Cn. V. 
OS' CORAL-REEFS. 
139 
refer to Plate I., which speaks more plainly to the eye, 
than any description to the ear. The authorities from 
which the figures have been coined, together with some 
remarks on them, are given on a separate page descrip- 
tive of the plate. At New Caledonia (Plate II., fig. 5) 
the barrier-reefs extend for 150 miles on each side of 
the submarine prolongation of the island ; and at the 
northern extremity these reefs appear broken up and 
converted into a vast atoll supporting a few low coral- 
islets. We may imagine that we see in New Caledonia 
the effects of subsidence actually in progress, — the 
water always encroaching on the northern end of the 
island, towards which the mountains slope down, and 
the reefs steadily building up their massive fabrics in 
the line of their ancient growth. 
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 hy 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 basin-like form 
of the central and 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, — 
