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 ha5 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. Tlieir 

 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 



