2^0 THE OCEAN". 



on the points of the reef, which, being commonly 

 tufted with cocoa-nut trees, is perceptible as far off 

 as the island itself, and forms a most convenient 

 landmark. 



Notwithstanding that the highest point of these 

 narrow islets is rarely more than a yard above the 

 tide, it is a remarkable fact that fresh water is fre- 

 quently found in them. It is probable that the coral 

 rock acts as a filter, allowing the sea- water to perco- 

 late through its porous substance, but excluding all 

 its saline particles held in solution. 



Though I have described the two parts of a Coral 

 Island, or Atoll, as it is called, as distinct, yet the 

 difference is only in appearance ; the foundation on 

 every side is the same, a coral reef rising to the sur- 

 face : but the side most exposed to the action of the 

 waves driven in by the trade-winds, is invariably the 

 first to be projected, and attains a higher elevation 

 than the leeward side. Neither must it be supposed 

 that the belt to windward is always continuous, 

 though the interruptions are comparatively few. A 

 close inspection will likewise show that the outline 

 of the whole reef possesses much less regularity of 

 form than its aspect from a distance indicated. The 

 form, however, is invariably a more or less close 

 approach to a circle. Sometimes the land is con- 

 tinuous through the whole circumference, with the 

 exception of a channel or two into the lagoon, which 

 presents the appearance of a circular pond with a 

 verdant border surrounding it ; again, another atoll 

 will be found which has brought its ring of reef 

 scarcely to the surface, exposing, perhaps, a single 



