m 
/ 
In the absence of any other evidence, it has been found con- 
venient to separate these rocks into two great lithological groups, 
viz., (a) the gneissic and granitoid rocks, and (b) the crystalline 
schists. 
The crystalline schists of which there are at least two distinct 
groups, which there is some reason to believe differ considerably 
in geological age; these are ( 1 ) phyllites. quartzites, conglomerates 
and arkoses, which have been designated the Mosquito Creek 
Series, and ( d ) an older, mica-quartz schist and marble group, 
associated with basic rocks which have been at times converted 
into greenstone schists. These rocks have been more or less irre- 
gularly folded and compressed concertina-fashion, along itig*-*, 
inclined axial planes: the folding is meridional, the prevalent 
strike being generally north-west and south-east. The broad 
geological structure of the western half of the State across some 
of these bands is shown in the section (Fig. 7), 
and will serve perhaps to make this clear. The 
dotted lines indicate the former extension of the 
strata. Some however, of the hornbendic rocks associated 
with these rocks may possibly represent original gritty heels made 
up of epidote and chlorite. In some instances quartzites in which 
original argillaceous impurities have re-crystallised as felspar and 
mica, render them easily mistaken for granite and its allies. 
Fig. 7 . 
Generalised section across Western Australia from the Coast to 
Premier bowns. 
A most noticeable feature in these crystalline schists is con- 
stituted by these bands of cherts and l)rilliantly coloured jasper 
( which often contain oxide of iron to such an extent as to warrant 
their lieing classeil as iron ores). These extend as roughly iiarallel 
bands, which owing to their serrated ridges stand out in bold relief 
{Fig. 8), and are very much in evidence in certain districts. 
These bands do not always occur in straight lines, but as they 
have also been subject to earth movement since their formation, 
they are often thrown into a series of gentle curves, which vary 
locally in general direction. 
