Vol. 62.] GEOLOGY OF DT7NEDIN (NEW ZEALAND). 397 



of sanidine, nepheline, aegirine, and cossyrite, the last-named 

 being rather abundant. The geological occurrence of this rock, 

 coupled with its miner alogical composition, proves that it is a 

 slowly-cooled equivalent of the Mopanui nephelinitoid phonolite 

 to be described later (p. 404). 



4. Camptonitic type of tinguaite (ulrichite). — This 

 rock occurs at the end of the Portobello Peninsula. It was described 

 by the late Prof. TJlrich as a porphyritic phonolite. 1 The nepheline- 

 phenocrysts measure sometimes 2 centimetres in diameter, and the 

 felspars 4 cm. Both are idiomorphic, and the latter are usually 

 sanidine, but some are anorthoclase ; they are very frequent. 

 Occasionally large crystals of brown hornblende occur, and smaller 

 ones of analcite, olivine, and aegirine : the first is certainly original, 

 for it is penetrated by microliths of felspar. The olivine is 

 sometimes idiomorphic, and the aegirine is always so. The ground- 

 mass consists of sanidine, hornblende, and aegirine. The extinction 

 and pleochroism of the hornblende prove it to be barkevikite ; 

 it is often fringed with aegirine. 



Chemically, the rock is not very different from several others that 

 have been called tinguaites, as the following analyses show: — 



I. II. 



Per cent. Per cent. 



Si0 2 53-64 53-28 



A1,0 3 18-26 16-38 



Fe;0 3 4-66 6-11 



Fe'O 2-72 4-52 



CaO 3-70 3-09 



MgO 1-53 2-50 



Na 2 5-51 6-42 



K 2 5-86 4-18 



P 2 5 012 015 



H 2 3-73 3-52 



Totals 99-73 100-15 



The two samples were taken from different parts of the same dyke. 

 This seems to me a type widely different from a normal tinguaite ; 

 and I propose for it the name ulrichite, after the original dis- 

 coverer of the alkaline rocks of Dunedin. 



The dykes of tinguaite penetrate the rocks near Portobello and 

 Port Chalmers in some number, and extend to the eastern coast 

 between Sandfly Pay and Wickliffe Bay. 



The tinguaite-dykes vary from 6 inches to 15 feet in width, and 

 they appear to radiate from a point near Port Chalmers. The mass 

 near the Limekilns on the Peninsula and the coarse type at Sea 

 View are much larger masses ; but their exact boundaries cannot 

 be defined, because of surface-vegetation. 



5. Hypabyssal trachydolerite. — The summit of Mount 

 Plagstaff and part of its southern and western slopes consist of a 



1 Trans. Austral. Assoc. Adv. Sci. vol. iii, 1891 (Christchurch) p. 128. 

 Q. J. G. S. No. 247. 2 e 



