110 
Hex T. Piuder. 
Plagioclase (10 to 15 per cent) is gonorally a slightly turbid and 
poorly twinnod oligo-albite (AboAn^) which is partly replaced 
by myrmekite when in contact witli microcline — it is never zoned, 
and is invariably replacc'd in part by tine fibrous soricit(‘. 
Biotite which, on the averages, forms 10 per cent of tliese rocks is 
of two types — (1) A strongly pleochroic greenisli brown variety 
with minute sagenitic rutile inclusions. The flakes are well 
formed parallel to 001, but have ragged terminations. It is 
partially replaced by green chlorite, especially along the cleav'age, 
leading to an intergrowth of these two minerals. (2) A bright 
greenisli completely chloritised biotite, usually developed in 
(‘ontact with microcline and intergrown with epidote. The biotites. 
often occur in small clustered aggregates- An the more crusht'd 
varieties they occur in well defined bands, curving around the 
lenticular granulated microcline aggregates. 
Apatite is the most abundant accessory in stout idiomorpliic prisms. 
Minor accessories are small zircons (with weakly pleochroic haloes) 
included in biotite, scattered grains of magnetite and epidote. 
Specimen No, 1213, an even-grained type, has been analysed as 
typical of these granite gneisses, and the analysis appears in 
Table 2, Column 1. It is a typical granite, and the appearance 
of a little corundum in the norm is due to tlie slight secondary 
alteration of the felspars. TJiere is insufficient excess alumina 
to suggest the possibility of a sedimentary origin for tliese gneisses, 
(Bastin, 1909, p. 401.) 
TABLE 2. 
Granite 
Gneisses from the Toodyay Area. 
1. 
2. 
SiOo 
• . • • . . 
71-85 
63-28 
TiO, 
0-25 
0-83 
AhO, 
15 • 00 
14-56 
0-55 
3-13 
FeO 
... ... 
1-20 
5-10 
MnO 
Tr. 
0-09 
MgO 
... 
0-42 
I -70 
CaO 
1-52 
3-51 
NaoO 
3-67 
3-66 
K^O 
4-45 
2-58 
HA)+ 
0-54 
0-52 
H,0- 
0-02 
0-15 
0-11 
0-45 
99-58 
99-56 
Norms. 
Q 
29-52 
20-40 
Or 
. • « » • • 
26-13 
15-57 
Ab 
• • • . ♦ . 
30-92 
30-92 
An 
... ... 
6-67 
15-01 
C 
1 -73 
0-20 
hy 
2-58 
9-74 
mg 
0-70 
4-41 
il 
... ... 
0-30 
1-52 
ap 
... 
0-34 
1-01 
1. Biotite-inicrocliiie granite gneiss (Spec. 1213), Toodyay, W.A. Analyst, K. T. Pridcr. 
2. Biotite-oligoclase granite sneiss (Spec. 15389), Toodyay, W.A. Analyst. R. T. Prider. 
