Geology and Petrology of Part of Toodyay District, W.A. 87 
northern boundary of the upper granite gneiss and is bounded on both sides 
by metasediments, the dip and strike of which conform to that of tlie gneiss. 
Regarding the dip of these rocks — in the western half of the area the dip 
is constantly south at amounts ranging from 15° to 40° ; in the centre of the 
area the general dip is at fairly flat angles to the south-east but there is a con- 
siderable amount of minor folding in the metasediments in this part ; at the 
place where the strike suddenly changes to the S.S.E. in the N.E. part of the 
area the dips are very steep to the south, and thereafter fairly steep to the 
east. 
From the fleld mapping the. structure of this area is interpreted as : — 
A major anticlinal structure (sec flgure 1), the axis of which traverses 
the centre of the area in a direction striking approximately N.N.W. and 
pitching to the S.S.E., which has, on its eastern limb, a recumbent syncline 
with its axial plane striking N.N.W. and dipping towards the east at approxi- 
mately 60°. The axis of this fold also pitches to the south. The sequence as 
it appears in the eastern part of the area is therefore inverted. These struc- 
tures are illustrated on the cross sections appearing with the geological map 
in Plato 1. 
This idea of the structure of tlie area Y^ould receive considerable support 
if it could be proved that the beds of the north and north-east parts of the area 
were older than those of the south-western parts. Up to the present, however, 
there is very little evidence othei' tlian that afforded by occasional drag folds, 
for determining the age relations of these rocks. Complete recrystallisation 
has almost obliterated graded bedding and similar structures which might 
have afforded criteria for determining the stratigraphical succession. (Some 
obscure current bedding structures have been noted but no certain interpreta- 
tion of these was possible). All the rocks are in the sillimanite zone, and 
consequently grade of metamorphism affords no information as to the succes- 
sion. Read (1936, p. 473) has pointed out that the abundance of andalusite 
in narrow politic bands may be due to graded bedding — -the lower gritty pos- 
tions of the band being almost free from andalusite, which becomes more 
abundant in the upper more politic part of the band, and he has applied this 
method to determining the stratigraphical succession in the Dalradian rocks 
at Banff. In the area at present under discussion this variation in size and 
abundance of andalusite in the upper politic rocks of the series is not present. 
As far as can be seen the rocks throughout this band are fairly uniform in 
composition. There is, as will be shown later, a constant variation in the 
development of the andalusite in this band, but this extends over a consider- 
able wddth and is not due to any variation in the argillaceous components of 
the original sediment, but rather to a variation in the temperature conditions 
to which the rocks have been subjected. 
Drag folded structures in the Cjuartzites at a position 140 chains east 
and 225 chains south of datum indicate that the easterly dipping metasediments 
at that locality are on the western limb of an anticline overturned to the west 
and that the se([uence in this vicinity in inverted. 
If, as suggested by the drag folds at 140 E., 225 S., the rocks of the north- 
ern and eastern part of the area are older than those to the south-west (if they 
are not, the whole succession over the greater part of the area surveyed must be 
