504 
SUBSIDENCE OF CORAL-REEFS 
CHAP. 
will go on vigorously growing upwards ; but as the island 
sinks, the water will gain inch by inch on the shore—the 
separate mountains first forming separate islands within one 
great reef—and finally, the last and highest pinnacle disappear¬ 
ing. The instant this takes place, a perfect atoll is formed : 
I have said, remove the high land from within an encircling 
barrier-reef, and an atoll is left, and the land has been removed. 
We can now perceive how it comes that atolls, having sprung 
from encircling barrier-reefs, resemble them in general size, 
form, in the manner in which they are grouped together, and 
in their arrangement in single or double lines ; for they may 
be called rude outline charts of the sunken islands over which 
they stand. We can further see how it arises that the atolls 
in the Pacific and Indian Oceans extend in lines parallel to the 
generally prevailing strike of the high islands and great coast¬ 
lines of those oceans. I venture, therefore, to affirm, that on 
the theory of the upward growth of the corals during the 
sinking of the land, 1 all the leading features in those wonderful 
structures, the lagoon-islands or atolls, which have so long 
excited the attention of voyagers, as well as in the no less 
wonderful barrier-reefs, whether encircling small islands or 
stretching for hundreds of miles along the shores of a continent, 
are simply explained. 
It may be asked whether I can offer any direct evidence 
of the subsidence of barrier-reefs or atolls ; but it must be 
borne in mind how difficult it must ever be to detect a move¬ 
ment, the tendency of which is to hide under water the part 
affected. Nevertheless, at Keeling atoll I observed on all sides 
of the lagoon old cocoa-nut trees undermined and falling ; 
and in one place the foundation-posts of a shed, which the 
inhabitants asserted had stood seven years before just above 
high-water mark, but now was daily washed by every tide ; on 
inquiry I found that three earthquakes, one of them severe, 
had been felt here during the last ten years. At Vanikoro 
1 It has been highly satisfactory to me to find the following passage in a' 
pamphlet by Mr. Couthouy, one of the naturalists in the great Antarctic Expedition 
of the United States : “ Having personally examined a large number of coral-islands, 
and resided eight months among the volcanic class having shore and partially 
encircling reefs, I may be permitted to state that my own observations have 
impressed a conviction of the correctness of the theory of Mr. Darwin.” The 
naturalists, however, of this expedition differ with me on some points respecting 
coral formations. 
