Vol. 70.] LAVAS AT NORTH HEAD, OTAGO HARBOUR. 405 



While the statements made and arguments advanced in this 

 paper give a satisfactory and reasonable explanation of the occur- 

 rence of phonolites in genetic association with basalts in the Dunedin 

 district, as also in Tahiti and probably in the other islands of that 

 part of the Pacific basin, it is not supposed that this explanation 

 necessarily accounts for the occurrence of phonolites in other 

 volcanic districts. There are not sufficient data derived from a 

 study of this district to justify a discussion of the origin of the 

 essexitic marina itself. 



'6V 



VI. Summary and Conclusions. 



(3 1 The series of volcanic lavas at the Xorth Head, Dunedin, all 

 flowed down the same slope, and probably all issued from, the same 

 volcanic orifice. 



(2) The large amount of scoria between the lava-flows shows 

 that the centre of eruption was not far distant from the localitv 

 of the North Head. 



(3). The fact that there was no erosion of any lava-flow 

 before the outflow of the next indicates that no interval of any 

 great length separated the eruption of the various lavas. 



( 4 l The lavas may be classified as trachyte, phonolite, kaiwekite, 

 basalt, and traehydolerite. 



The trachydolerites and kaiwekite have an intermediate 

 composition between the phonolites and the basalts. 



1 6) The trachydolerites and kaiwekite contain large crystals of 

 barkevikite and of anorthoclase or of nepheline, and are much the 

 most porphyritic rocks of the series. 



(7 ) Although these rocks are the most porphyritic of the suc- 

 cession, their intermediate chemical character marks them as 

 formed from the magma before it was differentiated. 



(8) This consideration necessitates a threefold eruption from a 

 deep-seated source to an intermediate level. 



( i) ) After a partial eruption of the magma at this intermediate 

 level complete resorption of the barkevikite and other minerals 

 would take place. 



(10) The eruption of the dense non-porphyritic phonolite 

 would take place from the upper part of this residue, which 

 would have become separated into portions chemically different by 

 the process of molecular differentiation acting under the force of 

 gravity. 



(Hi The basalts represent the heavier residue of the differ- 

 entiated residue at the intermediate level. 



1 12 j The barkevikite, violet augite, anorthoclase, and basic 

 felspar, which occur as phenocrvsts in the intermediate lava, as well 

 as the chemical composition of these lavas, prove that the original 

 magma had the composition of essexite. 



(13) The course of differentiation which is indicated in the 

 series of lavas exposed at the Xorth Head is apparently the 



Q. J. G. S. No. 280. 2 r 



