OCEAN CONTOURS, S.W. PACIFIC. 443. 



Tofoa and Falcon Shoal, in the Tonga group, are andesites, but 

 again there is an approach to a more basic t3rpe. 



In Fiji Woolnough says the most recent rocks appear to be a 

 series of andesites, but in the NeAv Hebrides Mawson regards the 

 andesitic eruptions as Miocene. 



Alkaline rocks also have a wide distribution. Limits of space 

 will not allow of a discussion of different alkaline groups. Those of 

 basic and acidic character are liere grouped together. The rocks of 

 Erebus appear to be alkaline if the samples forwarded by the Nimrod 

 expedition are a criterion of the composition of the whole mass. Rocks 

 of a sub-alkaline character are found at Campbell Island, and there is 

 a fine series of phonolites and related rocks at Duiiedin. At Auckland 

 the basanites are rather widely distributed, and their age is veiy late.. 



At Rarotonga a nephelinite occurs at the north and south end of 

 the island, and at Aitutaki there is a typical nepheline basalt. In 

 this connection the occurrence of alkaline phitonic rocks at Tahiti, as 

 i-ecorded by La Croix, ^.s of great intei'est. 



Basalts have, of couise, a wide distribution. They are now being 

 emitted at Savaii and the New Hebrides, and they occur in practically" 

 all the areas mentioned above, except in tlie volcanic plateau of New 

 Zealand, though even here examples are found to the west and to the 

 north of it. It appears that basalts have not yet been recorded from 

 Tahiti, though dolerites are abundant. It is worthy of mention in 

 tliis connection that in New Caledonia no Cainozoic volcanic rocks are 

 mentioned by Heurteau. 



Structure. 



The tectonics of the South-west Pacific region have attracted the 

 attention of many observers, and the following opinions appear tO' 

 express the divergent view^s which ai^e held : — 



Suess, it is well known, contrasts the Pacific coast type with the 

 Atlantic. It is important to quote the exact words of this contrast. 

 With two quoted exceptions : " The outer side of a folded range 

 nowhere determines the outlines of the Atlantic." '' The inner side of 

 folded ranges, jagged rias coasts which indicate the subsidence of 

 mountain chains, fractured margins of horsts and fractured tableland 

 form the diversified boundary of the Atlantic Ocean."' With one ex- 

 ception, " The whole boundary of the Pacific Ocean, wlierever it is. 

 known in any detail, is formed of mountain chains folded towards the 

 ocean in such a manner that their outer folds either form the 

 boundary of the mainland itself or lie in front of it as peninsulas 

 and island chains." Of the Pacific, he says: "The striicture of this 

 ocean shows that it was ah'eady in existence in the Trias epoch." He 

 appears to accept Dana's views " that the Loyalty Islands, New 

 Hebrides, Solomons, New Ireland, and the Admiralty group are the 

 outer arc of the Australian region," and to sympathise with Drasche's 

 idea, " It would be sound geology to draw the eastern boundary of 

 the Pacific Ocean outside the Island arcs from Kamchatka through 

 Japan and onwards through the Mac(]uarie Islands to Victoria Land 

 in the Antarctic region." He has al«o entered into a comparison 

 between the structure of Austi-alasia from west to east, and S. America 



