ON 



CORAL REEFS AND ISLANDS. 



CHAPTER I. 



STRUCTURE OF CORAL REEFS AND ISLANDS. 



1. General Features of Coral Reefs and Islands. 



The general features of coral reefs and islands have often been 

 delineated by travellers, and are probably almost as familiar to the 

 reader as the scenes of the land around us. Yet a few brief re- 

 marks on this point form a necessary introduction to the more 

 minute descriptions of structure which follow. 



Coral reefs. — A wide platform of rock covered by the sea 

 except at low tide, borders most of the high islands of the Pacific. 

 It is a vast accumulation of coral, based upon the bottom in the 

 shallow waters of the shores. This bank or table of coral rock, 

 is of varying width, from a few hundred feet to a mile or more ; 

 and although the surface is usually nearly flat, it is often intersect- 

 ed by irregular boat channels, or occasionally encloses large bays, 

 affording harbor protection to scores of ships. In very many in- 

 stances the reef stands at a distance from the shores like an artifi- 

 cial mole, leaving a wide and deep channel between it and the 

 land ; and within this channel are other coral reefs, some in scat- 

 tered patches and others attached to the shore. The inner reef 

 in these cases, is distinguished as the fringing reef, and the 

 outer as the barrier reef. The sea rolls in heavy surges against 

 the outer margin of the barrier; but the still waters of a lake pre- 

 vail within, affording safe navigation for the tottling canoe some- 

 times through the whole circuit of an island : and not unfrequent- 

 ly, ships may pass, as by an internal canal, from harbor to harbor 

 around the land. The reef is covered by the sea at high tide, 

 yet the smoother waters indicate its extent, and a line of breakers 

 its outline. Occasionally a green island rises from the reef, and in 

 some instances, a grove of palms stretches along the barrier for 

 miles, where the action of the sea has raised the coral structure 

 above the waves. 



