GEOGRAPHICAL DISTRIBUTION. 271 



bottom of a harbour. It has been further demonstrated that 

 in difterent groups, as the Ladrones, Sandwich Islands, Navi- 

 gators, New Hebrides, there is an inverse relation between the 

 extent of reefs and the evidences of recent volcanic action in 

 the island ; and that the largest reefs exist where there is no 

 proof of former igneous action, or where it has long ceased. 

 The existence of large reef-islands in open seas where the 

 neighbouring lands are mostly destitute of coral reefs, har- 

 monizes with the conclusions announced, since such islands are 

 in general removed from the deleterious influences just men- 

 tioned ; yet it is very possible that in some cases of this kind 

 the region of the open sea may have undergone a subsidence 

 not experienced equally by the lands on either side. 



The modifications of form and interruptions of reefs, arising 

 from abrupt or sloping shores, and tidal or local currents, have 

 also been exemplified. 



