174 PORT JACKSON SILT BEDS. 



easy on pressure, and on rubbing with the hands leaves a dust 

 stain on them. The next overlying bed is composed chiefly of 

 greyish sand mixed with a few whole shells. On drying it 

 becomes very heavy and sets like a cement, being very hard to 

 break with the hand. The third bed is composed chiefly of broken 

 shells mixed with sand ; it is of a bluish colour and breaks up 

 much more easily than the previous formation. The ooze on the 

 top of all, drys into fine clay and sand of a bluish colour, which 

 has little cohesion. 



The records of the borings between Fort Macquarie Point and 

 Beulah Street, North Shore, show that the average depth of the 

 whole channel is 67 '1 feet, and of the silt 30*7 feet. The silt 

 therefore fills "45 or nearly a half of the channel. The greatest 

 depth of water at low tide is 54 feet in the south shore channel, 

 44 feet in the centre, and 52 feet in the north shore channel. The 

 greatest depth of silt at the above points is 57, 61, and 64 feet 

 respectively. The records of the borings on the line between 

 Dawes Point and Milson's Point give an average depth for the 

 whole channel down to the bed rock of 65*52 feet, and an average 

 depth of 18*86 feet for the silt beds, consequently *29 of the channel 

 has silted up. The greatest depth of water in the channel near 

 the south shore is 74 feet, in the centre 51 feet, and near the 

 north shore 57 feet. There is therefore a large excess of silt in 

 the channel on the first line as compared with the last line, which 

 is undoubtedly due to the accumulation from city drainage- 

 delivered by pipe at Fort Macquarie Point and to street drainage 

 etc. from Circular Quay. 



The history of these beds, could it be truly traced, would lead 

 us through some of the most interesting epochs of the earth's- 

 existence. The bottom silt is apparently the sole relic of a very 

 thick bed of alluvium, washed into the valley by the action of 

 erosion on the softer rocks of the waters] led for ages, which as the 

 valley subsided was almost completely scoured out by tidal currents.. 

 The deposition of the grey heavy silt followed very slowly, and 

 this was again succeeded by the dark coloured broken shell silt 

 and ooze, which have evidently been deposited within quite recent 

 periods since the tidal currents lost much of their scouring power. 



As the age of the Hawkesbury sandstone and the accompanying 

 shales which prevail over the whole watershed of Port Jackson 

 have been pronounced by different scientists to belong to the 

 Triassic age, it may be presumed that the same system of events 

 has been at work during the life of that age in this country, which, 

 according to Professor Dana, was the cause of the tilting, fractures, 

 joints and ejections in Eastern North America. Whilst the 

 preservation of its tilted beds, (for as far as I am aware, there are 

 only very few and narrow limited indications of folding in the 



