270 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 
