128 CORAL FORMATIONS. 
sketches of the peaks, facts observed elsewhere, authorize in every 
essential point the transverse section given in figure 3, resembling 
closely, as is apparent, that in figure 1. The section is made through 
the line 50, 6'0’, of figure 2. It is unnecessary to add other illustra- 
tions. They may be made out from any of the eastern groups of the 
Feejees, the Gambier Group of the Paumotus, or Hogoleu in the 
Carolines. Wallis’s Island is another example of islets of rock in a 
large lagoon inclosed by a distant barrier. 
It has been asked why the interior channels do not become filled 
by coral reef, as the island sinks, and thus a plane of coral result, in- 
stead of a narrow belt; and this has been urged against the theory of 
Mr. Darwin. But it is a sufficient reply to such an argument, to 
state the fact that the subsidence admits of no doubt, and that the 
islands referred to as exemplifications of it, present this very pecu- 
liarity. It should be received, therefore, as a consequence of it, 
instead of an objection to the view, for it is the most common feature 
with all islands that have broad reef-grounds, or in other words, that 
show evidence of subsidence during the growth of the reefs. Broad 
channels, and even open seas within, as in Nanuku and the Exploring 
Isles, are therefore to be received as results of the subsidence, for 
which explanations should be sought. 
These explanations are at hand, and accord so exactly with facts 
ascertained, that the existence of inner passages becomes 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 influence of marine and fresh-water currents and trans- 
ported detritus. It is obvious, therefore, that the former may retain 
themselves at the surface, when through a too rapid subsidence the 
inner patches would disappear. Moreover, after the barrier is once 
begun it has growing corals on both its inner and outer margin, while 
a fringing reef grows only on one margin. Again, the detritus of the 
outer reefs is, to a great extent, thrown back upon itself by the sea 
without and the currents within, while the inner reefs contribute a 
large proportion of their material to the wide channels between them. 
These channels, it is true, are filled in part from the outer reefs, but 
proportionally less from them than from the inner. The extent of 
reef-grounds within a barrier, raised by accumulations, at the same 
time with the reefs, is often fifty times greater than the area 
of the barrier itself. Owing to these causes the rate of growth of the 
