132 CORAL-REEFS. 



for instance, on the S.W. coast of Madagascar, a reef 

 extends for several miles, within which there is a broad 

 channel from seven to eight fathoms deep, but the sea does 

 not deepen abruptly outside the reef. Such cases, however, 

 are open to some doubts, for an old fringing-reef, which had 

 extended itself a little on a basis of its own formation, 

 would hardly be distinguishable from a barrier-reef, pro- 

 duced by a small amount of subsidence, and with its 

 lagoon-channel nearly filled up with sediment during a long 

 stationary period. Between barrier-reefs, encircling either 

 one lofty island or several small low ones, and atolls includ- 

 ing a mere expanse of water, a striking series can be shown: 

 in proof of this, I need only refer to the plates in this 

 volume, which speak more plainly to the eye, than any 

 description could to the ear. The authorities from which 

 the charts have been engraved, together with some remarks 

 on them, are given on a separate page descriptive of the 

 plates. At New Caledonia (Plate III., Fig. 3) the barrier- 

 reefs extend for 150 miles on each side of the submarine 

 prolongation of the island ; and at their northern extremity 

 they appear broken up and converted into a vast atoll- 

 formed reef, supporting a few low coral-islets : we may 

 imagine that we here see 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 lines of their ancient growth. 



We have as yet only considered the origin of barrier-reefs 

 and atolls in their simplest form; but there remain some 

 peculiarities in structure and some special cases, described 

 in the two first chapters, to be accounted for by our theory. 

 These consist — in the inclined ledge terminated by a wall, 

 and sometimes succeeded by a second ledge with a wall, 



