PORT JACKSON SILT BEDS. 



179- 



and Granville would be provided with settling basins, and immense 

 bodies of silt would be precluded from entering the harbour, whilst 

 the period of slack water during which most of the silt is deposited 

 would be reduced. 



The fourth question as to the preservation of present channels 

 is of equal importance with the previous one and largely depends 

 on the satisfactory solution of it. It particularly necessitates a 

 careful training of the tidal currents. No precautions will absolutely 

 prevent drainage from entering the harbour, but if a plan could 

 be devised to strengthen the force of the tidal currents so that 

 they could transport it beyond danger of settlement inside it, the 

 preservation of deep channels would be assured through all time. 

 For this purpose it would be advisable to deepen and enlarge the 

 channels in the bays above Darling Harbour and in the Parramatta 

 River, as high up as the town of Parramatta so as to increase the 

 depth and volume of low water, which would increase the velocity 

 of the tidal currents, and enable them to carry off any ordinary 

 drainage into them beyond reach of mischief. At the same time 

 the South Channel on the bar should be deepened to 40 feet for a 

 width of 200 yards. This latter enlargement, estimating its length 

 at 600 yards and it its average depth at 5 yards, would require 

 the removal of 600,000 cubic yards only. By such means the 

 volume of tidal water entering the harbour would be increased, 

 inducing stronger tidal currents, and thus establishing the 

 equilibrium sought for, by which the action of deposit would be so 

 evenly proportioned to that of scour, that the silt left by the slack 

 water of one tide would be removed by the currents of the next 

 tide. Here, for the present I leave my subject, feeling far from 

 satisfied with the scant justice I have rendered it from want of 

 more accurate information on several important particulars. The 

 discussion it may provoke, may however make up for my short- 

 coinings, and may render good service in the treatment of a question 

 of such grave importance to our shipping interests. 



APPENDIX I. 



Abstract of boring records from Fort Macquarie 

 Point to Beulah Street, North Shore, across Port 

 Jackson reduced to low water mark. 





Distance i 



No. of 



froni Fort 



Boring. 



Macquarie 





Point in feet. 



1 



100 



2 



150 [ 



3 



250 



4 



350 



5 



400 



6 



500 | 



Depth 



Depth of 



of Water 



Silt in 



in feet. 



feet. 



22 



6 



36 



10 



52 



7 



54 



23 



51 



49 



40 



57 



Total Depth of 

 Channel in feet. 



28 

 46 

 59 

 77 

 100 

 97 



