248 Transdctions . < 



Tahiti. 



In 1904 Lacroix* described from Tahiti a series of plutonic rocks, em- 

 bracing gabbros, monzonites of alkaline affinities, as well as true nephelene 

 syenites ; but these may be the plutonic or hypabyssal equivalents of the 

 \ olcanic phonolites or nephelinites of the region. 



The other islands, as far as is known, are of volcanic origin, or are com- 

 posed of limestones. The occurrence of volcanic cones is not really opposed 

 to the existence of a former continent, as they may be the unsubmerged 

 peaks of the mountains which formerly covered its surface. However, 

 many of the volcanoes in the area are in process of construction, and have 

 had a submarine beginning — in fact, in some localities they have got no 

 further, and have not yet succeeded in establishing themselves in a sub- 

 aerial condition. These areas appear to be those where the crust of the earth 

 is rising, a special case being the line of volcanic vents parallel to the general 

 direction of the Tonga Islands, which undoubtedly show traces of recent 

 elevation. 



Thus, over a large area of the mid-Pacific region, and particularly that 

 part where Darwin demanded a sinking land to explain the formation of 

 coral reefs and atolls, there appears to be evidence of the former extension 

 of continental conditions, deduced from geological considerations. The 

 sinking crust practically impUes a former continent. As the boring at 

 Funafuti proved absolutely that the crust had been depressed relative to 

 sea-level in that locality by at least 1,100 ft., so we may suppose that sinking 

 has gone on in other places where there are coral islands of similar structure. 

 The fact of a recent elevation being known in numerous cases over the same 

 area does not dispose of the hypothesis of a general and prolonged sub- 

 sidence preceding this local elevation, and indeed it draws attention to the 

 instabiUty of the crust in that region. If subsidence has gone on to the 

 extent required by Darwin it would rapidly submerge any continent, and 

 leave little trace behind except that which chance geological discoveries 

 and a close study of the fauna and flora would reveal. 



An examination of the bathymetric map of the south-western Pacific, 

 modified by recent surveys, discloses certain well-marked submarine physical 

 features which suggest the lines of connection with the former Pacific con- 

 tinent. These may be easily followed if we suppose the whole area to be 

 raised stage by stage till it is above sea-level. Imagine, first of all, the 

 elevation to take place till the 1,000-fathom line is exposed. We should 

 then have a ridge extending north-west fi'om New Zealand towards the 

 mid-eastern coast of Queensland. Lord Howe Island would rise as a moun- 

 tain on its extreme western edge, and Norfolk Island would be a similar 

 mountain on its eastern edge. Sea would separate it completely from 

 Australia, which coast it would approach most nearly in the neighbourhood 

 of Rockhampton. New Caledonia would still be separated. 



To the north-east the coast of New Zealand would be extended somewhat, 

 but a long narrow ridge embracing the Kermadecs would follow the general 

 north-eastern trend of the islands, and would approach, but not junction 

 with, a similar ridge on which the Tonga Group would lie. A long tongue 

 of land would stretch down from the Fiji Islands and junction with the 

 Tongan ridge. A deep gulf would run to the south-west between Fiji and 



• Comptes Rendus," 1904. 



