Cotton. — Geology of Signal Hill, Dunedin. 119 



The largest phenocrysts arc feldspar of different varieties in slightly 

 corroded crystals. Anorthoclase is common in large crystals. It shows 

 fine lamellar twinning, crossed at right angles by another set of very fine 

 and rather indefinite lamellae. As will be seen from the accompanying 

 photograph (Plate X, fig. 2), these are too narrow and not regular enough 

 to be attributed to pericline twinning of a feldspar of the albite-anorthite 

 series. In thicker parts of the section extinction between crossed nicols is 

 irregular. For these reasons this feldspar is regarded as anorthoclase. 



As sections parallel to the cleavage are not obtainable, the extinction- 

 angle cannot be verified. 



Clear crystals of monoclinic feldspar with Carlsbad twinning are also 

 common. They are referred to sanidine. 



Oligoclase phenocrysts are also common, with polysynthetic albite 

 twinning. Sections perpendicular to 010, having equal extinction-angles 

 measured from the twin line, on either side of it, extinguish at an angle of 

 5° from the twin line. 



Perthitic intergrowths are rather common. 



The feldspars all occur in broad prisms, up to 3 mm. or i mm. in length. 



Numerous phenocrysts of a deep-brown amphibole occur. They have 

 definite crystal outlines when not entirely resorbed. Prisms and clino- 

 pinacoids are developed, giving hexagonal cross-sections. The crystals 



are elongated parallel to the c axis. The optical characters observed are as 



\ -L , 



follows: a — a, b = b, c/\c= 15°. The axis c lies in the plane of symmetry. 

 Pleochroism : {a) Pale yellovvash-brown ; (6) deep brown ; (c) deep brown. 

 These characters show it to be intermediate between barkevikite and basaltic 

 hornblende. The crystals are largely resorbed, being bordered, and in some 

 cases entirely replaced, by fringes of magnetite grains associated with augite 

 and a little calcite. 



This amphibole has evidently been one of the first minerals to crystallize, 

 having probably an " intratelluric " origin. It has afterwards become 

 unstable in the magma owing to altered conditions, perhaps of pressure. 

 The feldspar needles surrounding the amphibole phenocrysts have a 

 fluxional arrangement. 



An interesting comparison may be m.ade between this mineral and the 

 amphibole of a somewhat similar rock described from tlie *' Beagle " collec- 

 tion.* In that rock the amphibole is converted to tegirine-augite, and no 

 mention is made of magnetite. This is considered to be the source of all 

 the segirine-augite in the rock. 



The size of the amphibole phenocrysts in the Signal Hill rock varies 

 from 0-3 mm. in length and breadth up to 1 cm. or more. 



Pyroxene phenocrysts are rare, and of small size. They are augite 

 of a slightly sodic variety. The extinction-angle is 42°. The mineral is 

 rather pale in colour, with very slight pleochroism, as follows : («) pale 

 green ; (b) greyish, almost colourless ; (c) slightly yellowish-green. 



A few stout prisms of grey-coloured apatite occur, up to 0-2 mm. in 

 length. 



Olivine occurs rarely, and is largely altered to serpentine. A glomero- 

 porphyritic inclusion has been observed. In the centre is a large cross- 



* Geol. Mag., March, 1907, p. 100. 



