OCEAN CONTOURS, S.W. PACIFIC. 447 



and Dai-win mentions one at Tahiti. Within tliese groups tliere are 

 evidence of two movements, the atolls indicating subsidence, the 

 coral reefs elevation. It may be that in volcanic areas coral 

 reefs niay be formed round the margins of volcanic shoals as Lister 

 suggests is the case in the Tonga group. In other cases the elevation 

 may have preceded the depression. However, at Fiji, Woolnoug-h 

 I'ecoi'ds continuous elevation for a long period at Viti Levu, but we do 

 not at present know over what area this movement extends. 



In the neiglibourhood of the Cook islands, Palmerston, Suwarrow, 

 Penrhyn, Danger Islands are all isolated atolls, and may be held to 

 indicate elevation. The study of the land structure of these islands 

 prove that in some places where Darwin supposed subsidence in 

 progress there has in reality been elevation quite recently, but his 

 iiiain idea of differential movement is not traversed. 



Folding. 



It has been stated that many parts of the Pacific area under 

 review have been folded. New Zealand, Fiji, New Caledonia, all have 

 much folded rock masses. There is no reason to suppose that this 

 action has ceased. The very idea of folding suggests the elevation 

 and depression of two adjacent areas, an anticlinorium and syiicli- 

 norium, and such action would easily and simply account for the 

 observed movements in the South-west Pacific. It is Avell known 

 that actual folds are often quite gentle I'ises; it is only in regions of 

 maximum action that sharp mountain ranges or submarine ridges 

 are formed, and then only after the folded rocks have been submitted 

 to prolong subaerial erosion. 



The most important feature of the marine area is the great 

 trench east of Tonga and the Kermadecs. If this is regarded as a 

 '■ graben" or rift valley it presupposes a very remarkable want of 

 support within the earth's crust as a result of which a segment could 

 fall 6 inches below the sea surface. On the other hand it must be 

 remembered that the floor of the Pacific is 2,500 fathoms or more 

 below sea level. The bottom of the trench is about as much 

 below this level as the Tonga ridge is above it. It is, therefore, not 

 unreasonable to regard the trench as a syncline and the ridge as an 

 anticline of a huge earth fold of the progress of which the earth- 

 quakes are evidence. 



Mr. Hoben has showTi that an important centrum for earthquakes 

 exists near the end of the gi-eat trench which suggests that it was 

 Ktill extending. 



Conclusions. 



The ocean contours show that — 



1. The New Zealand Plateau extends to the outlying southern 



islands and has a north-westerly extension to the New 

 Caledonia plateau, and that another narrow ridg-e extends 

 to Norfolk Island. 



2. The Tonga-Kermadec ridge is a continuation of the 



dominant mountain structure of New Zealand and is 

 similar to it in arrangement of rock masses at its northern 

 extremity. - 



