258 C. #. Tilley—Para-Gneisses vn South Australia. 
Garnet may have a quite well-developed crystal outline. These 
grains reach 3mm. in diameter. Owing to a prominent develop- 
ment of the dodecahedron the crystals are mainly rhombic and 
hexagonal in cross section, although elongate types are also 
occasionally developed. Through the prevalent occurrence of many 
inclusions of quartz, orthoclase, and biotite, the garnet shows the 
typical sieve structure. Green spinel may be intergrown with the 
arnet. 
; Along cracks biotite is developed (PI. III, Fig. 1), and in some cases 
this is replaced by chlorite. This represents a degradation of the 
original garnet. In the angles of the grains biotite and quartz are 
often thickly developed, and this type of association is one of genetic 
significance. It is distinct from the development of biotite along 
cracks in the garnet, but is related to this change in that it represents 
a reaction which is a reversible one, the minerals involved being 
biotite and quartz on the one hand and garnet and orthoclase on 
the other (cf. equation in a later section). 
The biotite is the characteristic red-brown variety, with intense 
pleochroism. 
Sillimanite is an abundant constituent, and may form a zone 
of slender prisms at the periphery of garnet grains, a relationship in 
many cases of genetic significance. More characteristically it is 
present in streaks through the rock parallel to the banding and 
associated with orthoclase or biotite. The cross parting parallel 
to the base is a dominant feature. 
Plagioclase is present in quite subordinate amount. It shows the 
albite lamellation, but the twinning lamelle are usually bent owing 
to strain. The R.I. is greater than quartz. The majority of grains 
are positive, but a number show negative birefringence. Extinction 
angles from the albite lamelle are quite unreliable owing to the 
shadowy extinction. The composition of the plagioclase is therefore 
at least as calcic as andesine. 
This rock has suffered crushing, as indicated by the pranounees 
wavy extinction and granulation of the felspar. 
In a varietal type containing a larger percentage of plagioclase 
the abundant porphyroblasts of garnet are intersected by a set of 
parallel cracks coincident with the foliation, and not normal thereto, 
asis usually thecase. This structure implies the action of mechanical 
forces, the direction of the maximum pressure being at right angles 
to those in operation during the foliation and recrystallization of the 
rock (PI. III, Fig. 2). 
The fracturing of the garnets has evidently been induced in an 
epoch subsequent to the recrystallization, and may be connected 
with the pronounced crushing these rocks have undergone as revealed 
by the extensive granulation of the felspar. 
Interbanded with these rocks occurs a garnet gneiss, showing 
coarse felspar porphyroblasts as well as garnet. 
Under the microscope plagioclase is seen to be the dominant 
felspar, while orthoclase is the subordinate type. 
