Gp^ology and Petrology oi’ Part of Toodyay District, W.A. 115 
Biotite-oligoclase gneisses similar to those in the Upper Gneiss have 
been noted in narrow bands. Spec. No. 15389 is a type containing approxi- 
mately 25 per cent, of a brownish biotite. It has been analysed as typi(^al 
of these rocks (Table 2, No. 2). It differs from the microcline bearing gneiss 
in its higher iron and lower silica and ])otash (content. 
Under the microscope, oligoclase in subhedral grains up to 4 or 5 mm. 
<liameter is the dominant constituent. It shows line albite twhming, the 
lamellae often being slightly ciirved due to strain. A noticeable feature is that 
the (piartz moulded on such a strained crystal shows no undulose extinction. 
The brovv-nish biotite is in flaky aggregates winding around the Iarge?r oligo- 
clases ; they feather out against one another and have V>een squeezcnl together 
by movements in the semi-cTystallised magma. The microstructurcs recall 
in many ways the round grained ” gneisses of Glen Doll, Forfarshire (Ilarktir, 
1932, p. 298). 
Aj>atite is a very abundant ac.cessory, in small stout [jrisms, closely 
associated with the biotite ricli bands. 
These biotite-oligoclase granite gneisses a])])ear to be tlie result of the 
crystallisation of the residuum squeezed off! from the (uxrlier formed microcline, 
tlms containing a (H^ncentrate of biotite, oligoclase, and apatite. 
(e) XenoUlhs in the Lower Granite Gneiss. 
Numerous oval areas of foreign enclosures varying from several yards 
in diameter, to elongated masses up to 10 (diains in kmgth have been noted 
in the i^owor Gneiss. They are most abundant in the eastern part of the 
area (Plate 1). Most are poorly exposed and, witli several excej)tions, the 
contacts with the enclosing gneiss are obscured by soil. The mai>j)ing, hoW“ 
ever, indicates that they are lenticular bodies completely surrounded by 
granite gneiss. Tlie amphibolite enclosures are frequently traversed by 
quartz-felspar veins coming from the granite gneiss, thus establishing their 
pre-gneiss age. The longer axes and foliation of the entdosures strike jjaralle) 
to the banding in the surrounding gneiss. There is considerable variety 
amongst these xenoliths and the main ty[)es will be considered separately 
as follows : — 
(i) Kulysitic rocks.- Thono rocks are scarce in this area, but their occur- 
rciico is of interest since they boar similarities to rocks occurring as bands 
in the Bolgart greenstones to the n<jrth of Toodyay, and sinc^e they are rep- 
res(uitatives of the somewhat rare eulysites. 
They are found in a small xenolith 10 chains 8., 223 chains F. of datum 
(Plate I), entirely surrounded by gneiss. Near}>y xcmoliths are greenisli 
“ hornblenditos.” 
'^wo rocks were collect(;d from this locality and in view of their somewhat 
dif£(H’cnt cliara(T(T they will be described so])arately. 
Jianded (luartz- magnetite- hyper sthene rock (8'pec. No. 1 5451 ). 
This is a heavy, (^oarsedy- banded typ(j made u[j of bands (3 mm. wide) of 
silky lustr(‘d lamellar hyi)ersthene, alternating with darker magnetite bands 
fi'om 1 to 2 mm. wide. The rock is (^onsi(lerably weatliertid and iron stained, 
and its feeble magnetic (diaracter indicatt^s that th(i magnetite lias gone largely 
to fine granular martite. 
Th(j hypiTstliene is usually found with its longitudinal (d(;avago at about 
45° to the banding. It has a poikiloblastic structure, enclosing botli (piartz 
and magnetite, and the original bedding has been preserved in the wide bands 
of magnetite and the thin |)arallel bands of fine granular ore which traverse 
