5 2 4 
CCELENTERA TES. 
the channel ( e"). On the one side (/"), the fringing-reef is retained, while on the other 
it has disappeared, a fact due to currents and other such agencies. Finally, when 
the water is at the level {IV), two rocky islands are visible in a large lagoon 
surrounded by the reef (b'" b'"), with two small reef-islands {%" i'") developed on 
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mountain-peaks which have disappeared below the surface. The coral-rock has 
greatly increased in thickness, and almost entirely covers the former island. 
Such an ideal section corresponds exactly with the reality. In the following 
illustration the outline of the island of Aiva, one of the Fiji group, is given. In 
the lagoon there are two islands resembling the summits of mountains, just as in 
the diagram. The exact altitudes and depths are unfortunately not known, but it 
will be seen how well the theory of subsidence appears to explain the conditions. 
The chief objections which have been made to this theory of subsidence are 
the following. The simultaneous occurrence of atolls, barrier-reefs, and fringing- 
reefs in neighbouring regions does not coincide with the theory, nor does the 
appearance of atolls and barrier-reefs in regions in which recent elevation of the 
land has been proved. The discovery of extensive submarine banks of sediment 
formed of the calcareous portions of foraminifera, deep-sea corals, molluscs, etc., 
makes it possible to explain the formation of atolls and barrier-reefs without the 
help of subsidence, this explanation being more probable than that involving the 
sinking of extensive areas of land. The formation of atolls can be explained by 
