TKANSACTIONS OF SECTION C. 145 



A quantitative analysis of the rock by Mr. Thomas Bute- 

 ment, an Associate of the Dunedin University School of Mines, 

 gave the following results : — 



SiOo 



Al^Os 



FeO 



CaO 



MgO 



Na„0 



H,6 



52-30 



98-62 



Concluding Eemakks. 



The foregoing descriptions show that nepheline, both 

 allotriomorphic and idiomorphic, forms an essential comj)onent 

 of the volcanic rocks of a large portion of the Dunedin district, 

 and that all those concerned belong to the phonolite group of 

 rocks. Adopting the classification of Eosenbusch, the coarsely- 

 porphyritic rock first described, and 2, 3, 4, 5, 8, 9, 13, 15, 

 belong to " nephelinitoid phonolite;" whilst others (7, 10), 

 through increase of sanidine and corresponding decrease of 

 nepheline, come under his designation of " trachytoid phono- 

 lite ; " and of the remainder, 1, 6, 11, 14, and 6, 12, through 

 accession of plagioclase, and either absence or presence of 

 olivine, graduate respectively towards tephrite and basanite. 

 The great abundance of hornblende and olivine in some of the 

 rock-varieties described, especially the coarsely-porphyritic 

 ones, though an uncommon occurrence in phonohtes, would, 

 I think, not justify applying any special terms to these 

 varieties, such as olivine-phonolite, hornblende-phonolite, 

 olivine-hornblende-phonolite, for the reason that both minerals 

 are generally considered to be only casual accessories. It is 

 certainly, however, a peculiar circumstance that other minerals 

 of the nepheline group, especially nosean, which is so common 

 an accessory— in fact, nearly an essential constituent of Euro- 

 pean phonolites — is here completely absent, as far, at least, as 

 my observations have gone ; and that of the occurrence of 

 tibanite, also an accessory hardly ever absent in those phono- 

 lites, only one doubtful instance could be found. Another 

 singular feature in the case of our rocks is the scarcity of 

 zeolites, such as analcime and natrolite — species which are 

 generally so abundant in druse-cavities and fissures of decom- 

 posing phonolites, — for only a few small cavities coated with 

 them have so far been discovered in the decomposed rock of the 

 Portobello promontory. It seems, indeed, as if the nepheline- 



* 0-05 per cent, at 100° C, 4-28 per cent, on ignition. 

 10 



