134 THEOKY OF THE FOEMATION Ch. V. 



newly-forrned barrier-reef is represented by unbroken 

 lines, instead of by dots as in the former woodcut, let 

 the work of subsidence go on, and the doubly-pointed 

 hill will form two small islands included within one 

 annular reef. Let the island continue to subside, and 

 the coral-reef will continue growing up on its own 

 foundation, whilst the water gains inch by inch on the 

 land, until the last and highest pinnacle is covered, and 

 there remains a perfect atoll. A vertical section of this 

 atoll is shown in the woodcut by the dotted lines; — a 

 ship is anchored in its lagoon, but islets are not supposed 

 yet to have been formed on the reef. The depth of the 

 lagoon and the width and slope of the reef, will depend 

 on the different circumstances to which it has been 

 exposed, as just stated with respect to barrier-reefs. 

 Any further subsidence will produce no change in the 

 atoll, except a diminution in its size, from the reef not 

 growing vertically upwards. I may here observe, that 

 a bank either of rock or of hardened sediment, level 

 with the surface of the sea and fringed with living 

 coral, would be immediately converted by subsidence 

 into an atoll, without passing, as in the case of a reef 

 fringing the shore of an island, through the inter- 

 mediate form of a barrier-reef. As before remarked, if 

 such a bank lay a few fathoms submerged, the simple 

 growth of the coral, without the aid of subsidence, would 

 produce a structure scarcely to be distinguished from a 

 true atoll ; for the corals on the outer margin, from being 

 freely exposed to the open sea, would grow vigorously 

 and tend to form a continuous ring, whilst the growth 



