INTRODUCTION. 



from being obvious, and from including most of the 

 coral-reefs existing in the open sea, it admits of a more 

 fundamental division into barrier and atoll-formed 

 reefs on the one hand, where there is a great 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 

 adjoining land, there is no such difficulty. The two 

 blue tints and the red colour on the map (Plate III.) 

 represent this main division, as explained in the be- 

 ginning 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 geo- 

 graphical 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-building 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 to 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 



