3498 CCELENTERA TES 



have been frequent. For these and other reasons, the volcanic origin of coral reefs 

 was rejected. It is obvious also that the same objections dispose of another hy- 

 pothesis, namely, that nonvolcanic summits and banks of equal height were the 

 foundations on which corals built. 



Darwin believed that all forms of reef arise by the gradual sinking of 

 the land they surround. This theory has been confirmed in all essential 

 points by Dana, and recently by Langenbeck. Other authorities have, however, 

 differed. 



The condition of the reefs attached to the Fiji islands illustrates Darwin's 

 theory of subsidence. The Goro reef lies close to the land along whose submarine 

 coast it grows. The Augo reef is of the same nature, but lies further from the land, 

 having a channel between it and the shore, and forms what is called a barrier 

 reef, which name denotes merely difference of position, not of kind. The barrier 

 reef of the island of Nanuku incloses a large stretch of sea, the islands within it 

 being nothing else than the rocky summit of a mountain. Darwin's theory gives 

 an explanation of these differences. If, for example, the island of Ango were very 

 gradually to sink, two things would happen, the island would disappear little by 

 little, while the reef would remain at the surface of the water, that is, so long as the 

 land did not sink faster than the corals could build. When the subsidence had gone 

 so far that only the last mountain summit remained above water, the condition found 

 in the island of Nanuku would be realized. Instances are also found in the Fiji 

 islands of the intermediate stages, where only a single mountain ridge and a few 

 isolated peaks remain above water. 



It is a known fact that large countries, such as Sweden and Greenland, are in 

 the act of sinking, and we also have direct proofs that reefs and their islands have 

 subsided. The depth of a reef, although not directly measurable, can be approxi- 

 mately estimated, and must in many cases be at least three hundred yards. Since 

 the living portion of a coral reef cannot reach more than eighteen to twenty fathoms, 

 such a depth of reef can only be .explained by the sinking of the land on which it 

 stands. If, instead of sinking, the land rises, the reef would be lifted out of water; 

 raised reefs three hundred feet high being known. This enormous thickness of reef 

 can hardly be explained without a previous subsidence, inasmuch as such a height 

 is greater than the known depth at which corals can live. The assumption that 

 many reefs are the consequence of simple subsidence thus appears highly 

 probable. 



The diagrammatic section figured on p. 3499 through an island and its reef 

 illustrates the action of gradual subsidence. The island at the water line (/) has a 

 simple fringing reef (//), a narrow rocky terrace at the level of the water, which 

 first descends very gradually and then more steeply. Supposing the island to sink 

 to the level (77) , what would happen ? While the land has sunk, the reef has risen, 

 and there is a fringing reef (/') and a barrier reef (), with a narrow chanel (e'} 

 between them. A further subsidence to level (777) greatly increases the width of 

 the channel (e"~). On the one side (/") , the fringing reef is retained, while on the 

 other it has disappeared, a fact due to currents and other such agencies. Finally, 

 when the water is at the level (IV}, two rocky islands are visible in a large lagoon 



