214 
Maitland . — irestem A ustralian Geology. 
A ty])ical and iioteAvortliy member of tlie Pre-Cambrian group 
of rocks is the brilliantly coloured compact jaspilites and cherts, 
grading gradually into valuable liaematite deposits, niauy of nhich 
are several liundreds of feet Avide and some thousands of feet in 
length. There iron-ljeai'ing’ jaspers occur in more or less parallel 
bands Avhich at times stand out in bold relief above the ground 
surface and constitute important sti-atigraphical horizons, trace- 
able across country for considerable distances. 
The presence of such thick and extensiA’c deposits of jaspilite 
and iron ore in the Pre-Cambrian formations, not only in Western 
Australia, but also in Routh Africa, India, and the North American 
Continent, suggests, as has recently been ])ointed out by a leading 
geologist, that conditions at the period of their formation must 
haA'e been fundamentally different from those obtaining in Post- 
(’ambrian times and that “ uniformitarianism as a Avorkiug principle 
for the geologist cannot be pushed back indefinitely into the past.’* 
The jaspilites may ha\'e been originally highly ferruginous 
fine-grained grits of sedimentary origin, and tlie source from whicii 
the iron-liearing minei'als originated Avas basic igneous rocks. The 
fenuiginous bands generally follon’ very closely the bedding planes 
of tin* enclosing m('tamor])hic sediments, indicating that the solu- 
tions responsible for the de])Osition of the iron ore Avere to a very 
large extent controlled by the stratification. In many localities 
liands of siliceous doloniitic limestone are associated Avith and 
gradually jiass into the ribboned jaspilites; an association which 
suggests that some of the jas^iilites and their allies originated 
from the alteration of limestones. 
One of tlie outstanding features in connection Avith these Pre- 
Cambrian rocks is their distribution along Avhat geological investi- 
gation has shown to be one of the principal lines of Aveakness or 
mobility in the crust as dcAudoped in Western Australia. 
The sedimentary beds Avere deposited in a broad geo-syncline! 
and since their formation Inu’e been more or less irregularly folded 
and comjiressed, concertina-fashion, along higiily inclined axird 
planes. The folding is meridional Avith a tendency to an alignment 
ill a nortli-west and south-east direction. The cleavage or schist- 
osity cunnmonly follows a like direction. Tn this geo-synclinal basin, 
\\hi(di extends from tlie Hecherche Archipelago on the Routli Coast 
to that portion of the North West coastline into Avhich the T)e 
Grey, Yule, Fortescuo, ami Ashburton Rivers discharge their drain- 
age, AA-ere de])osited \-ast amounts of sediments carried by SAcift- 
rnniiing riA^ers issuing from the series of broad inoiuitain ranges, 
which have since been AA'orn doAvn to base level by a cycle of 
erosion. 
