C. E. Tilley—Para-Gneisses in South Australia. 257 
The garnet may sometimes contain minute rods of ilmenite (*), 
and scattered through the rock are opaque leucoxenic grains of the 
same mineral. Little prisms of sillimanite with their longer axes 
parallel to the banding are sparingly present in the orthoclase, or 
in some cases associated with the garnet. 
Fic. 2b.—x 50 diam. gn = garnet, sp = spinel, or = orthoclase. 
(iv) Type 6—Amongst the complex of metamorphosed sediments 
present in the Hutchison area, there are certain garnet gneisses, which 
in their field relations and microscopical character closely resemble 
the garnet gneisses of Sleaford Bay. 
They are typically developed in the Waterfall Creek, and bound the 
dolomites of this area on the east. 
These garnet gneisses are conformably bedded with the dolomites 
and strike north-east with a vertical inclination. 
Those types which are allied to the Sleaford Bay occurrences will 
be here described. 
They are dark-grey rocks with prominent garnets of a brownish- 
red colour, showing dodecahedral outline, and may be developed 
up to half an inch in size. Banding is a prominent feature. 
Silimanite may be detected in some specimens, with the aid of 
a lens, in long fibrous needles. 
In places they have been invaded by quartz and pegmatite 
solutions, giving rise to lenses parallel to the foliation. 
A rusty weathering appearance may be given to these rocks by the 
alteration of garnet to a limonitic product. 
Under the microscope the typical sections are seen to consist of 
orthoclase, quartz, garnet, biotite, sillimanite, plagioclase, ilmenite. 
Orthoclase is the most abundant constituent. The fibrous character 
is not in evidence, but perthitic spots are present, characterized 
by higher refractive index and birefringence—essentially an albitic 
plagioclase. 
The orthoclase shows considerably undulose extinction, and the 
periphery of the grains is marked by granulation giving the typical 
mortar structure. Quartz is never in large grains, but forms small 
individuals in bands separated by felspar. 
VOL. LVIII.—NO. VI. 7 
