C. HE. Tilley—Para-Gneisses in South Australia. 258 
weathering sometimes developed in these rocks is due to this 
degradation of garnet. The bands of quartz and felspar with garnet 
are separated by thin layers of biotite. The garnets reach tin. 
in diameter, but average under din. Lenses of pegmatite with their 
long axes parallel to the surface of banding are occasionally present, 
and are readily picked out by their coarse texture. 
In thin section the following minerals are seen to be present: 
Quartz, orthoclase, biotite, garnet, ilmenite (with leucoxene), 
plagioclase, graphite, and zircon. 
Within the limits of the field of view of the lin. objective, the 
parallel banding of this rock is a marked feature. The mineralogical 
composition of the bands separates them into quartzose and 
felspathic layers. 
Biotite occurs in elongated flakes, showing parallel orientation. 
The pleochroism is strong, and the scheme is— 
Z=Y, red brown. X, pale straw yellow. 
Faint pleochroic haloes surround small. inclusions of zircon. 
The principal felspar is orthoclase, often intergrown with quartz. 
It is very characteristically a fibrous type, and is the dominant 
feature of the orthoclase felspar of all the garnet gneisses of this 
area. 
The fibrous appearance is due to the presence of minute grains 
arranged parallel to the face (100), and the fibrous character dis- 
appears in many sections showing straight extinction. These features 
indicate the presence of an intergrowth which is probably perthitic, 
but the minuteness of the grains prevents the determination of other 
properties. In addition, in some of the orthoclase grains there are 
large spindle-shaped grains of a colourless mineral of higher relief 
and birefringence. They are oriented slightly obliquely to the 
perthitic type. In some cases these are certainly quartz, but in 
others represent an acid plagioclase. The quartz grains are 
characteristically elongated parallel to the banding surface, and may 
show rows of fluid pores at right angles to their elongation. In some 
sections the average elongation ratio reaches a value as high as 3:1. 
Plagioclase is not abundant. It shows albite and pericline 
lamellation. The refractive index is about the same as quartz, 
but lower than the highest of quartz, and the sign of the birefringence 
is negative. The properties are those of an oligoclase-andesine. 
Myrmekitic intergrowths of plagioclase and quartz occur at the 
junction of plagioclase and orthoclase and often extend in a bay 
into the potash felspar. 
Garnet is present in colourless porphyroblasts of rounded outline, 
with the typical quartz enclosures. It is usually quite isotropic, 
but an occasional grain shows anomalous birefringence, in one case 
with twinning lamelle. The grains reach 3mm. in size, but are 
usually smaller. The garnet is an almandine type, as might be 
expected in rocks of this class. Thisis confirmed by the characteristic 
