March 1948 The Queensland Naturalist 
91 
the three structural units of our first paragraph. In the 
erustal adjustments following the intrusions, the two 
stiffened regions seem to have aeted as units whose 
movement was upward relative to the intervening narrow 
Brisbane Valley strip. 
The adjustment almost certainly involved crustal 
fracture, at least along the western and southern margins 
of the D'Aguilar block, which are linear and angular. Tt 
affected the Permo-Carboniferous marine sediments of 
south-eastern Queensland, for we know of two small 
patches eanght tip along the western margin of the 
D'Aguilar block, one at Northbrook, another now partly 
inundated by the dam on the east bank of the Stanley 
River near Wyangy Creek. It saw S.E. Queensland finally 
emerge from the sea. never to be inundated again. 
Probably it occurred at the beginning of Triassic 
times; for in Triassic times coarse sediments like those 
deposited by young streams in low land at the foot of 
steep hills covered the Brisbane Valley floor and mantled 
vfhe southern margin of the D’Aguilar block; it is sug- 
gested that the steep scarps were fault scarps formed dur- 
ing this late Palaemie or early Mesozoic crustal adjust- 
ment. Some of these sediments show evidence of being 
laid in water, most likely in lakes dammed up by volcanic 
action or changes in the drainage by earth movement, 
others could be sub-aerial pediments. They are conglom- 
erates. sandstones and shales associated with volcanics. 
The most remarkable feature of this Triassic sedimen- 
tation was the great volcanic episode responsible for the 
rocks on which the dam wall is based. All along the 
western margin of the D’Aguilar block, from Northbrook 
to Kilkivan. there was poured out (it seems from vents 
arranged parallel to or perhaps coincident with this 
faulted margin) a gieat thickness of fragmental volcanic 
matter, mostly of intermediate chemical type, andesitic 
but in part trachytie. Tt consisted of large angular or 
rounded pieces of porphvritic andesite mixed in greater 
or lesser proportion with fine and coarse ash. and poured 
out over the floor of the valley, possibly in places as 
incandescent streams, or elsewhere, moving down the 
valley as great mud flows or lahars. The dam wall is 
based on just such rocks, but which have had a subsequent 
toughening by igneous action, as will be seen below. In 
