138 THEORY OF THE FORMATION Ch. V. 



basis for the attachment and subsequent upward growth 

 of a great bed of living corals. But the corals would 

 not be able to grow up from the sandy shores of the old 

 lagoon ; consequently the new annular reef would be 

 separated from the new lagoon by an abrupt edge or wall. 

 As the corals would grow upwards much more vigorously 

 on the outer side, and more detritus would be accumu- 

 lated there, the surface of the new annular reef would 

 slope gently inwards. Hence the summit of the new 

 annular reef on the inner side would probably never 

 rise above the level of the new lagoon, and in this case 

 would be covered with sand. If now a second and 

 'somewhat sudden subsidence were to occur, the same 

 results would follow as during the previous and similar 

 subsiding movement. Consequently the new lagoon 

 would be surrounded by two inwardly sloping ledges, 

 which once existed as the summits of two successive 

 annular reefs, both terminated on the inner side by 

 vertical walls or cliffs. 1 



The ring or bason-formed reefs of the northern 

 Maldiva atolls. — I must first observe that small reefs 

 within large lagoons or within broad lagoon-channels, 

 would grow up during subsidence ; and therefore such 



1 According to Mr. Couthouy (p. 26) the external slope round many- 

 atolls descends by a succession of ledges or terraces. He attempts, but 

 I doubt whether successfully, to explain this structure somewhat in the 

 same manner as I have attempted, with respect to the internal ledges 

 round the lagoons of certain atolls. More facts are wanted regarding 

 the nature both of the interior and exterior step-like ledges. Are all 

 the ledges, or only the upper ones, covered with living coral ? If they 

 are all so covered, do the species differ on the different ledges ? Do 

 ledges occur on the inside and outside round the same atolls ? &c. 



