GEOUltAPIIWAL DISTRIBUTION. 307 



therefore as we go northward, and the extent of the coral 

 reefs increase as we go southward ; no reefs occur about the 

 northernmost islands, while they are quite extensive on the 

 shores of Guam. This group, consequently, like the Hawai- 

 an and Navigator, illustrates the influence of volcanic action 

 on the distribution of reefs. 



A short distance southwest of the Ladrones, and nearly 

 in the same line, lie extensive reefs. Mackenzie's is an atoll 

 of large size. Yap (or Eap), Hunter's, Los Matelotas and 

 the Pelews (Palao), are high islands, with large reefs. In the 

 last mentioned, the reef grounds cover at least six times the 

 area occupied by the high land. Still farther south, toward 

 New Zealand, lie the large atolls Aiou, Asie and Los Guedes. 



South of the equator again : — The New Hebrides consti- 

 tute a long group of high islands, remarkable for the absence 

 of coral reefs of any extent, though situated between two of 

 the most extensive coral regions in the world, — the Feejees 

 and New Caledonia. But the volcanic nature of the group, 

 and the still active fires of two vents in its opposite extremi- 

 ties, are a sufficient reason for this peculiarity. Tanna is one of 

 the largest volcanoes of the Pacific ; and nearly all the islands 

 of the New Hebrides, as far as known, indicate comparatively 

 recent igneous action, in which respect they differ decidedly 

 from the Feejees. 



The Vanikoro Group, north of the New Hebrides, accord- 

 ing to Quoy, has large barrier reefs about the southernmost 

 island, Vanikoro ; but at the northern extremity of the range 

 there is an active volcano, Tinakoro, and no coral. Tikopia, 

 to the southeast of Vanikoro, is high and volcanic, according 

 to Quoy, though not now with active fires ; and it appears from 

 the descriptions given, to have no reefs. Mendana, northeast 

 of Tinakoro, according to Kruesenstern, as stated by Darwin, 



