On Areas of Subsidence in the Pacific. 348 



only have reefs 120 feet in depth, instead of the great thick* 

 ness they are believed to possess. It is my present object to 

 fix the area of this subsidence, and suggest something with re- 

 gard to the extent of it in different parts of the ocean. On 

 examining a map of the Pacific, between the Sandwich Islands 

 and the Society group, wo find a large area just north of the 

 Equator with scarcely an island. To the south, the islands in- 

 crease in number ; and off Tahiti, to the northward and east- 

 ward, they become so numerous, and are so crowded together, 

 as to form a true archipelago. They are all, too, coral islands 

 throughout this interval. This, then, is a rather remarkable 

 fact in the distribution of these islands. But let us look far- 

 ther. 



If we draw a line running nearly E.S.E., from New Ire- 

 land, near New Guinea, just by Rotumah, Wallis's Island, 

 Samoa or the Navigators, the Society Islands, and thence 

 bending southward a little, to the Gambier group, we shall 

 have all the islands to the north of it, with two or three 

 exceptions, purely coral, while those to the south are very 

 generally high basaltic islands. These basaltic islands are 

 bordered by reefs, and these reefs are most extensive about 

 the islands nearest this line. In the Feejees, the north-eastern 

 part of the group contains some coral rings, while the north- 

 western consists of large basaltic islands with barrier reefs. 



Again, to the north of this boundary line, the islands far- 

 thest from it are usually small, in many instances mere points 

 of reef, a fraction of a mile in diameter, while some of the 

 coral islands near the same line are thirty or forty miles 

 in length. 



Now, a growing coral island or atoll will gradually become 

 smaller in diameter as subsidence goes on, and by the same 

 process must finally be reduced to a mere spot of reef, or, if 

 the subsidence is too rapid, that is, more rapid than the growth 

 of the coral, the island will become wholly submerged and 

 leave nothing at the surface. 



On these principles I base my conclusions. Along the equa- 

 tor, as explained, there is a large area containing few islands, 

 and these small, while farther north, the coral islands are nu- 

 merous and large : Is this not evidence, that the subsidence 



