

 of the hill. The heavy rain experienced on this hill, however, inter- 

 fered with geological work. 



Nephelinite breccia is also exposed in great thickness on the 

 south side of the Avatiu valley, and the solid rock is exposed in 

 many places in the hills on the south-west side of the island. A 

 glassy basalt was found on the shcre of the Ngatangiia stream and 

 a dyke of dense basalt on the north spur of Hikurar gi, while blocks of 

 a coarse tinguaitic reck were found in the village of Avarua. This 

 rock contains much anorthoclase, but its origin could not be found. 



The structure of the island appears to be as follows : — 



(1) The lowest rock a fine grained, but much decomposed 



basalt, 500 ft. 



(2) Nephelinite breccia, 200 ft. 



(3) Nephelinite lava, 100 ft. 



(4) Intrusive masses and dykes of a coarse basalt or dolerite 



zltntte lava 



Jfi^Maungafea 660 Ft 

 Breccia 



Mostlu 'Vephe/imre. 



Decomposed 



BasaH- \ /t'oisea Coral 



(oral Rid Sea Ltve! 



Fig. 2. — Geological Section, Raratonga. 



