The Geology and Physiography of the Lawnswood Area. 
65 
Biotite is highly corroded and brownish in colour where associated with 
sericite and its pleochroisin is weak, X light brown, Y and Z = broAvn. 
Where associated with muscovite the biotite is invariably altered to green 
ehlorite. Sillinianite, variety tibrolite, occurs in bands of tine acicular 
aggregates and is largely alteiaul to sericite. Magnetite and sillinianite 
are accessory. Primary minerals appear to iiave been muscovite, biotite, 
and sillimanite; secondary minerals sericite, chlorite, and magnetite. 
The at)})roximate composition is muscovite 36 per cent., sericite 30 per 
cent., biotite 20 per cent, chlorite seven por cent., sillimanite five per 
cent., magnetite two per cent. 
(iv) Hybridised granitic gneiss. — Thei’e are several small occurrences 
of intermediate to basic rocks formed by the partial graniti^ation of the 
basic xenoliths. They vary from basic lioru])lende ])eg^^atites, Avith horn- 
blende, qiiartx. and felspar grains tAvo or three cm, in diameter, through 
coarsely Imnded dit>pside-plagioclas(' am])hihoHt(‘s Avith small scale ‘‘lit- 
par-lit’* injections of (iuart7. and microcdine from the granite, to strongly 
gneissic aci<l to intermediate rocks iiij(‘cted by numerous (inavtz A^eins 
(225291. 'file latter type is light grey in colour juid consists of an aggre- 
gate of tiiu*-grained, hlue-gi’ecn hornhlemle, (piartz, f('lspnr and diopside 
in bands one to three mm. Avide alternating Avith sill-like bands of (|uartz 
from one to four mm. Avidc'. A similar gneiss has l)een descrilied from, 
Toodyay (23, pp. 122-123). 
Origin of the (jraniti;' gncirfs on 'I assncl ded .rrnoUths . — This 
granitic gneiss is either a concordant acid intrusion or results from the 
granitizatioii of sediments. If the gneiss resulted from the granitization 
of an acid sediment (arkose) then such gi'anitization AA’ould he rxi)ected 
to transgr(*ss the stratigraj>hic horizons of the metas(‘diments and otlier 
metasediments such as mica scliists should also show the etfects of grani- 
tizaliou. As this is not llie (oise and as the granitic gneiss ('ontains 
different kinds of xenoliths it is considered to have been ititroduced ns 
a liighly viscous magma during the ])eriod of diasti’ophism. Plygmatic 
folding, protoclastie. structures, and the absence of preferred orientation 
in the quartz grains of these gneisses suggests that the magma Avas a 
viscous fluid, crowded Avith early formed jdienoci’ysts and xenoliths during 
the folding, the (juartz crystallising Avhen the tectonic activitv had ceased 
<23, p. 109). 
The Jiiagmn is considered to liave been inti'oduced at a considerable 
depth on tin* basis of the folloA\ing generalizalions of Bucher and Balk: 
^ ‘large concordant acid intrusi\ms in folded sediments are’ Avide spread 
only in early ])re-Cambrian terranes” (1, ]). 281); and ‘hh'ep levels in 
the earth.... and slow consolidation of tlie mass during continuous 
movement are probably amongst tin* factors which seem to result in thick 
foliated shells’^ (2, p. 81). The more pronounced platy parallelism of 
the gneiss near its contact Avith the metasediments is probably due to 
movemeu: of the quasi-solid granitic magma against the relatively 
stationary metasediments. 
The typical granitic gneiss A has an abundance of hydrothermal 
minerals — epidote, chlorite, and “saussurito’’ — Avhich Avere probably formed 
durijig the end stages of consolidation. Pegmatite and c(uartz veins are 
frequent, some ])Ossibly derived from the Younger Granite. 
