CORAL-REEFS. 157 



If during the prolonged subsidence of a shore, coral-reefs 

 grew for the first time on it, or if an old barrier-reef were 

 destroyed and submerged, and new reefs became attached 

 to the land, these would necessarily at first belong to the 

 fringing class, and, therefore, be coloured red, although the 

 coast was sinking; but I have no reason to believe, that 

 from this source of error, any coast has been coloured 

 wrongly with respect to movement indicated. Well charac- 

 terised atolls and encircling barrier-reefs, where several 

 occur in a group, or a single barrier-reef if of large 

 dimensions, leave scarcely any doubt on the mind respect- 

 ing the movement by which they have been produced; 

 and even a small amount of subsequent elevation is soon 

 betrayed. The evidence from a single atoll or a single 

 encircling barrier-reef, must be received with some caution, 

 for the former may possibly be based upon a submerged 

 crater or bank, and the latter on a submerged margin of 

 sediment, or of worn-down rock. From these remarks we 

 may with greater certainty infer that the spaces, especially 

 the larger ones, tinted blue in the map, have subsided, 

 than that the red spaces have remained stationary, or have 

 been upraised. 



On the grouping of the different classes of reefs. — Having 

 made these preliminary remarks, I will consider first how 

 far the grouping of the different kinds of coral-islands and 

 reefs is corroborative of the truth of the theory. A glance 

 at the map shows that the reefs, coloured blue and red, 

 produced under widely different conditions, are not indis- 

 criminately mixed together. Atolls and barrier-reefs, on 

 the other hand, as may be seen by the two blue tints, 

 generally lie near each other ; and this would be the natural 

 result of both having been produced during the subsidence 

 of the areas in which they stand. Thus, the largest group 



