140 THEORY OF THE FORMATION Ch. V. 



prevent their central spaces from being almost imme- 

 diately filled up with the sand and detritus driven 

 inwards by the waves from all sides. We can thus 

 understand how it is that few reefs less than half 

 a mile in diameter, even in the atolls where perfect 

 ring-formed reefs are found, include lagoons. This 

 remark, I may add, applies to all coral-formations. 

 The bason-formed reefs of the Maldiva Archipelago 

 may, in fact, be briefly described as small atolls 

 formed during subsidence over separate portions of a 

 large and broken atoll, in the same manner as the 

 latter was originally formed over a reef encircling one 

 or more mountainous islands. 



The disseverment of the larger Maldiva atolls. — 

 The apparent progressive disseverment of large atolls 

 into smaller ones in the Maldiva Archipelago, de- 

 mands an explanation. The graduated series which 

 marks, as I believe, this process, can be observed only in 

 the northern half of the group, where the atolls have 

 imperfect margins consisting of detached bason-formed 

 reefs. The currents of the sea flow across these atolls 

 with considerable force, as I am informed by Captain 

 Moresby, and drift the sediment from side to side 

 during the monsoons, transporting much of it seaward ; 

 yet the currents sweep with greater force round their 

 flanks. It is historically known that these atolls have 

 long existed in their present state ; it is intelligible 

 that they might thus remain, even during a slow sub- 

 sidence, owing to the continued growth of the corals, 

 and to the lagoon being kept at nearly its original depth 



