Ch. VI. DISTRIBUTION OR CORAL-REEFS. 
159 
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 harrier-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 harrier-reefs. The 
most obvious character of reefs of this class is the 
broad and deep-w T ater moat within the reef ; hut 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 
