(]1eology and Petrology of Part of Toodyay District, W.A. 99 
composition is quartz 10 per cent, oligoclase 35 per cent, silli- 
manite 20 per cent, biotite 30 per cent, muscovite 5 per cent, 
and accessory magnetite and zircon, but in certain bands the 
sillimanite content rises to 70 per cent or more of the rock which 
thus becomes a valuable sillimanite ore. 
In the psammitic bands both c(uartz and oligoclase are xenoblastic. 
I’he felspar is invariably zoned, the zoning being reverse gradational 
in character from Abg An^ centre to Ab 3 An^^ periphery ; twinning 
is seen in the central parts but the outer zone is generally un- 
twinned. The outer zones of the plagioclase grains also carry 
numerous vermicular inclusions of (juartz, forming a myrmekite- 
like structure. The development of this structure in the absence 
of microcline is interesting — it appears to be due to the growth 
of the plagioclase around crushed (juartz spindles. 
The sillimanite is in two generations (1) as fine acicular clusters, 
(2) stout cross fractured rods, the arrangfunont of which is unusual 
(figure 7B). In any section they appear to be arranged in bundles, 
some' lying in the plane of the slice, others arrangcnl normal to or 
at an angle with it. 
The biotite is a deep rc^ddish-brown lepidomelane in im‘gular shaped 
flakes with but little tendemey to parallel orientation. In addition 
to inclusions of sillimanite there are numerous minute zircon 
inclusions, surrounded by pleochroic haloes. 
Muscovite is in larger plates (Imm.) carrying poikiloblastic in- 
clusions of (juartz and oligoclase. It is pc'netrah'd by sillimanite 
rods but is idioblastic towards the biotite. 
Magnetite and zircon are the only accessories. 
(iii) Cordierite-sillimanite schists {e.g., specimen 15683). This sj)('cimen 
is the only metasediment of this area in which the preseiKfe of 
cordion'ite has been noted. Ther(‘ is no sign of schistosity in the 
rock, either in hand specimen or in the field occuirix'nce. Con- 
stituents noted microscopically were quartz, biotite, muscovite, 
sillimanite', and pinite (after cordierite). The sillimanite is found 
as small tufts of needles closely associated with a deep rc'ddish- 
lirown biotite and radiating out into the neighbouring ejuartz. 
This variety is abundant and appears to be developing at the 
('x])('nse of the biotite. Sillimanite occurs in a second g('neration, 
in stout('r rods haphazardly includc'd in a pale brownish practically 
isotropic pinite (after cordierite) ; in section these jiinitic areas 
mc^asure up to 0-5 mm. diameter. There is no definite oric'nta- 
tion in the micas. Plagioclase is absent. 
The ajiproximate mineralogical composition is -biotite 35 p('r cent, 
sillimanite 25 p('r cent, (juartz 25 p(T ccnit, pinite U) jier ceiit, 
muscovite five p(T (^ent, and ac(^('SSory zircon and magnetite. 
(6) (kirnvt-hiotite-plagloclase schist.- occunence of garnet in the mica 
schists has only been noted in one place (15 cliains >S., 122 chains \V. from 
datum). H('r«i the garnets are found in a very sandy schistose rock, which, 
under the microscope, is seen to consist largely of zoned plagioclase. The rock 
is medium grain('d, schistose and granular in structure, with porphyroblasts of 
pink garnet up to 3 mm. diameter. The greater part of the rock consists of an 
ecjuigranular completely recrystallised aggi*egate of jilagioclase, cjuartz, and 
biotite with an imperfect schistose structure due to tJie subparallel alignment 
