238 
JiRENDAN P, Thomson. 
TIL STRUCTURAL GEOT.OGY AND FIELD DISTRIBUTION OE 
THE ROCKS (seo Plate I.). 
A. Granite anei Gneis<i. 
1. Field Oeearrence . — Aplogranite and ‘iiieiss are intimately associateil, 
as at Armadale, hence mapping* ut' them separately was not attempted. Also 
no j)orsistent tyj^e corresponding; to the tine* banded gneiss of Armadalo 
(Prider, 1941) (*onld be found ditlering in strike from the hybrid gneiss. 
Aplog'i’anite tends to outcrop predominantly in the we.stern part of 
the area. Here near the Wongong the g'neissic foliation is usually vague 
altliough a few outcrops contain typical hybrid gneiss (Prider, 1941) with 
basic xenoliths. Farther south from the Rifle Range to the Brickworks 
Quarry, the gneissic structure is fairly well maiiitaiTied and basic xenoliths 
are common. Again in the south-west of the an^a, between the Cardup and 
Beenyu[) Brooks, a]dogranite predominates. 
The eastern part of the area exposed in the Wongong Brook consists al- 
most entirely of well-banded gneiss which is cut in places i)y aplite veins. 
Xenoliths are rare or abs(*nt, although biotitic clots and streaks in the 
gneiss appear to coiTc^'pond to the basic xenoliths found elsewhere in the 
area and at Armadale (Prider, 1941). 
2. Stracture of the f/nfuite gneiss area and the oeearrence o) Qaartz 
Veins , — Dips and steikes of the gneiss foliation were mapped, with the 
knowledge derived from Armadale that the readings in the hybrid gneiss 
may be uni'eliable for detailed intei^pretation. Such local revei'sals of dip 
and overturned, sti’uctures as observed at Armadale by Priilei* (1941) were 
found in the we.stern part of the granite gneiss ar(*a, but as mapping pro- 
gressed it was s(Hm that order was maintained in the strike and in a more 
general way in the dip of tin* gneiss throughout the entire area. The gneiss 
in the western pari within half a mile of its contact with the f ardup series 
strikes north-west to norlh-north-west, hence the contact instead of being 
parallel to the strike of the gneiss foliation as at Armadale, truncates it 
obli(|uely. The series of i|uartz blows which outcrops “(*n echelon’’ across 
the Beenyup Brook in a north-w'csterly direction, appears to be related to 
the structure of the gneiss in the western strip. For three miles north of 
these blows the general dip is east. South of the blows the gneiss dips 
steeply to the west. All this e\idence indicates that the structure of the 
gneiss in the western strips hears no relation to the structure of the Uardup 
series. The gneiss appears to form a major anticlinal structurt*, cored by 
quartz blows (steep dip inferred), which may have formed in cither a 
sheared zone or in tension openings along the crest. This anticlinal inter- 
pretation is supported by the majority of the dragfolds which also suggest 
that the anticline pitches gently to the mu’th-uorth-west. 
The gneiss in the eastern part of the Wongong Brook area dips Avithout 
exception to the west. Dragfolds are rare and they indicate an east limb 
of a normal .svncliiie. No structural break lias been observed between this 
gneiss and that to the west. On the south side of the Wongong Brook at one 
point near the broad laterite spur, the foliation is flat-lying, which suggests 
that the axis of the syncline passes beneath the spur. North of Y ongong 
Brook the strike of the east limb SAvings nortli-uorth-east possibly because 
of cross-folding. The synclinal structure may extend to Aianadale, the axis 
of the syneline lying cast of the area examined by Prider. 
