62 
J. K. H. McWhae. 
3. Granitic Gneiss and associated xenoliths. 
The granitic gneiss is in two thick sills continuous with the Toodyay 
Area. The lower granitic gneiss outcrops in the north-east of the Lawns- 
Avood Area while the upper granitic gneiss covers its centre and has been 
-Studied in some detail. 
(a) Upper (jranitio gneiss. 
This gneiss is almost identical with the one described by Prider at 
Toodyay (23, p[». 107-111) except for the absence of augen structure. 
There appear to be two main types; — (i) Granitic Gneiss A (ehloritic) 
which is a medium- to coarse-grained, greyish gneiss, with widely spaced 
microcline jihenocrysts (two cm. diam.) and is characterised by the presence 
of chlorite and (-pidotej (ii) Granitic Gneiss B (biotitic)^ which occupies 
the edge of the sill near the metasediments, being characterised by a finer 
grain, a nuu-e strongly developed giieissie structure and the presence of 
biotite. 
(i) Granitic Gneiss A (ehloritic) {22479)— The type rock has a 
coarsely gneissic structure sometimes Avith slight cataclasis and the fol- 
lowing minerals: — slightly saussuritiscd oligoclase, clear microcline, quartz, 
■chlorite, epidote {often in veinlets), and accessory apatite, magnetite, and 
zircon. The chlorite occurs in .Avell developed plates frequently Avith 
puri)l(‘ zircon inclusions surroiuuhul hy strong pleochroic luiloes. It is 
thought to 1)0 pseudomori)hic after biotite because ot* its form and the 
purple zircon inclusions which are common to both. Its optical properties 
are: — ])leochroism X = very pale yellow-green, V and Z = green, absorp- 
> tion Is S --- /, birefringence very low, anomalous blue colours frequent, 
■elongation ])Ositive, optically negative, j3 = 1.025. These data indicate 
the variety as aphrosiderite Avhich has a composition similar to biotite. 
Approximate composition is oligoclase 35 per cent., quartz 30 per 
cent., microcline 25 pev cent., chlorite fiA^e per cent., epidote fi\’e per cent. 
(ii) Granitic Gneiss B (biotitic) (225/4) is characterised by biotite 
in very elongated [)lates rather than chlorite and epidote psendomorphie 
-after biotite as in (i). The biotite ])Iates arc crowded with sagenitic rutile 
inclusions and have .strong phmchroisin X = yellow, Y = dark brownish 
green, Z — very dark broAvn, absorption X <. ^ * Z. Miei'ocline pheno- 
crysts are absent but some larger grains (tAVo to three mm.) slightly 
kaolinised and with myrniokite frequently developed in associated oligoclase. 
probably represent early- formed microcline, AA'hile smaller (0.5 mm.) 
clear grains, strongly eross-hatehed and containing microperthite are prob- 
ably a later generation. The quartz generally occurs in rounded grains 
poikilohla.stieally enclosed in all the other minerals. 
The approximate comjoosition i.s microcline 35 per cent., quartz 35 
per cent., oligoclase 15 per cent., biotite 15 per cent. 
(})) Xenoliths in the granitic gneiss. 
These are of amphibolites, except for one of sillimanite-mica schist 
and some of chlorite-epidote rock. Generally they are large, up to two or 
three chains in major diameter, and irregular, and are elongated parallel 
to the regional strike. 
