TKANSACTIONS OF SECTION C. 147 



It may in conclusion not be out of place to mention that 

 nepheline-bearing rocks are not confined to the Dunedin 

 volcanic district, but that the similar volcanic area of Banks 

 Peninsula (Province of Canterbury) contains at least one large 

 dyke of a rock which is no doubt a phonolite," whilst there 

 occurs, in addition to andesite, trachyte, and basalt, an im- 

 portant type of rock, viz., rhyolite, which still awaits discovery 

 in the Dunedin district. 



Explanation of Plate V. 



Figs. 1, 2, 3, arc drawn in ordinary light from thin slides of the 

 dark base of the coarsely-porphyritic rock of the Pnie Hill ; 1 and 2 are 

 magnified 26 diams. ; 3 is magnified 120 diams. 



Fig. 1. — Along upper right-hand edge are portions of three crystals 

 of sanidine, marked s, the centre one forming a Carlsbad twin, with the 

 twiniiing-liue running through the middle in the direction of the dotted 

 line towards s. A fourth sanidine (s), of narrow-columnar form, runs 

 upwards from near the broken end of the largest of the three crystals 



* With regard to this assertion, I have to state the following : Some 

 years ago I received from the late Sir Julius von Haast a specimen of 

 rock considered to be trachyte-porphyry, broken from a massive dyke 

 traversing the high andesite range on the eastern side of the Lyttelton 

 Railway tunnel on the Christchurch side of Banks Peninsula. During 

 my examination of the Dunedin rocks I remembered this specimen, and 

 also examined it closel)^ with the following results : The rock is of a dark 

 greyish-green colour, dense and compact, and rendered strongly porphyritic 

 by small crystals of sanidine, generally disposed parallel to the clino- 

 pinacoid, whence results on fracture in a favourable direction a glittering 

 aspect due to the cleavages of that face of the feldspars shining out 

 simultaneously. In fact, the rock very much resembles the green rock. 

 No. 9, described from the Pine Hill, being only more strongly porphyritic. 

 The powder gives a considerable amount of gelatinous silica on treatment 

 with cold HCl ; and according to a quantitative analysis by Mr. Thomas 

 Butement the composition of the rock is as follows : SiC, 59'87 ; AlOg, 

 21-22 ; FeoOs, 4-42 ; PeO, 1-16 ; MnO, 0-14 ; CaO, 258 ; MgO, 091 ; KoO, 

 4-06 ; NaaO, 5-34 : total, 99-70. 



Microscopic examination of thin sections discloses a clear ground-mass 

 showing all the characters of allotriomorphic nepheline as previously 

 described, but the granulation, so to speak, in light and dark resulting on 

 revolving of the section between crossed nicols is more minute than in the 

 Dunedin rocks. Sparingly distributed through this ground-mass occur 

 lath-shaped microlites of sanidine, and in very great profusion tufts and 

 dense groups of pale-greenish or greenish-grey microlites (oegirine ?), 

 imparting to the sections in ordinary light a finely-speckled dark-and- 

 bright appearance, which is rendered more pronounced by an abundant 

 even distribution of rarely large but generally small and even-sized grains 

 of magnetite. Porphyritic sanidiues, mostly in Carlsbad twins, are large 

 (up to 4mm. in length and 2mm. in breadth) and numerous, and the 

 larger ones generally show inclusions of microlites and dusty matter. 

 Augite occurs very sparingly in small ill-formed crystals and grains of 

 light-green colour and slightly dichroic. Apatite is also scarce. Idio- 

 morphic nephelines could not he discovered in any of the slides. 



Considering these observations in connection with the chemical 

 results, I have no hesitation in calling the rock a " trachytoid phono- 

 lite." 



