BY H. I. JENSEN. 87 



albite. The phenocrysts may be idiomorphic, or corroded, or 

 allotriomorphic. Several bundles of phenocrysts occur which are 

 inclusions of a medium-grained bostonitic rock consisting wholly 

 of felspar. There is no proper corrosion-rim round any of them, 

 yet some have been rounded, and strands of base have filled 

 hollows formed by corrosion. The extinction angle varies from 

 4° to 8°, and the cleavage is rectangular. Twinning is wholly on 

 the Carlsbad plan, and only the faintest traces of shadowy extinc- 

 tion are seen in some. The form of the felspars is orthorhombic. 

 Evidently the phenocrysts and groupings of phenocrysts are 

 derived from some previously consolidated bostoniteor lestiwarite. 

 The fabric of the groupings approaches that of lestiwarite, but 

 the crystals are not wholly allotriomorphic as in many lestiwarites. 

 The felspar crystals of the base are minute needle-shaped micro- 

 lites which are doubly refracting, and still finer crystallites which 

 are practically isotropic. These minute needles can be seen with 

 the high power in non-polarised light. The whole mass exhibits 

 a faint shadowy, pseudospherulitic extinction when the nicolsare 

 crossed. A cloudy grey, practically isotropic, base exists inter- 

 stitially between the fine needles; this may consist of still finer 

 crystallites and globulites of felspar, with or without either cryp- 

 tocrystalline quartz or some felspathoid. As the rock is slightly 

 decomposed, staining tests are not of much use, and, the SiOo 

 percentage not having been determined, the exact composition of 

 this groundmass must remain unknown for the present. The 

 acmite is greenish-yellow and feebly pleochroic as in the trachyte 

 from the summit of Mt. Flinders. It is very sparingly repre- 

 sented. Magnetite occurs in idiomorphic corroded grains, also 

 in minute amount. INIinute violet grains of a highly refracting 

 and doubl}^ refracting mineral also occur very sparingly. These 

 are probably a titanium mineral allied to rutile. 

 ISame : Pseudospherulitic Trachyte-Porphyry. 



Fl. 1 6. Loc. : Mount Flinders, east side, forming the main mass 

 of the mountain. 



Handspecimen : porpliyritic light-coloured whitish trachyte. 



