394 PROF. P. MARSHALL ON THE [Aug. 1906, 



2. Augite-Diorite. — Macroscopically, the rock is of a fresh- 

 looking light-grey type, penetrated by narrow bands of darker 

 colour. The bands are 1 centimetre thick and 7 cm. apart. 



In slices, the rock is found to consist almost entirely of triclinic 

 felspar, the extinction-angle of which is that of andesine ; the 

 structure is panidiomorphic. Small grains of a very pale augite 

 and crystals of magnetite are frequent. A violet-brown perofskite 

 in crystals, sometimes measuring 1 millimetre in diameter, is also 

 frequent. The occurrence of the titanium-mineral perofskite is 

 particularly noticeable, because no sphene has been found in any 

 rocks of the district, and the only other titaniferous mineral that 

 occurs is cossyrite. 



The large rock-mass placed in this class occurs almost in the 

 middle of the city of Dunedin, forming the Bell Hill. The diorite 

 clearly underlies all the volcanic rocks near it, and in the road- 

 cutting in Dowling Street it is seen that the diorite -surface 

 was eroded, uneven, and much decomposed, before the overlying 

 andesite flowed over it. The diorite is so much decomposed, indeed, 

 as to resemble a sandstone in all those portions which are near its 

 surface. The unaltered rock is exposed in the face of the Bell Hill 

 where, prior to the accumulation of sand and to European coloni- 

 zation with its consequent reclamation, marine erosion had removed 

 all the weathered portion. A very prominent spheroidal weathering 

 is to be seen, and its dark horizontal bands seem to indicate that 

 the rock has yielded to pressure and become crushed along these 

 planes. 



Except in the Bell Hill and its neighbourhood, this rock is 

 nowhere exposed, and no fragments have been found in the breccia 

 at Port Chalmers. 



3. Tinguaite. — Rocks that are classified under this division 

 occur quite frequently. Among them are fifteen well-defined 

 dykes of a compact light-green rock, which differs but little from 

 ordinary tinguaites. There are others that are coarsely porphyritic, 

 and one that is distinctly camptonitic in structure. 



In thin slices some examples (Hooper's Inlet and Acheron Point) 

 show crystals of dark-brown hornblende with the pleochroism of 

 basaltic hornblende. It is always fringed with segirine. The 

 needles of Eegirine vary in size, but are never large. Felspar is almost 

 entirely absent in crystals and microliths ; an exception is found 

 in the rock at Acheron Point, in which small crystals of sanidine 

 (twinned on the Carlsbad law) are not infrequent. Nepheline is 

 occasionally present in rather corroded phenocrysts, but crystals 

 are not distinguishable in the groundmass, which is quite isotropic 

 between the segirine -nee dies. In two examples (Eish-Hatcheries 

 and Hooper's Inlet) a peculiar spherulitic structure appears in the 

 groundmass. The mineral of which the spherulite is composed is 

 highly birefringent — as strongly as quartz — and quite transparent. 

 With ordinary light it is impossible to determine which part of a 

 section is spherulitic. The spherulites between crossed nicols 



