264 CORALS AND CORAL ISLANDS. 
existing islands is fully sustained by actual comparison. The 
figure beyond is a map of the island of Aiva, in the Feejee 
Group. There are two peaks in the lagoon, precisely as 
above; and although we have no soundings of the waters in 
and about it, nor sketches of peaks, facts observed elsewhere 
authorize in every essential point the transverse section here 
viven, which resembles closely, as is apparent, the preceding. 
The section is made through the line b db, b’ b’, of the map. 
It is unnecessary to add other illustrations. They may be made 



MAP AND IDEAL SECTION OF AIVA ISLAND. 
out from any of the eastern groups of the Feejees, the Gamb- 
ier Group of the Paumotus, or Hogoleu in the Carolines. 
It has been urged against the theory, that the process ap- 
pealed to ought to fill the channels inside as the island sinks, 
and thus a plane of coral result, instead of an outside barrier 
reef and narrow belts within. 
But the facts prove that the existence of ner passages 
is a necessary feature of such islands. It has been shown 
that the ocean acts an important part in reef-making, that 
the outer reefs, exposed to its action and to its pure waters, 
grow more rapidly than those within, which are under the in- 
fluence of marine and fresh-water currents and transported 
detritus. It is obvious, therefore, that the former may retain 
themselves at the surface, when through a too rapid subsi- 
