68 DISCOVERY REPORTS 



and cleavage [jointing?] did not look much different from the crystalline rocks above it.' [This 

 may have been a discoloration of the granite due to intensified weathering between tide-marks. — 

 G.W.T.] 



Dr Mackintosh collected four specimens from this locality, two from the main rock formation 

 (granite), and two from dikes. He remarks that the granite showed some variation within short 

 distances, especially in the proportions of the darker minerals, and that his two specimens may have 

 a smaller proportion of the dark minerals than is typical of the rock as a whole. 



The main rock is a true granite consisting of quartz, orthoclase, albite-oligoclase, and a very small 

 amount of biotite largely replaced by pale green chlorite. One of the specimens is verv coarse-grained, 

 the crystals ranging from ^ in. to h in. in greatest diameter. The feldspar is pinkish white and the 

 quartz milky blue in colour. The other specimen is finer in grain and shows a white vein of aplite 

 with a knife-edge contact against the granite. 



In thin section the feldspars are seen to be thickly dusted with kaolinitic and sericitic alteration 

 products. The orthoclase seems to be almost pure, with only obscure traces of albite lamellation. 

 The albite-oligoclase occasionally shows an approach to the typical chequer-twinning, and is sub- 

 ordinate in amount to the orthoclase. The quartz and feldspars are sometimes intergrown in a coarse 

 and obscure graphic structure, especially in the finer-grained specimen. The only ferromagnesian 

 constituents are a very few flakes of chloritized biotite. The aplite vein consists of a very fine-grained 

 base of quartz and sericitized orthoclase with a saccharoidal texture, which carries small micro- 

 phenocrysts of quartz, orthoclase, and albite. It is quite devoid of coloured constituents. 



Conspicuous dikes of a blackish rock traverse the granite. One of Dr Mackintosh's specimens is 

 ' probably characteristic of all the black dikes in the headland '. In hand specimen it is a fine-grained 

 dark grey rock with a few large fresh phenocrysts of feldspar and a sprinkling of pyrites. In thin 

 section it consists mainly of a panidiomorphic plexus of andesine feldspar and a pale green hornblende 

 in about equal amounts. In addition, there are a few phenocrysts of labradorite (extinction 30 ), 

 a little quartz, and numerous fine-grained irregular aggregates of a reddish brown biotite which, in 

 many cases, are apparently growing at the expense of the hornblende. As these aggregates are 

 invariably associated with pyrites, they are probably of secondary origin, and connected with the 

 ingress of sulphide solutions into the rock. This rock is identical with some of the lamprophyres 

 described by Rosenbusch as spessartite} 



The remaining dike specimen was taken from the inner portion of what is probably a composite 

 dike. This dike was of the same blackish tint as the others. It was about 8 ft. thick, and had a central 

 part of greenish colour and a foot in width. This, however, is only a surface coloration. When broken, 

 the fresh rock is of a greyish blue colour and is very dense, with a flow-banding delineated by the 

 alinement of small pink feldspar crystals. In thin section it proved rather hard to interpret owing to 

 its denseness and opacity. It appears to consist mainly of straight-extinguishing feldspar microlites 

 ( .^ oligoclase) arranged in a wavy flow-banding, with somewhat larger feldspars (? orthoclase), and 

 quartz in smaller quantity. The feldspars are all highly sericitized. In this ground-mass material 

 there are embedded micro-phenocrysts of quartz, oligoclase, and a few pseudomorphs in pale green 

 fibrous hornblende of what may have been an earlier amphibole. As some epidote is always associated 

 with the oligoclase, the original crystals were probably of a more calcic composition. On the whole, the 

 rock has the mineral composition of a dacite. Perhaps an earlier generation of petrographers would 

 have called it quartz-porphyrite. 



1 Osann-Rosenbusch, Elemente der Gesteinslehre, 4th ed. 1922, p. 333. 



