MAGNETITE GARNET ROCKS COULSON. 289 



Magnetite has crystallised out later than the biotite and it appears to have split 

 off fragments of biotite by crystallisation between the cleavage flakes. The magnetite 

 does not appear to have been derived from the biotite as there is a general absence of 

 any minerals accompanying such a change. Flakes of green chlorite with a magnetite 

 are rare. Garnets usually show round edges due to resorption. Quartz is abundant 

 and shows strain polarisation. Apatite is present, and possible cyanite. Little 

 silliiiianite and no felspar can be observed in the section on account of the very variable 

 distribution of the minerals in the rock. Fragments from the hand specimen show that 

 the felspar is an acid plagioclase. Cordierite gives rise to a yellowish alteration product. 

 ' )ne large zircon crystal, -20 x '15 mm., with uniaxial figure and high colours, is observed. 

 The structure is heteroblastic and the rock is best described as a Magnetite-Gamet-Gneiss. 



No. 181. This i:; ;: dii'iiuctivc and banded variety with numerous dark porphyro- 

 blasts of magnetite (Plate I, fig. 1). Their longer axes are parallel to the well-developed 

 sdiistosity and at times measure 14 x 7 mm. One large elongated crystal of tourmaline 

 is noted which possesses the trigonal cross-section and characteristic absorption. A 

 green chloritic mica is the only other mineral occurring in large masses. A similar rock 

 is illustrated (Plate 1, fig. 2), in which the magnetite porphyroblasts have been elongated 

 into a pronounced linear structure. 



Microscopically the rock is composed of bands of minute quartz and garnet, 

 quartz, and felspar and biotite crystals. The magnetite individuals are studded with 

 numerous inclusions of garnet, biotite, quartz, and altered felspar. A little rutile is 

 intergrown with the magnetite and occasionally there is a slight alteration to limonite. 

 Garnets are numerous but their size is small ; some of the grains, especially the smaller 

 ones, show anomalous double refraction. When a band of quartz and garnet meets a 

 magnetite porphyroblast, it divides into two and surrounds the porphyroblast. Green 

 biotite is common and associated with green chlorite. There is a distinctive yellowish- 

 brown biotite which is rare relative to the green variety. Muscovite is present in small 

 amount. Plagioclase and orthoclase are present and the former ranges down to andesine. 

 The felspar is frequently kaolinised. Epidote is present. Some crystals suggest 

 cordierite but there is an absence of brown pleochroic spots. Quartz is abundant, while 

 apatite is a common accessory. Xo tourmaline is present in this section. The rock is 

 a Porphyroblastic-Magnetite-Garnet-Gneiss. 



No. 304. This is a schistose rock, to which is attached, like No. 147, part of a 

 coarsely crystalline vein or lens composed of quartz, felspar, garnet, magnetite, and also 

 micaceous hematite. 



Magnetite forms 11-9 per cent, of the rock and it contains inclusions of garnet, 

 biotite, apatite, quartz, and cordierite. Hematite is present and a little limonite has 

 developed from the magnetite. No ilmenite is noted and no reaction obtained for 

 titanium. Garnets form 7-9 per cent, and are usually idiomorphic. Biotite is abundant, 

 forming 14-7 per cent., and frequently includes quartz and zircon. Quartz is abundant 

 and contains numerous inclusions of garnet, magnetite, and zircon and also fluid 



