TRANSACTIONS OF SECTION C. 129 



in this search he occasionally submitted to me specimens of 

 "likely-looking" rocks, as he termed them, because of their 

 showing fine impregnations of pyrite or being traversed by white 

 veins of zeolitic matter. Most of the rock-pieces he brought 

 were unfortunately too decomposed for determining their 

 original character ; only so much could with certainty be made 

 out, that they were of volcanic origin. However, on looking 

 more closely through one lot of coarse rock-pieces he left for 

 my inspection, I found, on breaking one of the largest ones, 

 that it contained beneath a thick yellowish-white decomposi- 

 tion-crust a hard nucleus, which, on fracture, revealed the 

 porphyritic rock under notice — a greyish-black base abundantly 

 impregnated with large crystals of two white or, rather, light- 

 coloured minerals. One of these, a feldspar, was found by 

 Szabo's method of flame-reactions (indicating potassium), and 

 from the absence of plagioclastic twin-striation, to be sanidine ; 

 whilst the other, by its gelatinising in HCl, flame-reaction for 

 sodium, its oily lustre, and crystal outlines, unmistakably 

 proved to be nepheline — conclusions subsequently confirmed by 

 microscopic examination. 



Considering the interest attaching to this discovery, I in- 

 duced Mr. A. Purdie to journey soon after to Portobello, 

 where, informed by Mr. Porterfield, he soon found the out- 

 crops of the rock ; and, on examining these later on in his 

 company, I observed the following : The rock forms two dykes 

 in the small promontory (about 350ft. high at its highest point 

 at the far end) which runs out north into Port Chalmers 

 Harbour, on the eastern side of Portobello Inlet. One 

 dyke (about 12ft. thick) is situated between half and three- 

 quarters of a mile north of Portobello ; the other, a good 

 15ft. in thickness, crops out a quarter of a mile further north 

 near the point, where the shore-line of the promontory turns 

 sharply eastward ; and they are both accessible only at low 

 water, being exposed in the nearly vertical line of cliff, from 

 30ft. to 80ft. in height, facing Quarantine Island. Both dykes 

 traverse decomposed yellowish-white soft rock, the first at 

 a strike of S. 25°E., the second at S. 20^E., with a slight 

 dip to the east ; but all traces of them disappear on the smooth 

 slope of the hill above the top-line of the clilfs. Along the foot 

 of the cliffs, at and near about the dyke-outcrops, blocks of the 

 dyke rock are seen of all sizes up to several feet in diameter, 

 resembling at a distance some coarse quartz-breccias, because 

 of the crowded outstanding large sanidine crystals, from be- 

 tween which the nepheline crystals and the dark base have 

 been more or less removed by weathering and sea-action. The 

 soft white rock which the dykes traverse is evidently the 

 result of decomposition of two other kinds of rock — one of 

 greenish-grey colour and dense, the other dark-grey and fine- 

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