90 The Queensland Naturalist 
March 1948 
until a point is finally reached at which the fish die. 
Other chemical factors, present in certain regions, 
probably also complicate the problem still further. 
It is suggested that the whole problem may be 
regarded as unavoidable, being possibly a method whereby 
is maintained in one phase of life, the “Balance of 
Nature.” 
NOTE ON THE GEOLOGY OF SOMERSET DAM 
By DOROTHY HILL 
Three of the most remarkable structural units of 
south-eastern Queensland are the D’Aguilar block of 
highlands, the Yarraman block of highlands and the 
intervening Brisbane Valley; and the geology of Somerset 
Dam will be considered in relation to them (see Map). 
In the Brisbane Valley we find outcropping to-day 
relatively soft, Mesozoic rocks laid down in fresh water 
or under subaerial conditions, while in tin* higher blocks 
on either side there are instead much harder and older 
rocks capped by younger lava flows. These older rocks 
were laid down originally as sediments ill the early Palae- 
ozoic seas which then covered the eastern margin of the 
present Australia — some three to five hundred million 
years ago. They are a very thick series known as the 
Brisbane Schists, which during the passage of time have 
been hardened by pressure and folded and broken by 
earth movements. In late Palaeozoic times they were 
penetrated from below by magmas — very hot pasty liq- 
uids — which baked them, engulfing large segments, and 
which on cooling solidified into great areas of granites. 
During the 150.000.000 years of erosion since these gran- 
itic instrusions, the granites have been laid bare in an 
eastern and a western belt; in the eastern, in the 
D’Aguilar block, we see them at Mt. Crosby, Enoggera, 
Samford. Dayboro and many other places further north; 
in the western belt, in the Yarraman block, we see them 
at Eskdale, Crow’s Nest, Auduramba, and Nanango. 
\Ye may perhaps conjecture that it was this stiffen- 
ing in two separated belts that led to the formation of 
