Rocks from Port Phillip Heads. 133 



little more than forty feet at the most. This reef ends abruptly 

 on the inner side as is shown by the rapid change to deeper 

 water, but on the seaside it shelves away much more gradually. 

 The block of stone under consideration came from a position 

 about midway between the Heads and a little on the seaside 

 of a line joining these two points of land and from a depth of 

 thirty-two feet. The outflowing tide meets suddenly with this 

 submarine shelf, and tlie rough choppy sea, well-known as the 

 •' Rip," is the result. This disturbed state of the Channel is 

 rendered more marked when a strong south-westerly wind is 

 blowing. The tidal current as it rushes out and in through the 

 entrance flows at a rate of about eight miles an hour, so that the 

 scour is considerable. Enormous quantities of sand, loosened from 

 the neighbouring beaches and sand banks inside the entrance, are 

 at times carried backwards and forwards through the Channel, 

 but it is hardly likely thab the rock under consideration has 

 resulted from the deposition of such sand under present 

 conditions, especially where the current runs fastest. 



It seems probable that the limestone represents a shallow 

 water marine formation which may pass under the Dune lime- 

 stone on either side. 



If such be the case, taking into consideration the recent record 

 of an extinct marsupial from the Dune rock\ this deposit may 

 represent in age the Bankivia beds on the Glenelg, recorded by 

 Mr. Dennant as Pleistocene.-^ 



We have to thank Captain Stalker of the Pilot Service, Mr. 

 McLean, the Chief Engineer of Ports and Harbours, and Mr. 

 T. H. Smith, Marine Surveyor, for opportunities afforded of 

 makincr these observations. 



1 Proc. Roy. Soc. Vic, vol. xiv., p. ii., N.S., p. 139. 



2 Trans, and Proc. Roy. Soc. Vic, vol. xxiii., p. 225. 1887. 



