Vol. 70.] LAVAS AT NORTH HEAD, OTAUO II A IMJOl'K. 408 



of the original undifferentiated magma; not from the complete 

 interminsrlinsr of two magmas widely different one from the other 

 in chemical composition. 



The occurrence of the alkaline and basic rocks in the island of 

 Tahiti is of special interest in this connexion. Here, as is well 

 known, Prof. A. Laeroix has recorded the occurrence of phonolites 

 and other alkaline rocks, as well as a great variety of basalts, some 

 of which are of extremely basic composition. There is also in the 

 interior of the island a mass of plutonic rocks which are mainly 

 monzonites, essexites, and theralites. 1 A recent visit to that 

 island, made for the purpose of studying these rocks, has enabled 

 me to make a comparison between the rocks of the two districts. 



On the outskirts of the plutonic area in the centre of the island, 

 G000 feet below the summit of the fantastic ruins of the great 

 island volcano, occurs a pure white syenite composed almost 

 solely of anorthoclase. At another place on its border a perido- 

 tite occurs, but over the greater part of the plutonic area there 

 is a large outcrop of essexitic and theralitic rock. Viewed under 

 the microscope, there is almost perfect identity between the 

 barkevikite, violet augite, and olivine of these rocks and the same 

 minerals found in the kaiwekites and trachydolerites at Dunedin. 

 Here, again, the phonolites that issued from the crater which at one 

 time reached more than 6000 feet above the surface of the plutonic 

 rocks (as at present exposed) are dense compact rocks. The nature 

 of the plutonic complex leaves no room for doubt that, in this 

 instance at least, the phonolites resulted from the differentiation- 

 processes within an essexitic magma. There is a close similarity 

 between the rocks of Tahiti, Karotonga, Moorea, Huaheine, and 

 Raiatea, in all of which dense phonolites are associated with 

 basalts often having the same extremely basic character as that 

 presented by the basalts in Tahiti. 2 These facts suggest that they 

 also have had a similar origin, and that here at least the usual origin 

 of phonolitic rocks is from processes of molecular differentiation 

 w r ithin a magma of essexitic composition. 



Difficulties in obtaining access to any large body of geological 

 literature have prevented me from making a comparative study 

 of the theories that have been advanced to account for the origin 

 of alkaline volcanic rocks. The only two that I can find — apart from 

 general statements in text-books of the relation of phonolites, for 

 instance, to nepheline-svenites — are those of Dr. Jensen and 

 Dr. Daly. Unfortunately neither of these authors quotes literature 

 on the subject. Dr. H. I. Jensen states, among other conclusions 

 in his summary :] ' — 



(1) All alkaline rocks are practically confined to the flanks of old plateaux 

 (continental areas) which have escaped heavy sedimentation in late Palaeozoic 



1 Bull. Soc. Geol. France, ser. 4, vol. x (1910) pp. 91-124. 



- For a general statement, see P. Marshall, ' Oceania ' in ' Handbucli der 

 Regionalen Geologic' vol. vii, pt. 2, Heft 9, p. 12 (Heidelberg, 1911). 

 r4 :i ' The Distribution, Origin, & Relationships of Alkaline Eocks ' Proc. 

 Linn. Soc. N.S.W. vol. xxxiii (1908) p. 584. 



