Lava Residuals, 137 



A line, represented approximately by the railway line between 

 Melbourne and Drouin, this extensive lava field was more than 

 thirty miles wide, but converged towards Flinders, and passed 

 through the bottle-neck there on to the now submerged area of 

 JBass Strait. The confluence of the confined and extensive lava 

 fields approximately in the vicinity of the railway line, marked a 

 change in gradient due in the first place to lava streams being con- 

 fined within the comparatively narrow limits of the valleys, and 

 in the second, to the lava rising above the Avatersheds and flooding 

 .an extensive area. 



So much of the Port Phillip lava field is buried beneath the 

 Upper Tertiary lavas and sedimentary deposits, or submerged 

 under the waters of Port Phillip Bay, that the evidence for recon- 

 :structing it is less direct than that of the Western Port area. The 

 sections available seem to indicate that the several tributaries 

 joined the trunk stream at more regular intervals than those of 

 Tthe Western Port system, and as a consec^uence the valleys were 

 more evenly graded. That a considerable volume of lava poured 

 •down the main valleys is evident from the thickness still existing 

 in the vicinity of Essendon and Bellarine. It probably first began 

 ito flood the watersheds in the vicinity of the railway line between 

 Melbourne and Drouin. Older volcanic lavas were pierced by bores 

 .at Mordi alloc and Frankston. 



From considerations of vis.cosity and cooling, it is obvious thai 

 lava flows confined to valleys and regulated as the Western Port 

 flows were by a single outlet, are relatively thick compared to those 

 that have welled out and flowed over a plain surface, as the lava 

 ■ did once the pre-existing watershedsi were flooded, and an extensive 

 ■lava field was formed. The implication is that wherever the valleys 

 were restricted the lava was to some extent banked up, resulting 

 in a greater thickness and extent of lava at and upstream from 

 ^he bottle-neck. Such a circumstance is implied by the Flinders 

 and Gembrook bottle-necks; the accumulation of lava at these 

 localities has had a profound effect on the subsequent development 

 ^of the streams of the Intermediate cycle by retarding it, and in the 

 case of the streams above the Gembrook bottle-neck, reversing their 

 ■direction. Bottle-necks undoubtedly marked a great change in 

 the gradients of the? lava fields. 



The gradient was materially affected by the proximity of a vent 

 or fissure to the valley. Prof. Skeats suggests^ that if a vent or 



1 Verb. cit. 



