152 CORAL-REEFS. 



the actual coral-formation is concerned, they have no dis- 

 tinguishing character. Fringing-reefs have been coloured 

 red, for between them on the one hand, and barrier-reefs 

 and atolls on the other, there is an important distinction 

 with respect to the depth beneath the surface, at which 

 we are compelled to believe their foundations lie. The 

 two distinct colours, therefore, mark two great types of 

 structure. 



The dark blue colour represents atolls and submerged 

 annular reefs, with deep water in their centres. I have 

 coloured as atolls, a few low and small coral-islands, 

 without lagoons ; but this has been done only when it 

 clearly appeared that they originally contained lagoons, 

 since filled up with sediment : when there were not good 

 grounds for this belief, they have been left uncoloured. 



The pale blue colour represents barrier-reefs. The most 

 obvious character of reefs of this class is the broad 

 and deep-water moat within the reef: but this, like the 

 lagoons of small atolls, is liable to become filled up with 

 detritus and with reefs of delicately branched corals : when, 

 therefore, a reef round the entire circumference of an island 

 extends very far into a profoundly deep sea, so that it can 

 hardly be confounded with a fringing-reef which must rest 

 on a foundation of rock within a small depth, it has been 

 coloured pale blue, although it does not include a deep- 

 water moat : but this has only been done rarely, and each 

 case is distinctly mentioned in the Appendix. 



The red colour represents reefs fringing the land quite 

 closely where the sea is deep, and where the bottom is 

 gently inclined extending to a moderate distance from it, 

 but not having a deep-water moat or lagoon-like space 

 parallel to the shore. It must be remembered that fringing- 

 reefs are frequently breached in front of rivers and valleys by 



