BY MARJORIE I. COLLIXS. 



377 



for some distance inland, are chiefly Triassie sandstones of the Hawkesbury 

 Series wbich yield upon denudation a coarse yellow sand. 



Since no important rivers flow to the sea in this region, the so-called 

 Parramatta and Lane Cove Rivers merely being arms of Fort Jackson, and 



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Text fig. 1. iMap of coast of \ew South Wales between Port Jackson and Port 

 Hacking showing the nature of the drowned river valleys, and the sheltered 

 arms and bays in which mangrove and saltmarsh occur. 



George's River (Botany Bay) and Port Hacking River (Port Hacking) being 

 insignificant streams, silt and detritus are not carried out to sea, but tend to 

 be deposited in sheltered arms and bays. 



That there has been comparatively recent subsidence on the coast near 

 Sydney is evidenced in the fact that a bore sunk in the mud at Narrabeen 

 lagoon (north of Port Jackson) disclosed at a depth of over 50 feet layere of 

 Peaty soil containing Casuarina cones, and remains of other Xerophytes which 

 now occupy the sandstone country surrounding the lagoon (David and Halligan, 

 1908, p. 235). 



This recent subsidence, together with the physiographic features mentioned 

 above, possibly accounts for the non-existence of such extensive areas of tidal 

 flats as are found in other parts of the world (Ganong, 1903; Harshberger, 

 1909; Carey and Oliver, 1918); unless the accumulation of silt were sufficiently 

 rapid to keep pace with and overcome the effects of gradual subsidence, the 



