INTRODUCTION. 
5 
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, hut 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 he seen, that both in 
atolls and barrier-reefs, the foundation to which the 
coral was primarily attached, has subsided ; and that 
during ibis 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 
