Geology and Petrology of Part of Toodyay District, W.A. Ill 
(ii) Microcline-granite gneiss.— -The mica is much less abundant than 
in type (i). These rocks are rare and occur in narrow seams. 
They consist essentially of microcline, quartz, plagioclase, and 
accessory biotite, and, therefore, probably represent an originally 
aplitic phase of the granite. The microcline is slightly micro- 
pc^rthitic, and the less abundant plagioclase is a clear, finely- 
twinned oligoclase (AbggAn^g). Such mica as is present is the 
same greenish-brown partially chloritised variety as described 
above. 
(iii) Biotite-oligoclase-granite gneAss . — In this type, which is of rare oc- 
currence, there is a much greater development of brownish biotite 
than in (i) above. The main felspar is oligoclase, and microcline 
is present to the extent of less than 10 per cent. The approxi- 
mate mineralogical composition of a typical specimen (9577) is — 
Quartz 
20 
volume 
Oligoclase (Ab 4 An|) 
65 
% 
Microcline .... 
5 
o/ 
0 
Biotite 
10 
0/ 
0 
Accessory rutile, zircon, and apatite. 
The oligoclase is in subhedral crystals averaging 2 mm. diameter, with 
oscillatory normal zoning and average composition Ab 4 Ani. Occasional 
plates of microcline are present and, when in contact with oligoclase, myrme- 
kite is developed. 
The biotite is a brownish, non-chloritised variety, but still carries the 
characteristic sagenitic rutile inclusions. 
(6) XenoUths in the Uppe?‘ Granitic Gneiss. 
(i) Schistose plagioclase amphibolites .- — These are found in narrow irregular 
bands, varying from small stringers, several inches wide, which taper out 
quickly, to well-defined bands up to one chain wide which usually run parallel 
to the foliation of the gneiss. The occurrence in them of acid veinlets suggests 
that they are inclusions, rather than sheet-like intrusions, in the gneiss, and 
they often show a minute crumpling which accords with the view that they 
are xenolithic, rather than post gneiss intrusions, as there have been no major 
tectonic movements since the final consolidation of the granite gneiss. 
There is little variation in these rocks and they are almost identical, both 
mineralogically and chemically, with the plagioclase amphibolites, already 
described, which occur interleaved with the meta-sediments. The main 
points of difference may be summarised : — 
(1) Xenoliths in the gneiss have hornblende with a much stronger 
pleochroism : — X yellow-green ; Y deep olive green ; Z deep 
bluish-green ; and absorption Y Z X. 
(2) The plagioclase is more saussuritised and less abundant in the 
xenoliths. 
(3) Microline is 'absent from the xenoliths and quartz more abundant. 
(4) Magnetite, rimmed with sphene, spread out into trains parallel to 
the schistosity, is more abundant in the xenoliths. 
A specimen of contorted hornblende schist (No. 1241) from a xenolith in 
the granite gneiss was analysed and the result is seen in Table 1, Column 2. 
The main points of difference from the previously described hornblende schists 
lie in the lower Si02 and corresponding higher alumina in the xenolith. The 
