74 SUNKLANDS OF PORT PHILLIP BAY AND BASS STRAIT 
due to the gentle tilting on a warp inshore of the almost level 
surface of the Werribee Plains and Keilor Plains lava fields. 
The Inner Basin north of the Nepean Bay Bar has a delta-form 
submerged level floor 78 feet below the surface of the Bay, and 
extending northwards for several miles ; from this level floor, the 
Bay shallows northwards until the Yarra Delta on the north 
shore of Hobson’s Bay is reached. 
Port Phillip was surveyed by Commander Cox in 1861 and his 
chart, the groundwork of the present Admiralty Chart, and also 
of the bathymetrical chart (Fig. 2), was published in 1864. Cox’s 
original chart is now in the possession of the Lands Department, 
Melbourne. Since 1861, various portions of the floor of Port 
Phillip have been the dumping grounds for silt from dredges, 
hoppers, and barges; but (yox’s chart gives reliable information as 
to the nature of the original floor. By far the greater number of 
his soundings bottomed on mud. Those on the 78 feet level floor 
north of the Nepean Bay Bar bottomed, with a single exception, 
on mud or mud and shells, the exception being a bottom of sand. 
A short distance north of the Bay Bar, the bottom is mud or sand, 
sometimes with shells: the bottom of the 17-fathom scour hole is 
sand and shells. Out from the Carrum Swamp for about a mile 
the bottom is sand, but further out it is mud. Inshore along the 
precipitous granodiorite coast below Mount Martha, the bottom is 
sand but it quicldy gives place to mud. At only one place in the 
central portion of the Bay is rock the bottom, namely, at 11 
fathoms Long. 144° 50' 12", Lat. 38° 0' 50". Westwards of the 
10 fathom line to the north-west shore of the Bay, and restricted to 
this portion, several soundings bottomed on stones. 
ITie question of the Decent emergence of the shores of the Bay 
has been discussed by Jutson (1931) and Hills (1940). Hills 
thinks that the date of emergence may be Recent, and that a 
eustatic fall of sea level may be suspected of having contributed to 
it; at Portarlington, Duck Ponds Creek, and between Sorrento 
and Dromana, however, he points out that there are deposits, 
indicating that, at least in part, the emergence was due to tectonic 
movements of uplift. 
I L Subsidence of Port Phillip SunMand. 
The Inner Basin of Poif Phillip Bay is a tectonic subsidence 
caused by tilting. It has been pointed out that the shore of Port 
Phillip Bay between Frankston and Dromana is a coast that has 
receded by erosion from the monoclinal flexure known as Selwyn’s 
Fault; this flexure has tilted the floor to the north-west. The 
tilting of the sediments between Hobson’s Bay and Mordialloc has 
