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NOTICES OF NEW BOOKS. 



the existence of the reef is only indicated by a line of breakers, 

 or a few small islets crowned with cocoa-nut trees ; and the reef 

 in this case is at a considerable distance from a central island, 

 which is not itself due to the labours of the coral polyp. 



Fringing, or jhore-reefs, differ from barrier-reefs in not having 

 within them a broad channel of deep water, and in not lying so 

 far from the shore. There are other kinds of reefs allied more or 

 less nearly to one or the other |of these ; but they possess com- 

 paratively little interest. With respect to the origin of all these 

 reefs, the great difficulty lies in explaining the foundation of the 

 barrier and atoll-formed class, the fringing-reefs requiring little 

 consideration. 



Keeling Atoll 

 (Chart and Vertical Section). 



a. Sea level at low water ; depth 25 fathoms at 150 yards from the edge of 

 the reef. 



b. Outer edge of the flat part of the reef which dries at low water ; this edge 

 is either a mound or consists of jagged rugged points. 



c. A flat of coral rock covered at high water. 



d. A projecting ledge of brecciated coral rock washed by the waves at high 

 water. 



e. Loose fragments of coral only reached by the sea during gales, and its 

 upper part (6 to 12 feet high) clothed with vegetation. 



f. Low water level of the lagoon. 

 Note. The dotted line in the chart marks the limit of shallow water. 



Mr. Darwin has described every existing coral-reef in geogra- 

 phical order so far as he possessed information (with the exception 



