5io 
DISTRIBUTION OF CORAL-REEFS 
CHAP. 
circles consist of atolls, originally by our theory formed during 
subsidence, but subsequently upheaved ; and on the other 
hand, some of the pale blue or encircled islands are composed 
of coral-rock, which must have been uplifted to its present 
height before that subsidence took place, during which the 
existing barrier-reefs grew upwards. 
Authors have noticed with surprise, that although atolls 
are the commonest coral-structures throughout some enormous 
oceanic tracts, they are entirely absent in other seas, as in the 
West Indies : we can now at once perceive the cause, for where 
there has not been subsidence, atolls cannot have been formed ; 
and in the case of the West Indies and parts of the East 
Indies, these tracts are known to have been rising within the 
recent period. The larger areas, coloured red and blue, are 
all elongated ; and between the two colours there is a degree 
of rude alternation, as if the rising of one had balanced the 
sinking of the other. Taking into consideration the proofs 
of recent elevation both on the fringed coasts and on some 
others (for instance, in South America) where there are no 
reefs, we are led to conclude that the great continents are 
for the most part rising areas ; and from the nature of the 
coral-reefs, that the central parts of the great oceans are 
sinking areas. The East Indian Archipelago, the most broken 
land in the world, is in most parts an area of elevation, but 
surrounded and penetrated, probably in more lines than one, 
by narrow areas of subsidence. 
I have marked with vermilion spots all the many known 
active volcanoes within the limits of this same map. Their 
entire absence from every one of the great subsiding areas, 
coloured either pale or dark blue, is most striking ; and not 
less so is the coincidence of the chief volcanic chains with the 
parts coloured red, which we are led to conclude have either 
long remained stationary, or more generally have been recently 
upraised. Although a few of the vermilion spots occur within 
no great distance of single circles tinted blue, yet not one 
single active volcano is situated within several hundred miles 
of an archipelago, or even small group of atolls. It is, 
therefore, a striking fact that in the Friendly Archipelago, 
which consists of a group of atolls upheaved and since partially 
worn down, two volcanoes, and perhaps more, are historically 
