952 C. #. Tulley—Para-Gneisses in South Australia. 
The members of the Hutchison series have been penetrated by 
sills of granite and pegmatite, and by veins of pegmatite and quartz, 
and composite rocks have in some cases thus arisen, consisting of 
bands of sediment alternating with igneous material. 
3. THE GARNET GNEISSES. 
(a) General Description. 
The typical exposures of the garnet gneisses occur in the section 
immediately to the east of the massive beds of diopside-rock, which 
forms here a prominent headland on the coast. The gneisses are 
interbanded with the calc-magnesian silicate rocks (diopside- 
microcline-scapolite type) already referred to. The rapid change 
in composition of the rocks across the strike in this locality is 
remarkable. The garnet gneisses are conformably bedded with the 
calc-silicate sediments, striking north and dipping at a high angle— 
vertical to 80 degrees to the west. The most marked feature of the 
garnet gneisses as seen in the field is the parallel banded structure, 
which is essentially a structure arising from initial differences of 
composition in the original sediments. In certain cases banding 
has arisen due to the lit-par-lit intrusion of pegmatitic veinlets in 
the series, but a parallel structure arising in this way can be readily 
distinguished from the parallel banding due to original hetero- 
geneity in the sediments. Additional garnet gneisses which enter 
for description here are developed to the east of the graphite gneisses 
(cf. section Fig. 1). 
Here the banding is still evident, but garnet does not form such 
a prominent constituent. These are associated on the east with 
gneissic aplites, which in the field resemble these rocks in some 
particulars, but a study of thin sections reveals an essential 
distinction. 
A bed of garnet gneiss with a well-developed parallel banding 
is present on the shore to the east of the sill of hornblende granite 
which borders the forsterite marble. Itis separated from the granite 
sill by a thin bed of graphite schist. 
(b) Petrography of the Garnet Gneisses. 
(i) Type a—The three types of garnet gneisses will be 
distinguished in this description by the prefixes a, 8, y, and this order 
is one of exposure from west to east. Their positions can be readily 
made out by reference to the sections. Type a is a banded rock 
of greyish colour and, often with rusty weathered faces. In hand- 
specimens quartz, an unstriated felspar, garnet, and biotite can 
be detected, and also sparing flakes of graphite. 
The parallel banding can be seen even in small fragments, as the 
width of the individual bands seldom exceeds in. The rose-pink 
garnets often show well-developed crystal outline (dodecahedra), 
and on weathered faces give rise to brown limonite. The rusty 
