136 ON CORAL REEFS AND ISLANDS. 



evidences of elevation, the Tonga Islands on the south have been 

 raised, and also the Fakaafo Group and others on the north. 



We cannot, therefore, distinguish any evidence that a general 

 rise is or has been in progress ; yet some large areas appear to 

 have been simultaneously affected, although the action has often 

 been isolated. Metia and Elizabeth Island may have risen ab- 

 ruptly : but the changes of level in the Feejees and the Friendly 

 Islands, appear to have taken place by a gradual action. 



3. Changes of Level in the Pacific preceding the Coral Reefs. 



The evidences of change of level previous to the growth of 

 coral are to be looked for in the topographical features of the 

 high islands, and the occurrence of conglomerate layers of rolled 

 stones in the structure of the mountains. 



To arrive at any general results on this subject, requires, there- 

 fore, a thorough knowledge of the surface of the islands, as well 

 as their interior structure : and as regards the last mentioned 

 point, the soil and vegetation over these tropical lands is every- 

 where an obstacle in the way of investigation. Our own sur- 

 veys have led to few results, and these can be stated in a single 

 paragraph. 



In our account of the island of Oahu, we have mentioned the 

 occurrence of layers of rounded stones and earth interstratified 

 with finer material at Ewa, and occurring at a height of sixty 

 feet above the sea. We were informed of similar deposits up a 

 valley on this part of the island, at a much greater elevation, but 

 did not have an opportunity to examine them. In crossing the 

 mountains of the western peninsula of Maui, Dr. Pickering ob- 

 served a basaltic pudding-stone, or conglomerate of half-rounded 

 stones, two thousand feet above the level of the sea. On 

 Mount Kea, similar beds were met with by Dr. Pickering at a 

 height of six thousand feet. On the island of Tahiti a coarse 

 conglomerate of partially rolled fragments was observed by 

 the writer up a branch from the Matavai valley, at an eleva- 

 tion of about one thousand five hundred feet above the sea. 

 From these facts, and others similar in the Feejees and Sa- 

 moa, we may infer that many of the islands were at a lower 

 level during some portion of their early history, while their 

 formation was in progress. But they do not prove their sub- 

 marine origin, nor anything definite respecting the actual con- 

 dition of the seas. This remains for future exploration. The 

 compact rocks of the interior of the islands, and especially the 

 crystalline syenitic rocks of Tahiti, were at one time considered 

 by the author evidence of their eruption beneath the pressure of 

 an ocean ; but this is not satisfactory, since the pressure required 

 for compactness would be afforded in the interior of a volcano, by 

 the molten lava itself. 



