Geology and Petrology of Part of Toodyay District, W.A. 01 
sections normal to the bedding the mica appears in innumerable minute paralh'l 
rods. Sections parallel to the bedding (figure 3B) show tliat the minute mica 
flakes are all euhedral and the marked lineation noticed in thi' hand specimi'n 
becomes evident under the microscope in the elongation of tlu^se flakes in 
a common direction. 
The mica, in hand specimen, is a pale bluish-grein colour, but under 
the microscope it appears colourless, except in thicker sections, when it has 
a bright light greenish colour with pleochroism - 
X pale bluish ; Y Z pale yellow-green ; absorption X ^ Y Z. 
^ Dti06 ; ( -)2V 31°~33° ; Dispersion distinct r > 
A determination of the Cr .>03 content of mica isolated from a narrow 
band (2 inches wide) of chrome mica in ojk' of the Lower Quartzites gave — 
0-22 p(T cent. 
This is considerably lower than the CT. 2 O 3 of tlu‘ normal clirome muscov'ites 
cjuoted by Do('lt('r (1917, p. 42S). Of the six analys(‘s (|uot(‘d, the lowest, 
^ is 0-S7 p(T cent, and it goes as high as 3*95 per cent in the original 
fuchsite from Zillerthal. Hutton (1940, p. 330H) has di'scribi'd a chromic 
musco\'it(' (H)iitaining 0-27 ])('r c('nt Gr .203 and has not(‘d liow a v(‘ry 
small chromic contcait is sufli('i(‘nt to )>ro(luce a bright gr(H 3 i colour in a thick 
flake of the miiK'ral. 
Partridge (1937, p. 457) has recently describt'd a similar mica from Maslii- 
shimala, Transvaal. It (‘arries 0-S5 p(‘r cint C'ro 03 and Partridge considc'rs 
that chrome muscoviti' is a bi'tter name for this mim'ral than fuchsite as it 
was })r(“viously termed. In its o])tics : Pleochroism X blue, yellowish 
p-eim, Z bright gret'ii (bluish), )3 = 1-590, (-) 2V 35° : it agrei's ciosi'ly 
with the mica in th(' Toodyay (juartzites. The (-hrome muscovite, ai^cording 
to Partridges occurs (juite commonly in the <{uartzit(>s of tin* Swaziland Systean 
in South Africa and it is tlu're'fore similar in occurrence' to the' e-hrome'-muscovite 
in the Toenlyay e|iiartzite*.s. 
The' source of the chromium, whie-li is so wiel(*ly disse'ininate'el througii 
these ejuartzite^s, is unknown -no chrome-be'aring detritals ha\e' be'cn nott'el 
in those rocks anel ne) ultrabasic re^cks (e)the'r than se'\ e‘ral small beielies youngea* 
than the (juartzite's) are known in the area. Partrielge' (1937, ]). 459) has 
noteel the' [ire'sence of traces of chromium in minerals e-onnecte'el with, and 
ceintaineel in, the granites e>f North-Last Trans\'aal, anel alse> that the oce-ur- 
reaiea's of chrome-musceivite are' conne'e'te'el with the' granite inti’usions anel are> 
founel einly in close proximity tei the' gi-anite'- -he cone-luele's, the'refore, that 
the e‘hrome-musce)vite is gene'tie*ally relateel to the granite'. 
An intere'sting assoe*iation of teiurmaline' with the e-hronu' muscovite was 
note'el in the' ejuartzite\s in the Gorge at Kew Farm -hea-o a fe*w platy crystals 
e)f tourmaline u]i to 2 cm. long, coateel with the' gre'e'nish mica, we're founel 
eai the be'deling surface's of the Cjuartzite's. The* teiurmaline is a sliglitly chromi- 
fe'rous pale breiwnish schorl with e ~ 1 • (559 + -002, o> = 1*(515 + *002, con- 
taining 0-35 pe'r cent (V. 2 O 3 . This oe-curreau'e seems tei support tlie possi- 
bility that the^ chromium was elea*ive'el from the granitie- gneiss intrusions 
(or freim its pe'gmatitic de'rivative >8 whie*h are ])artie*ularly nume'rous in the 
Gorge' Orea'k loe^ality at Ke'y Farm, whea-e the' gre'e'n niusce)\'ite is also more' 
abunelant than elsewlu*re). 
(b) The Lower Quartzites.- -Them' are' six distine-t banels in the' lower part 
of the series. They are inte'rle'ave'el with sillimanite schist, hornbk'iiele schist 
(plagioe-lase amphibolite), anel thicker granite gne*iss banels. 
