88 THE SHORE 



in a ring of boulders ; or (4) with a great shallow lagoon 

 flat a mile or more across ; or, lastly, (5) with a lagoon 

 flat and a deep lake, like the lagoon above. In the 

 latter case it is termed an atoll (Fig. 25). Then it is 

 essentially a ring of reef, bent as you will, surrounding 



Vf^ 



.lt*/S I*'*'-! ,,' \ 



'.afcrMJfep* 



FIG. 25. A TYPICAL ATOLL ; PEROS BANHOS, CHAGOS GROIIP. 

 SCALE, 4] MILLIMETRES = i MILE. (FROM THE ADMIRALTY 

 CHART.) 



a lake, the lagoon, which may be a mere pool or a great 

 enclosed sea (thirty or forty miles across) , which, how- 

 ever, never attains a greater depth than 50 fathoms.* 



* This is true for the lagoons of both atoll and barrier reefs, 

 with one exception namely, that of Vanua Balavu (Exploring 

 Group), Fiji. Here a bight of deep water, 60 to 100 fathoms, 

 extends into the lagoon, superficially reminding one of a sunk 

 valley. 



