ORIGIN OF BARRIER REEFS AND ATOLLS. 263 
The steps in the process are illustrated in the following 
sections of an island and its reefs. If the water-level be at 
I, the island enclosed would be, like Goro, one with a simple, 
fringing reef f, f Suppose a submergence to have gone on 
until II is the water line: —the reef growing upward may 
then have the surface represented by 0’ f’, b’ f’. There is 
here a fringing reef (/’), and also a barrier reef (6’), with a 
narrow channel (c’) between, such as exists on the shores of 
Tahiti (p. 149). Suppose a further submergence, till III is 
the water line: then the channel (c” ¢”), within the barrier 
has become quite broad, as in the island of Nairai or Angau; 

SECTION ILLUSTRATING THE ORIGIN OF BARRIER REEFS. 
on one side (f’”’) the fringing reef remains, but on the other 
it has disappeared, owing, perhaps, to some change of cir- 
cumstance as regards currents which retarded its growth 
and prevented its keeping pace with the subsidence. With 
the water at IV, there are two islets of rock in a wide lagoon, 
along with other islets (¢” 2”) of reef over two peaks which 
have disappeared. 6” b’” are sections of the distant enclos- 
ing barrier, and c” c”, and other intermediate spots, of the 
water within. The coral reef-rock by gradual growth has 
attained a great thickness, and envelops nearly the whole of 
the former land. Nanuku, the Argo Reef, and Exploring 
Isles are here exemplified ; for the view is a good transverse 
section of either of them. 
The supposed similarity between these ideal sections and 
