MUD ISLANDS, PORT PHILLIP BAY 137 



for treatment with sulphuric acid .... The following average 

 analysis will show its composition in 100 parts. 



Moisture 6- 12 



Organic matter 23 64 



Sand 44-82 



Phosphate of lime . . . . 20-62 



Carbonate of lime . . . . 3 31 



Other substances .. .. 149 



100-00. 



The organic matter is non-nitrogenous consisting chiefly of 

 vegetable debris." 



It has been mentioned here that on Ross's survey a waterway 

 (Fig. 1) is shown draining the tidal flat that formerly existed 

 between Middle Island and Eastern Island; this waterway was 

 joined by a short tributary from the north-west before it entered 

 the open water of Port Phillip Bay immediately north of the 

 guano deposit. On Cox's 1864 chart two other waterways are 

 shown on the north-east side of Boatswain's Island and another 

 was found on the eastern shore of Western Island. An examina- 

 tion of these shows that rney are actually tideways. At high 

 tide small basins behind the shore line are filled and as the tide 

 is falling, this water m the basins finds an outlet into the Inner 

 or Outer Lagoons by the tideways that have been suggestively 

 indicated as creeks on the charts; when the tide is rising the 

 water flows back through the tideways from the Lagoons into 

 the small basins. This disposes of the possibility of any of these 

 so-marked creeks being portions of a stream system that drained 

 the surface during the glacial stage when it was dry land. 

 Incidentally, it may be mentioned that on the Ncpean Peninsula 

 no streams have been formed on the dune rock and sand which 

 are too porous to hold up surface water; exactly the same condi- 

 tions are found on Mud Islands. 



Conclusions. 



The Mud Islands group has been formed by the incoming and 

 outgoing tidal streams behind a dune rock platform, portions of 

 which are exposed above high water mark on the islands. 



The islands are not more than 3,500 years old — all vegetation 

 and insect life found on them has appeared there within that 



period. 



That no evidence of the former occupation by the aborigines 

 was found is somewhat surprising in view of the fact that bird 

 rookeries exist. The tangled vegetation leaves few open spaces 



