REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



129 



brown hornblende well characterised by their contours, their cleavages, and their 

 extinction. Augite is present as greenish sections. These two minerals are rather 

 uncommon as large crystals in the rock just described. They bear traces of fusion or 

 corrosion, their outline being rounded by the action of the vitreous magma. By the use 

 of the highest powers little microliths of augite are seen in great abundance ; felspar is 

 extremely rare. 



There remain to be described some rocks collected in the bed of Charmer River 

 which flows into Eoyal Sound. The specimens are flat - rolled pebbles of augitic 

 trachyte, which contrast by their grey colour with all the other rocks that have been 

 described. Crystals of sanidine visible to 

 the naked eye appear in a greenish grey 

 ground-mass ; small prisms of augite may be 

 distinguished with the lens. These stones 

 have an indistinct schistoid structure, and 

 microscopic examination of thin slices shows 

 them to be microporphyritic. This structure 

 is determined by large sections of sanidine 

 of irregular outline, and by an aggregation 

 of little green crystals of augite, which 

 imitate by their general appearance crystals 

 of hornblende whose place they take. These 

 augitic pseudomorphs of hornblende are 

 accompanied by numerous grains of mag- 

 netite. The hornblende has, as a rule, 

 entirely disappeared, and zeolites fill the 

 spaces between the microliths of augite 

 (fig. 21). Sometimes, however, at the centre 

 of the aggregation there remains a brownish, 

 very pleochroic remnant of hornblende. 

 The ground -mass is composed of rather 

 elongated lamellae of sanidine, twinned according to the Carlsbad law, pressed against 

 each other, but still exhibiting a certain linear arrangement suggestive of fluidal 

 structure. The lamellae are sometimes less regularly disposed, forming a network ; the 

 forms of the felspar microliths in the ground-mass are less distinct. Almost all the 

 constituent minerals are surrounded by a zone of green microlithic augite. Titanite 

 is often present. A fibro-radial zeolite, showing the black cross of spherulites, lines the 

 hollows and penetrates the spaces between the minerals. 



Mr. Buchanan describes a peculiar hill at the other entrance to the Sound, almost 



Fig. 21. — Augitic trachyte from Royal Sound. 



Small grouped crystals of augite, imitating as a whole the 

 form of a hornblende crystal, whose place they fill. This 

 replacement of hornblende by augite has been accompanied 

 by the formation of numerous grains of magnetite, and in 

 the centre of the group of augites a small brownish pleo- 

 chroic remnant of hornblende may be seen. Usually, as in 

 the drawing, this mineral baB quite gone, zeolites filling the 

 interstices between the augite microliths. Js crossed nicols. 



(PHTS. CHEJI. C'HALL. EXP. — PART VII. — 1889.) 



17 



