140 
THEORY OF THE FORMATION 
Ch. V. 
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 hare 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 
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 1 ; 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 
he accumulated 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 pro- 
bably never rise above the level of the new lagoon, 
and in this case would be covered with sand. If now 
1 [This would seem possible in certain cases, though obviously the 
position would be an unfavourable one. See Appendix II.] 
