134 ■ KEPOET — 1891. 



kind of change of the mineral, as larger grains occasionally 

 occur broken up into a number of larger and smaller fragments, 

 each one surrounded by this rim (o, Pig. 4). There is also a 

 slight serpentinous change observable along most of the cracks, 

 though these are not as numerous as usually found in olivine, 

 comparatively large spaces in the inclusions being quite free of 

 them. 



Iron-Ore. — Judging from a number of the grains showing 

 square sections, this consists most probably of magnetite. The 

 grains go from O'Smm. in diameter down to the finest dust, 

 which latter, though very abundant in the base of the rock of 

 both localities, is generally less so in that of the Pine Hill than 

 in that of Portobello, as shown by the comparative clearness of 

 the sections (Pigs. 1 and 2 ; 4 and 5). 



Apatite. — To this, no doubt, belong sparingly-distributed 

 long and very thin acicular crystals, which seem to traverse 

 most of the other minerals and extinguish parallel to their 

 own length between crossed nicols. Bright specks of hexa- 

 gonal outline, sometimes seen in crystals of hornblende and 

 augite, are evidently cross-sections of these needles, as they 

 become dark between crossed nicols, and remain so on complete 

 rotation of the stage of the microscope. 



Titanite (?). — This mineral I add with some doubt, though 

 it is a very frequent accessory in rocks of the class under notice 

 in other countries. It seems to me to be represented in the 

 dark base of the Portobello rock by crystals of sharp wedge- 

 like outlines, which occur very sparingly amongst the brown 

 hornblende crystals, and are generally dark throughout, or 

 show only here and there fine specks of brown colour. 



Glass. — Particles of this, water-clear, perfectly isotropic, and 

 of all kinds of shapes, occur pretty frequently in the dark base 

 of the Portobello rock (g, Pigs. 4 and 5) ; in that of the Pine 

 Hill rock they are more rare. They always contain abundance 

 of larger and smaller gas-cavities, generally arranged in strmgs. 

 (See further on.) 



The Dense Compact Eock. 

 This forms the so-called Purakanui Cliffs, facing the Pacific 

 Ocean about eighteen miles north of Dunedin, and it no doubt 

 extends into the mountain at the back. The Dunedin-Christ- 

 church railway ran formerly in a cutting along these clifi"s ; but, 

 owing to frequent landslips, a tunnel has now been driven for 

 it through this rock. Large dykes of the latter, separated by 

 soft, decomposed, clayey rock of light colour, were also passed 

 through in the driving of the Deborah Bay railway-tunnel near 

 Port Chalmers, as shown in a collection of specimens of the 

 rocks met with during this work, made by Mr. W. N. Blair, 

 Chief Engineer of Eailways, and preserved in the Dunedin 



