396 PROF. P. MARSHALL ON THE [Aug. I906, 



wharf at Port Chalmers. In these felspar-phenoerysts are very 

 numerous, and are in part anorthoclase. A little olivine is present, 

 and there is much nepheline in the groundmass as well as in 

 phenocrysts. Small crystals of segirine frequently occur in the 

 groundmass, and cossyrite is much more abundant than in the 

 rock from Sea Yiew. This type with a nephelinitoid groundmass 

 has not been found anywhere in situ, nor has any analysis of it 

 been made. 



On the Peninsula there is a large intrusion of another, less strongly- 

 porphyritic, type. The only phenocrysts are of nepheline, and they 

 are comparatively small (2 mm.), but they are always idiomorphic. 

 JEgirine, nepheline, and sanidine are present in abundance in the 

 groundmass. The segirine is very green, and shows an extinction- 

 angle of 20°. There is no cossyrite or magnetite. 



Still another comes from Papanui. The large felspars and 

 nepheline-crystals embedded in a dark-green groundmass give the 

 rock an appearance intermediate between the specimens from the 

 breccia and that from the Peninsula. 



A rock that has been used for the breastwork of the road round 

 Hooper's Inlet is, in some respects, different from all of these. 

 Felspar-phenoerysts are not infrequent, and felspar-microliths are 

 the most important constituent of the groundmass. The nepheline 

 is all in the groundmass and is not very abundant, but it is markedly 

 less decomposed than the felspar. The coloured constituent is 

 nearly all aegirine. It occurs in groups of crystals with magnetite 

 and analcite, and in the middle of these groups there is often some 

 hornblende. It is evident that the aegirine, magnetite, and analcite 

 have been derived from hornblende-crystals. (See PI. XXXVIII, 

 fig. 1.) The change must have taken place after magmatic movements 

 had ceased, for no indication of flow-structure has been observed. 



A quantitative analysis yielded the following result: — 



Si0 ' 50-16 



A1 2 3 1975 



Fe 2 3 4-28 



FeO ' ;;-'o2 



CaO 310 



MgO 112 



Na o 763 



K o 6-73 



P 0- 013 



H 2 3-95 



Total 100-48 



This rock, from its mineralogical composition, must be classed as 

 a tinguaite, although its silica-percentage is so low. The change of 

 the amphibole to the minerals named was more a matter of 

 magmatic resorption than of decomposition. 



The last rock to be mentioned under this head occurs near the 

 Purakanui railway-station. Phenocrysts of sanidine, nepheline, 

 :and sodalite are very frequent, and the groundmass is a mixture 



