Cotton. — Geology of Signal Hill, Dunedin. 117 



length — up to 0-5 mm. — but all are very narrow. Some show Carlsbad 

 twinning. The crystals are often bent, and generally interwoven. 



The constituents next in importance are nepheline and a^girine-augite. 

 The nepheline is not distinguished without staining, but in a stained section 

 fully one-third of the rock is seen to consist of nepheline. chiefly in short 

 hexagonal prisms, and partly as fine allotriomorphous grains. The maxi- 

 mum size of the prisms is 0-1 mm. in length and breadth. Very small 

 isotropic spots with low refractive index are put down as sodalite. The 

 segirine-augite occurs as flakes and needles without definite crystal outlines 

 at Logan's Point. Near the summit better - defined elongated crystals 

 occur, which are also larger, being as much as 0-5 mm. in length. 



The maximum extinction-angle is 38°. 



The colour is bright green, and the pleochroism as follows : (a) grass- 

 green ; (6) lighter green ; (c) apple-green. With this may be compared 

 the pleochroism of pegirine from Laven, given by Rosenbusch :* (a) pure 

 green to blue-green ; (t») olivine-green ; (c) yellowish grass-green. 



Associated with the segirine-augite are numerous minute crystals of 

 deep-brown, almost opaque cossyrite, varying in size from minute grains 

 or needles to crystals 0-1 mm. in thickness. The form of the crystals is 

 rather indefinite, but as nearly as can be made out the prismatic angle is 

 that of typical cossyrite (66°). The mineral exhibits a deep-brown pleo- 

 chroism. The grains are scattered evenly throughout the rock, being often 

 associated with crystals of the green segirine-augite. 



Magnetite is fairly abundant, in fine grains, while apatite occurs very 

 rarely, in fine needles. No glass is present. 



The high percentage of both ferrous and ferric oxides may be noted 

 as peculiar to the rocks of this district. Treatment of the finely ground 

 powder with hydrochloric acid dissolves a considerable amount. This points 

 to the presence of a large proportion of nepheline ; and the unusual amount 

 of ferric iron obtained in solution by this method points perhaps to the 

 solubility of cossyrite. 



Variations of the Logan's Point Phonolite. — On the peak above Burke's 

 mossy aegirine-augite is very abundant, and phenocrysts also occur up to 

 0-3 mm. in length. Feldspar is very rare, and the rock should be classed 

 as nephelinitoid. Cossyrite and magnetite are abundant. 



Near the top of Signal Hill the feldspar is relatively abundant, showing 

 flow structure. At Normanby nepheline is abvmdant, as also is tegirine- 

 augite, both as mossy growths and as phenocrysts of considerable size. 

 These phenocrysts attain a length of 3 mm. or more, but are very narrow. 

 They have sometimes a core of pinkish-grey augite. The extinction-angle 

 of the segirine-augite is 36°, and of the augite 40°. Orthoclase needles are 

 moderately abundant. Co3syrite and magnetite occur. 



The Logan's Point rock has been compared by Rosenbusch, in a pri\'ate 

 letter to Dr. Marshall, with the apachite of Osann.t The chief resemblance, 

 however, appears to be the presence of cossyrite. According to Rosen- 

 busch,! ^^^ distinctiveness of the apachite type is stated by Osann to be 

 the richness in the younger amphibole minerals, the microperthitic nature 

 of the feldspars, and the presence of senigmatite. The last appears to be 

 the only point of similarity. 



♦Rosenbusch, Mikr. Phys., 1905. 

 tT.M.P.M., 1896, XV, 394. 

 t Mikr. Phys., vol. ii, p. 823. 



