346 T. G. TAYLOR AND D. MAWSON, 
difficulty to fix the age of many eruptive rocks in this divis- 
ion of the State further than to classify them as Post 
Triassic. We cannot depend therefore on paleontological 
data, but must rely on uncertain evidence, such as the 
amount of erosion, petrological resemblance, and tectonic 
position. It is preferable to deal with the subject under 
two heads—(a) Relative age (b) Absolute age. 
(a) The syenites and trachytes representing the more 
acid rocks of the district intrude the permo-carboniferous 
and triassic sedimentary deposits, tilting them up at con- 
siderable angles. The dolerite sills of the Nattai River 
and Jellore Creek intrude the coal measures and send off 
large dykes which intersect the Triassic sedimentary rocks. 
A fault occurs in Jellore Creek which seems to be connected 
with the effusion of the trachytes of Mount Jellore; the 
dolerites here intrude the coal measures only on one side 
of the fault which seems to be conclusive evidence that 
these rocks were intruded at a date subsequent to the 
faulting of the sedimentary formations ; this faulting being 
supposed to be due to the intrusion of the intermediate 
igneous rocks. Another minor point of evidence is that 
the field occurrence of the basic rock makes them parasitical 
on the masses of the Gib and Mount Jellore. 
The connection between the basalts and dolerites is more 
obscure, but there is strong evidence to prove that the 
basalt flows are subsequent to the dolerite intrusions. 
The basalts are in the form of approximately horizontal 
flows, and are found overlying gravel beds in some cases 
containing pebbles of an older basalt ; in turn these gravel 
beds overlie the eroded surface of tilted Wianamatta shales. 
The flows being approximately horizontal is proof that very 
little earth movement has taken place since the time of 
their formation, and their mode of occurrence in cappings 
on the tilted shales proves them to be the production of 
