38 



A NATURAUSrS WANDEMINQS 



liixui'iaiit and has grown to a leas height than more externally, 

 aE(l cousequently we have a LagoQUj which aometimes, though 

 rarely, ia enclosed by an unbroken ring of coral ; more com- 

 monly, however, (as in Keeling atoll) the reef is intersectetl by 

 aeveral channels commuuieating between the lagoon and the 

 outer ocean. These channels are produced by many causes, 

 «uch iis, swift enrreuts interrupting the growth, decay of 

 the coral from local canses, and natural or accidental dis- 

 turbances. 



On a aubsidiug or stationary foundation snch a reef, raised 

 to the level of low- water mark, can never by any luxuriance of 

 its own growth rise above the water level and become a coral 

 isimid. Great storms, however, by breaking oil blocks of its 

 living and ever seaward-growing margin, and throwing them 

 on the lagoonward purtiou of tlie reef, alone are able to 

 commence the raising above the surface of the ocean uf 

 future islets, on whieli after the gradual accumulation of soil, 

 consisting of sand and the decaying fltttsam and jetsam of the 

 ocean, and the germinating seeds t hat the winds, the sea currents, 

 or the birds of the air may chance to cast on its bosom, a 

 green clothing of vegetation inevitably grow s up, 



lu traversing the Keeling atoll it seemed to be uuacconnt- 

 able how the interior, or lagoon margins of the islets, which 

 must necessarily have been thrown up above water at the 

 earlii^et stage of the existence of the atoll, still contiuue (on 

 the supposition that the atoll is subsiding) seveml feet 

 elevated above high-water level, and show no indi&ition of the 

 water's encroachment. As a storm so violent as the cyclone 

 of 1876 was cajtable of piling the torn-off blocks of the reef- 

 floor — composed of a natural cencrete of worn coral, shells, 

 and the hard parts of pelagic animals, imbedded in a solid 

 calcareous matrix — onhj a few yards over the higher edge of 

 the island, it is impossible for the lagoon margins, in some 

 places more than 8UU yards distant from the sea, to be kept up 

 in elevation by the debris of the outer margin ; and the greatest 

 storms do not affect perceptibly or permanently the shores of 

 the lagoon, 



Mr. Ross informed me that what Mr. Darwin, from the 

 undermining of cocoa-nut trees seen by him, supposed to be 

 sea encroachments, was intermittently taking place during 



