CORALS. 523 



This hypothesis further requires the assumption that immense numbers of volcanoes 

 must have arisen over a limited area, and a still more improbable supposition, that 

 all these volcanoes rose to almost the same level, seeing that the coral animals 

 occur only within a depth of about twenty fathoms. Craters nearly fifty miles in 

 diameter must be assumed to have existed, and others of twenty to twenty-five miles 

 must 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 

 hypothesis, 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 Ango 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 encloses 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 realised. 

 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 accompanying diagrammatic section (p. 524) through an island and its reef 

 illustrates the action of gradual subsidence. The island at the water-line (I) 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 (II), what would happen ? While the land has sunk, the reef has risen, 

 and there is a fringing-reef (/') and a barrier reef (b), with a narrow channel (e') 

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



