Geological History during Tertiary Epoch . 
105 
Towards the close of the Middle Tertiary period volcanic action 
took place, and lava-flows poured down the river valleys, filliug 
or partly filling them in, and covering the deposits in their beds, 
spreading in wide layers over 
the lake deposits, and also 
partly filling up tlio straits or 
the estuaries along the coast, 
and covering the accumulations 
formed in them. This action 
appears to have been confined 
to the country lying eastward 
of the meridian of Geelong, as 
there are uo visible Older Vol- 
canic rocks in the Western 
District, or the country from 
Ballarat to Maryborough. 
Several old river systems of 
the Middle Tertiary period, 
marked by the now discon- 
nected vestiges of the lava- 
flows which filled them, are 
traceable Jrom the eastern part 
of the colony rouud to Mel- 
bourne. 
A few vestiges on the ranges 
or on their slopes show where 
valleys once existed between 
the Mitchell and the Snowy 
Divers. The lava of the Dargo 
High Plains, to a thickness in 
some places of 800 feet, covers 
the deposits in the valley of 
an old river, which had ap- 
proximately the same course 
as the present Dargo, but rose 
farther to the north-east, and 
whose outlet was about where 
the Mitchell River now enters 
the plain country. The basaltic 
plateaux of Connor's Plain, Ful- 
lar ton's Spring Hill, Mount 
Useful, Mount Lookout, at 
Aberfehly, a patch on the east 
side of Mount Matlock, and a 
basaltic area overlying gravelly 
deposits south of Mount Baw 
Baw, between the Tanjil and 
py 
