Geology and Petrology of Part of Toodyay District, W.A. 117 
The mineralogical composition (by volume) of the hyperstheiie rock 
is approximately — 
Hypersthene .... .... .... 60 % 
Ferro-anthophyllite 
Martite (after magnetite) 
Quartz 
Apatite 
15 % 
15 - 
-20 
5 % 
Accessory. 
Banded quaTtz-hematite-grunerite rock (15452). 
This specimen is from the same locality as (a) above — the field relations 
lietween the two types are obscure. 
It is a finely banded, even-granular rock, and consists of alternating 
dark coloured bands of recrystallised hematite and pale greenish ([uartz- 
amphibole bands varying in thickness from 0-5 to 1-0 mm. The sj^ecimen 
is non-magnetic — this, together with the deep red streak confirms tlie de- 
termination of the silvery grey, metallic mineral as hematite. 
Under the miscroscope the structure is even-grained granoblastic gneissic, 
the hematite 0''*f‘urring in recrystallised grains and aggregates showing a 
marked segregation into bands (figure 13A) — quartz is often included in 
these aggregates and shows no signs of reaction- with the liematite. 
The amphibole and (piartz form a granoblastic aggregate, the latter 
often occurring as poikiloblastic inclusions in the former. The amphiboles 
have a random orientation in the rock, although in ])articular bands they 
tend to show the same orientation througliout. 
This a-mpliibole is a })ale greenisli weakly pleochroic variety- in irregular 
shaped prisms with lamellar structure. Tlie ]:>rism faces are fairly well 
developed, but the terminations, whether in contact with (piartz or iron ore, 
are irregular. It has good 1 10 cleavage and commonly shows multiple 
twinning on 100. The oj^tics are a -= 1 -635 ; y ^ 1 -660 ; y - a ^ *025 ; 
Z Ac — 15° ; (-)2V near 90. Opt. ax. pi. || 010. 
The refractive indices indicate a cummingtonite with 42 per cent 
FeSi03, but the negative optical cliaracter suggests admixture of the actino- 
lite molecule. Richarz (1927, p. 700) has described a similar amphibole 
from the Lake Superior District. It has Zac = 15° ; y = 1-680 • 
/3 = 1 • 668 ; 
a = 1 • 665 ; 
y - a *025 ; (-)2V near 90° and com- 
position : — 
AI2O3 
5-3% 
Fe^Oa 
00 
0 
0 
FeO 
22-4% 
CaO 
•> ■ gC/ 
- ^ 0 
MgO 
a • 4 c/ 
thus differing 
from grunerite 
in its high FegO^ and Al^Og (nntent. In its 
optics it closely resembles the Toodyay mineral whicli is probably an alum- 
inous type of grunerite with admixture of actinolite molecules. 
The origin of the banded- eulysitic types. 
The [ireservation of well marked banding in these rocks is indicative 
of their origin from sedimentary, bedded iron ores. 
In their appreciable MgO content, and, in the case of type (b), the 
possible presence of CaO, the original rocks would appear to liave been impure 
banded siliceous ankerite rocks rather than greenalites. In the hypersthene- 
