(Ieology and Petrology of Part of Toodyay District, W.A. 101 
t ingui.shable only by the inclined extinction (Z/\c = 17°) of the oiimmingtonito 
as (compared with the straight extinction (Z = c) of the gedrite. The former 
oft(‘ji shows lamellar twinning. The optics of the latter : — 
Pale clove-brown, non-pleochroic, X = a, Y = b, Z -- c, (-) 2V 
large, y — DOOV, ^8 = 1*659, a = 1*653, indicate the aluminous 
variety, gedrite. 
'Fhe light-coloured bands are composed of cummingtonite (with gedrite) 
- plagioclase -|- ({uartz, while the darker-coloured bands are much richer in 
cummingtonite and contain little or no ({uartz, and in some still darker bands 
gre(‘n hornblende becomes an important constituent. 
vS])ecimens transitional in character between (6) and (c) are present in the 
same outcrop (see figim^ 8), and in these specimens narrow dark bands of 
garnet-biotite-plagioclase schist (often containing prisms of g('drite) alternate 
with light-coloured cummingtonite plagioclase^ rock. 
A noti(teable feature is that the ampliibole in the biotitic bands appears 
to b(‘ all gedrite, whereas this mineral is subordinate to cummingtonite in 
the lighter-coloured layers. 
All the above types («)» (^)» (c) found in the lower band of mica 
schist. They have been derived from argillaceous sediments of variable 
composition ; plagioclase becomes evident in the slightly calcareous bands. 
The various associations of sillimanite, biotite, (juartz, plagioclas(‘, and 
cordierite noted above are all well known. The g(^drite-cummingtonite- 
biotite-oIigoclas(^ association is much rarcT. Amongst the best known com- 
parable rocks are - 
(!) The gedrito-plagioclasites of the Kragero region, Norway, described 
by Brogger (1935, ])p. 213-325). Although generally massiv(', 
schistose types have been described from this district. Thcvse 
rocks ar(' (jonsidered to be derived from basic igneous ro(*ks in a 
similar fashion to the gedrite (and anthophyllite)-cordiorite rocks. 
(2) 'Phe ])lagioclase-gedrite gneisses of the Nesodden Peninsula n(‘ar Oslo, 
described by Brock (1926, p. 180) in which the association “ (juartz, 
plagioclase, biotit(', garnet, gedrite agrees very closely with that 
(>bserv(*d in the Toodyay rock. Brock considers that these rocks 
have b(‘cn derived from leptites by addition of FeO and MgO. 
(3) 'Phe garnet plagioclase amphibolites of Isopiia, Kalvola, Finland, 
(l(‘S(Tibed by Fskola (1936, p. 475). P^skola in this pa]K‘r d('scribes 
a paragenesis of gedi’ite and antho])hyllite, an association similai* 
to that noted in the Toodyay schist, and he considers that the 
g(‘drite-cinnmingtonite-garnet amphibolite of Isopaii has becui 
formed by metasomatic replacement of basaltic agglomcTates. 
'Pile gedrite-cummingtonitc’-biotite-plagioclase schist from Toodyay, from 
its int('rlamination with sillimanite schists and tlu' rhythmics banding obs(‘rved 
in the specimens appears to be a metascHlimentary rock, deri\’('d probably 
from a tliin bedded sediment such as an ini]')ure dolomitic shal(‘. 
{(1) Andahjsite schists. -These are confined to the uppermost })elitic band 
which outcrops in the S.W. portion of the ar(‘a. The rocks of this band are 
(juartz-muscovdte schists with a variable biotit(‘ cont(ait. All the outcro])s 
are highly weatla'red, the rocks aj)pearing as yellow and reddish iron-stained 
schists. Simpson (1936, p. 11) has noted the occurrence of sillimanite (fibro- 
lite) in these andalusite schists but it appears to be very rare, as I have noted 
the presence of sillimanite in one specimen only. 
