president's address. 37 



Previous to the present cycle, that of IVIt. Kosciusko, it is 

 beUeved by geologists that a great peneplain extended from New 

 Guinea, in the north, to Tasmania, in the south. Probably this 

 peneplain extended eastwards beyond the limit of the present 

 coast, and was continued seawards by a broad continental shelf. 

 Pi-obablv also the ocean eastwards was then shallower and 

 narrower than to-da3^ 



The theory is now advanced that the present cycle commenced 

 by the sinking of the ocean-floor, and by pressure upon the 

 border of the continent. In the zone of compression, folding on 

 a large scale ensued, by which the continental shelf was de- 

 pressed, and the coastal range elevattid, simultaneously. 



In support, it is demonstrated that harmony exisi s between the 

 margin of the continental shelf on the one side, and the line of 

 the Pacific watershed on the other. Where the margin of the 

 shelf approaches the coast, so does the divide AVhere the divide 

 retreats from the shoreline, so does the margin of the shelf. 

 Fi-om this it is inferred that a broad shelf serves as a buttress to 

 that portion of the continent that lies behind it. Sheltered by 

 this buttress, radial rivers persist as relics from the peneplain- 

 epoch. As an example, attention is directed to the Burdekin 

 and Fitzi-oy Rivers, the longest rivers of Eastern Australia, 

 which discharge upon the broadest shelf. In New .South Wales 

 even the broadest shelf is narrow, namely, that off Newcastle. 

 But here our rule holds good, for nari'ow though it be, that 

 shelf 25i"otects the longest river, the best example (»f a radial 

 river, in the State. The Hunter is claimed accordingly as 

 a survivor of the radial rivers that nuist have prevailed in 

 peneplain-times. 



To show that the continental shelf is still being diminished, a 

 remarkable in.stance is furnished by Captain Sharp, of how the 

 shelf has retreated from 5-10 miles within forty years, near 

 lireak Sea Spit. The coastal ranges and the streams they hear, 

 both in Queensland and in New South Wales, are regarded as 

 very new geologically. 



A peculiar featui'e of many rivers of our Pacific slope is that, 

 for part of their course, they run in \ alleys parallel to the shore. 

 Then they are apt to break away and run direct to the sea. 



