Geology and Petrology of Part of Toodyay District, W.A. 107 
gi'adation from normal plagioclase ami^hibolite. tlirougli epidosito to a diop- 
side-]>lagiocla.se rock in which grossularite is ])rescnt in small amount- — this 
latter type is developed right at the contact. 
The j)lagioclase amphibolite portion of the specimen is the normal type 
in which the felspar is very turbid with fine granular e])idote. Following 
this is a narrow band (} incli) consisting [predominantly of yellow epidote 
(pista<‘hite) with a minor amount of pale greenish hornblende and quartz — 
the hornblende then decreases in amount leaving an almost ])ure epidote 
band (] incli) in which lozc^nge shaped sphenes are a notable constituent. 
In tlie outer part of this band grossularite is occasionally seen. Jkwond 
this, the rock consists of epidote, slightly turbid plagioclase (Abc)Anj) and 
(juartz, tlie two latter occurring in micrographic intergrowth. On the outer 
limit of the specimen, the rock consists of diopside and saussuritised plagio- 
clase with a little grossularite, sphene, and ([uartz. Were it not so clearly 
related to the [plagioclase amphibolite, siu'h a rock could conceivably have 
resulted from the metamor[)hism of an im[pure argillaceous limestone. The 
develo[)ment of grossularite in this rock is interesting, as it lias developed 
from the epidotic alteration of the original felspar. 
The main features which elucidate the origin of these schistose [plagioclase 
nmplubolites are : — 
(1) They occur interbedded with metasediments and never transgress 
the bedding. 
(2) They are constant in character throughout the area. Rarely, 
more calcic diopside layers alternate regularly with layers of 
the normal amphibolite. A similar feature to this is seen in the 
Landewednack hornblende schists of tlie Lizard, Cornwall 
(Flett and Hill, 1912, [). -M>). 
(8) The refractive index — l-(>73) of tl le hornblende is indicative 
of hornblendes from e]pidiorites in the sillimanite zone (Wiseman, 
1934, [p. 394). 
(4) In their chemical com[Position they are normal igneous rocks, 
such as would result from the crystallisation of a (] uartz dolerite 
magma. 
i'lu'se [plagioclase am[Phibolites a|>[pear, therefore, to be m.etabasic igneous 
rocks, v'hich formed either a sill or flow in the metasediments [prior to tlie 
fprogenic [period, and have been folded along with the metasediments. In 
view of the complete recrystallisation’, no evidence is available as to whether 
thes(‘ were originally flows or sills. 
(5) The (hrmtfe Onetsses and iJte associated xenoliths. 
A^. d(‘s(;rib(-d above, tlien^ are two bands of light coloured gmnitic gneiss 
<levelo[H'd in the area. The U[Pper and Lower Gneisses form bands estimated 
to h(‘ a[)[proximately 2,000 feet and 5,400 feet thick respectively. Minera- 
l(pgir-all\-, there is but little difference between tlpe two. although the up[)er 
baud, in view of the greater abundance of microcline is somewliat tlie richer 
m [Potasli. It has also suffered more crushing than the Lower Gneiss and is 
<‘V(Tywhere a ty[ji(*al augen gneiss with large " eyes ” of microcline, sur- 
rounded by a granular ( juartz-biotite-microcline aggregate. The lower gneiss 
IS essentially a fluxion gneiss, in many [places a coarse [Porphyritie ty[Po with 
idiomor|)hic microcline [phenocrysts (iqp to I inch long), which have a linear 
orientation due to flowage. Both gneisses are penetrated }py late stagi' 
[pegmatitic [products of the granitic magma— these may be narrow sills or 
\eiiplfts traversing the foliation, usually only a few inches wide. 
