162 DISTRIBUTION OF CORAL-REEFS. Ch. VI. 



distinguishable. Such shores must either have remained 

 stationary since the period when they were first fringed; 

 or they may have been repeatedly upraised, with new 

 lines of reefs successively formed round them. If, how- 

 ever, coral-reefs became attached for the first time to a 

 shore which was subsiding, or if a barrier-reef was de- 

 stroyed and submerged with a new reef re-attached 

 to the shore, this would necessarily belong at first 

 to the fringing class, and would be coloured red, 

 although the land was sinking. So it would be with 

 a subsiding shore, if it plunged at a very high angle 

 beneath the sea, for in this case the reef would remain 

 closely attached to the land as it grew upwards, and 

 would resemble in all respects a fringing-reef. This 

 source of doubt applies especially to atolls which 

 have been upraised (sitch as Metia and Elizabeth 

 Islands), for from the steepness of their sub-marine 

 flanks, a reef growing up during a subsequent 

 period of subsidence round them, would still continue 

 closely to skirt the land, and would therefore be 

 coloured red. Well-characterised atolls or encircling 

 reefs, where several occur together in a group, or a 

 single barrier-reef if of large dimensions, clearly indicate 

 a movement of subsidence. The evidence from a single 

 atoll, or from a single encircling-reef, must be received 

 with caution, for the former may be based upon a sub- 

 merged crater or bank, and the latter on a submerged 

 margin of sediment or of worn-down rock. 



On the distribution of the different classes of reefs, 

 — Having made the foregoing preliminary remarks, I 



