Ch. V. OF COKAL-KEEFS. 137 



We have as yet only considered barrier-reefs and 

 atolls in their simplest form ; but there remain some 

 peculiarities in structure and some special cases, which 

 were described in the two first chapters, to be accounted 

 for by our theory. These consist, firstly, in the presence 

 of an inclined ledge terminated by a wall, and some- 

 times succeeded by a second ledge with a wall, round 

 the shores of certain lagoons and lagoon-channels ; for 

 this structure cannot be explained by the mere growth 

 of the corals ; — secondly, in the ring or bason-like form 

 of the centraland circumferential reefs of the northern 

 Maldiva atolls, — thirdly, in the disseverment of some 

 of the Maldiva atolls, — fourthly, in the existence of 

 irregularly formed atolls, some tied together by linear 

 reefs, and others with spurs projecting from them, — 

 fifthly, in the submerged condition of the whole, or of 

 parts of certain barrier and atoll-formed reefs, and in 

 the submerged parts being generally to leeward, — ■ 

 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, 

 the living corals would grow up on the outer margin 

 and on the deeper parts of the bare and hard surface 

 of the annular reef. Detritus would soon accumulate 

 and become agglomerated on this surface, after a time 

 forming islets. Consequently the whole atoll before long- 

 would recover its former structure and appearance. If, 

 however, an atoll were to subside somewhat suddenly to 

 the depth of a few fathoms, the whole annular reef, 

 consisting of solid rock, would constitute an excellent 



