54 
J. R. H. McWhae. 
4. Quartz Doierite. 
Quartz doierite d^'kes, from half to two chains wide, are intrirsive into 
all the above rocks. They have a general norlh-north-Avest trend and are 
most numerous in the centre of the Area. In some places there ai’e closely 
spaced fractures resembling fracture cleavage in the dykes giving them a 
platy structure. Slight shearing of their edges indicates movement sub- 
sequent to intrusion. 
5. Laterite. 
Laterite-capjied mesas and buttes are more common near the metasedi- 
mentary ridges and slo])e away from them at 4° to 8°. This sloping laterite 
is attributed to its formation on a gently inclined surface in the end stages 
of the jirevious cycle of erosion rather than to warping in late Kainozoic 
times, as in the case of an occurrence of dipping laterite in South Australia 
(31, pp. 32-33). 
n. STRUCTURAL INTERPRETATOON OF THE JIMPERDING SERIES. 
Text tig. 3 is a simi>lified diagram of the broader geological and 
structural features of the Toodyay and LaAvnsAvood Areas. The northern 
part of the map, the Iaa^o sections A-B and C-D and the interpretation of 
the structure are taken from Prider^s paper on the Toodyay Area (23, p. 87). 
hcreas fi'om the LawnsAVood Area to the southern part of the Tood- 
yay Area the strike is norlh-north-west, in the north of the Toodyay Area 
the strike is west and the dip is to the south at a fairly low angle. Again 
the quartzite near P.S. Trig has a Avesterly strike and a Ioav dip to the south. 
This unusual Avestei'ly strike, the drag fold evidence of iiiA’ersion of the 
lower metasediments and the discontinuous quartzite band running from 
near P.S. Trig to the intrusive granite at the south-east of the LaAvnsAvood 
Area, can be explained if Ave imagine the Toodyay and LaAA'iisAvood Areas 
to be in the main a major anticline pitching to the south-south-east, having 
a recumbent syncliue Avith an axial plane dijAping to the east on the eastern 
limb of this iniijor anticline. 
Tlie discontinuous quartzite band from near P.S. Trig is considered to 
be an infolded recumbent synclirie of the Upper Quartzites regardless of its 
petrological resemblance to the lower quartzites. Dr. Prider predicted the 
presence of thi.s infolded band of u]>per metasodiments at P.S. Trig, as a 
result of his structural interpretation of the Toodyay Area and he suggests 
that the discontinuous nature of this l)and is due to the presence of minor 
cross folds. The cross folds superimposed on the major north-AA^est trend- 
ing structure haA^e jAroduced a series of minor transverse synclines and anti- 
clines. Erosion has cut doAvn so far that only the transverse synclines 
remain as lenticular outcrops Avhile the transverse anticlines haA^e been 
removed. 
It is seen (Table 1.) that, Avhile the loAvest portion of the Toodyay se- 
quence is not represented in the Lawnswood Area, there is a considerably 
greater thickness of npjier metasediraents in the A\Tst of the area Avhicli 
probably correspond to the metasediments in the south-east of the Malkup 
Area (9, Table 1. p. 146). Probably of some importance in regard to the 
origin of the hornblende schists is the change of ^^llorizon 6” f]*om sillimanite 
schist in the Toodyay Area to hornblende schist in the Lawnswood Area. 
