C. BE. Tilley—Para-Gneisses in South Australia, 255 
the foliation are a typical feature. Graphite is invariably present 
in the serrated rod-like sections, the orientation being that of the 
banding. 
A finer grained rock of this type has the appearance in hand- 
specimens of a garnetiferous quartzite, but banding is still in 
evidence. 
In thin section the rock is seen to be considerably more quartzose 
than the type already described. Plagioclase is a more important 
constituent, and the fibrous character of the orthoclase is less 
marked. 
The garnets are characterized by a polygonal outline indicating 
a good crystal form. The banded character of this rock as seen 
under the microscope is strongly marked. The plagioclase shows 
curved albite lamelle and wavy extinction due to strain, and often 
contains blebs of orthoclase as inclusions. The composition varies 
in different grains from oligoclase-andesine to a calcic andesine. 
In a further variety the minerals sillimanite and green spinel enter 
as accessory constituents. 
The rock is very rich in quartz, and the fibrous perthitic character 
of the potash felspar, which is the next abundant constituent, is 
strongly developed. 
Fic. 2a.— x 50 diam. si = sillimanite, sp = spinel. 
Biotite is plentifully distributed with its elongation parallel to 
the foliation. The pleochroism is— 
Z = Y, dark brown. X, light yellow. 
The sillimanite is developed in colourless elongated prisms with 
the prismatic axis parallel to the foliation. 
These longitudinal sections show the characteristic cross parting 
parallel to the base. The elongation is positive. Usually the prisms 
are arranged in streaks and bands in the orthoclase, but may be 
associated with the garnet, which is present in irregular isotropic 
grains. 
Very closely associated with the sillimanite occurs a deep-green 
spinel, which must be regarded as a member of the pleonaste- 
hercynite group, and probably nearer hereynite in composition, as 
_the colour is a very deep green. 
Some magnetite grains are associated with sillimanite in a similar 
fashion. 
The sillimanite occurring in association with spinel forms a border 
