90 
The Queensland Naturalist May, 1938 
river and brought upstream by the tide in times of 
drought. 
During heavy floods the first effect would be the 
stripping and removal of these superficial mud deposits. 
This would be followed by modification of the under- 
lying gravel banks. In some places they would be 
eroded, in others aggraded, according to vagaries in 
strength and direction of the currents in the flooded 
stream. But the algebraic sum of these changes would 
be a definite increase in the amount of gravel. As each 
flood subsided, a fresh deposit of mud would be smeared 
over the gravel banks, and this would gradually be added 
to during normal conditions. 
To what extent the present stream may be regarded 
as strictly normal, and to what extent it has been in- 
fluenced by human activities, such as deforestation, 
deepening and regularising the river for shipping pur- 
poses, and the removal of gravel, can only be guessed at, 
but Mr. Bison’s observation recorded above suggests 
that the effect of man-made modifications may be far 
from trivial. Tn order that members may appreciate how 
different the lower reaches of the stream now are from 
their original state, T have brought here to-night a model 
o? the Brisbane River from St. Lucia to the mouth pre- 
pared by Mr. Elliott Bolland. and based on the earliest 
systematic survey of the river bed. 
If we wish to trace the history of our river back 
beyond these latest man-made changes, we must have re- 
course to physiographic inferences, and as Dr. Marks 
has pointed out in an address to you on a similar occa- 
sion to this, physiographic inferences are apt to be mis- 
leading. But at lease we can. in this case, deduce a story 
in which the several episodes are mutually consistent, and 
a story that is in close agreement with the geological his- 
tory of Eastern Australia as a whole. 
It would seem that the Brisbane River was once a 
sluggish stream similar in many respects to that of to- 
day, and following a course almost identical with its 
present sinuous curves, but in those distant days it was 
a hundred or more feet above its present level. Later, 
as a result of some influence that may be only guessed 
at, the torpid stream became rejuvenated and began to 
cut downward into its bed with renewed energy. Gradu- 
ally, the bed was carved deeper and deeper between its 
rock walls until instead of being a hundred feet above, 
it was a hundred feet below its present level. Then a re- 
versal took place. The whole eastern coast of Australia 
