ORIGIN OF HARBOURS. 119 
The existence of harbours should therefore be attributed, to a great 
extent, to the configuration of the submarine land; while currents 
give aid in preventing the closing of channels, and keeping open 
grounds for anchorage. This subject will be further illustrated in 
the following pages. 
The permanency of coral harbours follows directly from the facts 
above presented. They are secure against any immediate obstruction 
from reefs. Any growing patches within them may still grow, and 
the margins of the enclosing reefs may gradually extend and contract 
their limits; yet only at an extremely slow rate. Notwithstanding 
such changes, the channels will remain open, and large anchorage 
grounds clear, as long as the currents continue in action. Coral 
harbours are therefore nearly as secure from any new obstructions as 
those of our continents. ‘The growing of a reef in an adjoining part 
of the coast may in some instances diminish or alter the currents, and 
thus prepare the way for more important changes in the harbours; 
but such effects need seldom be feared, and results from them would 
be appreciable only after long periods, since the growth of reefs is very 
slow in the most favourable circumstances. 
When channels have a bottom of growing coral, they form an ex- 
ception to the above remark; for as the coral is acted upon by no 
cause sufficient to prevent its growth, the reef will continue to rise 
slowly towards the surface. 
Again, when the channels are more than twenty fathoms in depth, 
they have an additional security beyond that from currents, in the fact 
that corals will not grow at such a depth. The only possible way in 
which such channels could close, without first filling up by means of 
shore material, would be by the extension of the reef from either side, 
till they bridge over the bottom below. But such an event is not 
likely to happen in any but very narrow channels. 
In recapitulation, the existence of passages through reefs, and the 
character of coral harbours may be attributed to the following causes: 
1. The configuration and character of the submarine land ;—corals 
not growing where the depth exceeds certain limits, or where there 
is no firm basement for the plantation. 
2. The direction and force of marine currents with their transported 
detritus ;—these currents deriving their course, as in other regions, 
from the features of the land, the form of the sea-bottom, and the reefs, 
and being sometimes increased in force by the contributions of island 
streams, which add to the detritus and to the weight of accumulating 
waters. 
