OR (I AN 1C A GENCIES. 



101 



fringe. The existence and extent of such a reef are 

 revealed by the snow-white sheet of breakers which sur- 

 rounds the island like a snowy girdle (Fig. 52). Off the 

 mouths of large rivers breaks in the reef will occur. Fig. 

 53 is an ideal section showing the coral platform, cp, cp. 



So much for the agency of corals. The waves now 

 break off fragments from the outer part of the reef and 

 pile them up on the inner part against the land, and thus 

 form a low, level shore platform, s.p., s.p., above the 

 sea-level. Thus we have first the slope of the volcanic 

 island ; then the shore-platform of coral debris ; then 

 the submarine platform of living corals ; and, finally, the 

 deep water. In this case there is no coral island, but 

 only a coral addition to the volcanic island. 



2. Barrier Reefs. About the volcanic island there 



FIG 54. -Perspective view of volcanic island and barrier. 



may be little or no fringing reef, but at a distance of 

 five, ten, or fifteen miles away, in deep water, there rises 



FIG. 55. Section of volcanic island and barrier. 



a line of reef like a great rampart surrounding the island, 

 and, as it were, protecting it from the attacks of the sea. 



