42 RIVER YARRA, VICTORIA 
consider the Older Basalt on the Bellarine Peninsula and at Bal- 
combe Bay to be residuals on the flanks of the ancient valley. They 
indicate that the Older Basalt had a thickness of the order of 
1,000 feet, because the Older Basalt on the Peninsula reaches 470 
feet, and the bottom of the valley was something like 500 feet 
below present sea-level. Older Basalt over 1,000 feet thick occurs 
in Western Port. 
(4) The disposition of the Older Basalt residuals north-west of 
Melbourne suggests (as one would expect) that a tributary of the 
Melbourne River drained the country on the west side of the Mt. 
Gellibrand granitic intrusion. 
(5) Another stream flowed from the direction of Bacchus 
Marsh, called the Bacchus River in Fig. 6. Borings at Altona did 
not show any Older Basalt, but probably it was worn away from 
that area as from most of the lower part of the river system. 
Pleistocene low sea-levels resulting from glacio-eustatic emer- 
gence rejuvenated the post-Older Basalt streams so that most of 
the Older Basalt was stripped away. The low patch between 
Mornington and Frankston remains because the Wurunjerri 
River was diverted to form part of the Yarra system. No stream 
of any importance was therefore rejuvenated over this part of 
the Older Basalt lava field. 
(6) Yet another stream flowed from the direction of Maude, as 
shown by the Older Basalt residuals there. This is called the 
Maude River in Fig. 6. It is the ancestor of the Barwon River. 
The relationship of the basalt residuals to the Tertiary rocks 
shows (according to Singleton, 1941) that the lava flows at Maude 
and Curlewis (on the Bellarine Peninsula) were not contempor- 
aneous. Hither the two deposits belong to different valleys, or/and 
the lava to different eruptions. The palaeogeography of this area 
has not been worked out, and the course of the Maude River shown 
in Fig. 6 must be regarded as tentative. 
(7) Basalt and tuff at Airey’s Inlet indicate the presence there 
in early Tertiary times of a valley, as lava, like water, always 
seeks the lowest levels. Ash voleanoes are generally found near 
the coast, as they very often originate from hydro-explosions. It 
is interesting to note this general rule holding for the ?Oligocene 
vuleanism, for apparently all the inland voleanoes were effusive 
ones, while those producing tuff are to seaward. 
Noetling (1910), Dannevig (1915), and Keble (1946) have 
discussed the drainage of the Bass Strait area in "Tertiary and 
Quaternary times. 
