26 president's address. 



The lower ur tidal portion of the Hawkesbury Kiver reaches 

 the sea through a magnificent gorge, eight hundred feet 

 deep. If this gorge were artificially filled up, the river thus 

 dammed back could not overflow the obstacle, because before 

 the flood had risen to the necessary height it would escape as 

 a by-wash into Port Jackson by the Parramatta River. These 

 levels sviggest that the lower Hawkesbury is older than the 

 height of the country traversed by it. The great depth, two 

 hundred feet below sea level, of the drowned valley at the 

 Hawkesbury Railway Bridge is accepted as a proof of con- 

 siderable recent subsidence. But surely the height of the 

 land cut through by the river is an equally positive proof of 

 a penultimate movement of slow elevation recognised by Mr. 

 Andrews* during which the river sank its channel through 

 the rising ground. On a smaller scale the Parramatta repeats 

 the same history, cutting through high ground as it passes 

 Sydney. 



An inner fold is suggested by the Nepean Gorge which "is 

 due to the river gradually eating down its bed as the Blue 

 Mountain scarp was elevated."! The gravel bed at Peurith, 

 descending almost to sea level, might represent the trough 

 between these two folds. If these deductions be allowed we 

 find that ridges parallel to the coast have gradually risen and 

 after being sawn across by the streams they opposed, the 

 outer one subsided. These movements agree with the theory 

 of pressure forcing landwards from the sea. The coastal 

 range along Illawarra and southwards probably rose with the 

 Lower Hawkesbury country. 



Inland from the Upper Shoalhaven is a succession of 

 parallel rivers and ranges running in a northerly and south- 

 erly direction. This tract appears to have received a push 

 from the direction of the Ulladulla trough (Fig. 4). The 



* Andrews, These Proceedings xxviii., 190.S (1904), p.812. 

 t Taylor, These Proceedings, xxxii., 1907, p.328. 



