506 



CHARLES DARWIN 



near each other, I can show that there have been oscillations 

 of level; for in such cases the red or fringed 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 enor- 

 mous 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 consid- 

 eration 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 volcanos 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 



