114 
Rex T. Prider. 
proportions of (juartz, magnetite, and grunerite. These rocks are similar 
to some occurring as xenoliths in the Lower Gneiss, which are described in a 
later section ((5) (e) (i)) of this paper. 
(c) Sunwiari/ of Cone/ unions regarding the Origin of the Upper Gneiss. 
The main mass is constant throughout and is a normal hiotite-microcline 
augen gneiss, derived from a ])orphyritic microcline granite. The presence 
of both st'dirnentary and igneous xenoliths and the occurrence of small trans- 
gressive apo])hyses in the underlying ([uartzite, ])oint clearly to its intrusive 
<'haracter. 
Tlie gneissic banding was developed when the rocks had partially crystal- 
lised (/.e., it is a fluxional foliation due to the alignment of the microcline), 
during which time {jrotoelastic structures were developed. The final crystal- 
lisation of ([iiartz took place after the tectonic movements had ceased. 
The microcline granite was intruded as a thick sill which has produced no 
a|)])arent thermal effects in tlie associated sediments which are in every instance 
(juartzites. Such rocks, however, would not be expected to yield any in- 
formation regarding the degree of thermal metamorphism induced by the 
granite. Felspathisation of quartzose rocks, about which a good deal has 
been written, is not developed in the Toodyay Area. It is not clear why the 
<juartzites al)ove this gneiss have different microstructures from those below 
the gneiss. If the granite we're a sill we should expect them to be identical. 
[d] The Lower Granite Gneisses. 
These rocks, outcropping in the north and north-east parts of the aiva, 
^'arv from tine, even-grained, well-banded tyjDes, to coarse porphyritic granites 
in which the banding is visible only in a flow oritmtation of the microcline 
phenocrysts. 
The well-baiuU'd varieties all have granoblastic structure with but little 
sign of granulation. A gneissic structure is evident in the sub-])arallel orienta- 
tion of the biotite and in the elongation of the felspar and quartz allotrio- 
mor])hs in a common direction. In these finer-grained gneisses the (piartz 
grains have an average index of elongation of about 3:1. 
The mineralogical composition of the Lower Gneisses is fairly constant, 
irrespective of whether the rock is fine-grained or coarse por[)hyritic. Tlie 
av('rage composition by volume is : - - 
Oligoclase (Ab^Aiii) .... 
30°o 
Microcline (sliglitly micropcrthitic) .... 
30% 
Quartz .... 
30“„ 
Biotite .... 
I0°o 
with accessory apatite, magnetite, and more rarely sphene and e[adote. 
Minor variations due to a greater abundance of biotite together with an 
increase of the pro],)ortion of oligoclase to microcline (as in the Upper (ineiss) 
have been noted. Also a hornblende bearing granite gneiss is develoj)ed by 
liybridisation of the granite* by the hornblendic xenoliths which are present in 
some abundance. 
The main point of difference from the Upper Gneiss lies in the general 
absence of eataclastic (or protoclastic) structures. 
The microcline ])henocrysts which in the Upper Gneiss are represented 
by lenticular "eyes” are he*re seen to be well-shaped, uncrushed crystals (up 
to I inch in length). 
