326 Transaction s . 



Class II. — Igneous Rocks of Hypabyssal Origin. 



1. Porphyry. 



The typical rock is found across the channel of the harbour, opposite 

 Starling Point, where it forms a fringe bordering the tongue of sand 

 which bears the name of Tawaewae Point, and which is really the north 

 head of the harbour. 



The outcrop extends below low-water mark, but above that line its 

 width is only 15 or 20 yards. The rock is traversed by joints which 

 divide it into more or less oblong blocks of a variety of sizes. One set 

 of these joints strikes approximately north-west to south-east; the other 

 set crosses at right angles. The dip varies from 0° to 30° X.E. The 

 total length of the outcrop is about 16 chains. 



II and- specimen (specific gravity = 2"5). — The rock is dense, and when 

 freshly broken is of a light-grey colour. The weathered surface, how- 

 ever, is of a dirty brownish-yellow colour, and from it project numerous 

 crystals of feldspar. 



Under the Microscope. — Thin sections show phenocrysts of feldspar in 

 a groundmass consisting of feldspar, quartz, hornblende, and mica. 

 Magnetite also occurs, partly in masses of irregular size and shape, and 

 partly in small crystals. 



The phenocrysts of feldspar vary considerably in size, some going up 

 to as much as 2'4mm. by 1*2 mm., but the average size is 0*9 mm. by 

 0'6mm. They are chiefly orthoclase, and show twinning after the Carls- 

 bad law in nearly every case. Less common are phenocrysts of a plagio- 

 clase variety. These show the albite twinning very poorly developed, and 

 I have no section in which an absolutely satisfactory identification may 

 be made. The available evidence, however, points to albite. 



None of the phenocrysts are entirely fresh, while many bear in a 

 marked degree the signs of decomposition, and all stages between the 

 two extremes are represented. The first stage is a cloudiness which 

 spreads irregularly over the crystal, and associated with it is the deposi- 

 tion of a very fine dark-coloured opaque dust. Then appear minute pale 

 colourless microlites, which as they increase in size assume a pale-green 

 colour, and are distinguishable as hornblende. As the microlites increase 

 in size and number, larger and more definitely shaped crystals of mag- 

 netite appear. The needles of hornblende grow at the expense of the 

 feldspar, for they penetrate through and through the crystals of this 

 mineral, and also appear in great number round the edges of crystals, 

 where they finally arrange themselves in aggregates. As mineral change 

 becomes more and more complete small grains of quartz and flakes of 

 brown mica appear. Finally we see a cloudy space, recognizable by its 

 size and shape as the ghost of a feldspar, containing needles of horn- 

 blende, grains of magnetite, and quartz and flakes of biotite. 



The groundmass is partly crystalline and partly glassy. The crystal- 

 line portion consists of grains of feldspar and of quartz, crystals of 

 hornblende, and small flakes of brown mica. The grains of feld.spar 

 are rather rounded in shape, much decomposed, and many show undulose 

 extinction. The decomposition is associated with the deposition of the 

 fine dust above mentioned and with the formation of hornblende. Quartz 

 is in rounded grains, ranging up to 0"08 mm. in diameter. 



Chemical Composition. — It was found impossible to obtain an analysis 

 of a true porphyry similar to that of the Tewaewae Point rock. Com- 

 parisons with rocks from American and European localities are niven 



