Ch. VI. DISTRIBUTION OF CORAL-REEFS. 157 



these being in all essential respects closely related. 

 Fringing-reefs, on the other hand, have been coloured 

 dull red, for there is an important distinction between 

 them and barrier-reefs and atolls with respect to the 

 depth beneath the surface, at which, as we must believe, 

 their foundations lie. The two distinct colours, there- 

 fore, mark two great types of structure. 



The dark blue colour represents atolls and sub- 

 merged annular reefs with deep water in their centres. 

 I have coloured a few low and small coral-islands 

 as if they had been atolls, although not including a 

 lagoon ; but this has been done only when it clearly 

 appeared that they had originally contained one. 

 When no such evidence exists 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 lagoon of a small atoll, 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 far into a 

 profoundly deep sea, so that it can hardly be con- 

 founded 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 now include 

 a deep-water moat. But this has been rarely done, 

 and each case is distinctly mentioned in the Appendix. 



The red colour represents reefs which fringe the 

 land closely where the sea is deep, and extend to 



