148 



ORGANIC AGENCIES. 



Fig. 118. 



coral platform is indicated to the eye of the observer by a white sheet 

 of breakers which surrounds the island like a snowy girdle, and ex- 

 tends some distance from the shore-line (Fig. 117). The section Fig. 

 118 will give a clear idea of the contour of land and sea bottom. In 



this and the fol- 

 lowing sections 

 the dotted parts 

 represent coral 

 formation. If 

 the island is 

 large, and considerable rivers flow into the sea, breaks in the reef plat- 

 form will occur oj)posite the mouths of the rivers, the corals in these 

 places being destroyed by the fresh, muddy waters. In the case of 

 fringing reefs no islands are formed by the action of waves, but only a 

 shore-addition to the original island, as shown at a a in the section. 



Barrier Reefs . — In many cases besides the fringing reef there is 

 another reef surrounding the island like a submarine rampart at the dis- 

 tance of from ten to fifty miles. As the reef rises nearly to the surface 

 of the sea, its position is indicated by a snowy girdle of breakers sur- 



___^ rounding the island 



at a distance, and 

 this snowy girdle is 

 gemmed with wave- 

 formed green islets. 

 Within this girdle, 

 and between the 

 rampart and the 

 island, there is a ship-channel twenty or thirty fathoms deep (Fig. 119). 

 Through breaks in the coral rampart ships enter this channel and find 



secure harbor in a 

 stormy sea. The 

 section Fig. 120 will 

 give a clear idea of 

 the conformation 



Fig. 120. 



of bottom. On the landward side of the coral rampart the slope of 

 the bottom is gentle, but on the seaward side it is very steep, so that 

 it is almost unfathomable at a short distance from the reef. 



Circular Reefs, or 

 Atolls. — These are the 

 most wonderful of the 

 reefs of the Pacific. 

 In a circular reef there 

 is no volcanic island or 

 other visible land to fig. 121. 



Fig. 119. 



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