CORAL REEFS, AND ISLANDS. 255 



is still smaller, and in the Marshall Islands the dry 

 land is not more than the one-hundredth part of 

 the whole. 



Professor Dana has given so graphic a description 

 of an atoll that we are tempted to quote it, rather 

 than submit a more prosaic one of our own. " The 

 reef of the coral atoll, as it lies at the surface, still 

 uncovered with vegetation, is a platform of coral 

 rock, usually two to four hundred yards wide, and 

 situated so low as to be swept by the waves at high 

 tide. The outer edge, directly exposed to the surf, 

 is generally broken into points and jagged indenta- 

 tions, along which the waters of the resurging wave 

 drive with great force. Though in the midst of the 

 breakers the edge stands a few inches, and some- 

 times a foot, above other parts of the platform, the 

 encrusting Nullipores cover it with varied tints, and 

 afford protection from the abrading action of the 

 waves. There are usually eighteen to thirty feet of 

 water near the margin, and below, over the bottom, 

 which gradually deepens outward, beds of coral are 

 growing profusely among extensive patches of coral 

 sand and fragments. Generally the barren areas 

 much exceed those flourishing with zoophytes, and 

 not unfrequently the clusters are scattered like tufts 

 of vegetation in a sandy plain. The growing corals 

 extend up the sloping edge of the reef nearly to low- 

 tide level. 



