100 Tranmctions. 



The rock here described from Muri Point would, from the complete 

 absence of feldspar, be classed as a nephelinite according to Rosenbusch's 

 scheme. It is retained here amongst the phonolites because its chemical 

 and mineralogical composition show much closer relationship to the phono- 

 lites than to the basalts. 



The Native weapon forwarded by Mr. Brown is of quite different material 

 from the rocks described. The absence of olivine and the abundance of 

 feldspar place it among the augite-andesites. It is quite conceivable that 

 it was made from one of the Tongan rocks. 



AlTUTAKI. 



Mr. C. Cameron, E.M., kindly forwarded me samples of rock from this 

 island. The following notes were made by him on the specimens : " Sample 

 A, from the mainland, is fairly common, sometimes in large blocks, and was 

 occasionally used in ancient times for building round the priests' marae. 

 It is easily shattered by fire. Sample B is from a small island called Rapoka, 

 in the lagoon, about a mile from the mainland. It is often used by the 

 women for their native ovens, as it stands fire better than A, though not 

 entirely well." 



A. A relatively coarse basaltic rock, with much olivine and an abund- 

 ance of idiomorphic augite, and but little feldspar, which is labradorite. 

 The colour of the augite shows that it is strongly titaniferous. No nephelin 

 could be distinguished in section, but the rock-powder gelatinises readily 

 on treatment with dilute HCl. and crystals of salt are formed when the 

 solution is evaporated. 



B. Very fine grained compact rock. Olivine is very abundant, and is 

 often stained with limonite on the margin. Augite granular, and restricted 

 to the groundmass, where it is associated with much magnetite and a pre- 

 dominance of minute crystals of nepheline, without sharply crystalline 

 boundaries. The rock is clearly a nephelinite or nepheline-basalt. 



The accompanying map, copied from the Admiralty chart, shoAvs the 

 form of the island. It is at once seen that the mainland is at the northern 

 side of the interior of the circular coral reef, and is situated near the margin. 

 Rapoka Island is near the south-east portion of the interior of the reef. 



The form of the island suggests that submergence has taken place, but 

 it may be that the coral reef has arisen on the margin of a shoal formed 

 by wave-action dispersing the material of a loose scoria cone, as has been 

 exemplified in the case of Falcon Island, in the Tonga Group. It will be 

 remembered that this origin has been suggested by Mr. Lister to explain 

 the occurrence of atolls in the southern portion of the Tonga Group of islands, 

 although there is clear evidence of elevation of as much as 1,000 ft. in the 

 north-east portion of the group. 



The rocks from Aitutaki are not closely related to any types that I have 

 seen in the south-west Pacific. They resemble the Auckland basanites 

 more closelv than anv of the others. 



EXPLANATION OF PLATES III AND IV. 



Pl.ATK III. 



Sketch-maps of Rarotonga and Aitutaki. 



PLvrE IV. 

 Fig. 1. Micro, section of ne])heline-phonolite, Rarotonga. Tlie clear portion is nepheline ; 

 the daik material is Kgirine. Jlasnified 30 diameters, ordinary light. 



