356 CORALS AND CORAL ISLANDS. 
reef at the surface-level, during a slow subsidence, renders it 
impossible for the reef to rise above the waves and supply 
itself with soil, unless the subsidence is extremely slow, or 
has wholly ceased. 
From the above review of evidences of change of level, it 
appears that where there are no barrier reefs, and only fringing 
reefs, the corals may afford no evidence of subsidence. But 
it does not follow that the existence of only fringing reefs, or 
of no reefs at all, is proof against a subsidence having taken 
place. For we have elsewhere shown that through volcanic 
action, and at times other causes, corals may not have begun 
to grow till a recent period, and, therefore, we learn nothing 
from them as to what may have previously taken place. 
While, therefore, a distant barrier is evidence of change of 
level, we can draw no conclusion either one way or the other, 
from the fact that the reefs are small or wholly wanting, 
until the possible operation of the several causes limiting 
their distribution has been duly considered. 
The influence of volcanoes in preventing the growth of 
zodphytes extends only so far as the submarine action may 
heat the water, and it may, therefore, be confined within a 
few miles of a volcanic island, or to certain parts only of its 
shores. 
There are two epochs of changes in elevation which may 
be here distinguished and separately considered. 1. The sub- 
sidence indicated by atolls and barrier reefs. 2, Hlevations 
during more recent periods. 
