ALKALINE ROCKS OF THE COOK AND SOCIETY ISLANDS. 201 



From this account it will be seen that alkaline rocks have a 

 wide distribution in the Central Pacific. They certainly occur at 

 Raratonga, Aitutaki, Raiatea, Huaheine, Tahiti, Savaii, Upolu and 

 Tutuila. The aspect of Moorea, Tahaa and Bora-bora when com- 

 pared with Raiatea and Huaheine strongly suggests that these 

 islands also are crowned with phonolite lava flows, but no specimens 

 have yet been obtained from them. On Mangaia Island I found no 

 volcanic rocks except highly decomposed basalts. Specimens sent 

 to me from Atiu by Major Large are also basaltic. 



At Tahiti there is a great variety of alkaline rocks, both plutonic 

 and volcanic. It is highly desirable that this large island should be 

 more fully explored by geologists. At present we do not know the 

 relation of the plutonic to the volcanic types. Ellis has mentioned 

 granite from Maupiti and a quartz felspar rock from Bora-bora ; a 

 specimen of Gabbro was given me by Frere Alain as from the 

 island of Raiatea, so it is evident that work of an important nature 

 is to be done in these islands. Future researches may throw much 

 light on the antiquity of land areas in the Central Pacific. 



My discovery of the rock at Arue which corresponds with the 

 hauynophyre of Lacroix I regarded as of great interest, and it was 

 only eight months later that I found that the distinguished French 

 petrologist had received specimens of a similar rock, destitute, how- 

 ever, of hornblende, from the other parts of the island. 



In Raratonga, Huaheine and Raiatea the alkaline rocks are much 

 younger than the decomposed basalts on which they rest, and I could 

 find no rocks of other kinds on their surface. This appears to me 

 to prove that the alkaline rocks are the products of the latest 

 eruptions of this area, but still of considerable age, for denudation 

 has entirely destroyed all signs of the craters from which these 

 immense lavas issued. In each of the islands visited the alkaline 

 rocks are closety associated with extremely basic effusives. Some 

 of these lavas are so basic that Lacroix refers them to the class of 

 picrites. Whether the different species of rocks in Tahiti have 

 resulted from mamatic differentiation is a question for the future. 

 Lacroix evidently thinks this is the case, though he admits the 

 incomplete nature of the collections. The constant association of 

 the two main types mentioned in the islands visited tinily supports 

 this view. 



Literature. 



Ellis, W. — Polynesian Researches, 2nd ed. London, 183L Vol. I, p. 12. 

 Lacroix, A. — Les roches a nepheline de Tahiti. Comptes Rendus Ac. des 



Sci., 1904. CXXXIX, p. 953. 

 Lacroix, A. — Consequences generates a tirer de I'etude de la constitution 



petrographique de Tahiti. Comptes Rendus Ac. des Sci., 1910. CLI 



p. 121. 

 Lacroix, A. — Les roches alcalines de Tahiti. Bull. Soc. Geol. la France, -Ith 



serie, X, pp. 91-124. 

 Marshall, P. — Geology of Rarotonga and Oitutaki. Trans. N.Z. Inst. 



XLI, p. 98. 

 Marshall, P. — Geology of Oceania. Handbuch der Regionalen Geologic 



Bd. VIL, abt. 2. Carl Winter, Heidelberg. 



