26 RIVER YARRA, VICTORIA 
2. Hills (1934, p. 169) mentions the possibility of ejectamenta 
from a volcano at Lilydale blocking the pre-Older Basalt river and 
so causing flooding over the Wurunjerri Range to establish the 
present course of the Yarra. This theory was later abandoned. 
3. New Hypothesis. The Nillumbik Peneplain stretched east- 
wards to the Dandenong Mountains, and did not cease at the 
Wurunjerri Range as formerly believed (Jutson, 1911). The 
break in the Wurunjerri Range owes its genesis to lateral differ- 
entiation in the Western Quartzites facilitating reduction, and to 
some structural disturbance. In other words, the Wurunjerri 
Range did not have to be breached because it already had a very 
low saddle in it. The infilling of the Wurunjerri valley with basalt 
flows some 300 ft. thick raised the thalweg of the new marginal 
stream so that it was higher than the saddle in the Wurunjerri 
Range, and flowed over it with ease. Even after erosion through 
most of Tertiary time and all of Quaternary time, the residual at 
Lilydale stands 674 feet above sea-level, which is roughly 275 feet 
above the bed of the Wurunjerri River as exposed in the Cave 
Hill quarry. If once southerly drainage had developed again after 
the extrusion of the basalts, no factors were operative in this area 
sufficient to divert the river. 
THE YARRA PLATEAU 
Gregory (1903) defined the Yarra Plateau which '*onee ran 
from the Strathbogie Ranges, across the present main divide 
between Mt. Disappointment and Mt. Arnold. It forms the old 
platform under the Dandenongs" (p. 84, fig. 49, p. 109). He 
defined it more narrowly when he said that the eastern border of 
the Plateau may be drawn through Queenstown, Christmas Hills, 
and Mooroolbark. ‘Most of the Yarra Plateau may be regarded 
as a shelf on the eastern border of the Melbourne basin"' (p. 86). 
Gregory thus presented two definitions of the Yarra Plateau 
which in reality refer to two different surfaces: 
(a) The first definition refers to a pre-Dandenongs (viz., Upper 
Devonian) surface as shown in his fig. 49, i.e., a Palaeozoic 
terrain. 
(b) The second definition refers to a shelf, the remains of which 
are at present clearly visible, i.e., a Cainozoie terrain. The 
second definition also limits the Plateau to a small area 
near Melbourne while the first refers to a large section of 
the State. 
Gregory's first definition seems to have been largely disregarded 
by later writers. Jutson (1911), obviously taking the second 
