106 
Geology and Physical Geography ; 
Tycrs Rivers, indicate a river system whose course partly corre- 
sponded to that of the Thomson llivcr, but whose sources were 
further north than the present Main Divide. Similar belts of 
Older Volcanic rocks, underlaid by gravels, indicate the existence 
during Miocene times, of rivers rudely corresponding to the 
present Tanjil, La Trobe, and Tarwin Rivers. A succession of 
similar patches from Hoddle’s Creek to Melbourne indicates the 
ancient course of a river corresponding to the Yarra. (Fig. 48.) 
The wider areas of Older Volcanic rock around Neerini 
Brandy Creek, Western Port, Batman’s Hill, Emerald Hill, and 
North Melbourne, mark where the lava-flows of the period 
filled in lakes and estuaries to the depth in places of hundreds 
of feet. 
It is uncertain to what extent some of these last-mentioned 
portions of the Mioceno lava-flows which now constitute land 
surface may have flowed over what were at that time submerged 
areas. That such was the case in many places is evidcnced^by 
the occasional layers of lava found intervening between marine 
Miocene deposits. It may be surmised, however, that any of our 
Older Volcanic rocks, within 500 feet of present sea-level, were 
in all probability spread out under water. 
Connected with the volcanic activity of the period, hydrothermal 
action, or the agency of heated waters, probably charged with 
siliceous matter, appears to have been busy, and to have influenced 
the transmutation of gravels, sand, clays, infusorial earth, and 
other deposits into flinty siliceous rocks and hard conglomerates 
Wherever the Middle Tertiary valleys were filled, or partly ' 
filled, with lava, a diversion of the courses of the drainage channels 
was caused thereby, and, as tlie work of denudation proceeded in 
subsequent ages, new rivers cut their channels to deeper levels 
than the former ones, entirely removing some portions of the older 
gravels and their superimposed lavas, and leaving others standing 
in the form of elevated plateaux and ridges. In many places the 
sites where the Miocene streams found their outlet to the sea have 
thus been obliterated by the entire removal of the lava, the gravels 
beneath it, and also a considerable thickness of the subjacent bed- 
rock. Thus, it may now be observed, that that portion of the 
Southern Spur from Neerim to the low saddle between the Moe 
and Lang Lang Rivers consists of older basalt, which covers 
sedimentary doposits, showing that the watershed line of the 
Southern Spur was not, during Middle Tertiary times, in the 
same position as we now see it; and, in fact, it is* the rulo rather 
than the exception, that where remnants of lava-capped Middle 
Tertiary fluviatile deposits are found at any considerable elevations 
above sea-level, they form the upper portion of eminences high 
above the beds of existing streams. The following extract from 
the work of Professor Jukes (Manual of Geology, 3rd edition,. 
