ALKALINE ROCKS IN THE PACIFIC. 195 



3.— REPORT ON DISTRIBUTION OF ALKALINE ROCKS IN THE 



PACIFIC. 



By PROFESSOR P. MARSHALL. 



As a member of the Alkaline Rocks Research Committee it was 

 arranged that I should as far as possible study the distribution of 

 alkaline rocks in the Pacific Islands. 



I was able to visit Raratonga and Mangaia in the Cook Group, 

 and Tahiti, Raiatea and Huaheine in the Society Group. 



The petrographical descriptions of the rocks collected appear 

 in a separate paper submitted to the Geological Section. 



As a summary of general results of my work it may be stated 

 tjiat alkahne rocks have a wide distribution in the Central Pacific. 

 They certainly occur at Raratonga, Aitutaki, Raiatea, Huaheine, 

 Tahiti, Savaii, Upolo and Tutuila. The aspect of Moorea, Tahaa 

 and Borabora, when compared with Raiatea and Huaheine, strongly 

 suggests that these islands also are crowned with phonoUte lava 

 flows, but no specimens have yet been obtained from them. In 

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

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

 granite from Maupiti and a quartz-felspar rock from Borabora. A 

 specimen of gabbro was given to 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 rocks at Arue corresponding with the hau- 

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

 eight months later that I found that the distinguished French 

 petrologist had received specimens of a similar rock from other parts 

 of the island, destitute, however, of the hornblende. 



In Raratonga, Huaheine and Raiatea the alkaline rocks are 

 far later than the decomposed basalts upon 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 latest eruptives of 

 this area, but still of great age, for denudation has entirely destroyed 

 all signs of the craters from which these immense lava flows issued. 



In each of the islands visited the alkaline rocks were closely 

 associated with extremely basic effusives. Some of these are so 

 basic that Lacroix refers them to a species of picrite. Whether the 

 different species of rock in Tahiti have resulted from differentiation 

 is a question for the future. Lacroix evidently thinks this is the 

 case, though he admits the incomplete nature of the collection. 

 The constant association of the two main types mentioned in the 

 islands visited truly supports this view. 



