president's addrkss. 19 



While the subacrial crest is hacked by denudation, the sub- 

 ■marine trough lies undisturbed. Had the upper limb remained 

 intact, it might have reared a noble arph eighteen thousand feet 

 high, the symmetrical counterpart of the three tliousand fathom 

 trough off TJIladuIla. It was considered by Rev. W. B. Clarke 

 that Australia and New Zealand were " separated by a synclinal 

 curve of the rock formations forming the sea channel between 

 them."* But an ordinary syncline would have its maximum 

 depth in the centie, not close to one side as it is in the Tasman 

 Sea. 



For comparison with the pressure trough, we will glance at 

 another type of coast. The whole contour of the Great Austra- 

 lian Bight appears to be governed by the Jeffreys Deep, a linear 

 depression df three thousand fathoms, whose axis nearly corres- 

 ponds to the steamer track from Melbourne to Cape Leeuwin. 

 Bass Strait, it is now suggested, niay owe its origin to an 

 extension of this furrow. Recent surve3^s by Mr. H, C. Daunevig 

 on the Fisheries Investigation vessel, " Endeavour," show the 

 sea-floor in and east of the Bight to descend from the coast in a 

 flight of broad steps suggestive of block faulting. The western 

 shore of the Bight extends in a wall of cliffs, truncated Tertiary 

 beds, which may be held the topmost step, unless indeed the con- 

 centric mountain ranges of the interior be so regarded. 



Below and beyond the continental shelf, the soundings off 

 Sydney exhibit great irregularity, which, it is now suggested, 

 may indicate a range of deep sea volcanic cones. From a study 

 of the basaltic dykes which intrude the Triassic strata around 

 Sydney, it appeared to Mr. G. A. Waterhouse that the radii of 

 one system would converge to a focus about a point twenty-three 

 miles east of Botany Heads. This focus is marked by a star 

 under the centre of the continental shelf on Plates i. and ii. The 

 radiation of these dykes has been thus plotted in the "Geological 

 Sketch Map of the country in the vicinity of Sydney," Mines 

 Department, 1903. Their occurrence shows a centre of great 



•Clarke, Tians. Roy. Sec. N.S.W., ix., 1875, 1876, p 23. 



