38 A NATURALIST'S WANDEBINGS 



luxuriant and has grown to a less height than more externally, 

 and consequently we have a Lagoon, which sometimes, though 

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

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

 several channels communicating between the lagoon and the 

 outer ocean. These channels are produced by many causes, 

 such as, swift currents interrupting the growth, decay of 

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

 turbances. 



On a subsiding or stationary foundation such 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 

 island. Great storms, however, by breaking oif blocks of its 

 living and ever seaward-growing margin, and throwing them 

 on the lagoon ward portion ' of the reef, alone are able to 

 commence the raising above the surface of the ocean of 

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

 consisting of sand arid the decaying flotsam and jetsam of the 

 ocean, and the germinating seeds that the winds, the sea currents, 

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

 green clothing of vegetation inevitably grows up. 



In traversing the Keeling atoll it seemed to be unaccount- 

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

 must necessarily have been thrown up above water at the 

 earliest stage of the existence of the atoll, still continue (on 

 the supposition that the atoll is subsiding) several feet 

 elevated above high-water level, and show no indication of the 

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

 of 1876 was cai)able of piling the torn-o£f blocks of the reef- 

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

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

 calcareous matnx^only a few yards over the higher edge of 

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

 places more than 800 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. Eoss 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 



