256 ©. EH. Tilley—Para-Gnersses in South Australia. 
to the spinel grain, and the relationship is shown in the accompanying 
diagram (Fig. 2 (a)). 
The sillimanite shell is not optically continuous, nor does the 
optical orientation bear a constant relation to the external shape 
of the spinel core. 
(iii) Type y.—This type of garnet gneiss outcrops to the east of 
the graphite gneisses shown in the section. 
The rocks are fine-grained and microscopically have the 
appearance of quartzites. Garnet is present, but not in abundance. 
On the east they are associated with gneissic aplites, dis- 
tinguishable by their texture and appearance in thin section. Under 
the microscope the rock is seen to consist cf quartz orthoclase, 
biotite, garnet silliimanite, and a few grains of magnetite. 
Quartz is very abundant in the slides, and has characteristically 
arranged in rows at right angles to the banding inclusions which 
appear to be of the nature of fluid pores. 
These rows of dots are not only continuous through several 
contiguous grains of quartz, but also pass through adjacent grains 
of felspar. The phenomenon is clearly a result of shearing stress 
acting on the rock since its recrystallization, and the identical 
features of the Rohrsdorf granulites of Saxony are here recalled. 
The felspar is the turbid perthitic orthoclase already described. 
Biotite occurs in elongated flakes parallel to the foliation, and the 
garnet, usually quite colourless, is often elongated parallel to the 
foliation, indicating a development under considerable stress. 
Embedded in orthoclase, but with a streaky development, occur 
sillimanite needles, the orientation of the prismatic axes being 
parallel to the banding. 
There is no evidence of granulation along the lines oi silliimanite 
development. 
A variety of this type contains in addition a green spinel, and the 
rock is richer in garnet. Quartz is again the most abundant con- 
stituent. The felspar is the fibrous orthoclase, and plagioclase 
is rarely observed. 
The garnets are characterized by a habit, elongated in the 
direction of foliation. The elongation ratio of some of the apparently 
single grains may reach and exceed a value of 4:1. The significance 
of this will be dealt with hereafter, but it is clear that single grains 
have crystallized originally in a distinct elongated habit. 
Hercynite, or a member of the hercynite-pleonaste group of 
spinels, is present and invariably associated with the garnet. It 
usually forms a deep-green isotropic core to a garnet grain, or may 
appear intergrown with this mineral. The depth of colour is not 
constant, and a gradation to an intense dark-green almost opaque 
type is observed (Fig. 2 (0) ). 
In one core of a garnet grain the spinel appears to have undergone 
alteration to a serpentinous product, in which are set residual 
grains of the spinel. 
