The Geology and Physiography of the Lavvnswood Area. 
59 
The amphiboles are of two varieties^ both with a high relief, containing 
lileochroie haloes around zircon grains and tending to be idioblastic in form 
(17, pp. 327 to 328). One vaidefy is a pale green amxdiibole with weak 
pleochroism, X = colourless, Y = b = pale yellow gi’een to brown, Z = 
l)ale green-blue; absorption X Y ■= Z; Z c = 13° ; (-) 2 v near 9(J°. 
Polysynthetic twinning is common on 100. The mineral is a grunerite with 
a composition near cummingtonite prol)abiy containing over 75 per cent 
FeSiO^ (17, p. 327). The other variety ih a dark amphibole with very 
strong pleochroism X = light green, Y = dark olive green, Z = intense 
blue-green. Absorption is strong and masks the intei'fereiice colours, Z 
slightly ^ Y" > X, Z /, c =22°, Y ^ b, ufitical character biaxial -ve with 
large 2V (about 80°), twinning not seen. The mineral is probably an 
actinolitic hornblende. The garnet xenoblasts have an irregular form and 
a poikiloblastic character, They contain numerous inclusions of a highly 
l)irefringent mineral forming up to 60 per cent, of their volume. This garnet 
is a pink, }irobably iron-rich variety, ^lagnetite forms very irregular ])atches. 
The crystalloldastic order is: amphiboles, garnet, quartz and magnetite. 
Average composition — quartz 40 per cent., grunerite 30 per cent., dark green 
amphibole 15 per cent., garnet 15 per cent., magnetite accessory. 
(ii) Banded quartz-iron ore rocks (19954) are bluish grey, strongly 
banded, fine to medium-grained, willi a granular texture, and are reddish 
brown where weathered. 
The bands are quartz, four cm. wide (grain-size about two mm.), quartz 
and iron ore, one cm. wide (grain-size 0.25 - 0.5 mm.), and narrow iron ore 
bands up to one mm. wide (grain-size 0.25 mm.). Bands of amphibole 
occur rarely. 
A granoblastic structure is seen in tlie (piai’tz and quartz-iron ore bands. 
The quartz has an irregular mosaic structure and is ]>T’a(4icany free from 
inclusions except next to the iron <n*e Avhere it is crowded Avith fine, colour- 
less, needle-shaped crystals. These are indeterminable but are perhaps 
incipient grunerite resulting from reaction hetAveen quartz and the iron ores. 
The iron ore is almost entirely hematite but a minor amount of magnetite 
is present and quite a number of octahedral erystals Avere observed, some 
of which may be hematite pseudomoridiic after magnetite. 
(iii) Latcritised banded quartz-iron ore rock (19061) is a bi’ownish 
coloured rock which outcrops in the south-west corner of the area. It con- 
sists of alteimating bands of quartzite and limonit(‘, from one cm. to three 
cm. or more in width. The quartz (grain-size two to three mm.) has a 
mosaic structure. Sometirne-s the rock is almost pure, very fine-grained 
limonite Avhich is dark brown and has a metallic lustre, other specimens are 
composed of dull limonite and strikingly resemble the rocks of group (ii). 
Drag-folded and very eontorted tpiartzites, called “banded quartzite” 
by Mr. R. A. Hobson of the Oeologieal Survey of Western Australia who 
mapped the area in 1941, are very commonly associated Avith the banded 
iron ores. The data concerning the banded quartzite and banded iron ore in 
the south-Avest- corner of the geological map of this area are taken from 
llobson^s map. 
Origin of the mefajaspiJites.^Fjxtreme metamorphism in the 
sillimanite zone of iron-rich siliceous sediments has produced these banded 
