CORAL-REEFS. 133 



round the shores of certain lagoons and lagoon-channels; 

 a structure which cannot, as I endeavoured to show, be 

 explained by the simple growing powers of the corals, — in 

 the ring or bason-like forms of the central reefs, as well as 

 of the separate marginal portions of the northern Maldiva 

 atolls, — in the submerged condition of the whole, or of parts 

 of certain barrier and atoll-formed reefs; where only a 

 part is submerged, this being generally to leeward, — in the 

 apparent progressive disseverment of some of the Maldiva 

 atolls, — in the existence of irregularly formed atolls, some 

 being tied together by linear reefs, and others with spurs 

 projecting from them, — and, lastly, in the structure and 

 origin of the Great Chagos Bank. 



Step-formed ledges round certain lagoons. — If we suppose 

 an atoll to subside at an extremely slow rate, it is difficult 

 to follow out the complex results. The living corals would 

 grow up on the outer margin ; and likewise probably in the 

 gullies and deeper parts of the bare surface of the annular 

 reef; the water would encroach on the islets, but the 

 accumulation of fresh detritus might possibly prevent 

 their entire submergence. After a subsidence of this very 

 slow nature, the surface of the annular reef sloping gently 

 into the lagoon, would probably become united with the 

 irregular reefs and banks of sand, which line the shores 

 of most lagoons, Should, however, the atoll be carried 

 down by a more rapid movement, the whole surface 

 of the annular reef, where there was a foundation of 

 solid matter, would be favourably circumstanced for the 

 fresh growth of coral ; but as the corals grew upwards on 

 its exterior margin, and the waves broke heavily on this 

 part, the increase of the massive polypifers on the inner 

 side would be checked from the want of water. Conse- 

 quently, the exterior parts would first reach the surface, 



