Ch. V. 
OF CORAL-EEEFS. 
135 
dence and the length of the intervening stationary 
periods. 
It is evident in this section, that a line drawn per- 
pendicularly down from the outer edge of the new reef 
to the foundation of solid rock, exceeds, by as many 
feet as there have been feet of subsidence, that small 
limit of depth at which the effective polypifers can 
live, — the corals having grown up, as the whole sank 
down, from a basis formed of other corals and their con- 
solidated fragments. Thus the difficulty on this head, 
which before seemed so great, disappears. 
As the space between the reef and the subsiding 
shore continued to increase in breadth and depth, and 
as the injurious effects of the sediment and fresh water 
borne down from the land were consequently lessened, 
the greater number of the channels with which the reef 
in its fringing state must have been breached, especially 
those which fronted the smaller streams, will have 
become choked up by the growth of coral : on the wind- 
ward side of the reef where the coral grows most 
vigorously, the breaches will probably have first been 
closed. In barrier-reefs, therefore, the breaches kept 
open by draining the tidal waters of the lagoon-channel, 
will generally be placed on the leeward side, and they 
will still face the mouths of the larger streams, although 
removed beyond the influence of their sediment and 
fresh water ; — and this, it has been shown, is commonly 
the case. 
Referring to the following diagram (No. 6), in 
which the newly-formed barrier-reef is represented by 
