Ch. V. 
OF CORAL-BEEFS. 
141 
a second and somewhat sudden subsidence were to 
occur, the same results would follow as during the 
previous and similar subsiding movement. Conse- 
quently the new lagoon would be surrounded by two 
inwardly sloping ledges, which once existed as the 
summits of two successive annular reefs, both termi- 
nated on the inner side by vertical w T alls or cliffs. 1 
The ring or basin-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 
reefs would sometimes be found rising abruptly from 
a greater depth than that at which the efficient poly- 
pifers can flourish. We see this well exemplified in 
the small abruptly-sided reefs with which the deep 
lagoons of the Chagos and Southern Maldiva atolls are 
studded. With respect to the ring or basin-formed reefs 
of the Northern Maldiva atolls (see Plate II., fig. 4), 
it is evident from the perfectly continuous series which 
t xists, that the rings on the margin, although broader 
than the exterior or bounding reef of an ordinary 
atoll, are only modified portions of such a reef ; it is 
1 According to Mr. Couthouy (p. 26) tlie 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. [Some further information on this sub- 
ject has been obtained by recent investigations. See Appendix II.] 
11 
