54 J- T. Jutson : 



of hard, non-vesicular, dense basalt. Some blocks were polygonal 

 in shape. The writer has no doubt that these boulders are not 

 waterworu, but are the remains of basalt in situ, jointed into small 

 vertical columns, and then weathering into the observed forms bt 

 exfoliation, i.e., spheroidal weathering. 



The rocks were found near the crest of the ridge to the east of the 

 Darebin Creek, and high above and disconnected from the Newer 

 Basalt flow of that valley. 



The actual relation to the silicitied sediments could not be seen, 

 but the latter may be taken to represent a small altered patch of 

 the ordinary Kainozoic sediments of the district. The silicification 

 is apparently connected with the basalt, and the latter would prob- 

 ably therefore be younger. It is evidently older than the Newer 

 Basalt, and its most likely age is that of the Older Basalt of Greens- 

 borough, and of the cap of Mt. Cooper, if the latter ultimateh' turn 

 out to be of the same age as the former, the likelihood of which is, 

 in the writer's opinion, strengthened by the Ivanhoe example. The 

 possibility of the basalt being a volcanic plug of the Greensborough 

 Older Basalt period must also he borne in mind, although there is no 

 direct evidence on the point. 



It is interesting to observe that only at this locality (where basalt 

 is associated with them) are the Kainozoic sediments of Ivanhoe and 

 Heidelberg silicified to such an extent as to merit the term quartzite 

 being applied to them. Yet at Greensborough basalt caps are 

 numerous, and scarcely any silicification has taken place, but at 

 Kangaioo Ground both silicified and unsilicified Kainozoic sedi- 

 ments underlie the basalt. At Mt. Cooper the underlying sedi- 

 mentary rocks have been extensively silicified. Mr. Armitage^ has 

 suggested a cause for the silicification of similar rocks in the Essen- 

 don district, but the matter cannot here be discussed. 



The Physiographic Relations. 



As we have seen, the question of the age of the basalts and sedi- 

 ments is bound up with the physiography of the district. 



The following points appears to be well established. To the 

 north, east and south of Melbourne, tlie higher hills are capped by 

 Kainozoic sands and gravels, which were originally continuous and 

 rested upon a practically level surface of the older rocks, this sur- 

 face representing an old base-level of erosion, which stretched far 

 up the Yarra valley, and which for convenience has elsewhere been 

 called the Nillumbik Peneplain. These gravels and grits around 



1 Vic. Nat., vol. xxvii. (1910), p. 02. 



