FORMATION OF CORAL ISLANDS. 885 



Here the sea itself assists iu raising the elevation of 

 one part of the reef by disintegrating or breaking 

 up others. When the reef is of snch a height, says 

 Chamisso,* that it is left almost dry at low-tide, the 

 coral insects abandon their work. Above this line a 

 continuous stony stratum may be observed, com 

 posed of the shells of molluscs, of echinoidoB with 

 tlieir points broken, and of fragments of cora^ 

 cemented together by a calcareous sand produced b) 

 the pulverisation of the shells. The heat of the sun 

 often penetrates this mass when it is dry, and causes 

 cracks in different directions ; then the waves liave 

 sufficient power to break off masses of coral, some- 

 times six feet long, and four or five feet in tliickness, 

 and to throw them up on the reefs, whereby the crust 

 is so elevated that high-tide only covers it at certain 

 seasons of the year. The calcareous surface does 

 not, however, suffer any subsequent disturbance, but 

 offtjrs a soil to the seeds of trees and plants brought 

 by the waves, upon which the vegetables grow with 

 sufficient rapidity to form very soon a covering for its 

 dazzling white surface. Even before the trees 

 become sufficiently busliy to form a wood, the sea- 

 birds build their nests on the once bare reef; and 

 land-birds, lost in the ocean waste, fly to it as a place 

 of refuge ; and still later, long after the coral 

  Exi)editioa of Kotzebue. 



2 c 



