i8 CORAL-REEFS. 



apparent difficulty with respect to the foundation on which 

 they must first have grown ; and into fringing-reefs on the 

 other, where, owing to the nature of the slope of the adjoin- 

 ing land, there is no such difficulty. The two blue tints 

 and the red colour on the map (Plate V.) represent this 

 main division, as explained in the beginning of the last 

 chapter. In the Appendix, every existing coral-reef, except 

 some on the coast of Brazil not included in the map, is 

 briefly described in geographical order, as far as I possessed 

 information ; and any particular spot may be found by 

 consulting the Index. 



Several theories have been advanced to explain the origin 

 of atolls, or lagoon-islands, but scarcely one to account for 

 barrier-reefs. From the limited depths at which reef-build- 

 ing polypifers can flourish, taken into consideration with 

 certain other circumstances, we are compelled to conclude, 

 as it will be seen, that both in atolls and barrier-reefs, the 

 foundation on which the coral was primarily attached, has 

 subsided; and that during this downward movement, the 

 reefs have grown upwards. This conclusion, it will be 

 further seen, explains most satisfactorily the outline and 

 general form of atolls and barrier-reefs, and likewise certain 

 peculiarities in their structure. The distribution, also, of 

 the different kinds of coral reefs, and their position with 

 relation to the areas of recent elevation, and to the points 

 subject to volcanic eruptions, fully accord with this theory 

 of their origin. 1 



1 A brief account of my views on coral formations, now published in 

 my Journal of Researches, was read May 31st, 1837, before the Geo- 

 logical Society, and an abstract has appeared in the Proceedings. 

 (Reprinted at the end of this volume. — Ed.) 



