168 On the Sections of the Delta of the Yarra, 



viduals of all ages and sizes. The enormous number of 

 shells crowded into this layer, and the vast areal space occu- 

 pied by it, will be realised by anyone who sees large portions 

 of the new-made ground on the Port Melbourne side of the 

 river covered with the valves of the areas and oysters. 

 The bed had been previously reached in the river dredgings, 

 but its exact position and dimensions had not been deter- 

 mined. 



It was suggested to me that the areas and oysters were all 

 killed by an incursion of fresh water, as the upper limit of 

 the area bed is apparently definite, and the dense deposit of 

 silt commences above. But as the casts of the shells are of 

 the same silt as occurs in mass above — as marine shells occur, 

 more or less, throughout the silt, while scattered areas are to 

 be found, though rarel}^, amongst the shells of the lower 

 layer of the silt — it is clear that there is no evidence of fresh- 

 water influx. The deposit of shells ceased, probably because 

 the depth of salt-water became no longer sufficient, and the 

 areas and oysters migrated outwards to deeper waters. The 

 bivalve shells^ too, all contain a core of silt, which has con- 

 tracted on drying, so that the silt cast is smaller than the 

 cavity of the shell. This would not have been the case had 

 the shells been filled with mud from above. The oysters 

 are fine specimens of the mud-oyster, often 6 inches across. 



In addition to these, the two most prevalent forms, I also 

 obtained the following from the lowest 18 inches of the 

 silt : — Gardiwin tenuicostatum, Lam.; Mytilus latus, Lam.; 

 Natica conica, Lam.; Cooninella alveolata, Kilner ; Ain]pul- 

 larina fragilis, Quoy.; Cylichna arachnoides, Quoy.; 

 Philine Angasi, Sow.; Venus Icevigata, Lam.; Tellina 

 deltoidalis, Lam.; Nassa fasciata, Lam.; JV. pauperata, Lam.; 

 Balani, Polyzoa, Driftwood. 



Several fragments of driftwood were found on the area 

 horizon. One piece was covered with balani. I am indebted 

 to Mr. Meldrum for pointing out to me one fine trunk, ten 

 inches in diameter, in the part exposed, lying just on the top 

 of the yellow clay. The quantity of wood no doubt 

 indicates the estuarine origin of the silt, and the comparative 

 freshness and excellent state of preservation of the driftwood 

 bears witness to the imperviousness to water of the protect- 

 ing silt. 



The area band extends continuously throughout the 

 cutting at about the same horizon. Mr. Da vies also noted 

 it at the same level in a slip cut 500 feet north of the canal. 



