May, 1938 The Queensland Naturalist 
91 
was inundated by an ever rising* sea, possibly due to the 
melting of the extensive Antarctic ice cap which followed 
the Great Ice Age. As a result of this rise in sea level, 
the Brisbane River, among others, was invaded by sea 
water to an ever increasing depth. The velocity of the 
stream was slackened and its energy sapped. No longer 
could it scour out and deepen its bed. Instead it began 
to fill its channel with the rock detritus that it could no 
longer carry to the sea. And it was in this way that the 
gravel deposits we are considering came into being. 
This great and important submergence was succeed^ 
ed by, and its effects in small part counteracted by, a 
succeeding movement of emergence, as a result of which 
sea level appears to have been lowered several feet. 
This brings us to that point in the river ’s history repre- 
sented by Mr. Bolland’s model, the subsequent modifica- 
tions being due not to nature, but to various activities 
of man. 
Having dealt in rather sketchy outline with the 
nature of the gravel deposits, their distribution and their 
place in the history of the Brisbane River, let us now 
endeavour to trace the pebbles to their place of origin. 
A practice common among geologists is the attempt, 
from a consideration of the nature of a conglomerate, to 
interpret the geological structure of the hinterland from 
which it was derived. The interpretation is based on a 
study of the assemblage of pebbles in the conglomerate, 
the proportions of different rock types represented in 
these pebbles, and their relative sizes and degrees of 
rounding. The value of the inferences based upon such 
practice is doubtful. In some cases they may approxi- 
mate the truth very closely, in others they may tel! only 
part of the complete story, while in others they may be 
distinctly misleading. 
It might be of interest in our case, knowing as we 
do both the content of the gravel and the nature of the 
hinterland from which it was derived, to match the in- 
ferences against the facts and measure their degree of 
correspondence. 
In the first place let us recollect the assemblage of 
pebbles present in the river gravel. Earlier in this 
address, I spoke of recognising many old friends among 
the pebbles, bur just as notable are the friends who are 
conspicuous by their absence. Thus, not one pebble of 
basalt is to be seen, although the main stream and most of 
its tributaries head in basaltic tablelands. Again, al- 
though the Stanley River flows through several areas of 
