FORMATION OF CORAL REEFS AND ISLANDS. 247 



Again, when the channels are more than twenty fathoma 

 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 reefs from either side, till they bridge 

 over the bottom below. But such an event is not likely to 

 happen in any but narrow channels. 



In recapitulation, the existence of passages through reefs, 

 and the character of the coral harbors, 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 rocky basement for the plantation. 



2. The direction and force of marine currents, with their 

 transported detritus; — these currents having their course 

 largely modified, if not determined, as in other regions, by the 

 features of the land, the form of the sea-bottom, and the posi- 

 tions of 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. 



3. Harbors which receive fresh-water streams, or submarine 

 springs of fresh- water, are more apt to be clear from sunken 

 patches ; and the same causes keep open shallow passages to 

 the shores, where there are shore reefs. 



It should be remembered, that while the effects from fresh- 

 water streams are so trifling around islands, they may be of 

 very wide influence on the shores of the continents where the 

 streams are large and deep, and transport much detritus. 

 This point is illustrated beyond. 



