CORAL EEEFS AND ISLANDS. 



149 



Fig. 122. 



which the reef is attached. Imagine a circular Hue of breakers like a 

 snow-wreath on the sea, indicating a circular submarine ridge (the cor- 

 al reef) gemmed 



as before with . J^M. ^^^ 



wave-formed isl- 

 ets ; and within 

 the circle a la- 

 goon of placid 

 water twenty or 

 thirty fathoms 



deep (Fig. 121). It is a submarine urn standing in unfathomable 



water, as seen in the section Fig. 122. Through breaks in the reef 



_^_ ships enter the 



charmed circle 

 and find safe 

 harbor. By 



means of sound- 

 ing it is found 

 that on the in- 

 terior or lagoon 

 side the slope of 

 the bottom is 

 very gentle, but 

 on the outer or 

 seaward side is 

 very steep, often 



50° to 60°, and sometimes in places almost perpendicular to very 

 great depth. Fig. 123 gives a perspective view, and Fig. 124, a, 

 a map view, of an atoll, 

 showing the irregular cir- 

 cular form of the reef and 

 the little islands which gem 

 its surface. 



Small Atolls and La- 

 goonless Islands. — Besides 

 the atolls already described, 

 there are others, evidently 

 of similar origin, but much 

 smaller, in which the land 

 is continuous. Sometimes 

 the continuous line is open 

 on one side (Fig. 124, #), 



and the lagoon is still in connection with the open sea. Sometimes 

 the circle of land is complete, and the lagoon is isolated from the sea 



Fig. 123.— View of WWtsunday Island. 



