480 DISTRIBUTION OF CORAL-REEFS. [CHAP. xx. 



of the earth's movement. It deserves notice, that in more than 

 one instance where single red and blue circles approach near 

 each other, I can show that there have been oscillations of level ; 

 for in such cases the red or fringed circles consist of atoll-j, ori- 

 ginally by our theory formed during subsidence, but subsequently 

 upheaved ; and on the other hand, some of the pale-blue or en- 

 circled 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 pe- 

 riod. 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 Ame- 

 rica) 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 sur- 

 rounded and penetrated, probably in more lines than one, by 

 narrow areas of subsidence. 



I have marked with vermilion spots all the many known active 

 volcanos within the limits of this same map. Their entire ab- 

 sence from every one of the great subsiding areas, coloured either 

 pale or dark blue, is most striking ; and not less so is the coin- 

 cidence of the chief volcanic chains with the parts coloured red, 

 which we are led to conclude have either long remained station- 

 ary, or more generally have been recently upraised. Although 

 a few of the vermilion spots occur within no great distance ot 

 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 



