Lava Mefiiduals. 159 



EesiduaU of the Port Phillijy system. 



The connection between the Melbourne and Bellarine residuals 

 is problematical, but it is certain that an important stream 

 received tributai-ies from different portions of the system and 

 passed southwards over the Bellarine Peninsula. The extension 

 north of the Bellarine residual may be traced under the waters of 

 Port Phillip Bay by the bathymetrical contours. The trend of the 

 stream is represented by the Older Basalt lava, east of Sunbury, 

 Bulla, Broadmeadows, Essendon, Melbourne and South Melbourne, 

 .all situated on the Melbourne i-esidual. The western lateral of tlie 

 Melbourne residual lies beneath the Newer Basalt extensive lava 

 field, but its eastern lateral is now a confined Newer Basalt lava 

 field, and may be easily located by the laterals forming on either 

 ;side. (Fig. 4.) 



Apart from the Melbourne and Bellarine residuals, the Kan- 

 garoo Ground and some smaller residuals are all that remain to 

 .show the erosion antecedent to the Newer Basalt. 



A systematic classification of valleys. 



From the several types of erosion described, a tentative classifi- 

 -cation of many of the streams, according to the factors that started 

 the formation of their valleys, may be attempted. Starting with 

 the oldest, it is proposed to adopt the following classification : — 



(1) Pre-Older Basalt cycle. 



(2) Older Basalt cycle. 



(3) Intermediate cycle. 



(4) New^er Basalt cycle. 



(5) Post Newer Basalt cycle. 



(1) Pre-Older Basalt Cycle. 



The head-waters of some streams may still belong to this cycle; 

 -probably the Watts River is still occupying a Pre-Older Basalt 

 valley. Many of the valleys are still intact below tlie Older 

 Basalt rendual. 



(2) Older Basalt cycle. 



Short cycles which started aftei- each successive flow of the older 

 lavas — in mining phraseology, '* false bottoms." It may be 

 found possible to correlate the soft strata between tlie liard l)asalt 

 -disclosed in the Lang Lang, Phillip Island, Flinders and Cape 

 Schanck bores. 



