94 president's address — section c. 



on the sea floor between the separated islands are still but imper- 

 fectly known, though soundings have now revealed some of their 

 more important features. It is also true that the majority of the 

 islands contain no folded rock series and are formed mainly of 

 volcanic material or of limestone, which is but shghtly tilted. So 

 far as the nature of the rocks is concerned, their mineralogical and 

 chemical composition is becoming fairly well known, at any rate in 

 the western portions of the area. 



Any conclusion that may be arrived at after consideration of 

 these various matters must certainly be less problematical than 

 those which are based solely on the positions of the islands. 



Two years ago^ I endeavoured to point out some of the main 

 features of the ocean floor as sho . n by soundings. The same 

 features have also been shown by Murray and by Murray and Lee, 

 Inspection of these maps shows that there is a continuous area of 

 relatively shallow water extending from New Zealand through the 

 Kermadecs and Tonga groups and thence through the Fiji Islands, 

 afterwards striking west, afterwards turning again to the north, and 

 passing through the New Hebrides. North of this group the 

 soundings are extremely few, and our opinions of the natvne of the 

 sea floor is conjectural only. 



This shallow ridge separates the Gazelle Basin from the main 

 basin of the Pacific, and it is noteworthy that one of Suess' lines 

 of elevation that whirl from the north of New Zealand must pass 

 through the centre of this well-sounded deep basin. Westward of 

 the Gazelle Basin there are two other ridges that separate it from the 

 deep water of the Thomson Basin of the Tasman Sea. Obviously 

 then all the sea floor west of the New Zealand, Tonga, Fiji, New 

 Hebrides line is greatly varied, alternately sinking to great depths 

 and anon rising almost to the surface of the ocean. On the other 

 side of this line the deep water extends over wide areas. A mere 

 consideration of the soundings appears, then, to suggest that the 

 line mentioned is the true border of the Pacific basin at the present 

 day, and this statement in no way involves any theories as to the 

 antiquity of the great basin. 



It is perhaps necessary to explain what one means by the state- 

 ment that the island line so often referred to is the real margin of 

 the Pacific basin. By this statement it is intended to convey the 

 meaning that any movement of elevation or depression, or any 

 rock movements that have affected that portion of the earth's 

 crust that lies to the west of this hne, may have left all that which 

 lies to the east entirely unaffected. In other words, the structural 

 margin is supposed to mark the limit of that portion of the area 

 that according to geological ideas may in the past have formed an 

 eastern extension of the Australian continent. 



In this matter a recital of observed rock foldings will help us 

 but little. It is only in New Zealand, Fiji and the New Hebrides 

 that they have been observed. In all of these cases they are 

 parallel to the line of elevation here described, a result that at least 



1 Proc. .4.A.A.S., Brisbane, vol. XII., p. 432. 



