Yol. 62.] GEOLOGY OF DUNEDIX (NEW ZEALAND). 395 



show rather an irregular black cross, and the mineral of which they 

 are composed is platy rather than fibrous iu structure. I have 

 been quite unable, with the means at my disposal, to identify 

 the mineral or to satisfy myself as to whether it is original or 

 secondary. 



Only one of these numerous tinguaites has been analysed (A.). 

 Petrographically it is the most compact of all. 



Si0 2 



A. 



Per cent. 

 ... 56-17 



B. 



Per cent. 

 54-46 

 19-96 

 2-34 

 3-33 

 2-12 

 0-61 

 8-68 

 2-76 



5-20 



99-46 



C. 



Per cent. 

 52-40 



A1 9 3 



... 19-25 



1993 



Fe (X 



4-77 



3-83 



FeO 



... 2-72 



1-51 



OaO 



... 1-26 



1-34 



MgO 



Na 2 



K o 



... 0-21 



... 6-08 

 ... 466 



0-32 



11-71 



4-10 



PX>- 



.... 0-21 



trace 



H 2 



... 4-09 



3-94 



Totals ... 



... 99-42 



99-08 



A = Dyke, Acheron Point, Otago Harbour (N.Z.). Anal. P. Marshall. 

 B=Tinguaite, Uruptek, Kola. Rosenbusch, ' Elemente der Gresteinslehre r 



2nd ed. (1901) p. 223, Xo. 12. 

 C=Tinguaite. Mount Kosciuscko (X.S.W.). Anal. F. B. Grutbrie, Journ. & 



Proc. Roy. Soc. N.S.W. vol. xxxv (1901) p. 366. 



The analyses show the great similarity which the New Zealand 

 rock bears to that from Russian Lapland, although it is less alkaline 

 than that from New South Wales. The only other Australasian 

 tinguaites of which I have heard occur at Cygnet Cove (Tasmania), 

 but I am not aware that any analyses of these have been published. 

 Their sections show a much greater development of felspar and, of 

 course, much less isotropic matter. 



A porphyrinic type occurs at Sea View. Large crystals of 

 nepheline, 1 centimetre in diameter, are frequent, and felspar- 

 crystals (sanidine) of the same size are common. In thin slices, 

 both of these minerals are seen to be idiomorphic. Small crystals 

 of idiomorphic segirine-augite are also present, and the ground- 

 mass is composed of a very fine-grained mixtnre of aegirine-augite, 

 sanidine, magnetite, and a little brown hornblende (cossyrite). In 

 some slices olivine and brown augite may be seen, bat they are in 

 small quantity. There is every variation in this locality between 

 the porphyritic type with conspicuous nepheline-crystals and an 

 extremely-dense green rock without any phenocrysts, and showing 

 the same incipient spherulitic structure as that described in the 

 Purakanui phonolite on p. 402. 



Another porphyritic type forms a small dyke near Port Chalmers. 

 The sanidine-crystals are here relatively smaller, and nepheline less 

 frequent, but the groundmass is somewhat coarser. 



Large boulders of another type occur in the breccia near the 



