130 
Rex T. Prider. 
emphasised by the sliglit slieariiig out of the microcline phenocrysts into 
inicrocline augen. Earth movements had practically ceased by the time that 
the granite had finally consolidated. Late stage aplites and pegmatites were 
associated with this period of intrusion. This intrusive granite has picked 
up fragments of the older rocks, comprising : — 
(1) Plagioclase amphibolites and related types, derived from basic 
igneous rocks of doleritic composition. 
(2) Altered ultrabasic igneous rocks, now represented by anthophyllite- 
hypersthene and anthophyllite-cordierite rocks. 
(3) Metasediments in the form of banded eulysites (derived from banded 
iron ores) and biotite granulites (derived from psammitic sediments). 
Tliere is no evidence of any tectonic movements of later age than the 
granite gneiss intrusion. 
Later these rocks were intruded by a granitic magma of similar composi- 
tion to the earlier orthogneisses, together with its end stage products (aplites, 
pegmatites, and quartz veins). This granite has not effected any meta- 
morphism of the older rocks other than the retrogressive alteration of pre- 
existing andaliisite, by vapours advancing ahead of the intrusive mass. The 
intrusion of this later granite is considered to have taken place at considerable 
<lepth. 
A later period of igneous activity is represented by dyke intrusions of 
quartz dolerite, which in their constancy of trend appear to have come up 
along joints or other lines of weakness, approximately parallel to the tectonic 
strike. Certain ultrabasic dykes are believed to be also post-granite. 
Tlie structure of the area, which is one of abnormal strike in the Western 
Australian shield, is interpreted as a major anticline, with its axis striking 
a little W. of N. and pitching to the south. This structure has a recumbent 
syncline on its eastern limb and is cross-folded on axes trending N.N.E. 
The geological liistory of this region may be summarised thus : — 
Early Pre-Cambria?i (Kal- 1. 
goorlie-Yilgarn Period) 
o 
3. 
4. 
o. 
Late Pre-Cambrian (Nul- (5. 
lagine Period) 
7. 
? Aliocene .... .... S. 
Present .... .... .... 9. 
Deposition of argillaceous and aren- 
aceous sediments with a minor 
calcareous facies and with inter- 
calated lava flows (or sills). 
Diastrophic period accompanied by 
intrusion under orogenic stress, of 
a porphyritic microcline granite. 
Main period of regional metamor- 
phism. No orogeny after this period. 
Intrusion of end stage products of the 
microcline granite magma. 
Intrusion of the later granite. No 
further metamorphism. 
Intrusion of end stage products of the 
later granite. Period of formation 
of auriferous deposits. 
Intrusion of c|uartz dolerites. 
Erosion period. 
Region reduced to a p(meplain. For- 
mation of laterite (Woolnough, 
1930, p. 125). 
Erosion periotl to the present. The 
region is now a dissected, laterite- 
capped })lateau. 
